Corrosion Control Measures

Chapter 4 Corrosion Control Measures The main cause of corrosion of metals and alloys is their thermodynamic instability in the environment. Only some of them, gold, silver, and platinum are resistant to the environment of the Earth’s crust and exist as pure metals. Many “attractions” are present around the metals: water and aqueous solutions of electrolytes, gases (oxygen, ozone, sulphur oxides, nitrogen oxides, and hydrogen sulphide), salts, acids, alkalis, organic substances, and microorganisms. The conditions around the metals, such as high temperatures and temperature changes, high velocities of liquids or their stagnation, also contribute towards corrosion. Metals are not able to be apathetic to the environmental “attractions” and conditions. Our aim: to keep metallic structures in a good state, to prevent their oxidation, deterioration, loss of functional properties, damage, and failure. In order to select the correct measures of corrosion control, we have to study the corrosion mechanism, how metals react with the environment, the factors of metallic corrosion, how metals behave in different media (in water, in the atmosphere, in the presence of various salts and gases) and, of course, corrosion phenomena (general, pitting, galvanic, erosion, cavitation, MIC, and others). If a person feels ill, he goes to the doctor. What does a doctor do? He carries out all the necessary physical and biochemical analyses. Only on the basis of these results, can a doctor come to a conclusion about the illness and may provide the solution for the remedy. A similar situation occurs with the “illness” of metals – their corrosion. If we know the causes and factors of the corrosion of metals, we can select the correct methods of prevention and control. It is very important to predict the occurrence of possible corrosion problems and to plan correct anti-corrosion measures accordingly. For example, if we design new equipment with different alloys, it is important to select those with close electrode potentials, or electrically isolate different alloys, or to increase an anodic area towards a cathodic one, and to carry out all this – in order to exclude or to diminish the probability of galvanic corrosion. If the main factor of corrosion is the flow velocity of a liquid, we have to take measures for its change (increase in the case of low velocity and stagnation, or decrease in the case of turbulence), or to select a suitable alloy or coating resistant to erosion or cavitation. If it is impossible to use a corrosion-resistant alloy A. Groysman, Corrosion for Everybody, DOI 10.1007/978-90-481-3477-9_4, © Springer Science+Business Media B.V. 2010 151 152 4 Corrosion Control Measures or suitable coating (metallic, ceramic or polymeric) we can select the proper corrosion inhibitor, neutralizer, oxygen scavenger, use cathodic protection, or keep the temperature above the dew point. We differentiate all the corrosion control methods into six groups: (a) The use of coatings (organic, inorganic, and metallic). (b) The use of electrochemical methods: cathodic and anodic protection. (c) Change the environment: the use of corrosion inhibitors; removal of the aggressive components, such as oxygen (deaeration), hydrogen sulphide, chlorides, ammonia; neutralization (injection of alkalis into acidic solutions, or acids into alkali solutions); drying the atmosphere (removal of water vapors); use of biocides, etc. (d) Correct selection of materials: corrosion-resistant metals and alloys, polymeric materials, ceramics, glasses, and composites. We have to remember that there is no universal metal or other material resistant to all media and under all conditions. Polymeric materials are not resistant to high temperatures and high mechanical stresses. Ceramics and glasses are brittle. Composite materials are not resistant to high temperatures, some chemicals, and are relatively expensive. (e) Correct design. Metallic structures and equipment must be designed in such a manner that they would be convenient for drainage, cleaning, surface preparation, and painting; not to use different alloys in a general electrolyte solution, in order to prevent galvanic corrosion; not to use sharp elbows; etc. (f) Technological measures, namely, changes of process conditions. For example, keeping the temperature 20 to 30◦C above the dewpoint in order to prevent condensation of corrosive substances (for instance, H2 O with dissolved HCl or H2 SO4 ); decrease flow velocity of liquid in the case of erosion, or increase its flow velocity in the case of stagnation, in order to prevent formation of deposits (fouling and, as a result, a possible under deposit corrosion). Combining methods of corrosion control exist too. For example, we can use corrosion inhibitors in paints or in concrete; cathodic protection together with coatings (and corrosion inhibitors); keeping the temperature above the dewpoint and injection corrosion inhibitors and neutralizers in the processing streams. As we see, there is wide spectrum of corrosion control methods. We shall become familiar with some of them in the following sections in order to make the correct selection and to prevent corrosion of metallic structures. 4.1 Use of Coatings The use of any coating is based on the fact that metal must be isolated from an aggressive environment. Probably, use of coatings is one of the most ancient anticorrosion and waterproof techniques. We can mention the story of Noah, the ark and the flood from Genesis. Noah’s ark was waterproofed with pitch (bitumen, asphalt, or tar). The reed basket that carried the infant Moses into the Nile River in Egypt Corrosion for Everybody 153 was waterproofed with pitch. The Babylonians (7th century BC) and the Nabateans (“Oilmen of the Dead Sea”, 4th–3rd centuries BC) used bitumen for protection of various constructions. All coatings can be differentiated into three groups according to the nature of the basic material: organic, inorganic, and metallic. Organic coatings may be paints, polymer materials, greases, and other paraffinic mixtures. Inorganic coatings may be enamels, ceramics (among them cement), and glasses. Metallic coatings are any metal or alloy. We shall begin with organic coatings. 4.1.1 Organic Coatings Corrosion occurs only in the presence of an oxidizer in the environment of metal. The purpose of coatings is to isolate the metal surface from any oxidizer. It is possible to understand the protection mechanism of organic coatings if we accept the electrochemical mechanism of corrosion. The corrosion rate is equivalent to the electric corrosion current (Icorr ) formed between an anode and a cathode on the metal surface: Icorr = (Ec − Ea )/R, (4.1) where Ec and Ea represent the electrode potentials of cathode and anode on a metal surface; R is the electrical resistivity of the region close to the metallic surface (in electrolytes) between cathode and anode in the outer electric circle. Organic coatings have high electrical resistivity Rcoat relating to the resistivity of electrolyte Relectrolyte. Therefore, electric corrosion current Icorr decreases according to (4.1) or is even excluded fully in the presence of organic coatings. This mechanism relates to the protection by any non-conductive coating. Organic coatings are produced after the drying of liquid paints. From which components do organic coatings, or paints, consist of? Paint is a multi-complex system which consists of a mixture of resinous binder, pigment, additive, and solvent. Every paint component has its own purpose. Resinous binder can be called resin, or binder; it is the main component and essence of the paint which establishes most of its chemical and physical properties. It may range from the natural egg-white to the synthetic organic polymers such as epoxies, urethanes, or vinyls. A binder determines the drying characteristics of the paint and controls the chemical and physical properties of the hardened coating. The pigments are the substances of high dispersity (powders or fillers), nonsoluble in resins and water, added to the paints for the formation of a thick coherent film, for imparting color, for protection of the binder by absorbing ultraviolet light and for corrosion inhibition in primer paints. The pigments are differentiated into protective (anti-corrosive, or inhibitive pigments), decorative (coloring pigments), and special (bactericidal, anti-foulant, luminous, etc.). The pigments can be natural and synthetic. Typical pigments are oxides, sulphides, and salts of transient metals (Fe, Co, Cr, Ni, and others), powders of non-ferrous metals (Al, Zn, Cu, Ni) 154 4 Corrosion Control Measures and alloys (bronze, brass), and soot (carbon black – a dark powdery deposit of unburned fuel residue). The following pigments are used in paints. White pigments: lithopone (a mixture of barium sulphate BaSO4 and zinc sulphide ZnS), rutile and anastase (TiO2 ), zinc oxide (ZnO). Red pigments: haematite (Fe2 O3 ), minium (red lead Pb3 O4 ). Yellow pigment: ochre (Fe2 O3 + alumosilicates). Black pigment: soot. Green pigments: Cr2 O3 , emerald green (Cr2 O3 ·H2 O + B2 O3 ). The color of some pigments can significantly differ, for example, of ultramarine from green to violet. Some pigments are toxic, for example, white lead 2PbCO3 ·Pb(OH)2, red lead (Pb3 O4 ), calcium plumbate (CaPbO3 ) and zinc chromates (ZnCrO4 ). Many steel structures have been painted with primers based on these toxic pigments for the last 100 years. The use of toxic pigments based on lead salts and chromates is now forbidden. It is interesting to emphasize that France, Belgium, and Austria banned white-lead interior paints in 1909. Varnishes are paints that do not contain pigments, therefore they are transparent. The additives (sometimes called extenders) are substances of high dispersity (powders, or fillers) added for improving the mechanical and maintenance properties of coatings, to replace part of the pigment content, and for reducing the cost of organic coatings. Typical additives are talc (magnesium silicates), clay, or kaolin (aluminum silicates), soot (carbon black), quartz (SiO2 ), graphite (C, carbon), limestone, or chalk (CaCO3 ), dolomite (CaCO3 ·MgCO3 ), baryte (BaSO4 ), woolastonite (CaSiO3 ), and mica (silicate minerals). The solvents are the liquid components used for attaining the appropriate viscosity to paints for facilitating their application to metal surfaces. Traditional solvents contain volatile organic compounds (VOC), namely, hydrocarbons (white spirit, xylene, toluene), alcohols (methanol, ethanol), ketones (acetone), and esters. The selection of solvents includes cost, volatility, low toxicity, and acceptable fire hazard. Some other components can be added to paint for modifying the rheology of media, for assisting the dispersion of pigments, and for the acceleration of drying. To sum up, paint is a solution or dispersion of resinous binder with solid discrete phase of pigment and additives in a volatile solvent. Resin dissolved or dispersed in a volatile solvent is called the vehicle; this is glue holding the paint cohesively together and adhesively to the metal surface. During and after application, the solvent evaporates and the dried film consisting of the non-continuous pigmentary phase in a continuous phase of solidified binder, is formed on the metal surface. The solidification or curing process depends on the chemical nature of the vehicle. We have to differentiate coating from paint. After painting and drying (solidification or curing), a hard coating (film) is formed on the metal surface. When paint is drying, the hard film is called a coating. Protection efficiency (as a result, a duration) of organic coatings is defined by their physico-chemical and mechanical properties, metal nature to be protected, surface preparation, environmental media type (atmosphere, water, acids, alkalis), and environmental conditions (temperature and their changes, stresses, wear). Thus, physico-chemical and mechanical properties of organic coatings provide their chemical and mechanical resistance. Chemical resistance means resistivity to penetration of aggressive species, such as water, oxygen, hydrogen sulphide, chlorides, acids, Corrosion for Everybody 155 alkalies, organic solvents; resistivity to temperature and its changes, to solar energy, and keeping the color. Mechanical resistance means resistivity to wear, erosion, impingement, to be elastic and flexible, to have enough hardness, and of course, to maintain high adhesion to the metal surface. The last property is so important that we shall describe it. Adhesion is the pull-off strength between a coating film and metal surface needed for film removing. Adhesion is defined as the greatest perpendicular force that a surface area can bear before a plug of material is detached. Therefore, adhesion is measured in values of pressure. It usually decreases with an increase of the exposure time of coatings in the environment. We have to differentiate adhesion from cohesion. The latter is the bond strength between the particles of paint ingredients (inside the volume of the paint). Adhesion is one of the main coating properties defining the duration of the coating, and depending on the quality of the surface preparation, type of paint, coating thickness, and nature of a metal. Penetration of aggressive species through coating films from the environment to the metal surface depends on adhesion, and the latter, in its turn, depends on the penetrating properties (chemical resistance) of the coatings. The efficiency of coatings to protect the metal constructions from corrosion (resistance and duration) depends on their thickness, adhesion, uniformity, and porosity. All these parameters are interrelated. For example, the thicker the coating, the fewer the quantity of aggressive components that penetrate through the coating film and, correspondingly, the better its chemical resistance (Figure 4.1). Inner tensile stresses appear and are enhanced in the volume of organic coatings when their thickness increases. These tensile stresses cause weakening of the bonding strength between coating and metal surface and, as a result, adhesion decreases. Thus, the greater the thickness of a coating, the lesser its mechanical resistance (properties). As a result, dependence between protection efficiency and coating thickness can be described by two curves, as shown in Figure 4.1. There is an optimal thickness for every organic coating if we take into consideration the chemical and mechanical properties influencing the resistance (duration) of organic coatings. Thus, the assumption that greater thickness produces greater protective properties and duration is not valid. The general protective mechanism of organic coatings includes physical barrier to aggressive substances, inhibition by pigments (oxides, phosphates, and other salts), and galvanic action of zinc or aluminum powder if they are added to paints. Usually several layers are used in order to decrease the porosity of coatings. The first layer of coatings is sometimes called a primer. If the primer contains phosphoric acid, it reacts with the metal surface and chemical bonds are formed between metal surface atoms and the molecules of the primer. Many organic coatings are based on chemical organic substances containing polar groups such as hydroxyl (–OH), carboxyl (–COOH), and imine (–NH). If wetting the metal surface with paints is good, physical contact is formed between coating and metal surface. Any contaminants (salts, oil, grease, dust, dirt, water, old paint) remaining on the metal surface will decrease the adhesion of coatings. Thus, adhesion depends significantly on the surface preparation. 156 4 Corrosion Control Measures Fig. 4.1 Schematic dependence between protection efficiency (duration) and coating thickness. 4.1.2 Surface Preparation We can categorize surface preparation methods into three groups: mechanical (physical), chemical, use of rust converters, and thermal. Mechanical energy is used in physical methods for removing contaminants from the metal surface. In chemical methods, active chemicals are used for dissolving corrosion products and removing contaminants. Thermal methods use high temperatures (thermal energy) for transformation of corrosion products into dense inert layers, burning old paints and contaminants on the surface of the metal. 4.1.2.1 Mechanical Methods Mechanical (physical) methods include (a) Metallic blast-cleaning abrasives: cast steel and cast iron shot and grit, crushed slags of copper and nickel. (b) Non-metallic abrasives: ceramic materials (SiO2 , Al2 O3 , Fe3 O4 , Si3 N4 , SiC), glass bead, salts [NaHCO3 , garnet – (Ca, Mn)3 (Fe,Al)2 (SiO4 )3 , zircon – ZrSiO4 , novaculite – siliceous quartz rock, aluminum silicate, and iron silicate], crushed slags (electric power generating ash), agricultural shell products (walnut shells, cherry pits, and corncobs). These abrasives may be used with pressurized air (abrasive air blast cleaning with a pressure of about 7 atm) or with pressurized water (water blast cleaning, sometimes called hydroblasting). The latter may be carried out independently with pressures up to 140 atm (low pressure), up to 1,700 (high pressure), and above 1,700 atm. (ultra high-pressure waterjetting). Corrosion for Everybody 157 (c) Hand and power tool cleaning includes wire brushes, non-woven abrasive pads, scrapers, chisels, knives, and chipping hammers. The productivity of mechanical cleaning methods is slow: 10 to 20 m2 /hour. Therefore, it is impossible to clean at once all large surfaces needed for painting. Large surfaces are partly mechanically cleaned and must then be immediately painted. Dust remains on the metallic surface after cleaning. Industrial vacuum cleaners are used for removing such dust before painting. 4.1.2.2 Chemical Cleaning Chemical cleaning includes the use of: (a) Organic solvents – cleaning surface from oil, grease, wax, and other organic contaminants. They include petroleum distillates (kerosene, naphtha, mineral, or white spirit) or chlorinated solvents (trichloroethylene or perchloroethylene). The latter can be hydrolyzed in the presence of water and dangerous hydrochloric acid may form on the metal surface. Organic solvents are volatile, harmful, and dangerous to the environment and people. They are restrictedly used, especially in closed spaces (tanks, vessels, tankers, and reactors). Chlorinated solvents are banned for use in most countries because of their very high toxicity. Therefore, water-based cleaners, alkaline or acidic, are preferentially used nowdays. (b) Alkaline solutions – cleaning surface from grease and acidic salts. Alkaline cleaners contain sodium or potassium hydroxide, alkaline salts (silicates and carbonates), surfactants, inhibitors, and soaps. (c) Acidic solutions – cleaning surface from rust, metal oxides, mill scale. Acidic cleaners contain acid (usually sulphuric acid), surfactants, and inhibitors. They are primarily used to remove corrosion products from the metal surface. The cleaning process using acidic solutions is called pickling. Corrosion organic inhibitors are added to acids during cleaning (see Section 4.3). The disadvantage of pickling is hydrogen formation as a result of the cathodic process, and then possible blistering of coatings and hydrogen damages. Therefore, the metal surface should be carefully washed with water from the acidic solution after cleaning. (d) Detergents – cleaning surface from polymeric contaminants. Detergent cleaners contain mixtures of surfactants, dispersants, inhibitors, and soaps. Chemical treatment can be applied manually, by immersion, spraying, or suspension in the vapors of the medium. Sometimes use of alkaline cleaners is called degreasing because they remove grease from metal surfaces. High-pressure steam can be used for this purpose too. Now we know how to prepare a metal surface before painting. Sometimes it is impossible to use mechanical or chemical cleaning to remove rust from complicated structures or inside vessels and tanks, pipes, cars, window grating, or iron-barred 158 4 Corrosion Control Measures railings. What to do in these cases? Can we convert and modify rust without removing it from the metal surface? Next we shall discuss rust converters and surface tolerant coatings. 4.1.2.3 Rust Converters and Surface Tolerant Coatings The use of rust converters and surface tolerant coatings relates not to chemical cleaning, but to chemical surface modification. Rust on a carbon steel surface usually consists of three layers. The outer layer (far from the metal surface) is thick (from several hundreds µm to several mm thickness), loose, and may be easily removed by means of any hand tool instrument. The second (intermediate) layer is not so thick (10 to 100 µm), and may be removed by means of various mechanical tools. The inner layer, which is close to the metal surface, is chemically bonded with it, very thin (about 5 µm thickness) and can be removed only by means of bending of the metallic element. Rust converters work on the inner and sometimes on the second (intermediate) rust layers. It is necessary to remove loose rust (outer thick layer) mechanically, and then to treat the thin rust (thickness <50 µm) bonding to the metal surface with rust converter. Rust is a complicated mixture of corrosion products of iron and various contaminants which were present in the environment. Salts containing anions 2− 2+ 2+ (Cl− , SO2− 4 , CO3 ) and cations (Ca , Mg ) may be present in rust (if the rust is formed in water solutions); water, dust, dirt, soot, fats, oil, and sand may be present (if the rust is formed in the atmosphere). Thus, rust may be different in composition, thickness, and adhesion to the steel surface. We must convert rust consisting of ferric and ferrous hydroxides and oxides into inert non-soluble salts, and to neutralize aggressive species (Cl− and SO2− 4 ). Rust converters containing acids can react with rust and modify it. The first rust converters appeared in 1950s and consisted of phosphoric acid (H3 PO4 ) and zinc (Zn) powder. When the rusted (thickness less than 50 µm) surface is wetted with an acidic mixture, ferric and ferrous hydroxides react with phosphoric acid and zinc, and inert non-soluble phosphate salts are formed on the steel’s surface: FePO4 , Zn3 (PO4 )2 , Fe3 (PO4 )2 . Any rust converter chemically modifies rust which tightly adheres to the steel surface. Rust converters cannot convert mill scale (iron oxides FeO, Fe3 O4 and Fe2 O3 ). Usually the rust converters contain acids (phosphoric or tannic – resulting in the formation of nonsoluble salts), complex salts [K4 Fe(CN)6 , or K3 Fe(CN)6 , forming complexes with Fe2+ and Fe3+ cations of rust], corrosion inhibitors, surface agents (surfactants), and alcohols for better penetration of rust converter compounds through the rust layer, and barium salts for precipitation SO2− 4 anions. Rust converters on the basis of yeasts and polyvinylacetate dispersion were developed too, although they did not find wide application. Rust converters chemically react with rust, act as primers and cannot stop and prevent rust from returning. They are sometimes called rust killers. They result in the formation of an inert layer on the steel surface, and should be topcoated. If this layer is not topcoated, leaching could occur if exposed to atmospheric humidity or Corrosion for Everybody 159 water. Rust converters based on acids (phosphoric or tannic) are liquids and must be carefully washed with water to remove the residue of non-reacted acidic components after their application. After applying, most rust converters are not compatible with topcoats at temperatures above 40◦ C. They may be used for restoration of antique implements. Converters not only for rust, but also for corrosion products formed on copper, zinc, and aluminum alloys were developed too. We have to differentiate between rust converters and surface-tolerant coatings. Surface-tolerant coatings are coatings that are used on hand-cleaned surfaces, old coatings, and surfaces cleaned by high-pressure (∼150 atm) waterjetting. Surface-tolerant coatings can be applied over non-perfectly cleaned rusted surfaces. They do not contain acids, and were created on the basis of epoxy, polyurethane, and other resins, which can be used to treat dense rust (with thickness less than 100 µm) remaining on steel surface, and can be served as independent coatings. The mechanism of usage of surfacetolerant coatings is based on their penetration through rust and inertization of all rust components. Brush and airless application are the best methods for assuring that the right substances penetrate the surface-rust layer. After drying, surface-tolerant coatings form a protective thick film on rusted constructions, and are used under particular conditions in the atmosphere, water, and fuels. We may ask what kind of cleaning and surface preparation is better, the mechanical or chemical method? There is no ideal cleaning. When chemical cleaning is used, chemical compounds (for example, acids) remain on the surface of the metal. When mechanical cleaning is used, it is difficult to remove salts which are always present in rust. High-pressure water blast has proved itself to be a good surface preparation method and is widely used nowdays in industry. Mechanical methods such as shot and sand blasting remove scale and roughen the surface providing a useful surface for good adhesion of paints. 4.1.2.4 Thermal (Flame) Cleaning The third group of surface preparation methods is thermal, or flame, cleaning. Various lamps and flame cleaning are used for burning old coatings, oil, and grease, and detaching scale by differential expansion on the metal surface. Flame cleaning is usually followed by wire-brushing. A laser can be used for transforming ferric hydroxide into ferric oxide in rust. A dry surface is formed in this case which can be painted. Thermal (flame) cleaning has not found wide use in industry. Ultrasonic cleaning uses swept resonance frequency technology, and is used for the surface preparation of small metallic elements which are usually immersed in a bath with liquid for cleaning. 4.1.3 Selection of Coating System The main question is how to select the coating system? 160 4 Corrosion Control Measures First of all, we have to define the service conditions and requirements for the coating system to be used: temperature (and its changes), chemicals (type and their concentration) which will be in contact with the metal surface, immersion or splash, sunrays, wear, etc. Then it is important to carry out accelerated tests of the recommended coating systems under laboratory conditions. For example, if we need to select the right system for the protection of the bottom inside of gas oil storage tanks, we have to immerse the panels coated with the recommended systems in three phases: in gas oil (organic phase), in an aqueous electrolyte solution (imitating water separation on the bottom), and in the gaseous phase (included hydrogen sulphide, because gas oil can contain this gas). The examination accelerated test period must not be less than three months. We may examine at ambient (20◦ C) or at higher (40◦C or 90◦C) temperatures for enhancing corrosive conditions. We may add some salts (3% NaCl and 0.2% NaBO3 ) to water. The latter compound is an oxidizer and promotes the aging of coatings. It is important to check the references where particular coating systems have yet been used. Then, to check the safety conditions for use of the coating systems recommended. Most paints contain organic solvents which are harmful to the environment and people. In order to overcome these problems, water-based paints, powder paints, and solventless (solvent free) paints were developed. Water-based paints have water as the solvent, they dry purely by evaporation, while oil-based paints have chemical drying agents added. The greatest advantages of water-based paints are that they are “friendly” for the environment and brushes and rollers can be washed out in water. Powder paints are applied in powder form at a high temperature when they are melting or chemically cross-linked on the metal surface. Solventless paints do not contain organic solvents, and consist of 100% solids, therefore they have many benefits. A binder in solventless paints is a liquid. One layer of solventless coatings may be very thick: from 400 to 1000 µm. In order to reach such a thickness with conventional paints containing solvents, we have to produce 10 to 20 layers. As VOC (volatile organic compounds) is nearly zero in solventless paints, hazardous organic compounds do not evaporate into the atmosphere, and there is no danger for health, spark and explosions. When the solvent evaporates, pores remain in the coating film after drying. The porosity of solventless coatings is very low, significantly less than that of paints containing solvents. The curing process is faster for solventless coatings. Such additives (fillers) as aluminum oxides, fiber glasses, and chips in coatings increase their resistance to erosion and wear. To sum up, the protective properties of organic coating systems and, consequently, their durability are defined by the correct selection of the coating system based on laboratory examination and checking the references, and the quality of the surface preparation and application. Suitable and qualifiable inspection must be carried out at all these stages. Only then will we enjoy and get satisfaction from using the most ancient method of corrosion control. 161 Corrosion for Everybody (a) (b) Fig. 4.2 (a) Anodic and (b) cathodic coatings on metal surface. Metal does not corrode under an anodic coating, but corrodes under a cathodic one, if there are pinholes in the latter. 4.1.4 Metallic Coatings Throughout the whole of the book we have talked about corrosion of metals, and now we intend to talk about the paradox that metals may be protected with metals. Yes, we know that carbon steel structures and equipment are widely protected from corrosion and aesthetically coated with other metals and alloys. What is the protection mechanism in this case? There are less and more noble metals regarding to iron (see Tables 1.3 and 2.3). In relation to this, there are two main types of metallic coatings according to their protective mechanism. When a metallic coating is less noble (more negative electrode potential) than the metal to be protected, the coating is called anodic (it will corrode as an anode) and provides cathodic protection (see Section 4.2) in the case of scratches, edges, pores and any defects, that is, it protects non-covered metal in the presence of electrolytes (Figure 4.2a). For the protection of carbon steel, the anodic metallic coatings are zinc, aluminum, magnesium, and their alloys. Cadmium is also anodic to carbon steel and was widely used as metallic coating in the past, but it is an extremely toxic metal and is not recommended for use. The more electrical conductivity of an electrolyte, the better the efficiency of the protective properties of anodic coatings when its uniformity is destroyed and pores appear in the coatings. Typical galvanic corrosion of a less noble metal occurs in this case. Zinc and aluminum are widely used as anodic coatings for carbon steel structures and equipment. If a coating metal is more noble (more positive electrode potential) than the metal needing protection, a coating is called cathodic. Cathodic coatings are tin, chromium, nickel, copper and its alloys, stainless steels, and silver on carbon steel. If scratches, edges, pores, or any other defects appear in cathodic coatings on carbon steel, the latter will be anodic towards metallic cathodic coatings, and will corrode under these coatings (Figure 4.2b). This situation is very dangerous because steel will corrode under the coating and we will not be able to immediately 162 4 Corrosion Control Measures Fig. 4.3 Electroplating – producing electrolytic coatings, A – amperometer; V – voltmeter; Mn+ – cations; Az− – anions. Cathode – metal to be protected. detect the rust. Rust formed under cathodic coatings will cause a decrease of the adhesion and delamination of the coating from the steel surface. Metallic coatings may be divided into several groups according to the way they are prepared: electrolytic, hot-dip coatings, metal spraying, chemical plating, vapordeposited coatings, diffusion treatment, and coatings prepared by mechanical methods. Electrolytic coatings are produced by the electrochemical process that has several names: electrogalvanizing, electrodeposition, and electroplating. We can use any of these names. Thus, electroplating is the producing of a metallic coating on a metal surface by the action of an electric current (Figure 4.3). The object of a metal or alloy (for example, carbon steel) to be protected with a metallic coating (electrolytically plated) is connected to the cathode (negative pole of battery) of an electrolytic cell. This metallic object (cathode) is immersed in a solution which contains a salt of the metal to be deposited. For example, if we need a nickel coating, we may take NiCl2 salt in water. This dissociates in water and Ni2+ cations are formed. Organic and even fused salt electrolytes may be used for electrodeposition too. A metallic electrode made of nickel (anode) is immersed in the same solution and connected to the positive pole of the electrolytic cell. When an electric current flows between the anode and cathode from the direct current power supply (battery), positively charged Ni2+ ions are attracted to the negatively charged object (the cathode). It provides the electrons to reduce them (Ni2+ ) to pure nickel on a carbon steel object according to the reaction: − → Ni(s) . Ni2+ (aq) + 2e (4.2) 163 Corrosion for Everybody Table 4.1 The melting temperatures of metals used for producing hot-dip coatings. Metal Melting temperature, ◦C Zn Al Sn Pb 419 660 212 327 This reaction is opposite to corrosion, and a nickel coating is formed on the steel object. The positively charged nickel electrode (anode) is oxidized to the Ni2+ cations, which are applied to the solution. In this case, corrosion of nickel anode is a beneficial phenomenon. The thickness of the electrolytic coating formed on a metal is determined by the duration of the plating. Usually the thicknesses vary from 0.1 to 40 µm. Electrolytic coatings became possible after the invention the “voltaic pile” (electric battery) by Alessandro Volta in 1797. His friend the Italian chemist Luigi Brugnatelli was probably the first to plate silver medals with gold using the electric Volta pile in 1805. The Emperor of France, Napoleon Bonaparte (1769– 1832) was personally interested in electrolytic coatings. Russian ambassadors in Paris and London at the beginning of the 19th century, had to report about this issue every month to the Russian government. Probably, “scientific spying” during that period played a significant role, and at the end of the 1830s scientists in both Russia and England had devised electrolytic coatings of copper for printing plates. By the middle of the 19th century, electroplating of nickel, tin, brass, and zinc were applied for commercial purposes. One of the disadvantages of electroplating is that uneven current distribution results in a non-uniform metallic coating on the object. An even current distribution is reached by auxiliary anodes and current screens. Use of various organic additives in electrolytic solutions also helps to form a uniform coating on the objects. Hot-dip coatings are produced when a metal object (for example, made of carbon steel) is dipped into a bath of molten metal (for instance, zinc). Metals with low melting temperature are used for the hot-dip coatings: zinc, aluminum, their alloys, and tin (Table 4.1). Lead was also used in the past, but because of its high toxicity is now banned from use. Zinc is mostly used as a hot-dip coating and in this case such a process is called hot-dip galvanizing. Similarly to the electrolytic coatings, the thickness of hot-dip coatings is defined by the duration that the object is left in the bath with molten metal. Usually the thicknesses of hot-dip coatings vary from 10 to 400 µm. Metal spraying is the process of producing metallic coatings on metal surfaces by means of spraying with compressed air metals or alloys after their melting. History tells that the Swiss engineer Max Ulrich Schoop from Zurich liked to go hunting. This was probably around 1910 when he payed attention to the fact that after shooting, small bullets of lead penetrated into a tree and might remain in its bark. He asked “why could I not do the same with a metal object instead of a tree?” Thus, 164 4 Corrosion Control Measures Schoop invented the first flame spraying “gun” for metalization in 1913. Such a process has been called the “Schoop process” for many years, and it has received many synonyms: flame spraying, thermal spraying, metalizing, or metal spraying. It can be carried out by three methods: flame spraying, electric arc spraying and plasma spraying. Flame spraying is the process of melting metal with an oxygen–acetylene flame and dispersing with compressed air onto the metal object to be coated. The metal used as a coating material may be wire or powder form. Molten particles of metal or alloys move with compressed air onto the metal surface to be protected, impact and flatten. Molten particles of metals are oxidized by the air during their moving from the “gun” to the metal surface. The distance between the “gun” and the metal surface to be protected is usually about 1 to 2 meters. Therefore, a finished coating consists of a mixture of melted metal and its oxides which are solidified. The requirements for preparation of the metal surface are similar to those before painting. Electric arc spraying is the process of metal melting with an electric arc between two wires (connected to plus and minus electric poles) and dispersing with compressed air. Plasma spraying is the process of powder melting by a plasma beam and dispersing with compressed air. The temperature in the last method is very high: about 15,000 to 20,000◦C; therefore, high melting metals, alloys, and ceramics (oxides, carbides, and nitrides) are used for plasma spraying. All three variations of metalizing require a spraying “gun”. Robotic “gun” systems have been developed for metalizing large surfaces. Metalizing equipment is mobile, appropriate for many complex shapes, and not limited by size. The thicknesses of metal spray coatings usually vary from 40 to 500 µm. Metal spraying is used for coatings with aluminum, zinc, their alloys (85%Zn–15%Al), aluminum alloy with 5% magnesium (Al–Mg5), and stainless steels. A severely corroded carbon steel surface may be metalized with carbon steel for repair, namely, for increasing the thickness of the corroded metal. The main drawback and “advantage” of metal spraying coatings is their high porosity which depends on the type of metal spraying (its density) and process type. For example, aluminum coatings have a higher porosity (5 to 15%) than zinc coatings (1 to 3%) (see Section 3.6). The lowest porosity may be received by plasma spraying: 0.5 to 2%. Corrosives in the environment can penetrate through pores to the metal surface under a metalized coating. Because of the different porosity of various metal spray coatings, the minimum thickness needed for metal protection is also different. The minimum thickness of the coating is the thickness needed for closing of all the pores in the coating, that is, the thickness where electrolytes would be unable to penetrate through the metallic coating to the base metal surface to be protected. Thus, the minimum thickness for a zinc coating is 100 µm, for aluminum and stainless steel coatings is 300 µm (see Section 3.6). The lifetime of metallic coatings depends on their thickness, namely, on their mass. Adhesion of metalized coatings is higher than that of paints. Metalizing may be used alone or in combination with organic coatings. It also increases drastically the adhesion (because of relatively high porosity of metal spray coatings) of topcoats when they are applied over metalized coatings. Such “sandwich” coatings (metal spraying following by organic coatings) may serve for approximately 30 years (see Section 3.6). The process of filling the pores of metal sprayed coatings with paint is called sealing. Treatment of aluminum 165 Corrosion for Everybody metalized coatings with water at 90 to 100◦C results in good sealing because of the formation of aluminum hydroxides in the pores of the coatings. Good adhesion of zinc and aluminum metalized coatings to steel allows the shaping of constructions (for example, sheets for tanks) in different forms without delamination. Aluminum and zinc coatings are used for protection inside and outside surfaces of tanks used for the storage of fuels and water (see Section 3.6). The advantage of metal sprayed coatings is that sheets with such coatings can be welded and then coated (sealed) with paints. Chemical plating is the process of producing metal coatings on a metal surface by means of chemical reaction. The miners in the Middle Ages knew that if an iron object came into contact with the “vitriol water” (“blue vitriol”) of the copper mines (an aqueous solution of copper sulphate formed when copper sulphide ores have been oxidized), the surface of the iron will be covered by a red copper layer. The following oxidation – reduction reaction occurred: Fe(s) + CuSO4(aq) → “Red Iron”, (4.3) 2+ Fe(s) + Cu2+ (aq) → Fe(aq) + Cu(s) . (4.4) This is the chemical plating or metal deposition of copper on iron (steel) surface. Miners observed this metal plating many times. The copper coating is very thin (about 1 µm), porous, and is not well attached to the steel surface. Chemical plating of nickel was developed in the 1940s. A carbon steel object is immersed in a bath containing an acidic water solution of nickel cations Ni2+ and hypophosphite anions H2 PO− 2 . The following reaction occurs: − − + Ni2+ (aq) + H2 PO2(aq) + H2 O(l) → Ni(s) + H2 PO3(aq) + 2H(aq) . (4.5) Nickel coatings are produced by the deposition of nickel cations as a result of chemical reduction, and this process is similar to reaction (4.2), but without an outer electric current. Therefore, this process is called electroless plating and it has many advantages over electrolytic plating. One major advantage of electroless plating is that a uniform coating is formed even on objects having crevices and complicated forms, such as corners and others. Thicknesses of electroless coatings vary from 5 to 125 µm. When reaction (4.5) takes place on a carbon steel surface in the bath, nickel coating contains phosphorus (2 to 13%) and this element determines the mechanical properties of the coating. Diffusion treatment is the process of surface changing of metallic objects by diffusion of protective metal. When treating carbon steel with carbon powder in a furnace to 900◦C, carbon diffuses into the steel surface. This process is called carburizing. If nitrogen diffuses into the surface of a metal, the process is called nitriding. These diffusion processes have been used for many years to improve the mechanical properties of steels. Similar diffusion may be carried out with any metal. Probably, the Englishman Sherard Cowper-Coles was the first to heat zinc dust together with a carbon steel object to 350–400◦C in 1904. The melting temperature of zinc is 419◦C. Zinc atoms diffuse at a high temperature into the upper layer (sur- 166 4 Corrosion Control Measures face) of steel and partly react with iron. An iron–zinc alloy and the outer layer of pure zinc are fused together as a result of diffusion of zinc atoms into the iron surface. The coating thickness depends on the reaction time and varies between 10 to 50 µm. Such a process of the formation of a protective zinc coating on steel objects is called sherardizing and was introduced in the 1920s. If aluminum is used for the diffusion treatment, the process is called calorizing and if chromium is used, chromizing. In calorizing, aluminum is usually used for diffusion treatment together with alloy ferro aluminum (Fe–Al) or other alloys, alumina (Al2 O3 ), and salt ammonium chloride (NH4 Cl). In this case, high-temperature wear resistance of the formed coating is increased significantly. Diffusion treatment provides coatings of a uniform thickness. Vapor-deposited coatings are the coatings producing by processes in vacuum by physical vapor deposition (PVD) or chemical vapor deposition (CVD). The species are deposited in the form of individual atoms or molecules in these processes. They are carried out in special vacuum boxes, therefore are not damaging to the environment as are some other metal coating processes, for example, electroplating or metal spraying. The PVD process is differentiated into evaporation, sputtering, and ion plating. Historically, evaporation was the first PVD process. Practically all metals can be evaporated, but aluminum, chromium, and stainless steels are widely used. The thicknesses of metal coatings obtained by evaporation are several microns. The disadvantages are non-uniformity and relatively low adhesion. Such coatings are usually used for decorative and optical applications, and are not used as anti-corrosion coatings. Sputtering takes place in an inert gas (argon) at low pressure. The argon ions formed by electric discharge, beat the solid coating material connected to the minus electric pole of the power supply. Sputtered atoms of the coating material move to the metal object to be coated. Thin metal coatings of 0.1 µm thickness may be formed with good adhesion. High-chromium alloy coatings on turbine blades are produced by sputtering. This method was developed in the 1970s for semiconductors. The drawbacks of sputtering are thickness limitation and difficulties in forming uniform coatings on complicated objects. Ion plating is based on evaporation (or sputtering) and a glow discharge. The application of ion plating is the formation of nitrides or carbides on the steel surface. Therefore, this method is not exactly PVD. In ion plating, nitrogen or carbon ions are obtained from the ionization of nitrogen or hydrocarbon gases. Then these ions react with iron on the steel surface forming nitrides or carbides. Therefore, these coatings have the best adhesion, structure, and fewer imperfections comparing with coatings obtained by evaporation and sputtering. CVD is based on chemical reactions. For example, if chromium is needed as coating, it should be converted into a gas compound. For this purpose, ammonium chloride as the source of halide is used in the hydrogen atmosphere at about 1000◦C. A source of chromium is usually chromium powder. As a result of chemical reactions chromium converts into a volatile CrCl2 compound which in its turn reacts according to the following reactions: 167 Corrosion for Everybody CrCl2(g) + Fe(s) → Cr(s) + FeCl2(g) , (4.6) CrCl2(g) + H2(g) → Cr(s) + 2HCl(g) , (4.7) CrCl2(g) → Cr(s) + Cl2(g) , (4.8) and chromium atoms deposit on the carbon steel surface. Chromium atoms diffuse into the carbon steel surface under high temperature (≈ 1000◦C), and the content of the chromium reaches 13 to 30% in the carbon steel surface, which increases the corrosion resistance and mechanical properties of steel. Sometimes CVD with chromium is called chromizing. The greatest disadvantage of this technique is that halide gases at high temperatures are highly corrosive and corrosion-resistant materials must be used. CVD coatings contain chromium, aluminum, titanium, manganese, boron, and silicon. These are used as anti-corrosion coatings (including high temperature oxidation resistance), and improve wear, abrasion, and friction properties of the protected surfaces. CVD coatings are thick (up to 6.5 mm), dense with good anti-corrosion properties, and their cost is usually lower than that of PVD coatings. Titanium nitride is very hard, stable, has good corrosion resistance in the atmosphere, and has a pleasing gold appearance which allows the use of titanium nitride as a coating on steel instead gold, for example on the domed roofs of churches (see Figure 6.8). Mechanical methods of coating are the processes that use physical bonding of one metal to another for the creation of metal coatings. Soldiers during World War I (perhaps even before) observed that shrapnel from disintegrated metal shell casings traveling at high velocities, sometimes bonded with steel stanchions and other metallic surfaces that they stuck. Then the American engineer L.R. Carl reported in 1944 about the welding of brass discs under high velocity impact, and concluded that the weld was not a fusion weld but that it had been formed by a solid-state mechanism. In the 1960s explosion welding was used for producing the clad metals of Cu–Ni/Cu/Cu–Ni needed by the US Mint for new coinage. As the vice president of one company producing clad metals noted, “people made lots of money making money”. Mechanical methods of coating are divided into the following methods. Cladding is the method of cold or hot rolling of coating high resistant metals or alloys onto the metal to be protected. Clad products include stainless steels, titanium, copper alloys, aluminum, vanadium, tantalum, and zirconium to steel. This method is used for producing the protective metal coatings inside pressure vessels, autoclaves, reactors, distillation columns, condensers, tubesheets, and shells of heat exchangers. Explosive bonding (explosion welding) is the method of welding by a controlled explosion of the coating metal (high corrosion resistant) to the metal to be protected. Overlay welding is the welding of a metal highly resistant to corrosion to the surface of a metal to be protected. Extrusion is the process when the coating metal and the metal to be protected are extruded together. Generally speaking, extrusion is the process by which long, straight metal parts may be produced. Extrusion is done by squeezing the metal and 168 4 Corrosion Control Measures coating together in a closed cavity through a tool known as a die using either a mechanical or hydraulic press. Mechanical plating is a process for impact deposition of some ductile metals from a powder on a steel object. Ductile (soft) metals are zinc, aluminum, copper, tin, and their alloys. This method relies on the principal that if there are two pure metals hitting each other, the softer one will cold weld to the harder one. Other names for mechanical plating are peen plating, impact plating, mechanical or cold galvanizing. This method was developed in the early 1950s and takes place in a rotating barrel containing small carbon steel objects, powder of the coating metal (for example, zinc) suspended in an aqueous solution, and glass beads. Rotated glass beads “hammers or peens” the metal powder onto the objects. An adhered uniform layer of coating is formed with thicknesses of 2 to 100 µm. Recommended Literature 1. Hare, C.H., Protective Coatings, Technology Publishing Company, Pittsburgh, Pennsylvania, USA, 1994, 514 pp. 2. Mattsson, E., Basic Corrosion Technology for Scientists and Engineers, Second Edition, The Institute of Metals, UK, 1996, pp. 107–125. 3. Keane, J.D. (Ed.), SSPC, Vol. 1, Good Painting Practice, Second Edition, SSPC, Pittsburgh, USA, 1989, 580 pp. [SSPC – Steel Structure Paint Council]. 4. Revie, R.W. (Ed.), Uhlig’s Corrosion Handbook, Second Edition, Wiley-Interscience, Inc., 2006, pp. 1023–1059. 5. Talbot, D. and Talbot, J., Corrosion Science and Technology, CRC Press, USA, 1998, pp. 185– 191, 219–226. 6. Mühlberg, K., Surface-Tolerant Coatings – Some Experiences, Protective Coatings Europe (PCE), December, 2001, pp. 13–21. 7. Korb, L.J. and Sprowls, D.O., Metals Handbook, Vol. 13: Corrosion, Ninth Edition, ASM International, USA, 1987, pp. 456–458. 8. von Baeckman, W., Schwenk, W. and Prinz, W. (Eds.), Handbook of Cathodic Corrosion Protection, Third Edition, Gulf Publishing Company, Houston, Texas, USA, 1997, 567 pp. 4.2 Electrochemical Methods of Corrosion Control The electrochemical mechanism of corrosion in electrolytes allows the use of electric current and electric potential in order to protect metals from corrosion. Therefore, electrochemical methods work only in solutions of electrolytes. Electrochemical methods are a general term for cathodic and anodic protection. They are different and they should not be mixed. In cathodic protection, metallic equipment is connected to the negative pole of the power supply and turns completely into a cathode, which does not corrode. Anodic protection is based on the phenomenon on passivity (see Appendix D). In anodic protection, metallic equipment is connected to the positive pole of the power supply, and the electric potential changes in the positive direction, in order to reach the passive state. Thus, anodic protection is 169 Corrosion for Everybody only applied for metals and alloys capable of being in a passive state in a particular media. 4.2.1 Cathodic Protection There are three ways to show the principles of cathodic protection. Chemical explanation. How can we prevent the anodic dissolution, or corrosion process, of iron immersed in an electrolyte solution? − Fe(s) ↔ Fe2+ (aq) + 2e . (4.9) Chemists know that one of the ways to change the direction of this reaction is to increase the concentration of ferrous cations Fe2+ in the electrolyte or, alternatively, to increase the number of electrons in iron. It is simple to realize the latter. Thus, if we connect the iron to the negative pole of a direct current power supply, electrons will “flow” to the iron, and reaction (4.9) would slow down to a negligible value or even to stop it. Thermodynamic explanation of cathodic protection is based on the Pourbaix diagram (Figure 4.4). It shows three realms of the possible existence of iron in water: corrosion (Fe2+ , Fe3+ and FeO2 H− ), passivity (Fe3 O4 and Fe2 O3 ), and immunity (Fe). The electrode potential of iron and carbon steels in neutral aqueous solutions is about −0.44 V regarding SHE. One can detect from the Pourbaix diagram that if we diminish this potential under −0.52 V (the arrow AB), we enter the realm of immunity, where a reaction opposite to (4.9) takes place, that is, the anodic reaction of iron dissolution will be suppressed. Polarization curves can also explain the principle of cathodic protection. Let us represent the Evans diagram for corrosion of iron in water (Figure 4.5). For the reversible iron electrode we can write → i1 − −→ Fe(s) . Fe2+ (aq) + 2e ←− (4.10) ← i1 → ← Current densities i1 and i1 are the rates of the cathodic and anodic processes, re→ ← spectively, for the reversible reaction (4.10). Cathodic ( i1 ) and anodic ( i1 ) curves for this reversible iron electrode are shown by curves 1 and 2, respectively, in Figure 4.5. For the oxygen reversible electrode we can write → i2 O2(g) + 2H2 O(l) + 4e− ←−−→ 4OH− (aq) . ← i2 (4.11) 170 4 Corrosion Control Measures Fig. 4.4 Potential–pH (Pourbaix) diagram for Fe–H2 O at 25◦ C. Fig. 4.5 The Evans diagram for corrosion of iron in water. (EFe )rev and (EO2 )rev – reversible potentials for reactions (4.10) and (4.11), respectively. → ← Current densities i2 and i2 are the rates of the cathodic and anodic processes for → ← (4.11), respectively. Cathodic ( i2 ) and anodic ( i2 ) curves for the oxygen reversible electrode are shown by curves 3 and 4, respectively, in Figure 4.5. The corrosion current icorr on iron immersed in water occurs at corrosion potential Ecorr (cross-section of anodic curve 2 for iron dissolution and the cathodic curve 3 for oxygen reduction on the iron surface). We connect iron to the negative pole and diminish its electrode potential, that is we move, according to the polariz- 171 Corrosion for Everybody ation anodic curve 2, in the negative direction. In other words, cathodic polarization of the iron electrode (change of electrode potential in the negative direction) res← ults in retarding of the anodic dissolution of iron ( i1 value diminishes), acceleration → of the cathodic reduction of dissolved oxygen ( i2 value increases), and formation of hydroxide ions OH− on the iron surface. According to Figure 4.5, in order to completely prevent the anodic dissolution of iron (curve 2), we have to diminish its potential to the value less than the reversible potential (EFe )rev of the iron electrode in the electrolyte solution. Usually polarization of iron equipment in the cathodic direction occurs up to 100 mV more negative than the corrosion potential Ecorr . Therefore, in order to carry out correctly the cathodic protection by means of cathodic polarization of the potential in the negative direction, we have to measure Ecorr of the iron equipment or structure under real conditions, and then to polarize 100 mV more negative than Ecorr . Two ways exist for cathodic polarization in practice: connecting the main metal to be protected (for example, iron) to a less noble metal (aluminum, zinc, magnesium, or their alloys), or connecting to the negative pole of the outer power supply (rectifier or battery). The first method is based on the use of sacrificial anodes because they are sacrificed by being dissolved as an anode and turn the metallic construction to a cathode which does not corrode. Principally, any metal may be protected by using suitable sacrificial anodes in a solution of electrolytes, and the electromotive force series (Table 1.3) can help select them. Sometimes this method is called a passive one (there is no relationship to passivity!), because we connect equipment to be protected to a sacrificial anode, and “forget” about corrosion for some time. The second method of cathodic protection is based on the connection to the negative pole of the rectifier and use of an impressed electric current. This method is sometimes called an active method of cathodic protection. The more electrical conductance of media, the better cathodic protection works. Therefore, sacrificial anodes work better in cooling water of high hardness (more calcium and magnesium salts, and corresponding conductance), than in low hardness waters. Cathodic protection is one of many methods of corrosion control of underground and undersea metallic structures and equipment. It is not simple to define the border of the electrical conductance when sacrificial anodes work well or not applicable. Of course, the metallic structures in the atmosphere, in fuels and demineralized water (or other media of high electrical resistance) will not be protected by cathodic protection. Any system for cathodic protection includes anode, cathode, general electrolyte and electric current. We shall discuss separately two types of cathodic protection: sacrificial anodes and impressed current use. 4.2.1.1 Sacrificial Anodes The use of sacrificial anodes is one of the ancient methods of corrosion control (see Section 6.1) and is based on the principles of galvanic corrosion which has positive application in this case for the saving of metallic structures. In order to 172 4 Corrosion Control Measures Fig. 4.6 Polarization curves of mild steel (C1018), magnesium (AZ61) and aluminum (6063) alloys in 0.5% NaCl aqueous solution. 1, 2, 3 – cathodic curves; 1′ , 2′ , 3′ – anodic curves. 1 – Mg; 2 – Mild steel; 3 – Al. The chemical composition of alloys is given in Appendix F. select the type of anode we have to define the electrochemical characteristics of the anode and the cathode in specific media. Therefore, we have to measure not only the electrode potentials of the anode and the cathode, but also their polarization curves. Electrode potentials of sacrificial anodes and cathodes may be changed with time, therefore thermodynamic electrochemical characteristics may not always give the correct forecast for use of sacrificial anodes. How to select the material of sacrificial anodes for protection of carbon steel structures? The selection depends on the properties of the environment (its electrical conductance) and electrochemical characteristics of anodes. Magnesium as a sacrificial anode is not used for the protection of carbon steel in media with high electrical conductance: sea water, cooling water and other electrolyte solutions (with conductance >3,000 µS/cm). This is explained by the large difference in electrode potentials of iron and magnesium (about 1.0 V – not a bad battery!), and appropriate polarization characteristics (Figure 4.6). Polarization curves of magnesium and mild steel show the very high corrosion rate of a magnesium anode in cooling water and 0.5% NaCl aqueous solution: 200 mm/year, or 350 kg·m−2 ·year−1 ! Magnesium anodes will sacrifice (dissolve) quickly and we often have to change them. Magnesium anodes are used in hot water (about 80◦C) for the protection of inner surfaces of boilers (we may check their presence in the boilers on the roof!), and in soils with low electrical conductance. Zinc anodes are used in sea water, non-brackish waters, and in soils. Aluminum has one essential disadvantage: it has a very good passive film of Al2 O3 which is not conductive to working well with aluminum as a sacrificial anode. Such elements as tin, mercury, indium, and antimony are added to aluminum, in order to de-passivate it. Thus, aluminum alloys Al–Zn–Hg, Al–Zn–In and Al–Zn–Sn are used as sacrificial anodes. Aluminum anodes are sometimes used for the protection inside bottoms of tanks and tankers containing crude oil and fuels when sea water is used as ballast. Corrosion for Everybody 173 In order for sacrificial anodes to work well, the content of salts in aqueous solutions should be above 0.3 wt%. Aluminum anodes in such tankers may fall and cause sparking in the presence of explosive gases (hydrocarbons, etc.). Zinc anodes do not result in sparking when falling on the carbon steel surface. Therefore, zinc is preferred for use in tankers in the presence of flammable and explosive gases. Sacrificial anodes are used for the protection of the outer surfaces of pipelines for short distances, outer and inner surfaces of tanks in contact with soil (outside) and aqueous solutions of electrolytes (inside), inner surfaces of heat exchangers with cooling water, supports, and other carbon steel structures in different aqueous solutions and soils. The efficiency of sacrificial anodes depends on the impurities contained in them. Any cathodic inclusions, for example, iron in zinc anode, result in diminution of efficiency because of the existence of inner galvanic cells and electric currents between iron and zinc. In this case, zinc will be destroyed quicker. Therefore, any anode material has restrictions regarding quantities of cathodic impurities. For example, a magnesium anode must not have iron above 0.003 wt%. If a magnesium anode has 0.01% of iron, efficiency diminishes from 100% to 74%. The voltage between anode and protected metallic structure depends on the type of material and environment, and is usually about 1 V. Sacrificial anodes must be checked and periodically changed. Usually they are installed for 4 to 5 years. 4.2.1.2 Impressed Current System of Cathodic Protection Although use of sacrificial anodes is a relatively simple method of corrosion control, there are disadvantages. The first one is that they only work for a limited period, corrode and should be replaced. Usually sacrificial anodes do not work for more than 4 to 5 years. The second drawback is that they only work in media of significant electrical conductance. The third one is that corrosion products formed as a result of the active work of sacrificial anodes in the chemical and food industry can deteriorate the products (chemicals) through non-desired contaminants. If sacrificial anodes are used for the protection of long underground pipelines, it will be very complicated to replace them every few years. An impressed current system can decide all these problems. Such a system consists of a transformer of alternating electric current in a direct one. The metallic structure requiring cathodic protection is connected to the negative pole of the electric power source (Figure 4.7). In order to close up the electric current auxiliary anodes, called ground bed anodes, are connected to the positive pole of the rectifier. 4.2.2 Criteria for Cathodic Protection Intensive use of an impressed current for the protection of underground gas pipelines began in 1920–1930s. It was important to define the criterion for cathodic protec- 174 4 Corrosion Control Measures Fig. 4.7 Cathodic protection of an underground steel tank with impressed current: 1 – the tank to be protected; 2 – auxiliary anodes; 3 – rectifier; 4 – ground level). I – protective electric current. tion. Two criteria are used. The first one was suggested by the American electrical engineer Robert J. Kuhn in 1928–1933. The Belgian scientist Marcel Pourbaix invented his diagram 10 years later which allowed the definition of what electric potential must be applied to iron for its complete protection. Kuhn knew nothing about the criteria of thermodynamic – the Pourbaix diagram, which showed that iron became immune to corrosion at an electric potential of less than −0.53 V with respect to SHE. He practically proved that the electric potential of a protected buried pipeline in soil should be less than −0.85 V with respect to a saturated copper/copper sulphate reference electrode. As ECu/CuSO4 = +0.317 V with respect to SHE, and if we take the criterion value −0.85 V, suggested by Kuhn, E = −0.85 + 0.317 = −0.533 V with regard to SHE, and shows good agreement between thermodynamic and practical values! Electrical engineers who dealt with cathodic protection thought that the impressed current method should completely protect underground structures. Unfortunately, electrochemical principles show that it is impossible to provide complete protection of buried structures by means of cathodic protection. According to polarization curves (see Figure 4.5), small anodic currents remain on the steel surface. A bare steel surface requires large electric current values for cathodic protection. In order to decrease them, we have to use organic coatings together with cathodic protection. Corrosion for Everybody 175 Fig. 4.8 Aluminum sacrificial anodes (A) installed on the inside of heat exchangers coated with epoxy paint (green). The cooling water flows inside the tubes. (For a full color version of this figure, see the Color Section) 4.2.3 Use of Organic Coatings Together with Cathodic Protection Organic coatings possess high electrical resistance (R) and the electric current (I ) required for cathodic protection of coated structures drastically decreases according to Ohm’s Law: I = E/R. If the electric current diminishes, the number of required sacrificial anodes also decreases and their duration increases correspondingly. As an example, aluminum anodes are used in cooling water for the protection of the insides of heat exchangers coated with epoxy paint (Figure 4.8). Cathodic protection alone cannot provide complete corrosion control of underwater and underground structures. From another side, there are no organic coatings without defects in the form of pores, cracks, and discontinuities. Every coating system has its duration. Let us first apply a coating on the metallic structure and then use the cathodic protection in order to protect the metallic surface against defects in the coatings. If cathodic protection is used for an uncoated structure, the electric current should reach all bare surfaces of the structure. If cathodic protection is used for a coated structure, the electric current must not reach all the surface of this structure but only places with defects: pores, cracks, and other discontinuities. 4.2.4 Limitations and Disadvantages of Cathodic Protection People are generally well informed about the side-effects of many medicines. Use of some chemicals (corrosion inhibitors, passivators, biocides) and paints (coal- 176 4 Corrosion Control Measures tar, minium, etc.) can cause undesired effects on the environment and the health of people. Use of cathodic protection is not excluded. Cathodic protection has a negative side, and there is a philosophical approach to its use (see Section 6.2). Impressed current systems may cause stray current corrosion (if metallic constructions are in the region of the activity of cathodic protection and are not connected to an impressed current), coating debonding and hydrogen embrittlement (in the case of overprotection and as a result of water decomposition and hydrogen evolving), and corrosion of aluminum (because hydroxyl ions forming during cathodic protection are harmful for aluminum). Recommended Literature 1. Korb, L.J. and Sprowls, D.O., Metals Handbook, Vol. 13: Corrosion, ASM International, USA, 1987, pp. 466–477. 2. von Baeckman, W., Schwenk, W. and Prinz, W. (Eds.), Handbook of Cathodic Corrosion Protection, Third Edition, Gulf Publishing Company, Houston, TX, USA, 1997, 567 pp. 3. Uhlig, H.H. and Revie, W.R., Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, Third Edition, John Wiley & Sons, USA, 1985, pp. 217–232. 4. Mattsson, E., Basic Corrosion Technology for Scientists and Engineers, Second Edition, The Institute of Materials, UK, 1996, pp. 95–103. 5. Shreir, L.L., Jarman, R.A. and Burstein, G.T. (Eds.), Corrosion, Vols. 1 and 2, Third Edition, Butterworth Heinemann, UK, 1994, pp. 10:1–10:154. 6. Revie, R.W. (Ed.), Uhlig’s Corrosion Handbook, Second Edition, Wiley-Interscience, 2006, pp. 1061–1087. 7. Peabody, A.W., Control of Pipeline Corrosion, Second Edition, Edited by Ronald L. Bianchetti, NACE International, USA, 2001, 347 pp. 4.3 Change of Chemistry of the Environment The environment for solid metal may be solid, liquid, gaseous, and two- or threephase media. For instance, concrete is an example of a solid medium, cooling water is a liquid one, the atmosphere is a gaseous medium, fuel with drainage water is a two-phase medium, and soil is an example of a three-phase mixture of liquid water or aqueous solution, solid salts and oxides, and gases in pores of the solid phase. Water vapors (steam) may flow together with various gases. Liquid water may be mixed with air or suspended particles when it flows inside tubes. We can change the chemistry of any environment around the metal but, as a rule, water (or aqueous solution), organic liquids (usually, oils, and fuels), atmosphere, and concrete are treated or changed. Change of corrosive soil of high electrical conductance for noncorrosive sand is one of the methods for changing the chemistry of the environment around a metal, causing diminishing corrosiveness of the environment. Deaeration, or removal of dissolved oxygen from water, is another example. Regulation and control pH are based on neutralizing the acidic or alkaline species in aqueous solutions, and we can decrease their corrosiveness in such a manner. Purification of water from Corrosion for Everybody 177 organic substances, inorganic ions, and dissolved oxygen is the main procedure for producing non-corrosive boiler feed water. Otherwise, boilers would undergo severe corrosion and explosions. Softening is the procedure of diminishing the concentration of calcium (Ca2+ ) and magnesium (Mg2+ ) ions in cooling water. Hard water (high concentrations of Ca2+ and Mg2+ ) can give rise to carbonate deposits on heat exchanger tubes. Soft water (low concentrations of Ca2+ and Mg2+ ) can cause corrosion. Any addition of chemicals into the environment results in its chemical change and, as a result, influences its aggressiveness, diminishing or increasing the corrosion of metals. One of the most widespread methods of changing the chemistry of the environment is the use of corrosion inhibitors. 4.3.1 Corrosion Inhibitors In my childhood, I heard a story about a small mouse (a mouse probably is always small) which penetrated into an ear of a big elephant, and the latter died. Everybody has suffered from the sting of a small gnat (mosquito). Small non-visible bacteria can kill any person. Homoeopathists use very small concentrations of medicines for treatment of many illnesses. Thus, we are familiar with the powerful influence of very small objects on the existence of big ones. Inhibitors are the general name for substances that, being present in suitable concentrations, decrease the rate of chemical reactions, diminish growth in biology, and impede the proceeding of physiological processes or even stop them. Inhibitors got their names from the Latin word inhibere which means to suppress, to hold, or to retain. Corrosion inhibitors are chemicals that, when present in very low concentrations in a corrosive environment, retard the corrosion of metals. Concentrations of corrosion inhibitors can change from 1 to 15,000 ppm (0.0001 to 1.5 wt%). Inhibitors are substances that work oppositely to catalysts, namely, slow down the reactions. Catalysts are not spent in chemical reactions but inhibitors are spent in electrochemical corrosion reactions. Corrosion inhibitors can be solids, liquids, and gases, and can be used in solid, liquid, and gaseous media. Solid media can be concrete, coal slurries or organic coatings (paints). Liquids may be water, aqueous solutions, or organic solvents. A gaseous medium is an atmosphere or water vapor. There is no clear classification of inhibitors. Most authors divide them into anodic (passivators), cathodic, and mixed inhibitors according to the mechanism of the influence on electrochemical reactions occurring on a metallic surface resulting in corrosion. Corrosion inhibitors can be divided into those which depend on using a medium, or type of protected metal. The first division concerns the inhibitors which are used in water and aqueous media (cooling waters), in acids, in organic media, in the atmosphere, in paints, and in concrete. Another classification can relate to inhibitors protecting iron and steel, copper and its alloys, aluminum and its alloys, zinc and its alloys, etc. Corrosion inhibitors can be classified according to their nature: organic and inorganic. We shall describe the inhibitors according to their use in specific media: in water and aqueous solutions of electrolytes (for instance, cooling waters), in 178 4 Corrosion Control Measures acids (for example, aqueous solutions of sulphuric acid, hydrochloric acid, or citric acid), in two-phase media (hydrocarbons–water), in atmosphere, in paints, and in concrete. 4.3.2 Corrosion Inhibitors in Water and Aqueous Solutions of Electrolytes When we talk about water, we have to point out a particular kind of water: cooling water, potable water, boiler feed water, water in extinguishing lines. Of course, corrosion inhibitors may be different for various kinds of water. For example, for most cooling water systems, corrosion inhibitors must work together with biocides and anti-scaling agents; for potable water, corrosion inhibitors must be non-toxic for people; for boiler feed water, inhibitors (oxygen scavengers) must react with small concentrations of dissolved oxygen at high temperatures; for water in extinguishing lines, inhibitors must be effective under stagnant conditions; for engine coolant waters, inhibitors must be applicable for many different alloys (carbon steel, cast iron, copper alloys, aluminum alloys) which are present in the system. Even if we talk about cooling waters, they may differ significantly in chemical composition, for example, be of medium, hard, or soft hardness. This fact influences drastically the efficiency of corrosion inhibitors. Those for iron and carbon steels, copper and its alloys, aluminum and its alloys in water and in aqueous solutions are summarized in Table 4.2. 4.3.2.1 Iron and Carbon Steels Corrosion inhibitors for iron and carbon steels can be of the anodic, cathodic, and adsorbed (organic) type. Anodic inhibitors are compounds that suppress the anodic electrochemical reaction of dissolution of metals. They are chromates, phosphates, silicates, nitrites, carbonates, molybdates, borates, hydroxides, benzoates, dissolved oxygen, and peroxocompounds. Potassium chromates (K2 CrO4 ) and dichromates (K2 Cr2 O7 ) have been used for many years, but are now forbidden for use as they can cause cancer. Phosphates. Inhibitive properties of phosphates are based on their hydrolysis and increase pH above 8.0, and on formation of ferric and ferrous phosphate layers (called passive films) on an iron surface. 2− − Na3 PO4(s) + H2 O(l) → 3Na+ (aq) + HPO4(aq) + OH(aq) (pH > 7; 11 to 12), (4.12) − − + H PO Na2 HPO4(s) + H2 O(l) → 2Na+ + OH (pH > 7; 8 to 9). 2 (aq) 4(aq) (aq) (4.13) Phosphates protect carbon steel at temperatures up to 80◦ C, but their efficiency decreases compared with the efficiency at low temperatures (about 20◦ C). In waters of Corrosion for Everybody 179 Table 4.2 Corrosion inhibitors of some metals and alloys in water. greater hardness, the efficiency of phosphates is higher because of the formation of mixed phosphates of calcium, magnesium, and iron on the protected metal surface. The concentrations of inhibitors needed to protect metals depend on the chemical composition of the aqueous solution, temperature, and flowing regime. We should be cautious regarding the use of phosphates! Mono-phosphates (NaH2 PO4 ) do not protect at all because they reduce the pH of water as a result of their hydrolysis and they can even stimulate corrosion of carbon steel. 180 4 Corrosion Control Measures Table 4.2 Continued. 2− + NaH2 PO4(s) + H2 O(l) → Na+ (aq) + HPO4(aq) + H3 O(aq) pH < 7; 5 to 6. (4.14) When calcium and magnesium ions are present in water, di- and tri-phosphates at high pH (above 8) and temperatures above 50◦C may result in scale deposits [Ca3 (PO4 )2 and Mg3 (PO4 )2 ] on heat exchanger tubes and diminish the ability of heat transfer. Another problem is that phosphates are favorable to bacteria growth in water. The last problem with the use of phosphates is that there are restrictions on phosphates concentrations in sewage waters. In most countries, the allowable concentration is 1.5 ppm of phosphorous P (4.5 ppm PO3− 4 ) in sewage waters. If phosphates are used in cooling water systems, generally the concentration is less than 0.2 ppm P (0.6 ppm PO3− 4 ) in sewage water (after biotreatment). Silicates: Different types of silicates can be used as corrosion inhibitors. They differ by the ratio [Na2 O]/[SiO2 ]: sodium meta-silicate Na2 SiO3 (or Na2 O·SiO2 ), sodium di-silicate Na2 Si2 O5 (or Na2 O·2SiO2 ), and sodium tri-silicate Na2 Si3 O7 (or Na2 O·3SiO2 ). Silicates are effective inhibitors in waters at higher temperatures (60 to 80◦C). The inhibitive properties of sodium silicates are explained by the increase of pH and formation of protective films incorporating SiO2− 3 anions on steel sur- Corrosion for Everybody 181 faces, for example, FeSiO3 . These films even protect carbon steel after the removal of sodium silicate from the water solution. Sodium silicate can also protect a rusted steel surface. Complex compound Fe3 O4 ·FeSiO3 is formed on the surface in this case. Sodium nitrite (NaNO2 ) is a good corrosion inhibitor in closed cooling water systems and two-phase gasoline–water systems. The concentration of NaNO2 needed for protection of carbon steel depends on the salt content in water, temperature, and flow velocity, and usually varies between 400 to 1,000 ppm. Dissolved oxygen in water of high purity may be used as an anodic inhibitor (passivator). Oxygen at high concentrations passivates metallic surfaces and thus decreases anodic dissolution of carbon steels in pure water at high temperatures. This principle is used at power stations. If anodic inhibitors are added in insufficient concentrations they may result in localized corrosion because of an increase of the ratio of cathodic to anodic surface. Thus, anodic reaction is concentrated on small anodic sites and results in pitting corrosion. The presence of chloride ions in water influences the protective properties of all inhibitors and accelerate corrosion of iron. Therefore, more concentrations of anodic inhibitors are needed in the presence of chlorides. Cathodic inhibitors are the chemical compounds that suppress cathodic electrochemical reaction on metallic surfaces. They do not cause local attack if their concentration is less than the value needed for effective protection. Calcium bicarbonate Ca(HCO3 )2 , zinc salts (ZnSO4, ZnCl2 ), polyphosphates, and phosphonates are cathodic inhibitors. Some cathodic inhibitors form non-soluble substances with products of cathodic reaction hydroxyl ions (OH− ) after oxygen depolarization on cathodic sites of metallic surfaces and thus prevent access of oxygen to cathodic regions. − Ca(HCO3 )2(aq) + OH− (aq) → CaCO3(s) + HCO3(aq) + H2 O(l) , (4.15) − Ca2+ (aq) + 2HCO3(aq) → CaCO3(s) + CO2(aq) + H2 O(l) , (4.16) 2− ZnSO4(aq) + 2OH− (aq) → Zn(OH)2(s) + SO4(aq) . (4.17) Polyphosphates are the generic name of inorganic polymeric substances with the general formula (NaPO3 )n . Main representatives of polyphosphates are sodium tripolyphosphate (Na5 P3 O10 ) and sodium hexa-meta-phosphate (NaPO3 )6 . The latter is hygroscopic and after being kept in the atmosphere, is transformed into pyrophosphate (Na4 P2 O7 ) and orto-phosphate (Na3 PO4 ). Hexa-meta-phosphates are not only used as corrosion inhibitors but also for softening of water and to remove scale from steam boilers and washing machines. Phosphonates are the organic compounds having direct bonds between phosphorus and carbon atoms which are resistant to hydrolysis. Typical representatives of phosphonates are amino-tris-methylene phosphonic acid (AMP) and 1-Hydroxyethane-1,1-diphosphonic acid (HEDP). Phosphonates influence both anodic and cathodic electrochemical reactions on metal surfaces. Cathodic inhibitors are usually less effective than anodic ones. 182 4 Corrosion Control Measures Fig. 4.9 Corrosion rate of mild steel in different solutions of NaCl versus concentrations of sodium peroxocarbonate (Na2 CO3 ·1.5H2 O2 ). Peroxo-compounds. Probably, the reader is familiar with the bleaching properties of hydrogen peroxide (H2 O2 ) for textiles (wool and silk). If women want to change their hair color to white, they have to use hydrogen peroxide. It also has antiseptic properties. The presence of the bond (—O—O—) in the molecule H2 O2 is responsible for its oxidizing properties. Peroxo-compounds, such as peroxo-borates (NaBO3 , KBO3 ), peroxo-carbonates (Na2 C2 O6 or Na2 CO3 ·1.5H2O2 ), peroxodiphosphates (K4 P2 O8 ) are very strong oxidizing agents as they contain similar peroxide (—O—O—) groups. They may decompose in aqueous solutions with the formation of active oxygen atoms, which are able to passivate metal surfaces. Passivity relates to the formation of passive oxide, hydroxide, or salt layers or films on metallic surfaces (see Appendix D). Peroxo-compounds can work as corrosion inhibitors because of their oxidizing capability and formation of a non-soluble film and thus retarding anodic reaction on metal surfaces. Similar to other anodic inhibitors, peroxo-compounds increase the corrosion rate of metals at lower concentrations (0.1 to 0.5 g/l) and decrease the corrosion rate at higher concentrations (6 to 10 g/l) (Figure 4.9). Thus, critical minimum concentrations of peroxo-compounds are needed which must be kept in a solution for the effective protection of metals from corrosion. Peroxo-diphosphates (K4 P2 O8 ) do not possess such properties of anodic inhibitors and protect carbon steel surfaces without an increased corrosion rate at low concentrations. This fact may be explained by the direct oxidation of iron and immediate formation of phosphates on carbon steel surfaces. Organic corrosion inhibitors. Benzoates, carboxylates and other salts of carboxylic acids, amines (high molecular weight, liquids), amides, tannins, and lignins are used as corrosion inhibitors in aqueous media. They are adsorbed on metallic surfaces and protect them from aggressive components. Sometimes amines are called organic filmers, and form films on metal surfaces that generally are substan- Corrosion for Everybody 183 tially thicker than films established with the proper application of inorganic inhibitors. Synergistic mixtures of corrosion inhibitors diminish both anodic and cathodic reactions on metal surfaces. Sometimes such mixtures are called synergistic, as together they decrease the corrosion rate of metals more effectively than inhibitors being present separately. Synergism is the principle of some substances improving the performances of others. Many organic and inorganic compounds relate to this type. Zinc salts are usually used synergistically with phosphates, polyphosphates, phosphonates, silicates and molybdates. If the concentration of zinc salt alone must be about 60 ppm Zn2+ as the corrosion inhibitor of carbon steel in aqueous solutions, the concentration must be 0.5 to 2 ppm in a mixture with phosphate and phosphonate. If the concentration of phosphate as a lone corrosion inhibitor of carbon steel in aqueous solutions should be about 60 to 120 ppm PO3− 4 , the concentration of phosphate must be 5.5 to 7.5 ppm PO3− in a mixture with zinc salt. When phosphonates 4 are used alone, their concentration must be about 100 ppm. If they are used in synergistic mixtures, the recommended concentrations may be 15 to 20 ppm. Silicates can be synergized with polyphosphates and zinc salts. Three and more corrosion inhibitors may also be present in synergistic mixtures. 4.3.2.2 Copper Small concentrations of copper (at least 0.1 ppm) in water can deposit on steel surfaces and cause galvanic localized corrosion (see Section 2.4). Sodium mercaptobenzothiazole (MBT), benzotriazole (BTA), and tolyltriazole (TTA) with the general name aromatic azoles at concentrations ∼5 ppm, are usually used as corrosion inhibitors of copper in aqueous solutions. Their inhibitive properties are based on the formation of a protective film on the copper surface. 4.3.2.3 Aluminum Hexametaphosphate, calcium bicarbonate Ca(HCO3 )2 , mixture of borax (sodium borate, tetraborate, or disodium tetraborate) with mercaptobenzothiazole (MBT), sodium silicate, and nitrates are used as corrosion inhibitors of aluminum in aqueous solutions. 4.3.3 Corrosion Inhibitors in Acidic Media (Pickling Inhibitors) Aqueous solutions of inorganic and organic acids are used for chemically cleaning carbon steels, stainless steels, titanium, copper, and other alloys. Usually, organic substances having N, S, or O atoms with free electron pairs, or some quaternary ammonium compounds with no donor electrons, are used as corrosion inhibitors 184 4 Corrosion Control Measures of metals in acidic media. Thiourea [CS(NH2 )2 ], amines, polyamines, amides, imidazolines, orto- and para-tolylthiourea, quinoline, derivatives of thioglycolic acid and 3-mercaptopropionic acid, gelatine, casein, sorbitol, agar – agar, dekstrin, and katapin at concentrations of 0.01 to 0.2 wt% (100 to 2,000 ppm) are used in acidic media. Usually mixtures of various amines of high molecular weight are used in industry as pickling inhibitors. Some of them work as cathodic inhibitors and slow down cathodic reduction of hydrogen cations (see reaction (1.47)), others work as filmers and prevent penetration of hydrogen cations into metallic surfaces. 4.3.4 Mechanism of Corrosion Control with Inhibitors In order to select effective corrosion inhibitors and regulate their protective properties, we have to understand their mechanism. How they protect the metals from corrosion? When we talk about coatings, the main protective mechanism is to isolate the metallic surface from corrosive media. If people do not want to get drunk they eat bread and butter before drinking vodka. Butter coats the wall of the stomach, vodka cannot reach the main organs, and a person will not become drunk. A similar principle exists when corrosion inhibitors contact a metal surface. Many corrosion inhibitors can form protective films on a metal surface and diminish possible contact with aggressive components. In order to protect metals from corrosion, inhibitors must reach the surface of metals and react with the products of electrochemical reactions, or be adsorbed. The protective mechanisms of anodic, cathodic, and adsorbing inhibitors are different. The protective mechanism of anodic inhibitors (phosphates, carbonates, molybdates, and nitrites) is based on a reaction with a metal surface and the formation of passive layers of oxides, hydroxides, or salts. These inhibitors significantly influence the corrosion potential of protected metals. The protective mechanism of cathodic inhibitors is generally based on a reaction with the products of a cathodic electrochemical reaction (OH− ). For example, Zn2+ ions react with OH− ions with the formation of non-soluble Zn(OH)2 at cathodic sites of metallic surfaces (see reaction (4.17)). Inhibitors change the kinetics of electrochemical reactions that cause corrosion in such a way that their rate drastically decreases. Organic inhibitors are adsorbed on metal surfaces. Presence of polar bonds C—N, C—S, C—O and C—P in organic molecules of inhibitors with free electrons on N, S, O and P atoms promote their adsorption on metallic surfaces. The inhibition efficiency of the homologous series of organic substances differing only in the heteroatom is usually in the following sequence: P > S > N > O. The mechanism of adsorption may be physical or chemical. When weak Coulomb forces are formed between atoms of inhibitors (for example, N) and metallic atoms, the adsorption is physical. If chemical strong bonds are formed between atom of inhibitors (for example, P or S) and metallic atoms, the chemisorption occurs. Organic inhibitors are sometimes called non-passivating types. They nearly have no influence on the corrosion potential of metals. 185 Corrosion for Everybody 4.3.5 Factors Influencing Efficiency of Corrosion Inhibitors Following factors influence the efficiency of corrosion inhibitors: the chemical composition of aqueous solution, pH, flow rate, temperature, and metal surface conditions (roughness, presence of corrosion products, and other compounds). Inhibitors are most effective in a definite pH range. Nitrites lose their effectiveness below a pH of 5.5 to 6.0; polyphosphates should be used between pH = 6.5 to 7.5; Zn – phosphate inhibitors are usually used at a pH = 7.8 to 8.2. The greater the concentration of chloride and sulphate ions, the greater the concentration of corrosion inhibitors needed for protection. Generally more concentrations of corrosion inhibitors are needed for protection under stagnant conditions than under the circulation of media. The nature of a metal surface is very important. The presence of hydrocarbons (oil or grease) or corrosion products on the surface of the metal negatively affects the efficiency of the corrosion inhibitors, because the latter cannot penetrate the surface of the metal. The metallic surface must be cleaned and passivated before use of corrosion inhibitors. Most companies recommend carrying out passivation by special chemicals containing corrosion inhibitors before use in cooling water systems. Suspended solids are detrimental in the use of inhibitors because the first can be adsorbed by the metallic surface thus preventing the corrosion inhibitors from reaching the surface. Alternatively, corrosion inhibitors may be adsorbed by suspended solids in water and more concentrations are needed. Usually inhibitive properties are diminished with an increase of temperature. But some corrosion inhibitors, for example, silicates, are more effective at 60 to 80◦C than at 25◦ C. 4.3.6 Inhibitor Efficiency In order to compare various corrosion inhibitors and select the most effective one for corrosion control, we have to know how to calculate their efficiency. There are two values, E and γ , for determinating inhibitor efficiency: E, % = CR◦ − CRi · 100%, CR◦ γ = CR◦ , CRi (4.18) (4.19) where CR◦ – the corrosion rate of metal in media without inhibitor; CRi – the corrosion rate of metal in media with inhibitor. The corrosion rate can be measured by any available method: weight loss or electrochemical method (see Chapter 5). The inhibitor efficiency values E and γ are interrelated by the equation 186 4 Corrosion Control Measures
E, % = 1 − 1 γ  · 100%. (4.20) Most scientists and engineers use E values, because they show efficiency in percents, 100% being full protection. In order to select allowable (optimal, minimum, or rational) concentrations of corrosion inhibitor, we have to study the corrosion rate of a particular metal as a function of the concentration of corrosion inhibitor in a particular solution. It is very important to compare the efficiency of various inhibitors correctly, preferably using concentration units in mol/L. Secondly, if we select the corrosion inhibitor, the question is what is the allowable E value: 60%, 70%, 80%, 90%, 99% or 100%? Let us define the minimum allowable E value. First of all, the corrosion rate of a metal in a particular solution must be determined. For example, the experimental corrosion rate of carbon steel in cooling water is 0.6 mm/year. Now we have to determine the maximum allowable corrosion rate for this metal in the solution (see Chapter 5). The maximum allowable corrosion rate of carbon steel heat exchanger tubes in cooling water is 0.1 mm/year (in the oil and petroleum industries). The minimum allowable inhibitor efficiency in this particular case is 
0.6 − 0.1 Emin = · 100% = 83%. (4.21) 0.6 This means that our target for the selection and use corrosion inhibitors is E = 83%. Only inhibitors which give this value and more can be selected for use. 4.3.7 Application of Corrosion Inhibitors and Some Recommendations Inhibitors are rarely used in the form of a single compound nowdays. For example, sodium nitrite (NaNO2 ) alone at 400 ppm is an effective inhibitor of carbon steel in closed cooling water systems. The minimum concentrations of different inhibitors for efficient protection may significantly vary and depend on temperature, chemical composition of media, and flow rate. Synergestic mixtures of anodic and cathodic inhibitors are often used in cooling water systems. For example, zinc and magnesium salts are used with phosphates and phosphonates. If copper alloys are present in cooling water systems, organic azole compounds should be added. Mixtures of corrosion inhibitors containing borate, silicate, molybdate, nitrite, nitrate, and benzotriazole are used in closed water cooling systems of motor vehicles. Such mixtures can protect carbon steel, cast iron, aluminum alloys, and copper alloys. Mixtures of calcium gluconate and sodium benzoate, gluconate with tannin, calcium, and zinc gluconates are synergestic corrosion inhibitors of carbon steel in water. Inhibitors can prevent specific corrosion failures. For example, nitrates are used as inhibitors of stress corrosion cracking of SS 304. Specific corrosion inhibitors (usually organic amines) are used in the oil and gas industry, crude oil refineries, in lubricating oils Corrosion for Everybody 187 and hydraulic liquids, and for protecting metals in the atmosphere, in paints, and in concrete. Corrosion inhibitors in paints are usually pigments. Chromates (SrCrO4 ) and lead-based compounds (Pb3 O4 , PbCrO4 ) commonly used in paints in olden times are toxic and are now banned from use. Zinc powder is also restricted. “Green” inorganic and organic inhibitors have been developed for use in paints. Organic inhibitors include amines and phosphate esters; inorganic inhibitors include phosphates and molybdates of zinc, aluminum and iron, Na-benzoate, Zn-gluconate, phosphites, stearates, acetates, acrylates, and silicates. Nitrites, borates, benzoates, formiates, surface active agents, salts of organic acids, their mixtures and VCI (VPI) are used as corrosion inhibitors of carbon steel in concrete. Regarding the application of VCI for protection of metallic devices and constructions in the atmosphere, the reader is referred to Sections 3.2 and 3.6. 4.3.8 Inhibitors and Ecology There is no clear and accepted definition of “environmentally friendly”, or “green” corrosion inhibitors. They must be assessed from the health, safety, and environmental point of view. Corrosion inhibitors must be low toxic (better non-toxic), biodegradable, with low bioaccumulation, and should not contain harmful elements and components. One example is the prohibition of chromates which have been used for many years as effective corrosion inhibitors, or restricted use of phosphates in sewage waters. For instance, Zn2+ cations, as all in nature, must be present in water in optimal, or no-risk, concentrations. These values are between 0.5 to 50 µg/l. When the concentration of Zn2+ is less than 0.5 µg/l, zinc is deficient; when the concentration of Zn2+ is above 50 µg/l, zinc is toxic for many kinds of fish. The criterion for selecting corrosion inhibitors in the 20th century changed significantly. Before the 1960s it was efficiency, then until the 1980s it was economy, now the main criterion is ecology. Intensive research has been carried out into the substitution of toxic inhibitors for less toxic or non-toxic ones. For example, plant extracts aloe leaves and peels from oranges, mangoes, and pomegranates, tobacco, black pepper, castor oil seeds, acacia gum, and lignin are recommended for protection of steel from corrosion by acids. The first oxygen scavengers for treating boiler feed water were natural products, extracts of vegetation, that included catechol, pyrogallol, and tannins. Then they were replaced by efficient, but toxic hydrazine compounds. Tannin and other natural products are now back in favor. If we are talking about inhibitors in paints, lead-containing (Pb3 O4 ) and chromates (SrCrO4 ) are toxic, and Zn-containing compounds [Zn3(PO4 )2 and ZnO] possess some level of ecotoxicity. New “green” inhibitor magnesium oxy-amino-phosphate is recommended for use in paints. 188 4 Corrosion Control Measures 4.3.9 Conclusions 1. There is no theoretical way of forecasting inhibitive efficiency of known or newly synthesized inhibitors in a particular medium. Only experiments can produce results and make conclusions about the possible use of inhibitors in a medium. 2. Use of corrosion inhibitors is influenced by regulations that have been developed because of toxicity and environmental effects resulting from industrial effluents. Therefore, it is necessary to analyze the requirements for water, fuels or atmosphere when we begin to use corrosion inhibitors, even that which has been used for many years in other places, because new information about their toxicity may be available. Recommended Literature 1. Shreir, L.L., Jarman, R.A. and Burstein, G.T. (Eds.), Corrosion, Vol. 2, Third Edition, Butterworth Heinemann, UK, 1994, pp. 17:10–17:65. 2. Revie, R.W. (Ed.), Uhlig’s Corrosion Handbook, Second Edition, John Wiley & Sons, Inc., USA, 2006, pp. 1089–1105. 3. Bregman, J.J., Corrosion Inhibitors, MacMillan, New York/London, 1963. 4. Rosenfeld, I.L., Corrosion Inhibitors, McGraw-Hill, New York, 1977, 327 pp. 5. Mattsson, E., Basic Corrosion Technology for Scientists and Engineers, Second Edition, The Institute of Materials, UK, 1996, pp. 103–107. 6. Korb, L.J. and Sprowls, D.O., Metals Handbook, Vol. 13: Corrosion, ASM International, USA, 1987, pp. 478–486, 494–497. 7. Rosenfeld, I.L. and Persiyantzeva, V.P., Inhibitors of Atmospheric Corrosion, Nauka, Moscow, 1985, 278 pp. [in Russian]. 8. Uhlig, H.H. and Revie, R.W., Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, Third Edition, John Willey & Sons, USA, 1985, pp. 263–277. 9. Corrosion Inhibitors, The Institute of Materials, London, 1994.