Cathodic Protection Presentation
Cathodic Protection Presentation
Cathodic Protection Presentation
WHAT IS CORROSION ??
Gradual destruction or alteration of a metal and its mechanical properties caused by direct chemical attack or by electrochemical reaction.
A significant amount of energy is put into a metal when it is extracted from its ores, thus placing it in a high energy state. As per the principle of thermodynamic, material always seeks the lowest energy states. Hence all metals are unstable because of their high energy levels. Metals tend to revert to their natural lowest energy steady states through process of corrosion.
HOW CORROSION OCCURS ?? Corrosion cell consists of following elements 1. Anode 2. Cathode 3. Electrical contact between both Anode and Cathode 4. Electrolyte
Two basic mechanisms by which metals in electrolyte corrode
1. Galvanic corrosion 2. Electrolytic corrosion
HOW CORROSION OCCURS ?? Corrosion of metals in aqueous environment is almost electrochemical in nature. It occurs when two or more electrochemical reactions occur on metal As a result some of the elements change from metallic to non metallic state. Electrochemical reactions occur either uniformly or non uniformly on the surface
HOW CORROSION OCCURS ?? Corrosion cell consists of following elements 1. Anode 2. Cathode 3. Electrical contact between both Anode and Cathode 4. Electrolyte
O2 + 4 H - ----------> 2 H20
O2 + 2 H2O + 4 e- -----> 4 (OH)(Neutral solution)
Next
HOW CORROSION OCCURS ?? Galvanic corrosion When two metals (having different potentials) are connected electrically and are immersed in an electrolyte, current will be generated and the metal which is less noble will corrode. Current from the corroding metal will flow into the electrolyte, over to a non corroding metal and back through the connection between the two metals.
The The
metal where current leaves will get corroded and is known as anode metal which receives current is known as cathode.
HOW CORROSION OCCURS ?? Galvanic corrosion The differences in potential can be due to 1. Coupling of dissimilar materials ( CS and SS) 2. Non homogenity in same material (Holidays in coatings/inclusion in a metal) 3. Non homogenity in electrolyte (different salt concentrations in soil)
Electrolytic corrosion
This is a result of direct current entering from outside sources leaving a particular metal structure through electrolyte. The location where current enters the structure is provided with protection and is called cathode.
The location where current leaves the structure is known as anode and corrosion is occurred. This type of corrosion is often referred as stray current corrosion and is a result of currents entering the ground from sources of DC as street railways or DC machinery.
1. COATINGS (Primary means of preventing corrosion) 2. CATHODIC PROTECTION (Secondary means of preventing corrosion)
Holidays in coatings can also develop during service due to degradation of coating, soil stresses or movement of pipe in the ground.
Introduction
Cathodic protection is an electrical method of preventing corrosion
It operates by passing direct current continuously from electrodes which are installed in the electrolyte, to the structure to be protected. Corrosion is arrested when the current is of sufficient magnitude and is properly distributed It is used for protecting wide variety of metallic structures which are submerged in electrolytes such as water/soil.
Applications
Buried vessels
Sub sea pipe lines Tank bottom plates (Internal/External) Ship hulls Lock gates and dams Steel pilings
This is done by forcing the electric current to flow through the electrolyte towards the surface of the metal to be protected The current may be obtained from any convenient, external source, such as battery, rectified alternating current supply,DC generator or by galvanic action.
Methods of cathodic protection Cathodic protection to immersed and buried surfaces can be achieved by following two methods:
Sacrificial anode
This method makes use of galvanic action to provide required cathodic protection current.
The surface of the structure is made cathodic by connecting it electrically to a mass of more active metal buried or immersed in the common electrolyte, the more active metal then become anode. Magnesium, Aluminium, Zinc etc. are commonly used for this purpose.
Protective current
Galvanic Anode
This method uses anodes/ground beds which are energized by an external DC power source. The ground bed may consist of titanium, platinum, graphite, high-silicon iron rods, cast ion scrap, old steel pipe etc. In this anodes are installed in the electrolyte and are connected to the positive terminal of the DC source, the structure which is to be protected is connected to the negative terminal of that source. DC power source can either be a battery, a Transformer Rectifier unit, solar cells, DC generators etc.
COMPARISION BETWEEN SACRIFICIAL AND ICCP SACRIFICIAL CP SYSTEM 1. Independent of electrical power source 2. Limited current is available. Hence restricted to protection of well coated pipelines 3. Impractical except with soils or water with low resistivity 4. Easy to install 5. Replacement is required whenever anodes get consumed 6. No/very less interference effects 7. No control over the current output 8. They cannot be wrongly connected, so that polarity is reversed
COMPARISION BETWEEN SACRIFICIAL AND ICCP ICCP 1. External source is required to supply current 2. Can be used to protect un coated structures 3. Not restricted due to high resistivity of electrolyte 4. Need careful design and difficult to install 5. Less number of anodes are required and anodes do not consume 6. Interference effects are high
MONITORING OF CP SYSTEM
POTENTIAL MEASUREMENTS PRINCIPLE In measuring the pipe to soil potential principle used is to form an electrochemical cell, the second half cell being a standard reference electrode.(Cu-CuSo4 or AgAgCl)
Reference electrode
Cu/CuSo4 Ag/AgCl/sea water Zinc
Potential required
-850 to -2000 mV -800 to -1950mV 0 to 250
It is required to determine the corrosivity of the soil for the buried metal structure.
It is required to identify the suitable locations for the anode ground beds and to select suitable T/R ratings. It varies greatly with its water content and with the electrolyte dissolved in the water.It vary with the season of the year and the rainfall.
Insulating flanges: Pipelines which are to be cathodically protected should have an insulating flange at both ends to prevent current loss to other installations. In general gaskets and sleeves will be of phenolic products.
Anode Characteristics
Current output is of primary importance in selection of anodes. This depends on the driving force available and the circuit resistance. The driving force is the difference in potential difference between
Anode Efficiency It is defined as the ratio of ampere-hours actually supplied (at site) to the theoretical ampere-hour output per unit weight of metal consumed. Efficiency of magnesium alloy anodes is about 50% Efficiency of Zinc anodes is about 90% Efficiency of Aluminium anodes is about 94%.
Anode Characteristics
Anodes used as sacrificial anodes are Zinc, Magnesium and Aluminium
Magnesium
Used for higher resistivity electrolytes Where Zinc or Aluminium are not recommended Mg used for CP should be of high purity Gives High current output as its potential difference with steel is more than Zinc or aluminium Energy capacity : 1100 Ah/kg Efficiency : 50 %
Anode Characteristics
Zinc Zn used for CP should be of high purity Gives small current output as its potential difference with steel is less. Not recommended to use with electrolyte resistivity greater than 1500 ohm-cm
Anode Characteristics
Aluminium Used only for sea service. It is never used in its pure form as it tends to passivate with time resulting in under protection of the system. Widely used are Al-Zn-Hg, Al-Zn-In, Al-Zn-Sn Energy capacity varies from 930-2760 Ah/kg for various alloys. Efficiency of anode is 94 %.