Corrosion and corrosion control were discussed. Key points included the mechanism of metal corrosion involving an anode, cathode, electrolyte and metallic path. Common corrosion types like uniform, galvanic, and pitting were also covered. Methods to control corrosion involved using corrosion-resistant metals and alloys, coatings, inhibitors, cathodic protection and altering the environment.
Corrosion and corrosion control were discussed. Key points included the mechanism of metal corrosion involving an anode, cathode, electrolyte and metallic path. Common corrosion types like uniform, galvanic, and pitting were also covered. Methods to control corrosion involved using corrosion-resistant metals and alloys, coatings, inhibitors, cathodic protection and altering the environment.
Corrosion and corrosion control were discussed. Key points included the mechanism of metal corrosion involving an anode, cathode, electrolyte and metallic path. Common corrosion types like uniform, galvanic, and pitting were also covered. Methods to control corrosion involved using corrosion-resistant metals and alloys, coatings, inhibitors, cathodic protection and altering the environment.
Corrosion and corrosion control were discussed. Key points included the mechanism of metal corrosion involving an anode, cathode, electrolyte and metallic path. Common corrosion types like uniform, galvanic, and pitting were also covered. Methods to control corrosion involved using corrosion-resistant metals and alloys, coatings, inhibitors, cathodic protection and altering the environment.
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1-1 UNIT 1
Corrosion and Corrosion Control
1-2 UNIT 1 OBJECTIVES An Understanding of: • Mechanism of metal corrosion • Common types of corrosion • Methods of corrosion control 1-3 MECHANISM OF MEDAL CORROSION • The corrosion process • The corrosion cell • Relative corrosion tendencies of medal 1-4 Corrosion • Corrosion: The chemical or electrochemical reaction between a material and it’s environment that results in the loss of the material and it’s properties 1-4A TYPICAL CORROSION COST ITEMS • Replacement of deteriorated items • Maintenance of facilities • Shut-down of facilities • Product loss or environmental contamination • Damage/injuries from accidents 1-5 FIGURE: NATURAL CYCLE OF CORROSION 1-6 FOUR CONDITIONS NECESSARY FOR CORROSION • Anode • Cathode • Metallic Path • Electrolyte 1-7 FIGURE: FOUR BASIC PARTS OF A CORROSION CELL 1-8 Figure 1: CORROSION CELL 1-9 FIGURE: DRY CELL BATTERY (CORROSION CELL ANALOGY) 1-9A FIGURE: THE CORROSION CELL ON A METAL SURFACE 1-10 TABLE 1: GALVANIC SERIES Active Magnesi um Zinc Galvaniz ed Steel Aluminiu m Mild Steel Wrought Iron Cast Iron Chromiu m Stainless Steel Type 410 (Active) Stainless Steel Type 304 (active) Naval Brass Yellow Brass Nickel (active) Cooper Silver Solder Chromiu m Stainless Steel Type 410 1-11 POTENTIAL DIFFERENCES ON A SINGLE PIECE OF METAL • Chemical differences (e.g., contaminants in the metal) • Physical difference (e.g., cuts, hammering, etc.) 1-12 Common Corrosion Cell Chemistry • Anode Loses Metal • Cathode protected, become, becomes alkaline, may produce hydrogen • Oxygen cathode affects corrosion rate 1-13 FIGURE: CORROSION REACTION Anode: M Cathode: 2H2O + 2e- 2OH- + H2 Water Electrons Hydroxide Hydrogen Gas (alkalinity)
Or depending on localized conditions:
2H2O + O2 +4e- 4OH- 1-14 COMMON TYPES OF CORROSION • Uniform • Dissimilar metal (galvanic) • Concentration cell • Stray current • Dealloying • Erosion-corrosion • Exfoliation 1-15 UNIFORM CORROSION • Anodes and cathodes reverse • Not usually 1-16 UNIFORM CORROSION 1-16A Pitting Corrosion • Localized Accelerated corrosion • Caused by metal ion non-uniformity • Caused by localized breakdown of passive layer 1-16B DIVER USING A GAUGE TO MEASURE PIT DEPTH 1-17 DISSIMILAR METAL CORROSION 1-18 CORROSION CAUSED BY DISSIMILAR SURFACE CONDITION 1-19 STEEL PIPE IN ALUMINUM DECK 1-20 PIPE WITH VALVE OF A DIFFERENT MATERIAL 1-21 AVOID DISSIMILAR METAL CORROSION BY: • Choosing compatible metals • Using rubber or plastic insulators • Avoiding electrolyte 1-22 FIGURE: THE EFFECTS OF RELATIVE ANODE AND CATHODE AREAS ON CORROSION 1-23 ALUMINUM TRUSS (ANODE) AND STAILESS STEEL BOLTS (CATHODE) 1-24 COMMON TYPES OF CONCENTRATION CELL CORROSION • Difference in oxygen concentration • Difference in metal ion concentration 1-24A FIGURE: OXYGEN CONCENTRATION CELL 1-24B FIGURE: METAL ION CONCENTRATION CELL 1-12 Common Corrosion Cell Chemistry • Skip weld • Back-to-back angles • Areas under bolt heads 1-25A SKIP WELD 1-26 CREVISE CORROSION 1-27 SOURCE OF STRAY CURRENT CORROSION • Electric railways • Electric cranes • Welding generators • Adjacent CP system 1-27A FIGURE: EXAMPLE OF STRAY CURRENT CORROSION 1-27B METHOD OF REDUCING STRAY CURRENT CORROSION • Reducing Current flow by modifying source • Modifying electrical flow by bonding • Applying counterbalancing CP 1-28 DEALLOYING • Selective metal loss • Dezincification of brass • Graphitization of cast iron 1-29 DEZINCIFICATION OF BRASS 1-30 EROSION-CORROSION • Abrasive removes corrosion products • Metal surfaces actively corrodes 1-30A ROLLING OF ALUMINUM PLATE 1-30B EXFOLIATION OF ALUMINUM 1-31 METHOD OF CORROSION CONTROL • Design • Resistant metals • Non-metallics • Inhibitors • Altering the environment • Cathodic protection • Coatings 1-32 • Avoid contact of dissimilar metals • Incopatible environments • Water traps • Crevices • Rough surfaces and sharp edges • Limited access to work 1-33 DISSIMILAR METAL CORROSION - STEEL NUT ON COOPER FITTING 1-34 ALUMINUM POLE IN CONCRETE 1-35 WATER TRAP 1-36 AVOID WATER TRAPS BY: • Inverting configuration • Drilling weep holes 1-38 TWO WELDS, ONE GROUND AND ONE UNGROUND 1-39 SPRAYING PAINT WHERE THERE IS LIMITED ACCESS TO WORK 1-40 CORROSION-RESISTANT METALS AND ALLOYS • Titanium • Aluminum alloys • Stailess steels • Zinc (galvaniszing) • Weathering steel 1-41 WEATHERING (LOW-ALLOY) STEEL (ADVANTAGES) • Forms protective oxide • Coating my not be necessary • Reduced Corrosion rate 1-342 A588 BRIDGE IN GOOD CONDITION 1-43 WEARING STEEL (LIMITATIONS) • Protective layer destroyed by chlorides • needs open, dry exposure • May not be aesthetically pleasing 1-44 CORRODED A588 BRIDGE 1-45 HOT-DIP GALVANIZING OF STEEL • Barrier coating of zinc • Cathodic protection • Painted for added protection 1-45A GALVANIZING BATH 1-45B GALVANIZING LAYERS 1-46 GALVANIZED GUARDRAIL 1-46A WAYS OF COATING STEEL WITH ZINC • Hot-dip galvanizing • Electrogalvanizing • Tumbling with powdered zinc • Metallizing • Zinc-rich coatings 1-47 CORROSION-RESISTANT NON- METALLICS • Plastic • Elastomers • Composites • Ceramics 1-47A THERMOPLASTICS • Fluorocarbons • Polythylenes • Polypropylenes • Polyvinyl chlorides 1-47B THERMOSETS • Epoxies • Phenolics • Polyesters 1-47C ELASTOMERIC LININGS • Natural and synthetic rubbers • Polyurethanes 1-47D COMPOSITES • FRP and FGRP materials • Used in Piping, Process Vessels, tanks, etc. 1-47E CERAMICS • Chemical resistant • Temperature resistant • Electrical resistant 1-48 INHIBITORS • Thin, Protective films • Passive layer on metal • Remove aggressive constituents 1-49 CORROSION CONTROL BY ALTERING ENVIRONMENT • Dehumidification • Purification 1-50 CATHODIC PROTECTION • External anode • Entire structure becomes cathode • Immersed or buried conductive media (electrolyte) 1-50A USES OF CATHODIC PROTECTION • Ships • Waterfront Structures • Underground piping and tanks • Water tanks interiors 1-52 SYNERGISM OF COATINGS AND CATHODIC PROTECTION • CP protects at holidays • Coating reduce CP current requirements 1-53 TWO BASIC TYPES OF CATHODIC PROTECTION • Sacrificial Anode (Galvanic) • Impressed current (external power source) 1-54 Figure: GALVANIC SYSTEM OF CATHODIC PROTECTION 1-55 SECRIFICIAL ANODE METALS • Magnesium • Zinc • Aluminum 1-56 FIGURE: IMPRESSED CURRENT SYSTEM OF CATHODIC PROTECTION 1-56A FIGURE: A CATHODICALLY PROTECTED WATER TANK FIGURE 1-56 IMPRESSED CURRENT CP SYSTEM FOR SHIP HULLS 1-57 IMPRESSED CURRENT ANODE MATERIALS • High-silicon cast iron • Graphite • Aluminum • Scrap iron • Pure, alloyed, or planted platinum 1-58 TABLE 3: SACRIFICIAL ANODE VS. IMPRESSED CURRENT CATHODIC PROTECTION Sacrificial Anode Impressed Current No external power supply External power supply Limited current output Variable voltage Adjustable medium output Variable current OK for low resistivity OK fo high resistivity medium Requires electrolytes Requires electrolytes Lower installation cost Higher installation cost Few interference problems Can cause interference Low maintenance cost Monthly power bills Localized protection Protects larger structures 1-58A POSSIBLE ADVERSE EFFECTS OF CATHODIC PROTECTION • Deterioration of coatings not resistant to alkali • Disbonding of coating by hydrogen gas from excessive current • Electroendosmosis 1-59 CORROSION CONTROL BY COATING • Barrier • Inhibitors • Cathodic protection 1-59A ADVANTAGES OF COATINGS • Ease of Application • Ease of storage and handling • Range of Acceptable ambient conditions • Economics • Easy repair • Selection of color, gloss, and texture 1-59B LIMITATIONS OF COATINGS • Surface preparation requirements • Application requirements • Drying/curing requirements • Health/safety/environmental concerns 1-60A CORROSION CONTROL PROGRAMS • A successful corrosion control program utilizes as many corrosion control systems as appropriate and practical. 1-61 UNIT 1 Summary • Mechanisms of metal corrosion • Common types of corrosion • Methods of corrosion control
Oxy-Acetylene Welding and Cutting: Electric, Forge and Thermit Welding together with related methods and materials used in metal working and the oxygen process for removal of carbon