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TYPES OF CORROSION AND FACTORS INFLUENCING CORROSION

TYPES OF CORROSION AND FACTORS INFLUENCING CORROSION. Presented by: TEENA KATHPAL Lecturer Pt. J R Govt Polytechnic College Hoshiarpur. CORROSION. “ It is the process of deterioration

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TYPES OF CORROSION AND FACTORS INFLUENCING CORROSION

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  1. TYPES OF CORROSION AND FACTORS INFLUENCING CORROSION Presented by: TEENA KATHPAL Lecturer Pt. J R Govt Polytechnic College Hoshiarpur

  2. CORROSION • “It is the process of deterioration • ( decay) of a metal as a result of its reaction with air and water.” • In case of Iron, it is called Rusting and in case of silver, it is called Tarnishing.

  3. TYPES OF CORROSION • 1. DRY or CHEMICAL CORROSION • 2. WET or ELECTROCHEMICAL CORROSION

  4. DRY CORROSION This type of corrosion occurs by direct chemical attack of atmospheric gases like O2 , halogens, hydrogen sulphide etc. on the metal surface. it is further of following types: • (a) Corrosion by O2 or Oxidation corrosion. • (b) Corrosion by other gases.

  5. OXIDATION CORROSION • It is due to chemical attack of O2 on the metal resulting the formation of metal oxide layer as: • 2M + nO2-------- 2MOn (metal oxide) • This theory is called Wagner’s Theory.

  6. Nature of Metal oxide layer : • Stable: It acts as a barrier between metal and O2, So prevents further corrosion. • Unstable: It decomposes back to metal and O2. Ag, Au, Pt are protected by this manner.

  7. (3)Volatile: Layer volatalises and metal exposed for further corrosion. (4) Porous: Corrosion occur through pores and continues till entire metal gets corroded.

  8. Pilling Bedworth Rule: If the volume of oxide layer formed is equal or greater than the volume of the metal, the oxide layer is protective. However if the oxide layer formed has volume lesser than that of the metal, then the oxide layer is non-protective.

  9. CORROSION BY OTHER GASES • Corrosion is also caused by some other gases like Cl2 ,H2S, SO2 etc. This type of corrosion depends on chemical affinity between metal and gas.

  10. WET CORROSION • This type of corrosion occurs when a liquid medium is involved. One part behaves as anode and undergoes oxidation and other part acts as cathode and undergoes reduction.

  11. Liquid medium involved acts as electrolyte. It is further of two types: (a) Evolution of H2 type corrosion (b) Absorption of O2 type corrosion

  12. Evolution of H2 type corrosion • It occurs in the acidic environment. At anode: M-------- M n+ + ne- (oxidation) At cathode: 2H + + 2e- ----- H2 (reduction)

  13. Absorption of O2 type corrosion • E.g. Rusting of iron

  14. If the layer of iron develops some cracks, this damaged surface acts as anode while the other intact metal behaves as cathode. Presence of moisture acts as electrolyte.

  15. The reactions occuring are: • Fe -------> Fe+2 + 2 e- (oxidation) • 1/2O2 (g) + H2O + 2e- ------> 2OH- (reduction) • Fe+2 + 2OH- --------Fe(OH)2

  16. In the presence of excess oxygen, Fe(OH)2 oxidises to Fe(OH)3 as: 4Fe(OH)2 + O2 + 2H2O---- 4Fe(OH)3 This ferric hydroxide formed decomposes to hydrated ferric oxide as: Fe(OH)3 --------- Fe2O3.xH2O (Rust)

  17. OTHER TYPES OF CORROSION • 1. Galvanic Corrosion. • 2. Waterline Corrosion. • 3. Crevice Corrosion. • 4. Stress Corrosion. .

  18. TYPES OF CORROSION (contd.) • 5. Pitting Corrosion. • 6. Soil Corrosion. • 7. High temperature Corrosion. • 8. Filiform Corrosion

  19. GALVANIC CORROSION • This type of corrosion occurs when two dis-similar metals are connected to each other in the presence of an electrolyte. This constitutes a galvanic cell in which a metal with lower reduction potential undergoes corrosion.

  20. e.g. when Zn and Cu are connected to each other then Zn having lower reduction potential i.e. -0.76V acts as anode and is corroded while Cu with more reduction potential i.e 0.34V acts as cathode and remains protected.

  21. WATERLINE CORROSION • This type of corrosion is observed in the water tanks where water remains collected for a long time. It is found just below the line of water, so called waterline corrosion.

  22. CREVICE CORROSION • This is also known as Contact corrosion. It occurs at the place of contact between metals and metals or non metals. This type of corrosion occurs at crevices between the metallic objects e.g. bolts, nuts, washers etc

  23. PITTING CORROSION • This type of corrosion occurs due to cracking of protective film from the surface of metal. It results in the formation of pits or holes on the metal, so called pitting corrosion

  24. STRESS CORROSION • It is due to the combined effect of tensile stress and corrosive environment. This type of corrosion is observed in the fabricated articles. During fabrication , these are subjected to different type of stresses like rolling, drawing etc.

  25. SOIL CORROSION • This type of corrosion occurs on those articles which remain burried under soil e.g. underground water pipes. It occurs due to moisture, dissolved electrolytes and presence of micro-organisms.

  26. HIGH TEMPERATURE CORROSION • This type of corrosion occurs in those components which operate at very high temperature like jet engines, gas turbines etc. It involves degradation of metal which results in the deposition of salts or other compounds on the metal surface.

  27. It may be of following types: 1. Oxidation: It involves deposition of oxides on the metal surface. 2. Sulphidation: deposition of sulphides. 3. Nitridation: deposition of nitrides. 4. Carburization: It involves deposition of carbon.

  28. FILIFORM CORROSION This type of corrosion is observed under thin coatings in the form of thread like filaments. It occurs on the surfaces coated with organic film which is 0.1mm thick.

  29. Factors influencing corrosion 1. Nature of metal 2. Nature of corrosive environment.

  30. Nature of Metal: • (1)Purity of metal: Impure the metal, more is the rate of corrosion. This is because impurities act as “minute electrochemical cells.” • (2) Physical state of metal: Physical state of metal like grain size, stress affects the corrosion rate. Stressed areas undergo more corrosion.

  31. (3) Position in Galvanic series: More the oxidation potential i.e if metal is higher in the galvanic series, it is more anodic and hence more is the rate of corrosion. (4) Nature of Oxide layer: Corrosion rate depends upon the nature of oxide layer whether it is stable, unstable, porous etc.

  32. (5) Passive nature of metal: Some metals like Ti, Al, Cr show more corrosion resistance than expected from their position in the electrochemical series. These are called passive metals. This is due to formation of thin protective layer on metal surface.

  33. (6) Nature of corrosive products: If the corrosion products are soluble or volatile in nature, then corrosion increases. (7) Relative area of anode and cathode: smaller the anodic area, more is the corrosion rate because oxidation of anode occurs at faster rate.

  34. 2. Nature of corroding environment: • Temperature: Rate of corrosion increases with increase in temperature. • Moisture: More the moisture, more is the rate of corrosion.

  35. (3) pH Value: Lesser the pH i.e. acidic conditions, more is corrosion. (4) Nature of electrolyte: Presence of salts in the electrolyte increases the rate of corrosion.

  36. (5) Presence of impurities in atmosphere: Impurities increase the rate of corrosion. Corrosion is more in industrial areas and sea. This is because of presence of gases like CO2, H2S, SO2

  37. THANK YOU

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