metal corrosion and its prevention.pptx

Very essential topic

1. Corrosion and its Prevention By Mr Pankaj Sharma

2. Introduction Corrosion is natural phenomenon which attacks metal by chemical, electrochemical action converts metal into metallic compound , such as an oxide , hydroxide or sulphate. Destruction or deterioration and consequent loss of solid metallic materials , starting at its surface due to chemical or electro chemical attack, by their environment. Corrosion is major problem worldwide in all industrial sectors. It impacts on safety , health and environmental issues .

3. Corrosion cycle

4. Causes of Corrosion Pure or noble metals such as gold, silver, platinum, aluminum , copper do not corrode since they are chemically uncombined in their natural state . Following conditions must exist before corrosion. Presence of metal that will corrode (anode) Presence of cathode Presence of conductive liquid (electrolyte) iv)Electrical contact between anode and cathode Elimination of any one of these condition will reduce or stop corrosion.

5. Corrosion Triangle

6. Classification of Corrosion on the basis of working temperature Low temperature Corrosion High temperature Corrosion on the basis of working environments Dry or Chemical Corrosion Wet or Electrochemical Corrosion

7. i) Dry or Chemical Corrosion It occurs when oxygen in air reacts with metal , without presence of liquid. It is very sensitive to temperature Rate of dry corrosion varies from metal to metal , as a result of mechanisms involved. Oxide layer on steel & iron is known as rust.

8. In aluminum , copper oxide layer formed due to reaction with atmospheric oxygen ,stops further corrosion. As oxide layer formed , prevents further contact of oxygen as film is non porous. This stopping is known as Passivations. Active Corrosion- steel, CI , Mg.

9. ii) Wet or Electrochemical Corrosion It is electrochemical phenomenon , which occurs in galvanic cell. Particularly when two metals are in contact with a liquid , liquid containing salt and electric potential is formed between the metals. in some cases in single metal , when comes in contact with liquid or electrolyte , a anode and cathode areas are created. When anode area starts corroding with respect to cathode areas , is termed as wet corrosion.

10. Mechanism of Corrosion Electrochemical reaction – transfer of electrons Electrochemical corrosion cell- Electrical circuit and its associated chemical reaction are called electrochemical corrosion cell.

11. Anode- Oxidation occurs, metal dissolution takes place & production of electrons Cathode – electrons are consumed & reduction reactions occurs Metal Path – movement of electron from anode to cathode Electrolyte- ionic current flows

12. Anodic & cathodic reactions in corrosion cell  2  Fe  Fe  2 e O  2 H O  4 e   4 OH  2 2 At Anode-Oxidation At Cathode -reduction

13. Type of Metal Heat treatment & Grain direction Presence of dissimilar metal Anode & cathode surface area Temperature presence of electrolyte Availability of oxygen time of exposure to corrosive environment Factors Influencing Corrosion

14. Types of corrosion 1. Pitting Corrosion- Aluminum, Mg alloys, Cu alloy.

15. Mechanism of pitting Corrosion Factors influencing pitting Corrosion Alloying elements Surface finish 3.Heat treatment

16. 2.Stress corrosion & season cracking It is combined action of corrosive environment and mechanical stresses which often due to poor design or poor maintenance practice. Stress corrosion cracking (SCC) is intergranular cracking of metal caused by stress& corrosion.

17. stress corrosion cracking process

18. Mechanism of stress corrosion cracking Transgranular cracking in SCC

19. Control or Prevention of SCC Control of stress level and Hardness. Avoid chemical corrosion that causes SSC Control of temperature Modification in design , that will reduce SCC

20. 3. Cavitation Corrosion Protective films are removed from metal surface during fluid flow. Low pressure development due to discontinuity in flow path , causes vapour bubbles to form in fluid steam. Due to collapsing of bubbles shock waves produced and removes protective film from metal surface

21. Cavitation corrosion on pump impeller

22. Cavitation damage in pump impeller Control of cavitation Corrosion- Careful material selection i.e. high alloy steels containing Ni, Cr, W Use of soft rubber coating , it absorbs cavitation energy Smooth Surface finish

23. 4. Hydrogen Embrittlement Hydrogen blisters results from hydrogen atoms , formed during sulfide corrosion process on steel. Hydrogen atom combines to form hydrogen molecule. Hydrogen blisters on steel surface

24. Mechanism of hydrogen blistering 3 Fe  4 H 2 O  Fe 3 O 4  4 H 2

25. 5. Inter Granular Corrosion Form of corrosion where boundaries of grain of metal are more susceptible to corrosion than inside grain. Difference in Corrosion potential between grain and grain boundary. e.g. CuAl 2 in Al alloys, Cr 23 C 6 in stainless steel. Inter granular corrosion in Austenitic stainless steel

26. sensitization Formation of chromium carbide precipitants (Cr 23 C 6 ) at grain boundary occurs when stainless steel of type 300 are heated to a temperature of 400 to 900 C in rolling mill or fabrication or welding.

27. Prevention of Intergranular Corrosion Knife Line Attack (KLA ) Use of low grade stainless steel (304 L, 316L grades) due to which less carbon will be available for precipitation of chromium carbide. Use of stabilized grade of stainless steel (321, 322 grade) titanium Use of post weld treatment.

28. 6. Crevice Corrosion Localized attack that occurs in restricted areas , gaskets, flanges, fasteners, lap joints. Occurs between two metal plassets.

29. Prevention of crevice corrosion Avoid crevice during fabrication i.e. smooth weld. avoid solution get into crevice i.e. apply grease to nuts and bolts. routine cleaning Apply external coating.

30. 7. Erosion corrosion It results from loss of film under high velocity or high turbulent fluid flow. Corrosion reaction which is accelerated by relative movement between corrosive fluid and metal surface. High Turbulence intensity impact velocity , angle, relative density between particle and fluid, particle size.

31. Impingement corrosion erosion corrosion of condenser tube

32. control of erosion corrosion selection of resistant materials- high wear resistance , high hardness Decreasing flow velocity minimizing the turbulence

33. 8. General or Uniform Corrosion It is attack over entire exposed surface or large area of metal . Metal loss is distributed uniformly across exposed surface. Manhole cover

34. 9. Galvanic Corrosion It occurs when two metals , with different compositions are connected in presence of electrolyte. Current will flow from anode to cathode. Rate of galvanic corrosion depends on difference in electrolytic potentials.

35. control of Galvanic corrosion selection of metals that are close together in galvanic series. Insulate metal from each other Painting of both metals

36. Corrosion Prevention Methods

37. 1. Active corrosion protection To influence reactions which proceed during corrosion. e.g. Addition of inhibitors to aggressive medium. 2. Passive corrosion protection By mechanically isolating package contents from aggressive corrosive agents by using protective layers , films, coatings. This method neither changes ability of package contents to corrode nor aggressiveness of corrosive agent.

38. 3.Permanent corrosion protection To provide protection at the place of use. e.g. stable oxide layer (nonporous film)forming metals 4. Temporary Corrosion protection Stresses occurring during transport, handling, storage are much greater than those occuring at place of use. e.g. extreme variation in temperature which results in condensation during transportation.

39. 5.Corrosion Inhibitors Chemicals that react with metallic surface & gives certain level of protection. inhibitors has chemical bonding with base metal , it is more adherent to base metal , due to which corrosion reaction stopped.

40. Use of coatings 1. Hot dipping-

41. 2.Metal spraying

42. 3.Electroplating

43. Low and High temperature corrosion Creep failure, oxidation, High temperature hydrogen attack

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