CORROSION

1. Presentation on CORROSION Submitted by Abdul Bari 213118001 Aditya Kumar 213118002 Akshat Shrivastava 213118003

2. Coverage • What is Corrosion??? • Principle of Corrosion • Different forms of Corrosion • Stress Corrosion Cracking • Pitting • Biological Corrosion • Hydrogen Embrittlement • Corrosion Prevention

3. What is Corrosion? • Corrosion is defined as deterioration or destruction of material due to its interaction with environment . • It’s a reverse of metallurgy . • Any material when It is kept in a certain atmosphere their will be potential difference between the material and the surroundings. To complete the circuit ions of material comes to the solution . • Rate at which ions are coming out is called CORROSION RATE .

4. Principle of Corrosion • It is based on Electrochemical Reaction . • Reaction is as follows OXIDATION (Anodic Reaction) Zn Zn2+ +2e- REDUCTION (Cathodic Reaction) Hydrogen Evolution 2H+ + 2e- H2 Oxygen Reduction O2 + 4H+ + 4e- 2H20

5. Principle of Corrosion

6. Different forms of Corrosion Corrosion

7. Galvanic or Two metal Corrosion • Galvanic Effect – If two dissimilar materials are in a contact (electrically connected or physical contact), one will be undergo corrosion (act as a anode) in the presence of corrosive medium (electrolyte). • The metal with high standard oxidation potential act as a anode and other with low reduction potential act as a cathode. • Galvanic effect can be estimated from EMF series and galvanic series of materials .

8. Galvanic Corrosion Mechanism • Reaction is as follows • At anode • Fe Fe2+ + 2e- • At Cathode acidic medium (Hydrogen Evolution type) • 2H+ + 2e- H2 • Oxygen Reduction • O2 + 4H+ + 4e- 2H20 • Alkaline or Neutral medium(oxygen absorption type) • ½ O2 + H2O +2e- 2OH- • Fe2+ + 2OH- Fe(OH)2 • 4Fe(OH)2 + O2 Fe(OH)3

9. Examples of Galvanic Corrosion Anodic- cathodic behavior of steel with zinc and tin outside layers exposed to the Atmosphere.

10. Prevention from Galvanic Corrosion • Avoid contact between dissimilar metals. • For unavoidable situations • anodic area should be larger than the cathodic area. • the two metals should be as close as possible in the galvanic series. • an insulator may be fitted between the two metals. • avoid threaded joints between the metals.

11. Localized Corrosion • Localised Corrosion is the selective removal of metal by corrosion at a small area or zones on a metal surface in contact with corrosive environment • It takes place at a higher rate than the rest of original surface . • Occurs when corrosion works with other destructive process such as stress , fatigue , erosion and other forms of chemical attack .

12. Localized Corrosion • Chemical factor • Initiated in the metal- intergranular, pitting • Initiated in the environment- crevice • Chemical and mechanical factors • Mechanical process in metal-stress corrosion cracking, hydrogen embrittlement • Mechanical action of metal-erosion corrosion • Mechanical action of solid body of metal-fretting corrosion

13. Localized Corrosion • Crevice corrosion- • Corrosion that occurs in stagnant locations such as those found under gaskets. • Filiform corrosion- • Corrosion that occurs when water gets under a coating such as paint. • Pitting corrosion- • The creation of small holes in the surface of a metal. • Intergranular corrosion- • Where the boundaries of crystallites of the material are more susceptible to corrosion than their insides.

14. Intergranular Corrosion Intergranular corrosion (IGC) is a selective attack in the vicinity of the grain boundaries of a stainless steel. It is as a result of chromium depletion, mainly due to the precipitation of chromium carbides in the grain boundaries. • Intergranular corrosion (IGC) is a selective attack in the vicinity of the grain boundaries of a stainless steel. It is as a result of chromium depletion, mainly due to the precipitation of chromium carbides in the grain boundaries.

15. Crevice Corrosion • Crevice corrosion is an electrochemical oxidation-reduction (redox) process. • It occurs within localized volumes of stagnant solution(like dust) trapped in pockets, corners or rivet heads. • Crevice corrosion is much more dangerous than uniform corrosion. • Crevice corrosion is highly accelerated if chloride, sulphate or bromide ions are present in the electrolyte solution. • Mechanism of crevice corrosion is similar to that of Pitting corrosion: dissolution of the metal and gradual acidification of the electrolyte caused by its insufficient aeration (Oxygen penetration). 

16. Mechanism • Anodic reaction :- • Fe = Fe2+ + 2e- (dissolution of iron) • Cathodic reaction :- • 1/2O2 + H2O + 2e- = 2(OH-) • Hydrolysis :- • FeCl2 + 2H2O = Fe(OH)2 + 2HCl

17. Prevention from Crevice Corrosion • Use butt joint instead of lap-joints. • Clean thoroughly to remove stagnant. • Avoid sharp corner while designing • Use non-absorbable solid gaskets such as Teflon.

18. Filiform Corrosion • It is a special type of crevice corrosion , where the corrosion takes place under a thin film . • It appears on the steel , Magnesium and Aluminium surface covered by tin , silver , gold phosphates . • The corrosion appears as thread-like filaments under the coating.

19. Filiform Corrosion mechanism The mechanism has a number of characteristics that are similar to Crevice corrosion, e.g. differential aeration and hydrolysis of metal ions resulting in increasing acidity in the region of dissolution.

20. Atmospheric Corrosion • According to this theory, corrosion on the surface of a metal is due to direct reaction of atmospheric gases like oxygen, halogens, oxides of Sulphur, oxides of nitrogen, hydrogen sulfide and fumes of chemicals with metal. • The extent of corrosion of a particular metal depends on the chemical affinity of the metal towards reactive gas. • Oxygen is mainly responsible for the corrosion of most metallic substances when compared to other gases and chemicals. • There are main three types of atmospheric corrosion. • (1) Oxidation corrosion (Reaction with oxygen) • (2) Corrosion by other gases. • (3) electrochemical

21. Oxidation Corrosion Some of the metals directly react with oxygen in the absence of moisture. Alkali and alkaline earth metals react with oxygen at room temperature and form corresponding oxides, while some metals react with oxygen at higher temperature. Metals like Ag, Au and Pt are not oxidized as they are noble metals.

22. During oxidation of a metal, metal oxide is formed as a thin film on the metallic surface which protects the metal from further corrosion. • If diffusion of either oxygen or metal is across this layer, further corrosion is possible. Thus, the layer of metal oxide plays an important role in the process of corrosion. • Oxides of Pb, Al and Sn are stable and hence inhibit further corrosion. They form a stable, tightly adhering oxide film. • In case of porous oxide film, atmospheric gases pass through the pores and react with the metal and the process of corrosion continues to occur till the entire metal is converted into oxide. • Porous oxide layer is formed by alkali and alkaline earth metals. Molybdenum forms a volatile oxide film of MoO3 which accelerates corrosion. • Au, Ag, Pt form unstable oxide layer which decomposes soon after the formation, thereby preventing further corrosion.

23. CORROSION BY OTHER GASES • In dry atmosphere, these gases react with metal and form corrosion products which may be protective or non-protective. • Dry Cl2 reacts with Ag and forms AgCl which is a protective layer, while SnCl4 is volatile. • In petroleum industries at high temperatures, H2S attacks steel forming FeS scale which is porous and interferes with normal operations.

24. Stress Corrosion Cracking • Stress corrosion occurs due to tensile stresses (applied and residual stresses) on the susceptible material in the presence of a specific corrosive environment , resulting In the formation of crack which propagates .

26. Causes of SCC • Alkalis • Nitrates • Basic Chlorides • Acid Chlorides

27. Causes of SCC Due to certain metal forming processes probability of SCC will increase. • Rolling • Drawing • Annealing • Bending • Welding

28. Mechanism of SCC

29. Active Path Dissolution • Active path dissolution is a material failure due to stress corrosion cracking (SCC) that is characterized by the progression of corrosion along an intermolecular path of least resistance with high concentrations of tensile strength. • Active path dissolution is characterized by the significantly increased rate of corrosion along an intergranular sub-section or boundary that presents above average susceptibility to corrosion. Such granular boundaries on the molecular level within metals tend to display segregation of impure elements, while the bulk layer of the material contains a passive tendency. This makes it more difficult for passivation and protection of the bulk material. occur.

30. Film Induced Cleavage

31. Prevention • Control over operating temperature . • Control stresses while machining . • Avoid specific chemical environment . • Choose an inert material , that does not reacts with the specific environment .

32. Pitting • Pitting corrosion is a localized attack in the metal resulting in the formation of pit , hole , cavity (confined to a point or small area ) • Reason – When the anodic area is small and cathodic area is large , it leads to drastic corrosion at the anode to form a pit or a hole .

33. Pitting • Pitting is intermediate stage between general overall corrosion and complete corrosion resistance .

34. Causes of Pitting • Damage/cracking of protective film over the metal . • Scratches / cut edges . • Sliding under loads . • Chemical Attack (Chloride damages the protective oxide layer) • Surface Roughness . • Turbulent flow of fluid over metal .

35. Pit Shape and Growth • Pits usually grow in direction in gravity . • Lesser number start on vertical surface and rarely do they grow upward . • Pits tend to undermine or undercut the surface as they grow . • Subsurface damage is much more severe than is indicated by surface appearances • Pitting requires an extended initiation period , once started pit penetrates at an ever increasing rate .

36. Mechanism of Pit Formation • At the interface between the pit and the adjacent surface , iron hydroxide forms due to interaction between the OH- produced by cathodic reaction and pit corrosion product . • This is further oxidised by the dissolved oxygen in the solution to Fe(OH)3 , Fe304 and other oxides . • This rust rim grows in the form of a tube .

37. Environmental Factors • Most pitting corrosions are caused by chloride and chloride containing ions . • Most pitting is associated with halide ions , with chlorides , bromides and hypo chlorites being the most prevalent . • Fluorides and Iodides have comparatively little pitting tendencies . • Even most corrosion resistant alloys can be pitted by Copper Chlorides and Iron Chlorides .

38. Biological Corrosion • The interaction of organisms with corrosion processes, is a complex subject encompassing contributions from several disciplines ranging from chemistry and surface science to microbiology and bacteriology. • They are caused by microbes and fungi . • Aerobic bacteria decreases the concentration of oxygen in the medium in contact with the metal surface . • The main product of corrosion is Iron Sulphide .

39. HYDROGEN EMBRITTLEMENT

40. Introduction: • Hydrogen Embrittlement is the degradation of structural properties of solid due to hydrogen. • Primary impact on the metals takes the form of loss of ductility and reduced load carrying capacity. • This is often a result of accidental introduction of hydrogen during forming and finishing operations. • Occurs in most metal, but not usually in copper, gold, silver and tungsten.

41. During hydrogen embrittlement, hydrogen is introduced to the surface of a metal and individual hydrogen atoms diffuse through the metal. Solubility of hydrogen increases at higher temperatures, raising the temperature can increase the diffusion of hydrogen. If there is significantly more hydrogen outside the metal than inside, hydrogen diffusion can occur even at lower temperatures. Individual hydrogen atoms within the metal gradually recombine to form hydrogen molecules, creating pressure from within the metal. This pressure can increase to levels where the metal has reduced ductility, toughness, and tensile strength, up to the point where it cracks open(Hydrogen induced cracking –HIC).

43. THEORIES • Decohesion Theory • Reduced Surface Theory • Planar Pressure Theory

44. Decohesion Theory: • The absorption of hydrogen decreases the atomic binding forces of the metal lattice. • This results in the premature brittle-material fracture along the grain boundaries or network levels.

45. Reduced Surface Theory: • The absorption of hydrogen decreases the surface free energy of the metal. • Propagation of the crack tip is enhanced. • Explains crack propagation of high-strength steels in low-pressure hydrogen environments.

46. Planar Pressure Theory: • Occurs when metals are charged with hydrogen during solidification. • High-pressure hydrogen can form in micro voids. • Same mechanism as hydrogen blistering.

47. Occurrences • Hydrogen embrittlement can occur during various manufacturing operations or operational use anywhere that the metal comes into contact with atomic or molecular hydrogen. • Processes that can lead to this include cathodic protection, pickling, phosphating and electroplating.

48. Preventive Measures • Prevention in Design: • High-strength metals and alloys are more susceptible to embrittlement. • Common mistake is to overcompensate on strength requirements for service. • At ambient conditions : Minimize the stress acting on the material!

49. Prevention in processing: • Embrittlement can originate from poor production techniques. • Problems arise when hydrogen is allowed into production environment. • Remedies: • Maintain low hydrogen atmosphere • Heat treatment

50. Prevention in Welding: • Embrittlement can be localized around a weld. • Welding rods containing hydrogen are the source. • Remedies: • Low hydrogen welding rods stored in a dry place • Local heat treatment before & after welding