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

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 H20

5. Principle of Corrosion

6. Different forms of Corrosion
Galvanic Corrosion
Localised Corrosion
Atmospheric Corrosion
Stress Corrosion Cracking
Pitting Corrosion
Biological Corrosion
Hydrogen Embrittlement
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 H20
Alkaline or Neutral medium(oxygen absorption type)
½ O2 + H2O +2e OH-
Fe2+ + 2OH Fe(OH)2
4Fe(OH)2 + O 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 .

25. SCC
Susceptible Material
Corrosive Environ.
Tensile Stress

26. Causes of SCC Copper Mild Steel Stainless Steel Alkalis Nitrates
NH3
Ammonia Compounds
Copper
Alkalis
Nitrates
Mild Steel
Stainless Steel
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. Active Path Dissolution .
Mechanism of SCC
Active Path Dissolution .
Film Induced Cleavage
Embrittlement
Caustic Embrittlement
Hydrogen Embrittlement

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 Brittle film over ductile metal
Crack induced in the film and propagates into ductile metal
If brittle film is formed due to corrosion process , it repeats till trans granular fracture process .

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

51. Using Clean Steal: Rimmed steel tend to have numerous voids.
Substitution of killed steel greatly increases the resistance to hydrogen interstitials for embrittlement.
Because of less number of voids in this material.

52. Reducing Corrosion Rate:
Hydrogen embrittlement occurs frequently during pickling operations.
Where corrosion of the base metal produces vigorous hydrogen evolution.
By adding inhibitor, base metal corrosion can be eliminated during pickling by subsequent decrease in hydrogen pickup.

53. Baking:
Hydrogen embrittlement is an almost reversible process, especially in steels.
If hydrogen is removed, the mechanical properties of treated material are almost equal to Hydrogen free steel.
Common way of removing hydrogen in steel is by baking at low temperatures at F.

54. Substituting Alloys:
High strength steels are more susceptible to hydrogen embrittlement.
Alloying with Ni or Mo reduces susceptibility.
Nickel containing steel and Nickel base alloys have very low hydrogen diffusion rates.
Best way to prevent from hydrogen embrittlement.

55. Testing
There are two ASTM standards for testing embrittlement due to hydrogen gas.
Standard Test Method for Determination of the Susceptibility of Metallic Materials to Hydrogen Gas Embrittlement (HGE) test uses a diaphragm loaded with differential pressure.
If hydrogen containing environment at high pressure , high temperature, or both Uses a cylindrical tensile specimen tested into an enclosure pressurized with hydrogen or helium.

56. Example
In 2013, six months prior to opening, the East Span of the Oakland Bay Bridge failed during testing. Catastrophic failures occurred in shear bolts in the span, after only two weeks of service, with the failure attributed to embrittlement, possibly from the environment.

57. Corrosion Prevention Following Methods for Corrosion Prevention
Material Selection
Alteration of Environment
Design
Coating
Cathodic Protection
Electroplating
Hot Dipping
Galvanising
Tinning

58. Design
As riveted joints are sites for crevice corrosion, welding is preferred more than riveting tanks.
Excessive mechanical stresses or residual stresses are avoided to reduce stress corrosion cracking.
Eliminate electrical contact between dissimilar to prevent galvanic corrosion by using similar material or using insulate materials.
Avoid sharp bends in piping systems when high velocities abrasion materials are involved to prevent erosion corrosion.
Tank bottoms must be designed in some inclination towards holes for easy draining otherwise the left out liquids results in severe corrosion.

59. Altering of Environment
Altering the corrosive environment by reducing the aggressiveness of the medium by adding inhibitors.
Changing the medium :-lowering the temperature, decreasing velocity, removing oxygen and changing concentrations.
Different inhibitors are:-
Adsorption inhibitors:- organic compounds which adsorb metal surface and decrease the metal dissolution and reduction reactions.
Scavengers inhibitors (sodium sulphite and hydrazine):- remove dissolved oxygen from water solutions.
Hydrogen-evolutions poisons (arsenic and antimony ions):- reduce hydrogen evolution reaction and effective in acidic solutions.

60. Cathodic Protection
Metal to be protected is forced to act as cathode so that corrosion does not occur.
The principle is that supplying the electrons to metal through electrolyte.

61. Hot Dipping
It is the process of coating the base metal (metal with higher melting point ) dipping into a bath containing molten coating metal (metal with low melting point).
It is of two types:-
(A) galvanising (anodic coating) :- coating of more active metal ,Zn over Cu
(B) tinning (Cathodic Coating) :- coating of less active material ,Tin over Cu

62. Material Selection Most important method –
Generally people believe that SS is good , but in chlorine containing medium and stressed structures , these are more susceptible than ordinary steels.
Localised such as Pitting and Crevice corrosion attack more on SS than MS .
General combination of alloy and medium for better resistance is given below :-
SS – Nitric Acid
Tin – Distilled Water
Steel – Conc. Sulphuric Acid and many more……

63. Coatings Metallic or inorganic and organic coating can be used.
Metallic coating can be applied easily very easily and they are
quite deformable.
eg:-silverware, galvanized steel, tin cans.
Inorganic coating are to be applied by diffusion or chemical conversion and these are brittle.
eg:- chromized steel ,anodized aluminium
Organic coating protect more than the metallic coating on a tonnage basis.
Eg:-paints, varnishes, lacquers.

64. Electroplating
Electroplating is a process of coating a superior metal over the base metal(inferior metal) by passing direct current through the electrolyte containing the soluble salt of the coating metal.