Unit 2- Class 3- CORROSION (Types of Corrosion)

In this post let us discuss about the corrosion types.

Types of Corrosion: Corrosion on the metals taking place depending on the nature of metals and depending on the types of environment by different mechanisms, giving different types of corrosion.


1. Galvanic corrosion or differential metal corrosion:
This occurs when two dissimilar metals are in contact with each other in a corrosive conductive medium; a potential difference is set up resulting in a galvanic current. The two metals differ in their tendencies to undergo oxidation. The metal with lower electrode potential or more active metal acts as anode and the metal with higher electrode potential acts as cathode. The potential difference is main factor for corrosion to take place. The anodic metal undergoes corrosion where as cathodic metal gets un -attacked.

Egs: When iron contact with copper iron has lower electrode potential acts as anode and undergo oxidation as,
Fe -------> Fe2+ + 2e-
   Where as copper which is having higher electrode potential acts as cathode gets unaffected. The rate of galvanic corrosion depends upon potential difference between anodic and cathodic metals, ratio of anodic and cathodic area and environmental factors and tendency of the metal to undergo passivity etc.

Other egs: When Fe contact with Sn then Fe acts as anode and Sn acts as cathode but when Fe contact with Zn, Fe acts as cathode where as Zn acts as anode.


2. Differential aeration corrosion:
 Differential aeration corrosion occur when metal surface is exposed to the differential air or oxygen concentration, that develops galvanic cell and initiate the corrosion. The part of the metal exposed to lower oxygen concentration acts as anode and the part of the metal exposed to higher concentration acts as cathode, so that poorly oxygenated region undergoes corrosion.
When a metal strip of iron, partially immersed in an aerated solution of NaCl the concentration of O2 is higher at the surface than inside the solution. Since cathodicreaction requires oxygen, hence cathodic area tends to concentrate near the water line so that bottom portion of the specimen acts as anode where corrosion starts.
At anode: Fe-------------->Fe2+ + 2e-
At cathode (near water line): O2 + 2H20 + 4e-   ---------->4OH-


3.Water line corrosion:
This is a case of differential aeration corrosion commonly observed in steel water tanks, ocean going ships etc. in which portion of the metal is always under water
The part of the metal below the water line is exposed to only to dissolved oxygen while the part above the water is exposed to higher concentration of oxygen.
Thus the metal part below the water line acts as anode where as above the waterline acts as cathode and process of corrosion starts. The metal just below the water line is more anodic and the creep (meniscus) is the one which is more oxygenated acts as cathode and unaffected. Amount of creep of the water determines the rate of corrosion, but mass intense corrosion occurring at the water line; there a brown line is also formed due to the deposition of corrosion products. This type of corrosion is commonly observed in ships floating in seawater for a long period of time.




4.PITTING CORROISON:
Pitting corrosion is a localized and accelerated corrosion. When a small particles of dust or water etc are get deposited on a metal (like steel). The portion covered by the dust will not be well-aerated area compared to the exposed surface hence the covered surface becomes anodic with respect to the surface exposed. In presence of an conducting medium (moisture) corrosion starts below the dust part and forming a pit. Once pit is formed the ratio of corrosion increases, because of the formation of smaller anodic and larger cathodic area intense corrosion takes place.
Pitting corrosion is one of the most destructive forms of corrosion. It causes equipment to fail because of perforation with only a small percent weight loss of the entire structure. Because of the small sizes of the pits it is highly difficult to identify the pitting corrosion. Pitting corrosion is an autocatalytic process, and once the corrosion products arte formed, it further provides the condition for differential aeration below the corrosion product and surrounding metal parts. The pit grows and ultimately may cause failure of the metal.


5. STRESS CORROISON:
Stress corrosion of the metal formed by the combined effect of a tensile stress and a specific corrosive environment on the metal., during stress corrosion, the metal or alloy is virtually unaffected over most of its surface, while fine cracks progress through it normal to the direction of tensile stress.
The stress on the metal may be internal or external and these stress is due to some mechanical or service conditions. The metal atoms under stress are always at higher energy level so acts as anode and stress free parts of metal acts as cathode under specific corrosive environmental conditions corrosion process starts.
Egs: Brass undergoes corrosion in the presence of ammonia.
        Stainless steel in the presence of Cl- and caustics

But best example for stress corrosion is caustic embrittlement:

Caustic embrittlement: It is a form of stress corrosion takes place in boilers operating at high temperature and pressure. Caustic embrittlement focus at stressed part of boilers such as cracks, rivets, bents, joints etc.
 The boiler fed water usually contains some residual sodium carbonate (used for softening process). At high temperature and pressure it undergoes hydrolysis to form sodium hydroxide.

                Na2CO3 + H2O---------------->2NaOH + CO2                  

The alkali water sweeps through the minute cracks, crevices between the rivets and joints by capillary action. Inside the cracks water gets evaporated leaving behind NaOH. The concentrations of the NaOH gradually increase on these sites due to poor circulation of water. When concentrations of the NaOH reaches a value of 10% it attacks the metal at the stressed region dissolving it in the form of sodium ferroate ( Na2FeO2). Sodium ferroate undergoes hydrolysis-depositing magnetite as follows

     3Na2FeO2 + 4H2O---------------->6NaOH + Fe3O4 + H2

       6Na2FeO2 + 6H2O + O2--------------->12NaOH + 2Fe3O4

So NaOH is regenerated in the process and its concentration is keep on increasing maintaining a required environment. Thus corrosion process develops cracks and making the metal brittle by the deposition of the product.
The corrosion cell can be represented as

 Fe (under stress) / conc. NaOH / dil. NaOH/ Fe (stress free)
      Anode                                                           Cathode

Caustic embrittlement can be prevented by the addition of compounds like sodium sulphite, tannin, lignin, phosphates etc. which blocks the cracks thereby preventing the infiltration of alkali.


This is enough for today's class, we will discuss Factors affecting rate of corrosion in our next class untill then, just go through everything and revise it twice or thrice