What is Pitting Corrosion?
Pitting corrosion is a type of corrosion that attacks a local area of the metal and eventually leads to the formation of holes in the metal. Pitting corrosion can lead to stress corrosion cracking, an example for which is the collapse of the Silver Bridge in West Virginia, the USA in 1967.
This type of corrosion generally occurs when a small area is affected by the environment and becomes anodic. Meanwhile, another area in the metal acts as a cathode. This leads to a type of galvanic corrosion which begins at the surface of the metal but can spread downwards and eventually lead to the structural failure of the metal.
Damages Caused by Pitting Corrosion
Pitting corrosion can lead to the formation of pits on the metal. The shape of these pits can vary from shallow and wide to deep and narrow. An illustration depicting the different shapes that a cavity formed on the metal due to pitting corrosion can take.
The ratio between the deepest pitting corrosion-formed cavity and the average penetration of the metal (which can be calculated from the total loss in weight of the metal) is given by the pitting factor.
Pitting Corrosion Mechanism
In the mechanism of pitting corrosion, the oxidation of the metal occurs, resulting in an acidity which is localized. The generous separation of the cathodic half-reaction and the anodic half-reaction helps maintain this localized acidity.
The acidity leads to the electromigration of anions towards the pit which is formed. A potential gradient is also formed by localized acidity. The pits that are formed on the surface of the metal are often filled with the side products formed in the corrosion process.
An example to help in understanding the pitting corrosion mechanism would be the corrosion that occurs when the metal is exposed to an oxygen-rich sodium chloride electrolyte. The surface of the metal now acts as a cathode whereas the pit acts as an anode.
The local production of metal cations in the pit creates an excessive positive charge that in turn attracts the chlorine anions of the electrolyte. The resulting metal chloride molecules now react with the water in the environment to give the metal hydroxide and hydrochloric acid which further speeds up the rate of corrosion.
Pitting Corrosion Test
There are various distinct methods to test a steel grade’s pitting resistance. They are:
- CPT (Critical pitting temperature)- This method of determination is frequently used.
- ASTM G48, practice A and E- Since the acidic solution combines with oxidizing ability and chlorides, it is one of the toughest pitting corrosion test conducted for stainless steel.
- ASMT G48 practice A- It is a performance test at a single temperature.
- ASMT G48 practice E- Determines the CPT.
Pitting Corrosion Prevention
The following countermeasures can be adopted to prevent pitting Corrosion.
- Selection of materials which are known to possess a level of resistance towards the given environment.
- Exhibiting a level of control over the environment, including its pH, temperature, and the concentration of chloride ions in it.
- Employing cathodic protection or anodic protection techniques.
- Application of industrial coatings to the metals and other assets.
Frequently asked questions
1. Name the chemical species which causes pitting corrosion.
Ans: The chemical species which causes pitting corrosion is chloride.
2. Where does pitting corrosion happen?
Ans: It takes place in stainless steels either in neutral solutions or acid solutions which contain halides, etc.
3. What is the difference between the crevice and pitting corrosion?
Ans: In crevice corrosion, the attack starts in a narrow crevice when compared to an unshielded surface.
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