Pitting Corrosion

Introduction

We know that corrosions simply mean rusting. However, the question is what is pitting corrosion. Pitting Corrosion is the form of corrosion that particularly occurs at one spot. We can call it localized corrosion also. It only affects a particular part of the metal surface. Initially, it forms a layer of rust which later forms the holes in the material. It is proven to be more harmful than corrosion because it is hard to detect and work against. The depth of the corrosion is measured using a calibrated microscope. In this article, we will discuss what is pitting corrosion, its mechanism, test, damages caused by pitting corrosion, and how to prevent it.

Causes of Pitting Corrosion

Now, We know that Pitting corrosion is a kind of corrosion that is confined to a small area. However, the question arises what causes this pitting corrosion? The answer is the Environment. Gases present in the atmosphere, reactive chemical species in free form like chlorides. Chloride is the main source of this pitting corrosion. It attacks the passive layer of metal and breaks the bonds of oxides. 

If the metal surface is exposed to the water. Water droplets can form a layer over the metal surface. Ions present in the water droplets react and initiate the process of corrosion.

So, there are two factors - Environmental and metallurgy. These two factors determine if this can be stopped or not. If we restrict the aeration (oxygen supply to the surface) and prevent the surface from getting wet, Pitting corrosion can be prevented.

Pitting Corrosion Mechanism

Pitting corrosion is initiated by the oxidation process. Exposure of the passive layer of metal with air and water initiates the oxidation process at the localized part. This leads to the acidification of ions formed by oxidation. These two processes are part of an electrochemical reaction.

Oxidation occurs at the anodic part and reduction occurs at the cathodic part. These half cells constitute the electrochemical cell, which forms at a small site.

The reaction occurring at the anodic site is shown below:

Fe → Fe2+ + 2e- 

The reaction occurring at the cathodic site is shown below:

½ O2 +H2O + 2e- → 2(OH-)

Overall, the reaction is shown below:

FeCl2 + 2H2O →Fe(OH)2 + 2HCl

The HCl formed in the pit increases the acidity. A potential gradient is also set in this localized pit region. It attracts the ions from the other nearby sites. The holes which are formed at the surface of the metal, get filled with the side product of the corrosion process.

In the presence of chlorine ions, holes keep on growing via an autocatalytic mechanism. 

Pitting Corrosion Test

A number of tests are available to detect pitting corrosion. A few of them are listed below:

• ASMT-G48-Practice A

• ASMT-G48-Practice E

• CPT(Critical Pitting Temperature)

ASMT-G48 Practice A and E are the toughest tests that are being conducted on stainless steel, while CPT is the most commonly used method.

Damages Caused by Pitting Corrosion

As we already know, it affects a localized part of the metal. It creates a rusty layer over the metal surface, eventually forming holes in it. It reduces the thickness of metal. This leads to structural defects and metal cannot handle stress. Finally, cracking starts and metal becomes totally useless. Pitting corrosion causes pipe leakages, electric short circuits and major machinery faults if not prevented.

Corrosion damage causes an economical loss of about ＄300 billion annually in U.S. Industries. 

Pitting Corrosion Prevention

Till now, we understood what is pitting corrosion, what are the causes and damages. Now, we will discuss the ways to prevent it.

• We should use such materials that are resistant to environmental factors such as aeration and moisture exposure.

• We can apply anodic or cathodic protection layers over the metal surfaces.

• We can paint the metal surface or apply industrial coatings.

• The zinc spray metalizing process is extensively used to cover the metal layer with the layer of zinc because it is prone to environmental factors. 

• We should keep the metal materials in an environment with less moisture exposure, optimum temperature and aeration should be controlled.

Conclusion

In this article, we discussed the pitting corrosion definition, pitting corrosion examples, causes, mechanism, damages, and prevention methods. It is a localized kind of corrosion that is caused by environmental factors. It forms holes and causes thickness loss. There are a number of tests available to detect it. It could be prevented by using alloys rather than pure metals, painting the metal surface, using rust-prone materials, and maintaining the protective film over the metal surface.