Pitting corrosion is one of the most common types of corrosion found in storage tanks, especially underground tanks. It can also be one of the most serious.
What is Pitting Corrosion
Pitting corrosion is a localised form of metal deterioration. It displays as small pits or holes on the surface of the steel causing a loss of thickness to the metal.
Eventually, pitting corrosion will cause a breach in the tank. Leading to leaking product into the surrounding environment.
The pits produced by this type of corrosion can be a variety of shapes and sizes. They can be narrow and deep or shallow and wide. They can also undercut. This is where the pits go into the metal and then develop sideways underneath the surface.
Pitting corrosion can be either, uncovered with the mouth of the pit open, or covered with a semi-permeable membrane. This makes pitting corrosion very difficult to properly detect before blasting.
How Does Pitting Corrosion Occur?
There are several factors that can cause pitting corrosion. Localised mechanical damage, chemical damage to a metals oxide film, or poor application of a tanks protective coating.
Poor coating application can cause many discontinuities. Often referred to in the industry as ‘holidays’. These are breaks in the coating, such as pinholes or voids.
The term ‘holiday’ comes from a time when sailors coated the wooden ships they used with tar. If the wooden mast rotted due to a lack of coating, then the sailor was said to have taken a holiday at that point.
There are many reasons why these ‘holidays’ can happen. Environmental factors, incorrect application, air bubbles, or if surface preparation has been bad.
Usually, after application, a visual check of the coating will be made. To determine any breaks in the coating not immediately visible, holiday testing takes place. This is a non-destructive test carried out to the coating. The test involves an electric current passing along the coating surface. If the instrument detects an electrical flow this indicates the location of a gap or void in the coating. Fixing these with a patch repair or re-coating in that area then takes place.
If left, these coating gaps leave a path back to the steel for any corrosion.
Damage to a storage tanks internal coating can also have the same effect. Causing exposed paths back to the steel which corrosion can take advantage of.
The presence of MIC (Microbially Influenced Corrosion) also has an impact on the rate of pitting corrosion. Particularly, Sulphate Reducing Bacteria, as they speed up corrosion within the tank.
What are the Prevention Methods?
Preventing pitting corrosion in storage tanks is simple. Apply the correct industrial coating to the tank. Running the correct inspection program recommended, by the manufacturers, is also good management. This will help ensure the integrity of the coating over the tanks lifespan.
Using an experienced and reliable coating applicator or lining company is also recommended. This will ensure that steel preparation is to standard and any coating applied is without discontinuities. Any good lining contractor will carry out coating checks after application.
Pitting corrosion is a serious problem for unprotected storage tanks. Operators should consider it a priority to protect against. The alternative is to risk a major loss of assets and product, environmental damage, extensive repair bills, and large fines.