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Pitting corrosion is one of the most common types of corrosion we find in storage tanks, especially underground tanks, and can be one of the most serious.

What is Pitting Corrosion

Pitting corrosion is a localised form of metal deterioration which displays as small pits or holes on the surface of the steel causing a loss of thickness to the metal.

If nothing is done, eventually this will cause a breach in the tank and lead 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, shallow and wide, they can undercut whereby they go into the metal and then develop sideways underneath the surface.

Pitting corrosion can be uncovered with the mouth of the pit open, but can also be covered with a semi-permeable membrane. This makes pitting corrosion very difficult to properly detect prior to blasting.

Pitting corrosion found in above-ground storage tank
Pitting corrosion found on internal steel of underground storage tank

How Does Pitting Corrosion Occur?

Pitting corrosion can be caused by several factors including localised mechanical or chemical damage to a metals oxide film and poor application of a tanks protective coating.

Poor coating application causes discontinuities, these are essentially breaks in the coating where it hasn’t been put onto the steel in the correct way or surface preparation has been bad.

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 to the steel which corrosion can take advantage of.

The presence of MIC (Microbially Influenced Corrosion) can also have an impact on the rate of pitting corrosion within the storage tank as Sulphate Reducing Bacteria accelerate corrosion within the tank.

What are the Prevention Methods?

Preventing pitting corrosion in storage tanks boils down to one main factor, applying the correct industrial coating to the storage tank and undertaking the correct inspection program recommended by the manufacturers to ensure the integrity of the coating over the tanks lifespan.

Using an experienced and reliable coating applicator or lining company is also highly recommended. This will ensure that steel preparation is done to standard and any coating applied is without discontinuities. Any good lining contractor will carry out coating checks after application to make sure of this.

Pitting corrosion is a serious problem for unprotected storage tanks and 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.

Protective solvent free coating applied to above ground storage tank
Protective solvent free coating to underground storage tank