pinholes in epoxy coating

[yahoo123]

I remember reading that pinholes in the epoxy coating is all it takes to seriously corrode the underlying material and that the epoxy coating is basically useless when the coating is scratched up.

I'm thinking this might be true for galvanic corrosion between two dissimilar metals in contact. But what about in submerged applications? Will a small pinhole in the coating of a submerged steel plate be the cause of a serious corrosion failure? I thought it was the wetted area that matters.

 

[bimr]

If most of the surface area is protected with a coating, the corrosion will be intensified in an area with pinholes. The pinhole area is the anode in a galvanic reaction.

 

[MTPipeliner]

Do you have a cathodic protection system protecting the plate? It's important to fix as many pinholes as you can, but you should have a cathodic protection system and coating working together for the best protection.

The presence of a pinhole in an FBE coating will not necessarily cause a corrosion failure. There are thousands of miles of pipeline with FBE coating, and even though pinholes are fixed prior to backfilling not every single one gets fixed.

 

[yahoo123]

How well will an epoxy coated steel pipe work in a tank containing potable water?

 

[JedClampett]

For potable water tanks, they put two or three coats of epoxy on, which will really reduce the likelihood of pinholes. After that, they holiday test the coating and repair any flaws. After the tank is put into service for 11 months, the tank is drained, inspected and any rust spots are repaired.
A pipe in a potable water tank should have all the same requirements. Pinholes are not acceptable.