Corrosion under paint blisters
Corrosion under paint blisters
(OP)
We are investigating corrosion of permanently submerged steelwork. It usually has zinc anode protection, but the replacement interval has sometimes been too long in the past, with the anodes being totally consumed. The steelwork is located in the concrete intake chamber of Cooling Water pumps for a thermal power station, therefore it is subject to constant flow.
The paint system is of unknown type and about 20 yrs old. It has isolated blistering of 20 to 40 mm dia. These blisters contain water and the steel within is corroding into a black coloured byproduct. There is no sign of the usual (above water) red rust colour.
Is this unusual or cause for alarm?
Could there be accelerated corrosion within these bubbles where it may have low oxygen levels?
Thanks
The paint system is of unknown type and about 20 yrs old. It has isolated blistering of 20 to 40 mm dia. These blisters contain water and the steel within is corroding into a black coloured byproduct. There is no sign of the usual (above water) red rust colour.
Is this unusual or cause for alarm?
Could there be accelerated corrosion within these bubbles where it may have low oxygen levels?
Thanks





RE: Corrosion under paint blisters
Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdo/
RE: Corrosion under paint blisters
The issue re lack of CP within the blisters is not something I would have thought of.
John
RE: Corrosion under paint blisters
This is something I hadn't heard, and actually have the opposite in one or two cases where we inspected a poor paint system with blisters, and found only flash rusting after the blisters were popped during inspection.
Maybe we were just "lucky", but I'm curious what experience/examples others may have on this example.
RE: Corrosion under paint blisters
Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdo/
RE: Corrosion under paint blisters
I would have thought that bacteria wouldn't penetrate the paint film.
Are there any views on this?
John
RE: Corrosion under paint blisters
You are suffering from “We don’t need those silly things in the water because we don’t have any corrosion on the steel”.
There is a very high probably you have MIC or as it's better known "bugs". This type corrosion is quite common under the conditions you describe. The affected areas will only get bigger and start to join.
If you can get to an intact blister break it open and at the same time use your hand to waft some of the trapped gasses across your nose. There is a good possibility that you will smell rotten eggs, if so you SRB (sulfate reducing bacteria) and if not you probably have iron bacteria. Both will cause accelerated corrosion especially under anaerobic conditions. There are a couple of other scenarios like a differential cell or a differential aeration cell, but the results are essentially the same.
Could you state the type water that the steel was exposed to?
How is the paint at the splash line?
How much steel do you have underwater at this location?.
Check the prints and see if the steel is removable.
The above isn’t required but might help someone else analyze you problem.
My recommendation would be to get someone well versed in the art and science of cathodic protection in to evaluate you problem. I would get the cathodic protection back on as soon as possible. There are firms that specialize in remediation of power plant intakes, hopefully some on will come forward with a name.
RE: Corrosion under paint blisters
The Anodes were white and crusted (should be metalic looking due to erosion) and the paint system had blisters around the location of the anodes but the steel under the blisters was clean!!! Can't be certain but the fluid that was in the blisters seemed to be water.
I am at a loss what caused the blisters but am sure someone on this site can throw light on this
Never Assume, you make an Ass out of U and Me
RE: Corrosion under paint blisters
We have recommended further investigation by an expert in the field, or alternatively a complete paint system renewal.
The importance of continuous cathodic protection was also stressed.
And I should have mentioned; the steel is submerged in sea water.
John
RE: Corrosion under paint blisters
As zinc anodes wear away, they always look white, chalky, and usually pitted on the surface. Again, this means they are doing what they are supposed to do.
FYI, if you are involved in this type of application again, I would advise using Aluminum Anodes instead of zinc. They produce approx 3 times the voltage and last longer than zinc anodes. They also "turn on" faster.
RE: Corrosion under paint blisters
RE: Corrosion under paint blisters
If only the reinforced concrete pier beams and slab above the waterline looked as good.