Corrosion under paint blisters 1

[apsix]

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

 

[SJones]

The corrosion won't be "accelerated" but it will be occurring as you have noted by the corrosion product. For the paint to work, it needs a high integrity interface between itself and the substrate. Water (and the oxygen in the water) will be continuously permeating the paint film and feeding the process. The additional problem with blistered paint is that the blister will shield the steel from CP; so, the corrosion will continue even when the anodes are replenished. Could be a good idea to remove the blistered areas and either maintenance paint or leave bare and get the operator to keep up to speed with anode replacement.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[apsix]

Thanks Steve.
The issue re lack of CP within the blisters is not something I would have thought of.

John

 

[mshimko]

Paint blisters provide shielding from CP?

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.

 

[SJones]

It's the same concept as disbonded pipeline coatings. What was the cause of blistering as it could be different in the two cases? Apsix's case maybe because of age and water permeation whereas mshimko's blistering may be because of solvent retention and the water hasn't permeated through yet.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[apsix]

It's been suggested that I could be looking at Sulphate Reducing Bacteria (SRB) corrosion causing the black colour.
I would have thought that bacteria wouldn't penetrate the paint film.
Are there any views on this?

John

 

[unclesyd]

I wrote this on the 14 but didn't post it for some reason. Another senior moment.

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.

 

[AndyKayll]

I have come across the same result as mshimco on a steel hulled boat that was using Zinc Anodes in Brackish Water.
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

 

[apsix]

Thanks to all for the responses.
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

 

[ssreese1500]

Andy, in response to your post about the blisters, that could have been where the paint wasn't touched up properly after anode installation. The steel being intact under the blisters just means the anodes were doing their job and preventing corrosion of the hull.

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.

 

[MASE81]

find the original coating inspector and send him a thank you note for a system thats lasted 20 years with minor damage.

 

[apsix]

I agree MASE81, it has performed very well. It is suspected that a recent lack of continuous CP has brought on the the recent corrosion.
If only the reinforced concrete pier beams and slab above the waterline looked as good.