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June 22 - July 6, 2010
I have several water tanks constructed with carbon steel for demineralized water storage. The tanks are about 20 years old, and the original coating has started to blister. For refurbishment of the coating, what material should be used and at what dry film thickness? The original type of coating is unknown.
Selected Answers
From
Ted MacMillan of Brant Corrosion Control,Inc. on
June 25, 2010:
There are several questions that need to be answered prior to making a proper recommendation for this service, or which path to take with repair of the existing system. If the blistering is extensive, the lining needs to be replaced, if it's minor blistering, chances are the system can be repaired; however, you will have to constantly monitor it during your maintenance program (yearly). Depending on the type of coating system that was applied and the original DFT of the system, you should be able to determine if that particular coating performed the way it was intended to for the service.
If the material was a thin film coating, say 10-12 mils, then getting 20 years of life in demineralised water service was a success. If the coating was not maintained regularly, you did exceptionally well. As previously stated by others, demineralised water is very aggressive, and selecting a replacement system should be based on the full operating parameters of the actual service.
Our company has installed several systems for these services ranging from thin films to rubber lining. The success of any lining will be dependent on the condition of the existing structure, cleaning to remove contaminants, blast cleaning, application, curing, and final inspection prior to placing the equipment back into service.
From
Pieter van der Poel of Danieli Far East on
June 23, 2010:
I think you are having a problem with osmotic blistering caused by the demineralized water. Demineralized water tries to absorb minerals, salts, etc., to achieve an equilibrium. This could come from a contaminated substrate sealed by the membrane (this being the existing coating).
If the coating has been in service for the last 20 years, it has performed well, especially if localized repairs had been carried out during regular inspection intervals.
A coating manufacturer could recommend a lining suitable for the type of service, but wet service temperatures also play an important factor in its selection. Whatever the lining used, it should be impervious to the maximum extent for this type of service. Surface preparation to SSPC-SP 10 and, most important, salt testing should be carried out to avoid future problems with osmotic blistering. The coatings manufacturer will most likely recommend a solvent-free epoxy lining, perhaps even with glass flake.
Depending on the extent of blistering, it should be decided whether to go for complete refurbishment as you already stated in your question or to go for spot repairs. Take into consideration any factors related to CP.
From
Chuck Pease of PCG on
June 23, 2010:
At this time there isn't enough information to answer your question.
A couple of things need to occur before a solution can be offered.
First question in determining the answer: is the coating you speak of interior or exterior or both?
The next thing would be to determine if the existing blistered coating will need to be abated, and more importantly, is it a lead-bearing coating.
If the coating is only blistered in various areas but the remainder of the coating is tightly adhered, then one can determine if a surface prep and an overcoat would be appropriate.
If the coating is lead-bearing and lossely adhered all around, these factors would suggest complete abatement and prep of steel to SSPC- SP 10 prior to choosing a coating system.
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