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September 1 - September 5, 2014

Does the presence of non-burst blisters in a coating system always mean that the coating requires replacement?

Selected Answers

Coatings can be repaired in blisters areas, but full replacement is not a proper response because of the high cost and the likelihood that the existing film can still perform as specified in other areas.

From Warren Brand of Chicago Corrosion Group on September 8, 2014:
My understanding of blisters is that they are osmotic in nature and that once that osmotic equilibrium has been met, the blister will not change in size, as it has found stasis. That is, unless there is something drawing moisture through the coating system, the blister will not continue to grow. Does that make sense?

From robert turner of Newport News Shipbuilding on September 5, 2014:
I would like to add that immediate replacement of the coating system (even with corrosion occurring under the blisters) may not be the most appropriate or cost-effective decision. One tends to forget that the purpose of a coating is to protect the substrate material (and a lining is to protect the tank contents with a benefit of protecting the substrate). If there is enough design margin in the substrate (for example if Tmin is .5" and it was constructed of 1" material) and the periodicity or frequency allows for another inspection prior to reaching the minimum design basis, then deferring the decision to replace may be the best alternative. Corrosion rates need to be understood and analyzed.

From Tom Schwerdt of Texas Department of Transportation on September 5, 2014:
Certainly not. If an older paint film is intact and still doing its job (typically corrosion and/or product protection), the blistered film can be left in place. Certainly, the blisters are an indication that the film is having issues, and future replacement should be budgeted. If the film is no longer doing its job (corrosion under the blisters, product contamination),  it needs to be replaced ASAP. Blisters are also not acceptable in a new coating film.

From William Slama of International Paint/Ceilcote Products on September 3, 2014:
Blisters in a coating or lining system, of course, indicate that the coating under the blister is detached, either by loss of adhesion to the substrate or by inter- or intra-laminar failure. History has shown that the blisters tend to grow in size, especially if the temperature cycles. If the integrity of the coating or lining material in the blistered area is good, corrosion under the blister may proceed slowly, but ultimately will need repair. Inter-laminar blisters that do not expose the substrate may provide corrosion protection for an extended time. First step should be to investigate some of the blisters to see whether the substrate is exposed and what degree of substrate corrosion has occurred. If significant corrosion has occurred, immediate repair is usually performed. Then it is necessary to establish the frequency of the blisters and blistered area compared to the rest of the coating. Are the blisters localized, possibly due to localized conditions such as cold wall effect from external stiffeners? If the condition is widespread in the vessel, a complete recoating or relining is the only solution. Before embarking on that, it is important to re-evaluate whether the coating or lining system was sufficient for the operating conditions –chemical and especially thermal. An upgraded system may be required for optimum long-term substrate protection. If the condition is localized, and coating integrity is good overall in the vessel, local repair may be most cost-effective; possibly an upgraded system (permeability resistance, chemical resistance) should be used for those areas. Evaluation, understanding, and recommendations are useful.

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Tagged categories: Blistering; Coating failure; Quality Control

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