A corrosion blister is a raised area, often dome shaped, resulting from either loss of adhesion between a coating or deposit and the base metal or delamination under the pressure of expanding gas trapped in a metal in a near-subsurface zone.
|This particular type of corrosion can be quite damaging since it combines all the elements of a crevice corrosion scenario, i.e. small local anode/large external anode, formation of an acidic environment, production of gaseous hydrogen. Paints are not impervious to water penetration. If the initial bond strength is not real good, if the pollutants in the environment are particularly insidious for the type of paint system used, blisters can develop beneath the paint even though there are no discontinuities in the paint. As the blisters grow larger and begin to combine, the net effect can be gross flaking of the paint in large areas.||
To minimize the tendency for loss of paint adhesion through undercutting corrosion or blistering, one needs to take into account very specific recommendations from the steel supplier and paint manufacturers. The “best” coated-product design requires that the user pay attention to the type and thickness of the metallic coating, the type of pretreatment, the type and thickness of the primer, and the type and thickness of the topcoat.