When examining coating surfaces, what is the primary indicator of soluble salt contamination?

The primary indicator of soluble salt contamination on coating surfaces is the presence of cracks in the topcoat. Soluble salts, often from corrosion or moisture, can migrate to the surface, drawing moisture and causing stress that leads to cracking. This phenomenon occurs due to the expansion of salt as it absorbs water, which creates pressure under the coating, resulting in cracks.

While blisters, filiform corrosion, and discoloration can indicate various issues with coatings, they do not specifically correlate to soluble salt contamination as strongly as cracking does. Blisters might suggest issues related to moisture or application problems, filiform corrosion indicates corrosion under the coating, and discoloration can result from aging or environmental influences rather than salt contamination specifically. Thus, cracks in the topcoat serve as a more direct and relevant sign of soluble salt presence and its effects on coatings.