Fighting the Salts

Soluble salts, like chlorides and sulfates, are found on surfaces everywhere. These soluble salts pull moisture from the air, causing protective coatings to fail. It is believed that 80 to 90% of all premature coating failures are caused by improper surface preparation [1]. The coatings industry is beginning to understand that a large majority of these premature failures is attributed to non visible contaminants left on the surface.

Most of these non visible contaminants fall into the category of what is termed "soluble salts." The term itself is a misnomer in that these salts are often chemically bound to a surface and not so readily water soluble. If they were, a simple water wash would be sufficient to remove them. The salts that are most commonly encountered in surface preparation and the most often to be contended with are chlorides and sulfates.

What happens when a coating is applied over soluble salts left on a surface? (see some examples here) Field testing for chlorides by both qualitative and quantitative methods is common and the threshold for acceptable levels is dropping, but the cumbersome methods for testing have remained the same until the development of a 1-2-3 test by CHLO*RID:

1. Empty the content of an extractive solution into a sleeve;
2. Massage the extract against the surface where salts are suspected;
3. Snap a little pipette and plunge it into the extract to identify the level of chloride in parts per million or micrograms per cm squared of surface.

Reference

1. Johnson, J.R. "A Primary Cause of Coating Failure" Materials Performance, June 1999, pp. 4849. (back)