Salt Contamination will cause coating failures.

Salt is basically a chemical compound that dissolves in water to form a solution. The industrial evironment has many sources of salt contaminants, both natural and manmade. Chlorides (Cl¯) are the most common, being present in marine environments, water treatment and de-icing products. Sulphates (SO4²¯) are present in many natural sources and are generated from gas and diesel emissions. Nitrates (NO3¯) are present in fertilisers and auto emissions. There can also be salt contaminants present in the blast media used to prepare metal surfaces, and salts can be found on the surface of new metal which has been exposed to chlorides or sulphates from coastal environments during fabrication or transportation. The presence of soluble salts as contaminants on metallic substrates can result in premature coating failure when present in sufficient concentrations, particularly in hot immersion applications. Typically, salts are not completely removed by blast cleaning and, in fact, can sometimes be driven into the blast profile of the metal by blast cleaning. When coatings are applied over soluble salt  ontaminated steel, subsequent moisture migration (normal for coatings in humid or immersed environments) in and out of the coating film can result in moisture solubilising the salts and condensing on the steel surface under the coating film (Figure 1). Eventually this creates small osmotic cells pulling more moisture toward the concentrated solution and causes osmotic pressure to develop (Figure 2). This pressure pushes the coating off the surface, creating blistering of the coating with subsequent delamination

              Figure 1                                         Figure 2                                              Figure 3

The presence of soluble salts on the metallic substrate will also promote under-film metallic corrosion and further blistering/disbondment of the coating when the concentration of soluble salts exceeds a critical level. To ensure the success of elevated temperature immersion coatings on metallic substrates it is therefore necessary to measure surface salt levels, and then to reduce those salt levels if they are found to be unacceptable.