Surface Preparation of Soluble Salt Contaminated Steel Substrates Prior to Coating
|Publication Date:||1 March 2010|
Long-term coating performance is directly related to proper coating material selection, correct coating formulation, appropriate and adequate surface preparation, and satisfactory coating application. In addition, soluble salt contamination in the form of chlorides, nitrates, and sulfates sometimes has a deleterious effect on long-term performance of some coating materials in some exposure conditions. Coating specification writers, facility owners, and coating applicators have avoided problems by understanding when soluble salts could be an issue and the steps that could be taken to mitigate the consequences of salt contamination.
Soluble salts on a surface potentially affect the substrate or coating in two principal ways:
• By accelerating corrosion of an underlying steel surface.Dissolved salt solutions may accelerate oxidation of steel, resulting in undercutting of a coating system applied over that substrate. Corrosion can occur without the presence of any salts. Crevice corrosion, oxygen concentration corrosion, pitting, and general corrosion all occur without salt contamination in the presence of moisture on susceptible metal surfaces, such as steel. If salts are present, the rate of such corrosion accelerates.
• As a cause of osmotic blistering. Blistering of a coating sometimes occurs when moisture permeates through the coating film and dissolves entrapped water-soluble salts beneath the coating. The coating acts as a semipermeable membrane between a dilute salt solution (the liquid outside the coating), and a concentrated salt solution (liquid with dissolved salts within the blister). Coating systems in immersion or wet environments are sometimes susceptible to osmotic blistering, and blistering may occur at the steel surface and between coats. Osmotic pressure beneath the coating, in excess of the bond strength of the coating, may cause blisters. Ultimately, corrosion sometimes develops within the blisters, depending on oxygen availability. Blistering of a coating system also occurs in the absence of soluble salts. Entrapped polar solvents and overdriven cathodic protection may draw moisture through a coating film to cause blistering.
The best life-cycle performance is usually achieved when the coating system is applied over an uncontaminated or less-contaminated surface. However, a level of nonvisible soluble salt surface contamination that does not significantly compromise a coating system's life-cycle performance may also exist. At this level, the additional cost of removal of the salt contamination sometimes is not warranted. The owner or specifier sometimes conducts a risk assessment to evaluate the costs of soluble salt removal versus the risk of reduced coating performance.
The owner and specifier often determine whether the increased performance anticipated by achieving a nonvisible soluble salt decontaminated surface is justified from a cost standpoint.