Surface Soluble Salt Detection and Analysis
Chemical contaminants on a surface can include chlorides, ferrous ions, sulfates, and nitrates, among other types of soluble salts. Chloride may come from deicing materials or marine/coastal environments, ferrous ions are a by-product of corrosion, sulfates can be airborne, particularly in industrial environments (e.g., near coal-fired power plants) and nitrates may come from the soil (e.g., fertilizers). These chemicals are deposited onto surfaces while the structure is in service, or during transportation of new steel to the fabrication shop, or from the shop to the field. If the salts are not detected or are not adequately removed from the surfaces, they can become trapped beneath a newly installed coating system. Salt contaminated surfaces can corrode prematurely, and in the case of immersion, if enough water is present, and the concentration of the water-soluble contaminant trapped beneath the coating system is high enough, water can be drawn through the coating film by a process known as “osmosis.” This drawing force can be quite powerful and will continue until the concentration of salt in water is the same on both sides of the coating film (the concentration reaches equilibrium). This process creates a build-up of water and pressure beneath the coating film, oftentimes enough to cause blistering and premature coating failure.
Many specifications require inspection of surfaces for chemical contaminants after surface preparation operations are complete, but before application of the primer. Because this type of contamination cannot be detected visually, the salts on the surface must be collected and the “surface extraction” tested for the contaminant(s) of concern.
Test your knowledge of surface soluble salt detection and analysis by answering each of the questions below. Helpful references are included in the text box below each question. The answers to the questions can be found at the end of the quiz on the results section. If you need help during the quiz, please visit https://kta.com/kta-university/surface-soluble-salt-ion-specific/ or go to SSPC Guide 15.
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