Along with Osmotic Blistering and Microbially Induced Corrosion, Poor Surface Preparation and Application is the third reason for a failure of industrial coatings in water-immersion conditions. In our previous blog posts, we explored Osmotic Blistering and MIC. This blog post explores common issues that can lead to poor surface preparation and application and how to mitigate them. Poor surface preparation and poor coating application can directly lead to failures in all types of industrial coatings, not just those in water immersion conditions. An asset owner’s best defense against these types of failures is a well-written coating project specification that can eliminate many of the underlying issues that lead to poor surface preparation and application. The specification should clearly delineate all the appropriate measures to be taken to ensure proper surface preparation, the methods and techniques to ensure coatings systems are applied and cured correctly so that they can perform to their designed capability, and specifics for the inspection process during application. Within this article, we will highlight specific areas of concern relating to surface preparation and application. Poor Surface Preparation Surface preparation is crucial for the longevity of any coating project. Surface preparation includes all aspects referring to the cleanliness and physical readiness of the substrate to be coated. Coatings in immersion conditions have three basic requirements for surface preparation: proper anchor profile, specified surface cleanliness, and low level of impurities on the surface. Coating applied without any surface preparation. Grit blasting is a crucial step and should be done for all coatings on metal in immersion conditions. Two measures should be performed on the surface after grit blasting. First, the depth of the profile should be measured in accordance with the ASTM D4417-11 standard. A typical minimum profile depth will be two or three mils but they can vary based on the coating being used. Second, the cleanliness of the surface should be determined by visual comparison to a standard. Coating manufacturers often recommend at least an SSPC-SP-10 “Near-White Metal Blast Cleaning” standard for immersion grade coatings, if not a SSPC-SP-5 “White Metal Blast Cleaning”. Proper surface cleanliness increases the adhesion value of the coating system because it removes poorly adhered oxides that may be present on the surface. Delamination of coating most likely due to insufficient or improper surface preparation. Poorly adhered previous coating flake off during grit blasting. Ions on the surface above a certain level will reduce the longevity of the coating system. Water immersion conditions require specific attention to ions on the surface. These can be chlorides, sulfates, sulfites or even iron oxide. We explore the mitigation of ions in more detail in our blog post on osmotic blistering. Table 1 shows commonly used standards for soluble ion concentrations. Osmotic blistering most likely caused by presence of chlorides on substrate surface prior to coating application. Table 1. Soluble Salt Maximum Concentrations Immersion Conditions Atmospheric Corrosion Maximum Chloride Concentration 3 µg/cm2 5 µg/cm2 Maximum Sulfate Concentration 10 µg/cm2 15 µg/cm2 *From Chlor-Rid International’s Non-Visual, Soluble Salt Contamination of Steel Substrates If ions are present above these limits, they should be removed using common ion removal techniques like distilled water wash, Chlor-Rid wash or a similar product. Oil is also critical to remove from the surface of the metal prior to grit blasting. Oil will be embedded into the profile if left on prior to grit blasting. Oily surfaces should have the oil removed prior to grit blasting with an appropriate degreaser or solvent wipe. Previous coating and oil on substrate after grit blasting. Improper Application/Quality Control Numerous issues during application can also cause premature failures in immersion grade coatings. Common issues include; Pinholes/holidays, uneven film thickness, edge retention, amine blush, solvent entrapment, improper curing. We will discuss each issue along with some common quality control guidelines that can be put into place to ensure they do not happen to you. Pinholes/Holidays: Pinholes often occur because of air entrapped in the coating system. Pinholes can be reduced by using multiple layer applications and by spraying the coatings with airless spray equipment. Holidays occur in hard to coat areas where one or both coats simply didn’t reach the entire substrate. Both holidays and pinholes can be found and repaired using high-voltage spark testing. This testing should be done in accordance with ASTM D5162-15. Pinholes in coating allow for corrosion to occur underneath the coating. Corrosion then “seeps out” when exposed to moisture or rain. Uneven Film Thickness: The industry standard to determine the coating thickness is to take the average value of multiple points. If the average dry film thickness of a 1 sq. ft. area is 40 mils, but the readings range from 25-55 mils, water will permeate much more quickly in the thinner areas. There are two ways to mitigate this: Applicators should adhere to the best practice of using wet film thickness gauges during the application process, and Specifications should mandate a minimum dry film thickness Localized corrosion can occur when there is insufficient film thickness and damage to wet or uncured coating. Improper or missing stripe coating on weld seams can invite localized corrosion to occur. Edge Retention: Many coating systems will not maintain their full film thickness on sharp edges or corners. It is good coating practice to grind down any sharp edges and corners prior to grit blasting a substrate. If this is not done, thin spots will occur on these corners which can cause premature failure. They should be identified during the high-voltage spark testing. Amine Blush: This is a unique phenomenon for epoxy type coatings which are very popular in water-immersion conditions. Amine blushes tend to occur when the temperature is low, the humidity is high or high levels of carbon dioxide are present. An amine blush will completely prevent adhesion of a second coat and must be removed prior to overcoating. We have not found a quantitative test to determine if an amine blush is present. They tend to reduce the gloss and leave a cloudy finish to the film. Visual inspection for amine blush should be done prior to topcoating, especially if any of the high risk conditions are present. Delamination of coating most likely due to occurrence of amine blush. Solvent Entrapment: Coating systems that contain solvents must be applied at tighter thickness guidelines to ensure for complete solvent removal during the curing stage. Solvent becomes entrapped in the film if the film is applied too thickly, the temperature is too low, or the coating cures too quickly. Solvent entrapment is hard to detect in the film prior to exposure to immersion conditions or elevated temperatures. Monitoring of the film thickness or using a solvent-free coating technology is the best way to ensure that solvent entrapment does not occur. Improper Curing: Many coating systems require specific curing times and temperatures to ensure maximum performance. Coatings like phenolics, powder coats, and e-coats usually require an elevated temperature cure to ensure proper cross-linking of the polymers. Epoxies, vinyl esters, urethanes and polyureas generally do not require an elevated temperature cure but often need a specific amount of time at ambient temperature prior to submersion in water. Consult with your coating manufacturer for the specifics of any system being used. Need Coating Specification or Inspection? Contact Advanced FRP Systems There are numerous reasons that immersion-grade coatings can fail related to poor surface preparation and poor application. The best defense an asset owner has is a thorough specification and the presence of a third-party, NACE-certified coating inspector. Coating inspectors are available worldwide and should always be directly hired by the asset owner to ensure proper oversight. If you are writing a specification for an immersion-grade coating, contact Advanced FRP Systems and our team of coating experts will help you write a comprehensive specification, customized to your exact requirements. We offer full written specifications, consultations to assist your internal specification writer, or product recommendations all free of charge. If you find failures on your coatings we can inspect your system and determine the best possible corrective action. Whether the inspection is done in house or in conjunction with us, Advanced FRP Systems is here to thoroughly diagnose your corrosion problems and prescribe streamlined, custom solutions. You can trust us to take the time to dive into your unique problems and build the right custom solution or preventative measure.