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98 SEPTEMBER 2015 COATINGSPROMAG.COM Industry Insight C oncrete substrates in both water treatment and waste- water collection systems and treatment plants can be subjected to corrosive conditions. In water, this can include primary and secondary containment applications. In wastewater, it generally involves headspaces in which biogenic sulfde corrosion occurs or in secondary containment structures. Coating History In the past, it was commonplace to directly coat these concrete structures with relatively thin-flm epoxy, polyes- ter, or vinyl ester coating systems (15–30 mils, or 381.0–762.0 microns). Tese coating systems invariably were riddled with breaches and pinholes found over air voids or " bugholes." Tis lack of flm quality typically allowed a corrosive attack of the substrate to occur. Polymer-based surfacers (mostly epoxy) that were applied with broad knives were sticky and prone to run and sag, and they were difcult to apply. Since about the late 1990s, the use of thicker-flm epoxy and fexible polyure- thane coatings have been widely adopted. Tese products include rapid curing, high-flm-build (50–250 mils, or 1,270.0– 6,350.0 microns dry flm thickness) systems generally applied using plural component spray equipment. And while these more robust coatings could bridge many shallow air voids, deeper bugholes that remained untreated captured entrapped air beneath the coating flm. In either case, pinholes are formed and good flm quality is not achieved. Under corrosive gas phase or liquid phase exposures, the concrete is attacked at these breaches over time, causing subsequent undercutting and delamination of the coating system. More flm thickness does not make for good flm quality. A ll more flm thickness does, if not worked into the substrate voids, is increase the length of time for the pinholes to form in the coating flm. Te proper pretreatment of the substrate — using adequate surface preparation and diligent use of fller/surfacers — will prevent air entrapment and pinhole formation, hence assuring improved flm quality. Two Changes Improving flm quality on concrete substrates involves two important changes in everyday practices. First, abrasive blast cleaning or water jetting or whatever methods are used must target the complete opening up of bugholes. Tis means the width, length, and depth of the void. Secondly, the proper application of fller/surfacer materials must be performed. Tis includes complete flling of bugholes in both new and retroft construction. Tis work is less onerous and costly than fxing pinholes in the fnished coating system. In retroft applications, skim coating or surfacing the substrate is essential to ensuring good flm quality. So, fll the bugholes and resurface the substrate where neces- sary to achieve a relatively fat, coatable surface. Tis not only eliminates flm quality problems, it also helps you to more accurately measure wet flm thick- ness (WFT) of the spray-applied coating to follow. Normally, we do not attempt to measure WFT on concrete surfaces, but this is possible when we apply coatings over surfaced concrete substrates. Selecting Materials Water-borne epoxy-modifed cementi- tious fller/surfacers are user friendly to trowel apply and have good working properties for flling bugholes. Tey can be applied directly to the concrete without the use of primers. And there is little evidence to demonstrate that primer use alone prevents outgassing from bugholes in concrete. Te water-borne epoxy cementi- tious fller/surfacers have the added advantage of not requiring abrasive blast cleaning following cure as their pure cementitious-based counterparts do. Cement-based materials cure via hydra- tion alone and therefore form laitance at the exposed surface. Tat thin yet weak layer can promote adhesion-re- lated failures of subsequently applied coatings. Te water-borne epoxy resins solve this problem and impart strength and curing properties to these materials, which promote good adhesion to the epoxy coatings applied subsequently. Te use of water-borne epoxy-mod- ifed cementitious fller/surfacers is growing within the coating industry. But many applicators continue to approach this work conventionally and fght the pinhole war. CP Photo courtesy of Corrosion Probe, Inc. By R. A. Nixon, President and Principal Consultant for Corrosion Probe, Inc. Enhancing Coating Film Quality