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By Allen Skaja, Ph.D., PCS, US Bureau of Reclamation
Presented at SSPC 2017; Session: Mini Session: Maintaining Aged Infrastructure with Difficult to Coat Features; Session chair: Heather Gilmer
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Maintaining aged infrastructure can be challenging, especially when structures were designed with difficult to coat features, such as riveted construction, back-to-back plates, or skip welds. According to NACE SP0178 Design, Fabrication, and Surface Finish Practices for Tanks and Vessels to be Lined for Immersion Service, these construction methods are not recommended for immersion service. However, the existing structures remain and must be recoated occasionally. This begs the question; what are the proper procedures for a successful recoat of these structures? The difficult to coat features once were protected using lead based paints, solution vinyl resins, and coal tar enamel coating systems. Sometimes special mastic materials were used that contained asbestos and other reinforcing ingredients. Today's coating systems, including epoxies and polyurethanes, appear to have significant difficulties in protecting these areas. The standard practice within SSPC PA Guide 11 is to brush apply stripe coats to work the coating into the crevices prior to spray application. In the case where the gap is too large for a coating to bridge, 100% solids epoxy mastic fillers are used to fill the void All rivets and overlapping plates have crevices, which eventually crack the coating and result in corrosion. Potential causes of this damage are coating shrinkage stress and joint movement. The older coating systems protected these areas for 40-50 years before cracking. In contrast several of Reclamation's recent recoating jobs experienced failures of these features within months, despite following application procedures. This presentation reviews common practices for coating the infrastructure features described above, case history using modern coatings, specification language, and workmanship of contractors.
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