Protective organic coatings are the primary form of corrosion control for steel structures exposed in a marine environment. For more than fifty years, evaluation of suitable coatings for various service environments has relied substantially on exposure of coated steel panels followed by visual evaluation. Exposure may include accelerated testing or natural environmental exposure in immersion or atmospheric conditions. Common visual inspection practices include semi-quantitative evaluation of rust-through, blistering, and scribe creep (i.e., cutback). Coatings performing well in these evaluations versus specification standards are often deemed “acceptable.”
However, the subject research suggests that common visual inspection protocols, while useful for judging aesthetic concerns, may not be sufficient for identifying barrier coating properties and substrate corrosion control (i.e., actual material loss of the steel due to corrosion). Post-exposure destructive physical inspection for corrosion under protective coatings suggests that significant metal loss may be occurring in the absence of a visual indication and that actual corrosion/material loss does not correlate with the visual inspection data. Improved inspection methods will help identify coatings that provide the best substrate corrosion protection.
Over the past twenty years, the Navy has widely implemented the use of advanced ultra-high solids (UHS) coatings for the protection of ship tanks and voids to extend the life of tank coatings. (1), (2), (3) The Navy intends these coatings to provide twenty years of service. Experience to-date has been largely positive concerning these coatings, although they are not immune to breakdown prior to the end of their intended service life. In areas of breakdown, the maintenance community needs a suitable touch-up and repair coating. As part of this ongoing effort, the Navy sought methods to improve basic coating test methods to better assess repair coating performance. The program intended to conduct testing that would project a 10+ year service life.
The technical community identified repair coatings applied to hull-cuts and stringers as particularly problematic. Figure 1 shows such areas in the initial stages of coating failure in a tank.