Evaluating Coating Blistering in Ship Tanks and Voids
Patrick Cassidy, Elzly Technology Corp; Jay Ong and Bill Needham, Naval Surface Warfare Center Carderock Division
Problem Statement: According to the Navy’s Corrosion Control Assessment and Maintenance Manual (CCAMM), when intact blisters grow to a certain size and density, they are added to the percent coating failed in a tank. The percent coating failed in a tank then drives the decision to re-coat the tank (based on the tank condition rating), costing significant time and money. When maintenance planners are faced with budgetary restrictions for repair of tanks and voids, prioritization is necessary. The subject test program sought to prove that if the growth of intact blisters stabilizes over time then intact blistering should not be treated as a failed coating, and thus not contribute to mandatory corrective action (i.e., re-coating of the tank). This would allow maintenance planners to prioritize repair of tanks with failed coatings that do not contain intact blistering, and more effectively allocate their maintenance budget. Key Points: A laboratory study was conducted which successfully initiated blister growth and monitored growth in mock service environments over a seven-month period. For blisters created in a hot distilled water environment, and exposed in alternate seawater immersion, potable water and cathodic protection environments, there was no measureable change in blister diameter (within the sensitivity of the measurement tool). For the blisters that were created in the cathodic polarization environment, no blister growth was seen beyond the baseline inspection for any of the three exposure environments (within the sensitivity of the measurement tool, in this case a three dimensional laser measurement system). It was also observed that neither blister size nor density had a significant impact on the blister growth rate. This was concluded from the fact that blisters of different sizes did not show any differentiating growth patterns. To validate this research, a field study was conducted to determine if blisters grow in various real world ship environments over a two-year period. Three tanks were identified aboard the candidate ship for inspection, representing various service environments and coating types. A baseline inspection was carried out to characterize the blistering of each tank. A one year inspection was conducted prior to ship deployment. A two year inspection will be conducted once the ship returns from deployment.
Conclusions: Ship inspections will be used to validate that intact blisters stabilize. Then changes can be made to the way inspectors inspect for, and classify, blistered coatings. This can then aid the maintenance planners in determining the best method for allocating maintenance funding.