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Getting It Right the First Time

New Ship Inspection Tool Keeps Hard-To-Reach Places Safe From Corrosion

Researchers at the Naval Surface Warfare Center (NSWC), Carderock Division, have developed an enhanced ultraviolet (UV) inspection tool to ensure that the thousands of tanks and enclosed spaces on Navy ships are refurbished properly and fully protected from corrosion for years to come.

There are approximately 18,255 tanks on U.S. Navy ships. These spaces are categorized as ballast tanks, fuel or compensated fuel tanks, potable water tanks, and collection and holding tanks, among others. When ships are brought into dry-dock after several years of service for overhaul, their myriad tanks must be emptied, cleaned, decontaminated, and re-coated. Recently the Navy has learned that many of these tanks are presenting corrosion problems well ahead of their next maintenance interval.

A detailed analysis of the corrosion problem indicated that for ships in dry-dock, the surfaces of many of these spaces had been contaminated with hydrocarbons when they were coated. Over time, these hydrocarbons had undermined the tank coatings' performance. Because each of these spaces had previously been decontaminated, inspected to standard, and declared "contaminant free" prior to coating, officials determined that a small amount of hydrocarbons had been present in the tanks. This amount was too small to detect, but it was enough to inhibit bonding during the coating process.

The ultraviolet (UV) light kit is portable and can detect most hydrocarbons. Photo courtesy of Naval Surface Warfare Center (NSWC), Carderock Division.

To solve the problem, NSWC developed a tool that can detect traces of hydrocarbon contaminants that are below what can be visibly discerned on steel surfaces. UV light improves the ability of inspectors to detect some hydrocarbon contaminants, because most organic materials fluoresce when exposed to it. NSWC's new UV light inspection kit is a super-high-intensity black light that combines a Micro Discharge Light technology with a heat-resistant, polymer lamp head and works with both AC and DC voltage for portability. The technology represents the current state-of-the-art for detection of hydrocarbon contamination.

UV "Black Light" inspection requires a dark room and black light source for inspection of parts. This method is a pass/fail test that will work on any material with a contaminant that fluoresces. The inspector simply places the part under the black light and notes any areas of fluorescence. Experimental results have proven that the UV method can detect the presence of hydrocarbons on a steel surface in amounts far below the visible detection threshold, and thus makes a far superior detector of surface contamination, NSWC officials report.

The premature failure of coatings in one of these tanks can be very costly to repair in terms of manpower, mission performance, and ship availability. Photo courtesy of NSWC, Carderock.

The detection of hydrocarbon contamination is a growing concern as the Navy moves to higher-solids and single-coat paints. These new paints use less hazardous solvents, but are more sensitive to the effects of contamination. In service, most ship tanks are exposed to one or more hydrocarbon contaminants such as fuel oil, oily waste, machine oil, and grease from cooking operations, some of which are deposited on the surface of the tank. Also, when the coatings within these tanks degrade, the products of their decomposition, which have now become contaminants themselves, will be deposited on the steel. During repair operations, these contaminants must be removed from the steel prior to coating application in order to achieve acceptable coating performance and longevity.

The Naval Sea Systems Command's (NAVSEA's) current standard acknowledges that low-level and trace organic contamination is known to degrade coating adhesion, but presently, it only requires a visual inspection of surfaces to detect oil and grease contamination. So the Navy required a more effective technique to "detect the previously undetectable."

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