Traditional Holiday Detection Issues
Using “traditional” electric low voltage wet sponge testers and high voltage spark testing holiday detectors in water storage tanks can create some concerns. As guided by the typically referenced NACE RP0188, “Discontinuity (Holiday) Testing of New Protective Coatings on Conductive Substrates” standard recommended practice, the tester is given guidelines for the use and rate of movement of each device. Since most water tank linings are applied at DFTs below 20 mils, the low voltage instrument is to be used for holiday detection. For low voltage wet sponge testing, per the NACE recommended practice, “the sponge shall be moved over the surface of the coating at a moderate rate of approximately 0.3m/s (1ft/s), using a double pass over each area.” Depending on the size of the sponge being used, the rate would allow for approximately 30 square feet of coverage per minute, not counting the time needed to mobilize and move through the tank in order to keep the sponge in direct contact with the coated surface. The instrument needs to be grounded to the conductive substrate during use, which, on the interior of the coated tank, could make locating a bare surface difficult, further creating mobilization issues.
More important than simply getting a job done faster is the additional access for inspection that OAP coatings allow. With the configuration of tank interiors, it is not possible to put the wet sponge tester in direct contact with many interior surface areas. (Coincidentally, it is mostly in these areas where we find early coating failures and the most significant corrosion when we inspect tanks, resulting in forensic investigations of coating failures.) The holiday detection instruments used in inspecting OAP coatings do not require contact with the coating or the substrate. Using UV-A lamps, pinhole defects can be visible at a distance of over eight feet, depending on ambient conditions. Tanks do not have to be blacked out to perform this inspection, and the visual effects can be clearly captured on camera.
Consultants and owners are constantly looking at new technology in coating formulations to increase the life cycle of their water tank linings. These technologies have included the incorporation of organic zinc-rich primers inside steel tanks and have driven coatings specifications to products with higher volume solids and film thickness. Led by actions of the U.S. Navy to maximize the life of coatings placed in immersion, a pigment can now be incorporated into coatings that enable a coating to fluoresce. Added to coatings complying with the NSF Standard 61, OAP can increase the life cycle of a coating system by ensuring proper coverage and installation of these high volume solids coatings.