The current procedure for inspecting the coating for holidays in water storage tanks is to follow NACE SP 0188-06 Discontinuity (Holiday) Testing of New Protective Coatings on Conductive Substrates. This method involves passing an electric low voltage wet sponge or high voltage spark tester over the coating area that will be in immersion. This procedure has been called time-consuming, as it requires direct contact with the coated surface. Per SP0188-06 Section 3.2.1 Sufficient drying or curing of the coating shall be allowed prior to conducting a test. The length of time required for drying or curing shall be obtained from the coating manufacturer. Solvents retained in the coating could produce erroneous indications. It may take 7 days or more after coating application for the coating to cure and the solvent to be totally released from the dried film.
This method does not take into account low film thickness as well as other possible defects in the dried coating film. Technology has been developed that will allow the inspector to verify that the coating is holiday free and the proper film thickness has been applied by visually inspecting the area using an inexpensive ASTM E 2501 approved light source. This UV visual inspection method can be done during and after the application process. The applicator can make the necessary corrections while the coating is still wet, thus saving time and labor costs.
The system works by adding an optically active pigment (OAP) to the coating. It makes no difference if this is a single or multi coat application. When viewed under an ultra violet light, a single coat system with the optically active pigments will fluoresce even when still wet. Defects such as holidays will appear black in contrast to the fluorescing coating. Insufficient film thickness will not appear as bright blue as the surrounding coating.
In multi-coat system the optically active pigments are added to the first or primer coat, not the topcoat. The first coat will fluoresce as stated in a single coat system. Once the topcoat is applied the opposite effect will take place – areas that fluoresce are the defects or holidays. Pinholes as small as .25 mils can be identified and corrected.
One area that usually shows the effect of pinholes, holidays and other film defects is the roof and structural support areas in a tank. Rusting and premature coating failure occurs in these areas above the mean water level in the storage tank. This usually is caused by low film thickness on edges, inside corners and back-to-back angles. These defects can now be visually identified during and immediately after coating application.
Sherwin-Williams now offers a number of NSF Standard 61 approved products that contain this optically active pigment. Full details of these products can be found at the Protective and Marine website of the Sherwin-Williams Company.