Technology Simplifies Holiday Detection in Water Storage Tanks

Nov. 1, 2007
For many years the protocol for inspecting water storage tank linings for coating failures in immersion or wet and corrosive environments involved ...

by Robert Murphy

For many years the protocol for inspecting water storage tank linings for coating failures in immersion or wet and corrosive environments involved using electric low-voltage wet sponge testers and high-voltage spark testing holiday detectors. The procedure can be described as time consuming, requires direct contact with the coating, and is unable to identify insufficient film thickness of multiple coats reliably.

Optically Active Pigments in the primer glow under UV light. Pinholes and holidays in the top coat are clearly visible.
Click here to enlarge image

A new coating formulation developed by Sherwin-Williams Industrial & Marine Coatings makes that protocol a thing of the past and holds promise for increased longevity for coatings systems. The company is offering a new pigment system that fluoresces under ultraviolet light, making pinholes, holidays and other discontinuities clearly visible.

Virtually any coating formulation using the Optically Active Pigments (OAP) can be made to fluoresce. OAP can be added in very small amounts, similar to adding a color pigment to a coating.

For a potable water tank, the life expectancy of a two-coat epoxy system, if correctly applied, inspected and maintained, should be more than 40 years. But in the field, owners and engineers have been tolerating shortened service life at 12-15 years of service. When existing specifications are upgraded to require fluorescing pigments, service life can be dramatically extended.

Sherwin-Williams’ new formulation technology - Duraplate® UHS Ultra High Solids Epoxy with fluorescing Optically Active Pigments - is the only product of its type meeting the requirements of NSF Standard 61 for potable water tanks of at least 1,000 gallons. The coating uses Duraplate UHS Epoxy as the vehicle to hold the pigment.

How the OAP system works depends on whether the application is single- or multi-coat. Viewed under ultraviolet light, a single-coat system with Opti-Check OAP pigments will fluoresce or glow even when still wet. Pinholes, holidays and improper film thickness will either appear black in contrast to the fluorescing coating, or not as bright due to insufficient film thickness.

The opposite is true for multi-coat systems. When OAP is used in the primer and not the topcoat, pinholes will fluoresce under inspection of the topcoat. Pinholes as small as .25 mils can be identified and corrected before a tank is placed into service using an inexpensive ASTM E 2501-approved light source.

A “civilian” application developed from a proven Sherwin-Williams technology supplied to the U.S. Navy, the fluorescing technology has doubled the service life of marine ballast storage tank coatings simply by illuminating insufficient film thickness wherever it occurs.

Traditional holiday detection methods as prescribed by NACE RP-0188 identify bare substrate, but are not designed to pinpoint insufficient film thickness. Nor are these methods practical in areas where wet sponge detection instruments don’t fit or can’t be set flush to the surface, such as welds, inside angles, supports, stiffeners and piping. And by requiring direct contact to all surfaces and the grounding of the inspection equipment to bare steel, the coating film must be destroyed just to inspect it.

Another benefit of the coating technology is that it overcomes hurdles that contractor skill can’t plan for. Multiple coat systems are typically specified in contrasting colors to make visual inspection easier, but even then, holidays can be obscured by glossy coatings and poor lighting.

Actual field inspector results indicate improved productivity (area covered per minute) by 50-70 percent, locating 25 percent more defects, visually detecting low film thickness on edges and visually identifying pinholes .25-.50 mm in diameter.

About the Author:

Robert Murphy is Strategic Market Manager for the Water/Wastewater sector at Sherwin-Williams Industrial & Marine Coatings, Cleveland, OH.

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