Holiday Testing

Holiday testing, or more accurately termed “discontinuity detection,” is a separate test from coating thickness measurement. This crucial process is to check for the presence and locations of discontinuities, near voids or thin spots, entrapped contaminates and insufficient bubble structure in an otherwise non-conductive surface. Holiday testing is performed by generating a DC voltage potential or difference between the glass surface and the steel (or other conductive) substrate, and then detecting any current flow that occurs between the two surfaces.

Section 12.9 of ANSI/AWWA D103-09 by reference to ASTM D1562-01 Test Method A permits the use of inexpensive low-voltage 67.5V wet sponge testing for coatings with thickness less than 20 mil or less because certain coatings may be damaged if tested with high-voltage equipment. A higher-quality alternative is the more stringent higher voltage 1100V spark test per Test Method B of ASTM D1562-01. This higher testing is done to greater verify the quality of the glass fusing process and to better detect discontinuities. Holiday testing must be recreated for each and every sheet tested. One leading manufacturer re-blasts any sheet found with any interior holidays.

The reason for using the 1100V test is that failure to detect imperfections through the use of only a 67.5V test may lead to premature failure of the coating and thus appreciably degrade the tank’s critical, front-line barrier to corrosion. Also, that the higher-voltage test certifies that the dielectric properties of the glass-fused-to-steel coating is sufficient to allow the use of the higher testing voltage. As with any type of testing there are economic, technical and operational tradeoffs (see Table Benefits and Restrictions of Low Voltage vs. High Voltage Testing).

Because of the superior results of conducting 1100V holiday testing on tank coatings, the use of ISO 28765:2008 standards for 1100V testing is growing globally and can be considered a global benchmark.

Examination of Bubble Structure

In order to confirm an ultra-fine bubble structure that is critical to porcelain enamel, cross-sectioning and microphotography are utilized. Bubble structure cannot be left to field evaluation.

Impact Testing

Impact testing is done to check coating bond strength with the steel and is critical in evaluating high-quality porcelain enamel. Manufacturers should always reject a sheet if its underlying steel is exposed after the test is performed. Visual standards are used to determine degree of bond through examination of substrate after completion of impact test.