In corrosive environments it is considered essential to produce a continuous protective coating for long term performance. Non continuous films provide opportunities for premature failure of the coating through isolated corrosion and undercutting. To reduce the risk of premature failure for thick film systems, high voltage holiday testing is completed using specialized equipment. High voltage holiday testing is the practice of using high voltage equipment to detect discontinuities in a coating system.

The rule of thumb throughout the US industry has typically been a minimum of 100 volts per mil of coating (1/1000 inch) as the basis of voltage settings. It is believed 100 volts/mil allows for the detection of holidays while not posing a risk of damaging the coating system. Consequences of incorrect voltage settings can range from undetected holidays to damaged coatings. The purpose of this paper will be to discuss the concept behind high voltage holiday testing, the theoretical required voltage to bridge the air gap and experimental findings of the required voltages for holiday detection on a range of thick film coatings.


A holiday is defined as a pinhole or discontinuity in a coating lining. These discontinuities are frequently very small and not readily visible and create a pathway for oxygen and an electrolyte to cause deterioration of the underlying substrate. These defects will tend to reduce the life expectancy of a coating in service, particularly if the service is to include immersion, such as the lining of a tank or a pipe.

Due to the small size of most holidays, visual inspection can be an inadequate method on its own for locating all holidays. The use of holiday equipment paired with a visual inspection can help reduce the risk of missed or unfound holidays. Holiday detection can be used on metallic substrates, such as carbon steel, ductile iron, stainless, or concrete substrates with sufficient moisture content.

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