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In certain protective coating applications, it is vital to test the finished system for flaws and pinholes, as these defects can lead to premature coating failure in service. This is particularly important when the coating is used in an immersion or partial immersion situation such as for tank or pipe linings.
In certain protective coating applications, it is vital to test the finished system for flaws and pinholes, as these defects can lead to premature coating failure in service. This is particularly important when the coating is used in an immersion or partial immersion situation such as for tank or pipe linings. The main technique used for porosity testing of protective coatings is the high voltage test where a probe with a voltage, measured in kilovolts (kV,) is applied to the coating and detections of a flaw results in a flow of current, which can be used to create an alarm. The two ASTM documents for porosity or discontinuity detection, D 5162 for testing coatings on metal substrate and D 4787 for testing for coatings on concrete, both mention continuous DC and pulsed DC apparatus. The NACE recommended practice, SP 0188, also refers to these two-test method. This paper will discuss these two types of equipment and compare and contrast their use and performance.
In the past, the present and undoubtedly in the future, coating specifications will be written and included in bid packages that will end up on your desk, mine and our fellow colleagues. Often these specifications are poorly written, boiler plated or haphazardly thrown together to hopefully provide us with enough information to successfully win and complete a given contract.
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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.
Testing cured coatings for flaws and defects is often a crucial part of the acceptance process for a coating assessed against its specification. This is particularly the case for pipeline and storage tank coatings and for coatings applied for corrosion protection, where discontinuities in the coating can lead to premature failure.