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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.
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 practise, SP 0188, also refers to these two test method.
This presentation will discuss these two types of equipment and compare and contrast their use and performance.