【国外标准】 Standard Practice for Discontinuity (Holiday) Testing of Nonconductive Protective Coating on Metallic Substrates

4.1 A coating/lining is applied to a metallic substrate to prevent corrosion or reduce product contamination, or both. The degree of coating continuity required is dictated by service conditions. Discontinuities in a coating/lining are frequently very minute and may not be readily visible. This practice provides a procedure for electrical detection of discontinuities in nonconductive coating systems.4.2 Electrical testing to determine the presence and number of discontinuities in a coating/lining is performed on a nonconductive coating/lining applied to an electrically conductive surface. The allowable number of discontinuities should be determined prior to conducting this test since the acceptable quantity of discontinuities will vary depending on film thickness, design, and service conditions.4.3 The low voltage wet sponge test equipment is generally used for detecting discontinuities in coatings/linings having a total thickness of 0.5 mm (20 mil) or less. High voltage spark test equipment is generally used for detecting discontinuities in coatings/linings having a total thickness of greater than 0.5 mm (20 mil).4.3.1 Coatings/linings less than 0.5 mm (20 mil) in thickness may be susceptible to damage if tested with high voltage spark testing equipment. However, coatings/linings greater than 0.25 mm (10 mil) and less than 0.5 mm (20 mil) may be tested with high voltage spark test equipment provided the voltage is calculated and set correctly, and the coating manufacturer approves its use.4.4 To prevent damage to a coating film when using high voltage test instrumentation, total film thickness and dielectric strength in a coating system shall be considered in determining the appropriate voltage for detection of discontinuities. Atmospheric conditions shall also be considered since the voltage required for the spark to gap a given distance in air varies with the conductivity of the air at the time the test is conducted. Table X1.1 in Appendix X1 contains suggested voltages for high voltage spark testing of low dielectric strength coatings/linings.4.5 The coating manufacturer shall be consulted to obtain the following information that can affect the accuracy of this test to determine discontinuities:4.5.1 Establish the length of time required to adequately dry or cure the applied coating/lining prior to testing. Solvents retained in an uncured coating/lining may form an electrically conductive path through the film to the substrate and may be a fire hazard.4.5.2 Determine whether the coating/lining contains electrically conductive fillers or pigments that may affect the normal dielectric properties.4.6 This practice is intended for use with new coatings/linings applied to metal substrates. Its use on a lining previously exposed to an immersion condition has often resulted in damage to the lining and has produced erroneous detection of discontinuities due to permeation or moisture absorption of the lining. Deposits may also be present on the surface causing telegraphing (current traveling through a moisture path to a discontinuity, giving an erroneous indication) or current leakage across the surface of the coating/lining due to contamination. The use of a high voltage tester on previously exposed coatings/linings must be carefully considered because of possible spark-through that will damage an otherwise sound coating/lining. Although a low voltage tester can be used without damaging the coating/lining, it may also produce erroneous results.1.1 This practice covers procedures for determining discontinuities using two types of test equipment:1.1.1 Test Method A—Low Voltage Wet Sponge, and1.1.2 Test Method B—High Voltage Spark Testers.1.2 This practice addresses metallic substrates. For concrete surfaces, refer to Practice D4787.1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.