GB/T 17720-2025 Metallic coatings—Review of porosity tests

Scope

This document reviews the published methods for detecting pores (see GB/T 3138) and discontinuities in coatings.
This document is applicable to coatings such as aluminum, anodized aluminum, brass, cadmium, chromium, cobalt, copper, gold, indium, lead, nickel, nickel-boron, nickel-cobalt, nickel-iron, nickel-phosphorus, palladium, platinum, enamel or enamel-coated ceramics, rhodium, silver, tin, tin-lead, tin-nickel, tin-zinc, zinc, and other base metals such as aluminum, beryllium-copper, brass, copper, iron, nickel-iron-cobalt alloys, magnesium, nickel, nickel-boron, nickel-phosphorus, phosphor-bronze, silver, steel, tin-nickel, and zinc alloys.
The test methods described in this document detect discontinuities in coatings by reacting with the exposed base metal to form observable reaction products.
Note 1: Pores are generally perpendicular to the coating surface, but may also be inclined to the coating surface; they often take the form of cylinders, but may also be distorted, as shown in Appendix A.
Note 2: Pore sizes vary from sub-microscopic, invisible under ordinary microscopes, to microscopic, visible under magnifications of 10x to 1000x, and macroscopic, visible to the naked eye.
Note 3: Pores can be clearly displayed by discoloration of the coating surface.
Note 4: Pores in coatings are not always harmful. For example, in discontinuous chromium, microporous or microcracked chromium is beneficial, and electrical conductivity is required during testing to reveal pores.
Note 5: The results of porosity testing are expressed as the number of pores per square centimeter, which is a relative value related to the specific testing methods used and the magnification used during inspection. Appendix B provides typical reporting and evaluation methods.

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