The ISO 15721-2001 metallic coatings porosity testing of gold or palladium coatings on substrates by sulfuric acid/sulfur dioxide vapor method specifies a method for measuring the porosity of gold or palladium metallic coatings by treating them with sulfuric acid and sulfur dioxide vapor. Porosity is an important indicator for evaluating metallic coating mass, corrosion resistance and service life. Porosity is judged by exposing the coating to a specific sulfuric acid/sulfur dioxide vapor environment and observing changes in the coating surface. This standard provides a scientific test method for coating products to help manufacturers and users make accurate assessments of the performance and durability of metallic coatings.