Continuity, porosity
Coatings in industrial production: specific controls

A distinction must be made between two cases, depending on the nature of the coating-substrate pair. When the deposit has a lower corrosion potential than the base metal, the latter is protected in the event of corrosion, to the detriment of the coating (cathodic protection: zinc on steel, for example); porosity testing is not essential in these conditions. On the other hand, when the coating has a higher corrosion potential than the base metal, the presence of pores can lead to significant corrosion of the substrate by galvanic effect. In this case, the coating thickness can be increased until the pores disappear completely (from 30 µm in the case of nickel). If this solution cannot be adopted, it is paradoxically necessary to multiply the number of discontinuities during the manufacturing process, in order to increase the anodic surface area and thus reduce the corrosion current density...