Generally there are holes in the substrate and there are impurities in the holes, after the PVD treatment, the coating surface has surface defects, such as micro cracks, holes, pinholes, loose grain boundaries, etc due to the problem of substrates.
In the salt spray test, brine media permeate into the substrate through these defects, so the coating, brine media and substrate form a corrosion battery with the connection of the media. The coating and the substrate work as the electrode of the battery, because the coating has generally high chemical inertness and high electrode potential, the metal substrate and stainless steel has relatively lower chemically inert as well as a lower electrode potential.
For the electrode couple of corrosion cell, the stainless steel substrate is the anode, in which Fe loses electrons in the electrode reaction to be oxidized, and then generates Fe2O3 to make oxidation reaction in which the substrate is dissolved and eroded. The coating is the cathode, and it obtains electrons to ensure reduction reaction, so the coating will not be damaged by corrosion.
In this way, the substrate that near the plating defects meets local corrosion. If the corrosion is severe, and corrosion products expands, the coating will be damaged by accumulation and extrusion of it, or the substrate is etched and eroded, resulting in the upper plating subsiding and crushing due to the lack of support. This is reason why the product rusting when the salt spray test.