The coatings of Zinc (zinc and zinc-alloys) provide cathodic protection; when applied on a more noble metal (such as steel) behave as a sacrifical coating.
The passivations have to meet the requirements specified by the standards in terms of corrosion resistance. Passivations and eventual sealants will play the first resistant effect. When this effect is finished, the corrosion will start its first products, the white salts, which begin to attack the zinc coating.
Till it is intact, the layer of zinc, or zinc-alloy, protects as barrier effect . When the coating shows the first defects, the zinc, or zinc-alloys, starts to protect as sacrificial action. The anodic reactions of oxidation are localized on the coating surface preserving the metal underneath. When the protective effect of coating is finished, the corrosion process will start to nick the material underneath causing the formation of red salts and the resulting red rust.
The corrosion resistance is expressed in hours in neutral salt spray (NSS in according to UNI ISO9227) before white and red salts show on the surface of the pieces, on the 5% of the total area. It means that the resistance to the white salts regarding the action of passivation and the resistance to red salts including also the action of micron of zinc or zinc-alloys coating, for indicating the total protective capacity. There isn’t a direct and absolute relationship between the resistance to the artificial corrosion and the environmental one, but this method gives the possibility to verify if a coating is able to maintain durable qualities of resistance.
In the attached technical details is possible to compare many of the characteristics of corrosion resistance related to our treatments of zinc and zinc-alloys, static and in barrel.