In practice, the effectiveness of a finish is expressed in terms of its resistance to corrosion. This resistance is in turn expressed in terms of hours of exposure, in accordance with standardized tests known as ’salt spray tests’.
The salt spray test entails the exposure of surface treated workpieces to a salt spray chamber in which a controlled aggressive saline environment reigns.
Resistance to corrosion is expressed in terms of the number of hours without the appearance of white rust, and then red rust.

White rust (WR):

White rust corresponds to the corrosion of the zinc layer.

Red rust (RR):

Red rust corresponds to the corrosion of steel – in other words, where no coating remains on the steel.

Optional top coat (electrolytic or flake galvanizing)
Passivation (electrolytic galvanizing)
Zn coating (sacrificial protection)
Steel substrate
White rust (corrosion of zinc)
Red rust (corrosion of steel)