Steel frames are zinc coated or galvanised, which protects steel from cut-edge corrosion by cathodic or galvanic action. Where the underlying steel base becomes exposed at cut edges holes or mechanically damaged areas, the zinc coating will corrode in preference to the steel.
The ability of a hot-dipped zinc coating on steel to protect it against corrosion is known as sacrificial, or cathodic protection because the cathode metal is the one protected. In this way the zinc coating adjacent to the edge or hole protects the cut area. Furthermore, because corrosion is a surface phenomenon, the cut surface protects the steel inside from corrosion. Rust cannot spread through steel from a point on the surface or cut edge –unlike rot in surface treated timber in which all cut ends need application of a rot-preventing chemical to preserve the durability of the stud.