The white wax deposit which forms on steel is referred to as white rust. The formation of white rust is what occurs after zinc has reacted with moisture. It can also be seen as corrosion or the weakening of the protective zinc coating of the pre-galvanised (Z275) steel.
Causes:
Zinc that is in contact with moisture is a reactive metal and the moment it gets wet or is in the presence of moisture without proper air circulation, zinc hydroxide is formed due to chemical reactions. The white rust can be caused in a number of situations for example: through condensation in the stack of galvanised or pre-galvanised (Z275) steel tubes, coils or the sheets; water entering the stack of tubes, sheets or coils during transportation or storage and a combination of the weather and dust.
Prevention:
The following can be done to prevent white rust forming on galvanised or pre-galvanised (Z275) steel coils or tubes:
Steel tubes, coils or sheets must be stored indoors away from any possible chemical pollution, rain or variation in the temperature.
The pre-galvanised steel tubes or coils must be kept away from contact with the ground, this can be done by packing it either on a metal or wood.
The temperature inside the warehouse must be controlled and kept above dew temperature.
Steel products must be stored away from open doors and windows, the reason for this being that the chances of moisture reaching the steel are higher as well as the chance of condensation happening increases when stored close to open doors and windows.
Preferably the steel tubes must not be stacked on one another. The moment when water gets in between the tubes stack on one another, it does not get a proper chance to dry which results in a continuous zinc reaction and an increase of white rust.
Two treatments that can also be used to prevent white rust is surface passivation treatment and passivating oils. The surface passivation treatment is a water-based solution or any passivation treatment which is sprayed onto the coils or the coils can be dipped into the treatment. A thin layer is sprayed onto the galvanising line. Passivating oils that are used contains corrosion inhibitors which refers to specific chemicals. The oil is also applied to the galvanising line of the steel. The corrosion inhibitor protects the galvanised or pre-galvanised (Z275) steel from white rust.
A paper or special plastic wrapping can also be used for extra packaging to prevent moisture from getting to the steel products.
