Rust is an iron oxide, usually a red oxide formed by the redox reaction of iron and oxygen in the presence of water or humidity. To prevent rust in steel products you must eliminate the exposure of oxygen and moisture to the raw steel surface. The most common way to prevent rust is by painting a product, thus sealing the steel from exposure to the elements (oxygen and moisture). If during normal use, the painted surface is scratched or worn and has exposed the raw steel, rust can occur depending on the level of humidity. It is very critical to keep the painted surface in good condition and repair any scratches in a prompt manner.
To improve the corrosion resistance of steel products, manufacturers of steel doors and frames use Zinc-Coated Steel. The zinc serves as a sacrificial anode so that even if the coating is scratched, the exposed steel will still be protected by the remaining zinc. The zinc protects its base metal by corroding before the iron in the steel.
There are two types of zinc coating used in doors and frames. The first we will discuss is Continuous Sheet Galvanizing. This method of zinc coating is also referred to as Galvanized or regular “G” coating. The zinc is applied on continuous coating lines and uses a series of air knifes to uniformly spread the zinc coating over the steel. The level of zinc on the steel is designated by the number after the “G”. G90 designated .45 oz. of zinc on each side of a 1’ x 1’ piece of steel, G30 would be .15 oz. of zinc per side of the same square foot piece.
The other type of zinc coating is Alloyed “A” coatings or otherwise known as Galvannealed steel. This method is done by special processing which produces an iron-zinc alloy coating that is not spangled, but is a dull gray in appearance and suitable for immediate painting without further treatment other than normal cleaning. The amount of zinc coating is the same as explained above. An A40 would be .2 oz. of zinc per side of a 1’ x 1’ piece of steel.