Red Rust vs White Rust

When most people use the term “rust”, they are referring to the “red-oxide” rust that forms when iron and oxygen react in the presence of water. This is the kind of rust that looks pretty cool on an old barn, however it is corrosive. It will eventually deteriorate the metal. This is the type of corrosion that is prevented by a galvanized coating. It is worth noting that even a G90 zinc coating will eventually red rust if left exposed to the environment, but it will take many decades for the metal to deteriorate!

Another type of rust that we talk about in the sheet metal industry is “white-rust”. White rust is actually not really rust at all. It is a storage stain due to zinc, water and a lack of oxygen. For example, when a stack of skids gets wet and water gets in between the layers of sheets where there is no oxygen, a stain will appear on the metal. It is a flaky white substance.

Storage stain (white rust) is simply the chemical compound zinc hydroxide, which forms when zinc oxide comes in contact with moisture in an oxygen deficient environment. Its appearance does not necessarily indicate an inferior or poor quality coating. It is the result of environmental conditions during the storage of the galvanized steel. The stain is often superficial even though it may appear to have a buildup of white chalky powder. The protective properties of zinc are not impaired by the presence of superficial white rust. If the application is not aesthetically critical, the galvanized coating should perform very well and meet the requirements and expectations of the end user. In most instances involving outdoor exposure, the white rust will disappear over time, as it either is washed off by rainfall or is converted to zinc oxide and then a protective zinc carbonate.