Cleaning Galvanized Steel
Q. How do you clean white lime scale and rust spots on galvanize? J.R.
Q. What is the best way to clean galvanized steel? It may or may not be painted, depending on how good the cleaning is. There is no rust, just white residue. K.G.
A. Steel is galvanized or coated with zinc to corrode sacrificially to protect the underlying steel substrate. The zinc coating can be hot dipped or electrogalvanized. The microstructure, thickness and chemical make-up of the coatings are different, but they still function in the same way. If a galvanized steel is showing signs of red rust, that is an indication that the galvanized coating, regardless of the thickness or type, has been breached (mechanically by scratching or chemically by corrosion). In this case, a simple cleaning to remove the corrosion product will not extend the life of the product significantly unless it is painted.
However, if a galvanized steel is free of red rust deposits from the base steel substrate, then the white deposits (zinc oxide, the corrosion product of the zinc coating) can be cleaned to improve the look of it without additional painting. If the galvanized is showing signs of red rust, then it is best to clean it with a phosphoric acid cleaner to remove that first. If there are significant amounts of it, it may not be possible to recover this product. The white corrosion product can be removed with a weaker organic acid such as acetic acid. The galvanized steel should be thoroughly rinsed following any acid application. If the part is to then be painted, an acid wash primer is sometimes required prior to the painting.
Powder coating is one of the most durable finishes that can be applied to industrial manufactured products, and offers excellent corrosion protection and is very safe because of its lack of volatile organic compounds.
A more realistic way to perform salt spray tests.
Specific questions about zinc phosphate and pretreatment are answered in one article...