Post-galvanizing – sometimes referred to as ‘batch galvanizing’, ‘hot dip galvanizing’ or ‘general galvanizing’ – involves the application of a strongly bonded layer of zinc onto the steel, formed by immersing the fabricated steel product into a bath of molten zinc to produce a relatively thick, tough and abrasion-resistant zinc coating. Layers formed through this process are entitled to use the quality mark:
However, some steel components can be manufactured using an alternative zinc coating process known as pre-galvanizing, or more commonly ‘continuous galvanizing’. This method involves the coating of steels with zinc or zinc alloys in its steel sheet or strip form. The sheet is passed continuously through a bath of molten zinc, and when the product cools, the coating is then mechanically wiped to produce a thin layer of zinc or zinc alloy.
The use of pre-galvanized steel sheets to make components is generally limited to use in indoor or non-aggressive situations, due to the relative thinness of the coating and limited metallurgical bond to the steel. As a point of comparison, the typical thicknesses of post-galvanized coatings tend to be between 55 and 200 microns, while in stark comparison, pre-galvanized coatings, including the recently introduced ZM grades, are typically between 5 and 25 microns in thickness. Coating before fabrication also impacts on the steel’s ability to be bent during manufacturing, while the cutting or welding creates uncoated and therefore unprotected areas.
In recent years, however, some steel producers have sought to re-invent the use of pre-galvanized steel sheets by making small additions of aluminium and magnesium to the coating – so-called ‘ZM’ grades. Producers of ZM grade pre-coated steels have diverted attention away from their thinner coatings and the problems of cutting the pre-coated sheet during fabrication by citing laboratory accelerated testing and short-term exposure tests that can overstate the performance of these types of coating.
Read more here: