The process of hot dip galvanizing starts with rigorous inspection, ensuring suitability for galvanizing. This includes checking for pre-existing coatings and ensuring appropriate holes are in place for ventilation. Next step is to ensure steel is cleaned and prepped. Items are suspended on hanging frames and placed in a series of pre-treatment chemical baths to remove rust, contaminants and light pre-existing coatings. Once clean the items are moved into a bath of molten zinc with a nominal operating temperature of 450 degrees celsius.
The coating is achieved when zinc reacts with iron contained in the steel's surface. A unique layered protective system is formed as the zinc 'galvanizes' with the base metal; covering corners, sealing edges and penetrating all internal and external recesses. Unlike paint based systems, the coating metallurgically bonds with the entire surface area of the item and does not shrink from the edges of the steel sections.
Three alloying layers Gamma, Delta and Zeta form on the surface of the steel. Harder than the base metal which is typically 150 DPN (Diamond Pyramid Number), these layers provide the durability and the high resistance to abrasion for which hot dip galvanizing is recognised. Iron content through the layers ranges from 6% - 25% producing hardness level between 179 DPN - 244 DPN.
Eta, a fourth relatively soft, pure zinc layer forms on top. Should this outer layer sustain impact or damage, the harder inner layers continue to offer abrasion resistance together with cathodic protection if bare metal is exposed. In addition to the physical barrier provided by these four layers, a fifth layer called the patina forms over a period of time after the item is despatched.
The patina is a series of films on the surface initially consisting of zinc oxide and zinc hydroxide. A final transition occurs as the oxides react to carbon dioxide in the air forming a dull grey zinc carbonate coating.
It is the formation of the zinc carbonate film which changes the lustre of previous bright coatings to a matte weathered appearance. Subject to the conditions of the immediate environment the item is located, this transition may occur quickly or over a period of months. The formation of the patina (weathering) completes the protective armour of the hot dip galvanized coating and is a critical key to long term corrosion protection.