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Regardless of customer anticipations, hot-dip galvanized structural steel has a grey coating, rather than a shiny coating. This usually leads to disagreements between galvanizers and their customers. This article explains the cause of grey coatings and their impact on the performance of hot-dip galvanized steel.
Why are Certain Galvanized Coatings Grey?
Hot-dip galvanized coatings are made by the metallurgical reaction between zinc and steel. This reaction creates a variety of zinc-iron alloys resembling needle-like crystals that form on the surface of the steel. With traditional galvanized coatings, the alloy layer constitutes about 80% of the coating, while zinc makes up the outer 20%. This surface layer gives a lustrous appearance to galvanized coatings.
When this surface coating of free zinc is not present, the zinc-iron crystals become visible and its exterior gives the coating a matte grey or silver appearance. When the steel is taken out from the galvanizing bath, the coating always appears shiny. The coating’s appearance changes to grey as the residual heat from the galvanizing process facilitates the reaction between zinc and the steel to carry on, until all the free zinc on the surface is consumed, which leaves the coating with 100% alloy layers.
What Causes Certain Steels to Produce Grey Coatings?
The reaction between zinc and steel in the galvanizing process is a function of many factors. The most significant reasons with regard to grey coatings are:
- The chemical composition of the steel
- The thickness of the steel section
- The galvanizing bath temperature
- The cooling rate of the steel after galvanizing
Among these, the chemical composition of the steel is the most important. Two alloying elements, specifically phosphorus and silicon, will enhance the reaction rate of the zinc with the steel. If the silicon content goes over 0.20%, or the combination of the percentage of silicon is over 2x, or the phosphorus level goes over 0.25%, then a grey coating is likely to be formed.
Most of the Australian-made steels are “galvanizer friendly” with silicon and phosphorus levels kept within permissible limits. Since about 35% of the steel used in Australia is currently imported, it has become very difficult to control the grey coatings because of the change in steel chemistry.
More than 20-mm steel section thickness comes into play because the greater mass of steel can hold heat for a longer period of time. The reaction between zinc and iron will carry on even when the zinc has solidified (at 420 °C) as a solid-state reaction, until the temperature drops below 390 °C. Thus, the fabrications of heavy plates will form thicker, grey coatings, regardless of the steel chemistry.
The galvanizing bath temperature will have an impact only when the galvanizing bath is worked at above the standard 455 °C level. This can only be realized in special ceramic-lined galvanizing baths, as high operating temperatures will deform traditional steel galvanizing baths.
The cooling rate of steel after galvanizing impacts the appearance of the coating. Air-cooled galvanized items are more vulnerable to the creation of grey or partly grey coatings when compared to items that are quenched directly after being removed from the galvanizing bath. This occurs because the quenching halts the solid-state zinc-iron reaction before all the free-zinc on the surface of the coating is consumed.
What Effect Do Grey Coatings Have on Coating Performance?
Grey coatings without exception are thicker than shiny galvanized coatings on corresponding steel sections. Australian and International galvanizing standards have fixed a minimum galvanized coating thickness of 85 µm on structural sections that are over 6 mm in thickness.
Grey-galvanized coatings have nearly doubled this thickness and on heavier sections tend to increase above 200 µm in thickness. Since the life of the galvanized coating is more or less directly proportional to coating thickness, a substantial increase in service life can be expected from these heavier coatings.
The main issues associated with grey coatings are their aesthetics, and the fact that the zinc-iron alloy layers are also inflexible and may be vulnerable to mechanical damage if exposed to the effects of transportation and erection. In such conditions, traditional shiny coatings exhibit exceptional resistance.
An extra advantage of grey coatings on galvanized steel is that they provide a good substrate for painting because of the matte surface. BHP manufactures a galvanized sheet product called Zincanneal. Here, the mill produced shiny galvanized coating is converted to a 100% alloy layer coating by post-heat treatment to improve the paintability of the product for white goods production.