Hot-dip-zinc-coated steel sheet, also called galvanized sheet, is by far the most widely used coated sheet product. For general applications, the galvanized sheets have 19 µm of zinc thickness per side. This corresponds to a two-side coating mass of 275 g/m2. Heavier coatings are used in applications which require higher corrosion resistance, such as highway drainage culverts. In the automobile industry, where formability and weldability are key considerations, lighter coatings such as 90 g/m2 are more typical (Townsend, 1993). The galling, a form of adhesive wear, affects the cost of forming steel in automotive stamping since it increases die maintenance costs and scrap rates (Kim et al., 2008).

Most of the coating is nearly pure zinc. There is an intermetallic layer containing about 6 % Fe, between the substrate and the zinc coating.

Aluminium, typically in the range of 0.1 % to 0.2 %, is added to the zinc bath in order to prevent the formation of a thick, continuous zinc-iron intermetallic layer that could lead to poor coating adhesion during forming. Aluminium reacts preferentially with the steel to form a thin iron-aluminium intermetallic layer which acts as a barrier and delays the growth of the zinc-iron intermetallic layer.

Galvalume® is the trade name of steel sheets coated with 55%Al-Zn alloy. It is applied by a continuous hot dip process similar to that of galvanized coatings. This coating combines the durability of aluminium and the galvanic protection of the zinc, resulting in a product which shows excellent corrosion resistance in marine and industrial environments, high temperature oxidation resistance, heat reflectivity of the aluminium coatings and a pleasant and distinctive appearance.

The chemical composition of the coating is 55 % of Al, 43.5 % of Zn and 1.5 % of Si and it is composed of a three-phase structure as described below (BHP Steel, 1994a):

a) a thin quaternary alloy layer, composed of Al, Zn, Fe and Si, between the steel base and the main coating layer;

b) an aluminium rich dendritic phase (80 % by volume) and

c) a zinc rich interdendritic phase.

All the three phases are necessary to provide the improved corrosion resistance properties. The zinc-rich phase gives the coating its galvanic protection ability, a similar property to a normal galvanized coating. The aluminium rich phase and the alloy layer provide to the coating its durability once the zinc-rich phase is consumed. About 1.5 % silicon is added to the molten bath in order to control the growth of the alloy layer during the dipping process (BHP Steel, 1994a).

As the specific weight of aluminium is lower than that of the zinc, the 55%Al-Zn coating is lighter than the zinc coating, both with same coating thickness. For instance, the weight of the zinc coating designed as G90 (ASTM A653) is 275 g/m2 while the weight of 55%Al-Zn coating designed as AZM150 (ASTM A792) is 150 g/m2. Notice that both coatings have about the same thickness of 20 µm per side which results in higher covering area per ton of Galvalume® compared to a galvanized product. As an example of the difference, one ton of 0.45 mm Galvalume® can cover 4.5 % more area than the same thickness of a galvanized sheet (BHP Steel, 1994b).

The 55%Al-Zn coating shows long-term atmospheric corrosion resistance due to the combination of the durability of aluminium which acts as a mechanical barrier and the galvanic protection given by zinc (Gronostajski, 1995). Exposition tests of the zinc coated steel sheet (coating G90) and Galvalume® (coating AZM 150) were done by Bethlehem Steel. The test panels were exposed at rural, industrial, moderate marine and severe marine environments. The results showed that the corrosion resistance of the 55%Al-Zn is generally at least two to four times higher than that of an equal thickness of galvanized coating (BHP Steel, 1994a). Recent work found that the zinc coating experiences a degree of corrosion between 1.7 times (marine-industrial atmosphere) and 4.5 times (rural atmosphere) greater than that of the 55%Al-Zn coating, considering the results obtained after five years of atmospheric exposure (Palma et al., 1998).

The steel sheet coated with 55%Al-Zn alloy can be used at higher temperature conditions compared to the zinc coated steel sheet. In terms of appearance at higher temperatures, Galvalume® maintains its superficial brightness up to 320 ºC, while the galvanized sheet maintains its brightness up to 230 ºC (BHP Steel, 1994c).