92 Steels: Metallurgy and Applications
55% AI-Zn
In 1972, the Bethlehem Steel Corporation introduced steel strip with a coating
of 55% AI 43.5% Zn 1.5% Si under the name Galvalume. Since that time, the
manufacture of the product has been licensed throughout the world and it is
marketed in the UK under the name Zalutite. The 55% A1-Zn coating is applied
by the hot dip process route in a similar manner to that used for the conventional,
plain zinc coatings. On cooling from the coating bath, an aluminium-rich phase
is the first to solidify and makes up about 80% of the volume of the coating. The
remainder is made up of an interdendritic, zinc-rich phase and an AI/Fe/Zn/Si
intermetallic compound bonds the coating to the steel substrate, providing further
resistance to corrosion. Silicon is added to the AI-Zn alloy in order to restrict
the growth of the brittle intermetallic layer.
The coating was developed specifically to provide an improved corrosion
performance compared with plain zinc coatings and benefit is derived from the
separate effects of zinc and aluminium. In the initial stages of corrosion, attack
takes place preferentially on the zinc-rich phase until its corrosion products stifle
further activity in these areas. However, as well as acting as a barrier to the trans-
port of corrodents, the zinc also provides sacrificial protection at cut edges and
areas of damage. As the zinc-rich phase is leached away, corrosion protection
is provided by the alurninium-rich phase which forms protective films of oxides
and hydroxides on the surface of the material. As indicated in Table 1.10, the
55% AI-Zn layer provides between two and four times the life of conventional
zinc coatings, depending on the nature of the environment.
The material is produced with standard coating masses of 150 (AZ 150) and
185 (AZ 185)g/m 2, including both surfaces. These values equate to coating
thicknesses of 20 and 25 ~m respectively, on each surface. The forming proper-
ties of the material are generally similar to those of continuously annealed, hot
dip zinc-coated steel. However, it has an increased tendency to spring back and
lacks the self-lubricating properties of hot dip zinc coatings. The application of
an effective lubricant is therefore essential.
Table 1.10 Comparative corrosion losses as a decrease in thickness (microme-
tres) for 55% Al-Zn alloy coated and hot dip zinc-coated steel strip at Australian
test sites
Site Years exposed (A) Hot dip (B) 55% AI-Zn Ratio A/B
zinc-coated alloy
Otm) (ttm)Severe marine 2.5 a 16.8 5.2 3.2
Industrial marine 7 10.5 4.7 2.2
Industrial 7 9.8 3.4 2.9
Marine 4 5.9 1.4 4.2
Rural 4 1.4 0.8 1.8
aExposure was discontinued after 2 89 years because all the coating on the groundward surface of the
hot dip zinc-coated sample had been lost by that time. No rust on remaining 55% AI-Zn alloy-coated
samples still on exposure after seven years.
After galutite Technical Manual
(British Steel Strip Products).