55 % Aluminium-Zinc Coating on Steel Sheeting:
1. The aluminium-zinc (AZ) coating on steel sheeting consists of nominally 55 % aluminium, 43
applied by hot dipping in a molten bath
2. It is produced in accordance with ASTM A 792, and is sold in numerous countries worldwide,
Zincal, Zincalume, Aluzinc and Galvalume.
3.Galvalume actually is a trade mark name steel coil coated with a metal alloy name which wa
4.Galvalume is more corrosion-resistant than galvanized steel, because aluminum provides bar
protection, scratches and cut edges are less protected. Galvalume is offered in both bare and
5.Galvalume has an excellent performance life in bare exposures as well. Proven corrosion res
Galvalume sheet steel is intended for uses requiring superior, long term corrosion resistance
6.Galvalume can generally be expected to provide at least 35 years of performance before req
projected to have a service life between 40-60 years in industrial and rural environments, base
installations
Comparison with Hot dip Galvanised Steel
1.In the case of conventional galvanized steel, the coating consists essentially of pure zinc with
metallurgically bonded to the steel substrate. The thickness of the intermetallic layer is suppre
0,2 % aluminium to the moltenzinc bath. The zinc layer provides sacrificial protection where th
cut edges or where there is heavy scoring. The zinc surrounding the bare steel protects the ste
and by the production of insoluble corrosion productswhich impede further corrosion. The zinc
fully covering the steel, thereby acting as a barrier-type coating.Corrosion of the outer layer of
protect the zinc below fromcorrosion (basic zinc carbonate). In time, some of this protective lay
corrosion of the zinc below. Corrosion protection stops when all of the zinc has dissolved.
2.In the case of AZ coating, the layer essentially consists of two components: an aluminium-ric
mixed. The considerable improvement in corrosion performance is essentially due to the coatin
typical crosssectional view of an AZ coating in comparison with that of a conventional galvaniz
shown in figure 2.
Corrosion Resistance
The enhanced corrosion resistance provided by the AZ coating compared with the conventiona
principle that when corrosion occurs, the layer of the interdendritic zincrich phase, at the oute
corroded and the products of the zinc corrosion at the surfaces of the interdendritic areas beco
the aluminiumrich phase, thereby effectively stifling the further corrosion of the remaining zin
remaining surface is rich in aluminium, and the stable oxides of aluminium that form during no
continue to provide an effective barrier against corrosion. Conventional galvanized coatings co
atmospheric corrosionresistant protective life is directly proportional to the original coating thi
there is an additional aluminium barrier coating, as well as a zincrich component that effective
corrosion at cut edges and where there is other mechanical damage (deep coating surface scra
After thirty years exposure at moderate marine, industrial and rural atmospheric sites, cut edg
free of iron rust corrosion products, exhibiting only superficial staining effect
Case Study on Corosion
Conclusion
Although conventional hot-dip galvanized steel has been used for many years and pre-dates AZ
years of successful use with performance data in excess of 30 years being recorded. The publis
enhanced corrosion resistance of AZ coatings compared with the conventional galvanized coat
g:
ominally 55 % aluminium, 43,4 % zinc and 1,6 % silicon, and it is
merous countries worldwide, under a variety of brand names such as
a metal alloy name which was invented in bethlehem on 1972
ause aluminum provides barrier protection instead of galvanic
e is offered in both bare and pre-coated versions.
s well. Proven corrosion resistance, long service life expectancy
term corrosion resistance
s of performance before requiring major maintenance, and is
and rural environments, based on long term data from actual building
s essentially of pure zinc with a thin iron-zinc alloy layer
intermetallic layer is suppressed by the addition of approximately
acrificial protection where the steel substrate is exposed, such as at
e bare steel protects the steel by preferentially going into solution
e further corrosion. The zinc also provides corrosion protection by
orrosion of the outer layer of zinc produces corrosion products that
e, some of this protective layer dissolves in the rain,causing further
the zinc has dissolved.
mponents: an aluminium-rich phase and a zinc-rich phase, both well
essentially due to the coating microstructure. Figure 1 shows a
at of a conventional galvanized structure as
mpared with the conventional galvanizedcoating is based on the
c zincrich phase, at the outer surface of the coating, is preferentially
he interdendritic areas become entrapped between the dendrites of
orrosion of the remaining zincrich phase within the interstices.The
uminium that form during normal atmospheric weathering will
ional galvanized coatings consist primarily of zinc and its
nal to the original coating thickness. In the case of the AZ coating,
ich component that effectively provides sacrificial protection against
ge (deep coating surface scratches due to handling damage etc.)
al atmospheric sites, cut edges of the AZcoated test panels remained
ning effect
many years and pre-dates AZ coatings, the latter also have many
rs being recorded. The published data substantiates the claims of the
onventional galvanized coating.
