Low-carbon strip steels 93The 55 AI-Zn coating has a smooth, silvery appearance with a very fine
spangle and is said to be attractive in the unpainted condition. However, the
material is also supplied in the factory-painted condition and painting is recom-
mended for use in severe marine and very corrosive environments. The coating
exhibits good resistance to heat and can withstand discoloration in air at temper-
atures up to 310"C, whereas a limit of 230"C is prescribed for hot dip zinc
coatings.
95% Zn 5% AI
A coating of this composition was first introduced in 1982 and is marketed world-
wide under the name Galfan. The coating is again deposited by the hot dip process
and, in addition to good corrosion properties, it is claimed to have particularly
good forming characteristics. The structure of the coating is dependent on the
rate of cooling from the bath and cooling rates greater than 20*C/s result in
the formation of a fine eutectic of zinc-rich and aluminium-rich phases. Slower
cooling rates result in the separation of a primary zinc-rich phase.
Hot dip aluminium coatings
Aluminium-coated steel may be produced using a hot dip process and therefore
combines the strength and formability of steel with the corrosion resistance of
aluminium. The steel base may be an aluminium-kiUed steel, but the use of an
IF steel provides high formability and improved higher temperature oxidation
resistance. A product with the basic coating weight may be used for applications
such as motor car silencers, but a product with about twice the coating weight at
250 g/m 2 may be used for building applications. The thicker coating also contains
a silicon addition. This reduces the thickness of the alloy layer and imparts greater
ductility than would be obtained from a pure aluminium coating.
Aluminium-coated steels withstand temperatures up to 5000C for long periods
while retaining their original surface appearance. Above this temperature, the
coating gradually converts to an iron-aluminium alloy layer, but under certain
conditions, it may still retain good heat-resisting properties.
Organic.coated steels
During the 1980s there was a dramatic growth in the production of pre-finished
strip, coated with various types of paint or plastic. These coatings are available in
a wide range of colours and textures and are used to advantage where corrosion
resistance and a decorative appearance are of major concern. The coatings are
applied to a range of steel substrates, but generally to zinc-coated strip, and
are formulated specifically for various manufacturing requirements or end uses.
Coatings are available that provide long life in external applications, good deep-
drawing characteristics and resistance to heat or chemical attack.
The organic coatings are applied continuously as a liquid film or as a lami-
nate which is bonded to the substrate with an adhesive. In either case, the steel
substrate is thoroughly cleaned in a multi-stage process to ensure uniform and
optimum adherence of the coating. In the liquid film route, the substrate is first