Low-carbon strip steels 87The HDG process is also used to produce galvanneal strip in which the
plain zinc coating of the traditional galvanized product is converted to a Zn-Fe
alloy with an approximate composition of 90% Zn, 10% Fe. Galvanneal strip is
produced by reheating the strip after it has left the coating bath and giving it a
thermal cycle to develop a suitable alloy layer of Fe-Zn intermetallic compounds.
The reheating was formerly carried out in a gas-fired chamber but is now more
usually achieved by induction heating. The formation of this layer is critically
dependent on the thermal cycle if the most suitable phase distribution is to be
formed. The coating weights produced on galvanneal steel are at the bottom of
those produced on plain zinc coatings.
Galvanneal coatings have a matt grey appearance of low reflectivity and have
the following characteristics and have a major market in the motor industry:- Similar corrosion resistance as plain zinc coatings.
- Good paint adherence.
- Better resistance welding performance than plain zinc coatings.
Electro-galvanizing
Zinc coatings. In contrast to the HDG process, electro-galvanizing is carried
out at or near ambient temperature, the feedstock having been annealed in the
conventional manner. Prior to the coating operation, the strip is first passed
through a series of chemical or electrolytic cleaning baths to remove dirt or
oil. The plating solution is made up primarily of zinc sulphate and zinc was
previously supplied to the electrolyte by means of zinc anodes. However, zinc is
now more usually supplied by adding zinc compounds to the electrolyte in the
form of zinc oxide or zinc carbonate and the anodes are made of lead. Oxygen
is, therefore, released at the anodes during the plating process. After plating, the
strip is passed through chromate baths for passivation.
87% Zn 13% Ni. Both forming and welding problems are .introduced with the
application of heavy zinc coatings and therefore there is a demand for coatings
that will provide good corrosion resistance, even when applied in thin layers. One
approach to this problem is the incorporation of nickel into zinc-based coatings,
and a coating containing 87% Zn 13% Ni is now being used for automotive
applications. The coating is applied by the electroplating route and the material
is marketed in the UK under the name Nizec.
Fe-Zn coatings. An iron-zinc coating may also be produced by an electrolytic
process and is used mainly in Japan and North America in the automotive
industry. Care is needed, however, in controlling the plating conditions because
changes in temperature and concentration of the electrolyte may cause relatively
large changes in the composition of the coating. The usual coating weight is
about 50 g/m 2 and the iron content of the coating is often in the range 8-20%.
According to Denner, 147 electrolytic iron-zinc coatings are particularly good
for chipping and corrosion resistance and also for coating and paint adhesion.