122 Steels: Metallurgy and Applications 9 Nizec: electro-deposited 87% Zn 13Ni
9 Durasteel: Nizec + 1.0 ~tm organic coatingDurasteel is one of a number of proprietary steels with a thin organic coating
which is intended primarily as a lubricant for complex forming operations but
which also provides corrosion resistance. After forming, the organic coating is
on the inside surface of the car where it is subsequently electroprimed but not
finished with a surfacer or topcoat.
The substitution of uncoated steel by coated products in automobiles introduces
a number of production implications and the experience in the Rover Group has
been described by Thompson. lss One very important consideration is that the
zinc surface is significantly softer than an uncoated steel surface and is therefore
more susceptible to damage during decoiling and handling. However, these prob-
lems have been overcome at Rover by training programmes rather than facility
changes. The soft nature of the zinc coating can also cause zinc to be wiped off
the steel by the tools during deep-drawing operations. However, it is reported
that the wiping action has little effect on the corrosion behaviour as only a small
amount of zinc is actually removed and the coating maintains sacrificial protec-
tion. On the other hand, the pick-up of zinc on the tool necessitates more frequent
tool cleaning and a reduction in the output of the press.
Thompson reports that the blanking process can leave small particles of zinc
around cropped edges which could deposit on the tools and produce surface
imperfections during pressing. However, this problem has been overcome by the
installation of blank-washing equipment. Although there was concern initially
that the introduction of coated steels might require major press tool modifi-
cations because of the different frictional characteristics of the surface, no tool
modifications were necessary and the problems have been overcome by improved
housekeeping and routine maintenance. In relation to the welding implications,
the author comments on the need for changes in machine parameters to cope
with the increased current (10-20%) requirements of coated steel and also on
the greater frequency of weld tip dressing due to the build-up of zinc on the tip
surface.
Reference was made earlier to Durasteel, which has a 1 ~m organic coating
over a zinc-rich substrate. However, duplex coatings with organic components
of 5-7 ~tm thick are also finding application in car body construction in order
to improve the corrosion performance and resistance to stone-chip damage.
Bowen 189 has stated that the incidence of stone-chip damage has increased due
to the greater use of road salt and also because of changes in vehicle design.
The latter is related to the practice of body streamlining in order to reduce air
resistance and the drag coefficient. Thus the paintwork on a sloping bonnet has
become subjected to greater attack than was the case on unpainted radiator grilles.
This problem is also exacerbated to some extent by the fact that zinc coatings are
inferior substrates to bare steel from the point of view of achieving good paint
adhesion and therefore stone-chip damage is more evident. Bowen states that this
problem can be overcome by using duplex coatings and, as an example, describes
the use of Zincal Duplex, an electro-galvanized steel coated with Bonnazine 2000,
an epoxy-based primer, filled with metallic zinc and aluminium and containing