A process for coating plated steel with a protective resin film,
developed in an earlier Steel Research Project, has been extended
to steel plated with zinc-nickel alloy. The resulting product, designed
by an Italian research centre for use in the automotive industry,
has good resistance to corrosion and can be readily welded, pressed
and painted. Its cosmetic corrosion resistance is good and superior
to some similar Japanese protective resin films, making it suitable
for both internal and external parts of the car body.
Car owners know that the body of their
car is likely to wear out long before the engine and other mechanical
parts. Sooner or later, steel panels and frames will start to corrode.
Not only is the rust unsightly, but if left untreated it will eat
holes in thin panels, weaken structural members and lead eventually
to expensive repairs or the scrap yard. Is there any possibility
of car bodies lasting as long as their engines?
Modern cars are less prone to rust than their predecessors, thanks
to improvements in metal coating and painting technology, but researchers
are still seeking ways of prolonging the life of steel components
against the inevitable onset of corrosion without adding significant
Bare steel is treated with phosphoric acid solutions to produce
a thin coating of phosphate which provides a good surface for subsequent
painting. However, during the second half of the 1980s, car manufacturers
extended the use of galvanised steel, in which the surface is plated
with zinc either by immersion in the molten metal or by electroplating.
Galvanised steel, and other similarly 'pre-coated' steels, cannot
not be readily treated with phosphate. This was the problem that
Centro Sviluppo Materiali (CSM), the Materials Development Centre
in Rome, tackled as far back as 1986. How could pre-coated steels
be further protected against rust?
In this pilot project, CSM looked for a new type of coating that
could be applied to pre-coated steel sheet on an industrial scale
to create a barrier to corrosion. They tested a variety of organic
resins, applied in tough, thin films. Two types of sheet metal substrates
were used: electrogalvanised steel and Multizincrox, a specialised
steel plated with zinc, chromium and oxides of chromium used in
prototype design. Of 17 compounds tested, the best proved to be
a type of modified basic epoxy resin, named 'D1'.
In the process devised at CSM, galvanised steel is coated on both
sides as it leaves the electrogalvanising plant. First, a 'conversion
layer' of chromate or mixed oxides is applied to the surface, then
the resin is rolled on in a layer only one micron thick. Finally,
the sheet is baked at 190 C to set the resin.
Laboratory tests show that the new coated steel has good chemical
resistance (it needs to withstand alkali baths and acid phosphating
on the car production line), and can be welded and shaped without
problems. As a bonus, the lubricating properties of the film reduce
friction and so assist the pressing of steel parts during manufacture.
The new steel was used to make a number of components - side and
rear doors - which were installed on an Alfa 33 car. The car was
then driven around a standard test circuit built by car-maker Fiat
to expose vehicles to many hundreds of hours of corrosive conditions
including salt baths and showers, and high humidity. At the end
of the test, the doors were taken off and examined. Rust resistance
proved to be as good as that achieved with the best available phosphating
Competitor to Durasteel
Although the original research project finished in 1990, CSM returned
to the problem in 1994, after some Japanese transplants of car manufacturers
in Europe began to use a new type of coated steel developed in Japan
Durasteel is a mild steel plated with an alloy of zinc and nickel
(ZnNi) which is then coated with organic composites similar to D1,
developed by different Japanese steelmakers. It has impressive corrosion-resistant
properties and Japanese manufacturers use it for certain internal
components, such as door frames. Several European steel producers
are licensed to manufacture Durasteel, often under other names.
With the hope of developing a competitive product to Durasteel,
CSM was given funding to start this new project to investigate whether
the resin coating process developed in 1986-1990 could be adapted
for use on ZnNi-plated steel.
Using a coating plant, ILVA - Lavezzari coil coating line, the researchers
succeeded in applying a two-sided coating of the polymer to the
ZnNi-steel in the same way as they had to the galvanised steel.
Mechanical tests showed that the film gives the steel good lubricating
properties, reducing friction during forming processes, and less
powdering at deformed surfaces. It can be readily welded and joined
by adhesives, and can be painted in the same way as conventional
All-important corrosion resistance was tested not by making panels
and putting them on a car, but by exposing samples to corrosive
conditions in the laboratory. A 'mini-door', designed in consultation
with Fiat Auto, contained on a smaller scale constructional features
found on full-size doors: a frame and skin, hem joints and spot
welds, and holes for drainage and painting. The mini-door and other
samples were then exposed to a cycle that included 24 hours in a
salt-spray cabinet, 80 hours in a humidity cabinet and 64 hours
drying at room temperature. The cycle was repeated for a total of
1,500 hours. The resistance of the coated steel was approximately
twice as good as that of the coated steel without the organic film.
Although the new coated steel has an initial cost higher than conventional
steels, CSM points out that it gives economic benefits as it would
no longer be necessary for manufacturers to spray wax into enclosed
cavities to protect them against rust. It is also more versatile
than Durasteel, in that the film can be applied to both sides of
the metal, and the steel is suitable for use both inside the car
and for the cosmetically important external surfaces.
Recently, some European car makers have begun to show interest in
steel sheets pre-primed with organic coatings. In this context,
CSM's solution can make a valuable contribution towards European
competitiveness in terms of both quality and costs with similar