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Resin film keeps rust at bay

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 cost.
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?

Organic film

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 treatments.

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.
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 steels.
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 Japanese technologies.


Project Title:  
Body in white boxed parts construction using an EG coated steel sheet with a thin organic film applied upon

Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: ECSC 7210-ZZ-612

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record