Galvanized steel girds cars with corrosion-resistant properties
Attleboro, Mass. — Will the domestic auto industry's dash toward two-sided galvanized steel end car corrosion? Rust has long been a nagging and costly problem, not only for car buyers but for the manufacturers themselves, destroying mechanically young cars far before their time.
Zinc-coated steel is one more step toward winning the battle against rust, but there is still some distance to go.
While it should stop perforation rust, it could result in blistered paint, warns Dr. Robert Baboian, head of the electrochemical and corrosion laboratory for Texas Instruments (TI).
``The two-sided galvanized steel has a zinc coating,'' Dr. Baboian explains. ``On the inside, the zinc coating acts to prevent perforation from the inside out. But on the outside you paint on top of the zinc and zinc is a very active element.'' A few cars now on the road with double-coated steel show damage to the paint.
``The two-sided galvanized steel ought to be watched very closely,'' urges the TI chemist, ``especially in the areas where there are bolts, fasteners, and the like.''
A Chrysler survey a few years ago found that in the Northeast, a car that was on the road for two years had about a 20 percent chance of having rust perforation, a four-year-old car about a 60 percent chance, and a six-year-old car, 90 percent. A TI survey last spring, by contrast, indicated no perforation rust on vehicles that had been on the road for six years or less.
General Motors is already using the first-ever application of rust-resistant, double-coated galvanized steel on all body panels of the restyled 1986 Cadillac Eldorado and Seville, Buick Riviera, and Oldsmobile Toronado, with perforation warranties up to five years or 60,000 miles. Swedish importer Saab warrants its cars for six years against perforation due to corrosion.
By 1987, some car lines will be warranted for 10 years against perforation rust and five years for cosmetic corrosion on the outside of a car. Chrysler's minivans, the Dodge Caravan and Plymouth Voyager, are already built to resist rust-through corrosion for 10 years.
Cosmetic corrosion is a car of another color, however.
The TI survey also points to cosmetic corrosion, such as pitting, in many of today's low-mileage cars, both domestic and import.
At the root of the rust problem are not only the road deicing salts, but also acid rain. Combined, the results are devastating. ``When you combine the two,'' Baboian explains, ``the effect is much worse than when they are separated.'' The corrosion rate of automotive steel can be anywhere from 2 to 10 times as great when acid rain is combined with road salt. About 12 million tons of road salt are used each year, compared with 2 million tons in the mid-1950s.
``Acid rain is making the environment more and more severe,'' Dr. Baboian says. The average rainfall for Boston, for example, has a pH of 4, but perhaps five times a year there is a rainfall with a pH of 2. ``That's just like vinegar,'' the chemist explains.
All of this has had a severe impact on paint. To extend the lifetime of paint, automakers now are switching to base coat/clear-coat paint, but there are negatives to the clear-coat system as well. Not only is it more costly to apply, but also more costly and difficult to repair.
As for the trim area on cars, anodized aluminum has long been used by carmakers, but when it is exposed to acid, such as the sulfuric acid and/or nitric acid in rain, it loses its original brilliance and turns a milky white. As a result, automakers are starting to move to more expensive trim systems, such as stainless-steel-clad aluminum. The stainless steel on the inside protects the body from corrosion.
All carmakers are using stainless-steel-clad aluminum on some of their cars, especially the more costly ones. On other models, notably the lower-cost cars, the producers still use anodized aluminum as well as solid-stainless-steel trim. On a $6,000 or $7,000 car, for example, you would probably get anodized aluminum, so you can expect a corrosion problem.
``The problem with solid stainless steel is that it can cause galvanic corrosion to occur, which is very similar to what had happened on the Statue of Liberty,'' Baboian reports.
The Japanese have probably had more cosmetic-corrosion problems than any other country, because most of their attention has been diverted to perforation rust, a massive problem in the 1970s.
To combat the problem, Japanese automakers have covered the aluminum trim with plastic, but this allows the road-salt liquids to creep up into the interface between the metal and the plastic. Corrosion results.
Japanese carmakers now are looking at stainless-steel-clad aluminum.
Charles E. Dole is the Monitor's automotive editor. Attachment of trim makes a difference in car corrosion
No matter how corrosion-resistant a car's trim may be, the way it is attached to the car plays a part.
``If you are using anodized aluminum or stainless-steel-clad aluminum, the attachment of the trim is not as important as with stainless steel or plastic,'' says Dr. Robert Boboian, head of the electrochemical and corrosion laboratory at Texas Instruments.
``The aluminum provides protection to the body so you can successfully use metal clips,'' he says. ``If there is any paint damage in the area of the fastener, the aluminum protects the body, so you don't have to worry too much about corrosion.''
When it comes to stainless steel, however, ``you want to use all plastic fasteners so you don't electrically bridge the stainless steel to the body of the car.''
Up to now automakers have not been successful in isolating stainless steel from the body, despite the use of all-plastic clips, plastic underneath the trim, and the like. None of it has worked.
``It hasn't worked because after the automobile has been on the road for a year or so, contact is made between the stainless steel and the body in a number of ways,'' reports Mr. Baboian. ``You could scratch the paint and make contact. Sometimes the plastic clips themselves become conductive and form the bridge.''
Plastic is the same way, he says. ``If you attach the plastic to the body and break through the paint, the paint will corrode,'' he explains. ``In some cases the carmakers use an adhesive, but the trim piece sometimes falls off.''--