Classic Porsche


Early in the 911’s life, Porsche recognised the need to protect its products against body corrosion. This realisatio­n resulted in innovation­s in the field of zinc galvanisin­g…

- Words Shane O’donoghue Photograph­y Porsche

Porsche’s innovation­s in anti-corrosion.

Those embarking on the restoratio­n of an air-cooled Porsche appropriat­ely enough need to galvanise themselves in readiness for what’s to come. Never mind the mechanical­s — what’s lurking under the car’s paint? Luckily enough (though this depends on the age of your Stuttgart-crested classic), Porsche was the first series production manufactur­er to use body parts galvanised on both sides. Before we get to what this means, it’s worth refreshing our knowledge concerning rust. In short, rust is the word used to describe corrosion of iron or steel, the latter an alloy of iron. In the presence of moisture or a salty or acidic atmosphere, oxygen in air combines with the iron to make iron oxide (or ferrous oxide). This flakes away from the surface of the body as rust, exposing what’s underneath to the same fate. Not all metals react in this way, though. Stainless steel, for example, forms an oxide layer on the surface which resists further corrosion, as does zinc. Zinc is plentiful and hence cheap. Plus, it bonds well to steel, which is why it has become the default protection against rust in automotive bodies.

In a bid to reduce the onset of rust, galvanisin­g is the process of applying a protective coating of zinc to iron or steel. Electro-galvanisat­ion, incidental­ly, uses

electropla­ting to bond a very thin layer of zinc to steel, but for the purposes of this article, galvanisin­g is taken to mean hot-dip galvanisin­g, where car body parts — or the metal sheets they’re made from — are submerged in molten zinc, as they were by Porsche in the 1970s. The resultant coating provides the protection from corrosion.

Though galvanisin­g can be traced back to (and is named after) Italian scientist, Luigi Galvani, he only indirectly contribute­d to the science behind galvanisin­g through a discovery he made in 1780, relating to the flow of electric current between two different metals. There’s actually evidence of galvanisin­g being used in the manufactur­ing of armour as far back as the 1600s, while a French doctor and chemist, Paul Jacques Malouin, presented a paper to the French Royal Academy in 1742, outlining how iron could be protected by dipping it in molten zinc. At that stage, it was assumed iron was protected by the zinc coating solely because moisture couldn’t get to it. It wasn’t until a Brit, Humphry Davy, carried out further research, building on Galvan’s findings, that the full story was unveiled. Davy presented his research to the Royal Society in London during 1824, revealing that, through “cathodic protection”, a metal to be protected can be connected to one that is more easily corroded. The latter becomes sacrificia­l, extending the life of the core metal, even if it’s exposed to a corroding

substance, such as water. Using this knowledge, Stanislas Sorel, a French engineer and chemist, patented the process of coating steel in molten zinc. He’s credited with naming this ‘galvanisin­g’, referring to the sacrificia­l role of the zinc, rather than the process of coating the steel in it.

Galvanisin­g has been used around the world ever since, but it seems to have taken the automotive world an awfully long time to catch up. Thick-gauge steel bodies were used for decades, which meant more metal was available to corrode before rust caused a serious problem. It’s worth reminding ourselves these were the days when a car’s chassis and body were separate entities for the most part, meaning the structural integrity of the body was less important than its appearance. It might amaze many younger Classic Porsche readers, especially in the context of today’s automotive manufactur­ing processes, but once upon a time, it seemed perfectly acceptable that a car body might exhibit rust within two years of production. Indeed, it was the norm well into the 1950s.

Things began to change during the following decade, when unibody constructi­on became prevalent and car makers pushed the envelopes of design and manufactur­ing in equal measure. Not long after, however, a focus on fuel economy meant a desire for lighter body panels. This sounded the death knell for thick steel bodies. Nonetheles­s, manufactur­ers still didn’t invest in anti-corrosion processes to any great extent, which is why many cars from the 1970s and 1980s suffered terminal rust issues. Not only were the bodies made from thin steel, but the manufactur­ing processes evolved to allow more complex shapes with ridges, curves and other details which turned out to be the perfect breeding ground for aggressive corrosion.


The 911 could have suffered the same fate if Porsche hadn’t identified a need to proactivel­y reduce the onset of rust in its cars. The brand went so far as to create three prototype 911s with stainless steel bodywork. As mentioned at the start of this article, stainless steel forms an oxide layer at its surface, protecting the metal underneath, thereby preventing corrosion. It doesn’t even need to be painted! Indeed, at least one of the prototypes, a 1967 911 S, was never wheeled into a spray booth. Instead, the metal was polished for an attractive finish. The car in question now resides in the Deutsches Museum in Munich and was driven for five years and over 60,000 miles without any rust materialis­ing. Nonetheles­s, after careful considerat­ion, manufactur­ing 911 bodies from stainless steel was ruled out on the grounds of excessive cost — it’s likely the 911’s body would have been twice as expensive to make from stainless steel than it was from standard steel. On top of that, stainless steel is heavier, harder on the tooling shaping it and more difficult to paint. Of course, all of these challenges remain today. Instead, Porsche turned to galvanisin­g.

Porsche itself quotes 1975 as the year it introduced the process to its produce line, but in truth, the company had started using galvanised parts a few years earlier. From 1970, for example, the 911’s floor pan and wheel arches were made from galvanised steel and, over the next few years, more parts were treated until, from the end of 1975, Porsche moved to make the entire car from fully galvanised steel. German industrial engineerin­g conglomera­te, Thyssen (now Thyssenkru­pp), is credited with producing the zinc-coated steel for Porsche production. Thyssen dipped steel sheets into molten zinc at 500°C, varying the thickness of the coating from ten microns (a hundredth of a millimetre) to fifty microns, depending on the component the sheet steel was to be made into and how exposed to the elements or susceptibl­e to rust it was likely to be. Porsche had to experiment with these coated sheets of metal to ensure they could be formed into final body parts without losing their protective coating. It was also necessary to rethink how the components were welded together, not least because toxic gases are emitted during the process. All this cost Porsche a relative fortune, adding an estimated £50 to the manufactur­ing cost of each car, equivalent to £350 today. It also added some 10kg to the overall mass of each finished vehicle. Even so, it gave Porsche buyers

confidence in the quality of their purchase, bolstered by an industry-leading warranty (introduced in 1976) valid for six years against rust of the core chassis. This promise was upped to seven years in 1981, following an extension of the galvanised material to the 911’s roof. In 1986, Porsche raised its anti-perforatio­n warranty period to ten years.


Sadly, those warranties have long expired and, while seeking out a vehicle that was originally zinc-treated is a wise move by any would-be classic Porsche rescuer, finding such a car is no get-out-of-jail card. How much the zinc coating will have deteriorat­ed will depend on the ambient conditions the car was subjected to throughout its life. Was it left outside? Was it parked under trees where lichen could attack the bodywork? Did it live in an area where acid rain is particular­ly bad, or where the roads are regularly treated with salt? All of these things will have contribute­d to breaking down the zinc coating and, while there’s still some protection if the steel underneath is exposed to the elements, it’s only a matter of time before it too begins to corrode.

If you’re in the market for a production Porsche from this era, despite the galvanisin­g, rust is still an issue. Look for it in the side sills, then from the rear of the sills into the back wheel arches, the bottoms of the doors, the floor of the front luggage space, below the battery, around the windows, the rear seat pans, the whole floor pan and, on cars with sunroofs, the screen pillars (the drains like to block with dirt). Of course, by now, with the current value of an air-cooled Porsche taken into considerat­ion, it’s possible someone will have already tried to rectify corrosion, but look closely at the quality of any welding or anti-corrosion measures taken. Don’t panic too much about flaked paint, so long as the zinc coating underneath is intact — in areas heated up by the exhaust or engine, it’s not unusual for paint to come off.

On that subject, unless you have the facility to hot-dip your car’s shell, think twice before diving into sandblasti­ng the whole body to get it back to bare metal.

This process will remove the zinc coating. Instead, though it takes a lot longer, it’s worth considerin­g ‘gently’ stripping just the paint. Obviously, if there is clear and obvious rusting of the core metal, remedial action will be needed — whether you’re cutting out rust or replacing whole panels, if there’s welding involved, make sure zinc protection is considered. Weld-through primers are applied before welding to extend protection to surfaces which can’t be painted afterwards, and inorganic zinc primers are recommende­d because, for a given thickness, they contain more zinc.

And make sure you’re aware of the harmful gases that come from welding zinc-coated steel and, crucially, how to protect yourself from them. This all takes a lot of effort, but just like Porsche’s original investment in bringing galvanisin­g to market, it’ll be worth it. After all, we’re told some seventy percent of all Porsches manufactur­ed are still on the road today. Without galvanisin­g, that figure would be a considerab­ly less.

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 ??  ?? Facing page Porsche Classic’s recently restored 993 Turbo body shell undergoing the cathodic dip coating process
Facing page Porsche Classic’s recently restored 993 Turbo body shell undergoing the cathodic dip coating process
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 ??  ?? Above Though not guaranteed to present a car free from corrosion, galvanisin­g was a massive leap forward in the field of preserving automotive bodies for the long term
Above Though not guaranteed to present a car free from corrosion, galvanisin­g was a massive leap forward in the field of preserving automotive bodies for the long term
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