GALVANISED FOR ACTION
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…
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.
LIVE FOREVER
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.
THINK ZINC
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.