Tin worm, metal cancer, rust, corrosion — regardless of what you call it, it’s long been a problem for car enthusiasts. And eliminating or controlling it before it advances too far is the one challenge that unites all car enthusiasts.

First published in the February 2007 issue of Street Machine. The sale prices mentioned were market values at that time.

With the dramatic rise in the value of classic machinery, it makes good financial sense to keep the dreaded metal cancer at bay. Well-known Melbourne restorer Peter Tommasini points out that an HK Monaro recently sold for $220,000. And who would have thought that an XU-1 race car would go for half a million? Other notable sales include $400,000 for a GTHO and $720,000 for an XY with a racing pedigree. Prices like these, and the fact that you can no longer go out and simply pick up another shell, add a whole new dimension to the importance of preventing rust and other corrosion.

THE SOLUTION IS THE PROBLEM

The corrosion process is quite similar to the electrochemical process that takes place in a battery cell; metal atoms are stripped away and deposited elsewhere or transformed into a different material via electrolysis. This process requires an electrolyte; a chemical solution of atoms with either an excess or deficiency of electrons. Such atoms are called ions and they make the electrolyte electrically conductive.

Metals in contact with an electrolyte tend to lose positive ions into it. This action leaves electrons behind in the metal. These electrons then flow to a different region of the metal where they combine with other positive ions in the electrolyte. A metal that loses electrons is called an anode while a metal that gains electrons is called a cathode. This flow of ions consumes the anode. In the case of steel body panels, the metal ions combine with oxygen molecules in the electrolyte to form iron oxide — rust.

There are a couple of important factors that determine the rate at which corrosion proceeds. First, the greater the difference in composition between two metals immersed in an electrolyte, the greater the electron flow between them will be and the faster corrosion will occur. The other major factor that determines the rate of corrosion is the concentration of ions within various parts of the electrolyte. This is of great importance in the case of car panels.

WHAT’S THE DIFFERENCE?

Most of the panels in a car are made from steel, so there’s not a huge difference between any two. Still, there is some difference and there can even be sufficient differences within the same panel for the process to take place — especially when the metal is highly stressed from being formed.

Also, car panels are often joined together by lines of spot welds running along flanges sandwiched together at the edges. This creates narrow spaces that are capable of trapping water. Water trapped in these regions tends to become oxygen depleted, creating an electrolyte imbalance that acts upon metals of the same basic composition as though they were more different.

BEST DEFENCE

One way of preventing electrochemical corrosion is to simply ensure that no water gets into the sorts of locations we’re talking about. Or, if it does, to dry it out before corrosion sets in. This, however, is more easily said than done, as moisture is very invasive. If you get even the smallest chip in the paint, moisture can get under an edge and start the process. The resultant corrosion will remove metal, allowing more water to get in and helping the process continue and spread.

Similarly, the water-based electrolyte at the top of a crack in a metal surface will have a different composition than the electrolyte at the bottom of the crack. Accelerated corrosion occurs at the base of the crack and the problem gets worse. This is a particularly dangerous condition because the corrosion is restricted to such a small area that it may go unnoticed until the crack becomes so deep that a catastrophic failure occurs. Pitting is another form of corrosion that occurs in the same manner.

KEEPING DRY

You’d think that keeping your car in a garage and never driving it would be enough to solve the moisture problem. Not so, according to general manager and head restorer at the Fox Museum, Brian Tanti. He says that even with cars that never see liquid water, temperature fluctuations cause condensation wherever air can reach. This moisture accumulates and eventually rust begins.

Brian further explains that the only solution to this is a temperature, humidity and pressure-controlled, dustless environment. In a car that is driven, dust is a particular problem because it gets into tight areas where it combines with moisture to form a damp electrolytic mud that never dries out.

PASS THE SALT

In countries with severely cold climates, salt is used to clear snow off the road. This salt turns water into a particularly effective electrolyte, making for massive rust problems. Fortunately, snow-covered roads are rare in Australia.

However, we do have a long coastline and the salty air in these vicinities creates an ideal environment for the propagation of corrosion. Humidity makes things even worse. That’s why the best place to store cars, planes or other corrosion-sensitive things is in the middle of a dry region, like a desert.

PROPER INSPECTION

We’ve described what corrosion is but what do you do if you find it? First, the full measure of the problem has to be determined. If rust is showing through your paint, you’ll have to strip it back until you reach good, clean, unaffected metal. This means that stripping away all the paint and body deadener is the best way to get an idea of how bad the problem is.

CURING CANCER

Brian reckons that surface rust is best dealt with by applying rust converter. Different brands have different instructions — these must be followed extremely carefully. But basically the process is to first remove any loose rust with a woven steel brush in a reversible electric drill. Reversing the brush makes use of the directional qualities of a used brush and allows it to dig into the work. The polished surface left by the brush is then removed with a sanding disc and the rust converter is brushed on. When the rust is fully converted, the surface can be prepared for painting.

It’s likely that rust in some areas will have progressed beyond the surface, rendering rust converter ineffective. The only remedy for incurably corroded metal is replacement with clean, rust-free steel. But that’s not the end of it; wherever metal is to be welded and is likely to form a moisture trap, zinc-rich primer should be added or the rust will return, often at a faster rate. The zinc in such primers is more anodic than steel and will therefore be consumed by the corrosion process before the steel.

BEST PROTECTION

If you want your car to last, some sort of protective coating should be applied to every square inch of metal. Externally, paint does that job. For everywhere else, Brian recommends Wurth’s Bright Zinc spray. This offers similar protection to zinc-rich primer. The thing about restoration is that many of the original constructional faults have to be duplicated, which allows corrosion to occur again unless preventative steps are taken. One common remedy is to apply fish oil in such areas and although this works reasonably well, eventually it dries out and has to be reapplied.

An alternative solution is the application of a wax-based Cavity Protection Spray. The applicator wand allows it to be sprayed into tight places and it doesn’t dry out, so the protection it offers is extremely long-lasting. These products form the main defence against corrosion in the Fox Collection and in many new cars.

THE GOOD OLD DAYS

Brian also points out that the chemical composition and bonding qualities of newer types of stone-guard material are much better at preventing corrosion than earlier versions. In fact, almost everything about newer cars is better — as far as corrosion is concerned. That said, highly experienced restorer Gary Stott, from Hoppers Stoppers, believes that current generation cars will be much more difficult to repair when they do rust due to the differences in the steels from which they are constructed.

He points out that to make panels lighter, they’re made from high-tensile alloys that are far more complex than previously. The trade-off is that they can’t be beaten as successfully as older panels made from thicker, softer steel. And as far as corrosion is concerned, modern panels can’t be patched in the same way as the simpler metals. The mismatch of ordinary sheet-steel and modern high-tensile steel (dissimilar metals) would create fairly high potential anodic/cathodic regions and corrosion would set in quickly.

PROBLEM AREAS

Various cars that have achieved cult street machine status are also notorious as far as corrosion is concerned. Gary is a Chrysler enthusiast and points out that Valiants got worse, corrosion-wise, as the models progressed. There are two main problems with Valiants; the first occurs where the steering box attaches to the rail — it invariably corrodes away. The second occurs around older-style windscreens when the seals turn to dust. This lets moisture into the A-pillar, where it runs down and accumulates at the bottom. From there, it corrodes a hole through into the plenum, which becomes the next area for it to collect in and corrode.

The XA, XB and XC hardtops are also infamous for corrosion problems in the plenum area, though their real claim to fame is sills. Once these have rotted out, the cars’ overall structures become quite unstable — which was why high-powered versions needed substantial reinforcing.

This came as a bit of a surprise given that XW/XY models weren’t particularly rusty cars for their time. Holden suffered too; Peter says he was fixing rust in HK Monaro C-pillars by 1974 — fairly quick for a ’68–’69 model. Holden utes didn’t have such a structural problem with rust because the body was mounted on a sturdy chassis which provided support even when body rust became embarrassingly severe.

While all cars have their own problem areas, some of the more common places at which rust occurs is at the bottoms of the doors, at the trailing and leading edges of the rear wheelarches, in the boot-rubber channel, and at the base of the front and rear screens.

Modern manufacturers have to deal with corrosion much more effectively than previously because the bodywork and panels are thinner, meaning the tolerance for rust before structural integrity is compromised is lower. It will be interesting to see how long they last, how we’ll restore them and, as always, which ones will be deemed worthy of restoration.

You can reach Peter Tommasini on 0411 037 135 and Hoppers Stoppers on (03) 9748 6950.

In detail:

1. The lower reaches of a car are exposured to the most water and it’s also where it accumulates. Consequently, the bottom six inches of many older cars are heavily corroded. Peter is replacing just about everything at that level in this car.

2. Corrosion also occurs readily in an engine’s cooling system, particularly between dissimilar metals like aluminium and iron. Here you can see that it’s been eroding the underside of an inlet port.

3. Cavitation in a cooling system can intensify corrosion. Varying pressure due to inefficient flow paths creates bubbles that collapse at supersonic speeds and act as miniature explosions. It’s not related to stray current in the coolant but it’s very destructive.

4. This head has been immersed in salt water for some time. No current was present but there’s still
some pretty serious corrosion involved.

5. Most of the inside areas of this convertible Camaro are quite manageable. However, all those brown (rusty) areas will eventually become chronic if not treated and properly painted.

6. The lower edge from a Porsche door-skin. Note how the corrosion is particularly intense where the skin meets the frame of the door — where the narrow gap becomes a water trap.

7. Surface rust like this is the easiest to deal with. Rust converter does the trick.