The metal-munching menace can turn any vehicle to dust in time. Here’s how to fight corrosion and win. By Ben Wojdyla
An ominous brown stain on the fender, a bubble in the paint at the bottom of a door, suddenly soaked floors after hitting a puddle… these are signs the iron worm has been hard at work. Rust.
Otherwise-good cars are regularly sent to early graves because of rust, but the problem is preventable. With iron-based metals, battling oxidation can be a Sisyphean task; in spite of the advanced coatings and alloys developed by chemists and engineers, steel’s unstable chemical make-up means it will always succumb to rust in a natural environment. This doesn’t mean your car is doomed. Understanding the rusting process, the problematic areas, and the ways to address trouble means your pride and joy can stay on the road as long as you do.
Red dust defined
Rust is the layman’s term for the electrochemical breakdown of iron-based metals called oxidation. In this process surface molecules react with oxygen in the air and produce a new molecule, Fe2O3, otherwise known as iron oxide. Iron and most steel will completely reduce to iron oxide and constituent elements given enough time. Very poorly made cars in the ’70s began showing surface rust as soon as they hit the docks, and untreated, raw sheet steel can rust through in a matter of a few years.
Rust preys on the structural and chemical impurities in metal alloys at the microscopic and molecular levels. Pure iron doesn’t oxidise as aggressively; examine an old iron engine block and you’ll see a thin surface layer of rust, but little penetration into the metal. Unfortunately, iron isn’t a particularly good material to build cars out of. Adding a dollop of carbon to iron creates steel, which offers dramatic improvements in flexibility, tensile strength and formability. But by definition this adds impurities – impurities that accelerate the rusting process.
Exposed steel rusts at different rates depending on several factors: alloy components, thickness, the environment the steel lives in, and the type of heat treating the steel undergoes. Alloying elements such as nickel and chromium can be added to stave off rust, but nothing is foolproof; everything eventually corrodes. The effect is accelerated by the presence of any kind of salt. Salts and other contaminants dissolved in water act as electrolytes, and when introduced to the reaction site, they make the exchange of molecular components much faster. In the real world, this means dirty or salty water trapped somewhere in the car’s body makes that spot rust faster – no surprise there. It also explains why cars in northern climates, where salt is used in winter, are prone to rot.
Motor manufacturers do a lot to try to prevent corrosion. A huge amount of testing and materials science is dedicated to keeping your car from dissolving away beneath you. Aluminium and magnesium components are becoming popular because of their light weight, but they also corrode at rates that are unnoticeable within a human lifetime. Those metals, however, are expensive enough to be used sparingly. Modern sheet steel comes off the roll with highly durable coatings. Those are further augmented in the final assembly plants when the freshly made bodies are dipped in baths of anticorrosion agents applied before the painting process.
Many vehicles have a thick coating on the underside that chemically seals the steel against oxidising agents. Unfortunately, the road-facing side of the car is basically a sandblasting cabinet at highway speeds, and those dips and coatings wear off over time.
Regular inspection and covering spots worn bare will keep rust from advancing and causing additional damage. Use primer and paint for light body rust, loadbed liner to repair undercoating wear (yes, the pickup-bed stuff), and a rust neutraliser on frame and subframe rust.
Just a little extra vigilance reduces rust to no more of a problem than any other regular maintenance issue. Of course, rescuing a rust bucket by replacing floors and doors can be fun, but not on your much-loved daily driver.
Stopping rust at any stage
Catch rust early and it’s an easy touch-up job, but ignore the problem and it’s just going to get harder (and more expensive) to fix.
Vehicular decay is largely preventable. The best advice is the most obvious: wash your car regularly to keep the body and underside clean of the road grime and dirt that lead to corrosion. The not-so-obvious advice is to check the drain holes along the bottoms of doors and rocker panels, which allow rainwater to flow out. Use a pipe cleaner to clear these out , and keep the car’s nooks and crannies dry.
2. Surface Rust
Most surface rust happens when paint breaks down through mechanical or UV damage. Structurally, surface rust is not a problem, and depending on the metal’s thickness and alloy composition, a level of “passivation” may be reached (see Glossary below). Regardless, it’s best to correct surface rust as soon as you see it. The fix is not unlike general paint repair. Start by using an abrasive wheel or sandpaper [ 2 ] to cut through the paint and corrosion until clean, bright metal is visible. Next, apply primer, followed by paint [ 3 ], then clear coat. Buff to blend the finishes [ 4 ].
So you didn’t correct the rust when it was limited to the surface, and now you’ve got a bubble. Molecules of rust are physically bigger than those of iron or steel. As a result, rust self-propagates by expanding and flaking away, exposing fresh base metal that begins corroding in turn. When rust penetrates into the surface it causes a rough, pitted type of damage called scale. Correcting scale means getting through the rust with a wire brush [ 5 ], knocking down roughness with a grinding wheel, and attaining a smooth surface with sandpaper. Then apply a coat of primer and paint.
Eventually, the base metal flakes away and leaves holes. Now you’ve got a bigger problem, and you’ve got two options. You can completely replace the affected panel (tough), or you can cut the rotten parts out and weld “patch panels” into place [ 6 ] (tougher). A rusted-through frame means the structural and crash integrity of the car is questionable, and it should be inspected and repaired by a qualified repair facility.
Used-Car Rust Inspection Points:
● Top inside edge of the fender well
● Spare-tyre well in the boot
● Driver-side floor
● Front and rear corners of the rocker panel
● The lower section of all quarter panels
All raw metals oxidise. (Some, such as sodium, oxidise so fast the process is observable with the naked eye.) But a thin layer of oxidised metal actually slows the chemical reaction, making the surface “passive”. Among hot rodders it’s called patina.