When Is a Car Too Rusty?

Still, rust is just a part of life, like a storm rolling in two hours after you washed your car. It happens, and you just have to deal with it. It's up to you to fight the rust and prevent it from taking advantage of you and your car. This guide will explain what rust is, why it happens, and help you figure out when a car is just too rusty, specifically if you're inspecting a used car that you might buy. Take note.

Rust is an iron oxide that appears when iron or iron alloy metals (steel) oxidize, which is when a compound bonds with oxygen. When unprotected iron and iron alloy metals are exposed to air and water, they degrade, flake, and corrode in a characteristically reddish brown color. 

All metals do go through some form of oxidation, but certain metals like gold, copper, and silver don’t rust because of a process called passivation. In passivation, the oxidized layer bonds with the fresh metal underneath, protecting it from the elements. Iron does not do this, inherently. When it oxidizes, it does not bond to the good metal, it just flakes away.  

That oxidation is more than just an ugly nuisance because it affects the integrity of the metal. When the topmost layer starts to oxidize, it flakes away and exposes the good metal to the air. That good metal then starts to oxidize, and that cycle won’t stop until no metal is left. Salt and water accelerate the oxidation process.

Because cars are made of steel, an iron alloy, rust left unchecked can seriously compromise vehicle safety. It can eat through suspension members, subframes, and unibodies. Any unprotected steel piece will eventually rust, which is why extensive measures are in place to prevent it from happening.

Cars do have a form of passivation; paint on auto parts or body panels can stave off the oxidation process, but any sort of nicks or breaks in that layer or protection will expose the steel to the air and start the oxidation process.  

It’s silly to assume that the mere presence of rust is equivalent to a structurally compromised vehicle that can no longer be safely driven. Surface rust, or a small layer of oxidation, is unavoidable in many locales. Even if a part has surface rust, the metal underneath still has the vast majority of its integrity. Some sandpaper and paint would be adequate to passivize the metal and stop the corrosion process. 

But, when things move beyond the surface level is when integrity is compromised. Metal pieces that have rust holes, or intense loss of metal, have seriously lost their structural integrity.

Modern vehicles are often made of steel, but not all parts are made of the same steel (iron alloy) concoction. Some parts might be made of stainless steel, which has chromium to encourage a passivization process natural to the metal. Other parts might have other additives in the alloy to slow oxidation or may not be painted. 

Sometimes vehicles have design defects that inadvertently encourage corrosion. For example, early models of the Jeep Compass, Dodge Caliber, and Jeep Patriot’s AC condenser drained directly onto the front subframe. The water would pool on the subframe itself, which encouraged premature corrosion and rusting. 

In an ideal world, state inspection would give us concrete criteria that would let us know when a car is too rusty. States like Pennsylvania and New York check for rust, disallowing tattered rocker panels and subframes on the roads without repairing, but those inspections aren’t nationwide and aren’t infallible. People have been known to cheat them. 

A rusty frame, subframe, or unibody with severely compromised metal isn’t safe to drive. The vehicle’s subframe plays a crucial role in holding the suspension and engine in place and often absorbs some of the forces from a crash. A vehicle with a compromised subframe isn’t safe to drive because it provides the main part of the vehicle’s skeleton. Similarly, any vehicle that has structure-compromising rust on shock towers, suspension members, or too much on the unibody itself isn’t safe. The weakened metal pieces aren’t able to do their jobs effectively and are more prone to failure.

Check out this video from South Main Auto in Avoca, New York; a Subaru with clean-looking body panels caused a customer not to look closely at the copious rust underneath that made the car borderline unsafe to drive. 

Can and should are two different things. At its core, repairing rust involves stopping the oxidation process, passivizing the surface, and if necessary, replacing the lost metal with new metal to gain back that lost metal strength.

In practice, that whole process can be extremely complicated. Some parts, like suspension members, body panels, or even subframes, can be repaired with some sanding and a coat of paint to stop the rusting. If they’re in particularly horrid shape, replacement parts can be swapped out for new units.

However, other parts can be more complicated to repair. For example, a rusty unibody with holes in the floor will require welding, if not a custom fabrication of new parts, to fix the damage done by rust. This can cost a few hundred dollars for a basic repair to thousands of dollars for restoration-level work. For a classic, valuable, or sentimental car, this process is somewhat common. For a not-so-special car, the cost of labor and materials often isn’t worth the trouble or may amount to more than the vehicle is worth, and it essentially becomes scrap. 

These are some key areas to check for excessive corrosion and rusting. If these places have holes, or any signs of excessive, strength-reducing rust, you will need to consider repairing them very soon.

You must ask yourself what the vehicle is worth to you? Do I have the funds to make it, at the very least, safe enough to drive legally? If you can’t, it may be best to sell it to someone who can or send it to the great scrapyard in the sky.

Don’t be scared of rust; it’s a part of life. Cars are funny-shaped things made of materials we got from the earth, and they aren’t invulnerable to the elements. Keep your wits about you, and I promise, figuring out if a car is too rusty to work with isn’t that hard.