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Rust on Cars: What Causes It and How to Prevent It

By Michael O'Connor | September 24, 2021

When it comes to exterior vehicle care, rust prevention is one of the most important considerations. Rust not only looks unsightly, but it can also contribute to structural issues and reduce the resale value of your car. Keeping rust off your vehicle is one of the best ways to make sure it stays safe and retains its value.

Understanding why rust forms on cars can help you prevent it from appearing in the first place. There are some specific reasons that rust ends up on cars and avoiding them can keep your vehicle’s interior and exterior in good condition. With enough preparation and prevention, your car can stay rust-free no matter where you live or how common it is in your area.

What Causes Rust on Cars?

The technical term for rust is Hydrated Iron Oxide, or hydroxide of iron. It is an electrochemical reaction that occurs when iron is regularly exposed to oxygen atoms and water and presents itself as a brown, flaky coating. However, this red rust isn’t just an unsightly addition to your car. The reaction of iron to moisture and oxygen can also quickly eat away at the metal and cause structural issues if it is allowed to remain on the surface.

Rainy weather is one of the number one contributors to rust on vehicles. If you live in an area where rain is common, the iron in your car could potentially react with the moisture and start the rusting process. This is especially true if your area has a lot of pollution or smog, as this can cause more oxidation with the rain.

Salt is another common contributor to rust on vehicles. This is why rust is so common in areas where chloride salt is used to remove ice and snow from the roads. It can get lodged in the underside of the vehicle and when moisture reacts with it, rust forms. This is because electrons move more freely in salty water and can speed up the oxidation process.

Lack of Maintenance:
A general lack of maintenance on your car will make rusting all the more likely. This is partly because you could end up with leaks that you don’t even know you have, which could turn into rust. It is also because you might have smaller instances of rust that will eventually get bigger and become more of an issue as time goes on.

Difference Between Rust and Corrosion

Rust and corrosion are often used interchangeably. While they are connected, they don’t mean the same thing and it is important to understand the difference between the two. They are both an electrochemical process that results in oxidation, but the difference lies in the types of objects and materials that each of them affects.

The main difference between rust and corrosion is that rust is a type of corrosion. Corrosion is a catch-all, umbrella term for the deterioration of materials due to exposure to carbon dioxide and oxidation. This is also sometimes called green rust. It can affect almost any kind of surface including non-metals like ceramics and polymers. It also requires exposure to air or other chemicals, whereas red rust, which is what we typically think of when we think about rust on cars, needs a combination of exposure to oxygen and moisture to form on metal surfaces.

Where on a Car is Rust Most Likely to Occur?

While rust can form just about anywhere there is metal on a car, there are a few places where it is much more likely to occur. This is usually a result of the area being closer to the road or more exposed to the weather. Any time metal objects on a car are close to the elements, they have a better chance of developing rust and oxidation and starting to corrode.

Stainless steel is an exception. It can resist rust because it has a thin protective coating of the chemical compound chromium oxide on the surface. This protective layer acts as a rust inhibitor and can even repair itself when rust starts to form. However, most vehicles do not have stainless steel components, which is why rust can be such a common issue. Metal can also be put through a process called galvanization, which covers it in a layer of zinc. Unfortunately, galvanic steel is also uncommon in vehicle production.

Frame Rails:
Your vehicle’s frame rails are one of the most common places for rust formation. This is because they are so close to the road and will be one of the first areas to be hit by salt and water as you drive. The main problem with this is that the frame rails are an incredibly important structural element for your vehicle. Developing rust here could potentially cause more damage and structural problems down the line.

Wheel Wells:
The inside of your car’s wheel wells is another common area to get rust. This is because salt and moisture are thrown up into the wheel wells as the car drives down the road. The metal here is quite thin, so any rust could potentially turn into holes as the rust breaks down the body.

Your exhaust system is very susceptible to rust and oxidation. This is due to its proximity to the road as well as the fact that it is made of a thinner metal just like the wheel wells. Your exhaust system also produces moisture from condensation as gasses exit the engine. This moisture sits in the exhaust system until it is expelled, which can lead to rust holes and other issues.

Your vehicle’s suspension is also very close to the salt and moisture of the road. Because of this, it can develop rust oxidation, which is a major problem with the suspension. If the areas where the suspension meets the frame are rusted, they can become weak and end up causing structural and handling issues.

Trunk Floor:
You should always pull up the carpet in your trunk when checking for rust. This area can develop holes from rust and oxidation due to being on the underside of the car. It is also relatively out of the way and hidden, so you may not notice that this area is developing rust until it has significantly progressed.

The undersides of doors are another common area for rust to form and turn into holes and other deterioration. Like the frame rails, the bottoms of your doors are located close to the ground and will bear the brunt of road salt, water, and exposure to oxygen. This is a perfect combination for the development of rust.

Where in the United States is Rust Most Common?

While your car can rust almost anywhere, there are some regions of the United States where it is much more common. This is usually due to things like weather and other environmental factors. If you live in one of these areas, it is incredibly important that you watch out for rust spots and get them removed as soon as possible.

Near Seawater:
Coastal towns and areas that get a lot of ocean spray often have high instances of rust on cars. This is due to the high salt content in the air as well as the excess saltwater moisture that comes in off the ocean. Vehicles in areas like Florida or some of the coastal regions of California are much more likely to develop rust than cars that spend more time inland.

The Midwest and Northeast:
The midwest and northeast of the United States are known for having some of the heaviest snowfall in the country. Because of this, there tends to be a lot of salt on the ground during the winter months to help melt some of that snow and ice. This salt gets stuck in the frames and wheel wells of vehicles and when it combines with the moisture it is a perfect recipe for rust.

How to Prevent Rust

Store the Car Properly:
Keeping your vehicle in a garage or covered carport is a great way to prevent rust from forming on the exterior. This can be especially helpful if you live in an area that has a lot of salt in the air. Any time you can protect your car from the elements, it will offer corrosion protection and keep rust from having a chance to form.

Wash the Car Regularly:
Regularly washing your car, preferably with pure water, can do more than just keep it looking nice. Consistent car washes can also be a great method for all kinds of corrosion prevention including rust. Washing a vehicle removes excess salt and drying it thoroughly can keep moisture from accumulating in different areas of the vehicle. It will also give you a chance to give your car regular visual inspections so you can keep an eye out for small rust patches.

Avoid Road Salt:
If you live in an area where salt is used on snowy or icy roads, do your best to keep that salt from accumulating on your vehicle. Wash your car at least every ten days to remove any excess salt and try to avoid driving through deep puddles on salted roads. It can be difficult to completely avoid road salt but any little bit of prevention helps.

How to Remove Rust

While it is much easier to prevent rust, it can also be removed if you catch it early enough. One thing you should look for when it comes to rust on a car is bubbling paint. If you notice this, it is a good sign that rust is starting to form and you can take steps to remove it.

Many instances of surface rust can be removed with fine-grit sandpaper. It may take quite a bit of work, but if the rust hasn’t penetrated to the core of the iron, it can be removed. This is also a great way to make sure the rust doesn’t progress and become seriously deteriorated.

A penetrant like WD-40 can be a great tool in the removal of rust. If you notice rust in hard-to-reach places like the wheel wells or certain suspension parts, you can spray a penetrant in there instead of sanding it off. Penetrants lubricate the area and will stop the chemical reaction that produces the corrosion.

Citric Acid:
If you notice rust forming on smaller, removable parts of the car like the wheel lugs or drain plugs, citric acid can completely remove it. Soak the parts in a solution of citric acid powder and water and leave it for one to two hours. This is an effective way to remove the rust and is much safer than some other methods like electrolysis, which involves shocking the rust off with electrolytes using batteries and water.

Final Thoughts

Rust is a common issue in many parts of the world and can be detrimental to the safety of your vehicle. While it can be removed, it is always best to keep it from forming in the first place. By knowing how rust forms and how it can be prevented, you can take the right steps toward keeping your vehicle aesthetically pleasing and structurally sound.