Whether restoring an old Dutch oven or saving a rake left out in the rain, the ability to remove rust is essential to giving new life to objects of iron and steel. And few methods are easier or more effective than chemical rust dissolvers.
First, a word of caution: For antiques and collectibles, a cleaned item is often worth far less than the same item in original, as-found condition. If value is not an issue, then you are in luck. Start by removing loose rust and dirt. De-grease the object, and wash it with soap and water.
Before selecting the rust remover, consider the item to be cleaned. Does it have paint or plating you want to preserve or remove? Does it have parts of plastic, rubber, or wood that might be damaged? Is the rust light or heavy? Is it localized or widespread? Can the object be disassembled? Is it small enough to soak? How will you be sealing the metal after treatment? Do you have small children or pets to worry about? The answers to questions like these will inform your decisions and possibly narrow your choices.
Most chemical rust removers use acids to dissolve rust. If care is not taken, these acids can often erode sound metal as well. A large number of commercial rust removers are based on phosphoric acid. In very small concentrations, phosphoric acid is the substance that gives colas their bite. It acts quickly on rust, is not overly aggressive on underlying metal, and leaves behind a coating of iron phosphate that serves as a short-term rust inhibitor. Phosphoric acid can remove paint and strip or damage other surfaces and coatings. Although it is not considered a strong acid, phosphoric acid can burn eyes and skin and is harmful if inhaled. Therefore, it is important to wear goggles and gloves, and work in a ventilated area.
Rust removers are available in liquid or gel formulations. In general, liquids require that rusted objects be fully immersed. Gels, on the other hand, cling better to vertical surfaces, stay wet longer and don’t require a deep container.
Rust removers that use phosphoric acid include Rust-Oleum Rust Dissolver Gel, Boeshield Rust Free and Krud Kutter “The Must for Rust” Rust Remover and Inhibitor Gel. Instructions differ slightly. Rust Free should be left on for a minute or less. The others treat light rust in 10 to 30 minutes. For heavier deposits, all require additional applications. They may or may not suggest a final rinse with water, which washes off the acid but can expose untreated metal to oxidation. In some cases, wiping the product dry without rinsing affords additional protection. Krud Cutter, used this way, claims to protect bare metal from rust for up to 12 months.
Some commercial rust removers use milder organic acids or gentler chemical processes. They tend to be safer but somewhat slower. Bull Frog Rust Remover, for example, contains citric acid. Found naturally in citrus fruits, citric acid is used to flavour and preserve soft drinks, candies, jams and many other foods. Bull Frog is billed as environmentally safe and non-toxic. Nevertheless, it can irritate eyes and skin; therefore, goggles and gloves should be used. It doesn’t harm paint, so long as it is adhered to a sound substrate, or damage metal surfaces. For light rust, Bull Frog should be left on for 15 minutes, then rinsed with water. For heavy rust, it should be left to penetrate for up to 2 hours before rinsing. Bull Frog contains proprietary corrosion inhibitors to protect the treated metal from further rusting.
Evapo-Rust is one of the safest commercial rust removers. It claims to pose no environmental or health hazards, is non-corrosive, and will not harm plastics, rubbers, non-ferrous metals and many coatings. Unlike most acids, Evapo-Rust will not damage the underlying iron or steel if the object is left soaking too long. And it can be reused repeatedly, which makes it economical.
Evapo-Rust works by chelation. The word chelation comes from the ancient Greek term for claw. In essence, a chelator is a molecule that grabs on to metal ions. It works by wrestling away the iron from rust, which is an iron oxide. After that, another chemical frees up the chelator so that it can attack more rust. This is what makes the product re-usable. Because the chelator is not strong enough to break the iron-to-iron bonds of sound steel, it doesn’t harm uncorroded metal.
Evapo-Rust can take 5 minutes to 30 minutes to remove light rust; up to 6 hours for medium rust; and up to 24 hours for heavy rust. After treatment, the part can be rinsed with water, dried and sealed. Or the product can be left to dry. As a dried film, it will inhibit rust indoors for up to two weeks.
Some people like to create their own acid-based rust removers. For those who want to give this a spin, citric acid powder is effective, relatively safe, and easy to find in health food stores and online. Here’s the method followed to clean the wrench shown above.
- Brush off any loose rust; clean and de-grease the rusted object. Wear gloves and goggles.
- Add enough warm distilled water — one litre at a time — to a plastic container sufficient to submerge the artifact.
- Stir in one tablespoon of citric acid powder per litre.
- When the crystals have dissolved, gently place or suspend the object in the solution. (Try to keep it off the floor of the container to let the solution reach all surfaces.)
- Check periodically, rinse and lightly scrub with a very fine scuff pad or wire brush.
- Quit when it is clear that the acid has done its work.
- Scuff, rinse and dry.
- Prime and paint, or seal with oil or wax.
Other favourite rust hacks
No doubt, there are other products people believe in fervently to treat rust. Many kitchen-cupboard favourites involve weak acids in very small concentrations. These include lemon juice (citric acid), vinegar (acetic acid) and Coca-Cola (phosphoric acid). More than a few car restorers are die-hard fans of diluted vinegar as a pre-paint rust killer on bare body panels. Some farmers swear by sugar beet molasses (used in feed) to remove heavy rust from equipment. Turns out, molasses contains natural chelating agents. Cleansers like Bar Keepers Friend include both mild abrasives and oxalic acid, a good rust remover. And CLR Calcium, Lime, & Rust Remover contains sulfamic acid, another good rust remover. Both products work well on rust stains. The rest likely owe more to mechanical effort than chemical efficacy. But your mileage may vary.