Rust is a generic term for several different oxides of iron and iron alloys. In general, these oxides are red in color. When iron “rusts,” what it’s really doing is corroding. Electrochemical corrosion is the scientific term for rust. Iron isn’t the only metal to corrode electrochemically, but the resulting oxides of other metals are not commonly called rust. Rust offers no protection for the metal underneath. Surface rust will slowly eat its way through any iron present, and cause all kinds of structural issues.
One of iron’s alloys is stainless steel. To make stainless steel, iron is mixed with chromium at a minimum of 10.5% chromium to iron. Stainless does not stain or corrode as easily as iron, but it’s not stain proof. The iron in stainless steel is protected by a thin film of chromium oxide that blocks further corrosion. But given enough time, oxygen and water will convert any iron into iron oxide—even stainless or stain-resistant steel.
What causes rust?
There are numerous chemical equations required to illustrate the precise conditions under which rust forms. But essentially three things are required: iron, water, and air. When water has electrolytes in it, acids—hydroxide ion—are created, and the process of oxidation or rusting is greatly increased. Like near the ocean, for example, or on our cars in the winter. When iron is in the presence of pure water or dry oxygen, it is relatively unaffected. A thin passivation layer of oxide forms on the surface and protects the bulk of the iron from further corrosion.
Rust Removal, Treatment, and Repair
Get rid of rust as soon as it starts. Rust is not something that you can just let go on for a while. The longer a rust spot sits untreated, the larger it will get. A small rust spot may only require a bit of sanding or brushing. Then a coat of anti-corrosive primer and a final coat of enamel to make it as though it never existed. A large rust spot may require rust killer, a power brushing, followed by Bondo to repair the hole, and painting to blend it in. So take care of small spots ASAP.
Scrape and sand away the rust. It is important to remove all the rust from the metal before you attempt to repair or cover up the steel. There are several ways to remove the rust from the iron, but most involve physically removing it by hand or with a power tool using either a brush or sandpaper. The brush will work best to remove large pieces of rust. To remove fine layers of rust, use a fine-grain sandpaper, specially designed for use on metal.
Convert the rust into something else. Phosphoric acid, when applied directly to rusted iron, will convert the iron oxide into black ferric phosphate. Rust converter is sold as a gel commonly called naval jelly. It’s also sold under other names: “rust remover” or “rust killer.” After the rust has been treated and converted to ferric phosphate, it can be scrubbed off to expose the fresh, clean metal surface underneath. The black ferric phosphate can also be left in place to provide moderate corrosion resistance.
Large holes will need to be filled in. Sometimes the larger rust spots will leave large holes in the surface of the steel. A product called Bondo is usually used to fill in that hole. Bondo is made from fiberglass, and it can be used on its own to fill the hole in. But if the hole is too big, attach a wire screen to the back of the hole. This will give the Bondo a matrix to build from. After the product is dry, sand it smooth to match the contours of the rest of the surface. Prime and paint the Bondo just as you would steel.
Protect the freshly exposed iron and paint the surfaces.Use an anti-corrosive primer to cover up the fresh steel and protect it from becoming rusty. Primer will help to isolate the steel from the other two requirements for rust creation: water and air. This technique will of course rely partially on how thoroughly you removed the rust prior to priming. Once you have slapped on a couple layers of primer, finish the surface off with a good quality paint or enamel.
Rust Proofing and Preventing
- Painting metal prevents corrosion of steel by isolating it from the environment. The “Black Knight” of medieval times was usually “black” because it was the cheapest paint to keep the armor from rusting.
- Zinc plating, also know as galvanizing, is a process of dipping steel in a bath of molten zinc, which protects the steel from corrosion.
- Bluing is a process of creating a passive layer of oxidation on the surface of firearms. The oxide is magnetite, and it’s a black oxide of iron that offers a minimal protection against corrosion. It is most effective when a layer of oil is applied to the bluing periodically.
- Controlling the humidity of the environment can impact the amount of iron corrosion. Less humid equals less rust, and more humid equals more rust.
- After market rust-proofing kits are sold for vehicles. The compounds are usually wax based and applied with a pressure sprayer.
- Cathodic protection is a method of inhibiting corrosion of large metal structures by suppressing the electrochemical reaction that occurs in the process of oxidation.