A Primer on Rust

Some things cause steel or iron to rust faster than others. Water will cause iron and steel to rust. Dissimilar metals rust faster than single metals because of electrochemical reactions, so steel rusts faster than iron, and joints between dissimilar metals rust very quickly. Salt water will cause rust faster than water because salt water is a better electrical conductor. Like most chemical reactions, heat also speeds rust.

If steel starts to rust, it will puff up because iron oxide is a larger molecule than iron. The puffing causes cracks and voids, which expose more bare metal to the environment. So the rusting of iron can progress and is only limited by destruction of all solid iron. Other metals oxidize, but the oxides of some other metals are no larger than the metal themselves, so they don't puff up or flake. For example, aluminum doesn't puff up when it oxidizes. This helps make aluminum oxide a good protective coating, rather than the start of rapid degeneration.

Rust is really Fe2O3, a reddish form of iron oxide. Iron has another oxide, Fe3O4, which is sometimes called black oxide, black rust, or hammerscale. Black oxide is a good protection for steel. Like aluminum oxide, black oxide molecules are the same size as iron molecules, so black oxide does not grow or flake. Black oxide is true gun bluing and the oxide found on some drill bits. Black oxide is also seen on iron and steel that has been hot-worked.

You can coat steel with black oxide by a careful regimen of rusting the right amount and boiling the rusted metal in water to convert it. This is how non-caustic gun bluing is done, and although it is tedious, it produces very attractive and durable results after several treatments.

Is Rust Alive?

Removing Rust

When iron or steel starts to rust, it will puff up and expose clean metal to the open air, allowing rust to continue to the depths of the metal. If your favorite possession is already rusting and you want to clean it or at least stop the rusting, you have a few alternativ

There are a few products on the market which fall into the category of organic rust converters. These products contain acid to convert rust to black oxide and polymers which bond to rust. Instructions say to remove all loose rust, paint on the product, and let it dry. I've used one of these products and it did work. However, you sho clean and prime the repaired surface as soon as possible.. Some trade names for these products are: POR-15, Glean Tech Rust Killer, and RustX. Your local hardware store such as Lowes, Home Depot, Lowes, etc should all carry similiar rust removal products. One of my favorites is MFR Must For Rust.

Sand blasting or bead blasting

This is standard practice in auto body repair shops. Sand blast will remove some good metal and will work-harden the surface, so glass bead blasting is used for more delicate parts. Immediately after any blasting, metal is clean and exposed, so it is essential that some form of rust proofing goes next. In auto body work, this is often an acid metal wash (see below) followed by self etching primer, but could also be a coating of a product like Boeshield T9.

Sand paper and steel wool will also remove rust, but they don't get into tiny crevices. Rubber abrasive sanding blocks like those made by Cratex are good at removing a thin coat of rust and can also remove rust from minute pores in the metal.

Another technique for removing rust is etching with Phosphoric Acid. Phosphoric Acid has a unique property of dissolving iron oxide quickly while etching iron very slowly. This means that you can leave metal in Phosphoric Acid for much longer than necessary with very little damage. The acid will attack bare metal slowly and will start the process of hydrogen embrittlement, so use the minimum etch time that removes all rust.

Another unique advantage of Phosphoric Acid is that it leaves a fine coating of iron phosphate behind. Iron phosphate prevents rust. However, the iron phosphate coating is not very thick and not durable. Some additional protection is still required.

Phosphoric Acid etch will leave a hard, bright metal finish. This is because it will etch the surface slightly, exposing new, bare metal. Often this is desirable. It leaves an attractive surface and a surface ready to paint. A common product which contains Phosphoric Acid is Naval Jelly. And of courseCoca-Cola contains Phosphoric Acid, so Coke will etch rust. But Coke also contains carbonic acid and other nasty things.

Auto body shops treat metal with acid metal wash, a solution of Phosphoric Acid and alcohol before painting. This removes waxes and oils, removes slight amounts of rust that form between sand blasting and painting, and leaves a thin protective coat of iron phosphate. One commercial solution for this is DuPont Quick-Prep. Sherwin Williams has a similar product called Metal Prep.

Other acids will etch rust, but not as selectively. Hydrochloric acid can etch rust very quickly. Oxalic acid has also been used. However, if you leave metal in these acids a bit too long, you will lose a significant amount of metal. Also, neither acid leaves a protective film behind. Oxalic acid opeartes to remove rust by forming a water-solable complex ion (called a chelate) around each iron ion. So do cyanide compounds. Both react likewise with the iron in your blood, meaning the oxalates are just as poisonous as the cyanides.

I read of using a solution of 1 part black molasses and 9 parts water to remove rust. Perhaps this works because the molasses is slightly acidic. The procedure is very slow. After treatment, metal starts rusting quickly, so this is probably not a phosphoric acid treatment.

All acids contribute some hydrogen to the metal structure. This weakens steel by a process called hydrogen embrittlement. If the metal is a cutting tool or gun barrel, for example, this weakening can be dangerous. One person claimed that if you bake the metal at 400F for an hour after acid cleaning, then you can drive out the hydrogen and prevent this embrittlement. With all acids, use rubber gloves and splash goggles, no matter how weak the solution. When thinning acid, add acid to water. Never add water to acid.

One more technique for removing rust is Electrolytic Rust Removal. Rust can be electrically etched off of iron or steel in a bath of mild alkali, such as Sodium Carbonate. Connect the rusty part to the negative terminal of a 12V battery or battery charger and a scrap piece of steel or iron to the positive terminal. Use one tablespoon of Sodium Carbonate per gallon of water. This technique has many advantages. First, the alkaline solution is much safer than some of the acids mentioned earlier. It is still a chemical, so rubber gloves and splash goggles are recommended. Another advantage of electrolytic rust removal is that it will have no effect on good metal, so you can leave the work in the bath for a long time and not damage the metal. There is no risk of hydrogen embrittlement nor of etching unrusted metal.

Electrolytic rust removal will leave a black oxide surface, which is the result of a process that doesn't remove any good metal at all. The only thing removed is loose rust and embedded oxygen. If your goal is to keep as much of the original metal behind, this is the best technique. However, be prepared to immediately follow this technique with one of the rust prevention procedures, such as MFR, Boeshield T9 or VPI Paper wrap.

There's another rust removal technique used for restoring chrome . Rub the area vigorously with the end of an aluminum bar. This will scrape the iron oxide off and at the same time the aluminum dust formed will penetrate and react with the rust to form aluminum oxide and iron. I haven't tried this myself.