My 2000 Subaru is full of holes. Like so many other cars in Canada and the northern United States, it is exposed oxygen and water, which combine with iron to make rust. Add a dash of salt and you can see them all at work here.
I often drive it on an elevated highway, where rusting reinforcing bars cause concrete to spall off and drop on the road below; it is a lucky thing that nobody has been killed.
Rust can be a minor annoyance if it stains your clothing or your tools, or it can be a major disaster in buildings and infrastructure. Rust is the result of an electrochemical reaction that is really like a battery; Iron turns to iron oxide with water as the electrolyte, actually making electricity in the process. That's why salt water rusts steel faster than fresh water; the ions move more easily, it's a better electrolyte.
The cost of rust is huge; America's bridges alone need $76 billion worth of repairs, and much of that is due to rust. But it also hits us at the more personal level, most obviously in our cars, but also with tools and appliances.
Whenever you get iron, water and oxygen together, you get rust. So the best way to prevent it is to keep them apart; that's what paint does, or the spray-on wax and oil coatings that the car protection companies sell. Keep your tools dry; wipe down your bike after a ride; keep the water away and it can't rust.
The acid treatment
If you have got rust that you want to get rid of, there are a couple of methods, mostly involving some form of acid. In the home you can use lemon juice (citric acid) or vinegar (acetic acid). Apartment Therapy offered a few recipes including one I had never heard of, with some interesting side benefits:
Salt + Lime: Sprinkle a little bit of salt on the rust, then squeeze the lime over the salt until it is soaking. Let the mixture sit for 2-3 hours and then remove the rust with the lime rind. This can also be done with a lemon. We like the salt and lime a little more because they double as margarita ingredients as well
Other acidic solutions include lemon juice and vinegar. Both Apartment Therapy and the Farmers' Almanac recommend potatoes, suggesting that this is particularly good with knives. "Sprinkle a little salt or baking soda onto the potato and then rub it over the rust spot, or just insert the knife into a potato and let it sit. The oxalic acid in the potato helps to dissolve the rust."
In Russia, they use Coca-Cola; it's the phosphoric acid.
More heavy duty methods include muriatic and phosphoric acids, which I do not recommend. I have the burned clothing to show for it.
Another option is to live better electrically, and reverse the process of rusting. Our friends at Instructables show how you can use electricity to remove rust:
...basically you set up a conductive solution and insert some sacrificial anodes. You hang your rusted tool in the solution and attach it to the negative end of the power supply. You attach the positive end to the anode and turn on the power. The current travels through the solution and in the process flakes off the rust - the flaking/softening occurs because of the reaction at the surface of the good steel that pushes the rust off.
Looking into my 1944 copy of Fortunes in Formulas, one finds all kinds of toxic solutions for getting rid of rust (Potassium cyanide anyone?) but also another electrochemical one that doesn't require adding electricity to make it work; it is actually building a battery that appears to consume the rust.
"the rusty piece is connected with a piece of zinc and placed in water.....
The best way to deal with rust is to avoid it in the first place. Keep your stuff dry; paint it with high quality paints and touch them up when they get dinged; oil them regularly.