Rate of Rust Formation
Most recent answer: 10/22/2007
1) The iron can have additives to prevent rusting. Stainless steel has added nickel and chromium which bind to the iron atoms and keep them from oxidizing. I havent seen stainless steel rust even over long periods of time.
2) The iron can be painted or coated with oil, preventing oxygen and water from coming into contact. This can slow or halt rusting.
3) The air can be devoid of humidity and in some place it doesnt rain much. Cars last longer out in the desert because its so dry, rusting is slowed.
4) Hot iron rusts faster than cold iron -- typically heat speeds up chemical reactions. This is one reason why mufflers and exhaust manifolds in cars get rusty very quickly (unless they are coated or made out of non-rusting materials).
5) Thin iron can rust through (get holes in it) faster than thick iron. The rusting rate may be the same, but you may notice it sooner in thin metal sheeting than on a thick piece of iron because the former will have a hole in it sooner. Some kinds of steel wool also rust quickly (they are commonly exposed to water so this doesnt help), although other steel wools are made of stainless steel or coated.
6) The rate of rusting or corrosion in water can be affected by the electrical environment. If you have two different metals in electrical contact, and both in contact with salty water, then effectively a battery is made. Current flows, and the energy comes from the corrosion of the metals. Some companies sell blocks of zinc that you can attach to boats so that the zinc corrodes first, protecting the other metals.
7) Some questioners on this site have found that rusting rates in iron submerged in water are affected by dissolved impurities. Vinegar and bleach have been tried and seem to affect the rusting rate.
Fresh iron exposed to a hot atmosphere with plenty of oxygen and water will form a thin layer of rust immediately (although if you look at a very short time after exposing the iron surface, you will have a very small amount of rust). Any of the above variables can affect the rusting rate however. Can you think of more?
(published on 10/22/2007)
Follow-Up #1: rusting stainless
Melbourne Victoria Australia
The rate of stainless steel rusting is very sensitive to what type of stainless it is. The amounts of chromium, nickel etc. in different steels vary a lot, and so does the uniformity of the mixing of the different elements. When a 'stainless' steel has pockets of nearly pure iron, it will rust.
(published on 10/22/2007)
Follow-Up #2: contact rust?
- Scott George (age 31)
Smith Valley, Nv, USA
I wonder if there's a connection with the effect by which when two metals are in electrical contact, one will corrode much faster than the other. Ships use this to protect the hull by attaching a more easily dissolved metal.
(published on 02/28/2010)
Follow-Up #3: Danger of wet electrical fuse boxes
- Jose Sanchez (age 25)
Ohio, il, usa
(published on 06/29/2010)
Follow-Up #4: rusting rate vs. oxygen pressure
- Mark Johnson (age 35)
woodland, ca, usa
(published on 01/04/2013)
Follow-Up #5: battery corrosion
- Ryan (age 26)
Billings, MT USA
I'm not sure about the rates, but you could check that pretty easily. I did just notice that the contacts with the negative sides of some bicycle light batteries were much more corroded than the positive contacts. Clearly the electrical charges are playing a role, just as you guessed. (I put some bulb contact grease, available at auto stores, on the contacts after cleaning them up.)
(published on 05/17/2019)