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Q & A: Rate of rust formation

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Q:
how long does it take to form rust?
- Anonymous
A:
Rusting can happen quickly or slowly, depending on the material that's rusting, and the environment. Rust is the oxidation of iron along with the absorption of water to make Fe2O3 with water molecules attached. Here are some things that can affect the rusting rate:

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 haven't 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 doesn't rain much. Cars last longer out in the desert because it's 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 doesn't 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?

Tom

(republished on 07/13/06)

Follow-Up #1: rusting stainless

Q:
I read what Tom said about the rate of rusting and stainless steel does rust! I did an experiment on rusting stainless steel and it rusted in water, salt water, vinegar, lemonade and red cordial. I just thought Id add that to what Tom said before Beth
- Beth
Melbourne Victoria Australia
A:
Beth, thanks.

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.

Mike W.

(published on 04/16/07)

Follow-Up #2: contact rust?

Q:
I just wanted to add that contact with other metals will contaminate the Stainless at almost any grade. I see lots of stainless tubes where some pipe-fitter leaves the chain-fall lead laying on it and the next morning there is a rust layer on the s.s. surface. Although some maybe transfer, that s.s. has been contaminated by the carbon in the chain. I think that rust rates are also related to whether the metal is a ferrous or nonferrous, but that is pure opinion, and I am just a dumb irish welder. S-
- Scott George (age 31)
Smith Valley, Nv, USA
A:
You know more about this than we do.

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.

Mike W.

(published on 02/28/10)

Follow-Up #3: Danger of wet electrical fuse boxes

Q:
I have 2 metal cabnets with alot of copper fusses and there has been at least 6 days of rain hitting them and there in a confined space, i was just wondering what is the possibilities of getting rust in such a short time. my email is ## i just trying to find out if you need more information i could provide it thankyou
- Jose Sanchez (age 25)
Ohio, il, usa
A:
I wouldn't worry about rust too much.  The conducting elements in wires and fuses are mainly copper.  However, water and electricity don't mix.   There is always danger of shock.  Don't get your hands in that fuse box when it's wet.
 
LeeH

(published on 07/03/10)

Follow-Up #4: rusting rate vs. oxygen pressure

Q:
Does air pressure affect rust formation? This pertains to steel tanks that hold compressed air. Would increased air pressure increase, decrease or not effect rust formation on steel? Thanks
- Mark Johnson (age 35)
woodland, ca, usa
A:
I'd assume that higher oxygen pressure would increase the rate of oxidation, i.e. rusting. Chemical rates don't always follow such simple rules, but it would be surprising if it didn't happen this way.

Mike W.

(published on 01/06/13)

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