Chemistry of rusting

This is bugging me. I've asked lots of people but so far haven't found a clear answer. Cut or machined mild steel rusts in our shed. But it
doesn't rust in our house. At all. Conditions in our shed are approximately 0-10 deg. C and 70-80 % relative humidity. Conditions in our house are about 15-25 deg. C and 50-60 % relative humidity.
I can see why the reaction might occur more slowly in the house, but I can't see why it should stop completely.
Does anyone know?
Best wishes,
Chris
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On Wed, 23 Aug 2006 06:01:28 +0000, Christopher Tidy

Check your mild steel that's in your shed on some chilly morning, you may find condensation on it.
i
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Ignoramus20689 wrote:

I have, on numerous occasions. No condensation. If it occurs, it's pretty rare.
Chris
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I used to think that rusting had to do with getting wet. However, over time, I realized and understood that rusting is an actual electrical galvanic reaction. That is, there is a very very small electric current that causes the reaction. These electric currents can be caused by many things, salinity, dissimilar metals, metals coming in contact with a ground rather than being isolated on nonconductive materials, lots and lots of things.
Derusting is also a galvanic process where a minor electric current is passed through metals with that metal in a chemical bath. the reverse of rusting.
Rusting really has nothing to do with being wet, but with an electrical conductance that takes place. Rusting is an electrical process. Of course, lessening all the contributing factors ......... moisture, grounding, contact with things that improve conductivity, etc. have varied results from nil to extreme.
It is not a simple thing.
Steve
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Steve B wrote:

The articles I can find on the subject (e.g., http://en.wikipedia.org/wiki/Rusting ) suggest that water is necessary. It is unclear to me if liquid water is necessary, or if water vapour will do. From my experience it seems that water vapour can cause rusting.
Chris
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On Wed, 23 Aug 2006 08:53:10 +0000, Christopher Tidy

That article does mention water droplets:
"When steel contacts water, an electrochemical process starts. On the surface of the metal, iron is oxidized to iron(II):
Fe ? Fe2+ + 2e- The electrons released travel to the edges of the water droplet, where there is plenty of dissolved oxygen..."
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Here's something I wrote awhile back: http://www.artmetal.com/files/imported/project/TOC/material/corros.txt
Steve
Don Foreman wrote:

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Steve Smith wrote:

Thanks Steve. I'll take some time to read that tonight.
Chris
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wrote:

There are inconsistencies between the two charts. Your chart suggests that aluminum would protect both zinc and iron, and that iron would protect cadmium. Lange's chart suggests that zinc and cadmium will protect iron, and that zinc will protect aluminum.
Cadmium has been used as a sacrificial or galvanic plate for steel in the past, and aluminum outboard motors have zinc tabs below the waterline to protect the aluminum body, and maybe the aluminum boat it's attached to.
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Don Foreman wrote:

Can't help it Don. Both charts are out of the same reference.
Steve
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wrote:

Go figure. Beats me, I'm no electrochemist.
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The two charts are simple enough.
One - the first simply states the one that is more negative than the other - or higher on the chart takes the hit, lower less or none. And that tells the story...
If one has a copper bottom boat and puts a Magnesium bar against it - expect -2.34V-(-.34) = -2.00V differential and the most negative takes the hit. The higher number shows the intensity of the hit.
Now Iron and Cadmium are very close. -.44 - (-.4) or a -.04 difference for a nominal hit. So Cad plate on Iron bolts won't corrode the bolts. (notice the direction of subtraction...)
as to the other chart - it simply groups 'good' stuff together. The active indicates that the metal has not been passivated with Nitric or Critic acids. e.g. 18-8 steel won't by itself corrode to death even when still active.
Also - 18-8 touching 304(passivated) and/or with 18-8-3 (passivated).
I'm not a chemical Engineer, but a Physicist and Practicing Engineer and senior management.
Martin
Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net NRA LOH & Endowment Member NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member http://lufkinced.com /
Don Foreman wrote:

-
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On Wed, 23 Aug 2006 12:43:50 -0500, Don Foreman

The data you see are half cell potentials under standard conditions where the concentration of the cation is 1N. You can't assume that what you see in the real world is the same as shown in the tables. In addition complexing agents such as chloride or ammonia e.g. can shift some of these potentials drastically.
Differences of a few hundredths of a volt are within experimental error and shouldn't be used to conclude that one metal will displace another.
The tables "suggest" perhaps to a metallurgist but not to a chemist that aluminum will protect a lot of metals in the table. The chemist will realize that the oxide coating on aluminum will be part of the equation and one shouldn't jump to conclusions.

The aluminum in the outboard and the boat probably gets more protection from the adherent oxide layer than any sacrificial anode.
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I didn't get in on the original thread but my simple answer is: A condensing environment makes the difference. Inside the house, the temperature doesn't vary very much. Outside it does. When the sun goes down, the humidity goes up. The parts cool off. Water vapor condenses on the part and there you go.
Pete Stanaitis ---------------------
Steve Smith wrote:

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Someone mentioned grease or oil. Remember that oil floats on water. If you oil it, the oil will float away eventually.
Pete Stanaitis ------------------
Steve Smith wrote:

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Christopher Tidy wrote:

A couple of thoughts. One the US military puts a lot of effort into keeping the humidity between 20 % and 50%. The 20% is to limit electrostatic charge build up. The 50% is to keep stuff from rusting. The 50% is probably conservative, so conditions in the house are probably good enough to prevent rust. At 20% RH there is enough water vapor on things as plastics so that they are slightly conductive.
Another thought is that the things in the house may have a better surface finish. Polished steel does not rust as easily as steel with a rougher surface finish.
Third is what the stuff is stored on. For example concrete is hydroscopic. It will adsorb moisture from the air. So if you are storing things on a concrete floor, use some wood to keep it from direct contact with the concrete. In the house objects are likely to be on painted surfaces. ( Bare wood is not all that great, but much better than concrete.)
Fourth there is probably less dust in the house. Some of the dust is likely to be hydroscopic.
Lots of possible factors.
Dan
Dan
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snipped-for-privacy@krl.org says...

Although, in the wintertime, our RH level at home will drop well below 10 percent. This is mostly a result of the house not being well sealed, so there's a fair amount of exhange with the outside.
Below 10 percent, all sorts of minor respiratory problems crop up. When we keep the bedrooms above 20 percent at night, the number of colds really drops way down.
Jim
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snipped-for-privacy@krl.org wrote:

That sounds sensible. I tried putting a 60 W light bulb inside a power supply I'm trying to protect. It dropped the relative humidity from 71% to 44%.

No. I have various rough and smooth parts on my desk. Quite a variety, and they sit there a long time as they're either trial parts or parts I screwed up.

That could very well be a factor. Some of the stuff I noticed significant rust on was in a cardboard box.

That's possible too, although I've seen stuff get very dusty in some buildings without a trace of rust. I guess it's a combination of factors.
Best wishes,
Chris
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As you said the outside is more humid. Water itself doesn't rust steel, it's a catalyst in the process.
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Gunner wrote:

Nothing weird. The previous owner of the house fried everything, which deposited lots of grease everywhere, but the components I'm talking haven't witnissed any frying.
Chris
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