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

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

As you said the outside is more humid. Water itself doesn't rust steel, it's a catalyst in the process.

I have, on numerous occasions. No condensation. If it occurs, it's pretty rare.

Chris

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

Keeping any acids or chlorinators in there?

A thought I've had is that perhaps mild steel forms some kind of very weak passivation layer, which surives in the house, but is broken down by damp, acidity, temperature fluctuations, etc. in the shed?

Chris

If the rh in the shed is 70% when temp is 10C the dewpoint is 4.78 deg. If the temp cools to that or below, condensation will occur.

If there is any ferric chloride or HCL in the shed, it is very difficult to get bottles of that sealed so well that no vapor escapes. Just a little of those vapors will rust steel in a hurry.

Nothing that isn't firmly sealed.

Chris

There is a bottle of concentrated phosphoric acid in one shed. It is well sealed. There is some fertiliser in the other shed. It is less well sealed.

Rusting occurs fairly slowly. It takes years for a cut surface to acquire a significant coating of rust. But this does not happen at all in the house.

Chris

I'd guess it was about 10 C when I measured the relative humidity. It probably falls to about 7 or 8 C at night in summer. Maybe condensation does occur, but if so it's weird that I've never see it. I wish I had something which could tell me if condensation was occurring - some sort of recording hygrometer perhaps - but I don't.

Chris

The articles I can find on the subject (e.g.,

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

Might not be too hard to make one. A polished bit of stainless, an LED, a photosensor and a CMOS flipflop from Maplin's. Aim the LED at the polished stainless mirror and arrange the photosensor where it can "see" the mirror but neither the LED nor its reflection. If the mirror temperature gets down to dewpoint it will fog causing scatter that the photodetector can "see". The photodetector trips the flipflop to "record" the event.

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..."

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

Chris

I have noticed that vapours of HCl will rust steel even if it's protected with a thin layer of grease.

S.

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

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