Chemistry of rusting - update

Living in the semi-arid climate of Colorado, I have essentially no problems with rusting. One time I took a picture of some of my tools and sent it out on an email list related to metal. People asked me how I kept my tools so shiny. I told them that I do nothing at all, they just don't rust here. The local humidity is generally so low that condensation is not possible, the wet bulb temperature is well below the day's low temperature.

Of course, if water was dripping on them it would be another story.

I keep the tools in an unheated attached garage on the south side of the house.

Richard

Christ> Hi all,

Kinda; condensation might occur on a cold surface (below the dew point) when the atmospheric temperature is higher (above the dew point), of course, BUT ALSO a fingerprint or other salt or hygroscopic impurity can attract moisture into salt solution at temperatures high enough that water wouldn't otherwise condense. The 'dew point' is for pure water, impure mixes are associated with higher temperature moisture accretion.

Clean dry steel usually forms black oxide faster than hydroxide (because air has lots of oxygen and only a little H2O) and the black oxide is slightly rust-resistant.

I think that if it does, it probably rusts a lot less.

Anyway, in my garage, I simply oil everything that is going to stay for over a month, with heavy weight used rear differential oil.

I am going to soon install a desiccant wheel dehumidifier.

i

I have a corrosion textbook and one very low corrosion site that was used as an example was Norman Wells in the North West Territories. It is a small oil based community dating back to WW2 halfway up the Mackenzie river. It is very cold there but the humidity is low. It does not have to be hot and dry to limit corrosion rate. It can be cold and dry. The colder it is the slower the rate of corrosion. Below 40 F the rate slows significantly. Equipment tarped over with a hole at the top and a light bulb left on at the bottom will reduce humidity and create a chimney effect for ventilation. Randy

You might remember that a while back I posted a question about the chemistry of rusting. Some people suggested that things might rust in my shed because of condensation, but I said I'd never seen any.

Well tonight I did. I went into the shed just before 6 pm and everything was dripping with condensation. I guess it had been a fairly warm, damp day for winter and that the temperature had fallen quickly at dusk, causing the condensation. I guess the solution is a dehumidifier.

I'm still curious to know if steel rusts at all when there isn't condensing humidity. Anyone know?

Best wishes,

Chris

Condensation is actually more likely to happen when the temperature rises. The tools are metal, and change temperature more slowly than most other materials. When the warm (and probably humid) air hits the cold tools, water appears, literally out of thin air.

It will help. Heat will also help. If you can somehow keep your tools a few degrees warmer than outside air, you'll improve the situation noticeably.

If you get one of those electric dehumidifiers, you will both add heat and remove water, so that would be a good option.

Robert Roland wrote: Mostly right, except:

Generally, metal changes temperature faster than most other materials. That is why touching room temperature metal feels colder than wood, paper, cloth, formica, etc. The metal conducts heat out of your hand faster.

To be more technical, how fast an object changes temperature depends on

3 parameters: its mass, its specific heat, and its thermal conductivity. A large objects with high specific heat will hold more heat and take longer to change. Higher thermal conductivity will allow the heat to transfer more quickly.

So your lathe, mill, etc has a large mass and stores a lot of heat. It changes temperature more slowly because of its mass, not because it's metal.

Bob

I have a wooden shed in my back yard with all my tools in it . It's not particularly airtight , but doesn't leak (roof) . I do have plastic sheeting as a ceiling . Memphis Tn is quite humid as a rule , and tools left out rust quickly . I keep a 100 watt light bulb lit at all times , and the only time I've seen condensation was the day I tested my propane torpedo heater . For heat while working , I use a 1650 watt electric heater , no moisture source . Even mild steels in my cabinet under the lathe have very little or no rust . I believe the light keeps the temp above the dew point . Not usually even noticably warmer , but it must be enough .