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Anyone who stores rust-prone instruments knows that even in temperature-controlled air spaces rust can form on uncoated steel. Since rust requires water, this suggests that water will condense out of air onto surfaces even above its dewpoint.

How is surface condensation of water above the dew point characterized and quantified?

Or is "condensation" not the right term for this? There does seem to be a non-linear relationship between relative humidity and the observed effects of that airborne moisture on exposed solids.

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feetwet
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  • Water vapour is always in the air and so water molecules will come into contact with any and all surfaces. These will form your rust. You will have to keep the humidity in your room very , very low to prevent this; or rather to make the reaction rate so low that it can be ignored. Temperature controlled does not necessarily mean humidity controlled. – porphyrin Dec 03 '16 at 16:06
  • Right, and what I want to see is the relationship between relative humidity and this "reactive contact rate." Note that temperature controlled in this case means that relative humidity never exceeds 65%, and that no tools or surfaces are ever colder than their surrounding air. – feetwet Dec 03 '16 at 19:26
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    At 65% relative humidity approx 1% by weight of the air is water vapour; this is a lot. Even if surface is not colder than air the water vapour is still coming into contact with it just as the air does. One way to keep the surface clean is obviously to to remove all water vapour, another way is to heat it so that there is thermal gradient between it and the surroundings. I have kept hydroscopic crystals perfect in this way for years. – porphyrin Dec 03 '16 at 23:43
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    @porphyrin , isn't it necessary to have at least a small liquid layer present for some of the steps in the oxidation mechanism, like $\ce{4 Fe^{2+} + O2 → 4 Fe^{3+} + 2 O^{2−}}$? (This Wikipedia article) I do suspect that you can get enough of an adsorbed layer of water for this to happen at relative humidities below 100% though. – airhuff Feb 27 '17 at 20:19
  • @airhuff which is why heating is probably best if all the water cannot be removed. – porphyrin Feb 27 '17 at 20:43
  • @porphyrin, agreed. – airhuff Feb 27 '17 at 20:46
  • @porphyrin: This isn't about countermeasures to rusting. This question is about vapor/liquid dynamics in non-condensing but humid environments. For example: If a "liquid layer" is necessary for iron oxidation, then if rust forms in non-condensing conditions that means that there is a mechanism for a "liquid layer" to form in non-condensing conditions. What is that "liquid layer?" By what mechanism does it form and evolve? Of what is its formation and evolution a function? – feetwet Feb 27 '17 at 20:54
  • I think that 'liquid layer' is too strong a statement, water molecules will form a monolayer to fully or partly cover any surface in proportion to the strength of any physisorbtion interaction, which is primarily van der Waals in nature. In doing so they will be in equilibrium with water molecules in the air. (At high humidity multiple layers may in principle form). Rusting seems to rely on having water molecules present so will be greatest where their concentration is also greatest. – porphyrin Feb 27 '17 at 21:08

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