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?
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
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 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.
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?
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.
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
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.
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.
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..."
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.
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.