Painted headers

[sciguyjim]

Originally posted by 454Casull
Isn't iron oxide the product of oxidation between iron and oxygen? That is also what people call rust, right?

That's right. The oxygen doesn't come from the water, but in fact comes from atmospheric oxygen dissolved in the water. This is why an iron object partially submerged in water will show the most rust forming at a short distance below the surface of the water. This is where the dissolved oxygen concentration is highest.

I've seen very clean iron rust in just a few minutes from water vapor in the air. Heating the iron to evaporate any liquid on the surface just caused it to rust faster. The heat accelerated the chemical reaction. Also, hot air can hold more water vapor than cold air. That's why the air is so humid in the summer and so dry in the winter.

[ivymike1031]

Also, hot air can hold more water vapor than cold air. That's why the air is so humid in the summer and so dry in the winter

You might want to rethink that statement. I think you're right in a round-about way, but not in the way that you seem to be suggesting. Consider: if you have a mass of air mixed with a mass of water vapor, and you heat that air, does the humidity go up, stay the same, or go down?

[sciguyjim]

Originally posted by ivymike1031


You might want to rethink that statement. I think you're right in a round-about way, but not in the way that you seem to be suggesting. Consider: if you have a mass of air mixed with a mass of water vapor, and you heat that air, does the humidity go up, stay the same, or go down?

I haven't actually done the calculations, but whatever they say, the atmosphere isn't a closed system. Hot air will still hold more water vapor on an absolute basis, if not on a "relative" basis.
Ever wonder why static is only a problem in the winter and never in the summer? It's because the static is "shorted out" by the microscopic layer of water on surfaces in a humid environment.
As air is cooled, the first thing to condense out will be water vapor. Next will be the gases which make up our air (argon, CO2, nitrogen, oxygen, etc.)
Now, with air dissolving in water, it's just the reverse. More air is soluble in cold water than hot. That's why warm soda goes flat faster than cold soda.

[ivymike1031]

There isn't any calculating to do, if you're familiar with relative humidity. Relative humidity is the ratio of the partial pressure of water vapor under present conditions to the saturation pressure of water under the same conditions. In other words, it's how much water is in the air divided by how much water the air can hold. If you increase the amount of water that the air can hold, r.h. drops. If you cool the air, r.h. increases, until (as you said in the last post) the water starts to condense out of the air. The water starts to condense when the relative humidity reaches 100% (when the vapor pressure reaches the saturation pressure).

Evaporation rate, and the perception of humidity, are influenced by relative humidity (not absolute). Seasonal trends of relative humidity generally follow the seasonal trends of rainfall, so if you're in an area where it rains more in the summer than any other time of the year, it's likely that the r.h. is higher in the summer. I think that it rains more in the spring and fall, here in Chicago, than in the summer.

In the winter, the dryness that you feel indoors (and the need to run a humidifier), are a direct result of the humidity dropping when air is heated. If the relative humidity was 100% outside, where the temperature was 40degF, and indoors the air had been heated to 72degF, the rel. humidity indoors (without a humidifier) would be 31%. The air would feel noticeably drier, and you'd possibly have trouble with static.

[sciguyjim]

Your statements all sound correct. You are more familiar with relative humidity than I am, so I will defer to you.