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Adding humidity

 

I tested some Zinc die cast w/conversion coating samples two different ways. Some were in a chamber at 90 degrees C with steam being used to control the humidity level to 95%rh.

Others were on a rack, in a pan above some water with a cover at the same temperature. The samples in the steam chamber corroded severely.(White zinc product) The samples in the pan did not.

Did the steam cause the corrosion? Or are the two tests that much different?

Russ Cannon
- Imlay City, Michigan

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Russ, the difference in the performance of the samples in the two cabinets is a reminder that accelerated tests result can vary from one test cabinet to another. It is alway best to try to run all the samples being compared in a single cabinet if possible. If not, the test samples should be compared to the performance of a known sample to minimize cabinet to cabinet variations.

In your testing to two cabinet present somewhat different conditions. In the first cabinet with test samples above the water and the cabinet heated to 90C, the humidity should be 100% if the cabinet is airtight and insulated. Pure water vapor would be condensing on the test samples and everything within the cabinet would be at the same temperature.

In the second cabinet, steam from an outside source is being used to produce the humidity. The steam is probably hotter than 90C when it is introduced to the cabinet. As a result, additional heat is being introduced into the cabinet with the steam. As the test samples are cooler than the steam, the steam condenses on the samples producing liquid water and heating the parts above 90C. The increase in temperature may have increased the rate of corrosion on these test parts compared the parts in the other cabinet.

A second cause of the increase corrosion observed in the steam humidified cabinet could be due to contaminates in the steam. There could be traces of carbon dioxide gas dissolved in the steam that would produce carbonic acid. The carbonic acid would greatly accelerate the corrosion of the samples in this cabinet compared to the other cabinet. You could quickly test this by testing the pH of the liquid water in the cabinet being careful not to contaminate the sample.

Roy Nuss
Trevose, Pennsylvania, USA

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If you used boiler steam, my 2 cents worth is the problem was caused by the boiler treatment chemicals. they are minimal, but can be extremely damaging in open air.

Years ago, we built a humidity cabinet using a small stainless steel water heater element connected to a temperature controller. We used a very small fish tank air pump (thru a hose with holes for a sparger placed in the water) to provide agitation and also the air would be 100% humid. We used a wet/dry bulb thermometer for humidity readings and had a very small hole for the air to get out of the box. Would have liked to have recycled the air thru the pump, but did not have the courage to try it. Box was made out of polypropylene and lucite. Worked great.

James Watts
- Navarre, Florida