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Black Compound on Galvanized Salt Fog Samples

I recently removed some galvanized steel parts from a 5% salt fog after 450 hours. About 40% of the surface had turned black. Do you have any idea what this black compound could be? The water used in the salt spray was from a city water supply, not DI.Black

Scott L [surname deleted for privacy by Editor]
- Monroe, North Carolina
2001

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The first thing I would like to point out is that your salt spray testing does not conform to ASTM B117. Salt spray requires deionized water that conforms to Type IV water according to ASTM D1193. You can get a mixed bed filtration system from a local water purification company that can do the job for a few hundred dollars. My main recommendation is to fix your water system so you can perform quality testing.

There is black corrosion possible, but I can not tell you exactly what it is. I have seen red, white, black, green, and yellow corrosion in salt spray tests that I have performed in the past. Usually corrosion other than white and red is disregarded since there is no specification against having that type of salt spray result. Maybe someone else out there will have a more definite answer as to what the black corrosion actually is.

Tim Neveau
Rochester Hills, Michigan
2001

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Zinc corrosion products can be black. The initial corrosion reaction product on galvanized surfaces is usually white zinc hydroxide, which is often called white rust. When the corrosion is more severe, the corrosion products are black.

Has the zinc been consumed, or is the material deposited on the surface? If the material is a surface deposit, its composition could be anything. Some relatively straight forward analysis could give you a conclusive answer.

Larry Hanke
Minneapolis, Minnesota
2001