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What is Alkaline Blackening of Steel?

I need to know the information about "blackening process". What's black film that plated on steel? Because currently I have a senior project about chemical surface treatment of metal by alkaline blackening process and phosphating. Thank you very much for your help.

Weeranuch Stitwittaya
Chiangmai University - Thailand
2001

Alkaline blackening of steel is called black oxide. The solution that the steel is processed in is a caustic solution of sodium hydroxide, sodium nitrite, sodium nitrate ⇦this on Amazon [affil link] , and various proprietary blends of surfactants, rectifiers, and accelerators. The finish is a conversion coating of the steel - therefore there is no dimensional change to the part (for all practical purposes). This process happens at between 280 and 290 degrees F.

The part of your question regarding phosphate is another subject completely because you are depositing a phosphate coating that may have a black pre-dip step added to the process.

I hope this can get you started in your research.

Dan Brewer
chemical process supplier - Gurnee, Illinois
2001