Black corrosion-resistant finishes for socket head screws
Q. I need a black finish on socket head screws with corrosion resistance. I have heard about phosphate and oil finish, and I'm looking for more information. How does it rate corrosion wise? Does it perform differently with different phosphates? What is the difference in phosphates? Any information would be appreciated.Randy Dastrup
Icon Health & Fitness - Logan, Utah
A. Randy - you don't say how much corrosion resistance you need, but zinc phosphate with an oil after-finish is very common in fastener finishing. Also, black oxide with an oil after-finish is common and less expensive than zinc phosphate. The zinc phosphate will give better corrosion resistance than the black oxide, but the black oxide won't change the dimension of the part. There are pros and cons to each. Good luckDan Brewer
chemical process supplier - Gurnee, Illinois
A. Hi Randy.
In addition to the finishes mentioned by Dan, you could also consider zinc-iron plating with a black chromium conversion coating or black chromium plating.
The zinc-iron plating is sacrificial, so would be significantly more corrosion resistant than phosphate and oil or black oxide (not to mention free of smuttiness and oil). The black chromium would actually be nickel plating followed by black chromium, and could be quite corrosion resistant as well as beautiful, but would be expensive. Good luck.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
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