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Breakdown of Black Oxide Tank
We are experiencing a problem here at Amplate, that at this point is almost impossible to explain. We operate a black oxide process that the solution consists of sodium hydroxide, sodium nitrite, sodium nitrate, nickel nitrate, and water. It operates at 280 degrees - 290 degrees Fahrenheit. Three years ago we replaced a mild steel black oxide tank, that had been in operation approximately 25 years, with a new tank made from a mild steel A36 hot roll pickled and oiled and mig welded with ER 70S wire. Both, the old and new, tanks were double walled, a tank inside of a tank, with a 4" air space used for insulation. A few weeks ago we noticed that the 4" had filled up with black oxide solution and started leaking from the outer wall on a corner. After examining the tank closer, we found that the tank seemed to be corroding from the inside.
Our question is: "What would cause a high Ph caustic solution to attack mild steel in only three years?" There is no "stay current" what so ever checking from tank to ground.
Thank you,
David FrenchAmplate Inc. - Charlotte, North Carolina
Sorry, but this one is a mystery to me as well. It seems there are 3 possibilities:
First, that some other chemical like a fluoride salt or a dry acid salt got added at some point. But I would expect a very strong reaction had this occurred, and damage to the production work as well.
Second, that the corrosion actually started in the air space, despite the fact that it doesn't look that way to you.
Third, that there is a stray current at some times that you haven't found yet.
I would expect a black oxide tank to be resistant to attack from the black oxide solution indefinitely.
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 Ted Mooney, P.E. finishing.com Brick, New Jersey |
It's probably too late now, but the material was probably not heat treated prior to fabrication. If this is the case, the tank will probably start to leak at most of the welded seams (due to the additional heat stresses in those areas).
Ed Simpson- Hamden, Connecticut