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"Black Oxide's affect on 316 stainless corrosion properties"
February 18, 2010
Does anyone know if black oxide coating alters the corrosion resistance properties of 316 (or 316L, or any other) stainless steel? I'm curious as black oxide contains iron compounds (and iron is not corrosion resistant).Alvin Vue
Engineering - Minneapolis, Minnesota, USA
February 19, 2010
Well you know what that is something that no one has ever asked me and for that matter I have never thought about. The only reason I ever applied black oxide to stainless was to make it look black. I had always assumed that the black finish formed on stainless was the same as that formed on chrome molybdenum steel, magnetite or Fe3O4, but after reading your question and thinking about it I am not even sure if this is the case. My education in this particular case is simply inadequate. I can tell you for a fact that if you took a stainless object that is blued by conventional methods and an un-blued object made of the same material and exposed them to the elements for a period of time. ( I mean the normal climate found in Alberta Canada) The black finish on the blued object would degrade, dull and in some case even pit slightly. The un-blued object would for the most part remain unchanged. In an ocean front environment both objects would suffer although the blued example would suffer much worse. I'm just a gun plumber and I'm drawing on experience here not education. I think we need a chemist !
Edmonton, Alberta, Canada
February 22, 2010
If the coating is properly applied, it shouldn't decrease the corrosion resistance of the stainless. However, I have seen many cases at a particular shop where significant red rust and pitting on the stainless occurred during a 2-hour salt spray exposure. This obviously puzzled us, but after a lengthy investigation, we discovered that the problem came from the use of aluminum oxide blast media that had been contaminated with iron through use. The blast media apparently embedded this contamination which ended up corroding. When we switched to a no-blast pretreatment procedure or to a virgin media pretreatment procedure, the problem went away.
Jon Barrows, MSF, EHSSC