Black Oxide on stainless steel: problems, corrosion properties, RoHS, etc.

A. Hi Dave. You posted with a bad e-mail address so you may never see this, but for the benefit of other readers whose curiosity has been aroused ...

• Black oxided plain steel fasteners are probably available from specialty fastener suppliers even if local stores don't carry them.
• Stainless steel can be black-oxided, which is probably the best answer if these fasteners truly are available only in stainless steel.
• The reason that stainless steel must receive a nickel strike before zinc plating is that plating must take place on raw metal for a proper metallurgical bond, but stainless steel instantly oxidizes. The nickel strike is designed to simultaneously dissolve any oxides, and keep them dissolved, while applying a very thin layer of fresh nickel which you will be able yo plate upon. If you deposit zinc directly on stainless fasteners, it will chip off immediately because it has no metallurgical adhesion. Good luck! Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi Scott. 18-8 stainless means 18% chrome & 8% nickel. The most common 18-8 stainlesses are probably type 303, 304 and 316. They are non magnetic because of the crystal structure generated by this composition whereas 4XX stainless steels contain no nickel and are magnetic. The 3XX stainless steels are also more corrosion resistant than the 4XX.

I don't think their magnetism is the direct cause of them being more difficult to black oxide, but the nickel content probably is. I guess an easy way to look at it is that if a material is more corrosion resistant, i.e., less willing to participate in chemical reactions, it can be harder to perform chemical reactions like black oxiding on it.

But one more thing that may enter the equation though: on a rough matte surface, black oxide tends to look black; on a highly polished surface it tends to look more of a dark jewel blue color. Luck & Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hello, JP. True black oxide is a process conducted at about 290 °F (the process solution is so salty that this is the boiling point). I have never heard of it being applied in wiping-style, only by immersion, and the temperature precludes polypropylene tanks. The tanks are usually steel with gas-fired burners underneath them.

There are "cold blackening" processes which we introduce in our FAQ about Black Oxide and Cold Blackening. Please make sure that cold black oxide is the process you want to do before investing too much effort in figuring out how to do it though -- it is a process of limited capability.

A. Hi, Armando. Hot black oxide is a slightly modified form of rust, so it doesn't take much in terms of a corrosive environment for it to prove unsatisfactory. Hot black oxide is better than cold black oxide, but not by enough to be considered corrosion resistant. Still, if this is a hot black oxide process, it may need a "boil out" which is lacking. Whether hot or cold, it's the oil on it that offers corrosion resistance.
Black chrome is much better but requires a substantial underlayer of nickel plating. The best black coating in your instance might be salt bath nitriding..

Regards,

A. Hi Ronald. It sounds like a tough calculation, but the last step in the process, deliberately drying hexavalent chrome onto the surface of the parts (even though dilute), sounds like direct violation of the spirit of the RoHS law even if it complies with the letter of the law.

Even if you measured the chrome content, and it was below the RoHS limit, all it would take is for one part to be tested and found not in compliance for you to have an awful lot of 'splainin to do. I think you should suggest to your customer that this really doesn't sound like a good finish to use if you must have RoHS compliance :-)
Luck & Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. 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 !

A. 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.

A. Hi, Jaroon.

Cold blackening is less robust and reliable than hot blackening, and blackening of stainless is more difficult than blackening of plain steel, so it's not too surprising that you are encountering some problems. But we have dozens of threads on line about blackening of stainless steel. We'll need to start with what alloy you are you working, what heat treatment it has received if any, what is its surface condition, what wear and corrosive conditions it will see, and (perhaps most importantly) why you are blackening it. It is possible that there is a cold dip that will satisfy your needs, or that anodizing of stainless steel, or plating with a blacken-able metal may be more appropriate. But you'll need to define those needs for us please. Thanks. Good luck.

Regards,

? Which 300? They're all different beasts. 303 has high sulfur content and often is an issue, for example.

You need to really break down your issue into sources such as:
- grade of raw material
- batch of raw material (and/or supplier of raw material)
- batch of black oxide processing

For example, are some parts in a batch blotchy and others are fine? Or is the batch run on Tuesday fine and the one run on Wednesday all terrible? Or do you have good parts for a week but when you get a new batch of raw material it all goes to mud?

Get whoever is in charge of supplier quality to push back hard on your supplier, too. A lot of this information should be coming from them.