We are often asked: for functional black oxide coating, what is the thickness on a steel part? We also get asked what the thickness is on a blackened stainless steel part?
The hot black on steel is between 0.4 and 2.4 microns, and that is very dependent on a couple of parameters: the temperature that the bath is boiling, which typically is 285°F and will give you a slightly thicker coating. Probably the biggest contributing factor of the thickness is the dwell time; typical black oxide applications in a job shop is going to go 10 to 15 minutes; if it’s a bearing manufacturer and they really want to optimize the coating thickness of the black up to a point before you start getting diminishing returns on your dwell time, I’ve seen them go 30 to 60 minutes, and that’s where you get those thicker coatings.
With stainless steel, the problem you are running into is the thickness of the black oxide coating if you go too long is detrimental; you actually get a coating that exfoliates at sharp angles on the part. If you have a 90°bend on a stamped part and you go too long in the black, you could actually see a loss of adherence in that area. Unlike hot black oxide, where longer is better for coating thickness, it doesn’t hold true with stainless steel black. I would imagine if it were to be cross-sectionally looked at, the stainless steel black would probably be down closer to the low end of the coating thickness of hot black oxide on steel, which would be about 0.4 to maybe 0.6 microns.
If I process steel parts through a room temperature black oxide, what kind of corrosion protection can I expect from the functional black oxide coating? What about a room temperature blackened stainless steel part?
When it comes to room temperature, black oxide on steel — because it is a copper selenide type or a copper telluride type of coating — tends to be very prone to corrosion. If you are blackening a steel part in room temperature black oxide, you come out of your water rinse after black, and you could get flash rusting starting to occur before you get to your subsequent topcoat. Room temperature black oxide on steel is never used without a subsequent topcoat; it offers no corrosion protection whatsoever, and it probably detracts from the corrosion resistance of the base material. You always have to use a topcoat with room temperature black oxide; it’s just the nature of the coating itself not being a true oxide, but more of an immersion coating of copper that’s subsequently black. For stainless steel black, it probably offers no corrosion protection whatsoever; there is a room temperature black for stainless steel, but it’s rarely used. It is a selenium copper-based material, and it works well on small parts like fasteners; it probably distracts slightly from the corrosion protection offered by the base stainless steel alloy itself, but if you’re looking for color-coding, you are looking just to have a black part or to cut down on the reflectivity it has its application. For fasteners, as an example, it doesn’t distract so much from the corrosion protection that it instantaneously rusts; again, it is better to topcoat it with something to give it a little bit more protection. The trouble with room temperature black on stainless steel is that, if you go to larger substrates, it is difficult to get a consistent black across the entire surface area of the substrate without getting a smutty finish or a black sooty type rub off. Black sooty type rubs off with room temperature black on steel, and stainless steel is probably one of the biggest problems encountered in a production situation that customers complain about. Tune in the concentration, and tune the dwell time to try to lessen the chance of producing that sooty type coating.
Functional room temperature black oxide, what temperature range is the black oxide coating on the steel part stable?
If you were to take a room temperature black oxide coated steel part and rinse it and thoroughly dry it, so there’s no corrosion occurring whatsoever on the part before putting it in an oven, it would be stable may be up to 250°F. But with room temperature black oxide, you never put it on a part that would be subjected to high temperatures above 100°F; you would go to hot black oxide for that particular type of application. Being a selenide type coating — and with selenium having multiple oxidation states or valence states —you could get some unusual things happening. It’s rare that you would ever subject a room temperature black oxide part to a temperature above 100°F, but I suspect if the test was done under controlled situations, you might be able to get to 250°F.
For functional room temperature black oxide, what is the thickness of the black oxide coating on a steel part? What about a blackened stainless steel part?
In all the years the 44 some odd years I’ve been in this industry, no one has actually asked me to cross-section and measure the oxide thickness of room temperature black oxide, but I would suspect that it’s at the low end of what hot black on steel would be. You are probably looking at 0.4 microns in that range. Thickness is typically looked at for tolerance; of course, with black oxide, tolerance of the part rarely does come into play; on very rare occasions, it does. If you know that a micron affects the part from a tolerance standpoint, of course, then you have to consider it, but generally speaking, for the types of parts that are black oxide coated, tolerances are not that tight. If coating thickness is important, then it would normally be important because you are looking for enhanced abrasion resistance or enhanced anti-galling friction characteristics due to the fact that this micro sponge oxide coating will absorb the subsequent topcoat or a lubricant. It then becomes important to try to get a thicker coating, and you could do that with hot black oxide. Of course, it tends to plateau off and becomes self-limiting when you get up over an hour or 90 minutes, and it probably flat lines. On room temperature black, if you try to go longer to get a thicker coating, you will get smut or black rub off, which is objectionable. You don’t really look at room temperature black oxide for abrasion resistance because one of the characteristics of room temperature black is that it is much softer and less abrasion resistant than hot black oxide. You could take a room temperature black oxide part and rub it with a pencil eraser and remove the coating in a short period. You could rub hot black oxide for a much longer time before you get any type of removal of the black occurring. Thickness never really seems to come into play with room temperature black oxides, whether they’re on stainless steel, steel, copper brass, aluminum, or zinc. It’s just not a variable or a characteristic that’s important.
Robert Farrell is Vice President and Technical Director at Hubbard-Hall Inc. Visit https://www.hubbardhall.com.
