A customer once asked me: Why would you use the hot black oxide when the cold black is so easy?
Basically, it comes down to performance. The hot black oxide is truly a better, more durable, higher-performing coating. With room temperature black oxide, you constantly run into trouble with bath control and smut development; it doesn’t offer the durability, it doesn’t offer the corrosion resistance that you get with the hot black oxide. If performance and corrosion resistance aren’t important factors, then you could use the room temperature black, and you think maybe as point-of-sale decorative type finish of a drill bit in a plastic container, for instance, or something like that. The one area where I would preferentially use room temperature black would be on castings, forgings, and powdered metal. The finish is a good finish; I think in many ways it’s actually better than the hot blacks for those materials. and of course, with powdered metal, it tends to absorb the caustic, and you get what’s called caustic bleed out, and it becomes very problematic. It’s hard to deal with you don’t have those problems with room temperature black oxides.
Here are some other questions we often get asked, and our answers:
What is the difference between Infusion and ABM?
The difference between the two products when it is all said and done is the Black Magic Infusion has a little bit higher surface tension so that when you use that product, you tend to drag out more solutions with the parts as they exit the black bath into the rinse. You may ask why would you want to do that; by doing that, it controls the sludge formation or the buildup of colloidal iron oxide in the bath, so you’re taking that contaminant out at a higher rate, and what we find is that you get less of an issue with colloidal red iron oxide adhering to the part as it exits the rinse and exits the black and that typically shows up as a reddish-type film on the part. So black magic infusion helps to alleviate that problem. I think what I’ve also seen in the field with Black Magic Infusion is it requires less rectification than the activated Black Magic Plus. So to get rid of colloidal iron oxide, you’ll add a rectifier that will cause the colloidal iron to float to the surface where you could skim it off. You don’t need to have that need as much with Black Magic Infusion, so it’s really the preferred product at this point.
What are the standard procedure for RT black and hot black?
For the standard procedure, there’s two: it could be a 5-step procedure, which would be clean, rinse, black, rinse, and seal, or it could be clean, rinse, pickle, activate remove rust (whatever that intermediate step is), rinse, black, rinse, and seal, so that would be a 7-step procedure. Either one, we’ll employ either a 5-step or a 7-step procedure. It’s dictated by the part itself what type of contaminants on the surface as far as oxide scales, rust, and that type of thing where you need that acid or that de-rusting step that’s intermediate between the cleaner and the black.
Why do I have to use two hot products for brass to get it black?
The two products are what’s called Brass Activator solution, and then, of course, the Black Magic or the Black Magic CBL the black for the brass; what you’re doing with the activator solution is you’re causing what’s called parting corrosion, which means that you’re de-zincing the surface. You’re leaching zinc out of the surface of the brass substrate, and that leaves a copper-rich surface that will accept the black. If you don’t do the de-zincing step, you’ll tend to get reddish, dark brownish-type coated colors out of the Black Magic CBL when you subsequently go into it. If you skip that step, you’re not going to get the color you’re desiring, which is black.
Will hot black on steel give hydrogen embrittlement? How do I deal with that?
It’s well documented that it can. The military spec that calls out hot black oxide on steel actually stipulates that if parts are of a certain tense tensile strength or greater that they have to go through a post blackening bake-out to relieve any hydrogen that’s in the surface of the part. I’ve actually seen parts — think of gator clips, for instance. There was a situation where I was blackening a gator clip, and I came out of the hot black oxide water, rinsed it, and as I came out of the rinse, the parts actually fractured and broke because of the hydrogen buildup in it. Generally speaking, the harder the alloy — say if it is a spring made out of piano wire, for instance, or those gator clip type parts — could be very prone to hydrogen embrittlement. And of course, the thinner the profile, the more severe it’s going to be, and you’re probably going to see fractures earlier. But the way to deal with it is a post bake oven.
I need 96 hours of salt spray – which do I use, and do I need a topcoat?
Use the hot black oxide rather than room temperature, and you do require a topcoat. It’s typically a solvent cutback thin-film type water displacing corrosion inhibitor that’s used. The products that we have that might get you to 96 hours is a product called the Metal Guard 510, and then another product called Metal Guard 560 will definitely get you over 100 hours on most black oxidized parts done in the hot black oxide, not the room temperature. If you’re pushing 96, I don’t think room temperature is going to get you there consistently.
What are the Military specs associated with different blacks?
When it comes to hot black on steel, you’re looking at Mil C13924, and you’re looking at class one. Mil C46110 may also be applicable. If you’re looking at stainless steel black or cast iron ductile, malleable iron blackening in a hot solution, that same specification, but instead of class one, you’re looking at class four. The other one that there’s a military specification established for is black on copper and brass, and it is Mil F495 is that specification for black on copper and copper substrates.
Why does my stainless steel rub off? What is the correct procedure for stainless steel?
Stainless steel is an interesting substrate to blacken, and we do it in a product that’s specifically targeted for it operating at 255°F, and the solution is boiling. The standard procedure for stainless steel is to clean, rinse, activate in an acid solution, rinse, blacken, rinse, and then either seal or you don’t have to seal, of course, with stainless steel. The activator that’s used most frequently is 50% hydrochloric muriatic acid at 70°F to 80°F. The rub-off could come in two forms: it could be a smutty black that rubs off to have an inherent black underneath, and that’s probably a case of over activation. You could also get a black that the integrity of the black itself is good, but if you rub it on sharp edges, for instance, or 90 degree bends, you get some exfoliation actually of the black, and that’s typically due to going too long in the blackening solution itself. So, in that case, instead of going to a standard procedure, which is five minutes in the Black Magic for stainless steel, you’d go two minutes. In some cases on certain free machining alloys of stainless steel — and I ran into this on a part that was actually done for a dial on a submarine by a defense contractor — the hydrochloric acid, although required, was too aggressive and believed it or not we went through a 10% to 15% phosphoric acid pickle and got an acceptable black with no smut. So it’s either over activation or too long in the blackening solution itself that leaves the smut on stainless steel.
How do I get rid of the solution?
With room temperature black oxide, you could bleed and feed and try to treat it. But seeing as it contains selenium and copper, you’re better off having it hauled off to be treated. You could do a treatment of a 10% solution in-house; we used to do it, we had a procedure where you introduce steel wool, drop the pH, let it react, raise the pH, filter, and discharge. But the selenium levels are getting so low now that that doesn’t get you to the level you have to get to. With a hot black oxide tank, I’m one of those firm believers that you never have to dump a hot black oxide tank for steel unless it’s severely contaminated with something. The only two things I’ve seen that have killed a hot black bath are hexavalent chrome or cadmium in one instance. With a hot black oxide, you shouldn’t have to treat it and dump it. If you do, again, you’re better off having it hauled off just because it’s such a concentrated solution.
How do I determine the size of a hot black tank, and what should it be made of?
My response may differ from others because I always tend to have a more forgiving process. I’ve always said that you want one gallon for every half-pound of workload that’s going into the bath. So, in other words, if you’re putting 100 pounds of work per load into the bath, you’d want a 200-gallon bath, a half a pound per gallon. What you get with that is that it’s a very dense solution and when it’s boiling at 285°F there’s a lot of heat energy in there. You want to make sure that your load as it’s introduced to the black does not kill the boil. That’s what’s preferred, but if the boil is suppressed for no longer than two minutes, it typically doesn’t create a problem. If you put too large a load in and it suppresses the boil for five minutes or 10 minutes, there’s a good chance that you’re not going to get a good quality black.
Can I keep the tank running 24/7?
You can, and people do it. We had a good customer that was in the gun business in Connecticut that actually ran 24/7, three shifts a day, but of course, they’re processing work through it. If you’re running one shift, the downside of keeping it up at boiling temperature because the boiling point is really what you’re concerned about rather than the true temperature of the bath itself — there’s a distinction between the two, it takes a lot of energy to maintain it, so why have it at boiling temperature when you’re not using it. What a lot of people do is throttle it back a little bit maybe maintain it at 220°F or 230°F, then come in in the morning — or when you’re ready to run work through again — fire it up, establish the boiling point, and make sure that it’s correct. If it’s a little too high, the water’s introduced to reduce the boiling point, and then you’re good. The answer to the question is you can, but ordinarily, unless you’re running work through it 24/7, you probably want to idle it back and maintain it somewhere around 220°F or 230°F.
How do I add water to the hot black oxide?
First, want to make sure the solution is boiling; it’s mandatory that the solution be boiling when the water is added. And then, the water is added slowly and cautiously, typically against the back wall of the tank at a level that’s roughly in the neighborhood of maybe two inches above the solution surface. In some installations, some of the lines I’ve seen out there, they actually introduced the water maybe an inch below, which isn’t bad. You definitely don’t want to induce the water down onto the bottom of the tank, because you’ve got to understand that you’ve got a solution that’s boiling to 285°F, and water boils at 212°F. So if you put water in — this is why you want boiling actions for mixing — if you put water into a tank that’s not boiling or not being mechanically agitated and the water goes down the back wall of the tank and gets on the bottom, and that water vaporizes and expands 22 times it literally will blow the solution out of the tank. So that’s why you want to make sure that the tank is boiling and/or mechanically mixed, which you rare rarely see. And the water is added slowly and cautiously, not at too great a rate against the back wall of the tank, preferentially above the surface level.
I want a US 10B black on brass – which product should I use?
The product I always recommend for a US10B is one of our selenium-based room temperature antiquing solution. The one I recommend is our product called Mi-Tique 1791. that’s the one that’s used most frequently to produce the US10B out there.
What equipment should I have for standard maintenance of a hot black oxide?
Hot black doesn’t require a lot of maintenance other than — assuming that you have automatic water addition based on boiling point — the removal of the colloidal iron oxide that may build up in the bath. And if it’s a bath like Activated Black Magic Plus, where the rectification system is designed to really float that up, you want to periodically skim that off. What people use is stainless steel or a steel screen, similar to what a home pool skimming device looks like, one of those made of stainless steel to periodically skim that sludge that floats up. That colloidal iron floats up and is kicked up and accumulates, usually towards the back or in the sides of the bath. You skim that out into the rinse, and that’s how you maintain it and keep it going. That’s essentially all that’s needed, other than periodic calibration to make sure that your controller is actually reading the correct temperature to maintain the boiling point.
Why are my parts coming out mottled?
It depends on what process you’re looking at, but generally speaking, mottled parts are due to two things. Inadequate surface preparation, no matter what the process you’re using, whether it’s hot, or whether it’s cold. It could be a residual heat sheet scale that’s not removed from the part, and that’ll give you a mottled look. The other thing is nesting marks; if you’re doing parts in a basket, you could have nesting if you don’t agitate it. It will come out, and people say, “Well, the parts are mottled.” How are you doing them? “Well, I’m doing them in the basket.” What’s the shape of the part? “Well, it’s washers.” well, of course, they tend to nest, and if you don’t get them moved off each other, you get light areas and dark areas. Now when it comes to hot black oxide, another cause of a mottled finish — and this is true for black on steel and black on copper and copper-based alloys — is if the bath is operating outside its normal parameters; with a hot black oxide bath for instance, if the bath is not boiling and the temperature is 185°F, you may get modeled a mottled finished. But I’ve seen cases where the bath is not boiling, and the temperature is actually a little bit higher, too. So you don’t get the chemical action that you really want to get. I saw a bath once in the field in Florida; the customers having all sorts of problems with the mottled finish. I went down there and said, “Okay, do me a favor; make sure it’s boiling and check it with the glass thermometer to make sure it’s 285°F.” The customers say, “Yeah, I did.” I go down there, and we walk into the shop with the distributor’s rep, and I look at the bath; the controller’s off, it says 285°F, but the bath itself was at close to 300°F, the boiling point was well over 300°F,and he wondered why he was getting mottled parts. So what we actually did was bailed out half the bath and added water while mixing well. The bath was cool, and I fired it up and established that the boiling point came up the 285°F and ran parts that came out beautiful. On copper and brass, the blackening solution contains caustic soda and sodium chloride, and there’s a balance there. What you typically see in a black on copper bath is that caustic will build up over time, and as caustic builds up, it tends to want to break down the chlorite. And when that occurs, the balance goes off, and that could result in mottled parts as well. The higher temperature could be bad in both cases, so if you’re running a copper and brass bath at 230°F, you could get a mottled finish because it really performs best at 200°F to 210°F. It could be surface preparation, or it could be a chemical balance of the blackening solutions themselves.
Robert Farrell is Vice President and Technical Director at Hubbard-Hall Inc. Visit https://www.hubbardhall.com.
