Black zinc chromate plated on stainless steel

A. Thomas,
The way I see it there are three prime choices. Black zinc, clear coat over black oxide or powder coat. Black zinc should work on your stainless and would be almost as good as powder coat perhaps. Clear coat over black oxide would not be as hardy but should be an improvement over the current process.

A. Hello Matt. Obviously, even 'very high corrosion resistance' isn't quite quantitative. Is this for outdoor direct exposure?

Within the useful life of the zinc plating, the steel substrate should be okay. Zinc plating offers cathodic protection so that a scratch in the coating isn't fatal. So steel should be okay.

But yes, stainless can be zinc plated, it needs to be Wood's nickel struck first, which can sway the economics to where black nickel or black chrome plating may be attractive.

A. Matt, based on your requirements, here are some options for you. First, only use black if you want the color, not greater corrosion resistance. Second, you will get the same white rust whether you use S.S. or not since the Zinc will be corroding. The plain steel parts will have red rust once the zinc is compromised so in severe cases the higher cost of S.S. is worth it. Any shop specializing in fasteners should be able to run either metal. As you know, the chromate coating is scratched during the plating and drying process so you may want to request a seal after chromate. There are several that offer excellent corrosion resistance.

Finally, if you go with S.S. you can have them black oxide coated with a sealer and skip the whole problem of the Zinc corroding.

Q. Yes it is outside, direct exposure (will be exposed to temperatures up to 400 °F).

Could you be more specific about it having cathodic protection, and what that exactly includes? The way I see it is...SS plated with black zinc chromate has more corrosion resistance than steel plated the same. Although said steel (through the life of the zinc and with no scratches) is just as resistant. There isn't much information on the web about SS being plated with Black Zinc chromate...is there any reason why it couldn't be done?

Thanks for you all your help!

A. Hi Matt. Stainless steel can be plated, although it is more difficult to activate it for plating than carbon steel; You usually need to start with a Wood's Nickel Strike.

Zinc is a metal which is less noble than steel. That means that when a steel piece is plated with zinc, as opposed to other metals like nickel which are more noble than steel, the zinc will sacrificially corrode. There is effectively a "battery" in operation which is oxidizing zinc to provide a potential which is working to prevent the steel from oxidizing.

This battery begins operating the moment steel is exposed at a scratch, because now there are dissimilar metals connected by an ionic path and a metallic path. If the plating were copper or nickel, the battery would be operating in the opposite direction, trying to dissolve the steel to protect the plating. Zinc plating cannot necessarily absolutely protect steel under all conditions, no matter how severe the scratching, but the basic principle is that the battery is working to protect the steel, and the principle works. Looking at it from "inside", what happens is if the Fe⁰ steel is exposed and in danger of losing electrons to corrosive forces, which would cause it to become Fe⁺² or Fe⁺³ and dissolve into solution, it is more noble than zinc and able to "steal" electrons so the zinc corrodes instead of the steel.

People sometimes refer to platings like zinc as "sacrificial coatings" or "cathodic protection coatings", and platings like nickel as "barrier layer coatings".

But actually, I'm liking Jim's idea of black oxiding the stainless, or black nickel, or black chrome because 400 °F is too hot for a chromate conversion coating.:-)

A. Hi, Arthur.

Your posting is a bit confusing to me. Black zinc plating means zinc plating followed by a chromate conversion coating in black color. These chromate conversion coatings may be based on either hexavalent chromium or trivalent chromium. Although I don't know Singapore's position on hexavalent chrome, due to European regulations such as RoHS, the world is moving towards trivalent chromate conversion coatings even if there is no local regulation banning hexavalent chromate conversion coatings.

Stainless steel instantly grows a passive film that precludes most plating solutions from acquiring the necessary metal-to-metal bond. The conventional approach to dealing with this is a Wood's Nickel Strike, which is a highly acidic nickel plating which simultaneously dissolves this passive film while applying a fresh nickel coating.

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Regards,

A. Hi, Arthur. Sorry, I can't make sense of it, which is usually the case with slang because it evolves locally. Degreased means any grease is removed. But passivated could refer to an operation on stainless steel or an operation on zinc plating. Black chrome could refer to black chrome plating or it could refer to black chromate conversion coating. We get lots of questions like yours, and unfortunately it is just not possible for anyone to read a short, incomplete mix of slang and specification and expand it for you into what the customer actually wants :-)

It's like saying: "I'm thinking of a word and the first 3 letters are 'thr'; what word am I thinking of"? :-) Sorry, but the necessary information simply isn't there, and guesses will only add to the confusion rather than reduce it. Good luck.

Regards,

A. Hi Eric.

Plating can cause hydrogen embrittlement, and consequently weakness, in high strength, high hardness steels ... but I doubt that the stainless steel that you have been fabricating is such a special high strength, high hardness grade that it cannot be baked for embrittlement relief if necessary.

Black zinc (actually zinc electroplating followed by black chromating) is probably a good choice for the application. Black chrome plating could be a lot shinier but it is much more expensive and typically is only used for highly decorative applications.

Not all plating shops can plate onto stainless steel since it needs a special activation routine, and not all shops plate black chrome, so you may have some difficulty finding a shop which can plate stainless steel, and more difficulty finding one that can black chrome plate it. Good luck.

Regards,

A. Hi Scott. Black oxide is millionths of an inch thick, that's one of the biggest draws for some applications -- but it means you're not going to get the durability of long-lived blackening unless you clear coat it. And if you're going to do that, maybe it doesn't need to be stainless. If you tell the readers the application and the environment they can probably help you choose a material of construction and a finish.

Regards,

Q. This is a snow removal application. The fasteners in questions are bolting some black powdercoated carbon steel pieces together. We currently use SST fasteners - and the only issue is that we want them to be black. It's a very tough environment for zinc plated fasteners. They are not abraded at all - they are just exposed to the elements. I always hate creating "new" problems by solving old ones - so any input is appreciated.

A. Maybe I'm missing something here, if the fasteners are holding together "black powdercoated carbon steel" and that holds up well enough in the working environment, then it ought to work fine for the fasteners themselves to also be black powdercoated carbon steel?

A. Hi Scott. I suspect that the reason it's a tough environment for zinc plating is that the zinc will try to galvanically protect anything it's not insulated from. So the zinc is not only consumed in protecting the fasteners, but is trying to protect the whole carbon steel assembly anyplace a scratch or pinhole in the powder coating exposes it. Thus, sacrificial coatings don't work well.

I'd suggest black nickel plating, which is nobler than steel, so it's not sacrificial. I suspect that it will be fine for stainless steel fasteners. The black oxide coating on stainless steel ought to be okay as well, but less durable and probably less consistent in color. Good luck.

Regards,

Thanks - good feedback.

As far as why I don't just powdercoat the bolts - I don't really have an easy way to accomplish yet. I'd prefer to buy something ready for assembly.
Thanks for the suggestions -- this is an amazingly useful forum for real answers.

A. You can also look at black PVD (physical vapor deposition) or DLC (diamond like carbon). Both adhere well to stainless steel and are far more durable than black oxide.