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Black zinc chromate plated on steel vs on stainless steel


An ongoing discussion from 2004 through 2016 . . .

(2004)

Q. My company is in the process of selecting a screw for our products. We need something that has a very high corrosion resistance (this is very important). We are trying to decide between
A. Black Zinc Chromate plated steel, or
B. Black Zinc Chromate plated Stainless steel.

Here are our concerns.... br - Steel with Black Zinc Chromate: 1. Plating will be scratched and corrosion will occur. br - Stainless steel with Black Zinc Chromate: 1.Stainless steel will strip more easily. 2.More expensive. 3.Can Stainless even be plated with black zinc chromate?

If you could give me pro/cons of both of these two different situations, It would be greatly appreciated. Also, the reason we are interested in plating SS is because we need the black finish.

Matt Sommer
Safety Equipment - Cleveland, Ohio, USA


(2004)

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

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

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(2004)

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.

jim conner
Jim Conner
TCS Texas - Fort Worth, Texas USA


(2004)

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!

Matt Sommer
- Ohio


(2004)

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. Were the plating 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 and then clear coating it :-)

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



April 9, 2012

Q. Hello,

I have a stainless steel bracket that I want a black finish on. It's an automotive throttle body cable bracket that has been heavily modified, welded (with different pieces of stainless steel), and partially spray painted. With all the work on it, it looks kinda ugly and I rather have it professionally finished than just spray painted again.

I took it took a local anodizing and plating shop. I wanted the finish to be glossy black but they said they can only use "black zinc" on stainless steel.

1.) If I wanted a black finish (preferably glossy but I can settle for matte), would black zinc be the appropriate finish?
2.) Would black zinc or the process weaken the integrity of the stainless steel? Since it's a throttle body cable bracket, I'm going to be mashing down on it with the gas pedal.
3.) They said they would "bead blast and black zinc" the bracket. When they mean "black zinc", they mean black zinc chromate?

Thanks!

Eric Lam
motorsports - Glendale, California


April 9, 2012

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,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



Black oxide on stainless steel

January 20, 2016

Q. So if you are strictly looking for a cosmetic black finish on stainless steel - you can just go with black oxide? Will that go "white" as it corrodes off? I'm willing to pay for the stainless in my application - but I do want a black finish (matte is OK).

Scott Moorman
buyers products - Mentor, Ohio USA


January 2016

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,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


January 20, 2016

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.

Scott Moorman [returning]
buyers products - Mentor, Ohio USA


January 21, 2016

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?

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois



January 2016

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,

pic of Ted Mooney
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


January 22, 2016

thumbs up signThanks - 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.

Scott Moorman [returning]
Buyers Products - Mentor Ohio USA


January 25, 2016

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.

treglio portrait
Jim Treglio
- Vista, California



July 26, 2016

Q. Hello,

I have a question regarding the black zinc chromate finish on stainless steel fastener mentioned earlier in this thread. I was always under the impression that zinc and stainless steel are too far apart in the galvanic chart to be used together, or am I missing something here? The reason I ask, is I have an application where I need to have black colored stainless steel fastener that will be expose to the direct outdoor environment; and the black oxide stainless steel screw are not holding up. I am curious if the black-zinc-chromate on stainless steel will be the ticket.

Regards,

Thomas Wong
- Ottawa, Illinois


January 2, 2017

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.

blake kneedler
Blake Kneedler
Feather Hollow Eng.
Stockton, California


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