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Plating zinc onto stainless steel?

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Q. Hi,
Our old customer for commercial black product used to request steel hardware with black oxide, we buy hardware with vendor with no problem.

Lately for outdoors application, customer is requesting 304SS hardware with black Zinc CR3. We have received the first samples and they look really nasty compared to other black zinc plated or black oxide steel products: the zinc easy peels off in normal use. Do you think that Black zinc over 304 is a correct request, or should we suggest customer other material or coating? By now I will ask vendor for their plating certificate for product as it seems to be a post process.

Carlos Retamoza
- Tecate B.C. Mexico
May 24, 2023


A. Hi Carlos. Black zinc on stainless steel is not a premium finish like, for example, black chromium or a PVD process. But is should not be "nasty" and the plating should not "peel off in normal use".

Some plating shops are not experienced in plating onto stainless steel, and I strongly suspect that proper pretreatment was not done. Stainless steel must receive a Wood's Nickel Strike or other acceptable nickel strike before the zinc plating. The reason for this is that stainless steel forms an oxidized skin on itself, which cannot properly be removed with acids because the skin re-forms virtually instantly, whereas proper plating can only be deposited upon raw, clean, bare metal -- you won't get proper adhesion on the oxide.

The Wood's Nickel process was specially designed with lots of strong HCl to dissolve that oxide, but with nickel being concurrently electroplated onto the stainless steel as the oxide is dissolved.
Luck & Regards,

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


thumbs up sign Thanks for your assertive answer Ted. We have requested vendor for their plating certificate to have a hint of what path they followed and will request process based on your suggestion.

Cheers, Enjoy Aloha!

Carlos Retamoza [returning]
- Tecate B.C. Mexico
May 25, 2023




⇩ Closely related postings, oldest first ⇩



Q. Plating stainless steel.
I am confused whether it is possible to do zinc plating on stainless steel fasteners (green chromated). If this plating is possible how is the corrosion resistance of stainless steel fasteners after zinc plating on them?
Is there any ASTM standard to be used for plating stainless steel?
Regards

Fred Patrick
interested - Ankara, Turkey
April 28, 2008


A. Hi, Fred. It is possible to plate zinc onto almost anything, including stainless steel. For good adhesion you would probably need to strike with Wood's Nickel first. But I personally do not know of any spec for this. It doesn't seem to make too much sense to me except for very special needs since stainless steel can be permanently corrosion resistant whereas zinc plating offers only temporary protection.

Regards,

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


A. Hi Fred ,

s/s can be zinc plated using a nickel strike prior to the zinc plating.

I think the reason for plating the s/s with zinc must be to get the green colour - via the passivate?

Also if the s/s fasteners you are zinc plating are to be used against aluminium for example, the zinc plating is required so there is no reaction between the 2 metals (s/s and aluminium)

Jay Smith
- Essex, UK


thumbs up sign Hi Jay. I appreciate your answer but am having trouble agreeing with it. I've seen a lot of aluminum fastened with stainless steel fasteners (like the railings on probably most hotels and apartments) and my understanding is that the passivity of the stainless, combined with the self-oxidizing nature of aluminum, so greatly reduces this potential galvanic incompatibility that it's really not much of an issue except in aerospace. Further, objects being fastened are usually bigger in surface area than the fasteners, and it's best to have the cathodic material be the smaller surface and the anodic material (which is being corroded away) to have the bigger surface -- and stainless is cathodic to aluminum, whereas zinc plating is anodic to aluminum. I'm not insisting that I'm right, just explaining my limited current understanding. Thanks again.

Regards,

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




Q. I had the same question.

The primary purpose I had in mind was to match the OEM fastener look on a classic car with hardware that would keep the "new look" until the next rebuild. The secondary purpose was to prevent corrosion of the aluminum castings, but this is generally a minor problem; the oil or thread lock seem to pretty much eliminate anyway --so mostly it's a durability of appearance question to me.

Mark Eberhardt
auto modification - Elverson, Pennsylvania, USA
September 8, 2010




Corrosion of zinc plated parts due to stainless fastener

Q. I have a related problem where I am inserting a stainless steel fastener into a piece of flat, cold rolled steel (.060" thick.) The part is then zinc plated. Now and then the metal around the fastener corrodes to unacceptable levels. I am being asked to provide a corrective action to eliminate incidents of corrosion.

stainless fastener corrosion

Note that the insert needs to inserted before plating because after plating it leaves a noticeable mark on the front surface of the part, which is unacceptable to the customer.

Any suggestions or at least an idea or a good explanation of why the corrosion is inherent in the design.

I appreciate any help or ideas you can provide.

Richard Webb
- Bloomingdale, Illinois USA
March 21, 2013


A. I see a couple of possible reasons for the corrosion. Acid is being trapped in the hardware itself and leaches out or it is being trapped under the material of the spotwelded bracket next to the hardware and is leaching out. The plater would need to either plug the hardware during the process and/or add an extra process such as blowing the area with air or adding rinse time. These usually correct the issue.

Hugo Ley
- Hanover Park, Illinois USA
August 27, 2014


A. Hi Hugo. I am perhaps not understanding the picture and Richard's dialog, but that looks to me like a nut welded into the plate.

I don't quite understand it all, but I agree with you that it doesn't look like galvanic corrosion -- but rather that there was little or no plating under or around the fastener, and probably some acid trapped in some capillary space, so it's corroding?

My question would be why weld a stainless nut to steel sheet metal, with the possible welding issues ... then zinc plate the weldment when zinc won't stick to the stainless ... then have the galvanic corrosion possibility? Why not just use a steel nut and zinc plate the whole thing?

Regards,

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



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Ed. note: For additional situations, options, and opinions please see also:
• Thread 30286, "Need to do Zinc Plating on Stainless Steel"
• Thread 32579, "Black zinc chromate plated on stainless steel"



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