Plating zinc onto stainless steel?

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,


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

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,

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)

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,

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.

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,