Does hot dip galvanizing work for stainless steel?

A. Carey,

One does not weld plain steel (I assume you mean mild steel) galvanized or not to stainless steel. Galvanic corrosion will be the result. If you need to fix mild steel parts onto stainless parts, go the mechanical way: clamping, bolting etc. If the parts are exposed to a corrosive environment (outdoors, high humidity, etc.) be sure to avoid direct contact between the two materials. Separate them using non conductive (i.e., plastic) washers.

Q. Remmelt,

I was always told that if two dissimilar metals are to suffer galvanic corrosion, there must be an electrolytic path between them. By galvanizing the welded assembly, I would then be sealing the unit and removing the path of travel for the metal ions. Is this incorrect?

Thank you,

A. Hi Carey. I would agree with that. If the whole surface is coated with zinc, there is no galvanic action.

Although I don't know the feasibility of galvanizing stainless steel, lots of parts are made with dissimilar metals as long as the whole surface is coated with the same metal. Many bright decorative things like slot machines are made by brazing steel pieces together then nickel-chrome plating them so the different electrochemical potentials of brass and steel do not cause a galvanic problem. The weldment/assembly can be electroplated for corrosion resistance.

Regards,

A. The hot dip galvanizing may not stick to the stainless steel unless the zinc penetrates the passive oxide film on the SS to form a metallurgical bond.

HD galvanizing is typically not a good idea for 300 series SS because the zinc will preferentially penetrate the grain boundaries of the SS. (The same problem can occur from welding SS to galvanized steel.) If these assemblies are subjected to external forces, the mechanical properties, especially fatigue behavior, may not be what is expected. If these assemblies are used at elevated temperatures, catastrophic failure from solid or liquid metal embrittlement is possible.

Galvanic corrosion should not be problem if the whole assembly is coated. The zinc will be sacrificial for both the stainless and the plain steel. In most environments, galvanic corrosion of steel in contact with stainless is not a big problem, since the stainless surface should be passive and not available to participate in the corrosion process. The mixed weld area could be different story.

Hope this helps.

A. Thomas,
Refer to ASTM A380 for pickling mixtures. These are typically nitric or nitric/hydrofluoric. The exact parameters necessary would depend on the specifics of your parts.

And remember, pickling is for descaling. If what you are after is corrosion resistance, that is passivation. See ASTM A967.

Stainless isn't typically ever galvanized, though the responses above do imply that it's possible if for some reason you really really want to. I always say though, if you are going to put a coating on stainless and rely on that for corrosion protection, why use stainless in the first place?

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Thanks for the very helpful pickling & passivation specs and the suggestions, Ray!

But for readers from other disciplines who therefore might not know Dr. Cook, he is arguably the world's best known & most respected galvanizing expert. I'm reading his quest for details as mostly skepticism about the practicality of galvanizing stainless steel rather than activating it :-)

Regards,

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

A. Overpickle in hydrochloric for 3 days and turn the kettle down to 820 °F. Aircool - no water quench. It will galv.

A. Hi Folks. I think, where practical, I would use zinc-rich cold galvanizing compound on any steel parts, and also paint any stainless parts to reduce the galvanic incompatibility.

Regards,

A. Hi. As long as only one surface remains exposed (the galvanizing in this case), you can't have galvanic accelerated corrosion. But this page has been on the web for over a decade, getting 200 or more visits per month, with nobody yet claiming that they've seen galvanizing of stainless steel truly working properly, so I remain dubious :-)

Regards,

A. I agree. It isn't possible to galvanize stainless steels. Nor is it sensible to do so!

A. Hello Mr Ian,

On the contrary, my personal experience is that the blasting on SS wakes up the surface and improves adhesion.

A. Ian,
Yes, steel blasting of stainless will contaminate it (as opposed to blasting with stainless shot or glass bead), but since you're putting multiple coatings over the top of it, it doesn't really matter much at that point.

Which also raises the question, why bother paying the extra money for stainless steel in the first place?

A. Hi C.R.,
Can you tell us what the parts are? Some things we wouldn't dream of doing on aerospace parts may be rather routinely done on low pressure vats, simple linkages, and farm equipment. Mr. Hanke warns of the zinc weakening the stainless against fatigue loading at high temperatures. Mr. Crowley says 304 stainless can't be gainfully galvanized and Dr. Cook seems to me to be expressing skepticism.

Are you certain that the parts are 304SS, and that the coating is applied by hot-dipping rather than electroplating or another technology? Some earlier posters have told us that the reason they ended up with a mixed-metal weldment was because they added stainless components to a galvanized boat trailer. Why are your parts mixed metal? Thanks.

Regards,