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Zinc Plated Fasteners vs. Stainless Steel

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Q. My company is developing an outdoor product line which is primarily made up of 6063-T5 aluminum extrusions. We decided to switch to all 18-8 stainless steel fasteners for corrosion resistance, but there has been some second guessing as to why we didn't decide to use zinc plated steel fasteners. One reason for the questions is because of the higher cost of the stainless fasteners. In our application, a hex bolt is slid into a channel in the aluminum extrusion, and a the assembly is fastened together with stainless steel brackets. The concern that we have is with the potential for galvanic corrosion due to the fact that we have dissimilar materials in direct contact with each other.

My question is, would zinc plated fasteners be a suitable alternate to the stainless steel fasteners? How would the appearance of the bolts (rust) compare to that of stainless over a period of years in an outdoor environment? How much galvanic corrosion would we see with the zinc plated fasteners as opposed to with the stainless steel?

Thanks for any help you can provide?

Scott Liebert
- Milwaukee, Wisconsin


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A. I have seen stainless fasteners used on architecturally anodized aluminum in outdoor exposure without any substantial problems, and suspect that it will be acceptable. I am a bit confused by your somewhat offhand reference to stainless steel brackets because these would (theoretically of course) cause the aluminum to galvanically corrode regardless of the fastener material, and being bigger in surface area than the fasteners should probably be a bigger concern.

You probably know that, were it not for its toxicity, the ideal fastener material is cadmium plated steel. It is so close to aluminum on the galvanic scale that there is no galvanic corrosion, it is corrosion resistant itself, if thick enough, and it has great lubricity. Nonetheless, cadmium is a cumulative poison, should be reserved for only the most critical applications, and even there is being rapidly phased out.

The next best material is aluminum plating or coating, again because the galvanic potential is zero. You can investigate Ivadizing for vapor deposition of aluminum, or AlumiPlate for electroplating of aluminum. But I fear that you may find the cost unacceptable for anything but airplanes.

All of the above not withstanding, you didn't mention how the aluminum is corrosion proofed, so the question remains a bit open. But zinc plating is not a good idea in my opinion, and I think it will be short lived. Stick with the stainless :-)

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

Fastener Design Manual



July 30, 2009

Q. My company is producing small gasoline engines; we sell our product to Southeast Asian for the long tail boat. Recently we met a serious corrosion on outside fasteners produced by zinc plating; the material of fasteners is low carbon steel. It is been use for affixing some steel stamping parts and plastic panels. I am not sure if there are some galvanic corrosion between fasteners and other dissimilar parts, considering from cost, do we have any other proper choice or solution?

Nick Lu
- ChongQing China


July 31, 2009

A. Hi, Nick. There are many choices for fastener coatings including galvanizing, organic coating, mechanical plating, electroless nickel plating, zinc alloy platings, aluminum Ivadizing, and so on.

But I'm afraid that I don't understand your question, and don't understand the application in a "long tail boat". You need to determine whether there were or weren't galvanic compatibility issues (and you haven't told us what other metals are involved), and you have to determine whether the plating was defective in this particular case, or whether zinc plating is a bad specification. Zinc plating is rarely sufficient for exposed outdoor use, and certainly not in a marine environment.

No other corrosion resistant coating will be as inexpensive as zinc plating though.

Regards,

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


August 6, 2013

A. No doubt that Zinc plated fasteners are a cost effective solution. But during the acid dipping process prior to plating Hydrogen will penetrate into the fastener surface and shall remain entrapped due to Zinc plating. This will make the fastener brittle and is called HYDROGEN EMBRITTLEMENT. This will create micro cracks on the surface of the fastener which will propagate over a period of time, which will lead to failure of the fastener after a long time (say 6 months to one year). Hence you need to ensure that the plated fastener has undergone hydrogen de-embrittle treatment prior to use.

SANDEEP VIJAYAKUMAR
- BANGALORE, KARNATAKA, INDIA


October 2013

Hi. While Sandeep is right that the fasteners -- if they are hardened steel rather than low strength steel -- are subject to hydrogen embrittlement unless baked, and we appreciate that warning, his post seems to imply that zinc plated fasteners would otherwise be satisfactory. I think they won't be. I feel Nick's poor experience is probably not an anomaly, but what is to be expected from zinc plated fasteners in outdoor exposure.

Regards,

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

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