Letter 16063

Zinc Plated Fasteners vs. Stainless Steel  

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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, WI

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, 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. I fear that you may find the cost unacceptable though.

I am a bit confused by your reference to stainless steel brackets because these will (theoretically) cause the aluminum to galvanically corrode regardless of the fastener material. And, presumably being larger in surface area than the hardware, they will have a more powerful impact.

All of the above not withstanding, you didn't mention how the aluminum is corrosion proofed, so the question is a bit open. But I have seen stainless fasteners used on architectually anodized aluminum in outdoor exposure without any substantial problems. The zinc plating is not a good idea in my opinion because it will be short lived.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

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