I'd never heard of Galvanic Corrosion until I started asking why hardware manufactures always say not to mix different metals for fasteners and hardware. After all, if you have zinc coated hangers (like I do) and want to use stainless nails (yup, me) that would be great right? Maybe. Or maybe not. Depends on who you ask apparently.
The Escape Hatch
At a very high level, Galvanic Corrosion means different metals can exchange electrical currents, possibly causing one to degrade much faster than it would usually. That may be enough info for some. Got it, just use zinc nails with my zinc coated joist hangers... or stainless with stainless. Good to go. Jump off now before this gets complicated.The whole story is way more complicated.
The Standard Line
Here is a quote I've seen repeated in various forms throughout the internet:Can I combine stainless steel and hot-dip galvanized steel?this is from "Corrosion of Metal Connections in Preservative-Treated Wood Structures" on Toolbase.com, a research lab.
No. Stainless steel connectors or fasteners cannot be used in combination with galvanized connectors or fasteners. These two metals are sufficiently dissimilar that zinc will be protecting the entire stainless steel component and, therefore, will corrode at a more rapid rate.
And another from DeckMagazine.com
For instance, because stainless steel will degrade galvanized steel, especially in a marine environment, don’t install galvanized joist hangers with stainless steel nails.in a very good article "Fighting Fastener Corrosion".
So it sounds pretty cut and dry, even if they don't really site any scientific evidence. Don't mix Zinc coated and Stainless hardware. Except that isn't what everyone is saying.
The Heretics
So it gets a little more complicated from here. Here is a quote that essentially contradicts (or at least qualifies) the opinions above.When zinc-coated screws are used to fasten stainless steel plates, the screws will corrode rapidly. But when stainless screws are used to fasten zinc-coated steel plates, neither the screws nor the steel will corrode particularly fast.this is from a Metallurgical Engineer in "Understanding Galvanic Corrosion" at Design News. It's a very interesting article. It isn't taking a position on whether you should use stainless hardware, just discusses the metallurgy behind Galvanic Corrosion.
Another, even more technical resource, gives a remarkably similar answer
Does galvanic corrosion occur when these metals are in contact?
... when stainless steel (300 series) are in contact with zinc, neither material will suffer additional corrosion, or at the most, only slight corrosion. The slight corrosion is usually tolerable when in this bimetallic contact.
I'm not even going to try to summarize the science. I'll just say they make a pretty convincing case that as long as the stainless is smaller than the zinc, the corrosion effects will be minimal.
The Manufacturer
Since I've already selected my joint hangers - Simpson LUS210Z - what does Simpson have to say? I emailed their customer service. Here is their response:Simpson Strong-Tie® recommends fasteners with a coating that meets the specifications of ASTM A153 be used when using ZMAX/HDG galvanized connectors.
We also recommend the use of stainless steel fasteners when using stainless steel connectors. It is a known fact that the galvanic corrosion can occur when dissimilar metals are in contract in the presence of an electrolyte.
Simpson Strong-Tie has not performed any research on galvanic corrosion or the extent of corrosion when stainless steel fasteners are in contact with a ZMAX connector.Here is the chart they referenced from “The Specialty Steel Industry of North America”.
Attached is information on galvanic corrosion published by “The Specialty Steel Industry of North America (SSINA)”.
Thank you,
Customer Support Technician
Engineering Department
Simpson Strong-Tie Company Inc.
The Verdict
Well, I'm going to email some of the sources above, see if I can get any comments. Personally, if I could ask for one thing, it would be real field research on long-term effects of Galvanic Corrosion. Particularly in the context of a marine environment. I doubt I'll get it. So unless my inspector enforces his opinion on this, I'll be left with conflicting information to make my choice.
If I had to decide right now, I'd go with stainless nails on my zinc hardware. I think the risks of corrosion of the zinc hardware don't overwhelm the known effects of ACQ pressure treated lumber and marine air on non-stainless nails. Is it the right decision? I guess I'll let you know in about 15 years.