Any metallurgists in the crowd?

Monel is a metal that is high in nickel, at 52% to 67%, with most of the rest being copper. It is supposed to have better corrosion resistance in saltwater than just about anything else. For example, Hobie uses monel rivets (which are more expensive than stainless rivets) in places where the rivets contact aluminum. So, presumably, monel is better than stainless at resisting galvanic corrosion.

According to the InterWeb, "Galvanic corrosion refers to corrosion damage that occurs when two different metals are in electrical contact in an electrolyte...". This happens, for example, when aluminum and stainless are in contact in saltwater.

So, which metals are similar and which are different wrt galvanic corrosion?  Apparently, it all boils down to the "anodic index". According to this article,

https://ficientdesign.com/galvanic-compatibility-metals-anodic-index/

metals in a "harsh environment" (which I assume includes dunking in saltwater) should be within 0.15V in their anodic indices to prevent galvanic corrosion.

This is where I get lost: The table provided at that link shows monel having an anodic index of 0.15V or 0.30V (it's listed twice, for some reason that I don't understand), while aluminum is at least 0.75V (depending on the alloy), and "8% chromium type corrosion-resistant steels" (which I would interpret to mean stainless) is at 0.50V. So, this tells me that the difference between monel and aluminum is at least 0.45V, whereas the difference between stainless and aluminum is more like 0.25V, so contact between stainless and aluminum would be better than contact between monel and aluminum, from a galvanic corrosion perspective.

So which is better, monel-on-aluminum or stainless-on-aluminum, or something else entirely?

I have no idea, but I’m curious to hear the answer. Galvanic corrosion is definitely a problem for saltwater kayak fishing. For example, the chamberlain downrigger release uses corrosion-resistant components, but it has stainless steel in contact with brass, and the stainless corrodes quickly at this location, presumably due to galvanic corrosion.

Stainless and aluminum are both commonly available, and both many different alloys that can be tailored to very specific applications, so why not just use all stainless or all aluminum?

As for the range given for monel on the chart, that probably depends on which specific alloy is in question.

Sounds like you are planning a project or modification ...

Sounds like you are planning a project or modification ...

Ha ha, yes!

I want to build a kayak trailer modeled after the Yakima Rack and Roll

https://www.etrailer.com/Kayak-Trailer/Yakima/Y08106.html

and make it as rust-proof as possible. Even using the Rack and Roll wheels and suspension, I think I can build it for about 1/2 the list price using 6061 aluminum tubing for the frame. The only issue I have is that I'd like to bolt it together (as opposed to having it welded), so that I can make modifications.

I had always thought that monel would be the ultimate thing to use to bolt aluminum together (well, other than titanium, which is way too expensive), and I even found 1/2" monel rods that only cost about 1/3rd more than 316 stainless. But when I looked a little closer at the galvanic corrosion issue, I got confused.

Btw, my aluminum Trailex trailer is held together with galvanized bolts. That seems to work sorta OK--the threads get rusty, and sometimes other parts of the bolts too. It seems like there should be a better option...

Ah, sweet! Love me a DIY solution!

Most boat trailers are either aluminum with galvanized bolts (as you noted), or welded and painted steel.

Galvanized bolts should work fine for what you are doing.  Wash after every use, maybe hit everything with a little lightweight oil once in a while, or something like this: https://www.westmarine.com/meguiars-trailer-corrosion-protectant-17035742.html

Why not just repair your current trailer? Or is it not the right size/configuration for what you are trying to do?

 

Look into boat anodes. They are sacrificial metals (like zinc and aluminum).

Why not just repair your current trailer? Or is it not the right size/configuration for what you are trying to do?

I am going to re-assemble my current Trailex trailer (in fact, I just spent several hours yesterday with a wire brush removing as much rust as I could). But, I've now got 2 complete AIs (except only 1 mast), and if they both don't have their own trailer, I'm afraid that the un-trailered one will get an inferiority complex...

Besides, I've always been a huge fan of the Rack and Roll trailer, but could never spend the money to get one. By DIY-ing most of it, the cost should be comparable to a new Trailex.

Why not just repair your current trailer? Or is it not the right size/configuration for what you are trying to do?

I am going to re-assemble my current Trailex trailer (in fact, I just spent several hours yesterday with a wire brush removing as much rust as I could). But, I've now got 2 complete AIs (except only 1 mast), and if they both don't have their own trailer, I'm afraid that the un-trailered one will get an inferiority complex...

Besides, I've always been a huge fan of the Rack and Roll trailer, but could never spend the money to get one. By DIY-ing most of it, the cost should be comparable to a new Trailex.

Ah, good call, I think I would also want individual trailers.  Another bonus to housing it yourself - you get to add in extra storage.