Aluminum Screws

We are in final development of a new sport product for use in the ocean. In one location on the part it would be ideal to use aluminum screws. Corrosion combined with weight combined with aesthetics would make black anodized aluminum screws ideal over 316 stainless. I’ve not used aluminum screws before so any advice would be appreciated. As the screws are not easily found off the shelf, custom screws need to be made in a significant batch before testing.

Here are a couple images that explain the part and some of the dynamics as background.

This is the relative size of the assembly. Shoulder width. The forces involved are the torsional forces that can be applied foot to foot in a gas pedal direction, toe to heel force.

Underside of the assembly showing the twist lock of the nylon torsion bars into the aluminum extrusion, this assembly is locked into a larger assembly that provides a single axis of rotation and twist.

The highlighted parts are extruded nylon bars with machined flats on the end to lock into the baseplates which are aluminum extrusions.

M8 x 35 mm 6061 screws, four per end, threaded in anodized aluminum extrusion.

Any suggestions or opinions on the use of aluminum hardware are appreciated.

I for one would advise rather not to use aluminium screws here. For two reasons:

1.: They are much weaker than steel screws. Did your engineers calculate the forces those screws would have to take?
2.: Aluminum does also corrode in salt water (surface). The thread might get messed up after a while.

In applications like sailing or biking you sometimens find titanium screws which can also be anodyzed in color
hues to match the design.

For a marine product like yours titanium might be too costly and aluminium to weak.
One could use V4A and live with having just another surface color and finish or you could cap them with an
additional plastic part.



Before I read your post, mo-i, I was going to suggest Ti, too. Much more strength and corrosion resistance. Plus Ti just looks cool!

Although, it’s not without it’s issues. You’d need to spec some anti-seize or similar barrier between dissimilar metals. (I’m pretty sure, anyway)

Yeah, aluminum is generally not an ideal choice. Overall bolt strength is probably not an issue here, but the threads are very weak, so if someone comes along and overtightens them, they’ll just tear out (or the hex drive will cam out). M8 sounds big to me, at first I was going to suggest going down to M6 in stainless, 16 screws of which would only be 45g heavier than using aluminum M8. (The weight difference seems pretty trivial in this application, but I will assume it matters.)

However… your main concern here, being in a saltwater environment, is galvanic corrosion. You don’t want more than a .15V difference between any metals in contact with one another. (Basic list here: Galvanic and Corrosion Compatibility Dissimilar Metal Corrosion, another one here: Galvanic series)

Titanium fasteners, apart from being very expensive and having a tendency to gall, are also very far from aluminum on the anodic index. You’ll also get considerable galvanic corrosion between 316 stainless and aluminum (I’ve seen this even in a dry climate.). The aluminum anode will erode around the threads, and they will inevitably fail. Without testing, it’s hard to say if that will take 20 minutes or 20 years, but it is a certainty. You can delay this with coatings, anodizing, etc., and that might be enough for something like this with a relatively limited lifespan.

If the center part has to be an aluminum extrusion, aluminum fasteners may well be your best option. Get the coarsest thread you can, give it a torx drive, and use a strong thread locker. Option #2 (or maybe #1): smaller screws in 430 stainless (NOT 316), hard coat anodize the aluminum, and test it thoroughly in a saltwater bath.

Thanks for all the input gentlemen. Great information.

We normally address the galvanic issue between the 316 and the aluminum with a teflon gel for isolation. The four M8 bolts in this location are a bit overkill to compensate with the lesser strength of aluminum. The real stress is loaded into the key shape of the machined bar and the extrusion

Scott, good tip on the thread size and the Torx head. If we decide to go ahead with aluminum, will choose that path. The depth of the threaded contact is 16mm before tension on the bolt, should be harder to strip that amount of threads out.

The most reliable solution may be M6 Stainless with snap fit black caps for aesthetic reasons as Moi suggests. And telfon gel isolation.

Everything is hard anodized in this set of parts.

We dealt a lot initially with galling of 316 o0n 316, that metal just seems to love to weld itself together in nuts and bolts. Solved that through Loctite Anti-Seize.

Someday I will get to design something that all of the screws are titanium, this part does not quite have the budget for that. And did not think of the galvanic difference between Ti and Al, just assumed titanium was magically inert.

@Scott, why 430 over 316?

Thanks again for all the input.

It’s several hops closer to aluminum on the anodic scale (I don’t have the numbers though, so it may not be a huge difference in practice). Hard anodizing may be enough to prevent problems either way, you have way more experience in saltwater than I do.

What about aluminum rivets with a backing washer?

This arrangement needs to be removable for tuning and for maintenance of bearings in attached parts.

Over the course of my career in sports design, the question of screws of rivets comes up. The choice of a rivet is more economical, but unforgiving, the downside is repair or replacement. The companies I have worked for have opted 95% of the time for screws, until a product becomes so mature that not change or replacement of parts is almost guaranteed.