Rusty bolts and aluminium fittings...

[[XAP]Bob]

I have a wheelchair attachment which clamps onto the footplate of the wheelchair, it has a length adjustment to deal with different footplate depths/styles.

This adjustment comprises a piece of aluminium which slides along a channel, controlled by a bolt which goes into a threaded section of the main clamp structure (also aluminium).
This bolt is currently captive (it has a few threads removed and a circlip installed which stops the sliding piece moving up the bolt).

All very good, except that I've just had to soak the bolt in WD40 for three days to get it out - it's really rather rusty.


There is none of the white dust I'd associate with galvanic corrosion.

Is said bolt likely to seize rapidly if I reinsert with a dollop of grease?
It's not as if I'm likely to want to adjust it again, but I'd always rather have the option...

[tim-b]

Hi
Stainless bolt? More corrosion-resistant both in contact with alu and in general
Regards
tim-b

[[XAP]Bob]

I meant the existing bolt... Inpresume a replacement, installed with grease, would fare batter - if I can replicate the captive bolt mechanism

[Brucey]

grease can't hurt but if left without a coating of some kind, aluminium and steel tend to rot as soon as there is any moisture present. I think that depending on the pH of the arrangement, you can get corrosion mostly in one or the other metal.

cheers

[Gattonero]

Who's the cleaver manufacturer that installs adjusting bolts without greasing the threads?

[[XAP]Bob]

Who's the cleaver manufacturer that installs adjusting bolts without greasing the threads?

Second hand device, although it doesn't look like it's ever seen grease.

[ANTONISH]

If you are going to use grease make sure it doesn't contain lithium.
I would suggest silicon grease or teflon grease.

[[XAP]Bob]

Engage dense mode..

Why not a lithium base grease (which is what my general purpose grease says it is)?

[alexnharvey]

Galvanic corrosion concerns.

[[XAP]Bob]

Galvanic corrosion concerns.

Effectively lithium acting as a catalyst to one bout of corrosion or the other... cheers

[alexnharvey]

However, there's no lithium metal in the grease, there is a lithium soap.

[Brucey]

the thickeners in most greases are soaps of one kind or another. If the soap doesn't contain lithium it might contain calcium instead, and it isn't clear to me that this makes an intrinsic difference to corrosion rates. Copper greases vary but they often use a clay thickener instead. If you just want a barrier of some kind that won't instantly wash off, Vaseline may do the trick.

cheers

[CJ]

Grease is a mixture of oil and soap. When mixed, the soap forms a stringy sponge that contains the oil and when squeezed, lets a bit out to do its lubricating job. When all the oil is squeezed and/or evaporates out of grease, the now dry soap is free to absorb water and react with the metals it's in contact with. Lithium soap has the perplexing ability to form a very hard cement-like compound with aluminium and iron, so it's a particularly bad thing to spread on an alloy stem before inserting in a steel fork and then forget about for a few years! And anyway, there are far better compounds specifically designed to lubricate sliding rather than rolling contact whilst providing long-term corrosion protection. Stuff like Copaslip and Alumslip.

If the bolt is stainless steel that is, by the way, very bad for corrosion of the alloy, because chrome and aluminium are a long way apart in the electrochemical series. A galvanised bolt would be better, because zinc and aluminium are pretty close. Aluminium on aluminium however is not so good either, because similar metals have a high coefficient of sliding friction, but appropriate lubrication can overcome that and I'd choose Alumslip.

[Sweep]

So any grease with lithium best avoided full stop?

So that you don't have any around and tempted to use inapprooriately?

I understand folk often use it for bearings but then also understand that teflon greases are fine for that?

[Gattonero]

So any grease with lithium best avoided full stop?

So that you don't have any around and tempted to use inapprooriately?

I understand folk often use it for bearings but then also understand that teflon greases are fine for that?

There's many types of aluminium alloys, and their properties are incredibly different.
I would expect such parts not to be an exotic alloy, but one that is commonly used for cast&machined parts. For which I've always used Lithium-based grease and never seen any problem with it.