Chemistry question!

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Jonnywishbone
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Chemistry question!

Post by Jonnywishbone » Wed Oct 12, 2022 1:15 pm

I'm posting this here at least partly because Charles is a chemist and he may see it.

Some scrote nicked an el cheapo plastic dust cap from a valve on one of by wheels. Annoying and weird but hey, there's a cost of living crisis and who wants to fork out for plastic dust caps?

My understanding is that the TPMS valves are aluminium. I am considering buying some genuine BMW dust caps, part number 36122447401 which are also aluminium.

I'm aware that galvanic corrosion is an issue which can and will cause metal caps to seize on metal valve stems, at least when the metals are different, due (I presume) to galvanic corrosion.

My question is, is galvanic corrosion an issue if both stem and cap are both aluminium? If not, would there be another reason not to use these caps?

TIA.
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Charles
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Re: Chemistry question!

Post by Charles » Wed Oct 12, 2022 5:42 pm

No pressure!

I assume you are referring to an external exposed valve rather than one hidden behind the centre cap.

Unless you are leaving the cap untouched for years, then the chances of any galvanic corrosion is minimal. As a point of reference, the hidden valve/cap is metal/metal and these don't suffer.
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Re: Chemistry question!

Post by Jonnywishbone » Wed Oct 12, 2022 6:18 pm

Thanks Charles and sorry for the name drop.

Yes it's an exposed valve. I can be a bit lax with manually checking pressures due to the TPMS but the caps would never remain untouched for longer than a few months.

Would I be right in assuming then that the numerous reports online of seized caps/valves is due to cheap/non-aluminium caps being used?
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Charles
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Re: Chemistry question!

Post by Charles » Thu Oct 13, 2022 8:13 am

Jonnywishbone wrote:
Wed Oct 12, 2022 6:18 pm
Would I be right in assuming then that the numerous reports online of seized caps/valves is due to cheap/non-aluminium caps being used?
I'm afraid I have absolutely no idea on that one but my guess would be poor maintenance rather than anything else.

If you are concerned then you could smear a very thin coating of Vaseline on the thread of the valve stem and then screw the cap on. As well as reducing any metal on metal contact - and hence any potential for corrosion - it will also provide a barrier for water/salt/grit etc getting between the stem and the cap. However, make sure that it is only on the thread and is very thin, otherwise you might transfer some into the valve core which may cause leakage of air.
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Re: Chemistry question!

Post by Jonnywishbone » Thu Oct 13, 2022 8:26 am

Thank you Charles.
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Re: Chemistry question!

Post by Guy » Thu Oct 13, 2022 6:18 pm

I'd avoid any metal valve caps.

I had metal ones on my 640d and at the most recent tyre change they had corroded in place, could not be removed by the mobile fitter and so they had to go to a centre and part of the TPMS was replaced for two of my wheels.

The tyre fitter said they have to do it regularly and recommended to only ever use plastic caps, as it's not worth the risk for something you can barely see.

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Re: Chemistry question!

Post by PerryGunn » Thu Oct 13, 2022 9:39 pm

If you really want metal caps (and you hunt around a bit), you can get metal caps with a plastic liner that avoids the corrosion problems
caps.jpg
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Re: Chemistry question!

Post by ali » Fri Oct 14, 2022 12:17 pm

I fit loads of aluminium anodised valve caps to all the bikes I sell in my shop and as Charles mentions a smear of grease or vasoline will do the trick to stop them sticking
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Re: Chemistry question!

Post by Jonnywishbone » Fri Oct 14, 2022 1:19 pm

I’ve hedged my bets and ordered the aluminium ones and some el cheapo plastic ones (at least partly because I expect the nice ones to get nicked!). I might swap between metal and plastic every time I do a tyre pressure check which will reduce the risk. The Vaseline thing sounds like an eminently sensible additional precaution.

Thank you all.
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