Why has this happened to my tyre valves?
 

Had some different wheels on my S8 for a couple of months now, so I've given the original S8 wheels a deep clean and stored them since then. Went to unscrew the plastic valve caps today and they are all seized on.

Managed to get them all off easily with some pliers and the valve is a bit powdery, almost like when a battery goes furry.

Any idea what's caused this? :Confused:

http://i65.tinypic.com/21ocv8i.jpg

It does look like galvanic corrosion of some sort, usually happens when steel and brass touch. Odd that it should occur in a plastic dust cap.

Did you ever adjust tyre pressures on a wet winters day? Could be salt from the roads?

Or it could be the wet vapours from the tyre exiting slowly through the valve. I had that happen to mine too, but not quite a severely as yours.

They are TPMS valves so when the wheels were on the car, I never checked the pressures unless the car told me something was up.

They have just been stored in a shed and I'll get them back on the car soon so went to check the pressures the other day and that's what I couldn't unscrew the valve caps. I've had it with cheap nasty metal caps before on other cars and learnt my lesson but never just standard plastic ones :Confused:

I didn't really drive it much in the winter and when salt had been spread on the roads. Had a Lupo for that so I don't think that's the cause.

What did you use to clean them? Could that have caused a reaction if something had lurked under the valve covers, either from the last clean or prior?

We're the valves salvageable, did they clean up?

Kept the valves on when cleaning them. Used some Iron-X type stuff sparingly on the inside then snow foam and Fairy liquid/elbow grease so nothing too extreme

Well whatever caused it, I hope they cleaned up ok +++

I can't see the images, but if they are Audi caps with Audi logo then there are two types: one for standard valves and one for TPMS valves. Both types use different metal and if you use the wrong caps they will seize.

Had this on mrs.h's mini. Aftermarket caps reacted with the brass valve stems after only 8 weeks. So bad I had to get the valves cut off and replaced.