[skogas]

Hello,

I've been removing rust from the motorbike parts using Citric Acid (~15% solution with water, initial temperature of ~75'c to get the reaction going).

It's been working really well for me, but when I put a gas tank cap into it - it went all fizzy, and this reaction was going strongly for hours and hours. After about 5 hours the solution became all grey, so I just flushed and rinsed it.

I found that the most likely source of this strong fizzy reaction was a grey plate (that was hiding under the rubber cap), that seems to have thinned out after this.

I suspect that it might be lead, as it's relatively easy to bend thin bits of the plate and they keep the shape.
So I just wanted to check with the professionals if it could indeed be lead, and whether the fumes from the reaction (or water in the solution) could be toxic.

Thank you

[Arkcon]

Unlikely to be lead, but you could have a professional check.  Urm ... that is a motorcycle design profession.  My guess is the grey plate was zinc metal, meant as a sacrificial electrode welded in place to prevent gas cap corrosion.  And you've just lost it.  Get another cap. 

It could conceivably be lead, if it were an automobile water radiator cap, but its not.  Lead doesn't fizz with weak acids.

[skogas]

Thank you very much Arkcon, that makes sense now!
Luckily I still have a fair bit of this plate left (it's a 1955 BMW, so those caps aren't easy to come by)

Thanks again, and I'll be more careful now - just didn't expect that something like Citric Acid can have such a powerful reaction with metals.