VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 Strange turquoise gas tank deposit

I got around to pulling off the gas cap of the 1914 Baby Grand the other day - the car that has been discussed here recently. After some 90 years of sitting, I can only assume the gas cap has been on there all that time, too. It had pretty well "become one" with the filler neck, but careful tapping and a little WD and I finally got it to release.

Inside the cap and top of the filler neck has a turquoise colored, grainy deposit that I have not encountered before. It is only inside the cap and nearby area of the neck, but not all over the inside of the tank. A flashlight shows the inside of the tank to be bone dry of course, with some "varnish" scales, and general orange-brown dust like you might expect.

I would think from the color that this deposit is some kind of copper salt deposit. The gas cap was originally nickel plated, so again I would assume there was a copper plating under that, which probably played a role inside there over the decades.

Any of you chemist types (like Chipper) have an opinion on this?

Don,
That color is typical of several copper salts as you suspected. It could come from copper plating but my guess is the cap is brass or bronze. Copper is a major component in brass/bronze and its oxidation creates copper salts. My suspition without analysis is azurite, Copper (II) carbonate, basic 2CuCO2Cu(OH)2 if you want the formula. It is often found on brass water pipes and fittings particularly hot water applications near a leak or high humidity.

OK, thanks Chipper.

I think of the copper carbonate deposit as typically being more green, less blue than this. But we should have had no water on the INSIDE of the tank where this appears (other than atmospheric humidity) - just primitive gasoline, slowly evaporating. Source of copper would be the plating underlying the nickel plating. Or maybe that cap is made of bronze, not ferrous. I should look at that. Wouldn't think it is brass, though.

Don,
The color of the other form of copper (II) carbonate (malachite) is more green. The bluish one forms with less water. There is enough atmospheric moisture that gets past the threads and drawn into the tank with temperature changes. Yes it is a slow process but that car was stored a long time. I could describe the process but won't confuse most of you with the facts. Suffice it to write that alloys are subject to molecular migration and several interesting chemical reactions.

OK. And yes, on inspection the gas cap is cast bronze with machined threads, then nickel plated, and a gasket.

And - the hubcap wrench fits the gas cap!