What is hydronalium?
That is an aluminum manufacturing company in Norway. Amazing what you can find with Google.
Are you sure you don't mean Hydronalium, an Al - Mg alloy?
Thanks D B. I edited my post. Aluminum or aluminum alloy strings deposit on the fingerboard. Is this an oxide?
Yes most likely an oxide or oxide hydrate. But once that happens, if the surface of the fingerboard becomes abrasive, I suppose it is possible for small amounts of the native metal to be transferred as well. I often see that the marks underneath my strings are shiny, which makes me wonder about that.
If it is an oxide, perhaps there is a limit to the thickness that can develop on the string winding. That would suggest minimizing the amount of transfer to the fingerboard to preserve the function of the string.
Aluminium oxidises easily in air, a protective adherent layer of oxide forming on the surface, whereas, in contrast, iron oxide (rust) is not adherent and falls off, exposing the underlying iron to yet more oxidation.
Years ago we got bridge spans in weathering steel. The steel would only oxidize to a minimal thickness.
I think there's really very little you can do about that. Probably the best thing is to just make sure your hands are clean before you play. The amount of material that is being transferred there is extremely small. Your strings are not wearing away underneath from contact with the fingerboard.