Luger paint color

[ithacaartist]

Originally Posted by Olle

I believe they use a hot dip process, much like hot dip galvanizing (simply put: you dip it in molten metal). An automotive engineer told me that it often replaces zink on parts where the temperature varies a lot (like on exhaust systems), as the thermal expansion of aluminum is more similar to steel. The hot dip method is a bit "sloppy" and not really suited for gun parts where close tolerances are needed.

Ollie,

Yes, that figures. That would mean that one could aluminize a steel piece first, to establish an aluminum outer surface which could, in turn, be worked carefully down to a desired smoothness, and then anodized purple!

An explanation for why Al is used rather than Zn for repeatedly heated and cooled components may lie in aluminum's higher ductility than zinc. Aluminum is generally softer than zinc, more ductile/less brittle. It would tend to hang on during dimensional fluctuations of the steel below, whereas zinc would tend more to crack off.

[Olle]

Originally Posted by ithacaartist

Ollie,

Yes, that figures. That would mean that one could aluminize a steel piece first, to establish an aluminum outer surface which could, in turn, be worked carefully down to a desired smoothness, and then anodized purple!

An explanation for why Al is used rather than Zn for repeatedly heated and cooled components may lie in aluminum's higher ductility than zinc. Aluminum is generally softer than zinc, more ductile/less brittle. It would tend to hang on during dimensional fluctuations of the steel below, whereas zinc would tend more to crack off.

If you could apply a very thin coat of aluminum, yes. I don't know if that's possible with aluminum though. The hot dip process will more than likely leave a coat that's too thick, and with some build-up in the nooks and crannies. I'm comparing with the problems you can have when using other metal coatings and if you compare with nickel, the only process that gives you a uniform coat is electroless plating. Even electrolysis can cause problems, as it won't cover evenly on irregular shapes. Then again, there might be a process that allows you to get an aluminum coat that's good enough for guns.

And you're probably right about the ductility. It has been quite a while since I discussed this with the engineer, but the problem with zinc is that it can start flaking if the metal underneath is expanding. This could very well be a matter of ductility rather than thermal expansion rate.