Posted by Rick Denney on February 09, 2004 at 11:59:06:
In Reply to: Offbeat plating question posted by Frank Rosenau on February 09, 2004 at 00:20:57:
Chuck has forgotten more about plating than I'll ever know, so I'll defer to his answer on the titanium nitride question.
I suspect that both the copper and the zinc in brass oxidize. Copper oxide is green, and zinc oxide is gray, while brass turns a pleasant brown over time. That oxide, as Joe suggests, is quite stable. If you subject the brass to electrolytes (such as the salt in perspiration) plus a corrosive acid or base, the brass might turn green, which suggests to me that the copper is oxidizing more quickly than the zinc. And if zinc oxidizes greater than copper, it leaves metallic copper. This can be superficial, making the surface look red, or it can perforate the brass, which is red rot. Brown brass instruments kept clean on the inside are probably pretty safe from either of these, if your sweat is not too corrosive, or if you keep your perspiration off the instrument.
Gold plating is chemically stable, of course, and strongly resists corrosion. But it is mechanically weak. Still, Daellenbach's Yamaha tuba that he used for a long time was gold-plated, like all the instruments used by the CB. I never noticed any big bare patches on the instrument. Silver-plate will oxidize, and I think this oxidation causes bare patches over time for two reasons: 1.) the tarnish itself is silver oxide, and the silver in used to shine, and 2.) we polish off that tarnish, which carries that silver with it. Since gold doesn't tarnish, we won't need to polish it, and a non-abrasive wipe-down will restore the shine. But if we try to polish out scratches, the gold will go away in a hurry, it seems to me. But if you don't polish it and don't subject it to abrasion, it's probably reasonably durable.
Gold-plating a tuba would be expensive (especially a big tuba), but it's been done and for someone with a prized instrument who might have especially corrosive perspiration, it might be worth it.
On a related concept, aluminum is frequently anodized to keep it from forming the white chalky surface as it oxidizes. But anodization is just a controlled oxidation process. It demonstrates that a layer of oxide is actually quite protective, if we don't polish it off. Core-ten steel was innovative because it was treated to oxidize (i.e., rust) evenly to form a protective film of superficial rust.
Rick "who only worries when brass oxidation is green or red rather than brown" Denney