How about better rust tokens?
1. Oxidization type
[OXIDIZES]
This is the standard oxidizes token, which would go on Aluminum, Titanium (if it is ever added), and many other metals like Lead, Bismuth and Zinc. In this case, the metal tarnishes to a different colour in air, and while the oxide is stable and will not wear the metal (as it does not flake off: aluminum oxide is very strong and transparent and protects the rest of the metal, and titanium oxide has similar properties), it lowers the metal's value or goods made of that metal if the tarnish is very advanced. The solution is to melt them down or possibly make some sort of polish.
[OXIDIZES_SPALLING]
This is what Iron does. You see, iron oxide flakes off normal iron, exposing more iron surface to oxidize, which is called a "spalling oxide". But many other metals oxidize in less obvious ways. The weapon will occasionally leave behind "a dusting of [metal] rust" and get a bit more worn. A spalling, oxidizing metal will have anything made of it get ever so slightly lighter over time. Implies [OXIDIZES].
2. Oxidization rate and conditions
[OXIDIZATION_MIN_TEMP:value]
The metal only oxidizes when above this temperature. Optional. Zinc, lead and many more exhibit behaviours like this, especially molybdenum (which is more-or-less stable at room temperature, but starts oxidizing at 300 Celsius or more).
[OXIDIZATION_MAX_TEMP:value]
The metal only oxidizes when below this temperature. Optional.
[OXIDIZATION_RATE:value]
The metal oxidizes faster, or slower. The value is abstract right now as I came up with it, but follows a linear scale. If MIN_TEMP and MAX_TEMP are set, then this will peak between the two temperatures and decline as it approaches either. If one is not set, it will increase towards infinity (or its melting point...). If neither is set, the oxidization rate will stay the same no matter how hot the metal is. If it is very high, the metal will just catch fire in air and be completely consumed (like caesium). This is so modders can have fun just like everyone else.
3. Oxide properties
[TARNISH_COLOR:color]
Sets the colour of the oxide. Slightly tarnished iron could be "gray tinged with rust" (rust is a DF colour). The colours would proceed something like "gray", "gray, with a touch of rust", "gray tinged with rust", "gray, with rust all over", "rust tinged with gray", "rust, with a touch of gray", and "rust", which would play out over decades. At the point of "gray, with rust all over", the item would gain a "rusty" adjective (if it has the SPALLING token) or an "oxidized" adjective (if it OXIDIZES), and its in-game colour would change. Bismuth oxidizes pink quite quickly, which is why its bars are pink currently: with this change, they would start gray and rapidly become pink.
[TARNISH_DENSITY:density]
For the sake of people who want their oxidizing aluminum to get slightly more dense over time.
[TARNISH_VALUE_WEAPONS:value]
To make the tarnish carry a slight value modifier on weapons, which increases as the metal tarnishes. These values can be negative, but I'm not sure how well DF raws handle negative integers. So, a +rusty iron sword+ will be worth less than an +iron sword+, and the longer you leave it to rust, the less it will be worth. I can't think of any oxides that would increase value (while aluminum oxide may be beneficial, it is undetectable with the naked eye) right now.
[TARNISH_VALUE_GOODS:value]
To make the tarnish carry a slight value modifier on goods, which increases as the metal tarnishes. Some tarnishes could feasibly increase the value, like green patina-covered copper. Others would still be considered detrimental, like iron rust (this isn't modern art, your buyers prefer shiny statues).