The reversal of the rusty engine point implies that the uncorroded condition is the most stable. Probably not. All compounds slowly seek the state of lowest energy.
The entire basis of the photovoltaic industry is thin film oxides, their production, and their life cycle management. Billions of dollars worldwide are being invested into exactly what is being talked about here. For parallel reasons, energy absorption, light reflectivity, and lifespan when exposed to sunlight.
Because it is folly to argue with saints and ghosts, I would refer back to my earlier remark, that time is relative, and that there are molecular structures formed in the fire that are unstable. Their "desire" is to become something else that is more energy stable. Plenty of quality papers being written these days about trying to make photocells last. Good surface chemistry and group theory stuff (for those inclined).
In the interest of common ground, I would also point out that the same standards of uv stability necessary for an intergalactic mission don't have to be applied to decorative differential corrosion in firearms. In fact, our interest in case colors pre-dates interplanetary travel by a century. So,in my polite opinion, the stability of colour is about relative stability, not absolute stability. And that is good enough for me.
And for the purposes of collecting, enjoying, and using firearms, relative stability is more than adequate. No harm comes from less exposure to sunlight, less rubbing, or obsessively maintaining a thin film of anti-oxidant protection over case hardened metals.
I am always reminded that if we stand around long enough, we are all slowly becoming dust.
