Well, the alternatives to Zinc (die-cast, Zamak, etc.) are brass and plastic. If you can live with plastic (i.e., non-structural application), 3-D printing holds a lot of hope for the future. More hope than getting help from some manufacturers, IMO 
@Hawkshaw it's definitely possible for die-cast metal to develop localized exterior corrosion from being stored in a high-humidity environment. I believe zinc tabs are used as sacrificial anodes on salt-water marine drives. In this application they corrode completely away! Remember, O-gauge trains are indoor pets!
I've noticed that some locos are especially susceptible, perhaps depending on whether the castings were chemically treated at the factory, and how they are painted. I don't believe the exterior corrosion can be fully removed once it starts. I'm sure you've seen an old penny (back when they were made of pure copper) with green crystals on it. You can remove the green crystals using a vinegar bath, but I don't think you're supposed do this to a rare coin, because you're actually removing some of the copper!
I suppose you could media-blast a corroded loco shell down to bare metal and repaint. Inevitably there would be some loss of detail, and the pitting would remain, kind of like a woman who suffered from acne in high school laying a heavy foundation over her ravaged skin. 
There is probably some kind of sealer that you should apply to the zinc after blasting, and before the paint. I know that some locos receive this kind of treatment at the factory, but I don't know what a home hobbyist would use for sealer.
The exterior corrosion you're asking about is different from the classic Zinc Pest (intragranular corrosion) that comes from a contaminated batch of alloy. If there was excess lead in the mix, the metal will continue to react with itself until it crumbles to dust. All of Ben's hoppers didn't fail because of humidity (although I suppose moisture and a wide range of temperature changes could speed up the process.)
So sorry to see/hear this Ben, hope you're able to get at least some of them back on the rails!
