Looks like heavy aluminum corrosion, or a metal fire. The problem with the metal fire is that those create their own oxygen and are hard to burn themselves out. The problem with heavy aluminum corrosion, is what would corrode it that fast?
It would be interesting to see tests conducted on this tank.
Reverse image search pegs the earliest copy to this website http://armchairgeneral.com/panzer-tanks-found-in-norway.htm
Although it looks like all of the images have been scrubbed.
Here in North Carolina, I'm not sure what the pH of the soil is, but I've seen a Chrysler 727 Torque-Flite set on the ground for about 7 years and the soil had eaten completely through the case so you could see the clutch packs and bands inside. Also, metals don't "create their own oxygen" unless you're making components out of unrefined ore.
Also, metals don't "create their own oxygen"
Military grade and aviation grade alloys do. Pray you never see class delta fire. It's why aircraft carriers have bulldozers, and why you see them jettison aircraft during the forestall incident.
Some metals will burn quite vigorously, but they're using atmospheric oxygen. That's why they keep lab samples submerged in kerosene, to keep the oxygen away.
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