doesn't the water and air rust the metal in an electrochemical process called oxydation?
bacterial processes, like the ones that do most of the work in hot compost, don't rely on acids to digest food, but rather, on enzymes. They are so small, they don't have room for openings, so everything has to be able to move through the cell wall. The whole process results in a net loss of hydrogen for the soil solution, not a gain. (as far as I know)
In other words aerobic bacterial action raises pH, but because you have iron, water, and air, you will get rust no matter what you do, short of immersion in water and installation of a sacrificial node that gives up ions easier than the barrel (which needs replacing as it melts).
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