Two reasons.
The chemical in the extinguisher, Mono-Ammonium Phosphate, is mostly corrosive when it’s exposed to water. After release aside from an obvious water spill, this comes from humidity in the air. The extinguisher itself is sealed when it’s just sitting waiting to be used, so humidity won’t seep in over time.
Additionally manufacturers coat the inside of the extinguisher with a plastic coating to prevent corrosion.
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