Why is the extinguishing agent inside a typical abc dry chemical fire extinguisher corrosive to metals once it is discharged, but the extinguisher itself doesn’t rust when not discharged even though the agent is always in contact with the inside of the extinguisher?

Question

Why is the extinguishing agent inside a typical abc dry chemical fire extinguisher corrosive to metals once it is discharged, but the extinguisher itself doesn’t rust when not discharged even though the agent is always in contact with the inside of the extinguisher?

In: 1

Answer 1

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.