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.

Answer 2

The powder is ammonium phosphate. It does react with iron in a few different ways. If you mix it with iron and water you get iron phosphate hydrate which is a dust that just flakes off. But if there is no water present then you get iron phosphate which forms a tough coating to the iron. In fact it is used as a primer to make paint stick to the steel better. When refilling a fire extinguisher you need to make sure not to get any moisture in it as this may corrode it over time. This is also why you should get your extinguisher inspected and refilled every ten years.

In addition to this fire extinguishers usually have a coating inside made of chemical resistant plastic. This is to protect the steel even better in case there is some moisture in it. This is a common practice for a lot of metal containers.