Letter 6181

Dry Chemical Fire Supression Agent Corrosion of Aluminum 

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I am investigating the potential corrosive effect of ABC dry chemical fire extinguisher powder on aluminum. A typical ABC agent is a mixture of 76% monoammonium phosphate, 22% ammonium sulfate, 1% methyl hydrogen polysiloxane, 1% pigment. Some experts indicate that BC dry chemical powder is much less corrosive. Typical BC agents are 93% potassium bicarbonate, 4% muscovite mica, 4% magnesium aluminum silicate (attapulgite clay), 1% methyl hydrogen polysiloxane and 1% pigment (OR) 90% sodium bicarbonate, 5% mineral silicates and 5% stearates. Why are BC agents less corrosive to aluminum than ABC agents?

Clifton L Foster
- Orlando, FL, USA

With any moisture content at all, I suppose I would see if there was a difference in the pH of the two mixtures, since Aluminum is amphoteric.

Maybe the BC is not as hygroscopic as the ABC?

Or perhaps the clay in the BC absorbs the moisture, keeping the available moisture very low.

Tom Pullizzi platronica.com Falls Township, PA

 

Keep in mind the princlple ingredient in a BC extinguisher is sodium bicarbonate, baking soda. In an ABC extinguisher, the monobasic ammonium phosphate is used. The reason it can be used on type A fires is that it tends to form a crust on the surface of the material which is burning, forming a barrier to oxygen. If you will note on the label, ABC extinguishers only barely perform as type A extinguishers, as they are listed as such by the number preceeding the ABC on the label. Water or foam is still the best for type A fires.

Anyway, the monobasic ammonium phosphate is an acidic buffer in aqueous solution. Using damp nitrogen to pressurize an extingisher is never a good idea, and residual ammonium phosphate will severely damage electrical components--plus, the stuff is so fine when used in an extingusher, it gets into everything. (this is why ABC extingishers should never even BE IN rooms with electronic equipment--use halon or CO2, even water instead)

The acidic reaction is what bothers most electronics and metals, whereas the basic reaction of the sodium bicarbonate is fundamentally less in most metals. Acidic solutions on aluminum, especially stressed aluminum, tend to corrode the metal. Polymer coatings might solve this problem; if you need one, let us know.

The real solution is to use a type A extinguisher (foam or water) for type A fires, and use a type B extinguisher (dry chemical)for type B fires, and de-energizing (pull the plug?) a type C fire makes it a type A or B. An ABC extinguisher is fundamentally best for type B, flammable liquid, fires, and fundamentally, sodium bicarbonate or potasium carbonates are better than ammonium phosphate for supressing fires.

Dale Woika
Surface Conversion Sciences - Bellefonte, PA

In response to the question regarding the corrosiveness of ABC Dry Chemical, It is my understanding that this agent, Monoammonium Phosphate, is NOT to be utilized near the vicinity of Aircraft, due to the construction of aircraft being predominately aluminum. Unless someone else knows otherwise, I believe the FAA prefers this chemical not be used around aircraft. Sodium Bicarbonate, Pottassium Bicarbonate, Halotron, or some type of Halogenated agent are the recommended agents.

Brent M. Saba
U.S. Air Force - Minneapolis, MN

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