Flammability of Metals in Fluorine and Nitrogen Trifluoride

Fluorine and many of its compounds are more aggressive oxidizers than oxygen. Flammability test results for metals in fluorine and nitrogen trifluoride patterned after Air Products' tests of metals in oxygen are reported. Although, combustion in fluorine is distinctly different than combustion in oxygen, several of the thermodynamic properties and parameters used to understand metal flammability in oxygen in ASTM G 94 are presented for the fluorine-chemicals case. They suggest substantially different metal rank orders and combustion behaviors should be found for combustion in fluorine-containing oxidants. The data appear to support this prediction.

Despite the strong oxidizing nature of fluorine, nitrogen dilution levels at various pressures were identified at which test specimens did not propagate combustion in the configuration studied. Carbon steel rods 0.25-in (0.64-cm) diameter burned in atmospheric pressure fluorine but did not propagate upwards in fluorine/nitrogen mixtures of (%F₂): 35% at 115 psig (0.89 MPa), 30% at 565 psig (4.00 MPa), 25% at 1065 psig (7.44 MPa) or 20% at 2015 psia (14.00 MPa).

The nitrogen trifluoride molecule contains atomic nitrogen equivalent to a 25% nitrogen/fluorine mixture, but it absorbs energy upon dissociation. Ignition of 0.25-in. (0.64-cm.) diameter rods at various pressures was attempted for the metals: 304L and 316 stainless steel, nickel alloys C22 and C276, Nickel 200, copper, nickel alloys 600 and 625, Monel®² 400, Chrome-Moly 4130X, and aluminum. Interestingly, aluminum, as well as nickel 200, Monel® 400, and copper, appeared to have the highest thresholds, above 750–1250 psig (5.3–8.72 MPa), and 304L and 4130X each experienced sustained combustion at atmospheric pressure. Stainless steel 316 appears to have a threshold at or below 225 psig (1.65 MPa). Nickel alloys 600, 625, C276 and C22 burned at 1000 psig (7 MPa) but were not tested at lower pressures.