The influence of aluminum phosphates on graphite oxidation

Aluminum phosphates are of interest for use as inhibitors for the oxidation of carbonaceous materials. This investigation analyzes three aluminum phosphates of nominal compositions, Al₂O₃ · P₂O₅, Al₂O₃ · 3P₂O₅, and Al₂O₃ · 9P₂O₅, in terms of thermal stability and efficacy to inhibit the thermal oxidation of graphite flake in pure oxygen. Temperature programmed oxidation reveals that the onset temperature for oxidation is increased by 75-100 °C as a result of the aluminum phosphate treatments. Isothermal oxidation rate measurements show that while the overall oxidation rate constants are lowered by the aluminum phosphate treatments, the apparent activation energy remains constant (54-59 kcal/mol), which indicates that the reduction in rate constant with temperature is due to a lowering of the pre-exponential term.