THE EFFECT OF THERMAL STRESSING ON PERFLUOROPOLYALKYLETHERS AT ELEVATED-TEMPERATURES

Fultz et al. have reported that the thermo-oxidative properties of lin ear PFPAEs can be improved by stressing the fluid at elevated temperat ure (371 degrees C) in the presence of air. A study of M-50 steel coup ons exposed to unstressed and stressed linear PFPAE fluids at 260 degr ees C and 330 degrees C each reveal complex surface layers. For the co upon exposed to the unstressed fluid at 260 degrees C, a subsurface la yer is observed between the iron oxide and iron substrate that has bee n characterized as being composed of FeF2. In contrast, the coupon exp osed to the stressed fluid has a marked increase in the iron oxide thi ckness (similar to 2-3 times) when compared to the unstressed sample a nd shows no evidence of a buried fluorine-containing layer. An increas e in temperature (330 degrees C) in the stressed fluid O-C test was re quired to form a subsurface FeF2 layer. It is proposed that the elimin ation of the fluorine layer found on the M-50 substrate increases the upper temperature limit found from the oxidation-corrosion studies. Th e increase in the oxide layer thickness implies that the FeF2 layer fo und in the unstressed sample acts like a diffusion barrier which inhib its the outward movement of Fe-0 and the decreased rate of iron oxide growth.