The effect of thermal stressing on perfluoropolyalkylethers at elevated temperatures

Abstract

Fultz et al. have reported that the thermo‐oxidative properties of linear PFPAEs can be improved by stressing the fluid at elevated temperature (371°C) in the presence of air. A study of M‐50 steel coupons exposed to unstressed and stressed linear PFPAE fluids at 260 °C and 330 °C each reveal complex surface layers. For the coupon exposed to the unstressed fluid at 260 °C, a subsurface layer is observed between the iron oxide and iron substrate that has been characterized as being composed of FeF₂. In contrast, the coupon exposed to the stressed fluid has a marked increase in the iron oxide thickness ∼2–3 times) when compared to the unstressed sample and shows no evidence of a buried fluorine‐containing layer. An increase in temperature (330 °C) in the stressed fluid O–C test was required to form a subsurface FeF₂ layer. It is proposed that the elimination of the fluorine layer found on the M‐50 substrate increases the upper temperature limit found from the oxidation–corrosion studies. The increase in the oxide layer thickness implies that the FeF₂ layer found in the unstressed sample acts like a diffusion barrier which inhibits the outward movement of Fe⁰ and the decreased rate of iron oxide growth.