Hot corrosion behavior of cast Co-22Cr-11Al alloy and Co-22Cr-11Al-0.3Y electron beam PVD coating on in 738

Abstract

The initiation and early stages of propagation of low temperature hot corrosion (LTHC) attack at 700 °C have been studied on a cast CoCrAl coating alloy and on a commercially prepared electron beam physical vapor deposited (EBPVD) CoCrAlY coating on a drop-cast IN 738LC substrate. To evaluate the role of active elements such as yttrium and hafnium on the LTHC degradation process, some specimens of cast CoCrAl alloy were partially ion-implanted with either yttrium or hafnium or cobalt. The specimens of cast CoCrAl coating alloy were pre-oxidized for 1 h at 700 °C before exposure to conditions that simulate the LTHC attack typically observed on hot-end components of gas turbine engines operated in a marine environment. Specimens of the EBPVD CoCrAlY coating were not given the 1 h pre-oxidation at 700 °C before exposure to the LTHC environment. Hot corroded specimens were subsequently examined by scanning and transmission electron microscopies and by energy-dispersive X-ray analysis. The LTHC results obtained on the cast CoCrAl coating alloy and on the commercially prepared EBPVD CoCrAlY coating on IN 738LC substrate are presented and compared. Fundamental mechanisms of initiation and propagation of hot corrosion attack for this class of coatings have been formulated that are consistent with the experimental results.