Type II Hot Corrosion Screening Tests of a Cr2AlC MAX Phase Compound

Abstract

Low temperature hot corrosion tests were performed on bulk Cr2AlC MAX phase compounds for the first time. This material is a known alumina-former with good oxidation and Type I high temperature hot corrosion resistance. Unlike traditional (Ni,Co)CrAl alumina-formers, it contains no Ni or Co that may react with Na2SO4 salt deposits needed to form corrosive mixed (Ni,Co)SO4-Na2SO4 eutectic salts active in Type II hot corrosion. Cr2AlC samples coated with 20K2SO4-80Na2SO4 salt were exposed to 300 ppm SO2 at 700°C for times up to 500 h. Weight change, recession, and cross-section microstructures identified some reactivity, but much reduced (< 1/10) compared to a Ni(Co) superalloy baseline material. Layered Al2O3/Cr2O3 scales were indicated, either separated by or intermixed with some retained salt. However, there was no conclusive indication of salt melting. Accelerated oxidation was proposed to explain the results, and coarse Cr7C3 impurities appeared to play a negative role. In contrast, the superalloy exhibited outer Ni(Co) oxide and inner Cr2O3 scales, with Cr-S layers at the interfaces. Massive spallation of the corrosion layers occurred repeatedly for the superalloy, but not at all for Cr2AlC. This indicates some potential for Cr2AlC as LTHC resistant coatings for superalloys.

Cr2AlC MAX phase as bond coat for thermal barrier coatings: Processing, testing under thermal gradient loading, and future challenges

“…Even though most of the investigation has been focused on MAX phases as bulk component, coatings as protective material against oxidation and corrosion have gained considerable interest in the last years. For example, advanced turbine disks can be protected against Type I and II of low‐temperature hot salt corrosion (LTHC) . Cr 2 AlC thin films have been deposited by high power impulse magnetron sputtering on a low γ'solvus and a high refractory content alloy, named “LSHR” and developed at NASA Glenn Research Center.…”

“…For example, advanced turbine disks can be protected against Type I and II of low-temperature hot salt corrosion (LTHC). 8,9 Cr 2 AlC thin films have been deposited by high power impulse magnetron sputtering on a low γ'solvus and a high refractory content alloy, named "LSHR" and developed at NASA Glenn Research Center. Cr 2 AlC coatings with thicknesses of 25-30 µm prevented hot corrosion pitting, improving up to 90% the low cycle fatigue (LCF).…”

Low Temperature Hot Corrosion Screening of Single Crystal Superalloys

“…Low temperature, Type II hot corrosion (LTHC) was performed exactly as that in the companion study [4]. Small, roughly 1 × 5 × 10 mm samples were polished to a 2400 grit emery finish and ultrasonically cleaned in ethanol.…”

“…Accordingly, serious effort has been put forth to investigate corrosion resistant Ni-Cr-Y sputter coatings and their ability to diminish the corrosion pitting deficit [1]. The use of a corrosion resistant Cr 2 AlC MAX phase compound has also been explored for this application [2,3,4]. Damage tolerance and thermal expansion matching with superalloys were viewed as other positive attributes of this MAX phase.…”

“…In a recent LTHC screening test, Cr 2 AlC MAX phase samples and the NASA LSHR (low solvus high refractory) disk alloy were exposed to repeated salt coatings and exposed to 700 °C air with 300 ppm SO 2 , using 25 h cycles [4]. Weight change and appearance were followed with time up to 500 h. Elemental rasters were obtained for polished cross sections of samples exposed for 300 h. In general, the Cr 2 AlC samples produced moderate weight gains with some evidence of accelerated oxidation.…”