The role of HCl-impurities in the environment and Y for the high temperature corrosion of CoNiCrAl(Y)-coatings on a Ni-base-superalloy under complex loading conditions

Abstract

The influence of small concentrations of HCl in synthetic air on the high-temperature corrosion of an Y-free and an Y-modified CoNiCrAl-coating on the directly solidified Ni-base superalloy CM247LC DS was investigated at 1,000°C under isothermal and thermocyclic conditions, accompanied by acoustic emission measurements. Furthermore, thermomechanical fatigue tests were carried out. Both coatings formed an Al₂O₃-layer. Y had a positive influence on the scale adherence and changed the growth direction of the Al₂O₃-scale. Y-modified coatings showed substantial internal oxidation. Pore formation at the oxide/metal interface took place in the case of the Y-free coating during oxidation in synthetic air as well during oxidation in HCl-containing atmospheres leading to a decrease of scale adherance. No crack closure by oxidation was observed under thermomechanical fatigue conditions in the case of Y-modified coatings on the alloy CM247LC DS. For the Y-free coating in HCl-containing atmospheres scale damage could be detected by accoustic emission measurements even under isothermal oxidation.

Keywords:

• oxidation resistance
• CoNiCrAlY-coatings
• HCl-impurities
• cyclic conditions