Adherence of Al2O3 coating layers to steels: The detrimental effect of sulphur and the beneficial effect of rare earth metals☆

Abstract Based on our previous work of scale spalling during cyclic oxidation and the surface segregation of sulphur, we considered that the adherence of an Al 2 O 3 coating layer to substrate steel will be weakened by the segregation of sulphur at the Al 2 O 3 -steel interface. In this work, the surface segregation rate of sulphur was measured on type 304 steels with different sulphur contents, type 310S steels with and without rare earth metal (REM) addition and MA6000 alloy with Y 2 O 3 dispersion. The same types of steel were then coated with Al 2 O 3 and the spalling of the Al 2 O 3 layer during cyclic oxidation examined. It was found that the extent of spalling was large on steels having a large surface segregation rate of sulphur and vice versa. Thus we concluded that sulphur segregation is detrimental to Al 2 O 3 -metal adherence, and that the beneficial effect of REM is attributed to the suppression of sulphur segregation through trapping it as a stable sulphide in the matrix.