BEHAVIOR OF OXIDE SCALES ON ALLOY 800-H AND HK-40 DURING THERMAL CYCLING

Cyclic oxidation causes degradation of alloys used in the chemical and petrochemical industries. In this paper, the behavior of the protecti ve oxide scales formed on Alloy 800 H and HK 40 was investigated under thermal-cycling conditions with upper-hold temperatures of 900-degree s-C (Alloy 800 H) and 950-degrees-C (HK 40). The atmospheres in the te sts were air, air + 0.5% SO2 and Ar-5%H2-50%H2O. Tests were accompanie d by acoustic-emission measurements in order to detect scale failure i n situ during the experiments. During cooling the scales were under co mpression which led to spalling when critical stress values were reach ed in the scales. The outer-spinel partial layers are more prone to sp allation, and the presence of SO2 increases the amount of acoustic-emi ssion activity (scale damage). In the case of HK 40 the oxide scales o n the as-cast surfaces showed better spallation resistance than those on the ground surfaces. Quantitative model considerations were able to describe the spallation behavior of the protective scales investigate d, and critical-temperature-drop diagrams for scale failure are given. The model approach was supported by results from the acoustic-emissio n measurements.