BEHAVIOR OF OXIDE SCALES ON 12CR-1MO STEEL DURING THERMAL CYCLING

Soot-blower operation leads to thermal-cyclic-oxidation conditions of heat-resistant steels in conventional power stations. The consequence may be failure of the protective oxide scales and increased corrosive attack. The behavior of protective oxide scales on 12Cr-1Mo steel was investigated under isothermal conditions at 650-degrees-C and under th ermal cycling conditions between 650 and 300-degrees-C (200-degrees-C) . The tests were performed in air, air + 0.5%SO2, simulating the fire side, and Ar-5%H-2-50%H2O, simulating the steam side. Complete heat-ex changer tubes were used as specimens. The main instrument for the dete ction of scale failure was acoustic-emission analysis. In air and air + 0.5%SO2 the M2O3 scales with M = Fe, Cr were very thin and did not s how significant failure either during isothermal or during cyclic oxid ation. The thicker scales formed in Ar-5%H-2-50%H2O, consisting of sev eral partial layers, failed even during isothermal oxidation due to ge ometrically-induced growth stresses in the scale. Thus, in the thermal -cycling cooling periods there was only very little additional scale c racking. The scale behavior can be explained consistently by applying the existing quantitative models.