Oxidation kinetics, breakaway oxidation, and inversion phenomenon in 9Cr-1Mo steels

• DOI: 10.1007/bf00659183
• Publication Date: 2004-11-27T12:13:38Z
• Authors: A. S. Khanna; P. Rodriguez; J. B. Gnanamoorthy
• Keywords: Thermogravimetric analysis; Partial pressure

The oxidation behavior of 9Cr-1Mo ferrritic steels has been studied in air, oxygen, and steam at 1 atm pressure at various temperatures. Long-term experiments in air were carried out from 500–800°C by measuring the weight gains by interrupting the experiment at regular intervals of time. Short-term experiments in oxygen from 500–950°C and in air at 900 and 950°C were carried out by continuous recording of weight gain versus time in a continuousrecording thermogravimetric balance. Short-term experiments in steam were carried out using a special atmosphere furnace attached to the thermogravimetric balance. In air/oxygen, the weight gains at 700°C were lower than those at 600°C, while in steam, the weight gains at 800°C were lower than those at 700°C. This inversion phenomenon was observed for all the three steels viz. 9Cr-1Mo (high Si), 9Cr-1Mo (low Si), and 9Cr-1Mo-Nb steel. Examination of the oxide scales was carried out using SEM/EDAX, AES/ESCA, and X-ray diffraction techniques, and a mechanism is proposed for the occurrence of the inversion phenomenon.