Met. Mater. Int., Vol. 21, No. 2 (2015), pp. 251~259 doi: 10.1007/s12540-015-4168-5 High Temperature Oxidation Behaviour of Ferritic Stainless Steel SUS 430 in Humid Air Xiawei Cheng 1 , Zhengyi Jiang 1, * , Dongbin Wei 1,2 , Jingwei Zhao 1 , Brian J. Monaghan 1 , Raymond J. Longbottom 1 , and Laizhu Jiang 3 1 School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, NSW 2522, Australia 2 School of Electrical, Mechanical and Mechatronic Systems, University of Technology Sydney, NSW 2007, Australia 3 Baoshan Iron & Steel Co., Ltd., Baosteel Research Institute (R&D Centre), Shanghai 200431, PR China (received date: 30 March 2014 / accepted date: 13 October 2014) The high temperature oxidation behaviour of ferritic stainless steel SUS 430 was investigated over the tempera- ture range from 1000 to 1150 ° C in humid air containing 18% water vapour. Isothermal thermogravimetric analyses were performed to study the oxidation kinetics. The microstructure, composition and thickness of the oxide scale formed were investigated via optical microscopy (OM), X-ray diffraction and a scanning electron microscope equipped with an energy dispersive spectrometer. The results indicate that breakaway oxidation occurs at all temperatures and that its onset is accelerated by increasing temperature. The growth rate of the multilayer oxide scale follows a parabolic law with apparent activation energy of 240.69 kJ/mol, and the formation of FeO is decreased when the temperature is higher than 1120 ° C. The inner oxide scale, Fe-Cr spi- nel, grows mainly inward and internal oxidation is observed even in a short oxidation test at 1150 ° C for 105 s. The mechanism of high temperature oxidation of SUS 430 in humid air containing 18% water vapour is dis- cussed. Keywords: metals, hot working, oxidation, X-ray diffraction, humid air 1. INTRODUCTION In the hot rolling process of stainless steels, a common surface defect is caused by sticking. The rolls and strips may stick together due to the lack of oxide scale protection during the hot rolling process, and the surface quality of the final prod- ucts will decrease [1]. Ha et al. [2] found that the oxide scale formed on the stainless steel surface during hot rolling is the key factor associated with the occurrence of sticking. The sticking does not occur in the surface region that contains oxides. Rather, it occurs at high temperatures on the surface region without oxides [3]. There is a scale thickness (about 3.0 µm) which is critical for the suppression of the sticking phenomenon [4]. The composition of the oxide scale also affects sticking. The plasticity of FeO plays a major role in improving sticking resistance of the roll surface [5]. Due to their high Cr content, ferritic stainless steels have very high oxidation resistance. As a result, the sticking during hot rolling can be severe. Ha et al. [2] suggested increasing the thickness of the oxide layer formed in the reheating fur- nace in order to reduce this, but this does not work with some ferritic stainless steel grades. Sun et al. [6] investigated the secondary oxide scale on the hot rolled strip which is formed after descaling the primary oxide scale, and found that the thick- ness of the secondary oxide scale is greatly influenced by the heating. It is therefore necessary to investigate the oxidation behaviour of the stainless steels in a heating furnace. In the heating furnace, the temperature can be as high as 1200 ° C and the atmosphere contains moisture as a result of natural gas combustion and/or humid conditions. The forma- tion of non-protective iron-rich oxide scale marks the failure of the chromia scale and the start of breakaway oxidation. This end-point is a function of the oxidation temperature, the oxygen partial pressure, the oxidation time, the water vapour content of the atmosphere and the Cr content of the alloy [7-10]. The long-term oxidation behaviour of some ferritic stainless steels has been intensively studied because of their application to solid oxide fuel cell (SOFC) stacks [11-16]. However, the operation temperature for these studies was always lower *Corresponding author: jiang@uow.edu.au KIM and Springer