Temperature and water vapour effects on the cyclic oxidation behaviour of Fe–Cr alloys Norinsan K. Othman 1 , Jianqiang Zhang, David J. Young * School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia article info Article history: Received 15 December 2009 Accepted 21 April 2010 Available online 27 April 2010 Keywords: A. Fe–Cr alloys B. Cyclic oxidation B. Water vapour B. Temperature C. Breakaway oxidation C. High temperature corrosion abstract Binary Fe–Cr alloys were subjected to cyclic oxidation at 600, 700 and 950 °C in flowing gases of Ar–20O 2 and Ar–20O 2 –5H 2 O (vol.%). The minimum chromium concentration required to achieve protective scale growth decreased as temperature increased from 600 to 700 °C. This change is attributed to faster chro- mium diffusion at higher temperature. Conversely, this minimum chromium level increased when the temperature was raised from 700 to 950 °C. This is attributed to faster scale growth, leading to its rapid mechanical failure, along with formation of slow-diffusing austenite. Water vapour accelerated scaling, leading to a need for higher chromium concentrations to resist breakaway oxidation. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The performance of stainless steels in resisting high tempera- ture oxidation depends on their ability to form and maintain pro- tective chromia scales. When this ability is lost, other components of the steel oxidise if the oxygen potential is high en- ough. Because that reaction is so much faster, it is termed ‘‘break- away” oxidation. Whether or not selective oxidation of chromium (and passiv- ation of the steel) is achieved clearly depends on the alloy chro- mium concentration. The critical chromium concentration, N Cr,crit (mol fraction), required to achieve passivation has been shown experimentally [1–3] to decrease as the temperature increases. This phenomenon has been qualitatively attributed [1–5] to faster diffusion of alloy chromium at higher temperatures. Selective oxidation of chromium leads to its depletion in the al- loy sub-surface region. In this case, the interfacial concentration of chromium, N ðiÞ Cr , is determined by the rates at which the metal is consumed by oxidation and replenished by alloy diffusion. The case of parabolic scaling kinetics is considered: X 2 ¼ 2k p t ð1Þ with X the scale thickness grown in time, t, and k p the rate constant. If the scale is pure chromia, Wagner’s analysis [6] leads to the result N ðoÞ Cr  N ðiÞ Cr ¼ V A V CrO 1:5 pk p 2 e D  1=2 ð2Þ Here N ðoÞ Cr is the original alloy chromium concentration, V A and V CrO 1:5 the molar volumes of alloy and oxide, e D the alloy interdiffu- sion coefficient, and scale-alloy interface recession has been ig- nored. This shows clearly that higher values of e D relative to k p lead to less alloy depletion, consistent with the qualitative expla- nation given for the dependence of N Cr,crit, on temperature. Successful application of (2) requires knowledge of the temper- ature dependence of both k p and e D. Whilst some alloy interdiffu- sion data are available for high temperatures [7], a difficulty arises with the evaluation of k p , which is affected by the gas atmo- sphere. Comparative studies [4,8–12] of stainless steel oxidation in oxygen bearing environments with and without the addition of water vapour have shown that the onset of breakaway is acceler- ated by the presence of H 2 O (g). Similar results have been found for model Fe–Cr binary alloys. Chromia growth rates on high chro- mium alloys have been reported [13–15] to be higher in water va- pour than in oxygen, and the onset of breakaway corrosion is accelerated [14–17]. The latter observation has been attributed in the case of reaction at 700 °C [15,18] simply to the increased value of k p for chromia, and hence to the adverse effect on alloy depletion predicted from Eq. (2). Despite this apparent success, it must be recognised that several other effects are possible, depending on reaction conditions. In gases containing both oxygen and water vapour, chromium volatil- isation as CrO 2 (OH) 2 can sometimes be important [19–21]. Water vapour accelerates grain boundary oxygen diffusion in chromia 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.04.026 * Corresponding author. Tel.: +61 2 9385 4322; fax: +61 2 9385 5956. E-mail address: d.young@unsw.edu.au (D.J. Young). 1 Present address: School of Applied Physics, Faculty of Science and Technology, University Kebangsaan Malaysia, 43600 Bangi Selangor, Malaysia. Corrosion Science 52 (2010) 2827–2836 Contents lists available at ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci