Corrosion of a stainless steel and nickel-based alloys in high temperature supercritical carbon dioxide environment V. Firouzdor, K. Sridharan ⇑ , G. Cao, M. Anderson, T.R. Allen Department of Engineering Physics, 1500 Engineering Drive, University of Wisconsin–Madison, United States article info Article history: Received 8 August 2012 Accepted 28 November 2012 Available online 28 December 2012 Keywords: High temperature corrosion C. Oxidation Austenitic stainless steel Nickel-based super alloys abstract Corrosion of four alloys has been studied in supercritical carbon dioxide at 650 °C and 20 MPa, specifically AL-6XN stainless steel and three nickel-based alloys, PE-16, Haynes 230, and Alloy 625. The tests were performed for exposure durations of up to 3000 h with samples being removed for analyses at 500 h intervals. The corrosion performance of the alloys was evaluated by weight change measurements, and the surface oxide layers were characterized by scanning electron microscopy, X-ray diffraction and X- ray photoelectron spectroscopy. Weight gain measurements showed that the Al-6XN stainless steel exhibited the least corrosion resistance while the weight gains were nearly similar for the other alloys. The oxide layer in AL-6XN stainless steel was composed of large equiaxed grained outer layer of Fe 3 O 4 (magnetite) and an inner layer of FeCr 2 O 4 . Oxide spallation was observed in this stainless steel even after 500 h exposure. In all alloys, Cr-rich oxides phases of Cr 2 O 3 and Cr 1.4 Fe 0.7 O 3 were identified as the protec- tive layers. In alloy PE-16 a thin layer of aluminum oxide formed that promoted the corrosion resistance of the alloy. Cr 2 O 3 was identified as the main protective oxide layer in nickel base alloys Haynes 230 and 625. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The supercritical CO 2 (SC-CO 2 ) Brayton cycle is under consider- ation for power conversion in the Sodium Fast Reactor (SFR) and Very High Temperature Reactor (VHTR) and other energy-related applications [1–3]. As compared to the traditional Rankine steam cycle, SC-CO 2 has higher cycle efficiency due to less compression work resulting from higher SC-CO 2 densities, smaller components size (especially turbomachinery), fewer components, and simpler cycle layout [4]. The SC-CO 2 cycle is comparable in efficiency with the helium Brayton cycle but can operate at significantly lower temperatures (550 °C vs. 850 °C), but at higher pressures (20 MPa vs. 8 MPa). It is well suited for any type of nuclear reactor with core outlet temperature above 500 °C in either direct or indirect ver- sions. In addition to power conversion systems, a CO 2 cooled reac- tor is also being considered [5] which would operate at temperatures higher than typical light water reactors. In all these applications, materials corrosion in the high temperature CO 2 envi- ronment will be an important consideration. Studies on corrosion of materials in SC-CO 2 are limited. Only minor corrosion was observed in austenitic stainless steels 304L and 316 after exposures to pure SC-CO 2 at 50 °C and 24.1 MPa for 24 h [6]. Corrosion issues associated with impurities and con- tamination in low temperature SC-CO 2 were studied by Thodla et al. [7]. The results indicate that the presence of small amounts of moisture can result in high corrosion rates in carbon steel, and the presence of amines such as mono-ethanol-amine caused a sig- nificant decrease in the corrosion rate. Propp et al. [8] studied the corrosion of ferritic steels and austenitic stainless steels over a temperature range of 150–240 °C and pressure range of 8.7– 15.7 MPa. The study found that low-grade carbon steels may be suitable candidates for use in a pure CO 2 medium while 304 stain- less steel was judged the best candidate in a medium containing up to 1% water or methanol impurities. The corrosion rate in carbon steels was up to 10 times higher than austenitic stainless steels. Lim et al. [9,10] studied corrosion of 316 stainless steel, ODS steels (MA956, MA957 and PM2000), ferritic/martensitic steels HT9 and T91, and a Russian alloy EP823 in SC-CO 2 at 650 °C for exposure durations of 200 h. Their results showed that materials that form a protective alumina/chromia oxide layer at the surface (e.g., MA956) are the most resistant to corrosion while 316 stainless steel exhibited the least corrosion resistance. In the recent study by Zhang et al. [11], the corrosion of X65 pipeline steel at various temperatures and under the low and high CO 2 pressure with water impurity was studied. They found out that under the low and supercritical CO 2 pressure the corrosion mech- anism is the same; however more corrosion occurs under SC-CO 2 due to higher concentration of carbonic acid. 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.11.041 ⇑ Corresponding author. Tel.: +1 608 263 4789. E-mail address: kumar@engr.wisc.edu (K. Sridharan). Corrosion Science 69 (2013) 281–291 Contents lists available at SciVerse ScienceDirect Corrosion Science journal homepage: www.elsevier.com/locate/corsci