X-Ray Photoelectron Spectroscopy Depth Profiling of Electrochemically Prepared Thin Oxide Layers on Duplex Stainless Ste
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STAINLESS steel is one of the most widely used materials and has many different applications. Duplex stainless steels with a ferrite/austenite volume ratio of about 1:1 have been recognized as a good corrosionresistant material in various aqueous environments.[1] For this reason, it is often exposed to different and relatively extreme conditions. Because stainless steel are the alloys with many alloying elements, different oxides can form on the surface and these complex oxides may change the mechanical, chemical, and physical properties of the material. The corrosion resistance originates from a Cr-rich oxide layer, which works as a barrier against ion diffusion between the alloy and the ambient phase.[2–11] Custom steel grades are designed for specific applications by optimizing their properties using specific alloy compositions. The alloy selected for this study was a 22 wt pct Cr, 5 to 6 wt pct Ni, 3 wt pct Mo, 2 wt pct manganese, and N-alloyed duplex stainless steel (DSS 2205 also known as W.Nr. 1.4462). The alloy has high general and stress corrosion-resistance properties in addition to high strength and excellent impact toughness. This type of stainless steel is increasingly used because of its range of superior properties. The oxidation and corrosion resistance of stainless steel have already been the subject of many studies. The corrosion resistance of stainless steel is caused by Cr oxide (Cr2O3) at the surface, which is considered to act as a protective layer against corrosion because of its low CˇRTOMIR DONIK, Researcher, ALEKSANDRA KOCIJAN, Head of Laboratory for Analitical Chemistri and Corrosion, DJORDJE MANDRINO, Senior Researcher, and MONIKA JENKO, Director, are with the Institute of Metals and Technology, SI-1000 Ljubljana, Slovenia. Contact e-mail: [email protected] Manuscript submitted December 10, 2010. Article published online May 27, 2011. 1044—VOLUME 42B, OCTOBER 2011
diffusion constants for oxygen and metal ions. Abreu et al.[12,13] point out the stabilizing effect of Mo on the surface of the passive film, enhancing the formation of a layer on DSS 2205 with a higher Cr/Fe ratio. Molybdenum increases the stability of the passive film and, therefore, the ability of the stainless steel to resist localized corrosion, including pitting and crevice corrosion, particularly in environments containing chloride ions. However, the oxide layer formed on stainless steel surfaces is usually not uniform with depth. Double or even multiple layers can be formed on the surface, depending on the alloy composition and the oxidizing conditions (oxidizing atmosphere, solution potential, time, and temperature) as well as on the different Fe phases of DSS 2205.[14,15] Oxide films may, in theory, consist of different Fe and Cr oxides, as well as their mixtures.[2,6] The aim of the present study is to investigate the initial phases of oxide growth on DSS 2205 as a function of the potentiostatic oxidation in chloride-enriched solution. The oxide layers were produced by the controlled exposure of polished duplex stainles
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