Advances in local mechanoelectrochemistry for detecting pitting corrosion in duplex steels
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oudreuse Industeel, Centre de Recherches des Mate´riaux du Creusot, 71200 Le Creusot, France (Received 29 June 2004; accepted 28 September 2004)
The goal of this study was to demonstrate that a relationship exists between surface stress and pitting corrosion. The surface stress field generated by polishing was first calculated using a thermomechanical model and a finite element code. Pitting corrosion tests performed at the microscale along the austenite/ferrite interface using the electrochemical microcell technique were then analyzed considering the microstructure, and the residual surface stress field calculated numerically under the microcapillary. Mechanical criteria are proposed leading to an enhancement of pitting corrosion of duplex steels.
I. INTRODUCTION
Duplex stainless steels are being increasingly used as structural materials in chemical and petrochemical applications, in power plants, etc. This is mainly due to their high resistance to localized corrosion as well as to their high strength and toughness. They have a complex microstructure with a comparable volume of austenite and ferrite. The main alloying elements (chromium, molybdenum, nickel, and nitrogen) are not equally distributed in the austenite and the ferrite. The austenite is enriched in nickel and nitrogen while the ferrite is enriched in chromium and molybdenum.1 The partitioning of alloying elements may affect the corrosion resistance of each phase and the entire metallic alloy. Microelectrochemical techniques, which have been developed over the past years, appear as powerful techniques to study the electrochemical reactions in the micro- and nanorange on such materials. Some workers have investigated at the microscale the influence of the partitioning of alloying elements on the electrochemical behavior of metallic alloys. The electrochemical microcell technique was applied to perform potentiodynamic polarization experiments on sites in the range of 10 m, and the individual corrosion behavior of phases of various duplex steels was determined.2 The results showed a good correlation with the empirical pitting resistance equivalent number (PREN) of the correponding single
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0475 3688
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 12, Dec 2004 Downloaded: 13 Mar 2015
phase. Corrosion behavior of duplex steels was also investigated using in situ scanning tunneling microscopy (STM) in 4M H2SO4 + 1M HCl solution.3 Selective dissolution of the boundary regions between the two phases was clearly observed. It was also noticed that the ferrite starts to dissolve at lower anodic potentials compared to the austenite. Fine grains coupled with a high content of alloying elements Cr, Mo, and N (well balanced in the two phases) seemed to be quite effective to reduce selective dissolution of duplex steels. Differences in the dissolution rates of the austenite and ferrite phases of about 20% were measured under potentiostatic control in 2M H2
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