Influence of thermal aging on the reactivity of duplex stainless steel surfaces
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I. INTRODUCTION
MANUFACTURING
duplex stainless steel (SS) requires specific caution to keep their main behavioral properties,[1,2] particularly their local corrosion resistance in sea water or in salted environment.[3–7] It has been shown that the use of alloys with optimized chemical compositions[1,2,3] is imperative in such environments where damage by pitting corrosion is dominant. Some recent literature[8] has evaluated these damages at 6 to 8 pct of the total corrosion damages. Compared with the austenitic nuances,[9] this asset can be significantly reduced when the thermal effects related to elaboration operations, which are characterized by annealing condition, relaxation, and welding,[3,10–19] result in significant structure transformation. These transformations involve the precipitation of phases with high chromium, molybdenum, and nitrogen contents.[1,17,20–22,25,26] The growth of these phases affects the equilibrium ferrite/ austenite fixed by the solution treatment.[3,18,20,21,23,24,25] It also modifies the chemical composition of the initial phases (a and g)[3,23] and generates chromium and molybdenum depleted zones.[2,27] Such modifications can be fast and significant enough to weaken the intercrystalline corrosion,[4,28,30,32] pitting, and crevice resistance of duplex SS.[4,9,23,29,31] The aim of the present study is to assess the effects of structural modifications occurring during aging at temperatures ranging from 400 8C to 1050 8C on the local corrosion resistance of Z6 CNDU 25-6 Duplex SS by means of common electrochemical techniques. This kind of steel is usually used for sea water pump components (wheel and diffuser), generally obtained by casting. It is important to notice that the repair of these components is usually made by welding.
T. AMADOU, Postdoctoral Student, A. BEN RHOUMA, Lecturer, and H. SIDHOM, Professor, are with the Laboratoire de Mecanique et Materiaux ESSTT, 1008 Tunis, Tunisia. C. BRAHAM and J. LEDION, Senior Lecturers, are with the Laboratoire de Microstructure et Mecanique des Materiaux, CNRS ESA 8006, 75013 Paris, France. Manuscript submitted December 29, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
II. MATERIAL AND TESTS The complete chemical composition of the steel studied is given in Table I. The material is received after annealing (heating at 1170 8C followed by air cooling) and has a duplex structure (a 1 g) and a ferritic phase content of about 40 pct. The kinetics of the structural variation during various heat aging, from 6 minutes to 100 hours and at different temperatures from 400 8C to 1050 8C, was studied by means of optical and scanning electron microscopy devices. Cyclic potentiodynamic polarization tests were carried out in synthetic sea water (ASTM 1141-86[33]) in order to characterize the reactivity of the surface of the aged material. These tests allowed us to determine pitting potential (Ep), repassivation potential (Er), and pitting critical temperatures (PCT). The variation with aging conditions of activation pH was studied by means of potenti
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