Pitting corrosion resistance of austenitic and superaustenitic stainless steels in aqueous medium of NaCl and H 2 SO 4
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edro de Lima-Neto Department of Analytical Chemistry and Physical Chemistry, Science Center, Federal University of Ceará, Campus do Pici, bloco 940, Fortaleza 60440-900, Ceará, Brazil
Marcelo J. Gomes da Silva Department of Metallurgical and Materials Engineering, Technology Center, Federal University of Ceará, Campus do Pici, bloco 729, Fortaleza 60440-900, Ceará, Brazil (Received 21 January 2016; accepted 2 May 2016)
The pitting corrosion resistance of AL-6XN PLUS™ superaustenitic stainless steel, 304L, 316L, and 317L austenitic stainless steels was investigated using the cyclic polarization technique. These materials were evaluated in the as received condition and heat-treated at temperatures between 500 °C and 900 °C for 72 h. A thermodynamic simulation was performed using the software ThermocalcÒ to predict possible deleterious phases in selected temperatures. The simulations have predicted the sigma phase in the selected temperature range. An aqueous solution of sulfuric acid and sodium chloride was used as electrolyte in the corrosion tests. The results showed that pitting corrosion was not observed on the samples of AL-6XN PLUS™ steel. The 304L steel suffered pitting corrosion. All the polarization curves of this steel showed hysteresis characteristics of pitting corrosion. The 316L and 317L steels were resistant to pitting corrosion, but susceptible to crevice corrosion.
I. INTRODUCTION
Offshore and onshore installations are widely used to extract oil and gas requiring resistant materials to marine environments. The chloride content contained in seawater can cause pitting corrosion in offshore installations. Metal alloys extremely durable are needed especially in this type of installation. Among the materials used in the manufacture of plates and pipes are highlighted the 300 series austenitic stainless steels. Austenitic stainless steels are also widely used as components operating in high temperature ranges, such as boilers, super-heaters, chemical reactors, in flue gas desulfurization systems (FGD).1 Superaustenitic stainless steels, due to their increase in chromium (Cr) and molybdenum (Mo) content when compared with the 300 series austenitic stainless steels, have excellent resistance to localized corrosion. The pitting corrosion, which is one of the most harmful forms of corrosion, is very typical on metallic materials that form passive films. This results from the active–passive cell of the points where the passive layer breaks.2 The pitting corrosion is difficult to monitor and control, as it occurs in the interior of equipment and facilities. The loss
of mass and thickness of materials subject to this form of corrosion do not characterize the wear observed.3 When working with metal alloys at high temperatures, there is the precipitation of deleterious phases that can compromise the mechanical performance and their corrosion resistance.4 The precipitation of sigma phase (r) is a severe problem when using austenitic stainless steels at high temperatures. The amount of Cr and Mo improves the corrosio
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