Localized corrosion inhibition of 304 stainless steel in pure water by oxyanions tungstate and molybdate

By Barbero, Cesar; Planes, Gabriel A. & Miras, Maria C.
Published in Electrochimica Acta NULL 2004

Abstract

The aim of this work is to study the efficiency of the oxyanions tungstate and molybdate as pitting corrosion inhibitors for 304 SS in deaerated pure water typical from nuclear reactors circuit for vapor production in the presence and absence of chloride ions. The following electrochemical techniques were employed: open circuit potential (OCP) monitoring and potentiodynamic or cyclic polarization curves, at 90 °C and room temperature, with static and rotating disk electrodes. The experiments were performed in pure water with or without 200 ppm sodium chloride (NaCl) in absence and presence of 10-4 and 10-3 M ammonium molybdate [(NH4)6Mo7O24·4H2O] or sodium tungstate (Na2WO2·2H2O). The addition of molybdate and tungstate improved the quality of 304 SS passive layer in pure water at both temperatures, but at 90 °C all the OCP observed were more negative than at room temperature. In the presence of chloride ions, the corrosion inhibitors raised the 304 SS pitting potential to more positive values. The increase of pitting potentials occurred for increasing concentrations of tungstate and molybdate. Otherwise, the increase of temperature decreased the pitting potential, that is, the efficiency of the oxyanions decreased with temperature increase. The influence of hydrodynamic conditions was also investigated. It was observed that molybdate oxyanion have always showed better efficiency than tungstate.

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