“Investigation of Molybdate and Tungstate Oxyanions as Localized Corrosion Inhibitors for 347 Stainless Steel in High Pure Water Containing Chloride Ions at Controlled Hydrodynamic Conditions”

The most commonly employed stainless steels in the pure water circuit in nuclear power stations are AISI 304 and AISI 347. However, in the presence of chloride anions, even at low concentration pitting corrosion is developed. So, with the purpose of increasing the corrosion resistance inhibitors are added in the aggressive medium. Among other inhibitors, sodium molybdate and tungstate are very popular because of their low toxicity, being environmental friendly, so they are widely used for protecting steel in the cooling water systems.
            In this work the efficiency of the oxyanions molibdate and tungstate as pitting corrosion inhibitors and the influence of chloride ions for 347 SS in deaerated pure water were studied. Chronoamperometric measurements were performed as well as cyclic potentiodynamic polarization and EIS. All measurements were accomplished under room temperature using a RDE (at 1200 rpm). Moreover, SEM was employed to characterize the electrode surface after potentiostatic tests.
. In the presence of oxyanions a shifting of the pitting potential to more positive values was observed. A marked increase of pitting potentials occurred for the highest molybdate and tungstate concentrations. The inhibitors delay the pitting formation due the adsorption on the stainless steel surface making the passive film thicker and more homogeneous, consequently improving the protection against its breakdown.