Corrosion behaviour of cobalt-based coatings with ruthenium additions in synthetic mine water

Abstract

The corrosion behaviour of two cobalt-based coatings (ULTIMET™ and STELLITE™ 6) with zero, 0.3 and 0.6 wt% ruthenium were studied and compared with two cobalt-based bulk alloys (ULTIMET™ and STELLITE™ 6B) in synthetic mine water (pH values of 6, 3 and 1) using potentiodynamic polarisation. The coatings demonstrated wider ranges of passivation behaviour (from −250 mV to 750 mV) than the bulk alloys. The corrosion potential became more positive and the active-passive transition reduced with increased Ru. The best coating was STELLITE™ 6 with 0.6 wt% Ru, which exhibited the lowest corrosion rates: 3.6 μm/y at pH 6 and 6.4 μm/y at pH 3.