The influence of alloying on the passive behaviour of steels in a bicarbonate medium studied by electrochemistry and XPS

Passive films formed in 0.5 M NaHCO3 and 0.5 M NaHCO3 + 0.01 M KCl aqueous solution on carbon steel, chromium steel and high speed steel have been characterized by electrochemistry and XPS. The influence of heat treatment on the passive film properties of chromium steel and HSS was also studied. Polarisation curves show that the steel composition influences the onset of passive film formation. Chromium and high speed steels show a higher ability to passivate as a result of the higher alloying content, corroborated by open circuit potential measurements. Passive current density values, obtained from potentiodynamic curves and by chronoamperometry, were higher for HSS than for carbon and chromium steels, and are associated with the presence of Mo and W oxides in the passive film, as confirmed by XPS analysis. The presence of 0.01 M chloride ion in solution shifts the passivation. of chromium steel and HSS to more positive potentials but does not interfere with passive film stability. For carbon steel, chloride ion does not influence the initiation of passive film formation, but diminishes film stability. Tempered chromium steel and HSS show passive film behaviour with similar characteristics to the respective as-received steels.