Influence of alloying on the passive behaviour of steels in bicarbonate medium Val  eria A. Alves, Christopher M.A. Brett * Faculdade de Ciencias e Tecnologia, Departamento de Qu  ımica, ICEMS, Universidade de Coimbra, 3004-535 Coimbra, Portugal Received 4 January 2001; accepted 7 December 2001 Abstract The electrochemical behaviour of passive films on carbon steel, chromium steel and high speed steel (HSS) was studied in 0.5 M NaHCO 3 . The influence of the presence of 10 mM potassium chloride on passive film stability, and of the heat treatment of chromium steel and HSS on the respective film properties, was also investigated. Polarization curves showed that the alloy composition exerts a significant influence on the onset of passive film formation, chromium and HSSs passivating more easily as a result of the higher alloying content. Values of passive current, obtained both potentiodynamically and at fixed potential, were higher for HSS than for carbon and chromium steels, which was attributed to the contributions of Mo and W to the passive film composition. The presence of chloride ion delays the passivation of chromium and HSSs, but does not interfere with passive film stability, as shown by the chronoamperometric profile and passive current values. In the case of carbon steel, with no alloying elements, chloride ion diminishes film stability but does not influence passive film formation. Heat treatment, which alters the microstructure, does not alter the properties of the passive films except for a small effect on those formed potentiodynamically on HSS. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Corrosion; Passivation; A. Steels; Bicarbonate; Chloride 1. Introduction The passivation of steels is a subject of much interest due to its influence on the rate of corrosion. There is much work in the literature dealing with the influence of www.elsevier.com/locate/corsci Corrosion Science 44 (2002) 1949–1965 * Corresponding author. Fax: +351-239-835-295. E-mail address: brett@ci.uc.pt (C.M.A. Brett). 0010-938X/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0010-938X(02)00019-7