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Title: Characterisation of passive films formed on mild steels in bicarbonate solution by EIS
Authors: Alves, Valéria A. 
Brett, Christopher M. A. 
Keywords: Bicarbonate; Passivation; Electrochemical impedance spectroscopy; Semiconductor properties
Issue Date: 2002
Citation: Electrochimica Acta. 47:13-14 (2002) 2081-2091
Abstract: Passive films formed anodically on carbon steel, chromium steel and high speed steel (HSS) in 0.5 M NaHCO3 and in 0.5 M NaHCO3+0.01 M KCl have been characterised by electrochemical impedance spectroscopy (EIS). Carbon steel presents a wide passive anodic region (0.0-0.9 V vs. saturated calomel electrode (SCE)), while those of chromium steel and HSS, either as-received or heat-treated, are narrower because of transpassive anodic dissolution of Cr above ~0.4 V. This is clearly seen through changes in the impedance spectra. An equivalent circuit is proposed to fit the experimental data, leading to determination of oxide film capacitance and film resistance. The semiconductor properties of the passive films were investigated by Mott-Schottky plots. All steel samples behave like n-type semiconductors, showing that the passive film properties are dominated by iron. However, same differences are found concerning the concentration of donors, i.e. oxygen vacancies, and thickness of the space charge region, which are correlated with the different pitting corrosion resistances of the passivated steels.
DOI: 10.1590/S0100-40422005000200007
Rights: openAccess
Appears in Collections:FCTUC Química - Artigos em Revistas Internacionais

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