Oxide growth and conversion on carbon steel as a function of temperature over 25 and 80 °C under ambient pressure

Abstract

Surface oxide film growth and conversion on carbon steel at pH 25 °C 10.6 was studied as a function of temperature in a range of 25–80 °C. The current behaviour observed as a function of various cyclic voltammetry scan conditions and the current and EIS spectra as a function of time during 7-d potentiostatic experiments all present consistent pictures of an oxide growth and conversion mechanism that depends mainly on electrode potential. At pH 25 °C 10.6 where the solubilities of both Fe II and Fe III species are low, the chemical and phase composition of the oxide film formed on carbon steel depends on the electrode potential, but is independent of temperature. For the type of film formed, three potential regions can be defined: formation of a uniform and compact magnetite-like layer at potentials <−0.6 V SCE, a uniform and compact maghemite-like layer in the range from −0.6 to 0.0 V SCE, and small crystallites of γ-FeOOH on a compact layer at potentials >0.0 V SCE. Although temperature influences the rates of thermal processes such as dissolution and charge transport through the oxide phase, and the Schikorr reaction at temperatures >70 °C, these thermal processes do not influence the oxide composition, but instead influence only the thickness of the oxide film at pH 25 °C 10.6.