Determination of the limiting current density through potential step measurements

Abstract

<span style="font-size:14.0pt;color:#1A1A1A;mso-bidi-language:HI">Processes such as anodic dissolution, prepassive film formation (salt and/or oxide, hydoxide film), and passive film formation are in the initial stage controlled by diffusion. Information about the value of the limiting current density (<i>i</i>_d) is needed for understanding the kinetics of passive film formation. In this study an alternative method,suitable for the determination of the <i>i</i>_d through potential step measurements, is- proposed. The method is based on the assumption that the reaction cannot proceed faster than that allowed by the <i>i</i>_d. In this case, the maximum reaction

<span style="font-size:14.0pt;color:#1A1A1A;mso-bidi-language:HI">rate corresponds to the current density at the inflection point of the <i>i-t</i> curve, and this current density is identified as the limiting current density. This method was used to determine the <i>i</i>_dvalues of some Cr-13% stainless steels at five passive potentials in mixtures of 0.1 M H₂SO₄ + 0.1 M Na₂SO₄ with different <i>p</i>H values. The results for the limiting

<span style="font-size:14.0pt;color:#1A1A1A;mso-bidi-language:HI">current density were compared with the corresponding <i>i</i>_d values obtained from the diffusion overvoltage equation.The agreement between the two methods is satisfactory.

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