In situ X-ray absorption near-edge structure study of the active and transpassive dissolution of passive films on Ni and Ni-Cr alloys in 0.1 M H2SO4

The formation, reduction, and dissolution processes of passive films formed on Ni, Ni-9 atom % Cr, and Ni-18 atom % Cr alloys in deaerated pH 2, 0.1 M sulfuric acid were studied using in situ X-ray absorption near-edge structure during potential stepping from passive to cathodic or transpassive potentials. At passive potentials, Ni slowly dissolves from pure Ni and Ni-9 atom % Cr, while Cr from the Ni-9 atom % Cr alloy does not dissolve. At passive potentials, neither Ni nor Cr from the Ni-18 atom % Cr alloy dissolves. Stepping between passive and cathodic potentials results in the loss of Ni and Cr to different extents, depending upon the Cr content, No Ni dissolves from either the Ni or the Ni-Cr alloys when the potential is first stepped from the passive to the cathodic potential, Oxidized Ni is reduced to Ni metal in a solid-state reduction reaction. However, upon stepping the potential back to the passive range, Ni does dissolve, with the amount of Ni lost decreasing as Cr content increases: significant Ni dissolves at 0% Cr, a small amount dissolves at 9 atom % Cr, and almost no Ni dissolves at IS atom % Cr. In contrast, for both the Ni-Cr alloys, approximately a monolayer of Cr dissolves during the step from passive to cathodic potentials, while no Cr dissolves when the potential is stepped back to the passive range. In pH 2 sulfuric acid, Cr in Ni-Cr alloys is susceptible to reductive dissolution but not active dissolution. Transpassivity was also explored. For pure Ni, transpassive dissolution begins above +0.4 V referenced to mercurous sulfate electrode (MSE) and becomes rapid above +0.5 V. For the Ni-Cr alloys. transpassive dissolution of both Ni and Cr begins above +0.3 V MSE, becoming rapid at +0.4 V. Although Cr provides protection to Ni from active dissolution and from dissolution at passive potentials, Ni does not protect Cr from dissolution at transpassive potentials. (C) 2001 The Electrochemical Society.