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 struct ure during potential stepping from passive to cathodic or transpassive pote ntials. At passive potentials, Ni slowly dissolves from pure Ni and Ni-9 at om % Cr, while Cr from the Ni-9 atom % Cr alloy does not dissolve. At passi ve 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 m etal in a solid-state reduction reaction. However, upon stepping the potent ial 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 s mall 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 p H 2 sulfuric acid, Cr in Ni-Cr alloys is susceptible to reductive dissoluti on but not active dissolution. Transpassivity was also explored. For pure N i, transpassive dissolution begins above +0.4 V referenced to mercurous sul fate 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, becomin g rapid at +0.4 V. Although Cr provides protection to Ni from active dissol ution and from dissolution at passive potentials, Ni does not protect Cr fr om dissolution at transpassive potentials. (C) 2001 The Electrochemical Soc iety.