Abstract:The electropolishing behaviour of 73 brass was studied by means of a rotating cylinder electrode (RCE) in a 70 vol % H 3 PO 4 solution at 27 • C. Owing to the formation of a blue Cu 2+ -rich layer on the brass-RCE, an obvious transition peak was detected from kinetic-to diffusion-controlled dissolution in the anodic polarisation curve. Electropolishing was conducted at the potentials located at the transition peak, the start, the middle, and the end positions in the limiting-current plateau corresponding to the anodic polarisation curve of the brass-RCE. A well-polished surface can be obtained after potentiostatic electropolishing at the middle position in the limiting-current plateau. During potentiostatic etching in the limiting-current plateau, a blue Cu 2+ -rich layer was formed on the brass-RCE, reducing its anodic dissolution rate and obtaining a levelled and brightened brass-RCE. Moreover, a rod climbing phenomenon of the blue Cu 2+ -rich layer was observed on the rotating brass-RCE. This enhances the coverage of the Cu 2+ -rich layer on the brass-RCE and improves its electropolishing effect obviously.
Cr-Fe-C alloy deposits were successfully prepared on high-carbon tool steel in a Cr3+-based electroplating bath containing Fe2+ ions and suitable complex agents. A Cr-based alloy deposit was obtained with an electroplating current density higher than 25 Adm−2, and a Fe-based alloy deposit was obtained using a current density of 20 Adm−2. Following electroplating, these alloy deposited specimens were annealed via rapid thermal annealing (RTA) at 500 °C for different periods up to 30 s. The experimental results show that Cr- and Fe-based alloy deposits could be significantly hardened after RTA at 500 °C for a few seconds. The maximum hardness was that of the Cr-Fe-C alloy deposit annealed at 500 °C for 10 s. The maximum hardness of 1205 Hv was detected from the annealed Cr-based alloy deposit prepared with 30 ASD. The hardening mechanism of annealed Cr- and Fe-based alloy deposits is attributed to the precipitation of C-related membranes. The hardness values of the annealed Cr- and Fe-based alloy deposits increase with the increasing degree of crystallization of the C-related membranes.
The corrosion behavior of Cr-C-deposited steel specimens, which were prepared from the Cr3+-based bath, was investigated in the 0.5 M H2SO4 solution. After electroplating, the deposited specimens were annealed at 300, 450, 600 and 700°C for 1 h. Based on the results of corrosion test, the corrosion resistance of as-plated and annealed Cr-C deposits has strongly related to the surface cracks. However, the corrosion resistance of 450°C-annealed Cr-C deposit could be obviously improved when the deposit was pre-polarized in the passive potential region. Annealed at 700°C, the corrosion resistance of Cr-C deposit is markedly raised owing to formation of Cr oxides on the crack surface during annealing.
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