Hard chromium plating on cold swaged Cr-Mo steel using rotating cylinder electrode

Abstract

Hard chromium plating is widely used owing to its good wear and corrosion resistance. This plating technology has been practiced and well developed for many decades [1, 2]. Since reduction of hydrogen ions occurs at electrode surface during chromium plating, the current efficiency for the plating is normally under 25% [1]. However, Hoare [3] shows that rapid flow of the electrolyte over the cathode surface is essential for highspeed chromium plating due to the reduction of the diffusion path in a diffusion-controlled electrode reaction. Plating by using rotating disc electrode (RDE) method is useful to study this effect. Cross-section microstructure of the plated hard chromium was studied with optical microscope and scanning electron microscope (SEM) [4, 5]. The results showed that the grain size of as-plated chromium was so fine that the grain size cannot be estimated, and only the grains after annealing can be observed. Pina [5] showed that a fiber texture existed with 〈111〉 axis perpendicular to the chromium deposit surface by using X-ray diffraction method. The interface microstructure between Cr-Mo steel and chromium deposit has not been studied yet. Especially, little work has been done on the current efficiency and TEM-cross section microstructure of chromium plating using rotating cylinder electrode (RCE), by which a uniform mass transport over all of the electrode surface can be obtained. In this letter the aspects mentioned above are reported using RCE specimens. Cold swaged Cr-Mo steel was the chosen specimen with its chemical composition shown in Table I. The cylinder specimen dimension was 16 mm φ× 5 mm with an exposing plating area of 2.51 cm2. Platinized-Ti mesh with dimension of 25× 35 mm was used as anode. All chromium plating was performed in a conventional bath containing 250 g/l of CrO3 and 2.5 g/l of H2SO4 at 50 ◦C. Cathodic current densities applied were 30, 40, 50 and 60 A/dm2, respectively. Rotating speed of RCE was controlled in the range of 5–1000 rpm. Before electroplating, the RCE specimens were anodically polarized with current density of 40 A/dm2 at 200 rpm for 30 s. The current efficiency was evaluated with three RCE specimens after a constant applied cathodic charge of 1.33 A h under same plating condition. The weight increment of chromium plating was measured and compared with theoretical weight increment according to Faraday’s law. After plating, the RCE specimen was rinsed in acetone, dried with hot air, then prepared for current efficiency evaluation; TEM-cross section specimens were prepared as mentioned elsewhere [6] and their microstructure examined. The current efficiency of applied cathodic current density with respect to logarithmic rotating speed of RCE is shown in Fig. 1. The results show that all current efficiency increases with the increasing of cathodic current density, suggesting that higher electric field tends