Effect of annealing on the wear properties of amorphous chrome-plated steel

Abstract

Chrome plating is widely used for the manufacture of machines and instruments in the automobile and other industries to improve the wear and corrosion resistance of metallic surfaces [1, 2]. Sargent et al. developed the conventional chrome plating method using a plating bath based on chromium trioxide (CrO3) (which forms dichromic acid (H2Cr207) in aqueous solutions) and containing a sulphate catalyst [3]. The conventional Sargent bath produces a crystalline chromium layer on the metal substrate. The microstructure and properties of the chrome layer are significantly influenced by small amounts of chemical agents in the bath [4]. Furuya et al. observed that the electrodeposited chromium was amorphous when formic acid or other certain organics were added to the sulphate-catalysed chromic acid electrolyte [5,6]. The amorphous chrome layer has fewer defects and smoother surfaces, is more resistant to corrosion by hydrochloric acid solutions and has increased hardness at elevated temperature compared to the conventional chrome layer [7]. Although considerable literature exists concerning amorphous chrome deposits, little information is available concerning the wear properties of the chrome layer. In this study, the vacuum annealing effect on the wear resistance of amorphous chrome deposits was examined in terms of hardness and wear resistance. Chromium deposits were obtained on AISI 1025 steel substrate by the method described by Hoshino et al. [8]. The conventional chrome layers were prepared by electroplating chrome in a Sargent bath containing 250 gl 1 of chrome acid and 2.5 g1-1 of sulphuric acid at ~ temperamre of 55°C and 0.6 A cm -2 for 3 h. Amorphous chrome layers were prepared by electroplating chrome in the modified Sargent bath containing up to 20 roll -1 of formic acid at a temperature of 30 °C and 0.6 A cm -2 for lh . The plating cell was made of acrylic with dimensions of 120 × 200 × 200 mm. The anode was lead/5% antimony alloy plate with dimensions 100 × 100 × 5 mm. The anode and the steel cathode were vertically situated at the ends of the cell. The electroplating of each sample was carried out with a potentiostat (Hokuto Denko HAB-151). Fig. 1 shows the schematic diagram of the amorphous chrome plating apparatus. After deposition, the samples were ultrasonically cleaned in acetone and ethanol and then annealed at up to 600°C in 1.3 × 10 -3 Pa vacuum for 1 h. Knoop microhardness testing was