Optimization of bath composition for hard Fe–C alloy plating

Abstract

Fairly uniform Fe–C alloy deposits with bright appearance and characteristic black colour were obtained in Hull cell tests at 1.0A from baths with the following composition: FeSO4 0.5m or higher, citric acid 0.01m or lower, and pH around 2.0. The carbon content of the deposits from these baths ranged between 1.0 and 1.2wt% and the Vickers hardness was HV700 or above over a wide area of the Hull cell cathode. The thickness distribution of the deposits on the Hull cell cathode from these baths suggested that the current efficiency was 100% and the local current density obeyed the primary current distribution, except for the region near the low-current-density (LCD) end where the thickness decreased unexpectedly. The addition of chloride ion as NaCl, at concentrations of 1.0m or above, improved the thickness in the LCD region, and the thickness distribution on the Hull cell cathode suggested that current efficiency was 100% even at the LCD end. Chloride ion did no t affect the carbon content or the hardness of the deposits. Polarization measurements showed that the hydrogen ion reduction occurred at more noble potentials than Fe deposition. The unexpected thickness decrease near the LCD end, observed in Hull cell tests, and the improvement of this by adding chloride ion were both accounted for by the change in the preceding hydrogen ion reduction rate. The increase in citric acid concentration enhanced hydrogen ion reduction and therefore caused a lowering of current efficiency at low current densities. On the other hand, the addition of chloride ion inhibited hydrogen ion reduction and caused an increase in current efficiency at low current density.