Optimization of bath composition for hard Fe-C alloy plating

Fairly uniform Fe-C alloy deposits with bright appearance and characteristi c black colour were obtained in Hull cell tests at 1.0 A from baths with th e following composition: FeSO4 0.5 M or higher, citric acid 0.01 M or lower , and pH around 2.0. The carbon content of the deposits from these baths ra nged between 1.0 and 1.2 wt % and the Vickers hardness was HV 700 or above over a wide area of the Hull cell cathode. The thickness distribution of th e deposits on the Hull cell cathode from these baths suggested that the cur rent efficiency was 100% and the local current density obeyed the primary c urrent distribution, except for the region near the low-current-density (LC D) end where the thickness decreased unexpectedly. The addition of chloride ion as NaCl, at concentrations of 1.0 M or above, improved the thickness i n the LCD region, and the thickness distribution on the Hull cell cathode s uggested that current efficiency was 100% even at the LCD end. Chloride ion did not affect the carbon content or the hardness of the deposits. Polariz ation 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 a dding chloride ion were both accounted for by the change in the preceding h ydrogen ion reduction rate. The increase in citric acid concentration enhan ced hydrogen ion reduction and therefore caused a lowering of current effic iency at low current densities. On the other hand, the addition of chloride ion inhibited hydrogen ion reduction and caused an increase in current eff iciency at low current density.