Abstract
Fe‒C alloys were electrodeposited at high current efficiencies from 
solutions containing a dicarboxylic acid with a linear alkyl chain, represented by 
(n = 0 to 7). The carbon contents of the deposits ranged from 0.1 to 3.7 weight percent (w/o), depending on the acid added to the bath. The hardness of the deposits sharply increased to a maximum value of HV 850 with increasing carbon content up to 0.6 w/o and progressively decreased at higher carbon contents. The deposits having carbon contents above 0.5 w/o exhibited a black color, because carbon‐rich alloys deposited through the black oxide films in a state similar to FeO containing an organic acid or its degradation products. Fe‒C alloy deposits had a body‐centered tetragonal, martensite‐type lattice with interstitial carbon atoms, and the axial ratios increased with increasing carbon content of the deposits. The high hardness of the Fe‒C alloy deposits is mainly attributed to the solid solution hardening by interstitial carbon atoms in the martensite‐type lattice. The larger grain size and FeO occluded in the bulk of the carbon‐rich deposits caused the decrease in hardness with increasing carbon content.