Phase Structure and Composition of Fe‐W Alloy Electrodeposits

Abstract

Parallel studies have been made of the phase structure, composition, and current‐potential characteristics of iron‐tungsten alloy electrodeposition from alkaline aqueous ammoniacal tartrate electrolytes. The presence of sodium, introduced with tungstate in the form of , was found to have profound effects on the deposition process. The deposits formed at low current densities consisted of a solid solution bcc phase containing a maximum of 13 atomic per cent (a/o) W. As the limiting current density for iron deposition was reached, an amorphous alloy phase, having a constant composition of 22.2 a/o W, began to form. This amorphous phase was identified as saturated with excess iron atoms and having a structure based on tetrahedral units. The initiation of amorphous phase deposition was correlated with the occurrence of multiple nucleation associated with increasing accumulation of sodium atoms at the electrode. When the cathode potential was made still more negative, further increase in sodium accumulation appeared to have been responsible for an observed decrease in the rate of deposition of the amorphous. However, an amorphous phase may have formed at extreme values of cathode potential. Such a phase was prepared by elevation of the electrolyte pH. From analysis of these results, a prescription is developed to account for the formation of homogeneous stoichiometric electrodeposits. This prescription, which may be applicable to many amorphous and crystalline alloy deposition systems, consists of three conditions which describe material balance and atomic interactions at the electrode surface.