The Kinetics Peculiarities and the Electrolysis Regime Effect on the Morphology and Phase Composition of Fe-Co-W(Mo) Coatings

Abstract

The electrodeposition theory aspects of the ternary iron alloys with refractory metals are discussed. The kinetic parameters and characteristic criteria of electrode reactions occurring during the formation of ternary Fe-Co-W(Мо) alloys have been defined. It was shown that hetero-nuclear complexes [FeHCitMеO4]− [СоНCitМеO4]2− (where Ме is Мо, W) and [FeHCit]+ and [CoCit]− complexes are involved in cathode processes. The oxometalates show step-by-step reduction producing tungsten (molybdenum) oxides of variable oxidation states that are reduced electrochemically and chemically by hydrogen adatoms. The competitive reduction of iron with cobalt and iron with tungsten at the Fe-Сo-W alloy deposition and the competitive reduction of cobalt with molybdenum into the Fe-Сo-Mo alloy were established. A mechanism of iron and cobalt with tungsten and/or molybdenum co-deposition into ternary alloys has been substantiated as a set of the coupled reactions of intermediate irreversible reduction with the limiting stage of electron transfer and the previous stage of ligand release. It was found that the pulse current allows formation of a more uniform coating surface and a significant decrease in the oxygen contained in the coatings compared with the direct current mode. X-ray phase analysis showed the amorphous and crystal structure of Fe-Co-W(Мо) alloys with a zone of coherent dispersion of 2–8 nm and intermetallic compounds Co7W6 and Fe7W6 in Fe-Co-W alloy and the inter-metallic phases Fe7Мо, Fe7Co, FeCo in the Fe-Co-Мо alloy. A change in the phase composition of Fe-Co-W alloys obtained by a pulse current was found that consists in the additional formation of crystalline tungsten phase and inter-metallic Fe2W. The X-ray patterns of Fe-Сo-Mo alloys visualize lines corresponding to crystalline cobalt and molybdenum, but the lines of inter-metallic iron compounds with cobalt are disappearing.