THE EFFECT OF CONTINUOUS HEATING ON THE PHASE-TRANSFORMATIONS IN ZINC-IRON ELECTRODEPOSITED COATINGS

The crystal structure and phase transformations of electrodeposited zinc-iron coatings were investigated by several techniques. The phase composition of as-plated zinc-iron coatings was found to contain both eta-phase and delta-phase and, according to the equilibrium phase diagram, was in a nonequilibrium condition for the compositions investigated. The transformation toward equilibrium during heating was a two-stage process. The first stage of transformation, at temperatures between 25-degrees-C and 360-degrees-C, is accomplished with very little change in composition, and the phase transformation occurs within the coating itself. It is proposed, through a simplified analysis, that the phase transformation in this stage is supported by short-range diffusion. In the second stage of the transformation, at temperatures above 360-degrees-C, the iron content in the coating increases as a result of interdiffusion between the coating and the base steel, and the phase structure moves to the iron-rich side in accordance with the equilibrium phase diagram.