Abstract <p>The structure of modified layers in carbon steels after thermodiffusion zinc coating in an ammonia atmosphere was studied. Theoretical analysis of phase diagrams identified the possible phases in the Fe–Zn–N system, including solid solutions, intermetallics, and nitrides. Metallographic, electron microscopic, and Auger electron spectroscopic analyses showed that diffusion zinc coating in ammonia produces modified layers consisting of an outer coating based on chemical compounds and a diffusion layer of solid solutions with a transition zone. Parallel nitriding of steel and an increase in layer microhardness due to solid-solution strengthening were observed.</p>