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Studies in plasma electrolysis resulted in development of various surface treatments for metal components. These treatments include formation of protective ceramic layers on some metals (e.g. oxide coatings), saturation of metal surfaces with interstitial elements (e.g. nitrogen, carbon and boron), and plasma electrolytic deposition of extrinsic compounds, heat-treatments (e.g. hardening and annealing), surface cleaning and polishing. The main advantages of plasma electrolytic treatments are high processing speeds and low costs. The treatments enable production of surface nanostructures and local area processing. This review examines recent results in plasma electrolytic carburising, nitriding, and nitrocarburising (as the most common diffusion-based treatments), including treatment modes, electrolyte compositions, structures, and properties of hardened materials. Analysis of the results obtained up to date indicates that pulse plasma electrolytic saturation treatments leading formation of surface nano-structures appear to be the most promising to advance further this type of electrolytic plasma technology. Moreover, electrolytic plasma treatments provide considerable research interest in terms of fundamental science, in particular for development of models of heat transfer on flat vertically or horizontally oriented surfaces and electrochemical processes occurring in the studied systems. These processes include stages of liberation of saturating components, adsorption of active atoms, and their diffusion into the metal surface; therefore understanding associated kinetics and limiting factors is important for gaining proper control over these surface treatments.

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