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The synthesis of oxides in a low-temperature electrolytic plasma allows to cover surfaces of magnesium and its alloys with multifunctional protective oxide-ceramic coatings. The corrosion properties of these layers are strongly dependent on their porosity. In order to minimize the porosity and to optimize the corrosion properties of the layers, the electrolyte concentration and composition (addition of CrO 3 as corrosion inhibitor) were varied, and the influences on layer structure, composition, and properties with a main focus on corrosion behaviour were studied. The corrosion properties of various layers thus generated were studied in 5% NaCl solution by measuring electrochemical polarization curves and by electrochemical impedance spectroscopy (EIS) at pH 3 and 6. Using XRD, LM, SEM and EDX to evaluate the composition and microstructure of the modified surfaces, the corrosion results were related to the microstructure and composition of the specific layer. The better results were obtained for layers produced at higher electrolyte concentration, whereas the addition of CrO 3 had no significant beneficial effect.

The thorough mechanical, corrosion-mechanical, hydrogen permeation studies of X52 steel in as-received condition and after 30 years of service in gas pipeline have shown the degradation of the bulk metal properties due to the exploitation. The structure degradation of exploited steels decreased the resistance to crack propagation, to stress corrosion cracking (SCC), HE and HIBC. The exploited steel exhibited specific anomalies of mechanical behaviour: (1) decrease in resistance to brittle fracture with simultaneous decrease in hardness; and (2) the opposite change of the plasticity parameters: decrease in the RA accompanied by the increase in elongation. Deformation ageing and formation of micro defects, assisted by entering and accumulated hydrogen evolved in corrosion processes which takes place in brine deposited inside the pipes, have been proposed to be responsible for the metal degradation and specific modification of the properties during the long-term service of pipes. The extent of the steel degradation not being able to be detected by the standardized mechanical properties and routine microstructure observation can be efficiently determined by the specially modified methods and conditions. The information may be used in assessment of the conditions and of the life-time of operated and newly constructed gas pipelines.

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