LDH conversion coatings on light metals : Al vs Mg

The scientific work, conducted within the this dissertation, is aimed at the development and study of in situ formation of layered double hydroxides (LDH) on the surface of magnesium and aluminium alloys. The developed approach involves the use of chelating agents as an addition to the treatment bath during thermal treatment stage. The use of chelating agents is important for simplifying in situ LDH formation, especially on bare magnesium and magnesium alloys as it allows to overcome the problem of passivation of magnesium surface in the highly alkaline environment necessary for LDH formation. This effect is achieved by shifting the pH of precipitation of hardly soluble magnesium hydroxide species to higher pH values in the presence of suitable chelating agents.

This method can be considered as an alternative to previously known methods of in situ LDH formation on magnesium surfaces in carbonate saturated treatment bath or in autoclaves at elevated temperatures and pressure. The application of this method was further extended to plasma electrolytic oxidation (PEO) coated magnesium and aluminium alloys. The addition of chelating agents allows to regulate the etching rate of PEO layer, thereby promoting the formation of metal-containing species necessary for the LDH synthesis.

The analysis of the effect of chelating agents on the change of equilibrium species in the studied systems for LDH formation allowed to select the ligands suitable for in situ LDH formation on galvanically coupled PEO coated magnesium/aluminium materials.

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