Corrosion properties of plasma electrolytic oxidation coated AA7075 treated using an electrolyte containing lanthanum-salts

Plasma electrolytic oxidation (PEO) is a very promising process that can enhance the corrosion and wear resistance by producing a relatively thick, dense and hard oxide ceramic coating on light alloys. The improvement in the corrosion behaviour of the treated metals strongly depends on some process parameters: current density, voltage, treatment time and electrolyte composition. In particular, the addition of additives in the electrolyte produces significant changes in the corrosion behaviour. The use of rare earth salts, especially of cerium and lanthanum, during PEO treatment was already studied in literature and the use of these salts improved the corrosion resistance of the obtained coatings. However, only few works reported the dissolution of rare earth salts in the electrolyte, whereas in the majority of studies these salts are used in pre-treatment or post-treatment solutions. In this work, the effect of the dissolution in the electrolyte of various concentrations of lanthanum nitrate on the corrosion properties of PEO coated 7075 aluminium alloy was studied. The base electrolyte was an alkaline solution containing sodium silicate and different concentrations of lanthanum nitrate (0.025, 0.05, 0.075 and 0.1 g/l). All the samples were treated with high current densities and for short treatment times. The morphology, thickness and composition of the protective layer were studied with SEM, energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), glow-discharge optical emission spectroscopy and X-ray photoelectron spectroscopy (XPS). The corrosion resistance was evaluated performing potentiodynamic polarization tests and electrochemical impedance spectroscopy tests. PEO coatings obtained with additions between 0.05 and 0.075 g/l of lanthanum nitrate resulted more dense and adherent, as evidenced by SEM analysis. These additions of La(NO3)3 had a positive effect on the corrosion resistance causing the formation of a large passive zone.