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

Abstract

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(NO 3 ) 3 had a positive effect on the corrosion resistance causing the formation of a large passive zone.

Microstructure and Corrosion Resistance of PEO Coatings Formed on KBM10 Mg Alloy Pretreated with Nd(NO3)3

“…It can be seen from Figure 10 a that the coating exhibited high total impedance at the beginning of immersion, which indicated that the coating owned high corrosion resistance. Figure 10 d is characterized by a capacitive loop in high and medium frequency ranges, and an inductive loop in the low frequency range, which was similar to the previous reports [ 35 , 36 ]. The appearance of capacitive loop is owing to the charge transfer process, whereas the inductive loop is related to the dissolution of Mg and is indicative of pitting corrosion of the substrate [ 37 , 38 ].…”

Sealing of PEO Coated AZ91 Magnesium Alloy Using La-Based Solutions

“…The high resolution Si 2p peak is shown in Figure 13(b). It resulted from the sum of two peaks: the one situated at 101.8 eV BE attributed to the alpha Mg 2 SiO 4 and the one at 102.9 eV BE corresponding to gamma Mg 2 SiO 4 [15]. For each of these two peaks, also the two components 2p 3/2 and 2p 1/2 are shown in the figure.…”

“…The use of rare earth metals salts in the PEO process has been already reported in literature. In detail, one work reported the direct dissolution of lanthanum compounds in the electrolyte used to produce PEO coatings [15] and particles of insoluble cerium oxide were also used suspended in the electrolyte to obtain their coprecipitation during PEO process and improve the properties of the oxide layer [16,17]. Cerium salts were also used to produce solutions used as posttreatment to seal the characteristic pores on coatings obtained by PEO [18].…”

Phosphate Porous Coatings Enriched with Selected Elements via PEO Treatment on Titanium and Its Alloys: A Review

“…Other ways of modifying the surface are ion implantation [26][27][28][29][30][31], electrophoretic deposition [32], and laser surface treatment [33][34][35][36]. Development of materials engineering and technology in the following years has allowed new challenges to be posed regarding surface engineering, including, but not limited to, fabrication of porous coatings by the Plasma Electrolytic Oxidation (PEO) or Micro Arc Oxidation (MAO) [37] on light metals and alloys, such as aluminum [38][39][40][41][42][43][44] and its alloys [45][46][47][48][49][50][51][52][53][54], magnesium [38,39,[55][56][57] and its alloys [58][59][60][61][62][63][64], titanium and its alloys [57,[65][66][67][68][6...…”