Characterization of Porous Phosphate Coatings Enriched with Magnesium or Zinc on CP Titanium Grade 2 under DC Plasma Electrolytic Oxidation

Abstract

The aim of the paper is to study and determine the effect of voltage increasing from 500 up to 650 V DC on chemical and electrochemical properties of the obtained porous coatings with plasma electrolytic oxidation (PEO) processes, known also as micro arc oxidation (MAO). In the present paper, the chemical and electrochemical characterization of porous phosphate coatings enriched with magnesium or zinc on commercially pure (CP) Titanium Grade 2 under DC-PEO obtained in electrolytes based on concentrated 85% analytically pure H 3 PO 4 (98 g/mole) acid with additions of 500 g•L −1 of zinc nitrate Zn(NO 3) 2 •6H 2 O or magnesium nitrate Mg(NO 3) 2 •6H 2 O, are described. These materials were characterized using scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and glow discharge optical emission spectroscopy (GDOES). It was found that the voltage of PEO process has influence on the chemical composition and thickness of the obtained porous coatings as well as on their electrochemical behavior. The higher the potential of PEO treatment, the higher the amount of zinc-to-phosphorus ratio for zinc enriched coatings was obtained, whereas in magnesium enriched coatings, the average amount of magnesium detected in PEO coating is approximately independent of the PEO voltages. Based on XPS studies, it was found out that most likely the top 10 nm of porous coatings is constructed of titanium (Ti 4+), magnesium (Mg 2+), zinc (Zn 2+), and phosphates PO 4 3− and/or HPO 4 2− and/or H 2 PO 4 − and/or P 2 O 7 4−. On the basis of GDOES studies, a four-sub-layer model of PEO coatings is proposed. Analysis of the potentiodynamic corrosion curves allowed to conclude that the best electrochemical repeatability was noted for magnesium and zinc enriched coatings obtained at 575 V DC .

Characterization of Porous Phosphate Coatings Enriched with Calcium, Magnesium, Zinc and Copper Created on CP Titanium Grade 2 by Plasma Electrolytic Oxidation

“…[9] reported that it was also possible to obtain a PEO coating in aqueous solution, in which mostly phosphates were detected. In the present paper, the continuation of previous works [13], i.e., the characterization of PEO coatings formed in electrolytes containing the concentrated phosphoric acid (H 3 PO 4 ) with the addition of four nitrates (Ca(NO 3 …”

“…The XPS spectra were processed in CasaXPS sotware [31] and interpreted based on NIST X-ray Photoelectron Spectroscopy Database [32] and XPS handbook [33]. SEM, EDS, XPS and GDOES measurement techniques, which have been used to characterize the PEO coatings, are described in detail in the references [13]. The surface topography of PEO coatings was evaluated by confocal laser scanning microscopy (CLSM), using a LEXT OLS4000 3D Laser Confocal Microscope (Olympus, Japan).…”

“…The studies already carried out by the authors have shown that it is possible to obtain porous PEO coatings which are enriched only with copper [28] or calcium [29]. In addition, in reference to [13], new coatings obtained in electrolyte contacting two nitrates (Zn(NO 3 ) 2 ·6H 2 O, Mg(NO 3 ) 2 ·6H 2 O) were presented. It was found that their Zn/P and Mg/P ratios depend on the PEO voltage, and that the Zn/P ratio increases linearly with voltage; in contrast, the Mg/P ratio it is a constant function.…”

“…The porous coatings obtained with such a method are mostly used as biomaterials, and are enriched with calcium and phosphorus to form a structure similar to that of hydroxyapatite [10], which may be doped by the addition of bactericidal copper [11], as well as zinc [12] and magnesium [13], added in case of acceleration of wound healing. The PEO processes may be performed under DC [7], AC [14,15], and pulse regime [16,17], which yields coatings with different porosities and chemical compositions, and also with different coating thicknesses.…”

Degradation Behavior of Micro-Arc Oxidized ZK60 Magnesium Alloy in a Simulated Body Fluid

“…In general, the degradation rate of alloys can be controlled mainly by tailoring their compositions or by conducting surface treatments [24,25]. Surface treatment can effectively improve the corrosion resistance of alloys, and conversion coating, anodizing, micro-arc oxidation (MAO), and other technologies are representatives of this treatment [26][27][28][29][30]. Hakimizad et al [31] reported that micro-arc oxidation coatings are formed by numerous short-lived micro-arc discharges on the entire surface occurring at voltages higher than the breakdown voltages of the oxide coating, the hard, thick, and adherent oxide coatings on light metals (e.g., Al and Mg) and on their alloys can be obtained by micro-arc oxidation technology.…”