Abstract
The aim of the paper is to study and determine the effect of voltage increasing from 500 up to 650 VDC 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 H3PO4 (98 g/mole) acid with additions of 500 g·L−1 of zinc nitrate Zn(NO3)2∙6H2O or magnesium nitrate Mg(NO3)2∙6H2O, 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 (Ti4+), magnesium (Mg2+), zinc (Zn2+), and phosphates PO43− and/or HPO42− and/or H2PO4− and/or P2O74−. 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 VDC.
Highlights
IntroductionElectrochemical treatments, such as electropolishing [1,2,3] and plasma electrolytic oxidation (micro arc oxidation) [4,5,6] may be used for preparing nano- and micro-layers, respectively
Electrochemical treatments, such as electropolishing [1,2,3] and plasma electrolytic oxidation [4,5,6] may be used for preparing nano- and micro-layers, respectively
It is worth noting that the obtained porous coatings may contain elements originating from the electrolytes, which lead to a bone-like structure [45,46,47] and show antibacterial properties [48,49,50,51,52,53,54] with simultaneous high corrosion resistance
Summary
Electrochemical treatments, such as electropolishing [1,2,3] and plasma electrolytic oxidation (micro arc oxidation) [4,5,6] may be used for preparing nano- and micro-layers, respectively. In case of standard electropolishing (EP) [7,8,9,10], magnetoelectropolishing (MEP) [9,10,11,12,13,14,15,16] and high-current density electropolishing (HDEP) [17,18,19,20] it is possible to obtain passive layers with different chemical compositions [20,21,22,23,24,25,26] as well as hydrogenation [27,28,29], which affect the corrosion behavior [30,31]. It is worth noting that the obtained porous coatings may contain elements originating from the electrolytes, which lead to a bone-like structure (phosphorus and calcium) [45,46,47] and show antibacterial properties (copper and zinc) [48,49,50,51,52,53,54] with simultaneous high corrosion resistance. It may be noted that the chemical composition [36,37,41,51] of PEO coatings as well as their porosity [42,52] which undergo the description by roughness parameters [55,56], may be expected
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