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

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 PO4 and/or HPO4 and/or H2PO4 and/or P2O7. 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.

The aim of the paper is to determine the effect of DC voltage and PEO processing time on the creation of porous coatings enriched with magnesium or zinc on CP Titanium Grade 2. Electrolytes based on concentrated 85% analytically pure H 3 PO 4 (98 g/mole) acid, containing zinc nitrate Zn(NO 3 ) 2 •6H 2 O or magnesium nitrate Mg(NO 3 ) 2 •6H 2 O, were used in the study.
A scanning electron microscope Quanta 250 FEI with Low Vacuum and ESEM mode (Field Electron and Iron Company, Hillsboro, OR, USA) and a field emission cathode as well as an energy dispersive EDS system in a Noran System Six with nitrogen-free silicon drift detector (EDS, Silicon Drift Detectors: Keith Thompson, Thermo Fisher Scientific, Madison, WI, USA), were used.
X-ray photoelectron spectroscopy (XPS) measurements on studied sample surfaces were performed by means of SCIENCE SES 2002 instrument (SCIENTA AB, ScientaOmicron, Uppsala, Sweden) using a monochromatic (Gammadata-Scienta) Al K(alpha) (hν = 1486.6eV) X-ray source (18.7 mA, 13.02 kV).Scan analyses were carried out with an analysis area of 1 mm × 3 mm and a pass energy of 500 eV with the energy step 0.2 eV and step time 200 ms.The binding energy of the spectrometer was calibrated by the position of the Fermi level on a clean metallic sample.The power supplies were stable and of high accuracy.The experiments were carried out in an ultra-high vacuum system (SCIENTA AB, ScientaOmicron, Uppsala, Sweden) with a base pressure of about 6 × 10 −8 Pa.The XPS spectra were recorded in normal emission.For the XPS analyses, the CasaXPS 2.3.14 software (Casa Software Ltd., Teignmouth, Devon, UK) (Shirley background type, D. A. Shirley, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA, USA) [57], with the help of XPS tables [58], was used.All the binding energy values presented in this paper were charge corrected to C 1s at 284.8 eV.
Linear polarization measurements were carried out in Ringer's solution, which contains sodium chloride (8.69 g/L), potassium chloride (0.3 g/L), calcium chloride (0.48 g/L), through a Gamry Femtostat potentiostat using a 3-electrode configuration with a saturated calomel electrode (SCE) as reference electrode and platinum wire as counter electrode.Before polarization, the samples were immersed for 1 h in the electrolyte (OCP).Linear polarization was performed with 1 mV/s scan rate and the starting potential was set up for every sample as 100 mV lower than the OCP potential, while the ending potential was set up for 2000 mV (vs.SCE).One has to add that minimum 15 repeats were performed for each EDS experiment; one depth profile was performed for GDOES, and three corrosion measurements were done for each statistical plan.

Results and Discussion
In Figure 1, current vs. voltage curves of CP Titanium Grade 2 after PEO treatment in 500 g Mg(NO 3 ) 2 or 500 g Zn (NO 3 ) 2 in 1000 mL H 3 PO 4 electrolyte, are shown.The current density peaks in the voltage range from 0 till 200 V should be interpreted as plasma inflammation in solution.Above a voltage of about 200 V the stabilization of current densities at about 50 A/dm 2 is observed.This means that all the PEO treatments were carried out under the same current density conditions.The limiting values of voltage for the different PEO tests were defined: the lower voltage limit (500 V DC ) corresponds to the minimum current density for which it is possible to obtain a porous coating in the two solutions, whereas the upper voltage limit (650 V DC ) is due to the hardware limitation, i.e., the stabilized power supply.Linear polarization measurements were carried out in Ringer's solution, which contains sodium chloride (8.69 g/L), potassium chloride (0.3 g/L), calcium chloride (0.48 g/L), through a Gamry Femtostat potentiostat using a 3-electrode configuration with a saturated calomel electrode (SCE) as reference electrode and platinum wire as counter electrode.Before polarization, the samples were immersed for 1 h in the electrolyte (OCP).Linear polarization was performed with 1 mV/s scan rate and the starting potential was set up for every sample as 100 mV lower than the OCP potential, while the ending potential was set up for 2000 mV (vs.SCE).One has to add that minimum 15 repeats were performed for each EDS experiment; one depth profile was performed for GDOES, and three corrosion measurements were done for each statistical plan.

Results and Discussion
In Figure 1, current vs. voltage curves of CP Titanium Grade 2 after PEO treatment in 500 g Mg(NO3)2 or 500 g Zn (NO3)2 in 1000 mL H3PO4 electrolyte, are shown.The current density peaks in the voltage range from 0 till 200 V should be interpreted as plasma inflammation in solution.Above a voltage of about 200 V the stabilization of current densities at about 50 A/dm 2 is observed.This means that all the PEO treatments were carried out under the same current density conditions.The limiting values of voltage for the different PEO tests were defined: the lower voltage limit (500 VDC) corresponds to the minimum current density for which it is possible to obtain a porous coating in the two solutions, whereas the upper voltage limit (650 VDC) is due to the hardware limitation, i.e., the stabilized power supply.In Figures 2 and 3, the SEM micrographs of porous coatings formed on CP Titanium Grade 2 after PEO treatment at voltages of 500, 575, 650 VDC in electrolytes containing 500 g•L −1 of magnesium nitrate hexahydrate Mg(NO3)2•6H2O or 500 g•L −1 of zinc nitrate hexahydrate Zn (NO3)2•6H2O in concentrated phosphoric acid H3PO4, are presented.Based on EDS data recorded at a magnification of 500 times, the Zn/P ratios were found for the samples treated in the Zn-rich electrolyte.They were equal to 0.12 ± 0.01 (median: 0.12, range: 0.02), 0.16 ± 0.02 (median: 0.16, range: 0.06), 0.20 ± 0.01 (median: 0.20, range: 0.03) for 500, 575 and 650 VDC, respectively.For the coatings obtained in the electrolyte containing magnesium nitrate hexahydrate, the Mg/P ratios were equal to 0.16 ± 0.01 In Figures 2 and 3, the SEM micrographs of porous coatings formed on CP Titanium Grade 2 after PEO treatment at voltages of 500, 575, 650 V DC in electrolytes containing 500 g•L −1 of magnesium nitrate hexahydrate Mg(NO 3 ) 2 •6H 2 O or 500 g•L −1 of zinc nitrate hexahydrate Zn (NO 3 ) 2 •6H 2 O in concentrated phosphoric acid H 3 PO 4 , are presented.Based on EDS data recorded at a magnification of 500 times, the Zn/P ratios were found for the samples treated in the Zn-rich electrolyte.They were equal to 0.12 ± 0.01 (median: 0.12, range: 0.02), 0.16 ± 0.02 (median: 0.16, range: 0.06), 0.20 ± 0.01 (median: 0.20, range: 0.03) for 500, 575 and 650 V DC , respectively.For the coatings obtained in the electrolyte containing magnesium nitrate hexahydrate, the Mg/P ratios were equal to 0.16 ± 0.01 (median: 0.16, range: 0.02), 0.16 ± 0.01 (median: 0.16, range: 0.02), 0.17 ± 0.01 (median: 0.17, range: 0.04) for 500, 575, and 650 V DC , respectively.
In Figure 4, the Zn/P and Mg/P ratios based on EDS results for coatings formed on titanium after PEO treatment at voltages 500, 575, and 650 V DC in electrolytes containing 500 g•L −1 of zinc nitrate hexahydrate Zn(NO 3 ) 2 •6H 2 O and 500 g•L −1 of magnesium nitrate hexahydrate Mg(NO 3 ) 2 •6H 2 O in concentrated H 3 PO 4 , are shown, lying in the ranges of 0.12-0.20 and 0.16-0.17,respectively.
Based on these results, one can conclude that the Zn/P ratio increases linearly as a function of the amount of zinc nitrate, which on the other hand is not observed for the solution containing magnesium nitrate, where the average value of Mg/P ratio is approximately constant.In addition, it should be noted that the standard deviation of Mg/P ratio is approximately two times as high as that of the Zn/P ratio, which may indicate that in the case of phosphate coatings enriched with zinc, the repeatability of the PEO coating composition is greater than that for coatings enriched with magnesium.(median: 0.16, range: 0.02), 0.16 ± 0.01 (median: 0.16, range: 0.02), 0.17 ± 0.01 (median: 0.17, range: 0.04) for 500, 575, and 650 VDC, respectively.In Figure 4, the Zn/P and Mg/P ratios based on EDS results for coatings formed on titanium after PEO treatment at voltages 500, 575, and 650 VDC in electrolytes containing 500 g•L −1 of zinc nitrate hexahydrate Zn(NO3)2•6H2O and 500 g•L −1 of magnesium nitrate hexahydrate Mg(NO3)2•6H2O in concentrated H3PO4, are shown, lying in the ranges of 0.12-0.20 and 0.16-0.17,respectively.
Based on these results, one can conclude that the Zn/P ratio increases linearly as a function of the amount of zinc nitrate, which on the other hand is not observed for the solution containing magnesium nitrate, where the average value of Mg/P ratio is approximately constant.In addition, it should be noted that the standard deviation of Mg/P ratio is approximately two times as high as that of the Zn/P ratio, which may indicate that in the case of phosphate coatings enriched with zinc, the repeatability of the PEO coating composition is greater than that for coatings enriched with magnesium.In Figure 5a, the XPS spectrum of a coating formed on CP Titanium Grade 2 after PEO treatment for 3 min in the zinc-rich phosphoric acid electrolyte at a voltage of 575 V DC , is presented.The obtained XPS results of PEO coatings clearly show that the 10 nm top layer is enriched in zinc consisting of Zn 2+ , as well as titanium-as Ti 4+ , phosphorus as PO In Figure 5a, the XPS spectrum of a coating formed on CP Titanium Grade 2 after PEO treatment for 3 min in the zinc-rich phosphoric acid electrolyte at a voltage of 575 VDC, is presented.The obtained XPS results of PEO coatings clearly show that the 10 nm top layer is enriched in zinc consisting of Zn 2+ , as well as titanium-as Ti 4+ , phosphorus as PO4 3− , or HPO4 2− , or H2PO4 − , or P2O7 4− , which was confirmed by the binding energies of 1022.4 eV (Zn 2p3/2), 499.8 eV (Zn LMM), 460.8 eV (Ti 2p3/2), 531.8 eV (O 1s), and 134.2 eV (P2p), respectively.
In Figure 5b, the XPS spectrum of a coating formed on CP Titanium Grade 2 after PEO treatment for 3 min in the magnesium-rich phosphoric acid electrolyte at a voltage of 575 VDC, is shown.The binding energies of magnesium Mg 2s (89.4 eV), Mg KLL (306.8 eV), and titanium Ti2p3/2 (460.2 eV) most likely may be interpreted as Mg 2+ and Ti 4+ , respectively.In addition, it should be noted that in PEO coatings enriched in magnesium, the binding energies of phosphorus P2p (134.1 eV), and oxygen O 1s (531.6 eV) may suggest the presence of PO4 3− , HPO4 2− , H2PO4 − , and P2O7 4− ions in the obtained porous coatings, which is very similar to the case of coatings enriched in zinc.In Figures 6 and 7, the GDOES signals, with their derivatives, of zinc or magnesium, phosphorus, oxygen, hydrogen, carbon, nitrogen, titanium of porous coating formed on CP Titanium Grade 2 after PEO treatment at voltages of 500, 575, and 650 VDC in both electrolytes, are presented.Four sub-layers may be distinguished in each obtained coating, i.e., the first one very porous and organically contaminated, the second one and third one, semi-porous, which have different thicknesses, dependent on magnesium or zinc nitrate tetrahydrate amount in the electrolyte, and the fourth sub-layer is a transition one.To understand the influence of voltage on each sub-layer, authors propose describing them by mathematical equations.The regions were designated on the basis of GDOES signals and their first and second derivatives.The derivatives allowed us to determine mainly the beginning and the end of sub-layer intervals, where the rate of change of individual signals underwent significant changes.The thicknesses of the first top porous sub-layers of all the obtained PEO coatings enriched in magnesium or zinc are linear functions of PEO voltage U and may be described by the following mathematical models: I = 2.67•U − 1143 (R 2 = 0.992) and I = U − 141.6 (R 2 = 0.964), respectively.In both cases, they are growing functions, i.e., the minimum top and porous sub-layer thickness was obtained at 500 VDC, and the maximum one, at 650 In Figures 6 and 7, the GDOES signals, with their derivatives, of zinc or magnesium, phosphorus, oxygen, hydrogen, carbon, nitrogen, titanium of porous coating formed on CP Titanium Grade 2 after PEO treatment at voltages of 500, 575, and 650 V DC in both electrolytes, are presented.Four sub-layers may be distinguished in each obtained coating, i.e., the first one very porous and organically contaminated, the second one and third one, semi-porous, which have different thicknesses, dependent on magnesium or zinc nitrate tetrahydrate amount in the electrolyte, and the fourth sub-layer is a transition one.To understand the influence of voltage on each sub-layer, authors propose describing them by mathematical equations.The regions were designated on the basis of GDOES signals and their first and second derivatives.The derivatives allowed us to determine mainly the beginning and the end of sub-layer intervals, where the rate of change of individual signals underwent significant changes.The thicknesses of the first top porous sub-layers of all the obtained PEO coatings enriched in magnesium or zinc are linear functions of PEO voltage U and may be described by the following mathematical models: I = 2.67•U − 1143 (R 2 = 0.992) and I = U − 141.6 (R 2 = 0.964), respectively.In both cases, they are growing functions, i.e., the minimum top and porous sub-layer thickness was obtained at 500 V DC , and the maximum one, at 650 V DC .It should be also noted that the magnesium enriched coating in the first top sub-layer is depleted in magnesium, whereas the other one is enriched in zinc.The thicknesses of the second sub-layers for two examined coatings also have growing trends due to the PEO voltages.However, the biggest thickness of that second sub-layer was found for coatings created at 500 V DC , i.e., about 450 s of sputtering time, while for other voltages, i.e., 575 and 650 V DC the differences did not exceed 80 s of sputtering time.Most important is the fact that the magnesium enriched coating reaches its global maximum in that layer, while in case of zinc enriched coating, the maximum was found in the top first sub-layer.In the third sub-layers, the signals of magnesium and zinc revealed a decrease.For the magnesium spectrum, the lowest thickness (at 200 s of sputtering time) was recorded for coating formed at 500 V DC , while for the other voltages, i.e., 575 and 650 V DC , the thinnest ones were equal to about 800 s of sputtering time, which may be described by a power function 0.006•U 1.83 (R 2 = 0.781) or a linear one of 2•U − 450 (R 2 = 0.75).In case of zinc enriched coating, the trend of coating thickness versus voltage U may be also considered as a power function, i.e., 3 × −11 •U 4.73 (R 2 = 0.944) or a linear trend, i.e., 3.33•U − 1516 (R 2 = 0.892).The last transition sub-layer, which contains the limit of porosity which is visible as a peak on the hydrogen spectrum, that is halfway of the slope of titanium, for two tested coatings may be described by one 2.33•U − 641.6 (R 2 = 0.875).VDC.It should be also noted that the magnesium enriched coating in the first top sub-layer is depleted in magnesium, whereas the other one is enriched in zinc.The thicknesses of the second sub-layers for two examined coatings also have growing trends due to the PEO voltages.However, the biggest thickness of that second sub-layer was found for coatings created at 500 VDC, i.e., about 450 s of sputtering time, while for other voltages, i.e., 575 and 650 VDC the differences did not exceed 80 s of sputtering time.Most important is the fact that the magnesium enriched coating reaches its global maximum in that layer, while in case of zinc enriched coating, the maximum was found in the top first sub-layer.In the third sub-layers, the signals of magnesium and zinc revealed a decrease.
For the magnesium spectrum, the lowest thickness (at 200 s of sputtering time) was recorded for coating formed at 500 VDC, while for the other voltages, i.e., 575 and 650 VDC, the thinnest ones were equal to about 800 s of sputtering time, which may be described by a power function 0.006•U 1.83 (R 2 = 0.781) or a linear one of 2•U − 450 (R 2 = 0.75).In case of zinc enriched coating, the trend of coating thickness versus voltage U may be also considered as a power function, i.e., 3 × 10 −11 •U 4.73 (R 2 = 0.944) or a linear trend, i.e., 3.33•U − 1516 (R 2 = 0.892).The last transition sub-layer, which contains the limit of porosity which is visible as a peak on the hydrogen spectrum, that is halfway of the slope of titanium, for two tested coatings may be described by one 2.33•U − 641.6 (R 2 = 0.875).In Figure 8, the potentiodynamic corrosion curves of the porous coating formed on CP Titanium Grade 2 after PEO treatment at voltages of 500 (red lines), 575 (blue lines), and 650 VDC (green lines), in (a) magnesium-rich, or (b) zinc-rich electrolytes, are presented.The electrochemical properties of coatings containing magnesium and/or zinc and/or titanium phosphates may be characterized by the passive current densities measured at 1400 mV.The minimum passive current density was recorded for coatings enriched with magnesium obtained at 500 VDC (mean: 2.3 × 10 −7 ; median: 1.6 × 10 −7 ; range: 4.4 × 10 −7 ), while the maximum one (mean: 9.2 × 10 −7 ; median: 1.1 × 10 −6 ; range: 8.0 × 10 −7 ), was at 650 VDC.That linear trend of mean of passive current densities i vs. PEO voltages U may be described by the following function: i = 5 × 10 −9 •U -2 × 10 −6 (R 2 = 0.98).It should be also noted that the smallest passive current density range (max-min) was found for magnesium enriched coatings obtained at 575 VDC, what may be interpreted as the best electrochemical repeatability.In Figure 8, the potentiodynamic corrosion curves of the porous coating formed on CP Titanium Grade 2 after PEO treatment at voltages of 500 (red lines), 575 (blue lines), and 650 V DC (green lines), in (a) magnesium-rich, or (b) zinc-rich electrolytes, are presented.The electrochemical properties of coatings containing magnesium and/or zinc and/or titanium phosphates may be characterized by the passive current densities measured at 1400 mV.The minimum passive current density was recorded for coatings enriched with magnesium obtained at 500 V DC (mean: 2.3 × 10 −7 ; median: 1.6 × 10 −7 ; range: 4.4 × 10 −7 ), while the maximum one (mean: 9.2 × 10 −7 ; median: 1.1 × 10 −6 ; range: 8.0 × 10 −7 ), was at 650 V DC .That linear trend of mean of passive current densities i vs. PEO voltages U may be described by the following function: i = 5 × 10 −9 •U -2 × 10 −6 (R 2 = 0.98).It should be also noted that the smallest passive current density range (max-min) was found for magnesium enriched coatings obtained at 575 V DC , what may be interpreted as the best electrochemical repeatability.The minimum passive current density was recorded for zinc-enriched coatings obtained at 575 VDC (mean: 2.9 × 10 −7 ; median: 2.5 × 10 −7 ; range: 5.6 × 10 −7 ), while the maximum one (mean: 9.3 × 10 −7 ; median: 3.8 × 10 −6 ; range: 2.1 × 10 −6 ), at 650 VDC.That linear trend of the mean of passive current densities i vs. PEO voltages U may be described by the following function: i = 4 × 10 −9 •U − 2 × 10 −6 (R 2 = 0.66).It should be also noted that the smallest passive current density range was found for magnesium enriched coatings obtained at 575 VDC, which may be interpreted as the best electrochemical repeatability.However, it has to be pointed out that the values of passive current density averages and medians of zinc enriched PEO coatings are very similar.It may be concluded that the values of passive current densities may be correlated both with corrosion resistance as well as with charging of PEO coatings.In addition, an important fact is that the statistics were performed based only on three repetitions.

Conclusions
The studies of the effect of DC voltages increasing from 500 up to 650 VDC on creation of porous coatings enriched with magnesium or zinc on the CP Titanium Grade 2 allowed to formulating the following conclusions: (1) It is possible to form porous coatings by using of DC PEO oxidation in electrolytes containing concentrated phosphoric acid and magnesium or zinc nitrate.(2) The voltage of PEO process has an influence on the chemical composition and thickness of the obtained porous coatings as well as on their electrochemical behavior.(3) The higher the voltage of PEO treatment, the higher is zinc-to-phosphorus ratio for zinc enriched coatings; there is a trend for growing as a function of PEO voltage.(4) The higher the voltage of PEO treatment, the higher the range (max-min) of magnesium-to-phosphorus ratio with very similar averages for magnesium enriched coatings; the trend may be described as a constant function of PEO voltage.The minimum passive current density was recorded for zinc-enriched coatings obtained at 575 V DC (mean: 2.9 × 10 −7 ; median: 2.5 × 10 −7 ; range: 5.6 × 10 −7 ), while the maximum one (mean: 9.3 × 10 −7 ; median: 3.8 × 10 −6 ; range: 2.1 × 10 −6 ), at 650 V DC .That linear trend of the mean of passive current densities i vs. PEO voltages U may be described by the following function: i = 4 × 10 −9 •U − 2 × 10 −6 (R 2 = 0.66).It should be also noted that the smallest passive current density range was found for magnesium enriched coatings obtained at 575 V DC , which may be interpreted as the best electrochemical repeatability.However, it has to be pointed out that the values of passive current density averages and medians of zinc enriched PEO coatings are very similar.It may be concluded that the values of passive current densities may be correlated both with corrosion resistance as well as with charging of PEO coatings.In addition, an important fact is that the statistics were performed based only on three repetitions.

Conclusions
The studies of the effect of DC voltages increasing from 500 up to 650 V DC on creation of porous coatings enriched with magnesium or zinc on the CP Titanium Grade 2 allowed to formulating the following conclusions: (1) It is possible to form porous coatings by using of DC PEO oxidation in electrolytes containing concentrated phosphoric acid and magnesium or zinc nitrate.(2) The voltage of PEO process has an influence on the chemical composition and thickness of the obtained porous coatings as well as on their electrochemical behavior.(3) The higher the voltage of PEO treatment, the higher is zinc-to-phosphorus ratio for zinc enriched coatings; there is a trend for growing as a function of PEO voltage.(4) The higher the voltage of PEO treatment, the higher the range (max-min) of magnesium-to-phosphorus ratio with very similar averages for magnesium enriched coatings; the trend may be described as a constant function of PEO voltage.(5) The four-sub-layer model may be assumed to describe the obtained PEO coatings: (a) The first and top sub-layers having a thickness corresponding with the sputtering time equaling to 200-600 s and 350-450 s for magnesium and zinc enriched PEO coatings, respectively; (b) The top sub-layer enriched with magnesium is depleted in magnesium compounds, while the other one, containing zinc in the top, is clearly enriched with zinc; (c) The thickness of semi-porous sub-layers depends on the PEO treatment voltage; the thickest second and third sub-layers appear to set up at high voltages and they decrease with PEO voltage decreasing; (d) The transition sub-layers have thickness corresponding to a sputtering time of about 500-900 s.

Figure 4 .
Figure 4. Zn/P and Mg/P ratios of EDS results of coatings formed on CP Titanium Grade 2 after PEO treatment at voltages 500, 575 and 650 V DC in electrolytes containing 500 g•L −1 of Zn(NO 3 ) 2 •6H 2 O and 500 g•L −1 of Mg(NO 3 ) 2 •6H 2 O in concentrated H 3 PO 4 .
the binding energies of 1022.4 eV (Zn 2p 3/2 ), 499.8 eV (Zn LMM), 460.8 eV (Ti 2p 3/2 ), 531.8 eV (O 1s), and 134.2 eV (P2p), respectively.In Figure5b, the XPS spectrum of a coating formed on CP Titanium Grade 2 after PEO treatment for 3 min in the magnesium-rich phosphoric acid electrolyte at a voltage of 575 V DC , is shown.The binding energies of magnesium Mg 2s (89.4 eV), Mg KLL (306.8 eV), and titanium Ti2p 3/2 (460.2 eV) most likely may be interpreted as Mg2+ and Ti 4+ , respectively.In addition, it should be noted that in PEO coatings enriched in magnesium, the binding energies of phosphorus P2p (134.1 eV), and oxygen O 1s (531.6 eV) may suggest the presence of PO 4 3− , HPO 4 2− , H 2 PO 4 − , and P 2 O 7 4− ions in the obtained porous coatings, which is very similar to the case of coatings enriched in zinc.Metals 2018, 8, x FOR PEER REVIEW 6 of 13

Figure 8 .
Figure 8. Potentiodynamic corrosion results of the porous coating formed on CP Titanium Grade 2 after PEO treatment at voltages of 500 VDC (red lines), 575 VDC (blue lines), and 650 VDC (green lines), in (a) magnesium-rich, or (b) zinc-rich phosphoric acid electrolytes; (a,d) logarithmic Y axis; (b) and (e) linear Y axis; (c,f) statistical analysis results.

( 5 )
The four-sub-layer model may be assumed to describe the obtained PEO coatings: (a) The first and top sub-layers having a thickness corresponding with the sputtering time equaling to 200-600 s and 350-450 s for magnesium and zinc enriched PEO coatings, respectively; (b) The top sub-layer enriched with magnesium is depleted in magnesium compounds, while the other one, containing zinc in the top, is clearly enriched with zinc;

Figure 8 .
Figure 8. Potentiodynamic corrosion results of the porous coating formed on CP Titanium Grade 2 after PEO treatment at voltages of 500 V DC (red lines), 575 V DC (blue lines), and 650 V DC (green lines), in (a) magnesium-rich, or (b) zinc-rich phosphoric acid electrolytes; (a,d) logarithmic Y axis; (b,e) linear Y axis; (c,f) statistical analysis results.