Corrosion of Titanium Alloys Anodized Using Electrochemical Techniques

: The anodization of titanium has been an excellent option for protecting titanium and its alloys from corrosive environments such as acids and chloride systems, by generating a homogenous oxide layer. The objective of the current investigation was to evaluate the electrochemical corrosion behavior of alloys Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V anodized in 1M H 2 SO 4 and H 3 PO 4 solutions at a current density of 2.5 × 10 –3 A/cm 2 . The anodization’s electrochemical characterization was achieved in NaCl and H 2 SO 4 at 3.5% wt. electrolytes. Scanning electron microscopy (SEM) was employed to determine the anodized thickness and morphology. Cyclic potentiodynamic polarization (CPP) and electrochemical impedance spectroscopy (EIS), based on ASTM G61-86 and G106-15 Standards, were the electrochemical techniques mainly employed. The anodized samples presented a change in E corr values and a higher passivation zone. The EIS plot showed a higher resistance for samples anodized in H 3 PO 4 and Ti-6Al-2Sn-4Zr-2Mo.


Introduction
Industries, including biomedicine and aerospace, require materials such as titanium for their great properties, both mechanical and chemical.For this reason, the study of the oxide layer produced on titanium has increased in recent years, with a view to increasing the life of components, reducing costs for preventive and corrective maintenance, and replacing steel in zones where corrosion is the higher priority [1][2][3].
Nonetheless, titanium and its alloys can present degradation when exposed to chloride and acid media due to the defects in the layers generated by natural processes.Authors have reported that the oxide film generated by titanium is composed of multivalent titanium, which provokes layer degradation by galvanic or crevice corrosion [4][5][6].
The classification of Ti-alloys can be divided into four classes: α, near to α, α + β, and metastable β; the classification will depend on the percentage of β elements that are present.Gloria et al. and Peters et al. [7,8] indicated that the β elements are mainly Mo, Cr, V, Ta, Fe, and Ni; the presence of these stabilizers can change the mechanical and chemical properties of alloys.Song et al. [9] related the presence of vanadium with a reduction in the oxide layer's corrosion resistance to vanadium dissolution.Furthermore, for biomedical applications, elements such as V and Al are classified as toxic for the human body because they provoke genotoxicity, Alzheimer's, and peripheral neuropathy [10,11].trochemical impedance spectroscopy (EIS).The anodized layers were characterized by scanning electron microscopy (SEM).

Material
The materials used in this work were Ti-6Al-2Sn-4Zr-2Mo (Supra alloys, Camarillo, CA, USA) and, Ti-6Al-4V (Titanium Engineers, Stafford, TX, USA) used in the received condition.The chemical composition of these alloys was obtained by atomic absorption spectrometry and is listed in Table 1.

Microstructural Characterization
Titanium alloys were prepared by metallography technique [43].The materials were polished using various SiC sandpaper grades 400, 600, and 800; each sample was ultrasonically cleaned for 10 min in ethanol (C 2 H 5 OH) and deionized water.The samples were subjected to a chemical attack using a Kroll solution made up of 3 mL of hydrofluoric acid (HF), 5 mL of nitric acid (HNO 3 ), and 100 mL of water (H 2 O) for 30 s [44].
The surface and cross-section of titanium alloys were investigated using secondary electron (SE) and backscattered electron (BSE) detectors in a scanning electron microscope (SEM, JEOL-JSM-5610LV, Tokyo, Japan) operating at 20 kV and 8.5 and 12 mm work distance.The chemical composition of alloys was obtained by energy-dispersive X-ray spectroscopy (EDS, JEOL-JSM-5610LV, Tokyo, Japan).

Anodizing Process
Ultrasonic cleaning in ethanol (C 2 H 5 OH) and deionized water served as the pretreatment for 10 min.
The anodizing procedure was carried out in an electrochemical cell with a graphite rod serving as the cathode and 1 M electrolytes (analytical grade reagents (JT Baker)), with the anodizing electrolyte's temperature being 25 • C ± 1.Using a DC power source, the titanium samples' current density was 2.5 × 10 −3 A•cm −2 for 600 s (XLN300025-GL).The AMS2487 specification was followed during the anodizing procedure [45].

Electrochemical Measurements
Cyclic potentiodynamic polarization (CPP) and electrochemical impedance spectroscopy (EIS) were carried out at room temperature using a VersaSTAT 4 Princeton Applied Research (Ametek, Inc.Oak Ridge, TN, USA) in 3.5 wt.% NaCl and H 2 SO 4 solutions.A conventional three-electrode cell configuration was employed for electrochemical characterization at room temperature and all the corrosion tests were performed in triplicate.The working electrode (WE, with an exposed surface of 1 cm 2 ) was anodized for the current study, the reference electrode (RE) was saturated calomel (SCE), and the counter electrode (CE) of platinum [46].
The CPP parameters were a scan potential from −1.2 to 1.2 V vs. SCE of OCP.The potential sweep was 1 mV/s to complete 1 cycle [47].
EIS was realized with a potential amplitude of ±10 mV; the frequencies were from 100 kHz to 10 mHz.The equivalent circuit analysis was made in ZView [48].

SEM Microstructural Analysis
Figure 1 shows the microstructure of the initial samples in the superficial section of Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V.Figure 1a shows Ti-6Al-2Sn-4Zr-2Mo, presenting α phase grains deformed with triple junction zones for β phase.Ti-6Al-4V had an equiaxial and fine grain, the β phase had a spherical form, and α was the matrix.The CPP parameters were a scan potential from −1.2 to 1.2 V vs. SCE of OCP.The potential sweep was 1 mV/s to complete 1 cycle [47].
EIS was realized with a potential amplitude of ±10 mV; the frequencies were from 100 kHz to 10 mHz.The equivalent circuit analysis was made in ZView [48].

SEM Microstructural Analysis
Figure 1 shows the microstructure of the initial samples in the superficial section of Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V.Figure 1a shows Ti-6Al-2Sn-4Zr-2Mo, presenting α phase grains deformed with triple junction zones for β phase.Ti-6Al-4V had an equiaxial and fine grain, the β phase had a spherical form, and α was the matrix.

SEM Surface Analysis of Anodized Alloys
Figure 2 shows the surface characterization of the samples anodized in H2SO4 and H3PO4.Figure 2a shows the Ti-6Al-2Sn-4Zr-2Mo anodized in H2SO4; this sample presented a homogenous porosity, and the different levels can be observed, associated with a high rugosity.Figure 2b of Ti-6Al-4V anodized in H2SO4 shows lower porosity but little crack zones at a different level, associating this with high rugosity.Samples anodized in H3PO4 showed similar morphology with great porosities and heterogenous distribution and size.

SEM Surface Analysis of Anodized Alloys
Figure 2 shows the surface characterization of the samples anodized in H 2 SO 4 and H 3 PO 4 .Figure 2a shows the Ti-6Al-2Sn-4Zr-2Mo anodized in H 2 SO 4 ; this sample presented a homogenous porosity, and the different levels can be observed, associated with a high rugosity.Figure 2b of Ti-6Al-4V anodized in H 2 SO 4 shows lower porosity but little crack zones at a different level, associating this with high rugosity.Samples anodized in H 3 PO 4 showed similar morphology with great porosities and heterogenous distribution and size.The CPP parameters were a scan potential from −1.2 to 1.2 V vs. SCE of OCP.The potential sweep was 1 mV/s to complete 1 cycle [47].
EIS was realized with a potential amplitude of ±10 mV; the frequencies were from 100 kHz to 10 mHz.The equivalent circuit analysis was made in ZView [48].

SEM Microstructural Analysis
Figure 1 shows the microstructure of the initial samples in the superficial section of Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V.Figure 1a shows Ti-6Al-2Sn-4Zr-2Mo, presenting α phase grains deformed with triple junction zones for β phase.Ti-6Al-4V had an equiaxial and fine grain, the β phase had a spherical form, and α was the matrix.

SEM Surface Analysis of Anodized Alloys
Figure 2 shows the surface characterization of the samples anodized in H2SO4 and H3PO4.Figure 2a shows the Ti-6Al-2Sn-4Zr-2Mo anodized in H2SO4; this sample presented a homogenous porosity, and the different levels can be observed, associated with a high rugosity.Figure 2b of Ti-6Al-4V anodized in H2SO4 shows lower porosity but little crack zones at a different level, associating this with high rugosity.Samples anodized in H3PO4 showed similar morphology with great porosities and heterogenous distribution and size.

SEM Cross-Section Analysis of Anodized Alloys
Figure 3 shows the cross-section of the anodized samples with the measurements and an element mapping by EDS for Ti-6Al-2Sn-4Zr-2Mo.Figure 3a shows the anodized sample in H 2 SO 4 , where the average anodized thickness was 1.62 µm, with a major thickness of 1.88 µm and a lower one of 1.36 µm, having a uniform and continuous coating.At the top, the material's roughness can be observed, and cracks were not present.Figure 3b shows the anodization in H3PO4, where the average anodized coating was 1.65 µm higher than that anodized in H 2 SO 4 .However, this anodization presents more variability in the thickness with a lower measurement of 1.28 µm and a higher one of 2.08 µm.However, the thickness of the sample is within the specifications of AMS2487B.

SEM Cross-Section Analysis of Anodized Alloys
Figure 3 shows the cross-section of the anodized samples with the measurements and an element mapping by EDS for Ti-6Al-2Sn-4Zr-2Mo.Figure 3a shows the anodized sample in H2SO4, where the average anodized thickness was 1.62 µm, with a major thickness of 1.88 µm and a lower one of 1.36 µm, having a uniform and continuous coating.At the top, the material's roughness can be observed, and cracks were not present.Figure 3b shows the anodization in H3PO4, where the average anodized coating was 1.65 µm higher than that anodized in H2SO4.However, this anodization presents more variability in the thickness with a lower measurement of 1.28 µm and a higher one of 2.08 µm.However, the thickness of the sample is within the specifications of AMS2487B.
For that reason, the porosities of Figure 2a,b could be localized at the top, but after that, a compact oxide layer was present.The element mappings of Figure 3 showed the presence of alloying elements.In the coating, titanium was localized in all alloys, but in the coating zone, the top was reduced; elements such as Sn had a presence in all the samples, including the top.The oxygen in Figure 3 is concentrated in the coating zone, relating that result to the oxide layer of Ti. Figure 4 shows the cross-section of anodized Ti-6Al-4V with the measurements and the mapping of the chemical composition obtained by EDS. Figure 4a shows the sample anodized in H2SO4, where the average anodized thickness was 0.95 µm, with a higher thickness of 1.02 µm and a lower one of 0.88 µm.This coating also presented a high roughness, and the morphology could not be related to a compact layer.Figure 4b shows the H3PO4 anodization had a higher coating thickness than the Figure 4a sample, with an average of 1.53 µm.This coating also presented a higher thickness, and the layer was For that reason, the porosities of Figure 2a,b could be localized at the top, but after that, a compact oxide layer was present.The element mappings of Figure 3 showed the presence of alloying elements.In the coating, titanium was localized in all alloys, but in the coating zone, the top was reduced; elements such as Sn had a presence in all the samples, including the top.The oxygen in Figure 3 is concentrated in the coating zone, relating that result to the oxide layer of Ti.
Figure 4 shows the cross-section of anodized Ti-6Al-4V with the measurements and the mapping of the chemical composition obtained by EDS. Figure 4a shows the sample anodized in H 2 SO 4 , where the average anodized thickness was 0.95 µm, with a higher thickness of 1.02 µm and a lower one of 0.88 µm.This coating also presented a high roughness, and the morphology could not be related to a compact layer.Figure 4b shows the H 3 PO 4 anodization had a higher coating thickness than the Figure 4a sample, with an average of 1.53 µm.This coating also presented a higher thickness, and the layer was compact.In some anodized zones, some discontinuities could be observed.Only the sample anodized in H 3 PO 4 achieved the AMS2487B specs.
Metals 2023, 13, x FOR PEER REVIEW 6 of 21 compact.In some anodized zones, some discontinuities could be observed.Only the sample anodized in H3PO4 achieved the AMS2487B specs.Figure 4a,b show the presence of the chemical elements.Titanium was also present in all matrixes, but the coating had more presence at the bottom than at the top for both samples.These anodized samples showed the presence of aluminum and vanadium (more than in the H2SO4 coating).This behavior is related to the formation of secondary oxides.Oxygen had a presence in the coating zone, as previously explained.

Cyclic Potentiodynamic Polarization
Figure 5 shows the uncoated CPP for alloys and anodized samples in NaCl and H2SO4 at 3.5 wt.%. Figure 5a shows the behavior of Ti-6Al-2Sn-4Zr-2Mo uncoated and anodized when exposed to NaCl; the uncoated sample presents a higher Ecorr when exposed to NaCl compared with the anodized sample of Ti-6Al-2Sn-4Zr-2Mo, −0.397 V (see Table 2), meaning that corrosion is most likely.The uncoated sample did not present a passivation zone, indicating activation and material dissolution.The sample anodized with H2SO4 showed the highest icorr (4.53 × 10 −7 A/cm 2 ) in this media for the Ti-6Al-2Sn-4Zr-2Mo alloy; furthermore, the behavior corresponds to coating and means that the anodized sample presented a higher corrosion kinetics in comparison with the sample anodized in H3PO4.The H3PO4anodized sample presented better corrosion resistance and did not show a significant passive breakdown potential, compared to that anodized in H2SO4.
Figure 5b shows the behavior of uncoated and anodized Ti-6Al-2Sn-4Zr-2Mo when exposed to H2SO4.In this case, the uncoated and the H2SO4 -anodized sample presented similar values of Ecorr (−0.301 and −0.312 V) and icorr (1.86 × 10 −6 and 4.23 × 10 −6 A/cm 2 ), meaning a similar corrosion probability and kinetics.However, the H2SO4-anodized sample presented three unusual reactions in the anodic breach, associated with a fast electrochemical reaction on the surface due to the porosity.Furthermore, that reaction in the anodic breach is related to an unstable oxide layer generated on the surface, and the Figure 4a,b show the presence of the chemical elements.Titanium was also present in all matrixes, but the coating had more presence at the bottom than at the top for both samples.These anodized samples showed the presence of aluminum and vanadium (more than in the H 2 SO 4 coating).This behavior is related to the formation of secondary oxides.Oxygen had a presence in the coating zone, as previously explained.

Cyclic Potentiodynamic Polarization
Figure 5 shows the uncoated CPP for alloys and anodized samples in NaCl and H 2 SO 4 at 3.5 wt.%. Figure 5a shows the behavior of Ti-6Al-2Sn-4Zr-2Mo uncoated and anodized when exposed to NaCl; the uncoated sample presents a higher E corr when exposed to NaCl compared with the anodized sample of Ti-6Al-2Sn-4Zr-2Mo, −0.397 V (see Table 2), meaning that corrosion is most likely.The uncoated sample did not present a passivation zone, indicating activation and material dissolution.The sample anodized with H 2 SO 4 showed the highest i corr (4.53 × 10 −7 A/cm 2 ) in this media for the Ti-6Al-2Sn-4Zr-2Mo alloy; furthermore, the behavior corresponds to coating and means that the anodized sample presented a higher corrosion kinetics in comparison with the sample anodized in H 3 PO 4 .The H 3 PO 4 -anodized sample presented better corrosion resistance and did not show a significant passive breakdown potential, compared to that anodized in H 2 SO 4 .
pitting and repassivation process that occurs.Additionally, the sample anodized with H2SO4 presented a passivation range with a reduction of current demand, meaning that the corrosion process was reduced.The sample anodized with H3PO4 presented the highest Ecorr in this media, −0.002 V, and lower icorr, 3.24 × 10 −8 A/cm 2 , associated with a lower corrosion rate.The passivation range was more extended with 1.17 V, and a reduction of current was presented.Figure 5c shows the behavior of Ti-6Al-4V, uncoated and anodized, in NaCl at 3.5 wt %.The uncoated sample had the higher Ecorr, −0.144 V (see Table 2) when exposed to NaCl and compared with the anodized of Ti-6Al-4V; meanwhile, the anodized sample in H2SO4 and H3PO4 presented −0.237 and −0.287 V (see Table 2).However, the uncoated sample presented the higher icorr, with 6.71 × 10 −6 , relating this behavior with a faster corrosion kinetic.All the samples presented passivation, the highest being for the sample anodized in H3PO4, meaning that the passive layer was more stable against corrosion processes.The sample anodized in H3PO4 presented a passive region of 1.28 V, and also presented a decrease in the current demand, from values near to ×10 −6 to ×10 −7 A/cm 2 , meaning a reduction of electron transference, associated with an increase in the passive layer efficiency.The anodized sample of Ti-6Al-4V in H2SO4 presented a lower passivation range (0.77 V), Figure 5b shows the behavior of uncoated and anodized Ti-6Al-2Sn-4Zr-2Mo when exposed to H 2 SO 4 .In this case, the uncoated and the H 2 SO 4 -anodized sample presented similar values of E corr (−0.301 and −0.312 V) and i corr (1.86 × 10 −6 and 4.23 × 10 −6 A/cm 2 ), meaning a similar corrosion probability and kinetics.However, the H 2 SO 4 -anodized sample presented three unusual reactions in the anodic breach, associated with a fast electrochemical reaction on the surface due to the porosity.Furthermore, that reaction in the anodic breach is related to an unstable oxide layer generated on the surface, and the pitting and repassivation process that occurs.Additionally, the sample anodized with H 2 SO 4 presented a passivation range with a reduction of current demand, meaning that the corrosion process was reduced.The sample anodized with H 3 PO 4 presented the highest E corr in this media, −0.002 V, and lower i corr , 3.24 × 10 −8 A/cm 2 , associated with a lower corrosion rate.The passivation range was more extended with 1.17 V, and a reduction of current was presented.Figure 5c shows the behavior of Ti-6Al-4V, uncoated and anodized, in NaCl at 3.5 wt %.The uncoated sample had the higher E corr , −0.144 V (see Table 2) when exposed to NaCl and compared with the anodized of Ti-6Al-4V; meanwhile, the anodized sample in H 2 SO 4 and H 3 PO 4 presented −0.237 and −0.287 V (see Table 2).However, the uncoated sample presented the higher i corr , with 6.71 × 10 −6 , relating this behavior with a faster corrosion kinetic.All the samples presented passivation, the highest being for the sample anodized in H 3 PO 4 , meaning that the passive layer was more stable against corrosion processes.The sample anodized in H 3 PO 4 presented a passive region of 1.28 V, and also presented a decrease in the current demand, from values near to ×10 −6 to ×10 −7 A/cm 2 , meaning a reduction of electron transference, associated with an increase in the passive layer efficiency.The anodized sample of Ti-6Al-4V in H 2 SO 4 presented a lower passivation range (0.77 V), but in that short period, the current demand remained uniform without increasing, relating the process with passivation.
The high porosity and low thickness of anodized H 2 SO 4 are related to the faster passivity breakdown, shown in Figure 5c; that breakdown is of the passive layer generated in the corrosion process.The decrease in the current demand for the samples anodized in H 3 PO 4 is related to the possible development of a passive layer in the anodized sample corresponding to a diffusion process.
Figure 5c shows the behavior of uncoated and anodized Ti-6Al-4V in H 2 SO 4 at 3.5 wt %.The uncoated sample presented the lowest E corr and highest i corr (−0.475V and 6.71 × 10 −6 A/cm 2 ) when exposed to H 2 SO 4 media.For the anodized samples, the worst corrosion performance characterized by CPP was for H 2 SO 4 , which presented a higher i corr (1.69 × 10 −6 A/cm 2 ) but presented a higher E corr (−0.146V), associating this behavior with the demand of energy to begin an anodic process.The passivation range was higher for anodized in H 3 PO 4 , giving rise to the best coating layer.
Table 2 shows the values of CPP obtained by Tafel interpolation and the range and passive breakdown.All the samples presented a negative hysteresis, meaning that the corrosion process occurring on the surface is uniform.The Ti-6Al-2Sn-4Zr-2Mo anodized in H 3 PO 4 showed the best corrosion performances in both media with the lowest i corr values Metals 2023, 13, 476 9 of 20 (2.64 × 10 −9 and 3.24 × 10 −8 A/cm 2 ).Furthermore, the passivation zone did not present changes in current, which can be observed in the passivation range values.
In the case of Ti-6Al-2Sn-4Zr-2Mo, the sample anodized in H 3 PO 4 showed the lowest corrosion rate in both media with 0.0014 and 0.022 mpy in NaCl and H 2 SO 4 .Even the polarization resistance was higher for the H 3 PO 4 anodized sample, with values of 2.9 × 10 7 and 1.7 × 10 6 Ω•cm 2 for NaCl and H 2 SO 4 .In comparison with the sample anodized in H 2 SO 4 and the uncoated sample, the one anodized in H 3 PO 4 presented the best performance against corrosion.
For the Ti-6Al-4V, the sample anodized in H 2 SO 4 presented a better corrosion performance in NaCl than the Ti-6Al-4V anodized in H 3 PO 4 with corrosion rates of 0.0848 mpy.That performance was also higher than the Ti-6Al-2Sn-4Zr-2Mo anodized in H 2 SO 4 , meaning that samples anodized in H 2 SO 4 showed a better conduct against corrosion when it was applied to Ti-6Al-4V.However, the R p values presented showed a high dominance of the Ti-6Al-2Sn-4Zr-2Mo alloy in both anodized electrolytes, but it is important to consider the sample of Ti-6Al-4V anodized in H 2 SO 4 as a good option for applications in acid media.
These results can be associated with a more uniform, compact, well-adherent oxide layer.However, samples anodized in H 3 PO 4 had a higher porosity than those anodized in H 2 SO 4 .The former showed the best performance.The EIS technique may explain this phenomenon.

Electrochemical Impedance Spectroscopy
Figure 6a shows the Nyquist plot for anodized samples of Ti-6Al-2Sn-4Zr-2Mo exposed in NaCl at 3.5 wt.%.The uncoated sample presented titanium's typical behavior with the development of a natural passive layer.As shown in Figure 7a, the coated samples presented similar behavior when the process was governed by diffusion.The H 3 PO 4 anodized samples presented a higher resistance to porosity (R por ) 5.42 × 10 4 Ω•cm 2 .The porous layer is the first barrier, and afterward, the diffusion process occurs in the compact oxide layer, and the resistance increased for both samples.
In Figure 6b, the uncoated sample presented a different behavior compared to that presented in Figure 7b.This is related to the creation of a stable passive layer in H 2 SO 4 .The anodized samples showed processes dominated by diffusion.For those anodized in H 3 PO 4, the Warburg's resistance increased, meaning that oxygen diffusion occurred, increasing the passive layer.
Figure 6c shows the behavior of Ti-6Al-4V in NaCl at 3.5 wt.%.The uncoated sample showed a higher resistance due to the resistance of the metal-electrolyte interface.The anodized porous layer was lower for the sample anodized in H 3 PO 4 .After the resistance of the porous layer, a diffusion process governed the system.
Figure 6d shows the behavior when Ti-6Al-4V and the anodized sample were exposed to H 2 SO 4 .The sample anodized in H 3 PO 4 presented a higher value of Warburg's resistance, 9.16 × 10 5 A/cm 2 .Such a behavior is related to creating a passive layer in the compact oxide barrier.The resistance of the porous layer was lower for this sample due to the high heterogeneous porosity.Furthermore, the Warburg's impedance increased as the thickness of the anodized samples was higher for H 3 PO 4 than for H 2 SO 4 .This behavior was presented for NaCl.The porosity had the same conduct in NaCl and H 2 SO 4 .
Figure 7 shows the Bode diagrams; Figure 7a,b shows the Bode diagrams for impedance magnitude for Ti-6Al-2Sn-4Zr-2Mo.The samples anodized in H 3 PO 4 presented the higher impedance resistance with values of ×10 6 order; this can be related with a high corrosion resistance.For the samples of Ti-6Al-4V, Figure 7e shows that samples anodized in H 3 PO 4 exposed to NaCl presented higher impedance, associated with higher corrosion resistance.On the other hand, when samples of Ti-6Al-4V were exposed to H 2 SO 4 (Figure 7f) the impedance of the anodized sample decreased to ×10 5 order; the values obtained were so close to the uncoated sample, that it meant a possible anodizeddissolution. Figure 7 shows the Bode diagrams; Figure 7a,b shows the Bode diagrams for impedance magnitude for Ti-6Al-2Sn-4Zr-2Mo.The samples anodized in H3PO4 presented the higher impedance resistance with values of ×10 6 order; this can be related with a high corrosion resistance.For the samples of Ti-6Al-4V, Figure 7e shows that samples anodized in H3PO4 exposed to NaCl presented higher impedance, associated with higher corrosion resistance.On the other hand, when samples of Ti-6Al-4V were exposed to H2SO4 (Figure 7f) the impedance of the anodized sample decreased to ×10 5 order; the values obtained were so close to the uncoated sample, that it meant a possible anodizeddissolution.
Figure 7c,d show the Bode diagrams vs. angle phase; the uncoated sample showed one time constant in Figure 7c and two for Figure 7d.The anodized samples presented the superposition of signals, meaning that two processes were occurring at the same time; also, at low frequencies, a change in the slope was present.Figure 7g,d show Ti-6Al-4V; the uncoated sample presented the same behavior for one time constant.On the other hand, the anodized samples presented the behavior of two time constants, with a change in the process between 100 kHz and 1 Hz related to the behavior of intermetallic coatings.Both samples of Ti-6Al-4V showed the two time constants.Figure 7c,d show the Bode diagrams vs. angle phase; the uncoated sample showed one time constant in Figure 7c and two for Figure 7d.The anodized samples presented the superposition of signals, meaning that two processes were occurring at the same time; also, at low frequencies, a change in the slope was present.Figure 7g,d show Ti-6Al-4V; the uncoated sample presented the same behavior for one time constant.On the other hand, the anodized samples presented the behavior of two time constants, with a change in the process between 100 kHz and 1 Hz related to the behavior of intermetallic coatings.Both samples of Ti-6Al-4V showed the two time constants.
Figure 8 shows the equivalent circuit for the different systems.The uncoated samples present the typical R-CPE behavior of Figure 8a; only the Ti-6Al-2Sn-4Zr-2Mo sample presented the system from Figure 8b, related to the tendency to generate a passive layer in H 2 SO 4 , and the stability of this one.All the anodized samples showed an 8c circuit related to the resistance of the porosity barrier, and the diffusion in the compact barrier zone, associated with an increase in the barrier.Figure 9 shows the diffusion behavior of anodized samples, where the diffusion process begins in the barrier zone due to the compactness of the coating in that zone.present the typical R-CPE behavior of Figure 8a; only the Ti-6Al-2Sn-4Zr-2Mo sample presented the system from Figure 8b, related to the tendency to generate a passive layer in H2SO4, and the stability of this one.All the anodized samples showed an 8c circuit related to the resistance of the porosity barrier, and the diffusion in the compact barrier zone, associated with an increase in the barrier.Figure 9 shows the diffusion behavior of anodized samples, where the diffusion process begins in the barrier zone due to the compactness of the coating in that zone.Table 3 shows the parameters obtained by EIS.The anodized sample in Ti-6Al-2Sn-4Zr-2Mo presented lower capacitance values related to increased coating thickness.Furthermore, a porous barrier presented similar behavior.The samples anodized in H2SO4 showed lower values of n associated with a non-homogenous current distribution due to the heterogenous porosity of the samples, even Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V.The error of the equivalent circuit was acceptable for all the systems.The anodized Ti-6Al- Figure 8 shows the equivalent circuit for the different systems.The uncoated samples present the typical R-CPE behavior of Figure 8a; only the Ti-6Al-2Sn-4Zr-2Mo sample presented the system from Figure 8b, related to the tendency to generate a passive layer in H2SO4, and the stability of this one.All the anodized samples showed an 8c circuit related to the resistance of the porosity barrier, and the diffusion in the compact barrier zone, associated with an increase in the barrier.Figure 9 shows the diffusion behavior of anodized samples, where the diffusion process begins in the barrier zone due to the compactness of the coating in that zone.Table 3 shows the parameters obtained by EIS.The anodized sample in Ti-6Al-2Sn-4Zr-2Mo presented lower capacitance values related to increased coating thickness.Furthermore, a porous barrier presented similar behavior.The samples anodized in H2SO4 showed lower values of n associated with a non-homogenous current distribution due to the heterogenous porosity of the samples, even Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V.The error of the equivalent circuit was acceptable for all the systems.The anodized Ti-6Al- Table 3 shows the parameters obtained by EIS.The anodized sample in Ti-6Al-2Sn-4Zr-2Mo presented lower capacitance values related to increased coating thickness.Furthermore, a porous barrier presented similar behavior.The samples anodized in H 2 SO 4 showed lower values of n associated with a non-homogenous current distribution due to the heterogenous porosity of the samples, even Ti-6Al-2Sn-4Zr-2Mo and Ti-6Al-4V.The error of the equivalent circuit was acceptable for all the systems.The anodized Ti-6Al-2Sn-4Zr-2Mo presented this technique's best behavior against corrosion, with values of ×10 6 and ×10 7 A/cm 2 .The order of the porous barrier was 10 × 4 A/cm 2 , so the porosity was more homogenous and stable.

Sample Rs (Ω•cm
Ti-6Al-2Sn-4Zr-2Mo The following equation defines the calculation of the oxide film formed: In this case, the ε and ε 0 correspond to oxide film permittivity, and the vacuum permittivity (8.85 × 10 −14 Fcm −1 ) and C cc are the system's capacitance.δ ox is the thickness of the oxide film formed in the process.Table 4 shows the results of the thickness.The value of ε for TiO 2 was 86.The CPE can obtain the C cc value from Table 3.The results of Table 4 show that the anodized samples of Ti-6Al-2Al-4Zr-2Mo presented high values for the oxide layer generated when anodized in H 3 PO 4 .For Ti-6Al-4V, the H 3 PO 4 anodized samples showed a higher oxide generation in NaCl with 1.75 × 10 −7 m than all the anodized Ti-6Al-4V in any media.It is important to mention that the anodization of Ti-6Al-2Sn-4Zr-2Mo presented an easier generation in H 2 SO 4 and Ti-6Al-4V had easier growth in NaCl media.Figure 10 schematizes the process of oxide layer growth.It is important to mention that the diffusion process begins in the oxide layer generated by the corrosion process due to the porosities of that part.

Discussion
Previous researchers have emphasized the role of the porosity of titanium alloys concerning their mechanical and corrosion resistance.Both samples showed porosity in both samples, decreasing the mechanical and corrosion resistance due to the pores being stress concentrators.This, in turn, makes the material susceptible to localized corrosion.However, the material can repassivate it.The tiny pores help to reduce the diffusion process in the electrolytes [49,50].In this case, Ti-6Al-2Sn-4Zr-2Mo had a higher porosity, but the size was smaller than in Ti-6Al-4V and localized in β phases [51,52].
In Figure 1b, the microstructure of Ti-6Al-4V and the porosity can make the material vulnerable to problems in the coating.Kumar [53] mentioned that H2SO4, an anodized electrolyte, produces a stable oxide layer.For that reason, the thickness of the anodized samples in H2SO4 presented less variation in thickness measurements since the concentration of 1M helped the correct flow of current.
The reaction in the anodic breach of Ti-6Al-2Sn-4Zr-2Mo anodized in H2SO4 was explained by Cabral et al. [54] as a cathodic-anodic behavior in the system; however, various authors related the reaction to the change in the electrolyte concentration when there was a variation in pH and oxide reduction.Furthermore, it can be associated with a reduction of the protective layer [55][56][57][58][59].For this anodized sample, the behavior presented was related to the reduction of anodized protection caused by oxygen reduction, and OH − ions attacking the surface because of the heterogenous nature of the anodized sample.The continuity of the other passivation zones is related to forming an oxide film in the passivated or anodized surface [60,61].
However, the high thickness of the sample anodized in H3PO4 is associated with continuous oxygen evolution; when it occurs in the anodizing process, it increases the thickness, and it is also reported by various authors [62][63][64][65][66]. Conversely, El-Taib Heakal et al. [67] mentioned that anodized H3PO4 presented better behavior against dissolution than H3PO4.However, this research showed that anodizing in H3PO4 presented better behavior against dissolution by using CPP and EIS.It can be observed in the anodic breach of CPP, where the current densities of the passive layer in H2SO4 presented high current demand in the system.The breaking potential of the passive layer is also lower than samples anodized in H3PO4, indicating higher anodized stability.The facility of the diffusion process corresponds to the electrolyte resistance, and the low ionic resistance describes a faster kinetic [68].
Martinez et al. [69] related the increase in current density (icorr) to a passive layer that does not contribute to corrosion protection, and also associated the increase in current It is important to mention that the diffusion process begins in the oxide layer generated by the corrosion process due to the porosities of that part.

Discussion
Previous researchers have emphasized the role of the porosity of titanium alloys concerning their mechanical and corrosion resistance.Both samples showed porosity in both samples, decreasing the mechanical and corrosion resistance due to the pores being stress concentrators.This, in turn, makes the material susceptible to localized corrosion.However, the material can repassivate it.The tiny pores help to reduce the diffusion process in the electrolytes [49,50].In this case, Ti-6Al-2Sn-4Zr-2Mo had a higher porosity, but the size was smaller than in Ti-6Al-4V and localized in β phases [51,52].
In Figure 1b, the microstructure of Ti-6Al-4V and the porosity can make the material vulnerable to problems in the coating.Kumar [53] mentioned that H 2 SO 4 , an anodized electrolyte, produces a stable oxide layer.For that reason, the thickness of the anodized samples in H 2 SO 4 presented less variation in thickness measurements since the concentration of 1M helped the correct flow of current.
The reaction in the anodic breach of Ti-6Al-2Sn-4Zr-2Mo anodized in H 2 SO 4 was explained by Cabral et al. [54] as a cathodic-anodic behavior in the system; however, various authors related the reaction to the change in the electrolyte concentration when there was a variation in pH and oxide reduction.Furthermore, it can be associated with a reduction of the protective layer [55][56][57][58][59].For this anodized sample, the behavior presented was related to the reduction of anodized protection caused by oxygen reduction, and OH − ions attacking the surface because of the heterogenous nature of the anodized sample.The continuity of the other passivation zones is related to forming an oxide film in the passivated or anodized surface [60,61].
However, the high thickness of the sample anodized in H 3 PO 4 is associated with continuous oxygen evolution; when it occurs in the anodizing process, it increases the thickness, and it is also reported by various authors [62][63][64][65][66]. Conversely, El-Taib Heakal et al. [67] mentioned that anodized H 3 PO 4 presented better behavior against dissolution than H 3 PO 4 .However, this research showed that anodizing in H 3 PO 4 presented better behavior against dissolution by using CPP and EIS.It can be observed in the anodic breach of CPP, where the current densities of the passive layer in H 2 SO 4 presented high current demand in the system.The breaking potential of the passive layer is also lower than samples anodized in H 3 PO 4 , indicating higher anodized stability.The facility of the diffusion process corresponds to the electrolyte resistance, and the low ionic resistance describes a faster kinetic [68].
Martinez et al. [69] related the increase in current density (i corr ) to a passive layer that does not contribute to corrosion protection, and also associated the increase in current density of passivation with an oxygen and chlorine evolution.Furthermore, they correlated the incorporation of Mo to inhibit chloride ion absorption.For that reason, the increase in corrosion resistance calculated by CPP was higher for Ti-6Al-2Sn-4Zr-2Mo, decreasing the dissolution.
The low corrosion rate of Ti-6Al-2Sn-4Zr-2Mo anodized in H 3 PO 4 (0.0014 and 0.022 mpy) is related to lower anodized dissolution and the generation of a layer generated by the corrosion process.The EIS characterization can corroborate that behavior.
The EIS double circle shown in Figure 6b, for the uncoated Ti-6Al-2Sn-4Zr-2Mo at low frequencies, is related to the layer of the corrosion products and is called the double electrochemical layer [70].The value of "n" has different interpretations depending on the authors.Fouda et al. [71] related the value of 1 with an ideal capacitor, and reducing values reduces the charge of samples.Gomes et al. [72] related the CPE behavior with the resistivity distribution in the thin oxide layer thickness.This method is possible and easily determines the oxide thickness.The second time constant is related to the diffusion inside the oxide layer.For that reason, Gateman et al. [73] concluded that CPE was directly related to the interfacial properties of the system, and the correct analysis would depend directly on that.Meanwhile, the n value being near 1 is related to a more homogenous surface.Furthermore, the values are related to capacitive systems, where energy accumulation begins with the process of charge transference.
The physical meaning of CPE has been a discussion motif for diverse authors.Macdonald et al. [74][75][76] mentioned that CPE represents the conductive behavior of the dielectric; however, the modeling is complex, and it is not easy to give an exactitude of the nature of the system.On the other hand, authors relate the CPE with the phenomenon of surface roughness, and when "n" values are between 1 and 0.9, the system is dominated by a power law; meanwhile, when the value is 0.5, there is talk of a contorted surface (Warburg's diffusion) [77].Kim et al. [78] related the CPE with the homogeneity of the surface reactions, and once Schiller et al. [79] associated CPE with the thickness composition and variation.Córdoba-Torres [80] mentioned that CPE at high frequencies is related to a less resistive film (in zones) and, at low frequencies, is associated with the power law distribution, dominating the low resistivity behavior of the distribution function.In this work, the results of CPE are directly related to the surface's homogeneity and resistance; when the "n" value was near 1, the coating presented better properties against corrosion.
Additionally, that behavior can be observed in EIS with the increase in diffusion resistance represented by Warburg's element (W), meaning that the oxide layer is more stable and increased.Some authors relate the Warburg impedance at low frequencies with redox molecules that diffuse in the system, giving a high Warburg impedance.However, when the Warburg process occurs at high frequencies, it is associated with low impedance values [81].In this case, the high values of the Warburg resistance were consequent to a diffusion process that occurred in the compact oxide layer-electrolyte interface, increasing the resistance of the anodized samples.Rajan et al. [82] associated the disappearance in the coating of Warburg impedance with the inhibition of diffusion, so the coating layer is very protective.It is essential to mention that the electrochemical reaction governed by the Warburg impedance is absorption, penetration and diffusion [83].The absorption-penetration occurs in the porous zone and the interface of the compact barrier and the diffusion in the compact barrier; that process produces an increase in the properties against corrosion.
The process that decreases the corrosion rate by diffusion occurs with the continuous development of a passive layer by the following reaction [84]: Ti + 2H 2 O → TiO 2 + 4H + + 4e − For the electrolytes that contain Cl − ions, the colocation of the ions in titanium or the anodized surface to form titanium oxide actuates by the following reactions: Ti + 4Cl − → [TiCl 4 ] − → TiCl 4 + 4e − TiCl 4 + 2H 2 O → TiO 2 + 4Cl − + 4H + These results show that the diffusion process in titanium and titanium anodized surfaces occurs on the solid phase due to the diffusion of metal ions in the porous layer.The micropores of the first oxide layer act as preferent diffusion sites for the solution ions [85][86][87][88][89].
The use of the constant phase element in the equivalent circuits is related to the non-deal behavior of the capacitor.The physical explanation of this phenomenon is that the surface is non-homogeneous and presents porosities or differences in roughness [86,88].Therefore, the value of "n" in the CPE element is lower for anodized H 2 SO 4 .The imperfection in the anodized sample is higher; meanwhile, in H 3 PO 4 , the values of "n" are close to 1, so the surface is more homogenous.Anodized Ti-6Al-4V presented values of "n" related to heterogeneities in the surface.
Sadek et al. [90] mentioned that the formation of a hydroxide layer (Ti(OH) x O y ) is due to a limitation in the oxidation process by the increase in oxide.The layer created by the hydroxide is porous, resulting in diffusion [91][92][93].This behavior is observed in Figure 5b of CPP, where a fast reaction produced the reaction in the anodic branch.In EIS, the effects of hydroxide and secondary oxides are associated with the graphics of the capacitive response [36].

•
The anodized Ti-6Al-2Sn-4Zr-2Mo alloys presented the best properties against corrosion, as analyzed by the electrochemical techniques employed in this research work.This behavior is related to Mo and Zr presence in the alloy and the anodized forms.

•
The anodizing H 2 SO 4 solution showed a smaller porosity than the H 3 PO 4 anodizing solution.However, the lower porosity helped to prevent ion penetration by capillarity.

•
The samples of both alloys anodized in H 3 PO 4 presented the biggest thickness measurement by SEM of the anodization with a maximum value of 2.08 µm.The presence of oxygen was higher in the oxide layer.

•
The H 2 SO 4 -anodized sample for Ti-6Al-4V did not reach the minimum specifications to accomplish the thickness required for AMS2487B for anodized aeronautical titanium.Furthermore, both Ti-6Al-4V anodized samples presented the imperfections of high roughness and lack of adherence.

•
For characterization of the CPP, the alloys anodized with H 3 PO 4 presented lower i corr , meaning a lower corrosion kinetic.Additionally, both H 2 SO 4 -anodized samples exposed to NaCl presented current densities similar to uncoated samples, meaning that the Cl − could easily penetrate the anodizing layer.

•
All the anodized samples studied by EIS were governed by a diffusion process represented by the Warburg element.The diffusion occurred after the porous layer finished in the compact oxide layer of anodization, meaning that the anodization protects the titanium from the electrolyte.

•
The results obtained by CPP and EIS converged to characterize the anodized samples, where the results showed that Ti-6Al-2Sn-4Zr-2Mo anodized in H 3 PO 4 presented the best properties against corrosion from both techniques.Furthermore, the results matched with the SEM characterization where the anodized samples presented the higher thickness (1.62 µm on average).

Figure 4 .
Figure 4. SEM-SEI of the cross-section of Ti-6Al-4V in (a) H2SO4 and (b) H3PO4; the elemental mapping by SEM identified Ti, Al, V, and O.

Figure 4 .
Figure 4. SEM-SEI of the cross-section of Ti-6Al-4V in (a) H 2 SO 4 and (b) H 3 PO 4 ; the elemental mapping by SEM identified Ti, Al, V, and O.

Figure 9 .
Figure 9. Corrosion system of the anodized samples.

Figure 9 .
Figure 9. Corrosion system of the anodized samples.

Figure 9 .
Figure 9. Corrosion system of the anodized samples.

Table 3 .
Parameters obtained by EIS.

Table 4 .
Values of oxide film thickness generated by EIS.