Effect of Pr(NO 3 ) 3 doping on the characteristics of TC4 micro-arc oxidation coatings

Abstract

Micro-arc oxidation coatings were prepared on the surface of TC4 by adding different contents of rare-earth compound Pr(NO₃)₃ to the aluminate and phosphate-based electrolyte. The results showed that the change in oxidation voltage increased at first and then decreased with the raised of Pr(NO₃)₃ addition. At 0.05 g L⁻¹, the oxidation voltage reached the maximum, the number of discharge micropores decreased, while the size increased, the thickness and hardness of the coatings reached the peak values. The coatings mainly included Al₂TiO₅, anatase, γ-Al₂O₃ and a little of Pr₂O₃. Pr(NO₃)₃-doped coatings showed excellent corrosion resistance in 3.5% NaCl electrochemical workstation and oilfield simulation fluid containing 5% NaCl, and the corrosion resistance of the coatings reached the best at 0.05 g L⁻¹.

Keywords

TC4 titanium alloy micro-arc oxidation corrosion resistance

Introduction

Shallow oil and gas resources are gradually depleted with continuous development, the oil well depth of oil and gas resources exploitation is increasing,¹ and the content of Cl⁻, H₂S, and CO₂ corrosive media in the oil well is constantly increasing, the iron-based oil country tubular goods suffered severe corrosion.^2,3 Titanium alloy has good corrosion resistance, it has been used to manufacture oil country tubular goods for use in surroundings with high corrosive media.^4,5 However, titanium alloys are limited by their poor wear resistance, low hardness and low thermal conductivity, and are prone to suffer local corrosion in the oil and gas exploitation environment.^6,7 In order to improve the corrosion resistance of titanium alloys, surface technology is often used to modify, including surface heat treatment,⁸ laser cladding,⁹ micro-arc oxidation,¹⁰ but the surface heat treatment process is complex, laser cladding is limited due to its high cost. Now, micro-arc oxidation has been widely studied because it can prepare coatings with good corrosion resistance on the surface of titanium, magnesium, and aluminium.^11,12

The coatings prepared by high-voltage discharge have typical porous characteristics.¹³ The micropore size on the surface and coating thickness have a great influence on the corrosion resistance. Researchers use inorganic salt, alkali metal compounds, and rare-earth compounds in the electrolyte to amend the coating morphology and improve the corrosion resistance. Such as bismuth sulphide is used as the modifier for micro-arc oxidation, and the protection ratio of the substrate is increased from 63% to 97%.¹⁴ The introduction of NaF leads to the formation of a fluorine nanolayer composed of MgO nanoparticles and insoluble MgF₂ nanoparticles at the interface, the corrosion performance is optimal at 2 g L⁻¹.¹⁵ Adding Er element to the electrolyte makes the surface sediment more uniform, the pore diameter is reduced, and the corrosion resistance is improved.¹⁶ Cerium oxide is added to pure titanium micro-arc oxidation, the number of pores decreases but the size increases, and the corrosion current decreases to 1.584 × 10⁻⁷ A at 6 g L⁻¹.¹⁷ Studies have shown that the addition of a rare-earth microelement to the modified coatings can refine the crystal grain, reduce the oxidation rate, improve the corrosion resistance and anti-spalling performance of the coatings. The rare-earth nitrate salt can be used as a promoter of the oxidation reaction, which was beneficial to improve coatings formation efficiency. Among them, the corrosion resistance of praseodymium was better than that of lanthanum, cerium, neodymium and europium.^18–21 In this article, the effect of Pr(NO₃)₃ addition on the formation process, morphology and phase composition of micro-arc oxidation coatings on TC4 titanium alloy was studied. The corrosion resistance was tested in a 3.5% NaCl electrochemical workstation and oilfield simulation fluid containing 5% NaCl, and the effect of Pr(NO₃)₃ on the corrosion resistance of micro-arc oxidation coatings on TC4 titanium alloy was discussed. It is hoped that this research can provide a technical reserve for the application of titanium alloy in oil country tubular goods.

Materials and methods

Rectangular samples (15 mm × 15 mm × 3 mm) of TC4 titanium alloy (the chemical composition by wt-%: V 3.5–4.5, Al 5.5–6.75, Fe ≤0.3, N ≤0.05, C ≤0.1, H ≤0.015, O ≤0.2, and Ti balance), the samples were ground with sandpapers from 400 to 1500, then ultrasonically cleaned in anhydrous ethanol, finally washed with deionised water and dried in the air. In Figure 1, the experiment power supply equipment adopted constant current mode, the positive and negative direction current density of 6 and 1.25 A dm⁻², the duty cycle of 60% and 10%, respectively, frequency of 500 Hz, oxidised for 30 min and the electrolyte temperature at 25 °C ± 5 °C. The mixed electrolytes of aluminate and phosphate were selected in the experiment, and doped Pr(NO₃)₃ were 0, 0.025, 0.05, 0.075 and 0.1 g L⁻¹. The ions in the electrolyte were kept uniform by a stirring system.

The surface and cross-sectional morphologies and element distribution were investigated by a scanning electron microscope (ZEISS EVO MA15, Germany) and matched energy dispersive spectrometer (OXFORD 20, UK). Phase composition was analysed by X-ray diffraction (XRD; DX-2700B, China), the X-ray adopted 40 kV, 30 mA Cu-Kα Launch, scanning speed was 0.05° s⁻¹, scattering angle 2θ from 10° to 80°. The electron binding energy was detected by an X-ray photoelectron spectrometer (XSAM-800, China). The coatings’ hardness was measured by a digital microhardness tester (HXD-1000TMB, China) with the diamond indentation head, a loading force of 100 gf, continuously loaded for 15 s. The coatings' thickness and roughness were tested by Digital Thickness Gauges (TT230, China) and a high-precision roughness meter (HD350, China). The hardness, thickness and roughness of the coatings were tested at seven random locations on the sample, and the average value of the seven measurements was taken as the result. The wear resistance of coatings was tested by a Friction and Wear Testing Machine (Bruker UMT-TriboLab, China), a 5 mm GCr15 bearing steel ball was used for ball–plane contact reciprocating friction, with a load of 3 N, a displacement amplitude of 6 mm, and a cycle of 100 times. The potentiodynamic polarisation and electrochemical impedance spectrum were performed by the Electrochemical Workstation (Gamry Reference 3000, American). The exposed area of the sample of 1 cm², the scanning frequency range was 100 kHz to 100 mHz, and the disturbance signal was 10 mV sine wave. The potentiodynamic polarisation curve, the scanning potential range was ±0.25 V_SEC, and the scanning rate was 0.45 mV s⁻¹. According to JB/T 7901-1999, an oilfield simulation solution containing 5% NaCl was selected as the immersion solution, Table 1 for chemical composition, the experimental temperature was set at 90 °C, the container was sealed to isolate oxygen, the immersion solution was replaced once a week.

Table 1.

Chemical composition of an oilfield simulation solution.

Composition	MgCl₂	Na₂SO₄	CaCl₂	NaHCO₃	NaCO₃
Content (g L⁻¹)	4	2	1.5	0.5	1

Results and discussion

Figure 2 shows that the oxidation process can be divided into three stages. The initial stage was anodic oxidation, a nanoscale insulation layer was formed on the substrate,²² the oxidation voltage rose rapidly and linearly from 0 to 180 V approximately, tiny bubbles appeared on the sample surface. The second stage was the spark discharge stage, the coatings started to break when the voltage reached about 200 V, small and fast-moving electric sparks travelled on the sample surface, the insulation layer broke repeatedly to form micropores with the increased voltage, and the ionised ions of electrolyte and the titanium ions corroded by the substrate were transmitted through micropores, and new coatings were continuously formed under the dual action of arc melting and electrolyte quenching. As the coatings' thickness increased, the voltage increased to nearly 400 V and reached the micro-arc oxidation stage, the electric sparks became larger and the number decreased, the discharge degree was more intense, the voltage growth rate slowed down, and the coatings entered a stable growth stage.²³ Figure 2(b) shows that the micro-arc oxidation voltage increased at first and then decreased with the addition of Pr(NO₃)₃, and attained the maximum at 0.05 g L⁻¹. Because Pr(NO₃)₃ was easily soluble in water, Pr(OH)₃ was formed with OH⁻ in the electrolyte, under the action of thermal decomposition to form insoluble oxides dispersed on the surface. The oxidation voltage gradually increased since the oxidation current was unchanged, resulting in the increased coating resistance. However, upon excessive addition of Pr(NO₃)₃, cluster structures were generated on the surface, which hindered the micro-arc oxidation reaction between electrolyte and substrate at high voltage, reduced the efficiency of coatings formation, and the oxidation voltage started to decrease gradually.

Figure 3 shows the surface morphology and element distribution of the coatings. Figure 3(a) illustrates the coatings prepared without Pr(NO₃)₃. There were a large number of discharge micropores on the surface, and ‘volcanic’ sintered discs were accumulated around the micropores. In the process of oxidation, the high-voltage electric field formed between the two electrodes caused the high-temperature and high-pressure micro-regions in the micropores, the inner molten materials were expelled through the micropores, solidified and accumulated under the quenching of the electrolyte. Figure 3(c) shows the amount of Pr(NO₃)₃ was 0.05 g L⁻¹, the number of micropores decreased, but the size of the partial discharge micropores increased. As can be seen from the voltage curve in Figure 2, doped 0.05 g L⁻¹ Pr(NO₃)₃ led to the highest voltage, and the discharge was more intense, which made the size of partial discharge micropores increase. In Figure 3(d), with the increase of Pr(NO₃)₃ addition, the molten materials began to stack on the surface, and the size of the partial discharge micropores began to decrease. In Figure 3(e), the accumulation area of the molten materials increased, causing the coating surface to be rough. Coatings roughness test were 1.56(±0.04), 1.38(±0.09), 1.27(±0.06), 1.57(±0.16) and 1.83(±0.13) μm, respectively. It can be seen that the roughness of the coatings decreased at first and then increased. In the range of 0–0.05 g L⁻¹, the gradually rising termination voltage provided a larger electric field energy, and the volcanic rock-like asperities were repeatedly melted and cured, which can play the role of filling, and the coatings’ surface became flatter. Doped with too much Pr(NO₃)₃, the high-impedance molten materials adsorbed on the coatings’ surface induced the plasma current to preferentially choose the low-resistance path, so that the current converged in the local area of the coatings and caused high-frequency discharge, resulting in the uneven coatings.

In Table 2, the coatings mainly contained elements such as O, Al, Ti and Pr, and the elements were evenly distributed on the surface. The percentage content of the O element decreased entirely, while the Ti and Al elements increased. The percentage content of Pr element increased with the increase of Pr(NO₃)₃ in the electrolyte, indicating that Pr element was successfully incorporated into the coatings.

Table 2.

The composition elements of the coatings.

Atomic (g L⁻¹)	0	0.025	0.05	0.075	0.1
Ti	12.49 ± 0.68	23.30 ± 1.77	19.56 ± 0.74	18.48 ± 1.92	20.87 ± 0.14
Al	27.01 ± 0.24	33.41 ± 1.45	37.35 ± 0.57	38.39 ± 0.90	34.42 ± 0.80
O	60.50 ± 0.91	43.13 ± 0.23	42.83 ± 0.09	42.82 ± 1.11	44.25 ± 0.60
Pr	0	0.16 ± 0.09	0.26 ± 0.08	0.31 ± 0.09	0.46 ± 0.05

Figure 4 shows the cross-sectional morphology and line scanning results of the coatings. It can be seen that with the well-integrated bonding between the coatings and the substrate, the coating thickness increased at first and then decreased. Doped 0.05 g L⁻¹ Pr(NO₃)₃, the higher oxidation voltage led to the emergence of larger discharge micropores, accelerated the ions exchange between substrate and electrolyte, improved the coatings formation efficiency, the coatings thickness increased from 9.43(±0.81) to 12.23(±0.78) μm (0.025 g·L⁻¹) and 13.87(±0.77) μm (0.05 g·L⁻¹). In addition, the low thermal conductivity of titanium alloy made the inner layer heat unable to diffuse, which provided favourable conditions for the conversion of titanium oxide change to hard phase, the coatings hardness was increased from 357.99(±32.31) to 395.64(±23.54) HV (0.025 g·L⁻¹) and 438.51(±20.63) HV (0.05 g L⁻¹). When the doped Pr(NO₃)₃ was in excess, molten materials began to appear on the surface (Figures 3(d) and (e)), which affected the transport of titanium ions and other electrolyte ions, and reduced the coating formation efficiency. The thickness and hardness decreased, respectively, to 11.75(±0.51) μm, 372.81(±23.72) HV (0.075 g L⁻¹) and 9.72(±1.21) μm, 346.79(±24.31) HV (0.1 g L⁻¹). The line scanning results of the coatings showed that the content of Ti in the coatings increased along the direction from the coatings to the substrate, O and Al mainly gathered in the middle of the coatings, and the content of Pr in the coatings was uniform.

Figure 5 shows the XRD and X-ray photoelectron spectroscopy (XPS) test results of the coatings. The coatings were mainly composed of Al₂TiO₅, anatase and γ-Al₂O₃. The characteristic peak of Ti appeared due to the coatings having a porous structure, and the X-ray penetrated the coatings to reach the substrate. In the process of micro-arc oxidation, the aluminate ion in the electrolyte was easily hydrolysed to Al(OH)₃ and precipitated on the surface. The micro-arc discharges generated Joule-heat and the temperature of the coatings micro-zone rose rapidly, Al(OH)₃ was decomposed to Al₂O₃ by thermal decomposition. TC4 oxidised to amorphous TiO₂, the amorphous TiO₂ transformed to metastable anatase because of heat generated by discharge and low thermal conductivity of titanium alloy, and reacted with Al₂O₃ to form Al₂TiO₅ under the action of arc.²⁴ The characteristic peak intensity of Al₂TiO₅ increased at first and then decreased, which was consistent with the voltage variation trend in Figure 2. The XPS result of the Pr element in the sample with a concentration of 0.05 g L⁻¹ in Figure 5(b) showed two peaks of Pr 3d_3/2 and Pr 3d_5/2, binding energies of 953.8 and 933.3 eV, respectively. It is analysed that Pr(OH)₃ was formed by Pr³⁺ and OH⁻ in the electrolyte, Pr(OH)₃ decomposed at 220 °C to form Pr₂O_3.²⁵ The reactions occurred during the oxidation were as follows:

Ti - 4 e^{-} \to T i^{4 +}

(1)

T i^{4 +} + 4 O H^{-} \to Ti O_{2} + 2 H_{2} O

(2)

{AlO}_{2}^{-} + 2 H_{2} O \to Al (OH)_{3} + O H^{-}

(3)

2 Al (OH)_{3} \overset{Δ}{\to} A l_{2} O_{3} + 3 H_{2} O

(4)

Ti O_{2} + A l_{2} O_{3} \to A l_{2} Ti O_{5}

(5)

P r^{3 +} + O H^{-} \to \Pr (OH)_{3} \overset{220 \circ C}{\to} P r_{2} O_{3} + H_{2} O

(6)

Figure 6 shows the abrasion morphology and friction coefficient of the coatings. The results showed that the friction coefficient of the coatings decreased first and then increased. Figure 6(a) shows that the adaptation period between the coatings with 0 g L⁻¹ and the Gcr15 steel ball sustained for about 4 min, which was attributed to the higher porosity and roughness and lower thickness and hardness of the coatings with 0 g L⁻¹, but then the abrasive particles filled the pores, the loose layer was gradually flattened, the friction coefficient eventually fluctuated approximately 0.4203. With the increase of Pr(NO₃)₃ content to 0.05 g L⁻¹, the coatings thickness and the content of the hard phase were improved, which inhibited the development of the abrasion zone in the depth direction, and the abrasion depth became shallow, and the friction coefficient fluctuated around 0.2033 in Figure 6(c). When Pr(NO₃)₃ continued to increase, the coating thickness and hard phase decreased, and the friction coefficient rose back to about 0.4356 in Figure 6(d). Noteworthy, the coatings’ surface with 0.1 g L⁻¹ had the worst smoothness, the friction coefficient fluctuated seriously in Figure 6(e), and the friction coefficient increased to around 0.5349, the coatings’ hardness was even lower than 0 g L⁻¹, due to the influence of coatings hardness, fatigue wear occurred in the abrasion zone.

Figure 7 shows the TC4 and polarisation curves of coatings with different additive amounts. After doped Pr(NO₃)₃, the corrosion potential of coatings increased at first and then decreased, while the trend of corrosion current density was the opposite. The fitting results are shown in Table 3. At 0.05 g L⁻¹, the highest E_corr value was 0.121 V the minimum I_corr value was 1.68×10⁻⁸ A cm⁻², and the corrosion current density of the coatings was significantly lower than substrate. It is concluded that the increased coatings thickness prevented the migration of Cl⁻ to substrate and improved the corrosion resistance of coatings.

Table 3.

Analysis results of polarisation curves.

Sample	E_corr (V)	I_corr (A cm⁻²)
Substrate	−0.510	4.40 × 10⁻⁵
0 g L⁻¹	−0.330	7.29 × 10⁻⁸
0.025 g L⁻¹	+0.060	4.18 × 10⁻⁸
0.05 g L⁻¹	+0.121	1.68 × 10⁻⁸
0.075 g L⁻¹	−0.041	6.11 × 10⁻⁸
0.1 g L⁻¹	−0.120	4.42 × 10⁻⁸

Figure 8 shows the test results of electrochemical impedance spectroscopy. Figure 8(a) and (b) shows the impedance arc radius of the substrate was pretty small, but the impedance arc radius of the coatings increased significantly after doped Pr(NO₃)₃. The radius of the impedance arc was proportional to the corrosion resistance of the coatings.²⁶ At 0.05 g L⁻¹, the impedance arc radius reached the maximum. Figure 8(c) shows the relationship between impedance modulus and frequency of coatings. There was a semi-quantitative index in the low frequency (f = 0.01 Hz). The impedance modulus was greater in the low frequency and the corrosion resistance was better. When the dosage was 0.05 g L⁻¹, the maximum impedance modulus at the low frequency was 4.284 Ω cm⁻². In Figure 8(d), the phase angle in the high-frequency region was related to the coatings’ surface microstructure. The extremum of phase angle in the high-frequency region of the coatings was higher than the substrate, indicating that the coatings were flat. Furthermore, there was a small crest and trough in the mid-frequency region. Due to the formation of a nanoscale barrier layer at the interface of the substrate, the coatings were shaped in the early stage, and dense and porous coatings were constructed on this basis. According to the relevant literature research and the law of coatings growth, the electrical double-layer capacitor model was adopted.^27,28 The equivalent circuit diagram is shown in Figure 9, where Rs is the solution resistance, Rc is the outer layer resistance, Rd is the inner layer resistance, Qc is the outer layer capacitance, and Qd is the inner layer capacitance. In Table 4, where Rc and Rd represent the degree of charge transfer difficulty, the resistance values of the outer and inner layer both increased at first and then decreased after doped Pr(NO₃)₃, reached the maximum at 0.05 g L⁻¹, the charge transfer in the coatings was the most difficult, and the corrosion resistance was the best. The resistance of the inner layer was much greater than the outer, disclosing that the coatings blocked Cl⁻ penetration mainly depended on the inner layer.

Table 4.

Equivalent circuit diagram fitting results.

Sample	Rs (Ω cm⁻²)	Qc (Ω⁻¹ cm⁻² S ⁿ )	N	Rc (Ω cm⁻²)	Qd (Ω⁻¹ cm⁻² S ⁿ )	n	Rd (Ω cm⁻²)
Substrate	9.63	——	——	——	2.146 × 10⁻⁵	0.828	313.5
0 g L⁻¹	29.64	1.031 × 10⁻⁶	0.574	2352	7.550 × 10⁻⁶	0.976	1.44 × 10⁴
0.025 g L⁻¹	37.78	1.005 × 10⁻⁶	0.559	2459	6.442 × 10⁻⁶	0.899	1.62 × 10⁴
0.05 g L⁻¹	29.01	5.769 × 10⁻⁷	0.657	3860	4.851 × 10⁻⁶	0.490	1.685 × 10⁴
0.075 g L⁻¹	82.69	9.794 × 10⁻⁷	0.510	2680	6.828 × 10⁻⁶	0.925	1.502 × 10⁴
0.1 g L⁻¹	30.51	5.326 × 10⁻⁸	0.835	1895	3.469 × 10⁻⁶	0.498	1.357 × 10⁴

Figure 10 shows the microstructure of the coatings after 30 days of immersion in an oilfield simulation solution containing 5 wt-% NaCl at 90 °C. In Figure 10(a) and (b), the coatings’ surface without Pr(NO₃)₃ was adsorbed with loose corrosion products. In Figure 10(c) and (d), the coatings with 0 g L⁻¹ and 0.05 g L⁻¹ additives after the removal of corrosion products, the corrosion weight loss rate of coatings was 1.62 and 0.65 mg cm⁻² respectively. The coatings’ surface had local corrosion shedding at 0 g L⁻¹, which was an important feature of point corrosion. Because the ions in the oilfield simulation solution entered the coatings through the micropores, the corrosion products in the micropores gradually accumulated and the stress generated made the micro-crack propagation, leading to the outer layer falling off. However, the coatings remained relatively smooth at 0.05 g L⁻¹ and no large area of corrosion occurred on the surface. Figure 11 shows the XRD pattern of coatings before and after immersion corrosion. MgCO₃, CaCO₃ and a little of MgCl₂ appeared on the coatings surface after immersion, due to the coexistence of Ca²⁺ and Mg²⁺ with HCO₃⁻ and CO₃²⁻ in the oilfield simulation solution, and the porous characteristics of the coatings, the corrosion products MgCO₃, CaCO₃ and MgCl₂ generated by the chemical reaction were absorbed on the surface. At 0.05 g L⁻¹, the number of micropores Pr(NO₃)₃-doped coatings decreased, the adsorption capacity weakened, and the larger stress threshold could not be reached at the micropores with a slightly larger aperture, a layer of corrosion product film formed on the surface can prevent the penetration of chloride ions and produced a maze effect, thus slowing down further spalling.

Conclusion

After adding the proper amount of Pr(NO₃)₃, the micro-arc oxidation voltage was increased. The highest oxidation voltage at 0.05 g L⁻¹, the number of micropores on the coating surface was reduced, the thickness and hardness of the coatings arrived at the maximum, and the coatings were the smoothest.

Elements such as O, Al, Ti and Pr were uniformly distributed on the coatings. The oxidation reaction was promoted by the heat generation of micro-arc discharge, the coatings were mainly composed of Al₂TiO₅, anatase and γ-Al₂O₃, Pr(NO₃)₃ existed as the Pr₂O₃ phase in the coatings.

After doped Pr(NO₃)₃ improved the thickness and densification of the coatings, impeded the migration of Cl⁻ to TC4 substrate. The corrosion resistance of the coatings came the prime at 0.05 g L⁻¹ and has outstanding corrosion resistance in 3.5% NaCl and oilfield simulation solution.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Sichuan Science and Technology Program (grant number 2022YFSY0018), Open Fund of State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization (grant number 2022P4FZG08A), and Open Fund of Sichuan Provincial Engineering Research Center of Advanced Materials Manufacturing Technology for Shale Gas High-efficient Exploitation (grant number 2022SCYYQKCCL007).

ORCID iD

Ping Wang

References

Volozh

Abukova

Rybalchenko

, et al. Formation of oil and gas fields in deep hydrocarbon systems: outline of a universal search concept. Geotectonics 2022; 56: 586–606.

Kawano

Eguchi

Ishiguro

, et al. Passivity of martensite-based stainless steel oil country tubular goods in sour environment. Corrosion 2016; 72: 1556–1564.

Mubarak

Elkhodbia

Gadala

, et al. Failure analysis, corrosion rate prediction, and integrity assessment of J55 downhole tubing in ultra-deep gas and condensate well. Eng Failure Anal 2023; 151: 107381.

Zhu

Wang

, et al. Adsorption characteristics between Ti atoms of TiO₂(100) and corrosive species of CO₂-H₂S-Cl⁻ system in oil and gas fields. Materials 2023; 16: 3129.

Liu

Shen

, et al. Calculation model and experimental study of the collapse strength of titanium alloy tubing and casing. Sci Rep 2022; 12: 1–14.

Pustode

Raja

Dewangan

, et al. Effect of long term exposure and hydrogen effects on HSSCC behaviour of titanium alloy IMI 834. Mater Des 2015; 86: 841–847.

Khayatan

Ghasemi

Abedini

. Synergistic erosion-corrosion behavior of commercially pure titanium at various impingement angles. Wear 2017; 380–381: 154–162.

Yumak

Aslantas

. A review on heat treatment efficiency in metastable beta titanium alloys: the role of treatment process and parameters. J Mater Res Technol 2020; 9: 15360–15380.

Miah

Singh Chand

Malhi

, et al. Influence of laser scanning power on microstructure and tribological behavior of Ni-composite claddings fabricated on TC4 titanium alloy. Aircr Eng 2023; 95: 1165–1171.

10.

Wang

Xiao

Zhou

, et al. Effect of MgO microparticles on characteristics of microarc oxidation coatings fabricated on pure titanium. Int J Electrochem Sci 2019; 14: 287–300.

11.

Dou

Chen

, et al. Research status of magnesium alloys by micro-arc oxidation: A review. Surf Eng 2017; 33: 731–738.

12.

Dudareva

Zaynullina

Kiseleva

, et al. The influence of alloy microstructure on corrosion resistance and other properties of coatings formed by microarc oxidation. Mater Chem Phys 2022; 288: 126379.

13.

Wheeler

Collier

Paillard

, et al. Evaluation of micromechanical behaviour of plasma electrolytic oxidation (PEO) coatings on Ti–6Al–4V. Surf Coat Technol 2010; 204: 3399–3409.

14.

Liu

Wang

Tang

, et al. Effect of Bi₂S₃ on characteristics of micro-arc oxidation coating formed on TC4 alloy. Int J Electrochem Sci 2021; 16: 150919.

15.

Zhu

, et al. The mechanism for tuning the corrosion resistance and pore density of plasma electrolytic oxidation (PEO) coatings on Mg alloy with fluoride addition. J Magnesium Alloys 2021; 11: 2823–2832.

16.

Zhang

Luo

, et al. Effect of Er on the characteristics of an oxide coating prepared by micro-arc oxidation on a 7075 aluminum alloy. Int J Electrochem Sci 2020; 15: 3445–3452.

17.

Guo

, et al. Microstructure and properties of rare earth CeO₂-doped TiO₂ nanostructured composite coatings through micro-arc oxidation. Ceram Int 2015; 41: 6178–6186.

18.

Zhang

Gao

Jia

, et al. The effect of trace rare earth on grain size and properties of copper coating. J Phys: Conf Ser 2023; 2587: 012013.

19.

Brachetti-Sibaja

Domínguez-Crespo

Torres-Huerta

, et al. Bath conditions role in promoting corrosion protection on aluminum alloy using rare earth conversion coatings. J Electrochem Soc 2012; 159: C40–C57.

20.

Wang

Zhu

Xia

, et al. Effects of grain boundary ternary alloy doping on corrosion resistance of (Ce, Pr, Nd)-Fe-B permanent magnets. J Rare Earths 2021; 39: 979–985.

21.

Sheng-Lin

Xiao-Lin

Hua-Hui

, et al. The role of rare earth nitrate (REN) in phosphating of aluminum alloy. Met Finish 2007; 105: 39–45.

22.

Zou

Wang

Sun

, et al. Plasma electrolytic oxidation induced ‘local over-growth’ characteristic across substrate/coating interface: Effects and tailoring strategy of individual pulse energy. Surf Coat Technol 2018; 342: 198–208.

23.

Clyne

Troughton

. A review of recent work on discharge characteristics during plasma electrolytic oxidation of various metals. Int Mater Rev 2019; 64: 127–162.

24.

Park

Y-J

Kim

W-Y

Kang

Y-B

. Phase equilibria of Al₂O₃–TiOx system under various oxygen partial pressure: emphasis on stability of Al₂TiO₅–Ti₃O₅ pseudobrookite solid solution. J Eur Ceram Soc 2021; 41: 7362–7374.

25.

Niu

Hildebrandt

Schubert

, et al. Oxygen vacancy induced room temperature ferromagnetism in Pr-doped CeO₂ thin films on silicon. ACS Appl Mater Interfaces 2014; 6: 17496–17505.

26.

Song

Yan

Cai

, et al. Superhydrophobic composite coating for reliable corrosion protection of Mg alloy. Mater Des 2022; 215: 110433.

27.

Mamaev

Mamaeva

Beletskaya

, et al. A theory of a collective microplasma process for formation of nanostructural inorganic nonmetallic coatings through localization of high-energy flows in the nanolayers of the metalelectrolyte interface. Mathematical modeling. Part 1. Russ Phys J 2013; 56: 959–969.

28.

Kaseem

Fatimah

Nashrah

, et al. Recent progress in surface modification of metals coated by plasma electrolytic oxidation: principle, structure, and performance. Prog Mater Sci 2021; 117: 100735.