Effect of surface treatment process on micro arc oxidation film and galvanic current of TC4 titanium alloy

The micro morphology and galvanic current density of micro arc oxidation film of TC4 titanium alloy after different treatment were studied by scanning electron microscope and electrochemical workstation. The results show that high temperature oxidation does not significantly change the surface morphology of micro arc oxidation film; TC4 titanium alloy is prone to galvanic corrosion when contacting with steel, so it can not be used in direct contact; Micro arc oxidation of titanium alloy can reduce galvanic current density and galvanic corrosion sensitivity; Silane sealing treatment has little effect on the galvanic current density of micro arc oxidation film; After micro arc oxidation, high temperature oxidation and silylation sealing treatment significantly reduce the galvanic current density of TC4 titanium alloy.
Titanium alloy has the characteristics of light weight, high strength, good fatigue performance and certain corrosion resistance under high temperature conditions [1], and is widely used in aviation, aerospace, shipbuilding and other fields [2]. However, in seawater and marine atmospheric corrosion environment, galvanic corrosion will occur when titanium alloy is used in contact with dissimilar metals due to its high potential. Micro arc oxidation technology is an effective measure to solve this problem and an effective surface treatment method to improve the properties of titanium alloy [3,4].
Micro arc oxidation is a new technology for directly in-situ growth of ceramic layer on the surface of non-ferrous metals [5]. The film obtained by micro arc oxidation not only has the advantages of strong adhesion and good corrosion resistance [6], but also can greatly improve the hardness, wear resistance, electrical insulation and other properties of the surface [7], and can play a good role in protecting the matrix [8-10]. However, the micro arc oxidation coating carries out arc discharge on the workpiece surface under the action of high voltage. There are a large number of discharge channels in the obtained oxidation coating, resulting in a large number of micron scale micropores in the micro arc oxidation coating. In the corrosive environment, the existence of these micropores not only provides a channel for the corrosive medium to penetrate into the matrix, but also speeds up its erosion rate of the matrix [11,12]. Therefore, it is necessary to further treat the oxidation coating to isolate the contact between the substrate and the external environmental medium and increase its anti-corrosion performance.
In this paper, the micro arc oxidation film on the surface of TC4 titanium alloy was treated by micro arc oxidation, micro arc oxidation + high temperature oxidation, micro arc oxidation + silylation sealing, micro arc oxidation + high temperature oxidation + silylation sealing, and the effects of different surface treatment conditions on the micro arc oxidation film and galvanic current of TC4 titanium alloy were investigated.

Experimental method

The experimental material is TC4 titanium alloy plate with 2.5 mm and its main chemical components (mass fraction,%) are al 6.47, V 4.2, Fe 0.22, C 0.01, O 0.16 and Ti surplus. Take 120 mm × 25 mm × The 2.5mm TC4 titanium alloy plate is subject to high-temperature oxidation and sealing treatment after micro arc oxidation treatment. The specific process is as follows: 1# sample surface is subject to micro arc oxidation treatment, 2# sample surface is subject to micro arc oxidation + high-temperature oxidation treatment, 3# sample surface is subject to micro arc oxidation + silylation sealing treatment, and 4# sample surface is subject to micro arc oxidation + high-temperature oxidation + silylation sealing treatment. Positive constant current electric pulse control mode is adopted for micro arc oxidation, and the electrolyte temperature is controlled at 20 ~ 35 ℃. The high temperature oxidation temperature of micro arc oxidation film is 450 ℃, and the high temperature oxidation atmosphere is air. The hydrolysis solution of silane coupling agent was used for sealing. After the experiment, the micro morphology of micro arc oxidation film was observed by Fei quanta-200 environmental scanning electron microscope (SEM), and the galvanic current density of TC4 titanium alloy coupled with 925 steel was measured by par2273 electrochemical workstation.

Results and analysis

Effect of surface treatment process on micro morphology of micro arc oxidation film

The surface micro morphology of micro arc oxidation film under different process conditions is shown in Figure 1. It can be seen from the figure that after micro arc oxidation treatment of TC4 titanium alloy, the surface of the film presents porous morphology, and the film shows “volcanic” microporous morphology. The morphological characteristics of micropores on the surface of the film depend on the essence of micro discharge in the micro arc oxidation mechanism [13]. The different size and uneven distribution of pores are due to the relatively strong arc discharge in the process of micro arc oxidation. This porous morphology leads to the coarsest grain and loose structure of the oxide film.

Fig.1 Morphologies of MAO film (a) and MAO+high temperature oxidation (b)
The surface morphology of the 2# samples treated by micro arc oxidation and high temperature oxidation has no obvious change compared with the 1# samples treated by micro arc oxidation, and the morphology after calcination is relatively clearer. The surface oxides are connected, covered and remelted, and massive oxides are distributed in the pores, and the film is relatively dense. However, cracks occurred on the surface of the film, and most of the cracks passed through the microporous defects on the surface of the micro arc oxidation film. This is mainly due to the different thermal expansion coefficients between micro arc oxidation film and matrix during heat treatment [14]. Under the action of thermal stress generated during high-temperature oxidation, dislocations continue to form. Dislocations are blocked when they encounter microporous defects on the surface of the film. With the continuous accumulation of dislocations, crack sources are formed at the microporous defects on the surface. When the thermal stress accumulation reaches a certain degree, Cracking of micro arc oxidation film [15].

Effect of surface treatment process on hole sealing effect of silanization of micro arc oxidation film

The surface micro morphology of 3# and 4# samples after micro arc oxidation treatment and micro arc oxidation + high temperature oxidation treatment are shown in Fig. 2. It can be seen from Figure 2 that for the 3# sample whose surface is directly silanized after micro arc oxidation treatment, there is a gray film on the surface of micro arc oxidation film, and the micropore defects are closed to a certain extent; This is due to the loose structure of oxide particles on the surface of TC4 titanium alloy, which provides a favorable channel for the internal deposition of silane hydrolytic solution. However, due to the thin and uneven distribution of silane film, there are still some microporous defects on the surface of micro arc oxidation film. For the 4# sample treated by micro arc oxidation + high temperature oxidation and then silanized, a thick and uniform gray film is obtained on the surface of micro arc oxidation film, without micropore defects. The micro arc oxidation film treated by micro arc oxidation + atmospheric high-temperature oxidation, under the joint action of the porous structure of the micro arc oxidation film and the loose structure of oxide particles at the interface [16], makes the micro arc oxidation film obtain a thicker and more uniform silane deposition film on the surface after silylation treatment.

Fig.2 Morphologies of MAO+silanization treatment (a) and MAO+high temperature oxidation+silanization treatment (b)
Through the above comparison, it can be seen that the high-temperature oxidation treatment has a positive effect on the subsequent silylation sealing of the micro arc oxidation film, so that a thick silane film is deposited on the surface of the micro arc oxidation film, which completely seals the micropores of the micro arc oxidation film.

Effect of surface treatment process on galvanic current of micro arc oxidation film

The galvanic current of TC4 titanium alloy after different surface treatment was measured after coupling with 925 steel. The average corrosion current density of TC4 titanium alloy sample with untreated surface coupled with 925 steel is 1.27 μ A / cm2, galvanic corrosion sensitivity is grade C, which is not allowed to be used in contact.
When the surface of TC4 titanium alloy was treated by micro arc oxidation and then coupled with 925 steel, the galvanic corrosion current increased from 1.27 μ A / cm2 reduced to 0.19 μ A / cm2, its galvanic corrosion sensitivity is reduced from grade C to grade A, and its galvanic corrosion resistance is excellent, which can be used according to the standard. Further, the galvanic current density of TC4 titanium alloy after micro arc oxidation is only reduced to 0.18 after silylation sealing treatment μ A / cm2, indicating that further hole sealing treatment has little effect on reducing galvanic corrosion. The average galvanic current of TC4 titanium alloy after micro arc oxidation was reduced to 0.04 after coupling with 925 steel μ A / cm2, which is one order of magnitude lower than that of micro arc oxidation and micro arc oxidation + silylation sealing treatment, and two orders of magnitude lower than that of samples without surface treatment, indicating that the thicker and more uniform silane deposition film formed on the surface after micro arc oxidation + high temperature oxidation + silylation sealing treatment can significantly reduce the average galvanic current density, The corrosion resistance of titanium alloy is significantly improved.

The galvanic current time curve of TC4 titanium alloy-925 steel is shown in Figure 3. After TC4 titanium alloy without micro arc oxidation treatment was contacted with 925 steel, the galvanic current decreased gradually with the extension of time, and became stable after about 50000 s; The galvanic current of TC4 titanium alloy after surface treatment decreases rapidly and tends to be stable in a short time after contact. After contacting 925 steel, the galvanic current tends to be stable and the time is the shortest after about 2000 s. The reason for this phenomenon is that there is a stable oxide film on the surface of titanium alloy after micro arc oxidation, which can effectively reduce the galvanic current; After micro arc oxidation, silanization sealing treatment is carried out, so that some holes of the surface micro arc oxidation film are covered by thin silane film, which further reduces the galvanic current; After high temperature oxidation and silylation sealing treatment after micro arc oxidation, the galvanic current is greatly reduced due to the blocking effect of thick and uniform silane deposition film on the surface.

Fig.3  Galvanic current-time curves of TC4 Ti-alloy with 925 steels after different treatment

Conclusion

• (1) High temperature oxidation does not significantly change the surface morphology of the micro arc oxidation film, but has a positive impact on the subsequent silylation sealing treatment of the micro arc oxidation film, which can seal all the holes and microcracks in the micro arc oxidation film.
• (2) The surface treatment of TC4 titanium alloy can reduce the galvanic current density when TC4 titanium alloy is coupled with 925 steel, and can greatly reduce the contact corrosion sensitivity of TC4 titanium alloy and 925 steel. In order to avoid galvanic corrosion of TC4 titanium alloy and 925 steel in corrosive medium, micro arc oxidation + high temperature oxidation + silylation sealing treatment is an effective protective measure.

Authors: YANG Sheng, ZHANG Huijie, XIANG Wuyuan, OUYANG Tao, XIAO Fen, ZHOU Hui
Source: China Titanium Flange Manufacturer: www.titaniuminfogroup.com
Reference:

• [1] Xie N S, Wu L Z. Effect factor and its application of micro-arc oxidation technique on titanium alloy surface[J]. Hot Work. Technol., 2011, 40: 130
• [2] Jin H X, Wei K X, Li J M, et al. Research development of titanium alloy in aerospace industry[J]. Chin. J. Nonferrous Met., 2015, 25: 280
• [3] Yerokhin A L, Leyland A, Matthews A. Kinetic aspects of aluminium titanate layer formation on titanium alloys by plasma electrolytic oxidation[J]. Appl. Surf. Sci., 2002, 200: 172
• [4] Xin X J, Xue J F. Corrosion, Protection and Engineering Application of Titanium [M]. Hefei: Anhui Science and Technology Press, 1988: 65
• [5] Tu Z M, Li N, Zhu Y M. Application and Technology on Surface Treatment of Titanium and Titanium Alloys [M]. Beijing: National Defense Industry Press, 2010: 127
• [6] Wang Q L, Xu L L, Ge S R. Investigation on structure of microarc oxidation ceramic layer on titanium alloy[J]. China Surf. Eng., 2007, 20(5): 11
• [7] Shi S R. Study on sealing treatments of micro arc oxidation coatings on Al alloy 6061[D]. Harbin: Harbin Engineering University, 2013
• [8] Wang C J, Jiang B L, Liu M, et al. Corrosion characterization of micro-arc oxidization composite electrophoretic coating on AZ31B magnesium alloy [J]. J. Alloy. Compd., 2015, 621: 53
• [9] Cui X J, Yang R S, Liu C H, et al. Structure and corrosion resistance of modified micro-arc oxidation coating on AZ31B magnesium alloy[J]. Trans. Nonferrous Met. Soc. China, 2016, 26: 814
• [10] Wang P, Wu T, Peng H, et al. Effect of NaAlO2 concentrations on the properties of micro-arc oxidization coatings on pure titanium[J]. Mater. Lett., 2016, 170: 171
• [11] Xia S J, Yue R, Rateick Jr R G, et al. Electrochemical studies of AC/DC anodized Mg alloy in NaCl solution[J]. J. Electrochem. Soc., 2004, 151: B179
• [12] Duan H P, Du K Q, Yan C W, et al. Electrochemical corrosion behavior of composite coatings of sealed MAO film on magnesium alloy AZ91D[J]. Electrochim. Acta, 2006, 51: 2898
• [13] Rudnev V S, Lukiyanchuk I V, Kon’shin V V, et al. Anodic-spark deposition of P- and W (Mo)-containing coatings onto aluminum and titanium alloys[J]. Russ. J. Appl. Chem., 2002, 75: 1082
• [14] Luo Z Z. Study on silanization of anodized film on Al alloy[D]. Harbin: Harbin Engineering University, 2016: 34
• [15] Evans H E, Lobb R C. Conditions for the initiation of oxide-scale cracking and spallation[J]. Corros. Sci., 1984, 24: 209
• [16] Xiang W Y, Jiang H T, Tian S W. High temperature oxidation behavior of titanium and titanium alloys[J]. Metallic Funct. Mater., 2020, 27(3): 33