Effect of Initial Oxide Film on the Formation and Performance of Plasma Electrolytic Oxidation Coating on 7075 Aluminum Alloy

doi:10.1007/s40195-022-01378-7

Abstract

Abstract:

In the early stage of plasma electrolytic oxidation (PEO), the quick formation of the initial oxide film on the surface of the aluminum substrate is necessary for the subsequent discharge spark process. Furthermore, the compactness of the initial oxide film greatly affects the quality of the PEO coating, but the related mechanisms are not investigated in detail. In this paper, the status of the initial oxide film was adjusted by adding the (NaPO₃)₆ into the based electrolyte, and then the effect of initial oxide film on the formation of the PEO coating was compared. Microstructure and chemical composition were analyzed by scanning electron microscope, transmission electron microscopy and X-ray photoelectron spectrometer. The compactness of the initial oxide film was characterized by Mott-Schottky plots. The corrosion resistance was measured by electrochemical impedance spectroscopy. The results show that the addition of (NaPO₃)₆ can promote the co-deposition of phosphide compound into the initial oxide film and improve the film compactness, which is beneficial for the more uniform spark discharge in the later stage. As a result, the addition of (NaPO₃)₆ is helpful to obtain PEO coating with better performance.

Key words: Aluminum alloys, Plasma electrolytic oxidation, Initial oxide film, Growth process, Spark discharge, Compactness

Mingyu Zhu, Yingwei Song, Kaihui Dong, Dayong Shan, En-Hou Han. Effect of Initial Oxide Film on the Formation and Performance of Plasma Electrolytic Oxidation Coating on 7075 Aluminum Alloy[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(9): 1559-1571.

Figures/Tables 14

Table 1 Chemical composition of the 7075 aluminum alloy (wt%)

Zn	Mg	Cu	Sc	Zr	Al
9	2	1.1	0.25	0.3	Bal.

Fig. 1 Voltage-time response during PEO process a; b high magnification of one segment of a

Fig. 2 Surface morphologies of 7075 Al alloy after PEO treatment for different time in the basic electrolyte: a 40 s; b 80 s; c 150 s; d 240 s

Fig. 3 Surface morphologies of 7075 Al alloy after PEO treatment for different time in the electrolyte with (NaPO3)6: a 40 s; b 80 s; c 150 s; d 240 s

Fig. 4 Surface and cross-sectional morphologies of PEO coatings prepared in two electrolytes: a, b the basic electrolyte; c, d the electrolyte with (NaPO3)6

Table 2 EDX results of the outer and inner layer of PEO coating (wt%)

	Al	O	Si	P
A	21.14	71.24	2.36	5.27
B	27.31	64.79	6.65	1.25

Fig. 5 SEM cross-sectional morphologies and EDS element mapping of PEO coating prepared in the electrolyte with (NaPO3)6: a cross-sectional morphology; b Al; c P; d O; e Si

Fig. 6 TEM cross-sectional morphology (a) and EDS line scanning (b) of PEO coating prepared in the electrolyte with (NaPO3)6; c, d HRTEM micrographs of the PEO coating

Fig. 7 High-resolution XPS spectra of P 2p3/2 after PEO treatment for different time in the electrolyte with (NaPO3)6: a 40 s; b 80 s; c 150 s

Fig. 8 Mott-Schottky plots of the initial oxidation films formed in both electrolytes

Fig. 9 Nyquist plots a, Bode plots b of the PEO coatings formed in both electrolytes in 3.5% NaCl solution

Fig. 10 Equivalent circuit of the EIS plots

Table 3 Fitting results of the EIS plots

	R_s (Ω cm²)	Y_p (Ω^-1 cm^-2 s^-1)	n	R_p (Ω cm²)	Y_b (Ω^-1 cm^-2 s^-1)	n	R_b (Ω cm²)
Basic electrolyte	58.53	1.193 × 10^-7	0.893	6.024 × 10⁴	1.272 × 10^-7	0.931	2.215 × 10⁵
With (NaPO₃)₆	58.20	4.501 × 10^-8	0.953	3.17 × 10⁴	1.426 × 10^-7	0.788	5.915 × 10⁶

Fig. 11 Schematic diagrams of the influence of (NaPO3)6 on the initial oxidation film and the micro-arc discharge process: a, c, e the reaction at different PEO stages in the basic electrolyte; b, d, f in the electrolyte with (NaPO3)6

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