Effect of PEO power modes on structure and corrosion resistance of ceramic coatings on AZ91D Mg alloy

Preparation of coatings

Plate samples of AZ91D Mg alloy with dimensions of 30×15×1 mm were used as the working electrode and an electrolyser made of stainless steel served as the counter electrode. Before PEO process, the samples were polished with fine abrasive papers and then cleaned to obtain a uniform surface. The electrolyte system contained sodium hydroxide (7 g L^–1), sodium hexametaphosphate (4 g L^–1) and calcium acetate (0·4 g L^–1). PEO processes were carried out under three power modes (CV, CC and CP) with the same working frequency of 2000 Hz and initial peak current density of 2 A dm^–2. When the designated voltage (200, 300, 400, 450 and 480 V) was reached, the constant voltage was maintained for the CV mode for 30 min, while the PEO processes stopped directly for CC and CP modes. The reaction temperature was controlled to be below 30°C by adjusting the cooling water flow. After the treatment, the coated samples were flushed with water and dried in air.

Characterisation of coatings

Morphology images and the elemental distribution of the coatings were studied with SEM and energy dispersive spectroscopy (EDS). Phase composition of the coatings was examined using X-ray diffraction (XRD), with a Cu K_α source. The coating's thickness and roughness were measured using an eddy current based thickness gauge (CTG-10) and a surface roughness gauge (TR-100).

Corrosion resistance of coatings

The corrosion resistance of the coatings was evaluated using electrochemical impedance spectroscopy (EIS) and polarisation curve method using a CHI604 electrochemical analyser in simulated body fluid solution, using a three-electrode cell (a Pt plate was used as a counter electrode, a calomel electrode was used as an auxiliary electrode and the coated sample was used as a working electrode with the area of 1 cm²). In the measurement of EIS, a sinusoidal alternating current perturbation of 5 mV was applied to the electrode at the open circuit potential of the coated samples over the frequency range of 0·01 Hz–10 kHz. The measured EIS spectra of the coatings were fitted and interpreted by EIS fitting software. Furthermore, the polarisation curve's scanning rate was 0·5 mV s^–1, with a scanning range from −0·2 to +0·2 V versus open circuit potential. Based on the approximate linear polarisation at the corrosion potential E_corr, the corresponding corrosion potentials, corrosion current densities and polarisation resistance R_p values were determined.14

Thickness and roughness of coatings

The thickness and the roughness of the coatings prepared under different power modes are shown in Figure 1 Figs. 1 and 2 respectively. Both the thickness and the roughness of the coating abide by the following sequence: CV>CC>CP. Under CV mode, the coating was still growing, accompanying the spark discharging on the surface of the electrode sample when the constant voltage was maintained, which results into the increase in the thickness and the roughness, compared to the other two modes. Under CP mode, the output of the power is constant during the process, i.e. the cell voltage increases, while the current decreases during PEO process, and at CC mode, the current is constant, while the cell voltage increases. In this way, the actual output of CC mode is more than that of CP mode; therefore, the thickness and the roughness of CC mode are larger than those of CP mode in general.

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Figure 1

Thicknesses of coatings prepared under different power modes

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Figure 2

Roughnesses of coatings prepared under different power modes

SEM images of coatings

The surface and the section SEM images of the coatings prepared at 300 and 450 V under different power modes are shown in Fig. 3. From the surface SEM images, the coatings are all porous. Under the same power mode, the number of micropores is decreased, while the size is increased with the cell voltage. Under the same cell voltage, the number of micropores is decreased, while the size is increased in the sequence of CP, CC and CV modes. Comparatively, the surface morphologies of coatings prepared under CC and CP modes are basically similar, which are quite different from that of the coatings prepared under CV mode. The changes of the surface morphologies comply with the surface roughness of the coatings. As for the section SEM images, the thickness of the coatings is consistent with the results shown in Fig. 1. Furthermore, the micropores and the microcracks are clearly shown in the section images.

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Figure 3

Surface and section SEM images of coatings prepared under different power modes

Composition of coatings

Table 1 presents the relative content of the main elements of the coating prepared at 300 and 450 V under different power modes. All the coatings are composed of a large amount of O and Mg and a small amount of P, Al, Ca and Na, which shows that the elements, both from the substrate and the electrolyte, joined the PEO process and contributed to the formation of the coating. Furthermore, the relative contents of the main elements in the coating change less with different cell voltages or power modes. In order to investigate the crystalline substance of the coatings, XRD analysis was carried out, with the results shown in Fig. 4. The main crystalline in the coating is MgO. Moreover, the diffraction peaks corresponding to the Mg alloy substrate appear in the patterns because the coatings are thin and porous. Under the same cell voltage, the amount of MgO is increased in the sequence of CP, CC and CV mode. However, under the same power mode, the amount of MgO is increased with the cell voltage. Therefore, the coating prepared at the higher cell voltage under CV mode has better crystallisation. Moreover, P, Ca, Al and Na are not detected by XRD analysis, which means that they may exist in the form of amorphous state in the coating. Figure 5

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Figure 4

X-ray diffraction patterns of coatings prepared under different power modes:

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Figure 5

Electrochemical impedance spectra (Nyquist plot) of coatings prepared under different power modes

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Table 1

Energy dispersive spectroscopy analysis of coatings prepared at 300 and 450 V under different power modes

	Content/at-%
	Mg	Al	Ca	P	O	Na
CV-300	30·71	2·45	0·40	7·42	57·84	1·17
CC-300	32·31	1·41	0·36	6·86	57·65	1·42
CP-300	38·68	1·80	0·27	5·60	52·84	0·81
CV-450	33·44	2·66	0·32	3·57	59·25	0·76
CC-450	34·85	2·83	0·57	4·21	56·87	0·66
CP-450	33·26	2·13	0·57	5·08	58·05	0·92

Corrosion resistance of coatings

EIS measurement and analysis

Figure 5 is EIS (Nyquist plot) of the coatings prepared under different power modes. According to our previous study,15 the equivalent circuit of EIS spectra <R_s{Q¹[R₁(Q² R₂)]}> was adopted. R_s is the electrolyte resistance between the working and reference electrode, Q₁ is the constant phase angle element used to reflect the capacitance of the interfacial and pores in outer layer C_if, R₁ is the pore resistance of the coating's outer layer, R₂ is the electric charge transfer resistance corresponding to the inner layer and Q₂ is the constant phase angle element corresponding to the structure of the inner layer. The fitting values of equivalent circuit elements are shown in Table 2. According to the established ‘equivalent circuit’, the sum of R₁ and R₂ approximately equals the polarisation resistance R_p, which is able to reflect the corrosion resistance of the coatings. Under CV mode, the coatings prepared at 400 and 450 V are of better corrosion resistance, while the coatings prepared at 450 V under CC and CP modes both have better corrosion resistance. However, the coatings prepared at 480 V under different power modes were all very rough due to the long periods of large sparking discharging on the coating surface, which leads to the great decrease in the corrosion resistance. Among different power modes, the corrosion resistance of the coatings is generally increased in the sequence CV>CP>CC.

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Table 2

Fitting values of coatings prepared under different power modes

	Rs/Ω cm2	Q1–Y0/Ω−1 cm−2 s−n	Q1–n	Rpo/Ω cm2	Q2–Y0/Ω−1 cm−2 s−n	Q2−n	R2/Ω cm2
CV-200	19·37	8·795×10−7	0·8719	35·93	3·697×10−6	1	127·7
CC-200	20·46	7·597×10−7	0·8787	35·78	7·664×10−6	1	141·9
CP-200	19·04	5·004×10−7	0·9028	53·63	3·512×10−6	1	86·38
CV-300	19·06	3·968×10−7	0·9129	71·08	1·949×10−6	1	111·8
CC-300	18·63	7·808×10−7	0·8741	71·16	1·400×10−6	1	246·1
CP-300	18·92	9·648×10−7	0·8601	39·47	3·181×10−6	0·9924	406·5
CV-400	12·65	1·547×10−5	0·6239	750·8	6·529×10−7	0·9973	13 140
CC-400	17·91	1·934×10−6	0·7979	85·85	2·83×10−5	0·7167	3126
CP-400	17·75	1·213×10−6	0·8344	109·1	1·473×10−5	0·7496	6120
CV-450	19·5	7·650×10−7	0·8735	200·2	1·560×10−5	0·5757	13 130
CC-450	20·46	2·432×10−7	0·9567	102·2	2·050×10−5	0·6741	6006
CP-450	18·01	1·493×10−6	0·8165	151·5	1·473×10−5	0·7073	10 470

Polarisation curves measurement and analysis

To further verify the corrosion resistance of the coatings, the polarisation curves were measured, and the results are shown in Fig. 6. A summary of the electrochemical corrosion parameters derived from the polarisation curves is also listed in Table 3. Moreover, b_a and b_c are the anodic/cathodic Tafel slopes respectively. Compared to Table 2, it can be noted from Table 3 that the values of R_p from this polarisation curve method are a little larger than those from EIS tests, but have the same tendency or classification presented by EIS analysis.

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Figure 6

Polarising curves of coatings prepared under different power modes

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Table 3

Electrochemical corrosion parameters derived from polarisation curves

	bc/mV/decade	ba/mV/decade	Ecorr/V	Icorr/A cm−2	Rp/Ω cm2
CP-200	4·046	7·104	−1·462	2·113×10−5	5·297×104
CC-200	6·880	6·098	−1·771	1·012×10−4	1·387×104
CV-200	6·456	5·930	−1·793	1·630×10−4	7·858×103
CP-300	6·432	5·996	−1·799	5·931×10−5	2·272×104
CC-300	6·337	5·698	−1·660	1·217×10−5	1·070×105
CV-300	6·547	5·797	−1·706	1·325×10−5	1·008×105
CP-400	6·331	5·331	−1·670	5·687×10−6	2·210×105
CC-400	6·326	5·705	−1·655	9·674×10−6	1·346×105
CV-400	6·434	5·780	−1·660	2·401×10−6	5·506×105
CP-450	6·481	5·741	−1·658	3·457×10−6	3·824×105
CC-450	6·356	5·682	−1·658	5·313×10−6	2·452×105
CV-450	6·579	5·577	−1·607	2·196×10−6	5·957×105

Generally, the corrosion resistance of the coatings is related to the composition and the structure. The results of EDS analyses show that all the coatings have similar composition under different reaction conditions; therefore, the corrosion resistance of the coatings is mainly attributable to the structure and surface state of the coatings. During the CV process, the coating continued to grow, which may be liable for self-repairing of the coating and in the meantime presents a certain sealing effect for the inner layer of the coating. In addition, the coatings prepared under CV mode have the maximum thickness and better crystallisation. Therefore, the best corrosion resistance was obtained under the CV mode. As for CP mode, the coating shows the smallest surface roughness and the best density among the three kinds of coatings; therefore, such coating prepared under CP mode had the second best corrosion resistance. As for the coating prepared under CC mode, although the CC is liable for the growth, it also destroys the dense structure of the coatings to a certain degree, which results into the worst corrosion resistance. On the other hand, under the same mode, the proper increase in the cell voltage is liable for improving the thickness and the quality of the coating and, therefore, increasing the corrosion resistance.