Tribological and Electrochemical Corrosion Properties of CNT-Incorporated Plasma Electrolytic Oxidation (PEO) Coatings on AZ80 Magnesium Alloy

doi:10.1007/s40195-021-01363-6

Abstract

Abstract:

Plasma electrolytic oxidation (PEO) coatings were carried out in silicate-based electrolyte embedded with various amounts of carbon nanotube (CNT) additives (0, 0.5, 1, 2 and 4 g/L) on AZ80 magnesium alloy substrate. Microstructure, tribological and electrochemical corrosion properties of the coated specimens were investigated. The results demonstrated that the increasing CNT additions into the electrolyte resulted in a gradual increase in the thickness of PEO-coating layer. The CNT addition to the electrolyte by 0.5 g/L resulted in a slight decrease in the roughness of PEO-coating above which it continually increased. Wear resistance of the PEO-coated specimens showed a gradual improvement with increasing CNT-incorporation within the coating. The electrochemical corrosion tests revealed that the best corrosion resistance was found after the CNT addition into the electrolyte by 0.5 g/L due to the better roughness values, more homogenous coating layers and less pore formation.

Key words: Coating, Electrochemical corrosion, Magnesium alloy, Plasma electrolytic oxidation (PEO), Wear

Rabia Kara, Huseyin Zengin. Tribological and Electrochemical Corrosion Properties of CNT-Incorporated Plasma Electrolytic Oxidation (PEO) Coatings on AZ80 Magnesium Alloy[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(7): 1195-1206.

Figures/Tables 20

Table 1 XRF result of the magnesium alloy substrate

Alloy	Al	Zn	Mn	Si	Fe	Ni	Mg
AZ80	8.14	0.56	0.15	0.05	<0.01	<0.01	Bal.

Fig. 1 SEM microstructure of the CNTs

Table 2 Properties of the electrolytes for the PEO process

Specimen	CNT (g/L)	Na₂SiO₃ (g/L)	NaOH (g/L)	Na₂[SiF₆] (g/L)	pH	Conductivity (ms/cm)
0 CNT	0	10	1	3	12.0	17.74
05 CNT	0.5	10	1	3	12.1	18.56
1 CNT	1	10	1	3	12.2	19.17
2 CNT	2	10	1	3	12.2	19.32
4 CNT	4	10	1	3	12.0	19.96

Fig. 2 SEM micrographs of the as-cast AZ80 alloy at a low and b high magnifications, c XRD pattern of the corresponding alloy and d EDX results of the points indicated in b

Fig. 3 Macro appearances of the PEO-coated AZ80 alloy with different CNT additions

Table 3 Mean surface roughness values of the specimens

Specimen	R_a (µm)
Uncoated	0.2±0.02
0 CNT	1.3±0.16
05 CNT	0.8±0.08
1 CNT	1.2±0.15
2 CNT	1.9±0.21
4 CNT	2.7±0.29

Fig. 4 SEM surface micrographs of PEO-coated specimens with different CNT additions: a 0 CNT, b 0.5 CNT, c 1 CNT, d 2 CNT, e 4 CNT and f EDX results of the areas indicated in figures

Fig. 5 Measured pore ratio and pore size values of the PEO-coated specimens with different CNT additions

Fig. 6 XRD patterns of the PEO-coated specimens with different CNT additions

Fig. 7 Cross-sectional SEM micrographs of the PEO-coated specimens with different CNT additions: a 0 CNT, b 0.5 CNT, c CNT, d 2 CNT, e 4 CNT and f, g EDX results of the lines as shown in a and e, respectively

Table 4 Thickness and Vickers microhardness values of the PEO-coated specimens measured from cross sections

Specimen	Thickness (µm)	Hardness (HV_0.1)
Uncoated	-	64±2
0 CNT	4.7±0.9	481±7
05 CNT	5.5±1.2	533±8
1 CNT	6.9±1.5	538±7
2 CNT	7.2±1.5	579±9
4 CNT	10.1±2	624±10

Fig. 8 Specific wear rate and friction coefficient values of the specimens

Fig. 9 Wear depth profiles of the uncoated and PEO-coated specimens with different CNT additions: a AZ80 substrate, b 0 CNT, c 05 CNT, d 1 CNT, e 2 CNT and f 4 CNT

Fig. 10 SEM micrographs of the worn surfaces of the uncoated and PEO-coated specimens with different CNT additions: a AZ80 substrate, b 0 CNT, c 05 CNT, d 1 CNT, e 2 CNT and f 4 CNT

Table 5 EDX results of the areas as shown in Fig. 9

Specimen	Point	Composition (wt%)
		Mg	O	Al	C	Si	Na
Uncoated	1	81.9	11.4	6.2	0.5	-	-
0 CNT	2	58.5	34.7	3.4	0.7	2.2	0.5
05 CNT	3	59.8	31.9	3.9	1.5	2.5	0.4
1 CNT	4	47.6	41.7	5.5	2.1	2.6	0.5
2 CNT	5	40.2	47.5	4.9	3.8	2.9	0.7
4 CNT	6	36.3	49.3	5.0	5.6	3.3	0.5

Fig. 11 Potentiodynamic polarization curves of the uncoated and PEO-coated specimens

Table 6 Tafel analysis results of the uncoated and PEO-coated specimens

Specimen	i_corr (A/cm²)	E_corr (V)	E_bd (V)	β_a (mV)	- β_c (mV)	R_pol (Ω cm²)
Uncoated	1.4×10^-5	-1.42	-1.33	871.2	160.1	4.20×10³
0 CNT	2.5×10^-6	-1.38	-1.26	373.3	371.6	32.9×10³
05 CNT	2.4×10^-7	-1.29	-1.22	363.2	229.8	254.9×10³
1 CNT	5.9×10^-7	-1.26	-0.9	337.3	338.4	124.5×10³
2 CNT	5.6×10^-7	-1.25	-1.01	267.3	387.6	122.8×10³
4 CNT	2.5×10^-6	-1.30	-1.15	569.6	572.4	49.6×10³

Fig. 12 Nyquist plots of the uncoated and PEO-coated specimens

Fig. 13 Equivalent circuit model for the EIS data of the PEO-coated specimens

Table 7 Fitting results of the EIS data for the PEO-coated specimens

Specimen	R_s (Ω cm²)	R_p (Ω cm²)	R_b (Ω cm²)	R_ct (Ω cm²)	CPE_p (Ω^-1 sⁿ cm^-2)	CPE_b (Ω^-1 sⁿ cm^-2)	CPE_dl (Ω^-1 sⁿ cm^-2)	L (H cm²)
0 CNT	112.7	20.8×10³	16.4×10³	17.6×10³	840.8×10^-9	1.9×10^-6	12.3×10^-6	7.5
05 CNT	29.3	86.5×10³	128.9×10³	124.4×10³	12.2×10^-9	0.9×10^-6	2.4×10^-6	9.4
1 CNT	62.4	41.4×10³	109×10³	51.4×10³	459.6×10^-9	3.1×10^-6	24.1×10^-6	1.2
2 CNT	14.7	55.9×10³	46.1×10³	21.3×10³	47.2×10^-9	1.3×10^-6	6.2×10^-6	12.1
4 CNT	24.3	18.8×10³	17.7×10³	19.8×10³	777.4×10^-9	3.1×10^-6	15.4×10^-6	0.2

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