Balancing the corrosion resistance and through-plane electrical conductivity of Cr coating via oxygen plasma treatment

doi:10.1016/j.jmst.2020.06.012

Abstract

Abstract:

Developing an electrically conductive and corrosion-resistant coating is essential for metal bipolar plates of polymer electrolyte membrane fuel cells (PEMFCs). Although enhanced corrosion resistance was seen for Cr coated stainless steel (Cr/SS) bipolar plates, they experience a quick decrease of through-plane electrical conductivity due to the formation of a porous and low-conductive corrosion product layer at the plate surface, thus leading to an increase in interfacial contact resistance (ICR). To tackle this issue, the multilayer Cr coatings were deposited using the magnetron sputtering with a remote inductively coupled oxygen plasma (O-ICP) in the present study. After the O-ICP treatment, a Cr oxide layer (CrO*) is formed on the specimen surface. The CrO*/Cr/SS has a remarkably lower stable corrosion rate (i_ss) than that of the native Cr oxides (CrO_n/Cr/SS). Compared with CrO_n/Cr/SS, the excellent performance of CrO*/Cr/SS is attributed to a denser and thicker surface layer of CrO* with Cr being oxidized to its highest valence state, Cr (VI). More importantly, the through-plane electrical conductivity of the specimens treated by the optimized O-ICP decreases much slowly than CrO_n/Cr/SS and thus, the increament of ICR of CrO*/Cr/SS after the potentiostatic polarization test is considerably smaller than that of CrO_n/Cr/SS, which is benefited from the reduced i_ss that mitigates the deposition of corrosion products and hinders further oxidation of Cr coating. Therefore, CrO*/Cr/SS proves to be a well balanced trade-off between corrosion resistance and through-plane electrical conductivity. The results of this study demonstrate that O-ICP treatment on a conductive metal coating is an effective strategy to improve the corrosion resistance and suppress the increase of ICR over the long-term polarization. The technique reported herein exhibits its promising potential application in preparing corrosion resistant and electrically conductive coatings on metal bipolar plates to be used in PEMFCs.

Key words: Metal bipolar plates, Oxygen plasma treatment, Chromium coating, Corrosion resistance, Electrical conductivity, Polymer electrolyte membrane fuel cells

Xian-Zong Wang, Hong-Qiang Fan, Triratna Muneshwar, Ken Cadien, Jing-Li Luo. Balancing the corrosion resistance and through-plane electrical conductivity of Cr coating via oxygen plasma treatment[J]. J. Mater. Sci. Technol., 2021, 61: 75-84.

Figures/Tables 15

Fig. 1. Schematic diagram of the single layer and multilayer coatings. (a) Preparation processes and (b) sample information of the CrOn/Cr and multilayer CrO*/Cr coatings.

Fig. 2. Cross-sectional SEM images of (a) CrOn/Cr, (b) CrO*/Cr, (c) (CrO*/Cr)2 and (d) (CrO*/Cr)3.

Fig. 3. (a) XRD patterns of the CrOn/Cr and CrO*/Cr coatings and (b) Enlarged spectra of CrO*/Cr coatings.

Fig. 4. XPS spectra of Cr 2p of the (a) CrOn/Cr and (b) CrO*/Cr after sputtering with different times.

Fig. 5. Deconvolution of XPS spectra of Cr 2p of the CrOn/Cr and CrO*/Cr after sputtering with different times. (a) 0 s, (b) 10 s, (c) 20 s, (d) 30 s, (e) 40 s and (f) 70 s. The parameters used for deconvolution of XPS spectra and the corresponding results are presented in Table 1, Table 2.

Table 1 Binding enegy, full width at half maximum (FWHM) of the XPS spectra used for the deconvolution and the corresponding fitting parameters obtained for the main species presented in the prepared CrOn/Cr coating after sputtering with different times.

Sputtering time (s)		0	10	20	30	40	50	60	70
Cr (0)	Binding Energy (eV)	574.29	574.25	574.25	574.32	574.33	574.33	574.35	574.38
	FWHM (eV)	1.02	1.07	1.20	1.17	1.16	1.18	1.17	1.14
	Area	0.951	0.784	0.911	0.968	1.039	0.868	0.965	0.888
Cr (III)	Binding Energy (eV)	576.23	575.74	575.77	575.86	575.90	575.74	575.77	575.70
	FWHM (eV)	2.51	2.60	2.73	2.63	2.52	2.75	2.52	2.73
	Area	1.835	1.743	1.542	1.408	1.247	1.325	1.083	1.115
CrO_x	Binding Energy (eV)	577.93	577.94	577.94	578.07	578.13	578.05	577.94	578.00
	FWHM (eV)	2.13	2.13	2.28	2.13	2.08	2.18	2.16	2.17
	Area	0.643	0.445	0.347	0.311	0.287	0.311	0.285	0.233
∑x² (×10^-4)		6.615	6.597	11.554	8.574	6.538	10.966	9.671	9.194

Table 2 Binding enegy, full width at half maximum (FWHM) of the XPS spectra used for the deconvolution and the corresponding fitting parameters obtained for the main species presented in the prepared CrO*/Cr coating after sputtering with different times.

Sputtering time (s)		0	10	20	30	40	50	60	70
Cr (0)	Binding Energy (eV)	574.29	574.30	574.29	574.34	574.34	574.35	574.33	574.39
	FWHM (eV)	0.74	0.95	1.15	1.09	1.28	1.07	1.06	1.08
	Area	0.263	0.427	0.851	0.760	1.137	0.881	0.911	0.965
Cr (III)	Binding Energy (eV)	576.24	576.01	575.89	575.77	575.95	575.71	575.78	575.76
	FWHM (eV)	2.32	2.76	2.66	2.69	2.32	2.61	2.68	2.47
	Area	1.671	2.296	2.192	1.822	1.196	1.211	1.020	0.835
CrO_x	Binding Energy (eV)	577.95	577.94	578.10	578.07	578.08	578.07	578.16	578.05
	FWHM (eV)	2.63	2.40	2.11	2.20	2.07	2.16	2.01	1.88
	Area	0.993	0.696	0.472	0.413	0.328	0.269	0.189	0.165
Cr (VI)	Binding Energy (eV)	579.16
	FWHM (eV)	1.48
	Area	0.838
∑x² (×10^-4)		8.788	4.214	7.434	7.269	13.057	7.569	5.980	8.303

Fig. 6. ICRs of the specimens before and after potentiostatic polarization at +0.6 V for 3 h in 0.05 M H2SO4 + 2 ppm NaF solution.

Fig. 7. (a) Potentiodynamic and (b) potentiostatic polarization curves of the specimens at +0.6 V in 0.05 M H2SO4 + 2 ppm NaF solution.

Fig. 8. Electrochemical impedance spectra of the specimens in 0.05 M H2SO4 + 2 ppm NaF solution. The inset plot is the electrical equivalent circuit used for data fitting.

Table 3 Values of the components from the impedance fitting results of 304 SS, CrOn/Cr/SS, CrO*/Cr/SS, (CrO*/Cr)2/SS, and (CrO*/Cr)3/SS in 0.05 M H2SO4+2 ppm NaF.

Symbol	R_s (Ω cm²)	Q_dl (Ω^-1 cm^-2 sⁿ)	n_t	R_t (Ω cm²)	Q_f (Ω^-1 cm^-2 sⁿ)	n_f	R_f (Ω cm²)	Σx²
304 SS	2.865	5.135 × 10^-5	0.802	20.2	1.829 × 10^-4	0.959	3.579 × 10⁴	1.505 × 10^-3
CrO_n/Cr/SS	2.680	1.191 × 10^-4	0.835	14.8	5.557 × 10^-5	1	1.579 × 10⁵	1.822 × 10^-3
CrO*/Cr/SS	2.661	3.299 × 10^-5	0.853	12.5	5.158 × 10^-5	0.841	3.779 × 10⁵	2.888 × 10^-3
(CrO*/Cr)₂/SS	2.960	5.104 × 10^-5	0.934	19.4	2.061 × 10^-5	0.797	6.597 × 10⁵	2.212 × 10^-4
(CrO*/Cr)₃/SS	2.172	3.63 × 10^-6	0.901	10.6	5.061 × 10^-5	0.889	8.341 × 10⁵	2.010 × 10^-3

Fig. 9. Summary of the i0.6V, iss, and Rf derived from Fig. 6(a), (b), and Fig. 7, respectively.

Fig. 10. Surface morphologies of the specimens after 3 h of potentiostatic polarization at 0.6 V. (a) 304 SS, (b) CrOn/Cr/SS, (c) CrO*/Cr/SS, (d) (CrO*/Cr)2/SS, and (e) (CrO*/Cr)3/SS.

Fig. 11. Schematic plots of the effect of oxygen plasma on the coating and thus on the corrosion process.

Fig. 12. A comparison of ICRs of 304 SS, CrOn/Cr/SS, and CrO*/Cr/SS before and after 3 h of potentiostatic polarization. Schematic plots of the changes at the interface, including corrosion products formed on the specimen surface and oxidation of the coatings.

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