Corrosion Investigation by Scanning Electrochemical Microscopy of AISI 446 and Ti-Coated AISI 446 Ferritic Stainless Steel as Potential Material for Bipolar Plate in PEMWE

doi:10.1007/s40195-023-01653-1

Abstract

Abstract:

The components of proton exchange membrane water electrolysers frequently experience corrosion issues, especially at high anodic polarization, that restrict the use of more affordable alternatives to titanium. Here, we investigate localized corrosion processes of bare and Ti-coated AISI 446 ferritic stainless steel under anodic polarization by scanning electrochemical microscopy (SECM) in sodium sulphate and potassium chloride solutions. SECM approach curves and area scans measured at open-circuit potential (OCP) of the samples in the feedback mode using a redox mediator evidence a negative feedback effect caused by the surface passive film. For the anodic polarization of the sample, the substrate generation-tip collection mode enables to observe local generation of iron (II) ions, as well as formation of molecular oxygen. For the uncoated AISI 446 sample, localized corrosion is detected in sodium sulphate solution simultaneously with oxygen formation at anodic potentials of 1.0 V vs. Ag/AgCl, whereas significant pitting corrosion is observed even at 0.2 V vs. Ag/AgCl in potassium chloride solution. The Ti-coated AISI 446 sample reveals enhanced corrosion resistance in both test solutions, without any evidence of iron (II) ions generation at anodic potentials of 1.2 V vs. Ag/AgCl, where only oxygen formation is observed.

Key words: Ferritic stainless steel, Corrosion, Ti coating, Scanning electrochemical microscopy (SECM), Magnetron sputtering, Proton exchange membrane water electrolysis (PEMWE)

Andrea Kellenberger, Nicolae Vaszilcsin, Mircea Laurentiu Dan, Ion Mitelea, Alexandru Adrian Geana, Sigrid Lædre, Corneliu M. Craciunescu. Corrosion Investigation by Scanning Electrochemical Microscopy of AISI 446 and Ti-Coated AISI 446 Ferritic Stainless Steel as Potential Material for Bipolar Plate in PEMWE[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(4): 607-619.

Figures/Tables 14

Table 1 Elemental composition of tested and standard AISI 446 stainless steel

Stainless steel	C	Cr	Mn	Si	Al	P	S	N	Fe
AISI 446	0.14	24.90	1.08	0.72	-	0.032	0.026	0.18	Bal.
AISI 446 standard	≤ 0.20	23.00-27.00	≤ 1.50	≤ 1.00	-	≤ 0.040	≤ 0.030	≤ 0.25	Bal.

Fig. 1 SEM image of the Ti film deposited on the surface of the AISI 446 ferritic stainless steel a; thickness measurement of the Ti layer with the Dektak II profilometer showing a thickness of about 5675 Å b

Fig. 2 Cyclic voltammogram of Pt probe in 0.1 M Na2SO4 + 5 mM K3Fe(CN)6 solution far away from the AISI 446 substrate. Scan rate 20 mV/s

Fig. 3 Schematic representation of the reaction model for AISI 446 during SECM measurement in the feedback mode, in the presence of ferricyanide redox mediator, depending on the tip-substrate distance

Fig. 4 Schematic representation of the reaction model for AISI 446 during SECM measurement in the SG-TC mode, depending on the substrate potential

Fig. 5 SECM normalized Z-approach curves measured on the 25 μm Pt tip above: a the uncoated AISI 446 and b the Ti-coated AISI 446 substrate. Conditions: 0.1 M Na2SO4 and 0.1 M KCl solutions with 5 mM K3Fe(CN)6, tip potential Etip = − 0.25 V vs. Ag/AgCl, sample potential, Esample = OCP, scan velocity 50 μm/s, step size 10 μm

Fig. 6 SECM area scans obtained in: a 0.1 M Na2SO4 + 5 mM K3Fe(CN)6 solution and b 0.1 M KCl + 5 mM K3Fe(CN)6 solution, with the Pt tip placed at a large distance away from the AISI 446 substrate and at 20 μm distance from AISI 446. Conditions: tip potential Etip = − 0.25 V vs. Ag/AgCl, sample potential Esample = OCP, scan velocity 50 μm/s, step size 50 μm. Feedback mode

Fig. 7 Potentiodynamic polarization curves of AISI 446 and Ti-coated AISI 446 in 0.1 M Na2SO4. Scan rate 1 mV/s

Fig. 8 SECM line scans obtained in 0.1 M Na2SO4 at 20 μm distance from the substrate: a, b AISI 446 and c, d Ti-coated AISI446. Probe current (left) and sample current (right) were simultaneously recorded at different potentials of the substrate. Conditions: tip potential Etip = 0.6 V vs. Ag/AgCl, scan velocity 50 μm/s, step size 25 μm. Substrate generation-tip collection mode

Fig. 9 SECM area scan of uncoated AISI 446 in 0.1 M Na2SO4 solution with the tip biased at E = 0.6 V and the AISI 446 substrate at a OCP, b 0.2 V vs. Ag/AgCl, c 0.5 V vs. Ag/AgCl and d 1.0 V vs. Ag/AgCl. Conditions: substrate tip distance is 20 µm, scan velocity 50 μm/s, step size 50 μm. Substrate generation-tip collection mode

Fig. 10 SECM area scan of Ti-coated AISI 446 in 0.1 M Na2SO4 solution with the tip biased at E = 0.6 V and the Ti-AISI 446 substrate at a OCP, b 0.2 V vs. Ag/AgCl, c 0.5 V vs. Ag/AgCl and d 1.2 V vs. Ag/AgCl. Conditions: substrate tip distance is 20 µm, scan velocity 50 μm/s, step size 50 μm. Substrate generation-tip collection mode

Fig. 11 SECM line scans of uncoated AISI 446 in 0.1 M KCl solution with the tip biased at E = 0.6 V and the AISI 446 substrate at a OCP and b polarized at 0.2 V vs. Ag/AgCl. Conditions: substrate tip distance is 20 µm, scan velocity 50 μm/s, step size 25 μm. Substrate generation-tip collection mode

Fig. 12 SECM area scan of uncoated AISI 446 in 0.1 M KCl solution with the tip biased at E = 0.6 V and the AISI 446 substrate at a OCP and b polarized at 0.2 V vs. Ag/AgCl. Conditions: substrate tip distance is 20 µm, scan velocity 50 μm/s, step size 50 μm. Substrate generation-tip collection mode

Fig. 13 SECM area scan of Ti-coated AISI 446 in 0.1 M KCl solution with the tip biased at E = 0.6 V and the Ti-AISI 446 substrate at a OCP, b 0.2 V vs. Ag/AgCl, c 0.5 V vs. Ag/AgCl and d 1.2 V vs. Ag/AgCl. Conditions: substrate tip distance is 20 µm, scan velocity 50 μm/s, step size 50 μm. Substrate generation-tip collection mode

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