Effect of Al and Ni Contents on Anodized Oxide Film and Pitting Corrosion Resistance of Alcocrfeni High-Entropy Alloys
Authors: Pao Li , Muto Izumi , Sugawara Yu 
Published: December 2020

As a new alloy design concept, high-entropy alloys (HEAs) exhibit great mechanical and corrosion resistant properties. In particular, Al-containing HEAs have great potential to develop high corrosion resistant alloys by anodizing. In this research, to investigate the effect of alloy composition on electrochemical properties after anodizing, Al and Ni contents were changed from equimolar AlCoCrFeNi HEA, and Al25Co14Cr10Fe19Ni32 was made. Equimolar CoCrFeMnNi HEA was used as a comparison. Anodizing was performed in 1 M H2SO4, and the compositions of surface oxide films on these alloys were analyzed by Auger electron spectroscopy (AES). The corrosion resistance to pitting was studied by electrochemical polarization in 0.1 M NaCl solution.

Equimolar AlCoCrFeNi, Al25Co14Cr10Fe19Ni32, and equimolar CoCrFeMnNi, were fabricated by arc-melting, and melting atmosphere was Ti-gettered high purity argon. The ingot was re-melted at least three times to improve chemical homogeneity. Fabricated ingot was hot-rolled at 1200 ℃ to reduce the thickness to 2 mm, and heat-treatment was then performed by raising the temperature to 1200 ℃ at a rate of 20 ℃/min in 1 h, held at 1200 ℃ for 2 h, and water quenched. The crystal structures were analyzed by X-ray diffraction spectroscopy (XRD) to ensure whether solid solution structures were formed. Anodizing was done by polarization from -0.55 to 0.5 VSCE, which is in the passivation region, at a scan speed of 23 mV/min in 1 M H2SO4 at 25 ℃. Microstructures of anodized and non-treated samples were observed by an optical microscope and scanning electron microscope (SEM). The depth profiles of the anodized and air-formed passive films were analyzed by AES. In 0.1 M NaCl at 25 ℃, pitting corrosion resistance was investigated by measuring the potentiodynamic polarization curves of samples with and without anodizing. To identify the initiate sites of the pitting corrosion, in situ observations were conducted during electrochemical polarization. The size of the electrode area was approximately 100 μm × 100 μm. Pt was used as the counter electrode, and an Ag wire with AgCl on the tip was used as the reference electrode. The calibration of the reference electrode was made every time before measurements.

On the basis of the XRD results and surface images, both equimolar AlCoCrFeNi alloy and Al25Co14Cr10Fe19Ni32 alloy had a duplex structure includes BCC and FCC phases: one is Al and Ni rich; the other is Cr and Fe rich. In Al25Co14Cr10Fe19Ni32 alloy, the area of Al and Ni rich phase is larger than that of Cr and Fe rich phase. The results of AES analysis indicate that the surface oxide layer on the Al and Ni rich phase is slightly thicker than that on the Cr and Fe rich phase in both Al-containing HEAs. However, the morphology after electrochemical polarization tests shows that the Al and Ni rich phase is more readily dissolved in 0.1 M NaCl in both Al-containing HEAs, and this may be the reason why Al25Co14Cr10Fe19Ni32 alloy has a lower pitting potential than equimolar AlCoCrFeNi alloy.

From SEM images of Al-containing HEA after anodizing, the Al and Ni rich phase was oxidized comparing to Cr and Fe rich phase. In Al and Ni rich phase, the AES depth profile shows that the concentration of Al and Ni sharply decreased after anodizing. On the other hand, the depth profile of Cr and Fe rich phase did not show much difference even after anodizing. Polarization curves of both alloy after anodizing were measured in 0.1 M NaCl, and there was no pitting observed. In contrast, crevice corrosion occurred between the epoxy covering and base metal. It could be concluded that the pitting corrosion is inhibited by anodizing in 1 M H2SO4 for both equimolar AlCoCrFeNi and Al25Co14Cr10Fe19Ni32 HEAs.