Investigation of Metal Cation Effects on Corrosion Behavior of Aluminum Alloys
Authors: Sakairi Masatoshi , Islam Md. Saiful , Li Li 
Published: December 2020

Introduction

Due to excellent mechanical property and corrosion resistance, aluminum alloys are widely used in airplanes, automobiles, and kitchen utensils. The corrosion resistance of aluminum alloys relies on highly protective and reformative passive films. However, the passive films are destroyed in presence of chloride ions, and the corrosion rate of aluminum alloys depends on the concentration of chloride ions 1 - 3. A few studies have shown the corrosion behavior of aluminum alloys in freshwater; however, the corrosion rates of aluminum alloys were changed in freshwater which sampled different place 4. Because freshwater also contains some metal cations in dilute concentration. The authors expected that the metal cations changed the corrosion rate of aluminum alloy in freshwater. The authors have been investigated the corrosion of A3003 aluminum alloy in freshwater focused on the effects of metal cations5 - 8. The corrosion rate of A3003 aluminum alloy is changed with metal cations in model freshwater despites the concentration of chloride ions, dissolved oxygen and pH are the same. The researches also clarified that Zn2+ is the best metal cation to inhibit the freshwater corrosion of A3003 among the metal cations used6, 7. An Auger spectroscope was used to investigate the corrosion morphology and composition of corrosion products of A3003 formed in model freshwater with different Zn2+ concentration8. From the cross-sectional AES point analysis, it has been clarified that the corrosion products have multi-layer structure.
However, it is not fully elucidated the role of the metal cations on the corrosion behavior of aluminum alloys. The purpose of this study is to investigate the effects of metal cations on corrosion behavior of aluminum alloys in aqueous environment.

Experimental

Sheets of aluminum alloys (A7075, A6061and A2024) were used as the specimens. The sheets were cut into 7 7 mm for immersion corrosion tests, and 10 10 mm for electrochemical measurements. The specimens were grounded with SiC abrasive paper. Before the tests, specimens were cleaned in highly purified water and in ethanol using ultrasonic bath.
Model solutions of 1 molm-3 NaCl (NaS), 0.5 molm-3 MgCl2 (MgS), and 0.5 molm-3 ZnCl2 (ZnS) were used in this experiment. The pH of the model solutions was maintained at around neutral.
Electrochemical tests were carried out in a three-electrode cell using a potentiostat. An 4 cm2 Pt plate and an Ag/AgCl-saturated KCl electrode were used as the counter and the reference electrodes. Before the measurements, the specimens were immersed in the solutions for 3.6 ks (1 h) at 298 K. Electrochemical impedance spectroscopy (EIS) tests were carried out in the frequency range from 1 mHz to 10 kHz, and the modulation amplitude of the applied sinusoidal wave was 10 mV.
Specimens were immersed in the model freshwater for 2.59 Ms (30 d) and 0.6 Ms (7 d) at 25 ºC. The mass of a specimen was measured before and after the test. Surface analysis were carried out by scanning electron microscope (SEM) and X-ray photoelectron spectroscope (XPS).

Results

From the immersion tests, the mass of specimens increased after immersion in the solutions and corrosion behavior of aluminum alloys was changed with the metal cations present in the solutions. Independent of used alloy, SEM images clearly showed the deposited corrosion products on the surface. Uniform deposition of corrosion products was observed on the specimen immersed in ZnS as compared to the NaS and MgS. EDS results demonstrated the presence of zinc-related products on the specimen immersed in ZnS XPS results suggested that zinc ions were incorporated in the oxide films of aluminum alloy and increased the corrosion resistance property of the oxide films. EIS results showed the highest impedance in ZnS as compared to the other solutions.

References

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4) Editorial board of Japan Aluminium Association: "Aluminum Handbook. Tokyo", Japan Aluminium Association, (2007), p 73.
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