Online ICP-OES for Studying the Alloying Effect of Zn and Cu on Corrosion of Al-10 Mass% Si Alloy
Authors: Takano Natsumi , Iwai Mana , Kitano Sho , Habazaki Hiroki , Nakagawa Ryogo , Yamamoto Dai , Fushimi Koji 
Published: December 2024

Introduction

Alloying elements in Al alloy have important effects on corrosion properties as well as strength, formability, brazinability, and other properties. This is mainly due to the low solubility of the alloying elements in Al, which leads to the formation of precipitates along with various structures [1]. Al-Si is one of the major Al alloys, particularly applied in automotive industry due to high castability and machinability despite being lightweight. However, various studies have highlighted their poor corrosion resistance, which shortens the lifetime of alloy products. Previous studies have extensively investigated the effect of Si particle size and morphology on corrosion, suggesting that Si particles act as a cathode and selectively dissolve the surrounding Al matrix [2]. On the other hand, the actual dissolved mass and the effects of other alloying elements on corrosion have not been sufficiently discussed because of the difficulty of detecting the mass of corrosion for each element.Recently, online inductively coupled plasma-optical emission spectroscopy (ICP-OES), which combines electrochemical measurements with solution analysis, has attracted attention as an innovative method for analyzing the dissolution reaction of alloys [3,4]. Elements dissolved into the electrolyte solution through electrochemical reactions are carried away by flow and introduced to ICP-OES, enabling simultaneous qualitative and quantitative analysis of dissolved elements. In this study, we apply it to measure the reaction rate of Al-Si alloy corrosion and elucidate the effect of additive elements on the corrosion.

Experimental

Alloys of Al-10 mass% Si, Al-10 mass% Si-3 mass% Zn, and Al-10 mass% Si-3 mass% Zn-0.5 mass% Cu were employed as specimens. Potentio-dynamic polarizations of specimens in a 0.1 mol dm-3 KCl aqueous solution and solution analyses using a SPECTRO ICP-OES apparatus (ARCOS) were conducted at a solution flowing rate of 1 cm3 min-1. In the OES, the dissolution rates of Al, Si, Zn, and Cu were detected at wavelength of 176, 288, 206, and 219 nm, respectively.

Results and discussion

Specimen alloys exhibited hypoeutectic, which is composed of a primary α-Al phase and a eutectic structure consisting of α-Al and Si particles. Added Zn and Cu in the alloy were melted in α-Al phase, especially eutectic α-Al phase. In the simple immersion test in KCl solution, the formation of a pit was observed on Al-10 mass% Si surface after several hundred seconds. In the polarization experiment in anodic direction, anodic current density increased with increasing electrode potential, a detection rate of Al (vAl ) and Si (vSi) was also increased with increasing the current density. At anodic potential, a ratio of vSi to vAl was significantly larger than a solid-solubility limit of 1.6%. This suggests natural oxide film on eutectic α-Al phase initiates breakdown easier than those on other phases and allows to dissolve eutectic α-Al more than primary α-Al. In the polarization in cathodic direction, the dissolution of Al, which was above the detection limit of ICP-OES, was detected. The detection rate of Al was strongly depended on pH or buffering effect of solution, indicating that alloy surface dissolves Al species in local alkaline environment. In the presentation, we discuss the alloying effects of Zn and Cu on corrosion of Al-Si alloy as well.

References

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[4] A. Fujimura, S. Shoji, Y. Kitagawa, Y. Hasegawa, T. Doi, K. Fushimi, Electrochim. Acta, 433, 141246 (2022).