Corrosion analysis of Al–Zn–In–Mg–Ti–Mn sacrificial anode alloy

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Abstract

The corrosion behaviour of Al–5Zn–0.02In–1Mg–0.05Ti–0.5Mn (wt%) alloy was investigated by SEM, EDX and TEM. The results show that there exist different corrosion types of the alloy in 3.5% NaCl solution with immersion time. At the initial stage of immersion, pitting predominate the corrosion around precipitates. The major precipitates of the alloy are MgZn2 and Al6Mn particles. The corrosion potential of the bulk MgZn2 alloy is negative with respect to that of the α-Al and the bulk Al6Mn alloy, so the MgZn2 precipitate can act as activation centre, make the aluminum oxide film breakdown and cause the pitting corrosion. In the late stage, it turns out to be a uniform corrosion due to the control of dissolution precipitation of In and Zn ions.

Introduction

Aluminium alloys are widely used to cathodic protect due to their high current efficiency, low density and good economy. From commercially aluminium sacrificial anodes, there is little doubt today that Al–Zn–In base anodes present the best performance in seawater [1]. The activator elements (Zn, In) ensure a uniform dissolution of the anodes and avoid polarization. Yet high current efficiency of material is welcome. So many researches pay attention to improving the electrochemical properties of Al–Zn–In alloys [2], [3]. Some alloy elements are usually added, which can be very effective [4], [5]. The Al–Zn–In–Mg–Ti series anodes have become popular in China [6], [7], [8]. In order to further improve the electrochemical properties of the alloy, it is necessary reducing the harmful effects of impurity Fe element. Mn element can be come into being Al6 (Mn, Fe) in the alloy and the potential of Al6 (Mn, Fe) is almost the same with that Al based, thereby reducing the harmful effects of the impurities Fe [9], [10]. Thus the Al–5Zn–0.02In–1Mg–0.05Ti–0.5Mn sacrificial anode is developed.

Numerous studies have been carried out to study the corrosion behaviour of Al–Zn–In alloys and find the activation action of indium and zinc in the alloys [11]. The effects of Zn and In on Al activation was explained taking into account the displacement reactions that produced In accumulation and preferential Zn dissolution [12]. For Al–Zn–In–Mg–Ti–Mn sacrificial anode, the corrosion behaviour is expected to be related to the multielement and the distribution in the microalloy alloy. The elements may be either in solid solution or may be segregated as second-phase particles, intermetallic compounds or inclusions. Both the initiation of attack and the propagation of dissolution depend on local zones where the enrichment of the alloying elements occurs [13].

Aluminum is the best choice for the fabrication of sacrificial anodes to prevent corrosion of steel structures. Nevertheless, corrosion of aluminum occurs through pitting rather than uniform corrosion. This is due to the natural immediate formation of passive layer on Al surface. The oxide film contains micro-structural defects that may be leading to the film breakdown [14], [15]. The existence of highly conductive defective sites that are responsible for the observed electrochemical activity of film, leads to its breakdown and localized corrosion [16]. Pit corrosion of Al alloy results to the localized penetration of ions through the film at the micro-structural defect sites when Al alloy is exposed in aggressive electrolytes [17]. These are the case for pit initiation. Al–Zn–In–Mg–Ti–Mn sacrificial anode possesses excellent electrochemical properties, but there is little research about its corrosion behaviour. This work explored the corrosion behaviour of Al–Zn–In–Mg–Ti–Mn the alloy in 3.5% NaCl solution to gain a better understanding of the corrosion mechanism.

Section snippets

Material preparation

The nominal compositions of alloy in present experiment are 5 wt% Zn–0.02 wt% In–1 wt% Mg–0.05 wt% Ti–0.5 wt% Mn–Al. Raw materials are commercial pure aluminium, zinc, indium, magnesium, titanium, manganese ingots (>99.9%) for casting the above anode alloy. Raw material ingots were cut, dried, weighed the required amount of materials and melted in a graphite crucible in ZGJL0.01-4C-4 vacuum induction furnace under argon atmosphere at 760 °C. The molten alloy was poured in a preheated cast iron dye of

Microstructure and phase analysis

Fig. 1 shows a semiquantitative EDX analysis on the alloy surface. The EDX result confirms that the α-Al matrix at a location and the grain boundaries at position b all contain Al, Zn, and Mg elements, but the precipitates at position c contain Al, Zn, Mg and Mn elements. The amount of In and Ti added in the alloy is so small that the phases containing In or Ti phases cannot be detected by EDX.

Fig. 2(a) shows a TEM image of Al–Zn–Mg–In alloy. The precipitates appear as dark spot and could be

Conclusions

The corrosion behaviours of Al–Zn–Mg–In alloy after different immersion times in 3.5% NaCl solution were developed using SEM and EDX.

At the initial stage of immersion, some pits appear on the alloy surface. It indicates that there exists pitting corrosion. The main precipitates in the alloy are MgZn2 and Al6Mn phase. The Ecorr of the MgZn2 precipitates is negative with respect to that of the α-Al, so the precipitates can act as corrosion center and result in the pitting corrosion. With increase

Acknowledgement

This work was supported by Technology Creative programmer of Henan for Excellent Talents (grant no. 094200510019) and the Natural Science Foundation of Henan Province, China (grant no. 092300410132).

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