Aluminum is one of the most important metals worldwide, and its alloys are highly used in today’s engineering and industrial applications. According to Das. [
1], 45 million tonnes of aluminum are produced annually, where 31% of the production corresponds to recycled aluminum. As a result, aluminum is the most recycled material and the second most used metal in the world [
1]. Specific and expensive aluminum alloys (e.g., those alloyed with Hg, Ga, Sn, and In) are used in the cathodic protection industry [
2]. The Al-Zn type, alloyed mainly with Hg (mercury) and In (indium), is the most efficient anode for cathodic protection against corrosion of structures exposed to marine environments. It is commonly accepted that aluminum corrosion resistance is mainly due to a crystalline oxide layer formation [
3]. However, most electrochemical research is focused on the corrosion behavior of unalloyed aluminum in NaCl solutions. The success of such aluminum anodes is that both Hg and In prevent the formation of a continuous adherent and protective oxide film on the surface alloy, thus enabling continuous galvanic activity of the aluminum [
4].
AlMg-Zn based alloys have raised strong attention since the 1980′s. They are widely used in aerospace applications and manufacturing of high-speed boats and submarines due to the unique combination of lightweight and high mechanical properties [
5]. The main corrosion form of this alloy system in seawater and NaCl solutions is pitting [
6,
7]. The AlMg-Zn alloy is characterized by a very heterogeneous microstructure, consisting of an aluminum solid solution matrix and various intermetallic phases; their mechanical properties are due to the presence of these particles [
8]. The outstanding importance of the microstructure and the influence of the intermetallic particles on the corrosion behavior were extensively discussed by Campestrini et al. [
9] and Vander Kloet [
10]. Different local corrosion processes such as pitting corrosion, crevice corrosion or intergranular corrosion can be enhanced by the presence of intermetallic particles with cathodic characteristics, given the existence of a galvanic coupling with the aluminum matrix. This process produces a local increase in the pH, giving rise to the dissolution of the oxide layer in the area surrounding the intermetallic particle. Once this layer has been dissolved, the local alkalinity causes an intense attack on the interface between the matrix and the particles, as well as a detachment of the particles from the pit. Thereby, the presence of intermetallic particles with cathodic characteristics is the origin of pitting corrosion of aluminum-magnesium alloys. Previous research has described this phenomenon, where the reduction of oxygen occurs as the cathodic reaction on the intermetallic particles [
11]. Parallel to the cathodic reaction, the anodic reaction is necessary in order for the passive layer to grow on the matrix and the thickening of this layer. On the other hand, Al matrix reaction with chloride ions can also evolve corrosion products surrounding the intermetallic particles [
12]. However, Barbucci et al. [
13] proposed the AlMg-Zn alloy as a promising alloy system to be studied for cathodic protection of structures exposed to marine environments, due to its low electrode potential, high current capacity, and the absence of Hg and In, which might pollute the sea [
13]. More recently, it was reported that the presence and amount of Mg in Al alloys is important for cathodic protection effect, since it is the most active metal in the galvanic series and will always be the active anode when it is in contact with other metals [
14]. The AlMg-Zn alloy system has a relatively complex equilibrium diagram. The first investigation of the entire system was carried out by Eger in 1913 [
15]. From then on, AlMg-Zn alloys have been widely studied due to their excellent mechanical properties reached after age hardening [
16]. Age hardening AlMg-Zn alloys show a combination of low density and high strength, and as a result have become the primary material used in aircraft and automotive industries. Gonzalez et al. [
17] published that the magnesium in AlMg-Zn alloys played an important role on the τ (Al
2Mg
3Zn
3) phase particle distribution in α-Al solid solution. This distribution can promote a good surface activation of the anode, avoiding the formation of the continuous, adherent, and protective oxide film on the alloy surface once it is in use.
This research aims to describe the electrochemical corrosion behavior in two as-cast AlMg-Zn alloys with increased dispersion of the τ phase in the matrix, through several aging treatments, that withdraw the fast, kinetic reactions occurring in solid state at 200 °C, after the solubilization treatments. Additionally, the effects of Mg addition on the τ phase distribution in the microstructure and galvanic efficiency of the AlMg-Zn alloys were investigated by SEM and optical microscopy.