Electrochemical Impedance Spectroscopy Behavior of Aluminum Alloys 2024 and 6061 in Rainwater

In the present work the electrochemical resistance spectroscopy behavior of aluminum alloy 2024 and 6061 in rainwater, was studied before and after solution heat treatment at room temperature (25 ° C). If the resistance decreases, the corrosion becomes faster, and vice versa. The equivalent circuits was of type Constant Phase Element (CPE) and the highest resistance value gets for alloy 2024 aged at 250 ° C for 2 hours, and the highest resistance value was for alloy 6061 aged at 250 ° C for 1 hour. As for the values of capacitance, the highest amplitude value was for alloy 2024 aged at 300 ° C for two hours, and the highest value gets for capacity for alloy 6061 aged at 150 ° C for two hours.


Introduction
There are two Al alloy types considered, such as the wrought and cast aluminum alloys [1].In general, the heat treatment indicates any cooling and heating processes, which are carried out to change the metallurgical structure, mechanical characteristics, or residual stress state of the metallic product. In the case where the term has been applied to the Al alloys, nonetheless, its utilization is often restricted to particular processes which are utilized for increasing the hardness and the strength of the precipitation-hardenable cast and wrought alloys [2]. Solution heat treatment of the Al alloys provides the possibility for maximal hardening solute concentration for dissolving to the solution. Such a process is often performed through heating alloy to the temperature where there is a single, solid phase. By performing such a process, the atoms of the solute which have been, at rst, a portion of a 2 phase solid solution are dissolved to solution and produced a single phase. As soon as the alloy is heated to the suggested temperature of solutionizing, it will be quenched at a fast rate in a way that the atoms of the solute have no su cient time for precipitating out of that solution. Due to the quenching, a supersaturated solution is formed between the Al matrix and the solute [3,4].In heat treatment of the solution, this alloy has been heated up to the temperature which is su ciently high for putting soluble alloying elements into the solution. Post held at the treating temperature of the solution for su cient time for the diffusion of the atoms of solute to the matrix of the solvent to take place, it's quenched to a lower degree of the temperature (for example, room temperature) for keeping the elements of alloying trapped within the solution.
Throughout the aging, the elements of the alloying are trapped in the precipitate of the solution for forming an even distribution of highly ne particles. Some of the alloys of Al are hardened at room temperature in a few days in a process which is referred to as natural aging whereas others will be aged arti cially through the heating to the intermediate temperatures, in the case where it has been aged through the re-heating to an intermediate degree of the temperature, it will be designated as T6 condition (which indicates that the solution is heat-treated and aged arti cially) [5].The goal of the solution heat treatment of Al is obtaining maximal hardening solute concentration, like Zn, Mg, and Cu, in the solution through heating alloy to a degree of the temperature where the single-phase is going to be produced.
Through performing this process, the atoms of the solute which has been, at the rst portion of a 2-phase solid solution have dissolved back to solution and produce a single phase which is in the equilibrium. As soon as the alloy was held for a long time to ensure the full solutioning and the homogenous state, it's quenched fast, for the atoms of solute not having a su cient amount of time for the precipitation out of the solution. As a result, there is a supersaturated solution between the Al matrix and solute [6,7]. All non-heat-treatable alloys have a high resistance to general corrosion. Heattreatable wrought alloys have a signi cantly lower resistance to general corrosion. These include all 2xxx series alloys (Al-Cu, Al-Cu-Mg, Al-Cu-Si-Mg) in which copper is a major alloying element. As a result of the presence of copper in this type of alloy, the resistance decreases and therefore preventive measures must be taken [8]. However, those from the 6xxx series, which are medium strength alloys based on the quasi-binary Al-Mg2Si system, provide a high overall corrosion resistance equal to or close to a non-heat-treatable alloy. Aluminum alloys (2000 series) that contain copper as a major element used in alloying in general have less corrosion resistance than those alloys that do not contain these elements. For this reason, it is usually di cult to prevent corrosion of aluminum alloys in this series [9]. Corrosion in aluminum and its alloys, it is usually of a local nature and is most commonly caused by pitting or in areas that come into contact with dissimilar minerals in a conducive environment (sea water or road splash containing dissolving salts) [10]. (EIS) it is a distinctly touchy de nition technique used to non-destructively evaluate the electrical reaction of chemical structures. EIS systems re ect the chemical system's time reaction using alternating current (AC) voltages of low amplitude over a range of frequencies. Using an electrode setup including an operating, reference, and counter electrodes a known voltage is passed voltage of the counter electrodes is passed through an electrolytic solution from the working electrode and into the counter electrode. Quantitative measurements are provided through the EIS and allow the evaluation at the electrode interface and in the electrolytic solution of small-scale chemical mechanisms. Therefore, in the study of batteries, corrosion, and so on, EIS is useful in evaluating a wide variety of dielectric and electrical characteristics of components [11].

Aluminum alloys
The materials used in this study are aircraft aluminum alloys of a certain grade (2024 and 6061). The chemical compositions of these alloys were analyzed with spectromax, and the results are reported in " Table 1." The results were found to be consistent with the standard values [12]. This research was carried out in Rain Water. The plate was cut using a water jet to match the dimensions of the sample holder. The disc-shaped specimens are (φ25 X 2 mm) in size.

Water media
Rainwater was used as a corrosive medium in this study to see how solution treatment affected the corrosion and hardness of both 2024 and 6061 aircraft aluminum alloys. At room temperature, the work was completed (25 o C). With a salinity of 37.0 (milligrams per liter). The results of the chemical analysis of rainwater media are shown in Table 2.  (3) for both AA2024 and AA6061.From gure (1) and gure (2) by comparing the resistance for AA2024 and AA6061 it is clear that, In AA6061 the resistance is higher than AA2024 in rainwater.  (5) show the resistors and capacitors that had been gained from the Nyquist plots by using CPE t for both AA2024 and AA6061.
In Fig. (4) shows EIS test results of AA2024 alloy after heat treatment in rainwater for AA2024 of temperature   In Fig. (7) shows EIS test results of AA6061 alloy after heat treatment in rainwater for AA6061 of temperature    EIS test results of AA6061 alloys before solution heat treatment at rainwater Figure 3 shown a model of Constant Phase Element (CPE) (Rp= medium resistance, Ru= metal resistance, Y0= C = the capacitance, a=generally 0.9-1.0 (a=1 for an ideal Capacitor)

Figure 4
Page 10/12 shown Nyquist plot of AA2024 after solution heat treatment and aging at temperatures (150, 200, 250, and 300 oC) for 1hr in rainwater