Investigation of the Electrochemical Corrosion Property of 2xxx Series Cast Aluminium Alloy in 0.3 M, 0.6 M NaCl, and Seawater Environments

The electrochemical corrosion activities of 2xxx series Al alloys were investigated in NaCl of 0.3 M, and 0.6 M concentrations and seawater environments. Both Tafel polarization and electrochemical impedance spectroscopy (EIS) methods were followed to analyze the corrosion behavior. These results implied that corrosive attack is more aggressive in sodium chloride solution compared to seawater, as additional elements present in seawater are involved in forming various protective layers. Again, a higher concentration of NaCl solution damaged the alloy surface more drastically. The open circuit potential moved towards the nobler direction in the case of seawater environment, and 0.3 M solution for NaCl. The corrosion rate and corrosion current showed higher values in NaCl solution than in seawater. The surface morphologies of the alloys were characterized not only with an optical microscope but with a scanning electron microscope also. The scratch marks from polishing were removed after corrosion. Extensive damage to the surface was found in the NaCl environment, where 0.6 M solution created the most damage, which is evident by both optical and scanning electron micrography. A higher level of pitting corrosion occurred in NaCl than in seawater, identified by SEM images.


Introduction
Aluminum with copper as a major alloying element belongs to the 2xxx series aluminum alloy.2-7 wt.% of copper is alloyed to produce this type of alloy.A small amount of other alloying elements like magnesium, silicon, manganese, etc. are usually added to this type of alloy to improve strength [1].Aluminum alloys are demanded nowadays for their remarkable performance because they have a high strength-to-weight ratio, better weldability, appropriate hardenability, and enhanced corrosion resistance [2].Aluminum alloys are extensively used in lightweight automotive, aeronautics, aerospace, and fuel cell applications [3,4].Different properties of aluminum alloys with different chemical compositions corresponding to the applications are continuously being investigated to produce alloys with better strength, corrosion resistance, durability, longevity, and minimized cost [5].Aluminum alloys with high strength are especially important as most of the components where these alloys were being used have relative periodic movements with reduced fatigue lifetime [6].Moreover, these components are often exposed to various corrosive environments like salt, base, acid, bacterial environment, etc. that can corrode the materials away, accelerating the fatigue failure [7,8].Although these alloys show strong corrosion resistance by forming thin oxide films, when exposed to aqueous solutions, the alloys cannot resist corrosion that much [9,10].These alloys can be exposed to aqueous solutions like sodium chloride solution and seawater.The protective oxide film is depleted away in a heterogeneous way, resulting in pitting corrosion when the halide ions attack the surface, especially Cl- [11].There are HCO3-along with Cl-ions in seawater which degrades the alloy surface [12].The addition of alloying elements like Zr and Cu improves corrosion properties thus increasing the fatigue life of the components, thereby reducing material cost substantially.For electrochemical analysis, two characterization techniques are commonly used, electrochemical impedance spectroscopy and potentiodynamic polarization technique.In EIS, voltage and frequency data are fitted with analogous theoretical circuit elements to estimate the behavior of the electrochemical corrosion process.An AC voltage is applied to the terminals of an electrochemical cell and the current response is measured, and the phase shift and amplitude changes are recorded for a range of frequencies.In the potentiodynamic polarization technique, a potential difference is applied between the working and reference electrodes, and the produced current is measured, which represents the corrosion rate occurring on the exposed surface of the working electrode.The electrochemical corrosion behavior of aluminum alloys in different media is being investigated [13].Adams et al. conducted an experimental study adopting the polarization technique and microstructural characterization to understand the corrosion behavior of AA8011, AA4017, and AA1200 aluminum alloys exposed to nitric and acetic acid media and concluded that passive films that are formed in nitric acid are more stable than in acetic acid solution [14].Again, Cao et al. estimated the corrosion property of 2A02 Al alloy by electrochemical and microstructural analysis under an accelerated simulation marine environment and showed several corrosion product films like Al(OH)3, Al2O3, and AlCl3 [15].However, it is important to know how the 2xxx series aluminum alloy behaves when exposed to different concentrations of NaCl and seawater environment for proper material selection in designing relevant machine parts.Hence, this study aims to understand the corrosion behavior of 2xxx series cast Al alloy exposed to 0.3, 0.6 M NaCl and deep seawater by EIS, potentiodynamic polarization method, and microstructural analysis.

Material Preparation and Experimental Technique
The ingots of Al-50 wt.% Si master alloy, pure aluminum, magnesium, and copper were used for the study.The material composition of the prepared alloy is shown in Table 1.According to the composition shown in the table, the materials were weighed and melted in a graphite crucible on a natural gas-fired pit furnace.The molten metal was cast in a 20 mm × 200 mm × 300 mm sized mild steel mold.The mold was preheated to 250 °C.A flux cover was used to protect the alloy from oxidation.Inside a muffle furnace, the cast samples were homogenized for 12 hours at 450 °C.Then the samples were air-cooled to relieve stress.Later, they were kept at 535 °C for 120 minutes for solutionization.Afterward, the solutionized samples were subjected to machining to produce 5 mm × 5 mm × 12 mm rectangular samples.They were kept at 200 °C for 240 mins to gain the peak aged condition [13].The surfaces were polished with emery papers of 320, 600, 800, 1200, and 2000 grit sizes to get a smooth surface finish.The alloy composition presented in Table 1 was known from the optical emission spectrometer, Shimadzu PDA 7000.Three-electrode glass cells were prepared for both EIS and potentiodynamic polarization tests.Analytical reagent grade sodium chloride powder was dissolved in deionized water to get 0.3 M and 0.6 M NaCl solutions, and the seawater was collected from the Bay of Bengal, in the vicinity of Saint Martin's Island, Bangladesh.Three electrode electrochemical cells were prepared with 0.3 M, and 0.6 M NaCl solutions and seawater as electrolytes.The rectangular samples were used as working electrodes, wrapping the surfaces except for a 5 mm × 5 mm surface by PVC heat shrinkable tube.Platinum rod as counter electrode and as reference electrode Ag | AgCl was used.The electrochemical analyses were done with the CH Instruments -Electrochemical Workstation.The experimental impedance data were then modeled with an analogous circuit, which is shown in Fig. 1.The experimental setup for the potentiodynamic polarization technique is the same as the EIS analysis.The variation of electrode potential was between -1 V to +1 V and the scan rate was 1 mVs-1.The formula for corrosion rate is, EW = the equivalent weight, in g/equiv.
ρ = the density in g cm -3 A = the exposed surface area, cm 2   After corrosion, the microstructures of the affected surfaces were captured by a conventional digital microscope.The SEM images were taken by JEOL scanning microscope and an X-ray analyzer attached was used to generate the EDX spectra.

Electrochemical Impedance Spectroscopy
The Nyquist curves from the experimental data are shown in Fig. 1.The diagram has real portions of impedances along the x-axis, and imaginary portions along the y-axis, which is based on a capacitiveresistive semicircle model.The electrochemical corrosion of the 2xxx series Al alloy is governed by the charge transfer process, which is confirmed by the semi-circular shapes [16,17].The maximum value of Zreal is higher for the seawater environment than for the sodium chloride environment, which means that the seawater environment is quite protective against electrochemical corrosion.Among the two concentrations of NaCl, 0.3 M solution is found to be more corrosive as the maximum value of the real component of the impedance is the lowest.
Modeling the impedance data with the electric circuit shown in Fig. 1, the circuit components are generated, which are presented in Table 2, along with the open circuit potentials.The OCPs from Table 2 show a nobler value in seawater and then in 0.3 M NaCl between the two solutions of NaCl.A greater value of OCP generally indicates better corrosion resistance [18].Again, the polarization resistance is higher in the seawater environment than in the NaCl environment, and 0.6 M NaCl having the lowest value.As previous studies refer, the polarization resistance is inversely related to the corrosion rate [19].From all observations of EIS results and OCPs, it is apparent that the alloy shows the best corrosion performance in the seawater environment and the worst in the 0.6 M NaCl environment.Both seawater and sodium chloride solution contain a certain amount of chloride ions, which attack the surface of the alloy, break the oxide layer, and promote corrosion [11,20].However, several ions in seawater inhibit the corrosive attack [12].Aluminum and copper sulfate complexes are formed on the surface with the SO42-ions from the seawater and protect the surface from corrosion [21].Critical current passivation is lowered by copper nitrate ions formed with NO3-ions, present in the seawater [22].

Potentiodynamic Polarization Analysis
The Tafel plots obtained from the experimental data are displayed in Fig. 4. The parameters of corrosion derived from the potentiodynamic polarization tests are furnished in Table 3. Icorr denotes corrosion current, the dissolution current at the corrosion potential.Ecorr denotes the corrosion potential, at which the potential value of the anodic dissolution rate becomes the same as the cathodic reaction rate, meaning that the electrode is not conducting any current [24].When metal or non-metal surfaces are exposed to electrolyte solutions, the ability to lose electrons is expressed by the corrosion potential, Ecorr.The corrosion rate is denoted in mm year-1, the corrosive loss in thickness of metal per year.From the data Table 3, both the corrosion current and rate have higher values in NaCl solution than in seawater, as the Cl-attacks the surface more aggressively in sodium chloride solution than in seawater.The seawater serves protection by forming various films.The corrosion potential also shifts towards a positive direction in the seawater environment, indicating better corrosion resistance [25].A higher corrosion rate is found in the case of 0.6 M NaCl solution than the 0.3 M NaCl solution as there is an excess amount of Cl-ions in 0.6 M NaCl solution.

Optical Micrographic Investigations
The optical micrographic images of the 2xxx series aluminum alloy in three different media are shown in Fig. 5.Not very much information is discernable from this type of unetched microstructures.

SEM and EDX Observation
The SEM images of the surface of the alloy before and after corrosion in 0.  From the EDX analysis, it is observed that the percentage of Mg is highest in the seawater environment, as an additional layer of MgO is formed there.However, the MgO layer formed on the surface under 0.6 M NaCl is depleted away at different sites as more Cl-attacks.The Cl content shows that there is minimum Cl content on the surface under seawater, as the protective layers defend against the aggressive attacks of Cl-ions.The chlorine content is found maximum in the case of 0.6 M NaCl solution, as the concentration of chloride ion is higher.Some additional particles like Na, S, Br, K, and Ca are found in the sample under a seawater environment.

Conclusion
The corrosion properties of Al-Cu-Mg cast alloy in 0.3 M, 0.6 M NaCl, and seawater environments were investigated using electrochemical measurements, optical, SEM micrography, and EDX analysis.The summary of the results of the investigation is, • The alloy in seawater environments has shown a more positive value of OCP, higher polarization resistance, and lower corrosion current and corrosion rate.As ions like SO42-, NO3-, Na+, K+, Mg2+, etc. are present in seawater, they are involved in forming protective films on the surface.• The alloy in sodium chloride environments has displayed lower OCP, Rp, higher corrosion current, and rate as the attack from chloride ions is not protected that much as seawater.Again, a higher concentration of Cl-has resulted in a higher rate of corrosion.• The optical and SEM microstructures show that the corrosive attack is more devastating in the NaCl environment, especially in 0.6M NaCl environment, evidenced by an extensive number of microcracks and pinholes.A mixed layer is observed on the alloy surface after corrosion in seawater.
• The EDX analysis proves that there was reduced damage created by Cl-ions in the case of seawater environment, whereas more amount of Cl was found on the surface corroded in sodium chloride environment.Also, Mg content is highest in the case of seawater environment, depicting a thicker layer of MgO.

Figure 1 .
Figure 1.The analogous electric circuit for modeling

Figure 2 .
Figure 2. The Nyquist plots for the 2xxx series Al alloy in 0.3 M, 0.6 M, and seawater environments respectively

Figure 3 .
Figure 3.The Bode (a) magnitude, and (b) phase plots for the 2xxx series Al alloy in 0.3 M, 0.6 M, and seawater environment respectively

Figure 4 .
Figure 4.The Tafel plots for the 2xxx series Al alloy in 0.3 M, 0.6 M, and seawater environments respectively

Fig. 5
(a)   shows the uncorroded surface of the alloy, where polish marks are visible.The protective oxide films are visible on the surfaces in Figs.5(b) to (d), where Fig. 5(d) demonstrates different tones for different type of layers.Fig. 5(b) for 0.3 M NaCl shows a comparatively more compact and homogeneous oxide layer than Fig. 5(c) for 0.6 M NaCl.The most severe degradation is notable in the case of the surface exposed to 0.6 M NaCl.The increased amount of Cl-attacks the surface at various locations.Minimum surface demolishment occurs under a seawater environment, as there are MgO layers, complexes of sulfate and nitrate, and due to the Na and K elements in the oxide layer.

Figure 5 .
Figure 5. Optical microstructures of the polished Al 2xxx series alloy (a) before corrosion, after corrosion in (b) 0.3 M NaCl, (c) 0.6 M NaCl, and (c) seawater environments.

Figure 6 .
Figure 6.SEM images of the polished Al 2xxx series alloy (a) before corrosion, after corrosion in (b) 0.3 M NaCl, (c) 0.6 M NaCl, and (c) seawater environments

Table 2 .
EIS Test Results and ocps Environment OCP, V vs. SCE R p / Ω R s / Ω C p(eff)

Table 4 .
Surface Compositions According to EDX