Al-Si hypoeutectic alloys produced by casting are mostly used in the automotive industry, especially for engine blocks. They have the advantage of low weight associated with low coefficient of thermal expansion and excellent mechanical properties. The corrosion resistance of these alloys in coastal area, particularly in seawater environment is not well known. In this investigation, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation have been used to evaluate the corrosion resistance of Cu free and 0.5wt% Cu content Al-6Si-0.5Mg alloy in simulated seawater environment. The 0.5wt% Cu addition to the Al-6Si-0.5Mg alloy showed that Cu decreased susceptibility to electrochemical corrosion compared to the Cu free Al-6Si-0.5Mg alloy. The magnitude of open circuit potential (OCP), corrosion potential (E_corr) and pitting corrosion potential (E_pit) of Al-6Si-0.5Mg alloy in simulated seawater were shifted to the more noble direction due to 0.5wt% Cu addition and thermal modification.

Introduction

Owing to their light weight, suitable strength and strong resistance to corrosion, aluminum alloys are used in a broad spectrum of engineering applications. The corrosion resistance of aluminum is attributed to an exceptionally stable oxide film that forms on its surface. This film is resistant to attack from water and oxygen in a wide range of temperatures and pH levels, making aluminum alloys useful in a variety of environments [1]. However, the presence of aggressive ions like chloride creates extensive localized attack [2]. In addition, it is well known that the corrosion behavior of aluminum alloys is significantly affected by the presence of particles in the matrix. Particles that contain Al, Cu and Mg tend to be anodic relative to the alloy matrix, while those that contain Al, Cu, Fe and Mn tend to be cathodic relative to the matrix [3].

The increasing demand from many industries for improved properties in materials has stimulated the development of new materials. For the automotive industry, the properties most required are reduced weight, low thermal expansion coefficient and excellent mechanical properties; mainly wear resistance at high temperatures. In this context, various new materials such as the Al-Si alloys have been considered. Despite the excellent mechanical and physical properties of the Al-Si-Mg alloys, their corrosion resistance in aggressive environments is not yet well known. In recent years some work has been carried out to evaluate the corrosion resistance of these alloys in different media [4-7].

The alloys containing copper are the least resistant to corrosion [18]; but this can be improved by coating each side of the copper containing alloy with a thin layer of high purity aluminium, thus gaining a three ply metal (Alclad). This cladding acts as a mechanical shield and offers sacrificial protection [9].When aluminum surfaces are exposed to atmosphere, a thin invisible oxide (Al₂O₃) skin forms, which protects the metal from further corrosion in many environments [8]. This film protects the metal from further oxidation unless this coating is destroyed, and the material remains fully protected against corrosion [9]. A number of studies have been carried out to assess the effect of Cu content and the distribution of second phase intermetallic particles on the corrosion behavior of Al alloys. The distribution of Cu in the microstructure affects the susceptibility to localized corrosion. Intergranular corrosion (IGC) is generally believed to be associated with Cu containing grain boundary precipitates and the precipitates free zones (PFZ) along grain boundaries [13-15]. In heat treatable Al-Si-Mg(-Cu) series alloys the susceptibility to localized corrosion [pitting and / or intergranular (IGC)] and the extent of attack are mainly controlled by the type, amount and distribution of the precipitates which form in the alloy during any thermal or thermomechanical treatment performed during manufacturing processes [11-15].

The composition of an alloy and its thermal treatment are important do determine the susceptibility of the alloy to corrosion [10, 19]. Depending on the composition of the alloy and parameters of the heat treatment process, precipitates form in the bulk of the grain, or in the bulk as well as grain boundaries. As indicated by several authors, the precipitates formed by heat treatment in Al-Si-Mg alloys containing Cu are the θ (Al₂Cu) Q-phase (Al₄Mg₈Si₇Cu₂), β-phase (Mg₂Si) and free Si if Si content in the alloy exceeds the Mg₂Si stoichiometry [16-18].

In this work, the authors present a study on the electrochemical corrosion behavior of Al-6Si-0.5Mg alloy with low Cu content. The main attention is focused on the role on the Cu content precipitates and intermetallic phases, which are anodic with respect to the aluminum matrix and may improve, to a certain extent, the corrosion resistance of the Al-6Si-0.5Mg alloy.

Experimental Procedure

Materials preparation

The Al-6Si-0.5Mg(-0.5Cu) alloys were prepared by melting Al-7Si-0.3Mg (A356) alloys and adding Al, Cu and Mg into the melt. The melting operation was carried out in a gas fired clay graphite crucible furnace and the alloys were cast in a permanent steel mould. After solidification the alloys were homogenised (500^oC for 24hr), solution treated (540^oC for 2hr) and finally artificially aged (225^oC for 1hr). After heat treatment rectangular samples (30mm x 10mm x 5mm) were prepared for metallographic observation and subsequent electrochemical test. Deionized water and analytical reagent grade sodium chloride (NaCl) were used for the preparation of 0.1M solution (simulated seawater). All measurements were carried out at room temperature.

Potentiodynamic Polarization Measurements

A computer-controlled Gamry Framework TM Series G 300™ and Series G 750™ Potentiostat/ Galvanostat/ZRA was used for the electrochemical measurements. The Potentiodynamic polarization studies were configured in cells, using three-electrode assembly: a saturated calomel reference electrode, a platinum counter electrode and the sample in the form of coupons of exposed area of 0.50cm² or 10mm x 5mm as working electrode. Only one 10mm x 5mm surface was exposed to the test solution, the other surfaces being covered with Teflon tape. The system was allowed to establish a steady-state open circuit potential (OCP). The potential range selected was -1 to +1V and measurements were made at a scan rate of 0.50mV/s. The corrosion current (I_corr), corrosion potential (E_corr), pitting corrosion potential (E_pit) and corrosion rate (mpy) were calculated from Tafel curve. The tests were carried out at room temperature in solutions containing 0.1M of NaCl at a fixed and neutral pH value. The corroded samples were cleaned in distilled water and examined under optical light microscope (OLM) and scanning electron microscope (SEM).

Electrochemical Impedance Measurements

As in potentiodynamic polarization test, three electrode cell arrangements were also used in electrochemical impedance measurements. Rectangular samples (10mm x 5mm) were connected with copper wire and adopted as working electrode. EIS tests were performed in simulated seawater at room temperature over a frequency range of 100kHz to 0.2Hz using a 5mV amplitude sinusoidal voltage. The 10mm x 5mm sample surface was immersed in simulated seawater (corrosion medium). All the measurements were performed at the open circuit potential (OCP). The test cells were maintained at room temperature and the NaCl solution was refreshed regularly during the whole test period. The impedance spectra were collected, fitting the experimental results to an equivalent circuit (EC) using the Echem Analyst^TM data analysis software and evaluating the solution resistance (R_s), polarization resistance or charge transfer resistance (R_ct) and double layer capacitance (C_p) of the thermal treated alloys.

Results and Discussion

Impedance Measurements

Table 1 shows the Electrochemical Impedance Spectroscopy (EIS) test results.

OCP versus

time behavior – The open circuit potential (OCP) with exposure time of aged Al-6Si-0.5Mg(-0.5Cu) alloys in simulated seawater is shown in Table 1. Large fluctuations in open circuit potential for the alloys were seen during the time of 100s exposure. After a period of exposure the OCP fluctuation decreased and reached steady state. The steady state OCP of Cu free alloy (Alloy-1) is -0.8454V and it is the higher negative OCP value between the alloys under investigation. The occurrence of a positive shift in OCP in the Al-6Si-0.5Mg alloy containing 0.5wt%Cu indicates the existence of anodically controlled reaction. The OCP values mainly depend on the chemical compositions and thermal history of the alloys.

Impedance measurements – The data obtained were modeled and the equivalent circuit that best fitted to the experimental data is shown in Figure 1. R_s represent the ohmic solution resistance of the electrolyte. R_ct and C_p are the charge transfer resistance and electrical double layer capacitance respectively, which correspond to the Faradaic process at the alloy/media interface. Figure 2 shows the Nyquist diagrams (suggested equivalent circuit model shown in Figure 1) of the Al-6Si-0.5Mg(-0.5Cu) alloys in simulated seawater. In Nyquist diagrams, the imaginary component of the impedance (Z”) against real part (Z’) is obtained in the form of capacitive-resistive semicircle for each sample.

Figure 3 shows the experimental EIS results in Bode magnitude diagram for Al-6Si-0.5Mg(-0.5Cu) alloys. Bode plots show the total impedance behaviour against applied frequency. At high frequencies, only the very mobile ions in solution are excited so that the solution resistance (R_s) can be assessed. At lower-intermediate frequencies, capacitive charging of the solid-liquid interface occurs. The capacitive value C_p can provide very important information about oxide properties when passivation or thicker oxides are formed on the surface. At low frequency, the capacitive charging disappears because the charge transfer of electrochemical reaction can occur and this measured value of the resistance corresponds directly to the corrosion rate. For this reason, this low frequency impedance value is referred to as polarization or charge transfer resistance (R_ct).

The solution resistance (R_s) of the alloys varies from 40-44Ω (Table 1) and these values are very similar to each other. So there are insignificant changes of R_s values for the alloys during EIS testing. The R_s values are negligible with respect to R_ct and the electrolyte behaves as a good ionic conductor. Impedance measurements showed that in simulated seawater, addition of o.5wt%Cu in the Al-6Si-0.5Mg alloy increases the charge transfer resistance (R_ct). For the Cu free Al-6Si-0.5Mg alloy, the charge transfer resistance (R_ct) value in simulated seawater is 15.57kΩ, and this is increased to 25.75kΩ with the addition of 0.5wt% Cu into the Al-6Si-0.5Mg alloy. The increase in the charge transfer resistance indicates an increase in the corrosion resistance of the Al-6Si-0.5Mg alloy with Cu addition. The double layer capacitance (C_p) of the Cu free Al-6Si-0.5Mg alloy is 1.259µF, which is the lower value between the alloys investigated. The double layer capacitance of Al-6Si-0.5Mg alloy increased (1.793µF) with an addition of 0.5wt%Cu.

Potentiodynamic Polarization Measurements

Table 2 shows the potentiodynamic polarization test results obtained from the electrochemical tests.

Potentiodynamic polarization curves of Al-6Si-0.5Mg(-0.5Cu) alloys in simulated seawater are shown in Figure 4. Anodic current density of Al-6Si-0.5Mg alloy decreased with Cu addition. This is caused by the slowing of the anodic reaction of Al-6Si-0.5Mg-0.5Cu alloy. The addition of Cu caused the formation of micro-galvanic cells in α-aluminum matrix. The different intermetallic compounds (like Mg₂Si, Al₂Cu etc.) can lead to the formation of micro-galvanic cells because of the difference of corrosion potential between intermetallics and α-aluminum matrix. It was well known that the addition of Cu increased the corrosion potential of a number of Al-Cu-Si alloys. For the Cu free Al-6Si-0.5Mg alloy corrosion potential is -764mV, which is the higher negative potential between the alloys investigated. With addition of 0.5wt%Cu, the corrosion potential of the Al-6Si-0.5Mg alloy shifted towards more positive values. Pitting potential (E_pit) of 0.5wt%Cu content alloy also shifted towards more positive values (from -480 mV to -408mV). Potentiodynamic tests showed that in simulated seawater, Addition of Cu into the Al-6Si-0.5Mg alloy decreases the corrosion current (I_corr). For the Cu free Al-6Si-0.5Mg alloy, the corrosion current (I_corr) value in simulated seawater is 6.3µA, and this decreased to 5.640µA with the addition of 0.5wt% Cu to the Al-6Si-0.5Mg alloy and the corresponding corrosion rate decreases for the alloy (4.732mpy).

Microstructural Investigation

The microstructure of some selected as-corroded samples was observed under OLM and SEM. There was evidence of corrosion products of intermetallic compounds in all the samples examined. Besides, several pits were visible in all the samples examined. It is probable that the pits are formed by the intermetallics dropping out from the surface due to the dissolution of the surrounding matrix. However, it is also possible that the pits are caused by selective dissolution of the intermetallic/or particles of the second phase precipitates.

It was demonstrated that in Al-Cu-Si alloys a more finely and homogeneously distributed Al₂Cu and needle-like Si particles in the ternary eutectic mixture, tend to improve the corrosion resistance mainly due to the galvanic protection of both Al₂Cu and Si phases [21]. Although it has also been reported [21-22] that fine Si particles tends to decrease the corrosion resistance of binary Al–Si alloys when associated with the Al₂Cu intermetallic phase, a better galvanic protection is provided for finer Al-Cu-Si alloy microstructures. It was also reported that the ternary eutectic mixture consisting of Al + Al₂Cu + Si phases is nobler than the Al-matrix and Al-phase in the eutectic mixture [23].

The forms of corrosion in the studied Al-6Si-0.5Mg(-0.5Cu) alloys are not completely uniform and predominantly pitting corrosion as obtained by the OLM and SEM. Samples were characterized by OLM and SEM following potentiodynamic polarization tests. The peakaged Cu free Al-6Si-0.5Mg alloy exhibited pits on their surface (Figure 5), which apparently had nucleated randomly. Conversely, the exposed surface of the alloys exhibited a corrosion product covering the surface after polarization. There are more and severe pits in Cu free Al-6Si-0.5Mg alloy compared to Al-6Si-0.5Mg-0.5Cu alloy. All the optical micrographs (Figures 5-6) also showed that there was no corrosion in the fragmented and modified Al-Si eutectics.

Conclusions

The magnitude of the charge transfer resistance (corrosion resistance) value as an impedance parameter, increased with the addition of 0.5wt% Cu into Al-6Si-0.5Mg alloy in simulated seawater. The electrochemical parameters obtained from polarization curves showed that the corrosion current (I_corr) and corrosion rate (mpy) decreased with the addition of 0.5wt% Cu into Al-6Si-0.5Mg alloy. The open circuit potential (OCP), corrosion potential (E_corr) and pitting corrosion potential (E_pit) in the simulated seawater were shifted in the more noble direction due to 0.5wt% Cu addition into Al-6Si-0.5Mg alloy. The microstructures studied (by OLM and SEM) indicate the pitting corrosion in the investigated alloys.

Acknowledgements

The authors are acknowledged to the Pilot Plant and Process Development Centre (PP & PDC) at BCSIR laboratories, Dhaka for carried out the electrochemical corrosion tests.

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