Effect of chemical composition and heat treatment on intergranular corrosion and strength of AlMgSiCu alloys

1 Introduction

Aluminum and its alloys attract great attention in the automotive industry due to their superior properties such as high strength, excellent corrosion resistance, machinability, weldability, and low density. Nowadays, they are one of the most preferred materials in the industry, and it is expected that their use will continue to increase in the next decades with increasing electrical car production. Aluminum alloys reduce the weight of electrical car chassis and other parts, and this will help to increase the driving range compared to the high weight of batteries. Among the various aluminum alloys, 6xxx series aluminum alloys take the highest share in industrial processes (nearly 90%) [1]. They are actively used in architectural systems, the automotive industry, aircraft and space industry, truck and trailer applications, marine systems, and defense industries. Moreover, the demand for the 6xxx series of aluminum alloys increases in the new generation of automobiles since sustainability and clean mobility concepts have emerged in the automobile industry in recent years. These alloys can be extensively used in application areas that require high strength, high impact resistance, and good corrosion resistance (chassis, bumper, crash boxes, etc.) [1,2]. The 6xxx series of aluminum alloys contain Mg and Si as alloy elements in their structures. The main source of high strength and hardness in this series is the precipitation of Mg–Si phases [1,3]. Although Mg confers lightness and high strength as an alloying element, it can negatively affect the corrosion strength. During the determination of mechanical properties of alloys, the ratio of Mg and Si relative to each other and other alloying elements which are added to the alloy and their ratios are effective. In this study, besides Mg and Si elements, Mn, Cu, Zn, Ti, and other elements were added to the 6056 series of aluminum alloy. Although they improve the hardness and strength of 6056 series Al alloys, which have complex compositions, they can negatively influence plasticity. Therefore, the alloys should be annealed homogeneously to both improve plasticity and minimize dendritic separation. It is also known that homogenization heat treatment has a significant effect on mechanical properties. In addition, it was observed that the extrusion method used in the study affected the mechanical properties by contributing to the improvement of strength and plasticity. Generally, studies in the 6056 series of Al alloys have been based on the relation of applied heat treatment with the precipitate microstructure and mechanical properties [4]. In this work, the effects of various chemical compositions and different heat treatment applications on the corrosion mechanism of the 6056 Al alloy were investigated.

The use of aluminum alloys in chassis, bumper, and battery carrier systems has increased considerably in the last 10 years to reduce the battery weight of electric vehicles in the automotive industry [5]. In addition to high strength, high ductility of alloys is also required, especially in areas that are exposed to plastic deformation during a crash like bumper and sidewall profiles. For these cases, heat treatment processes applied to aluminum alloys gain significant importance. While the highest strength values for aluminum alloys are obtained with the T6 heat treatment process, the T7 (T76 and T79) heat treatment process is preferred in the automotive industry [6]. In T7, an over-aging process is applied to the alloy; as a result, the mechanical properties that can be reached with the T6 heat treatment process are compromised by approximately 10% [7]. However, the corrosion resistance, high-temperature usability, and ductility of the alloy can be increased with the T7 heat treatment process.

Among the acceptance standards of the profiles produced using aluminum alloys and extrusion techniques in the automotive sector, properties such as strength, ductility, thermal resistance, corrosion resistance, and shaping (form tolerance) are required [8,9].

Aluminum can be resistant to corrosion due to the oxide layer formed on aluminum structures. While aluminum dissolves as Al³⁺ in acidic conditions and as AlO₂ ^⁻ in basic conditions, the aluminum surface remains passive between pH 4 and 8. Although there is sufficient information about the corrosion mechanism of aluminum, there is a lack of information about the corrosion mechanism of alloyed and heat-treated aluminum alloys. Pitting corrosion is mostly seen in aluminum and its alloys, as the protective oxide film layer at the nanoscale on the surfaces deteriorates at certain intervals and can be corroded [10]. This situation causes the grain boundaries to be affected primarily by corrosion. When the amount of Mg and Si alloying elements in 6xxx series aluminum alloys are balanced, corrosion resistance can be enhanced. In cases where the Si ratio is higher, the resistance to intergranular corrosion decreases because Si is segregated at the grain boundaries [11,12].

Intergranular corrosion and pitting corrosion are generally observed in 6056 series Al alloys. Intergranular corrosion is electrochemical and occurs locally along the grain boundaries. While the precipitates may be directly corroded by intermetallic phases such as Si, Al₂Ni, Al₆(FeMn)Si, MgZn_2, or Al₈Mg₅ (shows anodic properties) at the grain boundaries, they may also corrode by being exposed to the corrosion mechanism in adjacent regions, although they do not show corrosion tendencies like Al5Cu2Mg8Si6, CuAl₂ (shows cathodic properties) [13]. It is known that this type of corrosion is affected by microstructure properties such as the number, shape, and distribution of second-phase particles in the alloy. Therefore, the corrosion susceptibility of the alloy can be improved by applying the heat treatment process [14]. On the other hand, pitting corrosion is a type of corrosion that occurs in the presence of halogen ions such as Cl in passivation metals or in anodic areas where aggressive aqueous solutions of heavy metals such as Cr, Ti, Sn, and Mn are present [15]. Corrosion, which starts anodic as small circular zones, can turn into sequential auto-cathodic corrosion, causing enlargement of the circular zone if no precautions are taken. If the amount of Cu and Si elements added to the 6056 alloys is more than Mg, intergranular corrosion resistance decreases considerably. This situation may also vary according to the type of heat treatment applied.

Cu reduces the corrosion resistance of an aluminum alloy similar to Si. It reduces corrosion resistance by forming Al₅Cu2Mg8Si6 (Q phase) in AlMgSiCu alloys [16]. Hence, in the 6xxx series aluminum alloys, it is necessary to reduce the copper phase [17], which forms a solid solution in the aluminum matrix, by applying the aging process [18]. In order to reduce corrosion, the usage of corrosion-sensitive elements can be decreased, and alloying elements that can create resistance to corrosion can be added to the aluminum. Moreover, neutral, weakly acidic, and alkaline solutions can be used to minimize material loss [15]. In addition, pitting corrosion cannot continue to expand in an active working environment. In this case, material loss can remain at a tolerable level.

JMatPro is a computer-aided engineering software that is used to calculate different kinds of material properties such as stable and metastable phases, solidification calculations, mechanical properties, phase transformations, thermo-physical and physical properties and is especially aimed at multi-component alloys [19,20,21,22].

Direct-chill casting is a semi-continuous process used since the 1930s for producing aluminum billets, which are the raw materials of the extrusion process [23,24,25]. In this process, molten aluminum enters the top of a water-cooled mold where it cools to form a solid ingot called a billet [13]. Fast solidification of aluminum alloy offered by this process provides lower macrosegregation and precipitation along the grain boundaries, with smaller grains affecting the properties of the final product [26]. However, higher cooling speeds can result in cold cracking, which makes billet unusable [25,27,28]. Thus, controlling the cooling speed is important for successful casting. Cooling speed is mostly determined by considering thermal properties and the solidus–liquidus temperature difference of the alloy.

The casting process is followed by a homogenization treatment. This heat treatment process includes three steps: heating the billet up to a certain temperature; holding the billet at that temperature for a pre-determined time; and cooling down the billet with a desired speed [29,30]. This process is necessary for changing the casting microstructure to the desired microstructure to obtain higher performance profiles with further plastic forming processes. These changes involve reducing the segregation; ensuring compositional homogeneity; transforming the formability and ductility by reducing β-AlFeSi particles (plate-like) to more acceptable α-AlFeSi particles (multiple rounded); and ensuring the formation of Mn, Cr, and/or zirconium (Zr) dispersoids to control the grain size and structure [31,32]. In this concept, metallurgical simulation programs, such as JMatPro, can be used to evaluate a wide range of material properties for multi-component alloys and can be used to calculate stable and metastable phase equilibria, solidification behavior, and properties, thermo-physical properties, and phase transformations. These calculations can help decide casting and homogenization parameters. In fact, there are an increasing number of studies using JMatPro for aluminum alloys including the effect of the addition of Ti and boron (B) on the solidification behavior [33], thermo-mechanical properties of wrought alloys [34], and so on [35,36,37].

Aluminum extrusion is one of the most widely used processes for producing solids and hollow shapes with complex geometries. This process transforms a metallic alloy billet into a uniform cross-sectional profile by forcing it to flow through a die hole under high pressure and temperature [38]. The production of hollow profiles in the extrusion of aluminum alloys is a special case. Square or round tubes are extruded through hollow dies with a fixed core or mandrel that defines the inner perimeter of the profile. The mandrel is held firmly in a fixed position by legs or bridges embedded in the back of the die. When the billet is pushed into the mold, the material flow is divided into different flows around the mandrel supports. After passing the mandrel, the metal streams recombine in the weld chamber and are welded in a solid state under the influence of pressure and temperature [39,40,41,42].

In this work, since the adequacy of AlMgSiCu alloys in terms of strength, ductility, and corrosion resistance in new-generation vehicles is still a matter of debate, 6056 alloys with two different chemical compositions and three different heat treatment conditions were used in corrosion tests according to EN ISO 11846 standard. The main aim of this study is to understand whether the corrosion resistance of 6056 alloys developed for high strength and high ductility meets the desired criterion in the automobile industry.

2 Experimental

The flowchart of the study is shown in Figure 1. In this study, 6056 aluminum alloys were produced in 254 mm (10 in) diameter billet form using the direct chill casting method in an industrial-type casting facility, and then 70 mm × 70 mm box profiles were produced by the extrusion technique. After that, the samples were subjected to T42, T62, and T76 heat treatment processes.

Figure 1

Flowchart of the study.

Within the scope of the study, two different chemical compositions of the 6056 Al alloy were determined by using the JMatPro metallurgical simulation program. The generation of phase diagrams and cooling curves was done with JMatPro. In the determination of chemical compositions, the Mg/Si ratio and mechanical strength values were taken as a basis. The chemical compositions determined are given in Table 1.

Liquid alloys (10 t) were prepared in a liftable reverberatory furnace in Wagstaff prototype casting facility in END Aluminium in Turkey, and the casting process was carried out with a 54 mm·min⁻¹ start casting speed and an 80 mm·min⁻¹ feed casting speed. These casting speeds were calculated according to cooling curves that were drawn by JMatPro analysis. During the casting process, the initial cooling water flow was 148 L·min⁻¹, whereas the feed cooling water flow was 177 L·min⁻¹. While the liquid metal temperature in the furnace was 720°C, the casting table temperature was 685°C. The billet formed samples with 254 mm (10 in) diameter by the direct chill casting method. The chemical composition of the alloys was determined by SPEKTROLAB A Model optical emission spectrometry. The chemical compositions of the alloys were designed to examine the effects of Mg–Si and other alloying elements (Cu, Zn) on the corrosion mechanism. Hence, in the 6056-1 Al alloy, the Mg content is higher as compared to other alloying elements; whereas in the 6056-2 Al alloy, the Si content is greater.

After the casting process, to complete the segregation and phase transformation in the internal structure, the homogenization process was carried out at 540°C for 6 h using a batch-type homogenization furnace. After the homogenization process, billets were cooled in air at a speed of 450°C·h⁻¹.

After the homogenization process, the preheating process for extrusion was applied at 440°C in a natural gas furnace and the induction furnace for homogeneous heating at 490°C before the extrusion process. The heated billets were extruded with an extrusion profile speed of 2.4 m·min⁻¹ and in the profile exit temperature range of 532–536°C; then the profiles were quenched with spray water (80% rate) at 2,750 t press in ONAT Aluminium in Turkey [43].

The extruded profile is a square box profile with a wall thickness of 2.5 mm (Figure 2). Profiles taken after the extrusion process were first subjected to the solutionizing process at 540°C under atmospheric conditions for 120 min and then quenched in a water tank. Afterward, T42, T62, and T76 heat treatments were applied to the profiles in a semi-industrial type aging furnace, which works under atmospheric conditions. The temperature and time data during the heat treatments are given in Table 2. Finally, the aging process was applied to samples at different times and temperatures.

Figure 2

Technical drawing and the 3D profile of the extruded profile.

After the aging process, the hardness of the aluminum profiles which were subjected to different heat treatment processes was checked and the tensile test was applied to the samples. The tensile test was performed with a Zwick/Roell Z150 tester according to EN 6892-1 standard. The EMCO TEST M4C G3M Brinell hardness device was used to measure the hardness of the profiles.

Before starting the corrosion test of the profiles produced from 6056-1 and 6056-2 alloys, five 20 mm × 20 mm square samples taken from each profile were polished starting from 150 grade to 2,400 grade. The roughness averages of the polished samples were calculated and their compliance with Ra ≤ 2.5 μm was checked. As a surface preparation process, the samples were immersed in a sodium hydroxide (NaOH) solution [5% (m·m⁻¹) to 10% (m·m⁻¹)] at 55°C (±1) for 3 min and then rinsed with water. For the corrosion test, the samples were immersed in a solution containing 30 g of sodium chloride per liter and (10 ± 1) mL concentrated hydrochloric acid (ρ = 1.19 g·mL⁻¹) at room temperature for 24 h. After the corrosion test, microstructure analysis was performed without etching. Then, in order to fully understand the type of corrosion, the microstructure was examined again by light metallographic etching. Finally, a scanning electron microscope (FEI Nova NanoSEM 430) and electrochemical workstation (Gamry Reference 3000) were used to examine the effect of the heat treatment process and chemical composition on the corrosion strength.

Potentiodynamic polarization tests were performed on a GAMRY Reference 3000 potentiostat/galvanostat/ZRA electrode. A three-electrode system was used to test the electrochemical corrosion resistances of the sample. The working electrode was the tested aluminum sample, and the reference electrode and counter electrode were saturated calomel electrode (SCE) and platinum wire, respectively. The electrolyte solution was 0.6 M NaCl aqueous solution. The potentiodynamic polarization curve was drawn in the range of −1.2 to 0.2 V and a scan speed of 0.5 mV·s⁻¹.

3 Results and discussion

3.1 JMatPro analysis

The cooling curve diagrams of 6056-1 and 6056-2 aluminum alloys are shown in Figures 3 and 4. The vertical axis of Figures 3 and 4 shows the weight% of the formation of liquid, solid Al, and other phases, and the horizontal axis shows the temperature. The phase diagrams of the alloys are shown in Figures 5 and 6. Figures 5 and 6 were drawn using JMatPro; in Figure 5, other alloying elements (Si, Fe, Cu, Mn, Zn) besides Mg were kept constant and, in Figure 6, other alloying elements (Mg, Fe, Cu, Mn, Zn) besides Si was kept constant in the chemical compositions are given in Table 1.

Figure 3

The 6056-1 alloy cooling curve diagram drawn using JMatPro.

Figure 4

The cooling curve diagram of 6056-2 alloy drawn using JMatPro.

Figure 5

Al–Mg phase diagrams drawn using JMatPro (a) 6056-1, (b) 6056-2.

Figure 6

Al–Si phase diagrams were drawn using JMatPro: (a) 6056-1 and (b) 6056-2.

As shown in Figures 3 and 4, the liquidus and solidus temperatures decrease as the alloying elements are added to the alloys. The solid–liquid temperature difference of the 6056-1 alloy is 77°C, while that of the 6056-2 alloy is 89°C. It is also possible to cast these two alloys with different parameters during the casting process. The 6056-2 alloy should be cast more slowly and with a less water flow rate. Within the scope of this study, the casting parameters were kept constant so that a feature other than alloying elements and heat treatment does not affect the microstructure. During the analysis of the cooling diagram of the 6056-1 alloy, it is observed that the Mg₂Zn phase is formed in the presence of Mg and Zn in the structure. This phase is one of the phases that adversely affects corrosion resistance [44].

These cooling curve graphs, in addition to solidus and liquidus temperatures, also show possible phases that can form and dissolve with temperature changes. For all four alloys, one can see the formation of solid solution matrix α-Al, Mg₂Si (α phase), which is the main strengthening precipitation phase for the 6xxx-series alloys and Al₃M (Ti, V, Zr) D022 phase, which is known as the compound formed by elements that have very low solubility in aluminum in solids and liquids [45]. Thus, this phase acts as a grain refiner by creating a nucleation surface during solidification [46,47,48,49,50].

The 6056-2 alloy contains Al5Cu2Mg8Si6 (Q-phase), which has positive effects on the mechanical properties of the alloy up to 200°C; however, above that temperature, this phase dissolves and affects the mechanical properties in a negative way. This phase also has a negative effect on corrosion resistance by creating microgalvanic coupling [29,51,52,53]. It was found that the amount of Al5Cu2Mg8Si5 (Q phase) increased as the amount of Si and Cu content increased as shown in the Al–Mg phase diagrams (Figure 5). In addition, it was observed that the area for the formation of the Mg₂Si phase increases during the solidification of the 6056-1 alloy with the increase in the amount of Mg.

As shown in the Al–Si phase diagrams (Figure 6), the Al20Cu2Mn3 phase is formed instead of the Al5Cu2Mg8Si6 phase with the decrease of Mn, Cu, and Si content in the alloys and the increase of Mg. Since Mg is more willing to form the Mg₂Zn precipitate phase by bonding with Zn, the Al, Cu, and Mn combine to form the Al20Cu2Mn3 phase. This phase does not affect the corrosion resistance as much as the Q phase and has a positive effect on the alloy [54].

3.2 Mechanical properties

The tensile strength, yield strength, and hardness of the six specimens are shown in Figure 7. There are two groups of alloys, i.e., 6056-1 and 6056-2, and three heat treatment processes, T42, T62, and T76. The basic difference between T62 and T76 is that T76 is over-aged to improve the corrosion resistance but mechanical properties slightly decrease. The strength of the specimens at T62 conditions is improved because of the artificial aging. The precipitates are not dissolved back and thus the stress concentrates around the precipitates [1]. On the other hand, solution heat-treated and natural aged (T42) samples’ precipitates dissolved in the matrix again. The precipitates coarsen during the over-aging treatment, and hence, the strength of the T76 heat-treated 6056 samples is slightly reduced. Moreover, the strength of the 6056-2 alloy is higher under all heat treatment conditions. It is because of the formation of the highest amount of MgZn₂, MgSi_2, and CuAl₂ precipitation phases. The hardness values of the specimens have the same trend as the strength values. An increase in the hardness for T62 samples because of artificial aging and a slight reduction for T76 samples due to the coarsening of precipitates during over-aging are observed.

Figure 7

Tensile and hardness properties of all specimens.

3.4 Corrosion tests

Aluminum 6056 alloys were tested by EN ISO 11846 standard to determine the intergranular corrosion susceptibility. The specimens were immersed in NaCl/HCl aqueous solution for 24 h. After 24 h, the specimens were cleaned with water, and intergranular corrosion was observed under an SEM. The dark lines on the SEM images are evidence of intergranular corrosion. The T42-6056-1 heat-treated sample reveals pitting corrosion rather than intergranular corrosion because corrosion was not concentrated at the grain boundaries and there are only black pitting regions. It is also observed in the electrochemical test because T42-6056-1 is the only specimen that shows pitting potential. There is a transformation of corrosion morphology from pitting corrosion to intergranular corrosion in the T42-6056-2 sample (Figure 8). The intergranular corrosion susceptibility of aluminum alloys is generally associated with the formation of electrochemical micro-couples which are continuously distributed along the grain boundaries. These approaches are also suitable for the intergranular corrosion mechanism of heavily over-aged samples. It can be concluded that the intergranular corrosion mechanism of the over-aged samples should be related to the formation of corrosion micro-couples between the grain boundary precipitates (cathode) and precipitate-free zones (anode) [4,15]. The T76-6056-2 sample exhibits the highest corrosion resistance. The corrosion appeared, and intergranular corrosion was minimal and homogenously distributed; also the corrosion depth was the shortest. T42 reveals the pitting corrosion mechanism, but T62 and T76 specimens reveal the intergranular corrosion mechanism. The corrosion of the T42 sample was not limited along the grain boundaries; it should be related to the line distribution of precipitates within the grains, as shown with white arrows in Figure 8. The density of precipitates is presumed to be accountable for precipitation strengthening under T62 and T76 conditions. These precipitates were homogeneously distributed over the whole sample. The elemental mappings in Figure 9 showed that the grain boundary precipitates mainly consist of Si-phase (Mg₂Si) and small amounts of Mg-phase (MgZn₂) and Cu-phase (CuAl₂). The Si-phase precipitates are covered by SiO₂ when in contact with the corrosive electrolyte; thus, these precipitates cannot effectively act as a cathode [55,56]. Therefore, the grain boundary Si-phase precipitates probably are not responsible for the corrosion of overaged T76 samples. The effective cathode can be a Cu-containing phase that is continuously distributed along the grain boundaries (Figure 9). It can be concluded that the reason for the intergranular corrosion of the overaged T76 samples may be related to the continuous corrosion of micro-couples between the cathode Cu-containing precipitates and anode precipitate-free zones. The 6056-1 alloy has more Mg inside compared to the 6056-2 alloy. It can be expected that 6056-1 has more corrosion resistance, but in this alloy, Zn alloying is also added. MgZn₂ formation affects the corrosion resistance negatively, more than the excess Si. Therefore, the 6056-1 alloy has low corrosion resistance.

Figure 8

SEM corrosion micrographs of (a) T42-6056-1, (b) T42-6056-2, (c) T62-6056-1, (d) T62-6056-2, (e) T76-6056-1, and (f) T76-6056-2.

Figure 9

EDS mapping of the T76-6056-2 specimen after intergranular corrosion attack.

3.5 Electrochemical tests

The open circuit potential (OCP) of the heat-treated samples is shown in Figure 10b. The OCP values are between −800 and −700 mV vs SCE. Almost all alloys reached a steady potential after immersing in 0.6 M NaCl for 20 min. The heat-treated alloys show noisy OCP potential because of the pitting process due to dispersoids and constituents in alloys [57]. Alloys that have the same heat treatment process and approximately have the same OCP values, such as T76-6056-1 and T76-6056-2; both were heat treated under T76 conditions.

Figure 10

Electrochemical results of six aged specimens: (a) potentiodynamic polarization curves in 0.6 M NaCl solution and (b) open-circuit potentials.

Polarization curves of alloys under T42, T62, and T76 conditions are shown in Figure 10a. The cathode part of the polarization curve is flatter because of the lower electrochemical rate. The polarization current increases sharply when the applied potential is higher than the corrosion potential. The polarization current’s rising trend decreases with the applied potential because the corrosion products accumulate on the surface and avoid electron exchange between the alloy surface and the electrolyte. Therefore, the corrosion process is inhibited and the corrosion rate decreases. Only the T42 heat-treated alloys have distinct pitting potential (E _pit); E _pit is the potential where there is a sharp current increasing. T62 and T76 heat-treated alloys regardless of their composition have a similar form of the polarization curve. The current of the alloys, other than T42-6056-1 and T42-6056-2, drastically increased at ∼−650 mV, indicating that the pitting potential is equal to the corrosion potential. Table 3 summarizes the values of the corrosion potential (E _cor), the corrosion current density (I _cor), the pitting potential (E _pit), and the polarization resistance (R _p). Alloys with a 6056-2 composition have better corrosion potential and lower corrosion current density than the 6056-1 composition for all heat treatment processes. Alloy T42-6056-1 has the highest I _cor and lowest E _cor; on the other hand, alloy 54 has the lowest I _cor and highest E _cor. Therefore, alloy T42-6056-1 exhibits the worst corrosion resistance, and alloy T76-6056-2 is the best in terms of corrosion resistance. I _cor values of alloys with 6056-2 compositions are similar, which indicates that the corrosion rate of these alloys is close to each other.

Table 3

Corrosion potential and corrosion current density of the six aged alloys obtained from potentiodynamic polarization curves

	Corrosion potential (V vs SCE)	Corrosion current density (µA·cm−2)
T4-6056-1	−1.018	0.548
T4-6056-2	−0.706	0.219
T6-6056-1	−0.768	0.947
T6-6056-2	−0.699	0.181
T7-6056-1	−0.730	0.447
T7-6056-2	−0.688	0.186

4 Conclusions

1. In this study, box profiles (70 mm × 70 mm dimensions and 2.5 mm thickness) are produced from aluminum billets with the extrusion technique and are solutions treated at 540°C for 2 h and quenched in a water tank. T42, T62, and T76 aging treatments are applied to profiles. The tensile test samples are taken from profiles to understand the difference in mechanical properties of alloys and aging conditions. After the aging process, 20 mm × 20 mm samples are taken from profiles and the corrosion test is applied according to the EN ISO 11846 standard Method B.

2. 6056-2 alloy has nearly 5% more strength when compared with 6056-1 alloy because the precipitation phases in the 6056-2 alloy are higher due to their amount of alloying elements.

3. Pitting corrosion formation is much more than intergranular corrosion in alloys that are naturally aged. It is known that pitting corrosion can be avoided with purer alloy (6056-1).

4. Excess Mg in 6056-1 alloy will tend to form a solid solution under normal conditions. When Zn is added to the matrix, Mg tends to combine with it to form the MgZn₂ phase which worsens corrosion resistance. As expected, the T76 heat treatment condition showed better corrosion resistance.

5. It is advised that as the expected strength is accepted as lower in automotive specifications, T74 or T73 thermal conditions, which have higher corrosion resistance and slightly lower strength, can be tried instead of the T76 condition. To obtain these, it is sufficient to increase the time at the same temperature. For 6056-1 alloy, the amount of Zn can be reduced to a minimum level (0.10%) in order to increase the corrosion resistance because the alloy has high strength for automotive specifications.