Combined Effects of Nb Content, Thermal Parameters, and Dendritic Scale Length on Electrochemical Corrosion in Horizontally Solidified Al3CuxNb Alloys

Abstract

In this work, the effects of thermal solidification and microstructural parameters, such as solidification rate (V_L), cooling rate (T_R), and secondary dendritic spacing (λ₂), were studied on the electrochemical parameters of corrosion potential (E_CORR), current corrosion resistance (I_CORR), and polarization resistance (R_P). The alloys were solidified in a horizontal solidification device and two as-cast samples from the heat transfer surface were subjected to electrochemical polarization and impedance techniques. It was verified that the experimental values found for E_CORR were relatively close for all the investigated alloys and in both analyzed positions, allowing to deduce that the V_L, T_R and λ_2, as well as the Nb content had little influence, but enough to lead to different behavior in the I_CORR and R_P. For the alloy with 0.5wt.%Nb, higher V_L and T_R values, and smaller λ₂ resulted in lower I_CORR values. On the other hand, for alloys with 3 and 5wt.%Nb, coarser microstructures (greater λ₂) have lower corrosion rates. Microstructural analysis showed that Nb acts as a protective element against corrosive actions in the dendritic matrix.

Keywords:
Al-Cu-Nb alloys; Solidification thermal parameters; Microstructure; Corrosion resistance

1. Introduction

Aluminum has high stability when exposed to certain oxidizing environments, which can be attributed to the natural formation of an oxide layer (Al₂O₃) that works as a barrier to corrosion¹-⁸. The electrochemical behavior of aluminum and its alloys is also influenced by this oxide layer, which forms spontaneously and governs their corrosion resistance. The rate of formation and the properties of Al₂O₃ depend on the chemical composition of the alloys and, therefore, the dissolution rate of the film will change with the addition of alloying elements to the metallic matrix. For example, magnesium enhances the protective properties of the oxide film of aluminum alloys forming mixed oxides which form 5XXX series alloys exhibit excellent corrosion resistance¹-⁸.

Despite the high resistance to corrosion, which may be associated with the presence of the passivation layer, aluminum alloys are subject to high current densities when subjected to aggressive media⁴-¹³. When in contact with the atmospheric environment, for example, it's highly resistant to corrosive processes, however, depending on the medium and condition to which it is exposed, this resistance will assume different behaviors since the aggressiveness of the medium can destroy the protective layer, leaving it vulnerable to corrosive processes¹-³. Certain elements reduce these protective properties, as is the case with copper³-⁸. Al-Cu alloys are generally less corrosion resistant than other alloys. In these alloys, intermetallics of Al₂Cu, Al₂CuMg, and Al₇Cu₂Fe are some of those responsible for initiating galvanic corrosion with the matrix⁴,⁵,⁷,⁸,¹².

Electrochemical techniques such as open circuit potential (OCP), electrochemical potentiodynamic polarization (EPP) and electrochemical impedance spectroscopy (EIS) are efficient ways to analyze and monitor the corrosion resistance of metallic materials. The ease of obtaining data and the versatility of applications are some of the characteristics that make these techniques the most used ones in this type of investigation¹⁴-⁴⁰, which have been applied in different situations of corrosive environments, more or less aggressive, ranging from electrolytes such as NaCl⁴,⁵,⁹,¹²,¹³,¹⁸,³²,³³,³⁵,³⁶,³⁹,⁴⁰, NaOH²²,²⁵,²⁶,²⁸,³⁸, HCl¹⁵-¹⁸,²⁰-²⁴,²⁷,²⁹,³¹,⁴¹, and H₂SO₄¹⁹,²³.

The study of the solidification phenomenon of multicomponent casting alloys has provided the development of metallic materials with differentiated and increasingly specific properties. In real applications, the transient conditions in which this phenomenon occurs lead to several possibilities for obtaining final structures for the same cast product and, consequently, for its performance⁴-⁹,¹²,¹³,⁴²,⁴³. The effects of the alloying element, and of unsteady-state solidification conditions, such as growth and cooling rates (V_L e T_R), and the length of microstructural scales, were evaluated on the electrochemical characteristics of aluminum alloys⁴-⁹,¹²,¹³. Directional solidification experiments were carried out with Al-Cu⁴,⁵,⁸,¹², Al-Cu-Si⁷, Al-Cu-Mg⁴,⁵, Al-Cu-Ni⁹ and Al-Si-Mg⁶ alloys, and the results showed that the microstructure plays a fundamental role on the mechanical and corrosion properties. The addition of Mg to an Al–Cu alloy has led to a considerable increase in Vickers Microhardness (HV), but reduced the corrosion resistance of the ternary Al-Cu-Mg alloy in a 0.5 M of NaCl solution⁴.

In turn, the combined effects of solidification microstructure refining scale and T6 heat treatment conditions on corrosion behaviour and HV were evaluated in the respective alloys, and the results reported that in both as-cast and heat-treated samples the coarser dendritic microstructure showed lower corrosion current densities, and that heat treatment improved the corrosion resistance of the Al-Cu-Mg alloy⁶. In a study⁸, it was depicted for as-solidified samples of Al-3wt%Cu alloy that due to lower V_L and T_R values and, consequently, higher λ₂ values the Al₂Cu intermetallic phase was more susceptible to the action of electrochemical corrosion phenomena in a 0.2 mol.L^-1 solution of HCl.

Investigations have shown that horizontally solidified as-cast samples of Al-3wt.%Cu-2wt.%Si⁷ and Al-7wt.%Si-0.3wt.%Mg⁶ alloys have higher corrosion resistance for smaller secondary dendritic spacing (λ₂). In the as-cast microstructure of the directionally solidified Al-4.5wt.%Cu alloy, smaller dendritic and eutectic interphase spacing provided a more extensive distribution of the protective barrier to corrosive action, since the Al₂Cu particles were enveloped by the α phase (α_E) that acted as a protection against corrosion¹². In the directionally solidified Al-3Mg-1Si alloy (wt.%) (for 0.15 M and 0.5 M in NaCl electrolytes)¹³, better electrochemical behavior was observed for finer cellular and dendritic arrays, due to lower current densities and more positive corrosion potential when compared with coarser microstructures.

In our recent studies, the effects of the solidification thermal parameters and the T6 heat treatment were evaluated on the length of the microstructural scale and on the microhardness HV⁴²,⁴³, as well as on the electrical conductivity of the investigated alloys in this work⁴². However, there is still a need for studies that seek to understand the influence of thermal and microstructural parameters on the corrosion resistance of Al-3Cu-xNb alloys (x=0.5, 3 and 5wt.%), which is the main focus of investigation of this work, aiming to add new scientific knowledge to the development chain of the respective alloys.

In this sense, observing the continuous development of research pertinent to niobium and its importance to Brazil, aiming to deepen these studies, the main goal of this investigation is to evaluate the effects of thermal parameters, microstructure and Nb composition, mainly regarding the role of the Al₂Cu and Al₃Nb intermetallic phases, on electrochemical characteristics, such as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), in two samples in the as-cast ingot from the heat transfer surface of horizontally solidified Al-3Cu-xNb (x=0.5, 3 and 5wt.%) alloys.

2. Materials and Methods

Horizontal solidification experiments were carried out with Al-3Cu-xNb alloys (x=0.5, 3 and 5wt.%) in our recent work⁴²,⁴³, and in order to evaluate the combined effects of Nb and microstructural scale length on corrosion resistance of the aforementioned alloys, samples of solidified ingots were submitted in this work to electrochemical tests.

The casting assembly used in the horizontal solidification experiments is shown in Figure 1. The solidification device was designed in such a way that heat was extracted only through one water cooled side, thus promoting solidification along the longitudinal length of the rectangular mold, as can be seen in Figure 1a. In order to perform a thermal analysis and determine the investigated solidification thermal parameters (V_L and T_R), continuous measurements of temperatures in the melts were monitored during solidification through the output of a group of thin type K thermocouples coated in stainless steel tubes of 1.6 mm outside diameter and strategically positioned in respect to heat extraction surface on the cooled part of the ingot mold, as shown in Figure 1b. All the thermocouples were connected by coaxial cables to a data logger interfaced with a computer, and the temperature data was acquired automatically. Once the thermal data was generated, the experimental V_L and T_R values were determined, as shown Figure 1c. More details on this experimental set-up can be found in a previous articles⁴-⁸,⁴²,⁴³.

Figure 1
Schematic representation of horizontal experimental solidification assembly: (a) HS device, (b) thermal data acquisition system, and (c) technique for determining of V_L and T_R.

Figure 2 illustrates one of the ingots of the investigated alloys, showing how subsequent cuts were performed to obtain samples for microstructural characterization and electrochemical tests. Selected longitudinal sections of the solidified ingot were polished and etched with Keller's reagent (10ml of HF, 15ml of HCl, 25ml of HNO₃ and 50ml of H₂O) to reveal the microstructure, as shown in Figure 2a, as well as two as-cast samples were selected for each alloy in the cross section at positions 10 and 70 mm from the cooled base to carry out the electrochemical tests, as can be seen in Figure 2b. The length of the microstructural scale was analyzed by measuring the secondary dendritic spacing, as shown in Figure 2c.

Figure 2
Illustration of the procedure for cutting solidified samples for microstructural analysis and electrochemical tests: (a) and (b) longitudinal and transverse sections, and (c) λ₂ measurement technique. Source: Adapted from Mendes et al.⁴².

MOTIC and the image processing software ImageJ were used to measure the secondary dendrite arm spacings, and at least 20 measurements were taken for each selected position along the casting length. Furthermore, a scanning electron microscope (SEM TESCAM, VEGA LMU) coupled to an energy dispersion spectrum (EDS X-MAX 20, Oxford) was used to investigate the microstructural phases of the samples before and after the corrosion tests.

Figure 3 shows the schematic procedure of the experimental assembly used for the application of the electrochemical tests. Initially, in an electrolytic cell (4), 500 mL of 0.2 mol.L^-1 of HCl solution was added at room temperature of 25°C. For the electrochemical measurements, a system of three electrodes (5), (6) and (7) was used for each test, fixed by electrical terminals (3) interconnected to the potentiostat/galvanostat (2), both connected to the microcomputer (1) with ESA 400 software from Gamry Instruments to obtain and analyze the anodic and cathodic potentiodynamic polarization curves and potential of corrosion. In addition to visualization, interpretation and adjustment of Nyquist diagrams obtained by electrochemical impedance spectroscopy. These electrochemical measurements during the corrosion process were obtained in accordance with the technical standard ASTM G3-14 ⁴⁴.

Figure 3
Schematization of the assembly of materials and equipment for carrying out the electrochemical test. Source: Adapted from Soares et al.⁸.

Aiming to develop a comparative study with works based on literature⁷,⁸, all electrochemical experiments were performed in HCl 0.2 M at room temperature. The use of HCl is justified by the fact that it is a strong inorganic acid, with a very low pH (~0.7) and easily ionizable in aqueous solutions. When it reacts with aluminum, it produces substantial loss of the metal through corrosion, where chloride ions (Cl-) attack the metal surface forming aluminum chloride (AlCl₃) and releasing hydrogen (H₂(g)). HCl solution is one of the most currently used acids in the pickling and electrochemical etching of aluminum capacitor foil, and it is widely used for acid cleaning and electro-polishing of aluminum. In addition, the passive Al₂O₃ layer is inert in most environments and resistant to a wide variety of chemical agents, however it is susceptible to hydrochloric acid (HCl) solutions that are used in acid pickling, a surface treatment of metals such as aluminum and its alloys⁶. More details on the preparation and execution of electrochemical tests can be consulted in our recent research⁶-⁸.

3. Results and Discussion

3.1. Heat flow parameters

Results of thermal analysis, including heat flux parameters, active in the solidification process, such as temperature records as well as growth and cooling rates (V_L and T_R), were discussed in our recently published studies⁴²,⁴³, which have been used in this work to establish a correlation with the electrochemical corrosion parameters. Thus, typical plots of experimental cooling curves for thermocouples placed along the length of castings resulting from horizontal solidification (HS) (see Figures 1a and 1b) of Al-3wt.%Cu-xNb alloys (x=0.5, 3 and 5wt. %) were used to determine the thermal solidification parameters (V_L and T_R), according to the methodology in Figure 1b, whose results can also be seen in our recent research⁴²,⁴³. Mathematical expressions have been proposed to predict the V_L and T_R variation along the length of the solidified ingot (P), given by V_L=C₁(P)ⁿ and T_R=C₂(P)^m according to Table 1. It was verified that the use of a water-cooled mold imposes higher growth and cooling rates values of Al-rich phase dendrite tips in positions (P) close to the heat transfer surface (cooled base of the ingot), and a decreasing profile of V_L and T_R along the length of the as-cast ingot due to the increase in the thermal resistance of the solidification layer with distance from the aforementioned cooled base.

Table 1
Experimental results of the solidification thermal parameters for the investigated alloys.

Alloy (Al-3Cu-xNb) (wt.%)	VL=f(P)	TR=f(P)
x=0.5	C1 = 2.17 and n = -0.30	C2 = 82.2 and m = -0.70
x=3	C1 = 1 and n = -0.25	C2 = 34.46 and m = -0.73
X=5	C1= 3.4 and n = -0.56	C2 = 43.82 and m = -0.78

In our recent research⁴²,⁴³, with the same investigated alloys and solidification conditions as in the present work, the results showed that the microstructure revealed for the Al-rich phase along the length of the solidified ingot was constituted predominantly by a dendritic network.. In these studies, thermal analysis for non-equilibrium and very close to equilibrium conditions, as well as the typical solidification microstructures showed, during the solidification path, the occurrence of a peritectic reaction represented by Liquid + Al₃Nb→Al_α for the Al-3Cu-xNb alloys (x=3 and 5wt .%), in which the primary Al₃Nb intermetallic phase was observed surrounded by Al-rich dendritic phases (Al_α). Both V_L and T_R were considered as the growth and cooling rates that governed the growth of secondary dendritic branches. Thus, the effects of thermal parameters were evaluated on the length of the dendritic scale (Al_α) for the three horizontally solidified Al-3Cu-xNb alloys. As expected, finer dendritic microstructures, represented by smaller λ₂ were achieved for positions closer to the heat transfer surface, whose ingot positions had the highest V_L and T_R values, as well as reported in the experiments of directional solidification of the references⁹,¹²,¹³,³⁵,³⁶. Power-type mathematical expressions were proposed, given by λ₂=10.1(P)^0.36, λ₂=23(V_L)^-2/3, and λ₂=55.3(T_R)^-1/3 .They characterize the λ₂ variation as a function of these solidification parameters, and the λ₂ values in the as-cast samples selected for the electrochemical tests are consolidated in Table 2. It was noted that the Nb content did not influence the growth laws of secondary dendritic spacing.

Table 2
Simulated experimental values for V_L and T_R from data presented in Table 1, and λ₂=f(P).

Alloy (Al-3Cu-xNb) (wt.%)	Position (mm)	VL (mm/s)	TR (oC/s)	λ2 (µm)
x=0.5	10	1.01	16.4	23,13
	70	0.60	4.20	46.61
x=3	10	0.56	6.42	23.13
	70	0.34	1.60	46.61
x=5	10	0.93	7.30	23.13
	70	0.31	1.60	46.61

3.2. Electrochemical Measurements

3.2.1. Open circuit potential

The study involves changing the potential of the working electrode and monitoring the current that is produced as a function of time or potential. The corrosion potential (E_OC) variations under open circuit potential (OCP) condition with time graphs recorded in 0.2 mol.L^-1 HCl solution for all the three types of specimens and two positions (10 and 70 mm) are shown in Figure 4a. In the initial moments, the E_OC curves for all alloys showed equal characteristics with profiles far apart in the two as-cast positions studied and descending in the first 100 s, and that after this time they grew and stabilized from 600 s, reaching the final values at time equal to 1200 s. Except for the first 100 s, similar open circuit potential behaviors are found in the literature for other aluminum alloys subjected to the same working electrolyte (HCl)⁶-⁸,⁴⁵. The observed trend indicates minimum shift of the E_OC in the negative potential direction and turning to achieving the plateau region after 600 s of exposure for all the investigated specimens. This movement towards less electronegative regions can be explained by the reduction of the active area due to the formation of a protective oxide layer on the metal surface. The initial spacing and then approximation of the curves and E_OC values may indicate that the composition and behavior of the passive films during acid immersion are similar. As can be seen, the final E_OC measurements indicated different values for both positions, demonstrating that the solidification parameters, represented by V_L, T_R and λ₂, as well as the Nb composition, exerted a strong influence on the electrochemical behavior. It was observed that for contents of 0.5 and 3wt.%Nb, lower E_OC values were found for a position equal to 10mm compared to a position equal to 70 mm from the refrigerated mold plate, that is, corresponding to higher and lower T_R and V_L values, whose positions secondary dendritic spacing assumed values equal to 23.4 and 46.61µm, respectively.

Figure 4
(a) Experimental open circuit potential (E_OC) curves as a function of time for the searched alloys in 0.2 M HCl at two positions, 10 and 70mm, from the heat transfer surface, and (b) comparison conducted with works from the literature.

The comparative study shown in Figure 4b has indicated more electronegative E_OC values for the ternary Al-3Cu-xNb (wt.%) and Al-3Cu-2Si wt.%⁷ alloys in relation to the values found for the binary Al-3wt. %Cu alloy⁸. This can be attributed to the presence of intermetallic phases within the interdendritic regions of ternary alloys with high corrosion potential, as reported in⁴-⁸,⁴⁰.

3.3. Polarization measurements

Through the potentiodynamic polarization test, it is possible to determine both the corrosion current density (I_CORR) and the corrosion potential (E_CORR), parameters of fundamental importance, since the corrosion rate is directly proportional to the corrosion current density, so it is often used in electrochemical corrosion analysis⁴,⁵. Thus, the effect of hydrochloric acid on the corrosion rate (I_CORR) of as-cast samples in the two studied positions, 10 and 70mm, of Al-3Cu-xNb alloys (x=0.5, 3 and 5wt.%Nb), was investigated by the method of Tafel extrapolation. The anodic and cathodic polarization curves obtained for the aforementioned ternary alloys in an acid medium of HCl 0.2M are shown in Figure 5a. In general, the polarization results shown in Figure 5a for all Nb compositions indicate that the behavior of the anodic and cathodic curves was similar in both studied solidification positions.

Figure 5
(a) Potentiodynamic polarization curves for Al-3Cux-Nb alloys in 0.2 M of HCl obtained for the two studied samples, 10 and 70 mm from the heat transfer interface, and (b) and (c) represent the electrochemical parameters, E_CORR e I_CORR, respectively, determined by Tafel.

For all the investigated alloys, small displacements of the curves were noticed in relation to the positions in the ingot, that is, in the Al-3wt.%-0.5wt.%Nb alloy the anodic and cathodic curves, position equal to 70 mm, suffered a small displacement to the right, while for the other Nb compositions (3 and 5wt.%) the displacement to the right was observed in the curves of the position equal to 10 mm. Such behaviors allowed observing differences in the results of the electrochemical parameters, potential and corrosion density (E_CORR and I_CORR), as shown in Figure 5b.

In concomitant observation between Figures 4 and 5, values close to E_CORR and E_OC were found, thus demonstrating the effectiveness of the electrochemical tests carried out under the assumed conditions. It was verified that the experimental values found for E_CORR were relatively close for all the investigated alloys and corresponding positions analyzed, allowing to deduce that they can assume values ranging from -694 to -711mV, and that the solidification parameters (V_L, T_R and λ₂ ) as well as the Nb content have little influences, as shown in Figure 5b, but enough to drive different behaviors in the corrosion rate (I_CORR), as can be evidenced in Figure 5c. Different effects of solidification thermal and microstructural parameters on I_CORR have been observed, since for the content of 0.5wt.%Nb, higher V_L and T_R, and lower λ₂ resulted in lower I_CORR, but for higher Nb compositions (3 and 5wt.%), coarser microstructures (higher λ₂) have lower corrosion rates.

Figures 5b and 5c present a comparative study with the Al-3Cu-2Si alloy (wt,%)⁷ solidified under the same solidification conditions of this work. As can be seen, the alloys investigated in this work showed greater resistance to corrosion due to lower corrosion rates (I_CORR). Studies with the Al-3wt.%Cu-2wt.%Si alloy⁷ pointed to the interference of silicon in the composition of the eutectic mixture with the intermetallic phase Al₂Cu in increasing the rate of the interdendritic corrosive process in an acidic medium of HCl 0.2 mol.L^-1. The increase in Nb content, consequently, and the length of the scale of the microstructure play a key role in corrosion resistance. Studies observed that finer dendritic spacing leads to a more homogeneous distribution of Al₂Cu, contributing to the increase in corrosion resistance⁴⁶. On the other hand, for the Al-Si alloy greater corrosion resistance has been attributed to higher λ₂ values due to the dissimilar growth behavior between the dendritic matrix and the eutectic Si crystals, which causes localized tensions⁴⁶. Table 3 consolidates the results obtained E_CORR, I_CORR, and anodic (b_a) and cathodic (b_c) inclinations from Tafel extrapolations, compared with values from the literature⁷. The achieved values of b_a (47.7 to 54.5) and b_c (108.31 to 121.1) for the alloys in this work indicate that they did not cause changes in the reaction mechanism of the corrosion process. It is important to highlight that these values are among those proposed for the Al-3Cu-2Si (wt.%) alloy⁷.

Table 3
Electrochemical parameters obtained by Tafel extrapolation for the Al-3%Cu-x%Nb alloy immersed in 0.2 mol.L^-1 HCl at 25°C.

Alloy (Al-3Cu-xNb) (wt.%)	Position (mm)	ECORR (mV/ECS)	ICORR (mA/cm2)	Tafel slope (mV)
				ba	bc
x=0.5	10	-706	0.492	54.45	121.12
	70	-711	0.883	72.32	108.31
x=3	10	-710	0.916	47.7	115
	70	-703	0.584	56.4	104
X=5	10	-703	0.908	56.48	106.58
	70	-694	0.559	42.5	113.34
Al-3Cu-2Si (wt.%)7	10	-683	1.496	90,8	129.3
	70	-694	1.038	84,1	117.9

3.4. Electrochemical impedance measurements

In this work, analysis by electrochemical impedance spectroscopy was also carried out, with the option of presenting the results in Nyquist graphical form, with real impedance values on the abscissa and imaginary values on the ordinate, as shown in Figure 6a. The Nyquist diagrams are presented as semicircles indicating that the corrosion process is mainly governed by charge transfer with only a capacitive loop at high frequencies and the onset of a small inductance at lower frequencies. Similar impedance characteristics have been reported in the literature for aluminum alloys in various electrolytes such as sulfuric acid, sodium chloride and composite solutions⁴¹,⁴⁷-⁴⁹. The impedance results used to characterize the HCl corrosion of the Al-3Cu-xNb alloys (x=0.5, 3 and 5wt.%) in the ingot positions at 10 and 70 mm were obtained after 2 h of immersion in the working electrolyte. The equivalent electrical circuit used in the modeling is presented in Figure 6b, in which the circuit components and the corresponding Nyquist diagram system are illustrated. This model is applied to spectra that present characteristics close to a semicircle.

Figure 6
Experimental Nyquist diagrams for the searched alloys in 0.2 M HCl at two positions, 10 and 70mm, from the heat transfer surface, and (b) equivalent circuit diagram.

Thus, through polarization resistance (R_P) and electrolyte/solution resistance (R_el), real data of high and low impedance frequencies were calculated, in which the real axis is intercepted by the spectra, considering the difference between these values to obtain R_P, these data are consolidated in Table 4, which presents the Rp and R_el values in series with capacitance elements (C_dl).

Table 4
Electrochemical impedance parameters obtained by simulation with corresponding equivalent circuit.

Alloy (Al-3Cu-xNb) (wt.%)	Position (mm)	Rel (Ω.cm-2)	RP (Ω.cm-2)	Cdl (mF.cm-2)
x=0.5	10	11.51	24.39	8.78x10-4
	70	9.46	27.57	6.28x10-4
X=3	10	6.72	29.45	3.97x10-4
	70	10.67	32.14	4.74x10-4
X=5	10	8.64	23.02	8.63x10-4
	70	9.38	34.25	4.44x10-4

In the presented curves, it was observed that the resistive behaviors highlighted, for the two positions in all the alloys investigated, revealed similar impedance characteristics with spectra in a semicircle format, indicating that the reaction mechanism remains unaltered. Despite the similarity of behavior, the results indicated for all studied compositions that the sample in the position 10mm (close to the heat transfer surface), where higher V_L and T_R were found, and λ₂ has been smaller, have presented lower impedance diameters at all data acquisition points during the time of the experiment. These results clearly demonstrate that the polarization resistance in the position of 70 mm, that is, for coarser microstructures, was superior to that in the position of 10 mm (small λ₂) in the used electrolyte⁷. Higher polarization resistance values were also found for coarser microstructures in Al-3Cu-xMg (x=0 and 0.5wt.%) alloys⁴ after corrosion tests in 0.5 M NaCl solution, but for the Al-3%Cu-3.2%Si⁷, under the same conditions as this work, with results shown in Figure 7, the most fine microstructure was the one that presented the greatest resistance to polarization.

Figure 7
Polarization resistance (Rp) as a function of time for the Al-3Cu-xNb alloys (x=0.5, 3 and 5wt%) at two positions, 10 and 70mm, from the heat transfer surface.

3.5. Typical Solidified and corroded - like Microstructures

Figures 8 to 10 show typical SEM micrographs with point microanalysis by EDS of investigated as-cast samples (70 mm from the heat transfer surface) of the Al-3Cu-xNb (x=0.5, 3 and 5wt.%) alloys before and after the corrosion tests for 24 h in 0.2M of HCl solution. In general, predominant localized corrosion in the interdendritic zones can be observed, characterized by a higher concentration of oxygen that surrounds the Al₂Cu intermetallic particles, as can be evidenced by the results of the microanalyses of elements represented by points 2 and 3 in Figure 8, and points 3 in Figures 9 and 10.

Figure 8
SEM/EDS micrographs for Al-3Cu-0.5Nb (wt.%) at position 70mm in the as-cast ingot from the heat extraction interface, after immersion in HCl (0.2 mol.L^-1) for 24 h.

Figure 10
SEM/EDS micrographs for Al-3Cu-5Nb (wt.%) at position 70mm in the as-cast ingot from the heat extraction interface, after immersion in HCl (0.2 mol.L^-1) for 24 h.

Figure 9
SEM/EDS micrographs for Al-3Cu-3Nb (wt.%) at position 70mm in the as-cast ingot from the heat extraction interface, after immersion in HCl (0.2 mol.L^-1) for 24 h.

The simulated and experimental solidification paths of the Al-3Cu-xNb alloys investigated in the present work were reported by Dillon et al⁴³ and Mendes et al.⁴², and during cooling a peritectic reaction was observed for 3 and 5wt.%Nb contents, represented by Liq + Al₃Nb → Alα⁴². Experimental XRD patterns for the Al-3Cu-5Nb (wt.%) alloy investigated in this work were presented, and intensities/cps of Alα as well as the intermetallic phase Al₂Cu were noted in the XRD patterns, but peaks of Al₃Nb were not detected due to the low fraction of this phase which is partially consumed during the peritectic reaction (Liq + Al₃Nb→ Alα)⁵⁰. On the other hand, thermal DSC and microstructural SEM/EDS analysis revealed the presence of Al₃Nb ⁵⁰, as well as XRD analysis results showed Al₃Nb intermetallic intensities in upward solidified Al-xNb alloys (x=0.8 and 1.2wt.%Nb)⁵¹.

In this work, the primary Al₃Nb-intermetallic phase has its presence confirmed in the microstructure surrounded by the Al-rich phase (Al_α), as can be seen from the element microanalysis performed on point 2 in Figure 9. This intermetallic appears to act as a protective phase of the Al_α matrix against electrochemical corrosion caused by the effects of the galvanic pile that occurs between the matrix and the Al₂Cu-intermetallic phase contained in the interdendritic eutectic mixture. In a comparative analysis with the Al-3wt.%Cu-2wt.%Si alloy⁷, shown in Figure 7, the dissolved Nb in the matrix in the Al-3wt.%Cu-0.5wt.%Nb alloy as well as forming the intermetallic Al₃Nb in the Al-3Cu-xNb alloys (x=3 and 5wt.%), seems to act as a protective phase of the Al matrix against electrochemical corrosion caused by the effects of the galvanic pile that occurs between the Al_α-Al₂Cu and Al_α-(Al₂Cu+Si) contained in interdendritic eutectic mixtures of Al-Cu and Al-Cu-Si alloys, respectively. Intermetallic phases within the intendendrite regions in Al alloys can perform functions as a cathode during the corrosion process, as they are more noble than the Al-rich matrix⁴-⁸,⁴⁰, since the aluminum matrix around these eutectic phases was dissolved in the form of ions, i.e, suffered corrosion in greater proportions when compared to the interdendritic region, as shown in Figures 8 to 10.

Figures 11 to 13 show typical optical micrographs of the two studied samples of all alloys, after solidified and immersion for 24 h in a 0.2 M HCl solution. As can be seen, localized corrosion in the interdendritic regions and in the Al-rich matrix (a) can be observed, as also noted in⁴-⁸. It has been more intensely evidenced for alloys with low Nb content, especially in the Al-3C-0.5Nb alloy (wt.%), as shown in Figure 11. This has reinforced that Nb acts as a protective element against corrosive actions in the Al-rich matrix.

Figure 11
Microstructures of Al-3Cu-0.5Nb (wt.%) alloy in the two investigated positions: (a) and (c) solidified-like samples, and (b) and (d) after 24 hour immersion time in 0.2 M HCl solution.

Figure 12
Microstructures of Al-3Cu-3Nb (wt.%) alloy in the two investigated positions: (a) and (c) solidified-like samples, and (b) and (d) after 24 hour immersion time in 0.2 M HCl solution.

Figure 13
Microstructures of Al-3Cu-5Nb (wt.%) alloy in the two investigated positions: (a) and (c) solidified-like samples, and (b) and (d) after 24 hour immersion time in 0.2 M HCl solution.

4. Conclusions

The following conclusions have been derived from the present investigation:

1. E_OC values found for the investigated alloys indicated that the composition and behavior of passive films during immersion in 0.2M of HCl were similar. It was observed that V_L, T_R and λ₂, as well as the Nb composition, exerted a strong influence on the electrochemical behavior.

2. The potentiodynamic polarization test results for all Nb compositions showed that the behavior of the anodic and cathodic curves was similar in both as-cast positions. Given the solidification conditions assumed for the investigated alloys, it was verified that the experimental values found for E_CORR were relatively close, allowing to assume values ranging from -694 to 706mV, and that the solidification parameters (V_L, T_R and λ₂) as well as the Nb composition had little influence, but enough to lead to different performance in the corrosion rate (I_CORR). Different effects of the thermal solidification parameters and the length of the microstructural scale on the I_CORR were observed, that is, higher V_L and T_R, and smaller λ₂ for the content of 0.5wt%Nb resulted in lower I_CORR, while the opposite behavior was noticed for higher Nb contents (3 and 5wt.%) assumed in the investigated alloys.

3. Analysis by electrochemical impedance spectroscopy in the Nyquist plot represented by semicircles showed that the corrosion process is mainly governed by charge transfer with only a capacitive loop at high frequencies and the onset of a small inductance at low frequencies, and revealed similar impedance characteristics with spectra in a semicircle format, indicating that the reaction mechanism remains unaltered.

4. It was also observed that the solidification parameters, under the assumed conditions, imposed a fundamental role in the behavior of the electrochemical polarization resistance (R_P), since a more accentuated corrosive process was observed for higher V_L and T_R values, and smaller λ₂.

5. The SEM/EDS analysis showed predominantly localized corrosion in the interdendritic regions of all investigated alloys, in which a higher concentration of oxygen enveloped Al₂Cu intermetallic particles. In addition, spot EDS microanalysis indicated the presence of the primary Al₃Nb intermetallic phase surrounded by a dendritic network characterized by the Al-rich phase (Al_α), which seemed to act as protective phases of the Al_α matrix against electrochemical corrosion due to the Al₂Cu intermetallic contained within the interdendritic region.

6. Finally, Nb as dissolved in the matrix forming a solid solution Alα, as well as forming the Al₃Nb-type intermetallics, seems to act as a protective phase against the deleterious effects of corrosion on the dendritic network, caused by the cathodic behavior of the Al₂Cu contained in the interdendritic region. Thus, the highest R_P values achieved in the alloys of this work when compared with those in the literature for the Al-3%Cu-3.2%Si alloy, seem to indicate that Nb acts to delay the corrosive process in the matrix.

5. Acknowledgments

The authors acknowledge the financial support provided by IFPA-Federal Institute of Education, Science and Technology of Pará, Postgraduate Program in Materials Engineering (PPGEMat/IFPA), UFPA-Federal University of Pará, CNPq-National Council for Scientific and Technological Development, Brazil, (Grant: 304924/2020-2), and CAPES – Coordenação de Aperfeiçoamento de Pessoal de Nível Superior- Brasil (PDPG/CAPES (Processo: 88881.707312/2022-01).

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Publication Dates

• Publication in this collection
  14 June 2024
• Date of issue
  2024

History

• Received
  22 Sept 2023
• Reviewed
  07 Jan 2024
• Accepted
  28 Mar 2024