Evaluating the Performance of Aluminum Sacrificial Anodes with Different Concentration of Gallium in Artificial Sea Water

Abstract

In this manuscript, the influence of gallium content additions of Al-Zn-In-Mg alloy was investigated through electrochemical techniques and microstructure observation in 3.5 wt% NaCl solution. The results indicated that Al-Zn-In-Mg-0.03Ga alloy has the best discharge performance among all alloys. We propose that this is due to the fact that gallium addition to the Al-4Zn-In-Mg alloy improves the discharge activity of the alloy as well as elevating its anodic efficiency. In particular, the effect of gallium addition to improve discharge activity tends to be a parabolic curve, in which there is an increase when the gallium is first added that rises to the maximum anode current efficiency of about 98.25% whenever gallium content is 0.03 wt%.

1. Introduction

In the corrosion protective area, cathodic protection (CP) is an electrochemical technique to control the corrosion of iron and steels. This technology is applied worldwide and across several industries to protect metallic structures, including buried piping, exposed marine structures, ship hulls as well as heat exchangers [1,2,3,4,5]. It is an extremely crucial electrochemical method to protect metallic structures by connecting them to either more electrochemically active metals or by supplying negative current from an external circuit. CP has a long history, which dates back to the early 19th century. Nowadays, the demand for effective control techniques is higher than ever due to staggering socioeconomic corrosion effects [6]. The direct corrosion costs of bridges, pipelines, steel construction, etc., are estimated at over USD 1 trillion yearly. The requirement for civil infrastructures’ routine maintenance and replacement is proposed to increase by a factor of 2–5 by 2050 [7]. The cathodic protection method is the key to polarizing metal structures, which requires corrosion protection in the cathodic direction by imposing an electrical current; otherwise, it can be accomplished by introducing a galvanic component with a sacrificial anode. Generally, for sacrificial anodes, it can be explained as a less noble metal connecting to the metal structure or impressing an electricity current with the aid of a DC current source. Aluminum sacrificial anodes are the most utilized method in marine environments due to their high theoretical energy density, rich anode resources, low cost, high current efficiency and capacity. Furthermore, aluminum anodes also have better environmental compatibility, recyclability, and safety [8,9]. However, as is well known, the outermost oxide layer on the surface of aluminum hinders its capability to achieve a reversible and retarded anode activation. In addition, another drawback is hydrogen evolution in the discharge process, in which the anode’s efficiency is significantly decreased. In particular, the most effective strategy to fight against these difficulties is alloying, which can be produced through the casting process [10].

To improve the efficiency of aluminum anodes, a series of alloying elements such as Zn, Sn, Mg, In, Bi, ND Ga have been selected to enhance Al-based anodes [11]. The most common alloying element in aluminum sacrificial anodes is Zn and its constitution percentage ranges from 2.5% to 5.75 wt%. The passive layer of the aluminum damage mechanism with zinc addition is the second phase (β-particles) formation, which can be compressed to avoid the formation of a homogeneous layer on the surface of aluminum [12]. Due to environmental protection and other technical reasons, the aluminum-based sacrificial anode that is mainly used in the oil and gas fields is the Al-Zn alloy, which destroys the oxidation film on the surface by adding Zn, In, Mg and other metallic alloy elements to ensure the continuous corrosion [13]. Another mechanism of the Al-Zn-Mg alloy is to imitate the formation of second phase particles, while lowering the electrode potential of Al and increasing its corrosion current by way of acting like an anode in the Al matrix [14]. However, with the continuous increase in corrosively produced liquid, there is a current efficiency decline and local block falling of Al-Zn-Mg in the aluminum alloy, which results in significantly reduced service life, ineffective protection of the inner wall of the separator, and great potential safety hazards to production [15]. These Zn, In, Sn, and Mg elements when alloyed with aluminum act as activators for aluminum dissolution, suppress the formation of protective oxide films and shift the potential toward negative values. Accompanied by a negative shift in the aluminum anode potential by 0.1–0.3 V, Zn can make the aluminum anode alloy composition uniform and corrosion products fall off easily [16,17,18]. Nestoridi et al. [19] investigated the Al-Mg-Sn-Ga alloy anodes in brine electrolytes; they reported that Sn and Ga together enhanced the dissolution rates of the anodes and made them suitable as anodes for high power density Al–air batteries. The addition of Zn and In increases the hydrogen overpotential of Al anodes to retard parasitic hydrogen evolution, and simultaneously makes the electrode potential of alloy anodes more negative than that of Al anodes in alkaline electrolytes [20,21,22]. Numerous studies have been conducted to find a synergistic interaction between In and zinc (Zn) in activating Al anodes [23,24,25]. The presence of Zn formed a surface enrichment of In by a displacement reaction. Zn lowers the corrosion rate active sites and moderates the localized attack caused by In by forming ZnAl₂O₄.

As a consequence, it was of interest to investigate other activation elements—gallium and zinc—which seem to enhance the efficiency of the sacrificial anodes. When alloyed with aluminum, there is a larger negative shift in free corrosion potential (to −1.3 VSCE) than other active elements; otherwise, the effects of combinations of zinc and gallium were not yet researched. Based on identical results, no significant change in electrochemical behavior was observed when the content of gallium had 0.025–0.1 wt% alternation. However, the corrosion potential was significantly more negative by content of up to 2–6 wt% [26]. Xia et al. [27] investigated Cd and Si elements added to Al-Zn-In anodes, which caused the actual capacity and current efficiency of aluminum alloys to increase from 2000.6 Ah/Kg and 70.7% of Al-Zn-In alloy to 2539.4 Ah/Kg and 88.6% of Al-Zn-In-Sn; 2437.3 Ah/Kg and 85.5% of Al-Zn-In-Cd; and 2500.4 Ah/Kg and 88.8% of Al-Zn-In-Si alloys.

Recently, more researchers have found that gallium content not only affects the corrosion potential of sacrificial anodes, but also the microstructure of the aluminum matrix with the help of zinc elements. With the addition of zinc, a modification of the solubility of gallium is induced as a single phase. There seems to be better metallurgical homogeneity due to zinc resulting in higher efficiencies [28,29]. Muazu Abubakar et al. [30] have found that the highest anode efficiency of Al-Zn-Mg Alloy for sacrificial anodes is 89% by the formation of a Zn intermediate alloy. In contrast, Jingling M et al. [31] investigated the discharge behavior of Al-Mg-Sn/Ga and Al-Mg-Sn-Ga anodes. It can be reported that Sn and Ga together elevate the rapid dissolution of Al anodes and easily improve them to be suitable for working at high power densities. He et al. [32] found that appropriate amounts of Ga or Bi addition in Al-Zn-Sn anodes will promote their electrochemical performance, and adding In and Ga elements reduces the resistance of the passive layer and enhances the Al-Zn anode’s discharge performance.

Thus, in the present manuscript, the authors intend to investigate the influence of gallium content alternation on corrosion potential and improvement of the current efficiency of sacrificial anodes.

2. Materials and Experimental Procedures

The main component of Al anodes was 99.99% pure Al (China Aluminum Co., Ltd., Beijing, China). The sacrificial anodes were made by the cast process. In the first procedure, pure Al was melted in an argon gas resistance furnace at 1073 K. Secondly, pure Zn, Mg, and Ga ingots (>99.99 wt%) were added to the liquid Al at 973 K. During the procedure, the elements were all melted and electromagnetically stirred for 30 min. Finally, the melt was poured into a carbon mold. The entire production process of the alloy anode was carried out under argon gas protection. The actual composition of the anodes is shown in

Table 1. The composition of Al-Zn-Mg-Ga aluminum sacrificial anodes (wt%).

Specimen	Zn	Mg	Ga	Al
1#	4	1	0	Rest
2#	4	1	0.01	Rest
3#	4	1	0.02	Rest
4#	4	1	0.03	Rest
5#	4	1	0.04	Rest
6#	4	1	0.05	Rest

.

The electrochemical experiments were carried out in a traditional three-electrode system. In this system, a Pt sheet (2 mm × 2 mm) was employed as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. Electrochemical experiments were conducted at 298 ± 1 K by an electrochemical workstation based on the current national standard GB/T 17848-1999 “Test method for electrochemical properties of sacrificial anode”. The exposed surface was mechanically abraded with 400-, 600-, 1000-, and 2000-grade grit sandpapers. Subsequently, the sample was ultrasonically cleaned in ethanol before measurement. The current efficiency of the anode sample was evaluated according to the GB/T 17848-1999 accelerated test method for the electrochemical performance of sacrificial anodes. A Ps-268a electrochemical tester was used to adjust the current density; a carbon steel cylinder was used as the auxiliary cathode. The saturated calomel electrode (SCE) was used as the reference electrode, the total area of internal and external working surface was 840 cm², the test medium was 3.5% NaCl solution, and the working temperature was 298 K. For the potentiodynamic polarization tests, the scanning range was from −1.8 to −0.2 V, starting from the negative potential direction, and the potential scanning rate was performed at 0.1 mV/s. When the anode sample was immersed in the corrosion solution for 3 h, the initial open circuit potential was measured, and the corrosion tendency was judged according to the principle of corrosion kinetics. The impedance spectra of the aluminum-based sacrificial anode at working potential were measured. The amplitude of disturbance was 10 mV, and the frequency was 100,000 Hz–0.01 Hz. The working potential conditions were −0.98 V (vs. SCE) and −1.05 V (vs. SCE) polarization potentials. According to the equivalent circuit fitted by the impedance spectra, the electrochemical performance was analyzed. Microstructures were observed by a field emission scanning electron microscope (FE-SEM, FEI Quanta600, USA) equipped with an energy dispersive X-ray spectrometer (EDS, FEI, USA). The 3D measurements were carried out on a confocal laser scanning microscope (LEXT, OLS4000, Olympus, Japan).

3. Results and Discussion

3.1. Microstructures

The microstructural characterization of the cast aluminum base alloy with variations in gallium content was carried out using an optical microscope and is shown Figure 1. These microstructures show that increasing the Ga content of the alloy (in the considered range of 0–0.05 wt%) has a constant effect on the grain size of the alloy. The average grain size measured using the linear intercept method and for each alloy is around 400 μm, 200 μm, 150 μm, 120 μm, 100 μm, and 80 μm, respectively. A gradual decrease was observed for the average-sized alloy. The optical micrograph of the alloy reveals the fragmentation of deleterious second phase particles into finer sizes. When there is no gallium added as in Figure 1a, the second phase particles of alloying elements are easily detected. Diffuse boundaries are eventually observed and indicate that micro-segregation mainly contains higher zinc contents, particularly zinc concentration at the grain boundary during solidification of the alloy. These second phase particles can lower the electrode potential of Al to a more negative value and increase the corrosion current by acting as anode sites in the Al matrix. It can also be considered that there is an even distribution of second phase particles in the microstructural characterization of Figure 1b,d and the Al-based matrix has equally columnar crystals.

3.2. Polarization Behaviors

The potentiodynamic polarization curves of six aluminum alloys were obtained from the potentiodynamic polarization tests, which are represented in Figure 2. The E_corr and i_corr values were calculated by using the Tafel extrapolation method. The mean values of E_corr and i_corr obtained from three independent measurements along with standard deviations are given in

Table 2. Corrosion parameters obtained from potentiodynamic polarization for different aluminum alloys.

Specimen	Ecorr (mV)	icorr (A/cm2)
1#	−1351.58 ± 0.2	3.62 × 10−5 ± 0.6
2#	−1140.45 ± 0.1	9.72 × 10−5 ± 0.4
3#	−1162.85 ± 0.3	1.28 × 10−6 ± 0.3
4#	−1051.56 ± 0.2	6.58 × 10−5 ± 0.5
5#	−1236.93 ± 0.4	5.85 × 10−5 ± 0.7
6#	−1097.78 ± 0.5	7.21 × 10−5 ± 0.4

. From the perspective of self-corrosion potential, the self-corrosion potential of five new sacrificial anode samples showed a small positive shift by adding Ga element, indicating that the addition of gallium slightly reduced the activation performance of 1% Mg-4% Zn series Al-based sacrificial anodes in 3.5 NaCl solution, and the self-corrosion potential of 4# samples increased up to −1.05 mV. The Gallium-rich SEI layer can effectively passivate the highly active Al anode surface and suppress the Al dendrite growth, forming a uniform structure on the Al/electrolyte interface that reduces interfacial impedance. With respect to the current density, after adding a certain amount of gallium, the self-corrosion current density value of six groups of aluminum-based anodes essentially increased, which indicated that the addition of the element gallium can decrease the corrosion resistance of alloys. Based on the above analysis, adding a certain amount of gallium can effectively improve the corrosion resistance of the new sacrificial anode. When the content of Ga is 0.03%, its self-corrosion potential is at the highest level.

3.3. Electrochemical Impedance Spectroscopy

The Nyquist plots of the anodes measured at OCP (open circuit potential) in 3.5% NaCl solution are presented in Figure 3. The proposed equivalent circuit embedded in Figure 3 is employed to analyze the EIS results. From the EIS results, after adding gallium element, the capacitive reactance arc diameter of the 6# sacrificial anode sample is the smallest, and the capacitive reactance arc of the 4# sacrificial anode sample is the largest, which indicates that the dissolution of the 6# anode in seawater is the easiest, and the dissolution of the 4# sacrificial anode in seawater is more difficult than other anodes. For the corrosion resistance investigation, it depends on the capacitive loop in the first quadrant. It can be seen that from the 4# to 6# sacrificial anodes, there is a clear reactance arc, which reflects that ion continuously reacts during the dynamic corrosion process. Accordingly, after element gallium addition, the capacitive reactance arc diameter of the 1% six-group series anodes decreased, indicating that the addition of Gallium in seawater effectively improved the solubility of this series of sacrificial anodes, made the charge transfer easier in the reaction process, and effectively promoted the activation performance of the anodes. Thus, the inductive loop is not accurate for investigating gallium addition. Meanwhile, the Bode diagram and the relationship between phase angle and frequency are also reflected in Figure 4 and Figure 5. It can be found that it was a typically uniform corrosion for all alloys, and the magnitude of modulus |Z| shows that 4# exhibits the best corrosion resistance followed by the other anodes. Furthermore, 4# has the thickest product film, which works as a barrier for its activity dissolution and worse electrochemical activity.

The open circuit potential and current efficiency of the anode sample by adding Ga are shown in

Table 3. Open circuit potential and current efficiency of anode sample by adding Ga.

Specimen	Open Circuit Potential(V)	Theoretical Capacitance Ah/kg	Working Potential V	Weight Loss g	Current Efficiency %
1#	−1.1189	2881.07	−1.0076~−0.8581	0.9586	90.02
2#	−1.0990	2875.45	−1.0880~−0.9587	0.9308	92.89
3#	−1.0391	2875.15	−1.0910~−0.9434	0.9841	87.88
4#	−1.1020	2874.86	−1.0440~−0.8261	0.8802	98.25
5#	−1.1041	2874.55	−1.0410~−0.8842	0.9025	95.84
6#	−0.9901	2874.26	−0.9870~−0.8287	1.0115	85.52

. Electrochemical performance specimen 4# is in the leading position. To sum up, the aluminum-based sacrificial anode can significantly promote the solubility of the anode sample in seawater after adding a certain amount of gallium element. When the content of gallium element is 0.03%, a better corrosion morphology can be obtained to avoid block falling. According to the current efficiency test results of six groups of sacrificial anode samples, the current efficiency of all samples is higher than 85%, meeting the type I requirements specified in GB/T 4948-2002 standard (Sacrificial anode of Al-Zn-In series alloy 29 August 2002). The current efficiency of 1#, 2#, 4#, and 5# is higher than 90%, which meets the requirements of type I. Specifically, the current efficiency of the 4# sample is the best, reaching 98.25%. It can be compared with the bibliographical references that state the current efficiency of Al-Zn-In alloys are around 70 to 90% [33,34,35,36].

3.4. Surface Micromorphology Analysis

The surface morphologies of the different anodes after potentiodynamic polarization tests are shown in Figure 6. Based on Figure 6, the sites around grain boundaries are corroded first, where grain boundaries have defects with excess free energy per unit area. From the corrosion surface of specimen 1# without gallium addition in Figure 6a, it can be seen that there are very large corrosion pits with heterogeneous distribution. With the increased gallium content in the alloys, it can be inferred from Figure 6b–f that the size differences and position distribution of the corrosion pits present diverse properties. According to Figure 6a,f, the entire surface of alloys 1# and 6# after discharge is very rough with many isolated and randomly distributed pores. In contrast, there are some small pores on the surface of alloys 4# and 5#, and the entire surface is smooth. From the macroscopic appearance of the 6# sacrificial anode in Figure 6f, there are obvious problems such as falling block and uneven, which is mainly caused by the rapid dissolution of the 6# sacrificial anode in seawater. However, the 4# and 5# anode surfaces are corroded evenly, with little corrosion trace and no block falling. It can be concluded that the content of 0.03 wt% gallium addition could reduce the difference between the crystal and the grain boundary, promoting the uniform discharge of the alloy in the 3.5 wt% NaCl solution. Furthermore, Figure 7 reflects the SEM images of surface morphologies and the corresponding elemental distributions for the six investigated anodes with different Ga percentages. From the resulting elemental distributions of Ga element, a typical “dissolution pit” with distinct gallium decreases around the corrosion pit on each alloy surface. In particular, the serious intergranular corrosion on the surface was mainly attributed to the component segregation of gallium at the grain boundaries. In fact, the content of 0.03–0.04 wt% gallium in the alloy can effectively improve the properties of the anodes.

Corrosion pits by laser confocal scanning microscopy for six alloys in the optical images of corrosion surface can be observed in Figure 8a–f, a cross-section profile of the corrosion surface is shown in Figure 8g–l and the 3D structure of the corrosion surface is presented in Figure 8m–r. We found that the morphology of the corrosion products varies significantly when the gallium content increases from 0.01 wt% to 0.05 wt%. Typically, localized corrosion and uniform corrosion are considered to be two of the main corrosion types, discussed separately. Both pitting morphology and the products of uniform corrosion have been clearly observed. If a corrosion pit is initiated, it will grow both vertically and horizontally. The dissolution of metal in the initiation process produces anodic ions around the corrosion holes, resulting in the migration of chloride ions from the electrolyte towards the anodic sites. From the 3D structure of the corrosion surface in Figure 8m–r, it is found that the average vertical and horizontal dimensions of corrosion pits on the 4# and 5# specimen surfaces are uniformly distributed.

4. Conclusions

(1) Six investigated anodes with different Ga percentages were successfully fabricated under argon gas protection. The increased Ga content of the alloy has a constant effect on the grain size of the alloy. There is an even distribution of second phase particles in 2# and 4# and the Al-based matrix has equally columnar crystals. The current efficiency of the 1#, 2#, 4#, and 5# anode samples is higher than 90%, which meets the requirements of type I. Specifically, the current efficiency of sample 4# is the best, reaching 98.25%.

(2) The self-corrosion potential of five new sacrificial anode samples showed a small positive shift by adding Ga element, indicating that the addition of gallium slightly reduced the activation performance of the original Al-based sacrificial anode in seawater, and the self-corrosion potential of sample 4# increased up to the highest point, to −1.05 mV. From the EIS results, after adding gallium element, the capacitive reactance arc diameter of the 6# sacrificial anode sample is the smallest, and the capacitive reactance arc of the 4# sacrificial anode sample is the largest.

(3) From the 3D structure of the corrosion surface in the six specimens, it is found that the average vertical and horizontal dimensions of the corrosion pits on the 4# and 5# specimen surfaces are uniformly distributed. The content of 0.03–0.04 wt% gallium in the alloy can effectively improve the properties of the anodes.