Introduction

Pure Pb is too fragile to be used in batteries. It is easily corroded upon immersion in H₂SO₄. Within the battery, pure Pb anode will stretch and deform, which causes PbO₂ layer to crack, exposing it to oxidation and accelerated CR. This has been proven by previous researchers ¹. Pb-acid batteries also suffer from sulfation phenomenon. This is indicated by the formation of PbSO₄ that covers the Pb surface and prevents electrochemical reactions to occur, thus reducing the battery life ². This is why extensive research has recently been made to develop some alloys based on Pb with better corrosion resistance and hardness, allowing batteries to last longer. Some researchers have chosen to add some chemical products into the battery electrolyte, e.g., P compounds such as H₃PO₄ and its salts, which have given some very remarkable results for decreasing sulfation intensity ^3-6. Other scientists have elaborated Pb-based alloys that will give not only stronger corrosion resistance but also more hardness to the metal, which will add up to the battery lifetime. Pb-Ag alloys are among the most effective ones. Ag has shown remarkable abilities to harden Pb anodes, offering them better resistance to H₂SO₄ attack. Researchers have done some studies about Ag mechanical effect on Pb anodes. It has been found that Pb hardness and Pb-Ag alloy structure essentially depend on the Ct of Ag. For Ct from 0.1 to 0.3 wt%, Pb-Ag structure is very similar to that of pure Pb ⁷. It has been found that Ag addition to Pb/PbO₂ electrode significantly decreases OER surge and E_a, of which effect is stronger with higher Ct ⁸. ⁹ have discovered that Ct of 1 wt% Ag decreases Pb anodic corrosion. Because Ag is expensive, the authors have added small quantities of Ca to Pb-Ag, which has improved even more corrosion resistance. These results have been confirmed by other researchers who have found that alloys of Pb with 0.5 and 1% Ag are the only ones that can be widely used in industry, due to their better corrosion resistance ^10-12. Also, Ag effect on pure Pb and Pb-Ca-Sn ternary alloy has been studied ¹⁴. The research has shown that 0.1% Ag has a delaying effect on overvoltage phenomena appearance, increasing Pb-Ca-Sn corrosion resistance in conditions that simulate the charge end, when Ag is added at a Ct of 0.05 wt% ¹³. Some researchers who have evaluated Pb anodes reactions in H₂SO₄ have concluded that, concerning corrosion resistance level, higher Ct of Ag offer better resistance to the alloy. They have classified certain alloys according to their corrosion resistance: Pb/Ag > Pb/Ca/Sn > Pb/In > Pb/Ag/Ca > Pb/Sb > Pb/Ca > Pb.

Prior to this work, the effect of P chemical compounds on the CR of Pb has been assessed. It has been proven that they strongly reduce the CR of Pb in batteries. The electrochemical and metallurgical behavior of Pb-Al alloys immersed in 4 M H₂SO₄ has also been studied. The results were remarkable, since it was possible to increase the battery lifetime up to 51.11%, by adding only 1.5% Al. Al-Pb also caused a decrease in I_pass, and in passivity bearing size (especially for Pb-1% Al). This means that Al facilitated PbO and PbSO₄ transformation reactions into PbO₂, which significantly reduces Pb-acid battery sulfation phenomena ¹⁵. T was also studied, and it has been found that its negative effect decreased, and that the battery lifetime was longer than that of pure Pb ¹⁶.

Herein, it was decided to study the effect of a different kind of addition, i.e., Ag at small quantities, on Pb corrosion and hardness. The difference between the previously published paper and the present work is that Ag is more resistant to corrosion than Al, since Pb-Ag alloy is more stable than the Pb-Al one. Therefore, the CR for Pb-1% Ag (17.13 mm/year) was lower than that obtained of Pb-1.5% Al (20.78 mm/year). The goal of this research was to study the electrochemical and metallurgical effect of Ag addition to Pb anodes in a H₂SO₄ environment similar to the battery electrolyte, to increase its life and improve its performance. Most research carried out on this subject has used gravimetric procedures. However, these methods do not provide enough information on the continuing evolution of corrosion phenomena, unlike polarization methods that instantly give CR ¹⁷. To evaluate Ag effect on Pb corrosion and mechanical stress resistance, PDP and EIS electrochemical methods, and OM and hardness metallurgical techniques were used.

Experimental procedures

Alloys preparation

The studied alloys were made of pure Pb and Ag (99.99%). They were Pb based with Ag addition at different Ct of 0.5, 0.8, 1 and 1.5%, respectively. In order to prepare these alloys, appropriate quantities of Pb and Ag were introduced in a silica glass tube of 1 cm diameter. Then, the tube was sealed under high vacuum to prevent Pb oxidation. The alloy and the tube were introduced in an oven at 500 ºC. After fusion and cooling, the assembly was soaked in cold water (≈10 ºC). The prepared alloys were studied right away, or preserved in liquid N.

To obtain reliable and reproducible results, before each test, the alloys were polished with 400, 600 and 1200 abrasive papers. Then, they were rinsed with distilled water and dried.

As shown in Fig. 1, Pb-Ag binary system presents a eutectic bearing at 304 ºC.

The composition of the eutectic liquid was approximately 2.5 wt% Ag. The solubility limit in eutectic T was 0.l wt% Ag. Before and after the alloys elaboration, the chemical composition of pure Pb and Pb-Ag alloys was analyzed, to determine the exact quantities of each element, including impurities. In doing so, portable Thermo Scientific Niton XL5 XRF spectrometer was employed. It is the newest and faster device that gives accurate results and exact composition on any metal surface. Each chemical composition represented in Table 1 is the result of a punctual analysis.

Hardness

The physical properties of Pb alloys quenched solid solutions evolve from room T. Hardening mechanisms are continuous/discontinuous transformations. In fact, this T corresponds to 0.5 TF (alloy’s melting T: 330 ºC). It is known that, from 0.4 to 0.5 TF, the alloy elements can diffuse, whenever the discontinuous transformation kinetics is rapid at room T. The alloys used in this study were polished, soaked in a chemical solution consisting of one to three parts part of 30% H₂O₂ and glacial CH₃COOH. The immersion process duration was varied from 20 s to 2 min, depending on the sample state. The chemical polishing was followed by repeated chemical attacks/etching using a mixture based on C₆H₈O₇ and H₈MoN₂O₄.

Electrochemical techniques

A classic cell of 3 electrodes was used to carry out electrochemical measurements. Saturated calomel, Pt wire and a Pb sample were used as reference, auxiliary and working electrodes, respectively. Before each test, the sample was left under OCP, for 5 min, to achieve a steady state. PDP was carried out at a SR of 2 mV/s, in the range from -1500 to 2500 mV. Various kinetic parameters such as I_corr, E_corr and I_pass were obtained. I_corr was measured by Tafel straight lines extrapolation. EIS measurements were carried out using a margin of frequency range from 100 kHz to 10 Hz, at E_corr (450 mV). For the analysis, a 10 VoltaLab model (PGZ100) connected to an HP computer was used. Data acquisition and processing was performed by using VoltaMaster 4 and OriginLab software ¹⁶.

Results and discussion

Hardness

Fig. 2 represents Pb-Ag alloys hardness values evolution in comparison with those of pure Pb.

It is clearly seen that, with higher Ct of Ag, the alloy hardness increased, going from 5 HV, for pure Pb, up to 30.2 HV, for Pb-1.5% Ag. This effect was probably due to the decrease in the alloy grains initial size after Ag addition.

PDP

Ct of Ag effect on Pb corrosion

To study the electrochemical effect of Ag addition on the CR of Pb in 4 M H₂SO₄, PDP and EIS were used. To do so, four electrodes (Pb with 0.5, 0.8, 1 and 1.5% Ag) were prepared by the above described procedure. So as to study Ag effect, it was required to compare polarization curves and EIS diagrams obtained by Pb-Ag electrodes with those obtained by pure Pb immersed in 4 M H₂SO₄.

Fig. 3 shows polarization curves of pure Pb and Pb-Ag alloys with different Ct of Ag in 4 M H₂SO₄, at 25 ºC.

Tafel lines intersection points gave I_corr. Polarization parameters are shown in Table 2.

The intense decrease in I at polarization curves beginning corresponds to HER. At this E margin, Pb is at the activation state. This oxidation resulted from the reaction between Pb and H₂O:

E_corr2 corresponds to the reaction of Pb with H₂SO₄, which forms PbSO₄, according to the following eq.:

PbO and PbSO₄ precipitate on the Pb surface, preventing H₂SO₄ infiltration, thereby inhibiting any reaction between Pb and the electrolyte. This is the passivation process phase represented by an I_corr bearing, which remains virtually constant for a given period of time. This phase ends with transpassivation. At this stage, the corrosion products (PbO and PbSO₄) are transformed into PbO₂, by reaction with H₂O, as follows:

This transformation is indicated by a polarization peak characterized by E_tra, followed by a strong increase in I that corresponds to OER.

By observing polarization curves in Fig. 3, and corresponding data in Table 2, it is seen that Ct of Ag up to 1% reduced E_corr1 and E_corr2 from -536 to -493 and -463 to -421 mV, respectively, which means that Pb was protected from corrosion. Also, Ag addition strongly reduced I_corr from 149 to 19 µA/cm², and I_pass from 125 to 17 µA/cm², indicating that, at this Ct, Pb corrosion resistance significantly increased. The rate of impermeable elements that turns Pb into a passive mode also decreased and, consequently, the battery lifetime increased. This can be concluded from higher IE(%) of 87.24% after adding 1% Ag (as shown in Fig. 4), which confirms that the battery lifetime will be significantly longer.

¹⁸ has done some research on the effect of Ag and Ca addition on Pb corrosion in H₂SO₄. The author has found that the alloy with the best IE(%) was Pb-1% Ag. Also, ¹⁹ has found that this quantity of Ag has given the alloy the best resistance to corrosion.

By adding 1% Ag, Pb oxidation to PbSO₄ was strongly inhibited. This can be associated to the fact that the anodic film higher density prevents atoms at the PbSO₄ membrane from accessing O.

E_tra decreased with the increase in the Ct of Ag up to 1%, meaning that the metal facilitated PbO₂ formation followed by OER. As polarization curves in Fig. 3 show, Ag had no noticeable effect on OER.

Fig. 4 illustrates the increase in IE(%) as a function of Ct of Ag.

IE(%) was calculated using the equation:

where C_R’ and C_R are the pure Pb and the alloy CR, respectively.

E_tra bearing decreased with the increase in the Ct of Ag, which means that the metal facilitated PbSO₄ and PbO conversion into PbO₂. This decrease was followed by OER. The parts of polarization curves which indicate these reactions did not change, i.e., Ag had no significant effect on OER.

Effect of T on pure Pb and Pb-Ag alloys

The maximum T that a Pb-acid battery can achieve is approxim. 80 ºC. That is why it was necessary to study T effect on Pb and Pb-Ag alloys at different Ct.

Fig. 5(a) shows Pb polarization curves in H₂SO₄ at T range from 25 to 80 ºC, while Table 3 shows polarization parameters.

It can be seen that the increase in T from 149 to 162 µA/cm² had a significant effect on I_corr. This means that the battery lifetime shortened every time T was risen inside it. The small fluctuations noticed in other parameters might have been due to the analysis conditions.

According to the obtained results, which are represented in Fig. 5 (b, c, d, e) and Table 3, it is clearly seen that Ag addition had no negative effect on Pb when T was increased up to 80 ºC. On the contrary, fluctuations in I_corr decreased for alloys containing from 0.5 to 1% Ag.

Some conclusions about the inhibition action mechanism can be taken by comparing measured E_a with and without inhibitor. Fig. 6 represents Arrhenius coordinate plot for CR of Pb in 4 M H₂SO₄ with and without Ag.

I_pass, E_corr1, E_corr2, E_tra and CR remained practically constant, and, consequently, also the battery lifetime. To be able to study T influence on the corrosion inhibition process, many authors have used Arrhenius equation:

where k and k’ are constants (Arrhenius’s pre-exponential parameters), E’_a is E_a with inhibitor and R is universal gas constant. T is in Kelvin.

The variation in i_corr logarithm, as a function of T^-1, indicates that Arrhenius law was obeyed, which was confirmed by R² and R^2' > 0.90 good linearity.

E_a values were obtained from the slopes of Ln (I_corr) lines, as a T^-1 function. These values are given in Table 4.

Table 4 shows that E_a value decreased after 1% Ag addition. This behavior is due to the presence of a solid solution phase that increased E_a barrier for corrosion, and therefore, decreased the alloy CR. These results reinforce those obtained for the decrease in the CR of Pb after Ag addition.

EIS

Ct of Ag effect on CR of Pb

Fig. 7 shows Nyquist representations of pure Pb and Pb-Ag alloys in 4 M H₂SO₄, at 25 ºC. R_ct and C_dl values at E_corr = -536 mV/ECS are listed in Table 4.

Nyquist representations in Fig. 7 have the shape of a unique semi-circle, which means that the main reaction mechanism present at C_dl was R_ct. With an increase in the Ct of Ag up to 1%, it is seen that the semi-circles diameter was wider. This indicates a stronger corrosion resistance at this Ct of Ag. As shown in Table 5, R_ct increased from 18 to 150 ohm/cm² with a raise in Ag up to 1 wt%, while C_dl decreased from 75 to 9 µF/cm². This reveals that the CR of Pb decreased and, consequently, its lifetime increased. Beyond this Ct, CR began to increase again ¹⁶.

Conclusions

The results of the present study on the electrochemical and metallurgical effect of Ag addition to Pb can be resumed as following: Ag addition up to 1.5% enhanced Pb hardness from 5 to 30 HV; Ag addition up to 1% reduced I_corr, increasing the battery lifetime; Ag decreased I _pass, i.e., it facilitated PbO and PbSO₄ oxidation to PbO₂; Pb-1% Ag alloy proved to be the most resistant to corrosion. The battery lifetime became two times longer; the reaction mechanism that governs the metal surface is R_ct mechanism; every time T rises inside the Pb-acid battery, the CR of its grids increases, but Ag addition up to 1 wt% significantly reduced this effect.

Authors’ contributions

Salma Khatbi: carried out the experiments and wrote the manuscript. Youssef Gouale: provided metals for the alloys elaboration. Abdeslam Lamiri and Mohamed Essahli: gave directions and necessary resources for this project.

Statements and declarations

On behalf of all authors, the corresponding author states that there was no conflict of interest.