# Role of S2− ions on the microstructure change and the pitting behaviour of aluminum in saline solution

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## Abstract

The electrochemical behaviour and the passive film microstructure of aluminum during its exposure to 3.5 wt% NaCl solution in the absence and presence of S2− ions are investigated using potentiodynamic polarization curves, electrochemical impedance spectroscopy measurements, XRD, XRF, SEM and AFM. Electrochemical measurements show that the presence of S2− ions enhances the uniform corrosion of aluminum in NaCl solution, but delay its susceptibility to the pitting corrosion. In addition, EIS analysis illustrate that the formation of more compact and protective passive layer in the presence of S2− ions compared to its rough surface in the absence of S2− ions as evidenced by the lower value of constant phase element (CPE) and higher value of phase shift (N). Cracks, non- homogenous and open large pits with high degree of roughness are clearly observed on the aluminum surface in the absence of S2− ions, compared to oriented grooves, elongated ridges with the accumulation of the corrosion products inside the pits in the presence of S2− ions. The inhibitory effect of S2− ions for the pitting corrosion of aluminum is interpreted on the basis of the change in its microstructure of the passive film in the absence and presence of S2−ions.

## Introduction

Aluminum and its alloys are widely used in industrial applications, because of their good mechanical and industrial properties as well as, their highly corrosion protection1,2,3,4. Aluminum when immersed in chloride solutions, it will expose to the pitting corrosion, that limits its industrial and marine applications5. The pitting corrosion of 7A60 aluminum alloy in 3.5 wt% NaCl solution was investigated using electrochemical impedance spectroscopy, electrochemical noise, scanning electron microscopy and energy dispersive spectrometer6. Results revealed that the intense pitting corrosion in 7A60 aluminum alloy is explained to the presence of electrochemical active MgZn2, Al2MgCu and Mg2Si, while the contribution of Al7Cu2Fe to the pitting corrosion is little. Zhang et al.7 studied the relationship between the intergranular corrosion and the crystallographic pitting in AA2024-T351 aluminum alloy in 3.5 wt% NaCl solution. It was found that the intergranular corrosion occurs firstly, and the crystallographic pitting initiates from the crevice wall behind the intergranular corrosion front.

Sulphide polluted saline solutions are very aggressive medium for the corrosion of many metals, that used in many industrial installations processing sulfides. Corrosion behaviour of Cu-Al-5Ni alloy in 3.5 wt% NaCl solution containing different concentrations of sulphide ions was studied using electrochemical impedance spectroscopy measurements8. Data showed that, increasing the sulpide ion concentrations leads to increase the corrosion rate of this alloy that can be explained on the basis of the presence of Cu2S that decrease the protective efficiencies of Cu2O film. Dissolution of aluminum in 0.01 M NaOH solution as a function of sulphate, nitrate and sulphide ion concentrations was evaluated using electrochemical noise measurements9. It was determined that the corrosion rate of aluminum in the alkaline solution was occurred more seriousness in the presence of these additives, as concluded from the positive shift in the frequency to more highly frequency region. Sherar et al.10, studied the effect of the sulphide ions on the aerobic corrosion of steel in near neutral saline solution using scanning electron microscopy, energy dispersive X-ray analysis and Raman spectroscopy. They found that the addition of the sulphide ions to the aerobically-exposed surface doubles the corrosion rate compared to its value in the sulphide free solution. Electrochemical performance of Cu, Cu-10Al-10Ni, Cp-Ti and C-steel in 3.5 wt% NaCl containing 2 ppm Na2S was studied11. Results clarified that the addition of S2−, shifted the corrosion potential for all alloys to more negative values except Cp-Ti that be shifted to more positive.

Morphology and microstructure of the passive film play an important role for its mechanical and electrochemical properties12. Liu et al.13 studied the effect of the abrasion on the microstructural and pitting corrosion of 2297Al-Li alloy in borate-buffered 0.001 M NaCl. Data revealed that the pitting corrosion of this alloy was strongly affected by the size, the population density and the area fraction of intermetallic particles as well as its smoother surface has lower pitting ability and better corrosion resistance compared to the rougher surface. In addition, the crystallographic orientations within a microstructure was observed to be considerable effect on pitting corrosion. It was observed the relation between the polarization and pitting behaviors with the surface orientation in pure aluminum and aluminum alloys14,15,16,17,18,19,20.

Effect of the microstructure on the corrosion behaviour of cast Mg-Al alloys in 5 wt. % NaCl solution saturated with Mg(OH)2 was studied21. Data showed that the corrosion protection of homogeneous α-phase increases with increasing Al-content, due to the higher chemical stability of α-phase with higher Al contents compared to its stability with lower Al contents. The effect of heat treatment on the electrochemical corrosion and microstructural behaviour of API X70 line pipe steel in sea water containing thiosulphate solutions was evaluated22. Data implies that the presence of different phases of ferrite during heat treatment. Polygonal ferrite and fine grained ferrite microstructures formed at 600 °C inhibits the corrosion rate compared to the tempered martensitic microstructure formed at 300 °C. The inhibition effect was explained on to the difference in the diffusion rate of the corrosion product across the fine and coarse grained microstructure. Corrosion protection of PEO coating on extruded Al6Cu alloy in 3.5% wt NaCl was evaluated23. The corrosion protection of Al6Cu alloy was enhanced in the presence of PEO coating compared to as-cast alloy due to the change in its microstructure.

Yang et al.24 studied the effect of sulphide ions on the passivation behaviour of titanium TA2 alloy in simulated seawater solutions. They found that the passive film was composed of TiO2 top layer with TiO sub-oxide layer and the highly resistance of Ti substrate was correlated to the presence of TiO2 top layer. In addition, the presence of low concentrations of highly amount of TiO2. While at highly concentration of S2− ions (>2 mM/L) decreased the corrosion resistance due to the highly fraction of TiOS and TiS2 compared to TiO2. The influence of sulphide ions on the passive behaviour of super 13Cr martensitic stainless steel in borate/NaCl solution was investigated25. Results showed that the addition of sulphide ions increased the values of the corrosion current and shifted the pitting potentials to more noble values. Moreover, the presence of sulphide ions changed the microstructure and the crystallinity of the passive layer.

By far, to our knowledge, little work reported the effect of the microstructure of the passive film on the pitting behaviour of aluminum in sulphide polluted saline solution. Therefore, the goal of our present work is to investigate the influence of the microstructure of the oxide passive film on the uniform and pitting corrosion behaviour of aluminum in 3.5 wt% NaCl solution in the absence and presence of different concentrations of Na2S using atomic force microscopy, scanning electron microscopy, X-ray fluorescence, X-ray diffractometer, potentiodynamic polarization and electrochemical impedance spectroscopy techniques.

## Experimental

### Materials

Sodium chloride, sodium sulphide and acetone are provided from Merck Chemical Co. (Germany). Working electrode is made from aluminum rod with the following chemical composition (wt%): 99.57% Al, 0.31% Fe, 0.07% Si, 0.015% Ti%, 0.0016% Zn, 0.0003% Cr, 0.0019% Mg, 0.0021% Mn and 0.0007 Cu.

### Microstructure characterization

The surface roughness of aluminum samples is examined using AFM measurements (Nanoscopic III E controller, Digital Instruments, Santa Barbara, CA). All AFM experiments are performed with soft cantilevers: n+- doped Si cantilever from Nanosensors (PPP-CONT-10), kn = 0.09 N m−1. Scanning electron microscopy is achieved using JSM-6510LA (JEOL, Tokyo, Japan). Surface composition, crystallinity and phase identification of aluminum samples are investigated using X-ray fluorescence (ARLTM Perform’x Sequential XRF spectrometer, Thermo Scientific) and X-ray diffractometer (PANalytical Empyrean, Netherlands).

### Electrochemical measurements

All the electrochemical measurements are carried out using the Potentiostat/Galvanostat (AUTOLAB PGSTAT 128 N), using a standard three-electrode cell with Al (1.0 cm2) working, saturated Ag/AgCl reference and Pt sheet (1.0 cm2) counter electrodes. NOVA 1.10 software is used to records and fits the electrochemical measurements. Potentiodynamic polarization measurements are achieved in the potential range between −100 to 200 mV vs. EOCP values at 30 °C with the scan rate of 1.0 mV s−1. Electrochemical impedance spectra at the respective EOCP values are recorded using AC signals of amplitude 5 mV peak to peak in the frequency range of 10 kHz to10 MHz. Prior to each experiment, working electrode is polished successively with fine grade emery papers, cleaned with acetone, washed with bi-distilled water and finally dried.

## Results and Discussion

### Electrochemical measurements

#### Polarization measurements

Figure 1a represents the potentiodynamic polarization measurements of aluminum electrode in x wt% NaCl solution (x = 0.5–3.5), while its electrochemical parameters derived from the polarization curves (Ecorr, Epit, Icorr, βa, βc) are listed in Table 1. Results show that the presence of active/passive/pitting as the electrochemical behaviour of aluminum for all studied concentrations. It is observed that, increasing the concentrations of Cl ions, shifts both the corrosion potential (Ecorr) and the pitting potential (Epit) to more negative values as well as increases the values of the corrosion current densities (Icorr) from 0.6 μA cm−2 in case of 0.5 wt% NaCl solution to 2.1 μA cm−2 for 3.5 wt% NaCl solution. This finding explains that the rate of both uniform and pitting corrosion of aluminum increases as a results of the aggressive attack of Cl ions. Mechanism of the anodic and cathodic reactions associated with the uniform corrosion of aluminum in Cl ion solutions based on the previously reported is explained as follows26,27,28:

• 1Cathodic reaction:

$${{\rm{O}}}_{2}+2{{\rm{H}}}_{2}{\rm{O}}+2{{\rm{e}}}^{-}\to 4{{{\rm{OH}}}^{-}}_{(\mathrm{sol})}$$
(1)
• 2Anodic reactions:

$${{\rm{Al}}}_{(s)}+{{\rm{3e}}}^{-}\to {{{\rm{Al}}}^{3+}}_{(\mathrm{sol})}$$
(2)
$${{{\rm{Al}}}^{3+}}_{({\rm{sol}})}+{{\rm{3OH}}}_{(\mathrm{sol})}^{-}\to {\rm{Al}}{({\rm{OH}})}_{3({\rm{s}})}$$
(3)
$$2{\rm{Al}}{({\rm{OH}})}_{3(s)}\to {{\rm{Al}}}_{{\rm{2}}}{{\rm{O}}}_{3(s)}+3{{\rm{H}}}_{2}{\rm{O}}$$
(4)

Moreover, the pitting corrosion of aluminum is explained on the basis of the reaction between the adsorbed Cl ions with Al3+ in the crystal lattice of the dual nature Al2O3 passive film, that consists of an adherent, compact and stable inner film covered with a porous, less stable outer film to form soluble oxychloride complex as shown in Eq. (5)29,30.

$${{{\rm{Al}}}^{3+}}_{(\mathrm{crystal}\mathrm{lattice})}+2{{{\rm{Cl}}}^{-}}_{(\mathrm{ads})}+2{{{\rm{OH}}}^{-}}_{(\mathrm{sol})}\to {[{\rm{Al}}{({\rm{OH}})}_{2}{{\rm{Cl}}}_{2}]}^{-}$$
(5)

The presence of the adsorbed chloride ions on the Al2O3 passive film is confirmed using XRF analysis as shown in Table 2. Our results are in a good agreement with Soltis20 and Tang et al.31, who reported that the pitting potential (Epit) was shifted to more negative values with increasing the concentration of the aggressive ions according to the following relation:

$${E}_{{\rm{pit}}}=A-B\,\mathrm{log}\,[{{\rm{Cl}}}^{-}]$$
(6)

where A and B are constants, whilst A measures the aggressiveness of the chloride ions at a given concentration. Explanation of this trend can be illustrated as follows:

• At low Cl concentration, Al3+ ions in the crystal lattice of the passive layer prefers to hydrolyze with OH rather than reacts with the Cl ions, therefore the growth and the propagation of the pits are decreased27.

• At high Cl concentration, Al3+ ions react with Cl ions producing soluble complex, that accelerates both the nucleation and the propagation of the pits29,31.

Figure 1b represents the effect of Na2S concentrations on the anodic and cathodic polarization curves of aluminum and the detailed electrochemical parameters are listed in Table 3. The results imply that the uniform corrosion of aluminum increases with increasing the S2− ion concentrations without any sign for pitting corrosion within the studied polarization range. Mechanism of the uniform corrosion of aluminum in S2− solution can be summarized as previously reported8,32,33,34, S2− ions is hydrogenated to give HS, that reacts with aluminum to produces Al2S3 and finally Al2O3 according to the following equations:

$${{\rm{S}}}^{2-}+{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{HS}}}^{-}+{{\rm{OH}}}^{-}$$
(7)
$${{\rm{Al}}}^{3+}+{{\rm{HS}}}^{-}+{{\rm{OH}}}^{-}\to {{\rm{Al}}}_{2}{{\rm{S}}}_{3}+{{\rm{H}}}_{2}{\rm{O}}$$
(8)
$${{\rm{Al}}}_{2}{{\rm{S}}}_{3}+{{\rm{H}}}_{2}{\rm{O}}\to {\rm{Al}}{({\rm{OH}})}_{3}+{{\rm{H}}}_{2}{\rm{S}}$$
(9)
$$2{\rm{Al}}{({\rm{OH}})}_{3}\to {{\rm{Al}}}_{2}{{\rm{O}}}_{3}+{{\rm{H}}}_{2}{\rm{O}}$$
(10)

The presence of Al2O3 as a passive film instead of Al2S3 is confirmed using XRF analysis (c.f. Table 2). On the other hand, the production of OH during the hydrogenation of S2− ions as shown in Eq. (7) increases the solution pH, that explains the enhancement of the uniform corrosion rate of aluminum with increasing the concentration of S2− ions.

Addition of different concentrations of Na2S (1–5 ppm) to 3.5 wt% NaCl solution, enhances the uniform corrosion of aluminum, compared to 3.5 wt% NaCl and 5 ppm Na2S solutions as indicated from the values of both Ecorr and Icorr as shown in Tables 1, 3 and 4. In contrast, the presence of Na2S in NaCl solution inhibits the pitting corrosion of aluminum as can be seen from Fig. 1c. The inhibition effect of S2− ions on the pitting corrosion of aluminum can be seen from the decreasing in the values of the Ipit and increasing the difference between the values of Ecorr and EpitE = Ecorr − Epit) with increasing S2− ion concentrations (c.f. Fig. 1c).

#### Electrochemical impedance spectroscopy measurements

Electrochemical impedance spectroscopy measurements are performed to evaluate the Al/electrolyte interface with different electrolyte composition. Figure 2 represents the Nyquest plots of aluminum surface recorded at EOCP in the presence of 3.5 wt% NaCl, 5 ppm Na2S and mixture of 3.5 wt% NaCl + 5 ppm Na2S solutions. Results point out that the presence of two consecutive capacitive semicircles in 5 ppm Na2S solutions, that have been interpreted to the dual nature of Al2O3 passive film including an inner compact layer covered with outer porous layer35. Whereas a single semicircle is observed in both 3.5 wt% NaCl and 3.5 wt% NaCl + 5 ppm Na2S solutions. This observation has been explained as a results of the adsorption of Cl ions on the Al2O3 passive layer, which reacts with Al3+ in its crystal lattice, forming soluble oxychloride complex as discussed before, leads to the dissolution of the outer porous layer. Also, the deviations of Nyquest plots in cases of 3.5 wt% NaCl and 3.5 wt% NaCl + 5 ppm Na2S solutions from a perfect circular shape refers to frequency dispersion of interfacial impedance arising from the inhomogeneity of the electrode surface due to roughness phenomena36. On the other hand, the enhancement in the uniform corrosion of aluminum in 3.5 wt% NaCl + 5 ppm Na2S solution compared to 3.5 wt% NaCl and 5 ppm Na2S solutions is observed from the semicircle diameters as shown in the Fig. 2.

Figure 3 represents the fitted electrochemical equivalent circuit as a function in 3.5 wt% NaCl, 5 ppm Na2S and 3.5 wt% NaCl + 5 ppm Na2S solutions. It can be seen that the Nequest plot of aluminum in 5 ppm Na2S solution shows [R(RC)/(RC)] equivalent circuit with two time constants, that agreed with the dual nature of the Al2O3 passive film formed in the Al/electrolyte interface. Whereas [R(RQ)] equivalent circuit with one-time constant is fitted to the experimental data obtained in 3.5 wt% NaCl and 3.5 wt% NaCl + 5 ppm Na2S solutions due to the dissolution of the outer Al2O3 passive layer under the influence of its attack with the adsorbed Cl ions. Moreover, the replacement of the double layer capacitance (Cdl) in the [R(RC)/(RC)] equivalent circuit with constant phase element (CPE) and phase shift (N) in [R(RQ)] equivalent circuit explains the dispersion effect and the degree of heterogeneity resulted from the microscopic roughness of aluminum surface due to pitting corrosion37,38,39. The highest value of N accompanied with smallest value of CPE for aluminum in 3.5 wt% NaCl + 5 ppm Na2S solution (N = 0.56 and CPE = 54.9 µMho) compared to their values in 3.5 wt% NaCl solution (N = 0.52 and CPE = 66.1 µMho) is calculated from the fitted experimental measurements using Boukamp model40,41. This means that, Al/electrolyte interface in 3.5 wt% NaCl + 5 ppm Na2S solution behaves as a more ideal capacitive rather than that in 3.5 wt% NaCl, which illustrates the role of the S2− ions in decreasing the roughness and increasing the homogeneity of the aluminum surface.

It is concluded from the electrochemical measurements that, even the presence of S2− ions in NaCl solution enhances the uniform corrosion of aluminum but delay its susceptibility to pitting corrosion. This finding can explained on the bases of the accumulation of the corrosion products on the aluminum surface that increases the stability and the protectionist of Al2O3 passive layer and consequently, inhibits the rate of the pitting corrosion.

### Microstructures of the aluminum surface

Figure 4 and Table 2 represents the surface composition, crystallinity and phase identification of as received aluminum surface immersed in 3.5 wt% NaCl, 5 ppm Na2S and 3.5 wt% NaCl + 5 ppm Na2S using X-ray fluorescence and X-Ray Diffraction analysis. Data shows that the phase composition of the passive layer is Al2O3 in all studied solutions, with 3.23, 0.154 and 2.127 µm average crystal size formed in 3.5 wt% NaCl, 5 ppm Na2S and 3.5 wt% NaCl + 5 ppm Na2S solutions respectively. SEM images for as received aluminum surface immersed in 3.5 wt% NaCl, 5 ppm Na2S and 3.5 wt% NaCl + 5 ppm Na2S solutions are represented in Fig. 5. Cracks, non- homogenous and open large pits with high degree of roughness are clearly observed on the aluminum surface exposed to 3.5 wt% NaCl solution in the absence of S2− ions, compared to oriented grooves, elongated ridges with the accumulation of the corrosion products inside the pits in the presence of S2− ions. Whilst, an intact surface with no cracks or pits are observed in case of 5 ppm Na2S free Cl ions solution.

Topography of the aluminum surface immersed in 3.5 wt% NaCl, 5 ppm Na2S and 3.5 wt% NaCl + 5 ppm Na2S solutions is represented in Fig. 6, while its morphological parameters (RMS and Ra) are tabulated in Table 5. Data shows that the extensive pitting corrosion throughout the aluminum surface exposed to 3.5 wt% NaCl solution in the absence of S2− ions is indicated by the large values of both root mean square roughness (RMS) and average roughness (Ra) compared to their values in the presence of S2− ions. This behaviour is in a good agreement with the electrochemical measurements.

The enhancement in both the roughness and the homogeneity of aluminum surface in the presence of S2− ions is due to the formation of smoother, fine crystalline sized Al2O3 passive layer that accumulates inside the pits and inhibits their propagation, therefore decreases the susceptibility of the aluminum surface towards aggressive pitting corrosion in NaCl solution as schematically represented in Fig. 7.

## Conclusion

It can be concluded that the electrochemical behaviour and the microstructure of the passive film of aluminum during its exposure to 3.5 wt% NaCl solution is strongly affected by the presence of S2− ions. Increasing the concentrations of S2− ions in 3.5 wt% NaCl solution increases the pH values of the saline solution, thus increasing both the anodic reaction of Al dissolution and the dissolution kinetics of Al2O3 passive layer resulting in an increase of the uniform corrosion. At the same time, the lower kinetic formation of smoother, compact, fine crystalline sized Al2O3 inhibits the pitting corrosion. The inhibitory effect is interpreted on the basis that the smooth, fine crystalline surface of Al2O3 increases its stability and decreases the aggressiveness of the Cl ions. Consequently, decreases the rate of the initiation and the growth of the stable pits.

## Data Availability

The dataset generated or analyzed during the current study are available from the corresponding author upon request.

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Correspondence to Mohamed M. EL-Deeb.

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