1 Introduction

For more than a century, the cyanidation process for gold leaching has been extensively used primarily due to its simplicity, cost effectiveness, and rapid leaching kinetics. However, growing environmental and public concerns over the safe usage of cyanide have demanded an alternate environmentally friendly gold leaching process.[1,2] In this effort, the use of environmentally benign thiosulfate solutions for gold leaching has gained widespread research and commercial interest in the last two decades; in fact, these thiosulfate salts have been used as fertilizers in agricultural industries.[3] Moreover, the use of thiosulfate becomes attractive in the leaching of copper–gold complexes and carbonaceous gold ores, which could not be satisfactorily leached using cyanidation.[4] Thiosulfate leaching may also present economic benefits, as thiosulfate salts are considerably cheaper than the cyanides.[5]

Thiosulfate salts are the metastable species and can decompose, oxidize, and reduce under varying electrochemical conditions.[1,2,4] Hence, it presents a very complicated electrochemistry during gold leaching. It is known that thiosulfate-ammonia-copper (II) leaching solution brings gold into the solution by oxidizing it to a gold-thiosulfate complex at an acceptable rate.[6–9] The addition of ammonia during the thiosulfate leaching process helps in increasing the alkalinity (which is necessary for the stability of thiosulfate ions) and also aids in preventing the passivation of the gold surface.[6,9]

In the past, it has been reported that the thiosulfate solution can cause localized corrosion and pitting of structural materials such as steels and nickel-based alloys in acidic and neutral environments.[10–17] The localized corrosion/pitting of these alloys in thiosulfate solutions is mainly attributed to the reduction/decomposition of thiosulfate in absorbed sulfur and/or sulfide ions, which normally inhibits the repassivation process of the alloy surface.[10,11,18] Newman et al.[11] reported that stainless steels undergo accelerated pitting in the solutions containing thiosulfate, chloride, and sulfate ions in a specific potential and concentration range. In pulp and paper industries, even the minute presence of thiosulfate (as low as 10 ppm) ions were recognized as a dangerous corrosion promoter, which led to the pitting of stainless steels used in print paper machines.[14] Moreover, the stress corrosion cracking of stainless steels and nickel-based alloys has been observed in thiosulfate-chloride solutions.[19–21] A recent study by Kappes et al.[22] suggested the use of thiosulfate solutions to simulate the sour gas environment present in oil and gas pipelines, which is known to promote their environmentally assisted cracking. Xia et al.[23,24] recently reported the effect of thiosulfate on the passivity degradation of Alloy 800. The magnitude of passivity degradation depended on the thiosulfate concentration. In acidic solutions, thiosulfate can also be reduced to hydrogen sulfide (H2S), and H2S is known to catalyze the anodic dissolution of stainless steel.[25]

In the light of the above discussion, it can be said that many of the relevant engineering alloys show some corrosion susceptibility in the solutions containing thiosulfate ions. It is important to note that most of the studies concerning corrosion due to thiosulfate ions were employed in the neutral and/or acidic solutions,[10,11,18,26] however, thiosulfate gold leaching is performed in alkaline solutions (pH 9 to 11).[1] To the best of our knowledge, there have been no studies reported in literature that are related to the electrochemical corrosion of stainless steels in thiosulfate solutions relevant to gold leaching industries. Moreover, the thiosulfate gold leaching process is still in its developmental stages and the long-term impact of the thiosulfate species and leach conditions on plant infrastructure is not yet well known. While the previous research studies may provide basic understanding of corrosion due to thiosulfate ions in a neutral/acidic environment, the exact electrochemical corrosion behavior in concentrated alkaline thiosulfate solutions needs further exploration. Hence, the main objective of this work was to evaluate the electrochemical corrosion performance of the stainless steel 316L alloy in the solution containing thiosulfate ions, ammonia, and chloride ions using electrochemical techniques in conjunction with X-ray photoelectron spectroscopy and electron microscopy.

2 Materials and Methods

2.1 Materials and Environment

Stainless steel 316L (SS316L) with nominal chemical composition (18 wt pct Cr, 8 wt pct Ni, 2.2 wt pct Mo, and 0.022 wt pct C) was used in this study. The SS316L was specifically chosen since this alloy is generally used in hydrometallurgical applications on a wide scale as a construction material for building leach tanks and relevant infrastructures. The electrochemical experiments were performed in the solutions containing thiosulfate (S2O3 2−) ions, ammonia, and chloride (Cl−) ions. The solutions were prepared using distilled water and used immediately after the preparation at their natural pH. The measured pH of all the solutions used in this study is presented in Table I.

Table I Measured pH of All the Solutions
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2.2 Electrochemical Experiment

All the electrochemical experiments were carried out in a conventional three-electrode glass cell that has a saturated calomel electrode (SCE) as the reference electrode and a graphite rod as the counter electrode. All the potential herein is reported with respect to the SCE. Before starting the electrochemical experiment, the working electrode (SS316L mounted in cold-epoxy resin and connected through a conducting copper wire) was progressively ground up to 1200 grit emery paper followed by a thorough rinsing with distilled water and ethanol. All the experiments were performed at room temperature and in naturally aerated conditions.

The potentiodynamic polarization scans were carried out starting at 250 mV below the open circuit potential at a scan rate of 0.5 mV/s. A potentiodynamic polarization scan was also performed at a scan rate of 0.1667 mV/s under similar experimental conditions and it was observed that the scan rate does not significantly affect the corrosion kinetics, as shown in Figure 1(a). Electrochemical impedance spectroscopy (EIS) experiments were carried out by applying a sinusoidal potential wave with an amplitude of 10 mV. Impedance response was measured over frequencies between 100 kHz and 10 mHz, recording 10 points per decade of frequency. All EIS and potentiodynamic polarization tests were performed after 1 hours of immersion, the time required for the stabilization of the system. A typical representative potential vs time plot for SS316L is shown in Figure 1(b). Impedance analysis was carried out based on appropriate equivalent electrical circuit (EEC) using a ZSimpWin software, which used complex non-linear least squares (CNLS) fitting method for simulating the experimental impedance data. The Mott–Schottky experiments were performed by stepping the potential in the negative direction from 1 V SCE to −1 V SCE in a 25 mV decrement. All the electrochemical tests were duplicated in order to examine the reproducibility of the results.

Fig. 1
figure 1

(a) Potentiodynamic polarization scans for SS316L at different scan rates and (b) typical potential vs time plot for SS316L showing the stability of the system (fluctuation in potential less than 10 mV over 1000 s)

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2.3 Post-corrosion and Surface Film Analysis

Corrosion morphology after potentiodynamic polarization (up to 1 V SCE) in different solutions was observed using a scanning electron microscope (SEM). The surface chemical composition of the passive film was analyzed by X-ray photoelectron spectroscopy (XPS), using a monochromatic Mg Kα X-ray source and pass energies of 192 and 48 eV for wide and high-resolution scans, respectively. XPS tests were carried out at the take-off angle of 90 deg with no sputtering or charge neutralization.

2.4 Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)

The solutions were analyzed by VARIAN 725-ES ICP-OES to detect chromium metal release from the specimens after potentiostatic polarization for 6 hours at various potentials (0.3 V SCE, 0.7 V SCE and 0.9 V SCE). After the potentiostatic polarization, SS316L specimens were taken out and solutions were stirred thoroughly. A fraction of the solution was then stored in sterilized polypropylene test tubes for ICP-OES analysis. Before analyzing, standard solutions containing known amount of chromium were used to calibrate the instrument.

3 Results and Discussion

3.1 Electrochemical Corrosion Behavior in Only-Thiosulfate Solutions

Concentration of thiosulfate ions may vary in the gold leach circuits with time. For that reason, the electrochemical behavior of SS316L was evaluated in the thiosulfate solutions with varying concentrations (0.01 to 0.5 M). The potentiodynamic polarization curves for SS316L in solutions containing only-thiosulfate ions are shown in Figure 2(a).

Fig. 2
figure 2

Potentiodynamic polarization scans in solutions containing varying concentrations of only-thiosulfate ions: (a) for SS316L and (b) for Pt

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The corrosion potentials and cathodic current densities are primarily similar for all the thiosulfate concentration solutions (Figure 2(a)). The main cathodic reactions which could possibly occur in this system are hydrogen evolution (Eq. [1]), oxygen reduction (Eq. [2]), and thiosulfate reduction (Eq. [3])[16,27] reactions. The standard reduction potentials (E 0) are also shown in the corresponding equations as given below

$$ 2{\text{H}}^{ + } + 2e^{ - } = {\text{H}}_{2} \quad E^{0} = - \, 0.242\,V_{\text{SCE}} $$
(1)
$$ {\text{O}}_{2} + 4{\text{H}}^{ + } + 4e^{ - } = 2{\text{H}}_{2} {\text{O}}\quad {{E}}^{0} = 0.987\,V_{\text{SCE}} $$
(2)
$$ {\text{S}}_{2} {\text{O}}_{3}^{2 - } + 6{\text{H}}^{ + } + 4e^{ - } = 2S + 3{\text{H}}_{2} {\text{O}}\quad {{E}}^{0} = 0.223\,V_{\text{SCE}}. $$
(3)

Based on thermodynamic calculations, the hydrogen evolution and reduction of thiosulfate ions are not favorable in the solutions containing thiosulfate ions only. Thus, the main cathodic reaction might be the reduction of dissolved oxygen. It is reasonable to assume that the amount of dissolved oxygen is similar in all cases, which may lead to the similar cathodic current densities irrespective of concentration of thiosulfate ions in the solution (Figure 2(a)). Tsai et al.[15] also observed the similar cathodic current densities, while performing potentiodynamic polarization testing on Alloy 690 in the solutions containing thiosulfate ions. The interesting observation to note from the potentiodynamic polarization is the increase in anodic current density with increasing thiosulfate concentration. This could be attributed to either the availability of more thiosulfate ions for oxidation in higher concentration solutions or increased dissolution kinetics of SS316L, thus leading to increased anodic current densities. To isolate this phenomenon, potentiodynamic polarization scans were performed on an electrochemically noble platinum (Pt) sheet in the thiosulfate solutions (Figure 2(b)). Even in the case of materials as noble as Pt, a similar increase in anodic current densities is observed with an increase in the thiosulfate solution concentration. Hence, it can be confirmed that the increase in anodic current density for SS316L with increasing thiosulfate concentration (Figure 2(a)) is primarily due to the availability of more thiosulfate ions for oxidation in higher concentration solutions. In addition, the appearance of oxidation peaks at ~−0.025 V SCE (marked by an arrow) in the polarization curves for high thiosulfate concentration ([S2O3 2−] ≥ 0.05 M) may be as a result of the oxidation of sulfur species.[26] These anodic oxidation peaks are more pronounced for the higher concentration thiosulfate solutions.

Corrosion morphology was observed under SEM after potentiodynamic polarization testing (up to 1 V SCE). The representative corrosion morphology for the specimens tested in only-thiosulfate solutions is shown in Figure 3, which shows a shiny, bright, and spotless surface with no pitting in only-thiosulfate solutions.

Fig. 3
figure 3

Representative post-corrosion morphology of SS316L after potentiodynamic polarization testing in the solutions containing thiosulfate ions only

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Based on the potentiodynamic polarization scans and SEM morphology (Figures 2(a) and 3), only-thiosulfate ions do not seem to initiate the breakdown of the protective quality of the passive film. This also suggests that the thiosulfate ion alone does not have any deleterious effect on the electrochemical corrosion of stainless steels. It is further reported that the sulfur species, such as thiosulfate ions, activate the anodic dissolution at the pit initiation sites but do not assist in the initial breakdown of the passive film on the surface of stainless steels.[26] The presence of anions such as Cl− (acidic in nature) are required in conjunction with thiosulfate to cause the breakdown of the passive film and, subsequently localized corrosion/pitting, which is followed by passivation retardation due to the presence of thiosulfate.[11,26] Hence, the electrochemical behavior of SS316L in the thiosulfate-chloride solution was also evaluated.

3.2 Electrochemical Corrosion Behavior in Thiosulfate-Chloride Solutions

Several studies were performed to understand the electrochemical corrosion of stainless steel in thiosulfate-chloride solutions, but most of the studies have employed a high concentration of chloride ions (≥0.1 M).[18,26,28] The chloride ions are also likely to be present at low levels in any hydrometallurgical circuit. For example, some gold leaching plants in Australia even use saline water during the leaching process.[29] Hence, a fixed amount of 0.05 M NaCl was added to the electrolyte while performing the electrochemical testing. Figure 4 shows the potentiodynamic polarization scans for SS316L in varying concentrations of thiosulfate ions in the presence of 0.05 M NaCl. The potentiodynamic results are discussed for these conditions below:

Fig. 4
figure 4

Potentiodynamic polarization scans for SS316L in the solutions containing thiosulfate and chloride ions

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3.2.1 Potentiodynamic polarization in only-chloride ion (0.05 M) solution

SS316L may be corrosion resistant in many aggressive environments but suffers pitting corrosion in chloride environments.[30–33] As shown in Figure 4(a), a sudden increase in anodic current density is observed at an anodic potential of 350 mVSCE, which suggests that SS316L undergoes pitting at ~350 mVSCE. The high diffusivity and small size of chloride ions enable the quick transportation of Cl− followed by a penetration through the passive film. The mechanism of pitting by Cl− is autocatalytic because of the increased acidity (due to the hydrolysis of corrosion products), which retards repassivation and continues the dissolution in the pits.[34] The corrosion morphologies for the specimens tested in only-chloride solutions after potentiodynamic polarization are shown in Figures 5(a) and (b), which clearly show the pitting of SS316L in only-chloride solution. The pits were typically deep, bright, and did not contain any visible deposits.

Fig. 5
figure 5

Post-corrosion morphologies of SS316L after polarization: (a) and (b) in only-chloride (0.05 M) solution (c) and (d) in 0.01 M Na2S2O3 + 0.05 M NaCl solution, and (e) in 0.1 M Na2S2O3 + 0.05 M NaCl solution (representative post-corrosion morphology in solutions containing [S2O 23 ]/[Cl−] ≥ 1)

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3.2.2 Potentiodynamic polarization in solutions containing [S2O3 2−]/[Cl−] ≥ 1

The potentiodynamic polarization scans for SS316L in the solutions containing a higher or equal concentration of S2O3 2− in comparison to Cl− ions are shown in Figures 4(b) through (d). The cathodic current densities are similar and independent of concentration of any solute in the solution. The corrosion potentials are in the range of -200 to -250 mVSCE. Anodic current densities are very much comparable until 400 mVSCE and increases steadily afterwards (Figures 4(b) through (d)) which may be attributed to the oxidation of thiosulfate ions.[26] It is noted that chloride pitting is inhibited for the solution containing higher concentration of S2O3 2− ions in comparison of Cl− ions. This was further confirmed by SEM images of post-corrosion morphology after the potentiodynamic polarization scans. A representative post-corrosion morphology of the specimen tested in solution containing [S2O3 2−]/[Cl−] ≥ 1 is shown in Figure 5(e). Similar to the only-thiosulfate solutions, no pitting was observed for the solutions containing [S2O3 2−]/[Cl−] ≥ 1. The pitting inhibition in these solutions can be attributed to the competitive electromigration of the S2O3 2− anions to the developing pit nuclei in comparison to Cl−, thus decreasing the deleterious effect of chloride ions on the passive film breakdown.[11] The similar competitive anionic adsorptions of SO4 2− and NO3 2− in comparison to Cl− have been shown to decrease the Cl− absorption,[33] thus explaining their action as pitting inhibitors. A similar effect was observed by Newman et al.[11] and Laycock[26] for the solutions containing higher concentrations of thiosulfate ions in comparison to the chloride ions. It could also be said that for high thiosulfate/chloride ratios, it became impossible for electromigration to produce a high enough chloride concentration in the developing pit to sustain pitting.

3.2.3 Potentiodynamic polarization in solutions containing [S2O3 2−]/[Cl−] < 1

In contrast, the corrosion behavior of SS316L was drastically different for the solution with the lesser S2O3 2− concentration in comparison to the chloride ion (Figure 4(e)). The anodic current density is considerably higher in comparison to the solutions with [S2O3 2−]/[Cl−] ≥ 1, and SS316L starts to pit at ~175 mVSCE.

Interestingly, the dramatic increase in anodic current density for the solutions containing lower concentrations of thiosulfate ions in comparison to chloride ions can be due to the insufficient electromigration of thiosulfate in the pit nuclei. Thus, more chloride ions reach the pit and break down the passive film by increasing the acidifying effect of chloride ions in the pit which, in turn, causes pitting even at the lower anodic potentials. Furthermore, the increase in localized acidity in the developing pit (due to break down of passive film by electromigrated chloride ions) may also lead to the decomposition of available thiosulfate to form a discontinuous layer of absorbed sulfur/sulfide.[11,14,18] This formation of a discontinuous colony of sulfur/sulfide may retard the repassivation process once the chloride-activated pitting is initiated.[13] Hence, for the solutions containing [S2O3 2−]/[Cl−] < 1, the role of thiosulfate ions could be to promote the conversion of the metastable pits initiated by chloride ions into stable pits.[23]

Post-corrosion morphologies also reveal severe pitting when the SS316L is tested in the 0.01 M Na2S2O3 + 0.05 M NaCl solution (Figures 5(c) and (d)). However, a black-colored corrosion product was observed in the pit macroscopically, possibly of FeS or FeS2. This is in agreement with the reduction of thiosulfate in the pit nuclei into S or S2−, which may subsequently form iron sulfide as a corrosion product. This iron sulfide is defective and, in turn, may cause accelerated anodic dissolution.[35] It is also noted that the pitting propensity is quite high in the 0.01 M Na2S2O3 + 0.05 M NaCl solution in comparison to the only-chloride solution. This is in agreement with the potentiodynamic polarization results (Figures 4(a) and (e)) which also indicates a high pitting occurrence.

3.3 Electrochemical Testing in Ammoniacal Thiosulfate Solutions

Highly alkaline ammoniacal thiosulfate solutions (pH 9 to 11) have been proposed for the leaching of gold.[1] Hence, the electrochemical corrosion behavior of SS316L in ammoniacal thiosulfate solutions has also been investigated. Since the [S2O3 2−]/[Cl−] ratio has been shown to affect the corrosion performance of SS316L, the electrochemical experiments are performed in two different solutions with varying concentrations of thiosulfate ions (0.01 M and 0.1M), 0.3 M NH3, and 0.05 M NaCl. The potentiodynamic polarization curves for SS316L in both the ammoniacal thiosulfate solutions are shown in Figure 6. No significant differences are detected in the shape of the potentiodynamic curves, suggesting identical anodic and cathodic corrosion kinetics. A well-defined passive range is observed between 0.1 and 0.6 V SCE. The increase in current density for the potential above ~0.6 to 0.7 V SCE may be due to an additional anodic reaction, i.e., the oxygen evolution reaction. Importantly, the alloy does not undergo pitting in the ammoniacal thiosulfate solutions even when [S2O3 2−]/[Cl−] < 1. It can be explained on the basis of high pH (~11.6) of the solution (due to the addition of ammonia), which decreases the localized acidity necessary to cause the pitting.

Fig. 6
figure 6

Potentiodynamic polarization scans for SS316L in ammoniacal thiosulfate solutions containing chloride ions

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The long-term corrosion kinetics (up to 5 days of immersion) of SS316L in ammoniacal thiosulfate solutions is performed using electrochemical impedance spectroscopy (EIS), which also provides mechanistic understanding of the electrochemical response of SS316L. In using EIS for understanding electrochemical degradation of metals, the Bode and Nyquist plots are the most common representations of the electrochemical processes occurring at various interfaces. In Bode plots, the magnitude of the impedance (|Z|) and phase angle are plotted as a function of frequency (ω), usually over a wide range of ω, on a logarithmic scale. While in Nyquist plots, the negative of imaginary components of impedance (−Z img) are plotted against the real component of impedance (Z real). In the case of the Bode impedance plot, the magnitude of the impedance at the lowest frequency represents the polarization resistance, whereas the diameter of the impedance loop represents the polarization resistance in case of the Nyquist plot.[36]

Figure 7(a) shows the Nyquist plots after different immersion periods in ammoniacal thiosulfate solution in the presence of chloride ions (0.1 M Na2S2O3 + 0.3 M NH3 + 0.05 M NaCl). The diameter of the loops in the Nyquist plot (Figure 7(a)) increases with the immersion time, which suggests an increase in the polarization resistance with the increased immersion time. The EIS results reveal that the highly protective and stable surface films are formed on the steel surface with increasing the immersion period in this solution.

Fig. 7
figure 7

EIS response of SS316L in 0.1 M Na2S2O3 + 0.3 M NH3 + 0.05 M NaCl solution after different immersion periods: (a) Nyquist plots (the electrical equivalent circuit used for simulation of experimental data is shown in the inset) and (b) Bode impedance and phase plots

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Figure 7(b) shows the Bode impedance and phase plots for the SS316L alloy in the ammoniacal thiosulfate solution. The Bode impedance plots also suggest the increase in polarization resistance with increased immersion time, while the Bode phase plots show typical capacitive-type behavior (phase angle maxima at ~80°) with a one-time constant. For developing a mechanistic understanding of the electrochemical degradation of SS316L, an electric equivalent circuit (EEC) with a constant phase element (CPE) has been proposed, as shown in the inset of Figure 7(a). The CPE is generally used to account for dispersion from ideal capacitive behavior. Such behavior is considered to be due to local inhomogeneity, surface reactivity, roughness, electrode porosity, mass transport, and relaxation effects.[36,37] In this EEC (Figure 7(a)), R s is the solution resistance, R p is the polarization resistance, and Q b is the CPE related to the capacitance of barrier layer. In the past, several EECs have been proposed for the interpretation of experimental impedance results on materials with passive film,[38–40] however, the simulation of experimental EIS data with proposed EEC resulted in a close fit with the measured data. The proposed EEC has been extensively used in the literature for simulating experimental EIS data of stainless steels.[41–44] The simulation of experimental data using this EEC presents low error (<5 pct) and high Chi-square values (>10−4), suggesting this model to be an appropriate choice. In a physical sense, the proposed EEC is generally used to describe the electrochemical reaction at the electrode/electrolyte interface of a passive system and takes into account only one time constant.[41,43,45]

The time-dependent evolution of Q b and R p with immersion time is shown in Figure 8. The interfacial resistance or polarization resistance clearly increased with increased immersion time, as also suggested by Nyquist and Bode impedance plots (Figure 7). The CPE value (Q b) associated with the interface decreases with time, which may indicate the formation of a compact passive film with increased immersion time. This also explains the increased polarization resistance of SS316L with increased immersion in ammoniacal thiosulfate solutions.

Fig. 8
figure 8

Parametric evolutions of R p and Q b with immersion time in 0.1 M Na2S2O3 + 0.3 M NH3 + 0.05 M NaCl solution

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The Nyquist and Bode plots for the SS316L tested in 0.01 M Na2S2O3 + 0.3 M NH3 + 0.05 M NaCl solution are shown in Figures 9(a) and (b), respectively. The EIS spectra show similar behavior as observed for the specimen tested in the 0.1 M Na2S2O3 + 0.3 M NH3 + 0.05 M NaCl solution (Figures 7(a) and (b)). This is in agreement with the potentiodynamic polarization results (Figure 6) in the ammoniacal thiosulfate solutions. The EIS results suggest that the synergistic presence of thiosulfate and chloride ions is not detrimental to the corrosion performance of SS316L in ammoniacal solution regardless of the [S2O3 2−]/[Cl−] ratio. The higher R p values suggest the formation of a compact passive layer in both ammoniacal thiosulfate solutions. This does not facilitate pitting primarily due to the increased alkalinity and avoidance of thiosulfate reduction/decomposition on compact passive films formed in these solutions (which is explained below in the XPS results). The negative effect of chloride ions on the electrochemical corrosion of SS316L is also suppressed in the ammoniacal thiosulfate solutions.

Fig. 9
figure 9

EIS response of SS316L in 0.01 M Na2S2O3 + 0.3 M NH3 + 0.05 M NaCl solution after different immersion periods: (a) Nyquist plots and (b) Bode impedance and phase plots

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As suggested earlier, the observed increase in anodic current density above 0.6 VSCE during potentiodynamic polarization testing in ammoniacal thiosulfate solution (Figure 6) may be due to the additional anodic reaction of oxygen evolution but it could be argued that the increase in anodic current may be due to the transpassive dissolution of the SS316L by converting stable chromium oxide film into soluble chromates.[46–49] In the literature, the transpassive dissolution is widely characterized by the presence of inductive loop in the middle and low frequency regions of EIS Nyquist spectra.[46,48,50] To confirm this, potentiostatic EIS tests were also conducted in the ammoniacal thiosulfate solution (0.1 M Na2S2O3 + 0.3 M NH3 + 0.05 M NaCl) after potentiostatic polarization at 0.7 VSCE and 0.9 VSCE for 6 h. The Nyquist plots for these two conditions are shown in Figure 10. The absence of inductive loop in the Nyquist plots under these conditions may suggest the absence of transpassive chromium dissolution. However, very low impedance values observed in the Nyquist plots may primarily be due to additional oxygen evolution reaction.

Fig. 10
figure 10

Nyquist plots for SS316L tested in ammoniacal thiosulfate solution (0.1 M Na2S2O3 + 0.3 M NH3 + 0.05 M NaCl) after potentiostatic polarization for 6 h at (a) 0.7 VSCE and (b) 0.9 VSCE

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Furthermore, the solution was analyzed for the presence of chromium after potentiostatic polarization for 6 hours at 0.3 V SCE, 0.7 V SCE, and 0.9 V SCE, using ICP-OES. The amount of chromium released in the solution for the specimen tested potentiostatically at 0.3 V SCE (passive region, Figure 6) was found to be similar in comparison to the solution for the specimen tested at 0.7 V SCE and 0.9 V SCE, as shown in Table II. The ICP-OES results may also suggest that the transpassive dissolution of chromium was not favorable under these experimental conditions. The post-corrosion morphologies of the specimens tested in ammoniacal thiosulfate solutions in the presence of the chloride ions were similar to the specimens tested in only-thiosulfate solutions (Figure 3), and no pitting or transpassive breakdown was observed. Although, it could also be argued that if the chromium dissolves transpassively, and the other metals (Fe and Ni) do not, then the surface would enrich in Fe and Ni oxides, and hence, it would not be possible to detect the transpassive dissolution of chromium by impedance and/or solution analysis.

Table II Estimation of Chromium Release (in 250 mL of the Solution) After Potentiostatic Polarization of SS316L for 6 h at Different Potentials Using ICP-OES
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3.4 Passive Film Characterization in Ammoniacal Thiosulfate Solutions

As observed in the EIS and potentiodynamic polarization results, the SS316L did not undergo pitting in ammoniacal thiosulfate solutions, which could possibly be attributed to the higher stability of passive films in ammoniacal thiosulfate solutions (pH ~11.6). The passive film of stainless steel is reported to exhibit semi-conductive behavior.[51] The semi-conductive properties are of great importance in understanding the structure of the passive film. Mott–Schottky measurement could be applied to analyze the semi-conductive properties, such as the carrier density and flatband potential, of the passive films. The Mott–Schottky relationship for the n-type and p-type semi-conductors is described by Eqs. [4] and [5], respectively:

$$ C_{\text{SC}}^{ - 2} = \frac{2}{{\varepsilon \varepsilon_{0} eN_{\text{D}} }}\left( {E - E_{\text{FB}} - \frac{kT}{e}} \right) $$
(4)
$$ C_{\text{SC}}^{ - 2} = \frac{2}{{\varepsilon \varepsilon_{0} eN_{\text{A}} }}\left( {E - E_{\text{FB}} - \frac{kT}{e}} \right) $$
(5)

where C SC is the space charge capacitance, ε is the dielectric constant of the semi-conductor film, ε 0 is the vacuum permittivity, e is the electron charge, N D is the donor density for n-type semi-conductor, N A is the acceptor density for p-type semi-conductor, E is the applied potential, E FB is the flatband potential, k is the Boltzmann constant, and T is the absolute temperature. Here, ε is set to be 15.6 and 25 for the Fe oxide and Cr oxide, respectively.[52,53] The values of the carrier density and flatband potential could be obtained by fitting the linear region of the Mott–Schottky plot.

Figure 11 shows the Mott–Schottky plots of the passive film formed at different applied potentials for 6 hours in 0.1 M Na2S2O3 + 0.3 M NH3 + 0.05 M NaCl solution. Both n-type and p-type semi-conductive behavior could be observed, which are attributed to the Fe oxides and Cr oxides,[52,53] respectively. The slope values of the n-type semi-conductor were not changed significantly with the increase of film formation potential, whereas, that of p-type semi-conductor decreased. Figure 12 shows the values of N D, N A of the passive film as well as their E FB derived by the Mott–Schottky plots. The N D values are almost independent of the film formation potential, however, the N A values increase linearly with the increase of the film formation potential. The E FB values are not significantly varied with the film formation potential for both the Fe oxides and Cr oxides, and their values for both Fe oxides and Cr oxides in this study are consistent with the previous reports.[52,53]

Fig. 11
figure 11

Mott–Schottky plots of the passive film formed on SS316L after polarization at different potentials for 6 h in 0.1 M Na2S2O3 + 0.3 M NH3 + 0.05 M NaCl solution

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Fig. 12
figure 12

(a) The carrier density and (b) the flatband potential of the passive film formed on SS316L after polarization at different potentials for 6 h in 0.1 M Na2S2O3 + 0.3 M NH3 + 0.05 M NaCl solution

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If the chemical composition of the passive film does not change during the film formation process at different potentials, a decrease in the carrier density is generally observed with increase in the film formation potential, such as reported in the case of titanium (Ti), niobium (Nb), zinc (Zn), and tungsten.[54–57] However, in this study, surprisingly, the acceptor density of the Cr oxide increased with the increase of the film formation potential. Figure 13(a) shows the XPS survey scans of the passive films formed on SS316L in 0.1 M Na2S2O3 + 0.3 M NH3 + 0.05 M NaCl solution at different potentials (0.1 V SCE and 0.7 V SCE), whereas Figures 13(b) and (c) show the XPS high-resolution scan spectra and peak deconvoluted results of the Cr and O elements. According to the deconvoluted results and binding energy, the Cr oxide is composed of Cr2O3, Cr(OH)3, and CrO3.[58]

Fig. 13
figure 13

(a) XPS spectra and peak deconvoluted XPS spectra results of (b) Cr and (c) O of the passive film formed on SS316L after polarization at different potentials for 6 h in 0.1 M Na2S2O3 + 0.3 M NH3 + 0.05 M NaCl solution

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Tables III and IV show the percent of the compounds for Cr and O in the passive film formed at 0.1 V SCE and 0.7 V SCE, respectively. The amount of Cr(OH)3 in the passive film formed at 0.7 V SCE is much higher in comparison to the passive film formed at 0.1 V SCE, and the amount of OH− group is doubled, which suggests an increased hydration degree of the Cr oxide in the passive film formed at a higher potential (0.7 V SCE). A previous study on pure Cr surface showed that the N A value for the passive film formed on pure Cr metal was increased with applied potential because of the increase in hydration degree of the passive film.[52] As a result, it could be concluded that the increase of the N A value in this study with the increase of the film formation potential is caused by the higher hydration degree of the Cr oxide, as the bond water in the film might act as acceptor impurities.

Table III Percent of the Chemical Compounds of the Cr in the Passive Film Formed at Different Potentials for 6 h
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Table IV Percent of the Chemical Compounds of the O in the Passive Film Formed at Different Potentials for 6 h
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It is also worth noting that the thiosulfate can only be reduced to the absorbed sulfur species only on the active corroding surfaces (but not on the passive oxide surface of SS316L), and hence, a prerequisite for thiosulfate-activated pitting would be a breakdown of passive film by aggressive ions like Cl−.[13,23] Interestingly, no sulfur peak was observed in the XPS spectra of passive film formed on SS316L in ammoniacal thiosulfate solution at both potentials (Figure 13(a)). This corroborates the results of Duret-Thual et al.[13] as they reported no reduction of thiosulfate to sulfur in the presence of intact passive film (which is primarily present on SS316L in ammoniacal thiosulfate solution, as shown by potentiodynamic and EIS results) on the stainless steel surface.

3.5 Significance of the Results to Gold Leaching Plant Infrastructure

The results discussed in this paper present an overview of the electrochemical corrosion performance of the SS316L alloy in solutions relevant to gold leaching industries. The fact that thiosulfate gold leaching is generally performed in highly alkaline solutions (pH 9 to 11), the corrosion performance of SS316L in ammoniacal thiosulfate solution is encouraging. It could be said that the increase in alkalinity of the thiosulfate solutions with the addition of ammonia might present a dual role, such as increasing the gold leaching rate in addition to suppressing the electrochemical corrosion of plant infrastructure materials in thiosulfate solutions. The small presence of chloride in the ammoniacal thiosulfate solutions was also found not to be detrimental on the corrosion performance of SS316L. It should be noted that the actual service condition may have a presence of other secondary ions, such as cupric, tetrathionate, and dithionate,[1] which could affect the corrosion performance of SS316L.

Avoidance of localized corrosion/pitting is the foremost condition for the structural materials used in hydrometallurgical plant infrastructure. The occurrence of localized corrosion and pitting of the structural materials could be quite dangerous in service conditions. The localized pits can sometimes show a crack-like behavior insidiously and cause sudden environmentally assisted cracking in the synergistic presence of a conducible environment and service stresses. Even though SS316L does not show any signs of pitting and forms a compact passive film in the ammoniacal thiosulfate solutions, the occurrence of stress corrosion cracking in these solutions cannot be ignored. Accordingly, it is essential to characterize the environmentally assisted cracking of these structural materials in thiosulfate solutions relevant to gold leaching industries. The authors are currently pursuing these studies.

4 Conclusions

The electrochemical corrosion behavior of SS316L was evaluated in different solutions containing thiosulfate, ammonia, and chloride using potentiodynamic polarization and EIS. The following conclusions can be drawn:

  1. 1.

    The SS316L did not undergo pitting in the solutions containing thiosulfate ions only. Thiosulfate salts alone were not able to cause any initial breakdown of the passive film formed on the SS316L.

  2. 2.

    Thiosulfate-activated pitting was only observed in non-ammoniacal solutions when the thiosulfate ion concentration was less compared to the chloride ions. The chloride ions migrated to the pit nuclei, causing the breakdown of passive film. This could be followed by the thiosulfate decomposition/reduction on the active surface of SS316L, which hindered the repassivation process and accelerated further pitting (as evidenced by post-corrosion morphology).

  3. 3.

    Ammoniacal thiosulfate solution inhibited the pitting corrosion of SS316L regardless of thiosulfate/chloride ratio. The increased pH of the ammoniacal solution may increase the alkalinity in the developing pit, thus avoiding the film breakdown by chloride ions. The EIS results showed the increase in polarization resistance in ammoniacal thiosulfate solutions with time, indicating the formation of more compact film with time.

  4. 4.

    The passive film was mainly composed of Fe oxide and Cr oxide in ammoniacal thiosulfate solutions. The acceptor density of the Cr oxide film increased with the film formation potential, which was caused by the increased hydration degree of the Cr oxides. Further, the absence of adsorbed sulfur in the XPS spectra of the passive film formed in ammoniacal thiosulfate solution confirmed the absence of thiosulfate reduction/decomposition to sulfur on defect-free passive film.