Evaluation of Corrosion Inhibition of 316L Stainless Steel by Permanganate Ions in Chloride Solution

The efficiency of permanganates to inhibit the scale deposit captured the attention for more investigation on their role as corrosion inhibitor. In this article, the effect of permanganate as corrosion inhibitor on 316L stainless steel in NaCl solution is investigated. The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) have been performed by varying the electrode stirring speed, the concentration of permanganate ions, pH and the temperature. The results show that the permanganate ions increase the cathodic and anodic currents under effect of stirring speed, due to oxygen reduction reaction and the reduction of permanganate ions. Electrochemical results indicate that the deposit of manganese oxide (MnO2) inhibits the pitting corrosion. The inhibition efficiency is up to 98 % for 10−4 mol.dm−3 of permanganate. The temperature reduces the effectiveness of permanganates against pitting corrosion, the pitting potential shifts cathodically from +0.395 V vs. Saturated Calomel Electrode (SCE) at 298 K to +0.275 V vs. SCE at 343 K. Surface morphology of the deposit oxide films and electrode are studied by emission scanning electron microscopy, X-ray diffraction, Fourier transform infrared and Differential Scanning Calorimetry. The analysis of the deposit layer by X-ray diffraction revealed the presence of δ-MnO2 form, with a crystallite size of 3.17 nm. Copyright © 2021 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).


Introduction
In a nuclear installation, operational safety is a major and permanent concern. The issue concerns all the installation's components, ble region of the circuit [3]. This interface can be the site of both corrosion phenomena and scale deposits. The challenge is to reduce the risks of both corrosion and scale deposits [4].
To prevent corrosion of the cooling circuits, it was recommended the use of corrosion inhibitors, which must be efficient, stable, inexpensive, but also non-toxic to the environment of the installation. Oxyanions such as molybdate, tungstate and vanadate which are similar to chromate are known to have a corrosion inhibiting effect on various materials [5][6][7][8]. The majority of these corrosion inhibitors increase the pH, which reduces the oxidation of the metal and consequently promotes the formation of scale deposit.
Permanganates are oxyanions known to be corrosion inhibitors with low toxicity and strong oxidants power [9][10][11][12][13]. It is used in acidic medium to decontamination of radionuclide in heat exchanger of nuclear power plant and it can inhibits pitting corrosion through the deposition of manganese oxide MnO2 [14]. Several recent studies use the permanganate ions such as corrosion inhibitor on different materials. Osipenko et al. have showed that the permanganate in acidic media containing chloride ions inhibit the corrosion of aluminum alloy AD31 [15]. Sung-Mao Hung et al. studied the coating of a magnesium-lithium alloy LZ91, using a strongly acidic permanganate solution, the corrosion resistance of this alloy was significantly improved by the thin and uniform coating, which consisted of MgO, MnO2 and Mg(OH)2 [10]. Yanqi Wang et al. showed that the permanganate ions exhibited a promising inhibition (>98%) on galvanized steel in concrete leading to the formation of a protective film of CaMnO4 and Ca(HMnO2)2 [16].
In our previous study, we demonstrated that the electrochemical reduction of permanganate on a stainless steel surface can lead to the formation of manganese oxides which can substantially reduce nucleation and growth of scale deposits. The manganese oxide appears at a more anodic potential than the potential for scale formation. This oxide is sufficiently conductive; it does not affect the reduction of dissolved oxygen which is necessary for the growth of scale deposits. Despite this, the scale deposits formed in the presence of permanganates are very poorly adherent and also very porous [17].
In view of the effect of permanganate ions for the inhibition of scale deposit on 316L stainless steel, we study in the present work the corrosion inhibition of 316L stainless steel using rotating electrode by permanganate ions in a chloride solution. We discuss the influence of various parameters on the open circuit potential and anodic and cathodic currents. We analyze the reduction of permanganate and the formation of manganese oxides on the steel surface. These oxides are then characterized by electrochemical impedance (EIS), scanning electron microscopy (SEM-EDS), X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Differential Scanning Calorimetry (DSC).

Materials Preparation
The sample selected for the study was a 316L stainless steel with an alloying element composition of 0.018 %C, 0.400 %Si, 1.740 %Mn, 0.038 %P, 0.029 %S, 16.550 %Cr, 10.060 %Ni, 2.030 %Mo, 0.014 %Nb, 0.067 %N, 0.450 %Cu, 0.160 %Co, and Fe remainder (in wt%). The stainless steel sample is placed in a rotary motor Model EDI101 connected with a stirring speed controller Radiometer; it is coated with PTFE shroud as shown in Figure 1.

Electrochemical Methods
All the electrochemical measurements were performed in a thermostatic cell (250 ml) containing three electrodes: the rotating disk electrode (RDE) as working electrode (WE) with a diameter of 5 mm, the reference electrode (RE) was the Saturated Calomel Electrode (SCE) and the auxiliary electrode (CE) made of a platinum sheet as shown in Figure 1. The surface of working electrode was prepared using abrasive silicon carbide emery papers (SiC) 1200 grade and then rinsed with deionized water, cleaned and degreased with acetone.
All electrochemical Polarization tests were performed in a Parstat 2273 (EG&G PARC) Potentiostat/Galvanostat, and controlled by Pow-erSuite software. The open circuit potential (OCP) of 316L stainless steel sample was recorded for six hours for each solution. Potentiodynamic polarizations tests were performed from −1 to 1 V vs SCE with a rate scan of 1 mVs −1 for each solution. The electrochemical impedance spectroscopy (EIS) measurements were performed after different time of immersion at the OCP potential in the frequency range 100 kHz to 10 mHz and with potential perturbation amplitude of 10 mV.

Characterization of Deposit
The formed deposit on the electrode surface after potentiodynamic polarization tests from −1 to +1 V vs SCE with and without permanganate ions was characterized by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD). The potentiodynamic tests were performed in solutions containing 10 −2 mol.dm −3 of NaCl, stirring speed 1500 rpm, temperature 298 K, at pH = 6.50 and 10 −2 mol.dm −3 of permanganate ions. The morphologies of electrode surface and deposits were investigated by scanning electron microscopy SEM-EDS Quanta 250 microscope. This equipment also gives the possibility to characterize qualitatively the composition of different deposit layers with the energy dispersive spectroscopy (EDS) technique.
The diffraction patterns were registered with a diffractometer model PANalytical X-Pert PW1500 equipped with a hot diffraction chamber (HTK1200 Anton Paar) with a Cu-K radiation ( = 0.15405 nm), (45 kV/40 mA) and the counting time was 3 s per 0.02° 2θ angular step. The scan was performed between 20°≤2θ≤75°. These data were processed to determine the size of the crystallites and the crystallographic parameters, including the micro strain.
The deposit obtained on the electrode surface, under the same previous conditions, was scraped and collected in order to be characterized by Fourier Transform infrared spectrometry (FTIR) and Differential Scanning Calorimetry (DSC). Infrared (IR) measurements were carried out by a Fourier transform infrared spectrometer FTIR (INFRALUM FT-02). The deposit is ground and mixed with KBr (with high purity 99.90% used as the diluent) dried at 393 K for 24 h to remove the adsorbed water. The mass ratio is: 99.70% KBr and 0.30% deposit. The mixed powders are pressed by a Tablet Press Machine (Specac) at 10 MPa, to obtain pellets with diameter of 13 mm. The spectrum was collected under ambient conditions in the range of 4000 to 400 cm −1 with a resolution of 4 cm −1 . It was saved and treated by SpectraLum software. Differential Scanning Calorimetry (DSC) was carried out on a Setsys-Evolution 1500 with heating rate of 15 K.min −1 from 298 K to 1073K, the experiments were carried out in a highly pure argon atmosphere, data are collected and processed by SETSOFT 2000 Software. nate ions the OCP displaces anodically from +0.475 V vs SCE to + 0.688 V vs SCE.

Results and Discussions
The anodic evolution of OCP potential in the presence of chloride ions during the immersion time indicates the formation of a passive film which contains two layers: an outer layer containing Fe and Mo and an inner layer containing chromium according to Wang et al. [18]. With permanganate ions, the OCP evolve to +0.688 V vs SCE, which indicates that the steel is more resistant to corrosion. This displacement can be attributed to the permanganate ions which promote the formation of a more protective layer [19]. Figure 3 shows the effect of stirring speed on the evolution of the potentiodynamic polarization curves with permanganate ions 10 −2 mol.dm −3 and NaCl 10 −2 mol.dm −3 , pH 6.50, and at 298 K. In stagnant solution, permanganate ions 10 −2 mol.dm −3 shifts anodically the corrosion potential (Ecorr) of stainless steel from −0.148 V vs SCE to +0.290 V vs SCE. It is observed that the stirring speed influence slightly the values of Ecorr which is located between 0.290 and 0.360 V vs SCE due to the oxygen reduction reaction (ORR). The current density of cathodic and anodic parts is proportional to the stirring speed. At cathodic potentials two kinds of reactions could take place: − The ORR, dependent on the diffusion of oxygen under the effect of stirring speed, according to two mechanisms: 2 electrons at low speed and 4 electrons at high speed. These reactions make the local pH more alkaline [17]. − The successive reduction of permanganate to manganate and hypomanganate takes place according to the reactions [20]: The current density of anodic peak appears around +0.465 V vs SCE, increases with stirring speed. This peak could be assigned to the reduction of permanganate into manganese oxide such as the follow reaction [20]: Figure 4 shows the effect of permanganate ions on the potentiodynamic polarization curves of stainless steel, by fixing the speed stirring 1500 rpm, the pH=6.50, the temperature 298 K and the concentration of NaCl 10 −2 mol.dm −3 . When the concentration of permanganate ions increases Ecorr shifts anodically from −0.148 to +0.400 V vs SCE. In cathodic parts of voltammogram, it is noted that the permanganate ions increase the cathodic current. The anodic current without permanganate ions is greater only compared to solution containing the smallest amount of a MnO4 − in electrolyte.
The increasing of inhibitor concentration beyond 10 −3 mol.dm −3 , the anodic peak of reaction (3) becomes more apparent. It can be seen that permanganate ions displace anodically the pitting potential from +0.443 V vs SCE (without permanganate) to total inhibition at a concentration of 10 −2 mol.dm −3 . SEM micrograph of electrode surface after potentiodynamic polarization test in NaCl solution 10 −2 mol.dm −3 without permanganate in Figure 5, shows the presence of pits on the surface of stainless steel electrode.   Table 1 gathers the electrochemical parameters according to the Tafel extrapolation to obtain the polarization resistance Rp, the corrosion current Icorr, the anodic and cathodic slopes and the inhibition efficiency IE%, which is calculated by comparing the corrosion current recorded with and without permanganate ions. The inhibition efficiency is expressed according to Kuznetsov et al. [21]: (4) For permanganates ions concentrations of 10 −4 mol.dm −3 and 10 −3 mol.dm −3 , the inhibition efficiency is 98.58% and 90.87% respectively. This confirms that permanganates can be used as general corrosion inhibitors. However, for high concentrations above 10 −3 mol.dm −3 , calculation of the inhibition efficiency gave outliers, this is due to the anode peak at +0.465 V vs SCE which is very close to the corrosion potential and which makes the calculation of Tafel parameters impossible in the potential range ±0.3 V about Ecorr. Figure 6 gathers potentiodynamic polarization curves of 316L stainless steel obtained in a solution of 10 −3 mol.dm −3 permanganate and 10 −2 mol.dm −3 chlorides, by fixing the stirring speed at 1500 rpm in different pH values: 2.17, 7.00 and 11.55. In acidic medium pH = 2.17, the corrosion potential shifts anodically to +0.455 V vs SCE, with an increase in anodic and cathodic currents compared to the voltammogram at pH=7.00 (+0.342 V vs SCE). In alkaline solution at pH=11.55, the corrosion potential shifts cathodically to +0.074 V vs SCE.
Permanganate ions in aqueous solution are stable regardless of the pH for potential greater than the potential of water oxidation [22]. In acidic medium the permanganate ions are reduced to MnO2 and oxidize the metal according to the following [14]: (5) This reaction takes place in several stages of transfer of one electron giving rise to intermediate oxidation states: Whatever the oxidation state of the manganese, it contributes to the formation of MnO2 [22]. However, in acidic medium, in addition to the dissolved oxygen reduction reaction and those permanganates, one should consider the hydrated proton reduction reaction H3O + according to the reaction [22]:   In alkaline medium the permanganate ions are transformed into manganite ions followed by a transformation into MnO2 [23][24][25][26]: In acidic medium, it was noted a considerable increase in the anodic current after the pitting potential, unlike permanganate ions in an alkaline or neutral medium. Figure 7 shows the voltammograms of 316L stainless steel, obtained in a solution of 10 −3 mol.dm -3 permanganate and 10 −2 mol.dm −3 chlorides, by fixing the stirring speed at 1500 rpm, for different solution temperatures: 298, 303, 313, 323 and 343 K. Under effect of temperature, the Ecorr shifts cathodically from +0.400 to +0.275 V vs SCE. It can be seen that the anodic currents increase with temperature, due to the decrease in the energy barrier with temperature for iron oxidation reactions [27]. The pitting potential decreases from +0.620 V vs SCE at 298 K to +0.320 V vs SCE at 343 K. In the cathodic part of the voltammograms, there are disturbances of the cathodic currents at high temperatures. The observed peaks in cathodic currents prove the presence of reduction reactions of permanganate [17]. The analysis by Electrochemical Impedances Spectroscopy (EIS) provides additional information concerning the steel/film/electrolyte system. Figure   8 presents the Nyquist diagrams recording of stainless steel and the fitted curve, obtained in a solution of 510 −3 mol.dm −3 permanganate and 210 −2 mol.dm −3 chlorides, at OCP potential and without stirring in different immersion periods between 1 hour and 24 hours.
The equivalent circuit chosen with two loops is similar to the equivalent circuit used for modelling the inhibition of cupronickel steel by permanganate ions [14], with solution resistance Rs. The first circuit describes the deposition of manganese dioxide with a resistance Rf and a capacitance Cf, the second circuit describes the reactions that take place at the interface with the steel where Rct represents the charge transfer resistance and Cdl the double layer capacitance. It is remarkable that the Nyquist diagrams progress to the axis of the imaginary impedances (i.e. dominance of the capacitive effect) due to porous morphology of manganese dioxide deposit. The results of the equivalent circuit parameters by fitting Nyquist plots are presented in Table 2.
The fitting of the Nyquist diagrams was obtained with errors  2 not exceeding 5%. The increased capacity and resistance of the deposited film testifies the evolution of film formation as a function of immersion time. The resistance of the film changes from 66 kW after 1 hour of immersion to 142 kΩ after 24 hours.

Bulletin of Chemical Reaction Engineering & Catalysis, 16 (2), 2021, 240
Copyright © 2021, ISSN 1978-2993 Figure 9 shows a SEM image accompanied by an EDS elementary analysis of a deposit on 316L stainless steel surface. The deposit layer is formed over the entire surface of the steel; the bursting of this layer is due to its dehydration. The thickness of this layer is about 8 µm. The white deposit on the layer results from NaCl crystals. Elementary analysis by EDS shows an important presence of manganese, the other elements: Cr, Fe and Ni come from the chemical composition of the steel.
The X-ray diffraction of the deposit presented in Figure 10, highlights the presence of an octahedral phase where the diffraction peaks located at 25.40°, 37.81°, and 65.54° corre-sponding to the planes (002), (006), and (119) of the -MnO2 Birnessite species [24]. The average crystallite size and micro strain are 3.17 nm and 4.24 %, respectively. In the work of Marafatto et al. for the same species was between 3 and 7 nm [25]. Figure 11 illustrates the FTIR spectrum of the deposit between 400 and 4000 cm −1 . Several distinctive peaks appear on this spectrum. The two peaks at 563 and 713 cm −1 correspond to the vibration of Mn−O according to Jin et al. [26], as reported by Wang et al. that bands in the region above 400 cm −1 are attributed to Mn−O vibrations [28]. The peaks at 3100 cm −1 and at 1600 cm −1 correspond to the O−H vibra-   tion. This is due to the existing low humidity, despite the sample being dried in an oven at a temperature of 393 K for 24 hours. The peak at 1100 cm −1 corresponds to the Mn−OH vibration according to Dubal et al. [29]. The existence of the peak at 2361 cm −1 is related to the carbonate fragments observed during an open-air FTIR analysis [30]. Figure 12 present the Differential Scanning Calorimetry (DSC) curve, it shows the enthalpy change of the deposit according to temperature. We can see, the endothermic peak located at 423 K [31], can be attribute to the loss of existing water in the sample. The three exothermic peaks appearing at 598, 679, and 800 K, are characteristic and correspond to the transformation of the -MnO2 phase, to Mn2O3 and Mn3O4 then to MnO, respectively [32,33]:

Conclusions
In this work we are interested to study the inhibition efficiency of permanganate ions as corrosion inhibitor of 316L stainless steel using rotating disk electrode. In presence of permanganate ions, the cathodic reduction reactions lead to a considerable displacement of the OCP to positive potentials and the formation of a deposit protecting layer, this result is confirmed by (EIS) where the Rct increases with immersion time, indicating the growth of deposit layer. Stirring speed promotes the appearance of reduction peak of permanganate to manganese oxide at +0.465 V vs SCE, this contributes to the anodic displacement of the pitting potential. For smallest concentration of permanganate 10 −4 mol.dm −3 , the inhibition efficiency is up to 98 %. Increasing the permanganate ions concentration improves the stainless steel's re-sistance to pitting corrosion which is reflected by anodic displacement of pitting potential. At high temperatures, the pitting potential shifts cathodically, making the surface more vulnerable to pitting corrosion, which requires further investigation for its use in cooling circuits. Whatever the pH of medium, permanganate ions are reduced to MnO2. The characterization of deposit layer by X-ray diffraction shows the formation of a single form of the manganese oxide -MnO2, with average crystallite size and micro strain: 3.17 nm and 4.24%, respectively.