The corrosion characteristics of SS316L stainless steel in a typical acid cleaning solution and its inhibition by 1-benzylimidazole: Weight loss, electrochemical and SEM characterizations

Acid cleaning, an inevitable industrial practice used to descale chemical reactors, usually causes serious corrosion attack on underlying alloy substrates. Ameliorating this phenomenon requires the addition of e ﬀ ective corrosion inhibitors into the acid solution. Current global regulations encourage environmentally-benign molecules as corrosion inhibitors. Consequently, 1-benzylimidazole has been investigated for its inhibitive characteristics against the corrosion of SS316L stainless steel in a typical acid cleaning solution containing 2 % HCl + 3.5 % NaCl. Weight loss measurements conﬁrm that the corrosion inhibition property of 1-benzylimidazole increases with concentration but depreciates with increased temperature. Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) measurements conﬁrm that 1-benzylimidazole adsorb on the stainless steel surface to isolate its surface from the acid solution. 1-benzylimidazole is a mixed-type inhibitor with greater anodic inﬂuence, and its adsorption enhances the formation and protectiveness of a passive ﬁlm. Weight loss and the electrochemical measurements agree to an average inhibition e ﬃ ciency > 70 % at 1000 ppm. The inhibitor adsorbs via physisorption and obeys the Temkin isotherm model. SEM surface characterization conﬁrm the ability of 1-benzylimidazole to protect the surface microstructure of the stainless steel during the corrosion.


Introduction
Stainless steel is heavily deployed in the construction of many reactors in the chemical industry. It exhibits good corrosion resistance because it readily forms an iron/chromiumenriched passive layer [1]. Unfortunately, the passive film metallic equipment and diminish their performance. While the metal surface is descaled, a simultaneous acid corrosion attack on the underlying substrate usually ensues. The extent of this corrosion is even aggravated by the seawater sources used to prepare the acid solutions, since seawater is naturally enriched with chloride ions, especially as NaCl [3,4]. A practical industrial approach to ameliorate this corrosion attack is to add highly effective corrosion inhibitors into the acid cleaning solution. Currently, most industries apply propargyl alcohol-based inhibitors which are very toxic, both, to humans and other environmental species [5].
Imidazole and its derivatives are nitrogen-based heterocyclic compounds largely used for the production of numerous cheap environmentally-safe drugs [6,7]. In the past decade, imidazole derivatives have been reported as formidable corrosion inhibitors for various metals. Singh et al. [8] reported that 2-(4-methoxyphenyl)-4,5-diphenyl-imidazole exhibited 93 % efficiency for the inhibition of J55 steel corrosion in CO 2 -saturated brine solution. Ting et al. [9] reported that 4-phenylimidazole acted as a mixed-type corrosion inhibitor for copper in sulphuric acid and provided 95.84 % efficiency; adsorbing both by physical and chemical mechanisms. In other reports [10], thiazole-4-carboxylic acid, 2-methyl-1,3thiazole-4-carboxylic acid, and some imidazole zwitterions have been reported as highly efficient for carbon steel during acid corrosion [11]. Particularly, dimethylbenzodiimidazole (2,7-dimethyl3,6-dihydrobenzo [1,2-d;3,4-d'] diimidazole acted as an anodic inhibitor for SS316L stainless steel in 0.5 M H 2 SO 4 and provided 77 % at optimum concentration [12]. Also, some synthesized dicationic imidazolium ionic liquids were also reported for SS304 stainless steel in 0.5 M H 2 SO 4 solution whereby 92 % efficiency was recorded [13]. Apparently, not much work has been reported for imidazole derivatives as stainless steel corrosion inhibitors. Even so, the reported derivatives are expensively synthesized compounds with cumbersome molecular structures that disturb their adsorption. Identifying a cheap and simple-structure imidazole derivative which can deliver high inhibition efficiency will be very beneficial to the industry. In the present work, 1-benzylimidazole has been identified as a promising imidazole derivative for the inhibition of stainless steel acid corrosion. 1-benzylimidazole is a major pharmacological ingredient with environmentally-safe characteristics, according to the Paris Commission (PARCOM) standard [14], because it exhibits log P o/w = 1.93 and LD 50 = 75 mg/kg [15]. As Figure 1 shows, 1-benzylimidazole has a simple structure whereby a phenylmethyl group is attached to the imidazole nitrogen. Computational simulation previously revealed that this simple structure enables 1-benzylimidazole to inherently orient in a flat position and achieve good metal surface coverage during corrosion inhibition [16]. The most visible reports of 1benzylimidazole as corrosion inhibitor have been presented for carbon steel [16,17,18] in acid solutions. The inhibitor exhibited more anodic behaviour on the carbon steels, causing us to suspect its ability to enhance anodic passivation when applied for stainless steel. Furthermore, the inhibitor efficiency has not been reported in a practical acid cleaning solution which contains HCl + NaCl, and is usually conducted at elevated temperatures [3]. Filling this research gap will be highly beneficial towards the development of greener corrosion inhibitors for industrial acid cleaning.
Consequently, we apply weight loss technique to screen the effect of concentration and temperature on the performance of 1-benzylimidazole as inhibitor against the corrosion of SS316L stainless steel in 2 % HCl + 3.5 % NaCl solution. Thereafter, we employ electrochemical techniques like electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) to provide mechanistic interpretation of 1-benzylimidazole performance at maximum concentration. Scanning electron microscopy (SEM) was employed to probe the extent of stainless steel surface damage by corrosion and its mitigation by the 1-benzylimidazole.

Materials
All chemicals and reagents used in this work were purchased from Sigma Aldrich and Fischer Scientific, respectively. Stainless steel SS316L, having the elemental composition previously reported [19], was used. The bulk alloy bar was machined into corrosion test coupons with total surface area 24.89 cm 2 and 1 cm 2 , respectively, exposed for the weight loss and electrochemical experiments. The test solution (2 % HCl + 3.5 % NaCl) was prepared by, firstly, diluting concentrated HCl (37 w/v%) with distilled water to obtain a 2 % HCl solution and, secondly, preparing 3.5 % NaCl solution using the 2 % HCl as solvent. Stock solution of 1-benzylimidazole was prepared in isopropanol and calculated volumes were subsequently retrieved and made up to 200 mL with the acid solution, so as to obtain working inhibitor concentrations of 200, 400, 800 and 1000 ppm. A thermostatic water bath was employed to maintain experimental temperatures.

Methods
Prior to any experiment, the surface of the stainless steel coupon was polished with #400, #600, #800 and #1000 grit silicon carbide papers. This was performed with simultaneous washing under running tap water, degreasing with acetone and mechanical drying with warm air. Weight loss experiments, after 24 h immersion in the uninhibited and inhibited acid solutions, were conducted, initially, at 25 • C. Using the optimum inhibitor concentration, the effect of temperature (45 and 65 • C) was investigated. The stainless steel coupons were retrieved after 24 h immersion and cleaned according to the ASTM G1-03 standard procedure [20]. The coupons were then re-weighed and the weight difference (∆W) before and after immersion was calculated based on Equation (1). From the weight loss, the corrosion rate (CR) in millimeter per year (mpy) was calculated, based on Equation (2); where A = total surface area of coupon The electrochemical experiments such as EIS and PDP were conducted on a Gamry 600 potentiostat equipped with the Echem Analyst software for data analysis. The electrochemical set-up was a 3-electrode system which contained the stainless steel as the working electrode, a graphite rod as counter electrode and Ag/AgCl as the reference electrode. All measurements were performed at the end of 1 hour free immersion to ensure that the corrosion coupons attained steady-state open circuit potential [21]. The EIS measurement was conducted by applying ± 10 mV sinusoidal signal amplitude perturbation over a frequency range of 10 5 Hz to 10 −1 Hz. The PDP experiment was conducted by polarizing the stainless steel from −0.25 V (vs. open circuit potential) to +1.2 V vs. Ref. at a rate of 0.2 mV/s . The SEM surface characterization for the polarized inhibited and uninhibited electrode was achieved using the JEOL JSM-6610 LV scanning electron microscope.

Weight loss results
The weight loss, and associated corrosion rate, experienced after 24 h immersion of the stainless steel coupons in 2 % HCl This result can be interpreted by considering that the weight loss experienced by the immersed coupons is caused by the loss of surface atoms like Cr and Fe via oxidation into species like Cr 3+ , Fe 2+ and Fe 3+ . On the other hand, organic inhibitors impede corrosion by interacting the reactive electrons n their heteroatoms and C = C functional groups with the d-orbitals of metal atoms. These electron-rich sites donate electrons that fill the d-orbitals in the metal atoms, thus, supressing their oxidation. The resultant inhibitor adsorption increases the hydrophobicity of the metal surface and isolates it from the corrosion environment, as the schematic in Figure 3 elucidates. The more inhibitor in the acid solution, the more metal surface 216 is covered and protected.
As temperature increases to 45 and 65 • C, Table 1 shows that the weight loss and corrosion rate increase for, both, the uninhibited and inhibited acid solutions. The corrosion inhibition efficiency provided by the maximum concentration of 1000 ppm, initially, decreased rather moderately to 67.80 % (45 • C) and, thereafter, quite drastically to 37.61 % (65 • C). Although the presence of 1-benzylimidazole, somewhat, protected the stainless steel at these elevated temperatures, the depreciation in inhibitor efficiency may be attributed to the competitive mechanisms of increased surface dissolution and weakened inhibitor adsorption. Increased temperature promotes the transfer of corrosive agents, especially the chloride ions, from solution bulk to the metal surface, thus, promoting surface dissolution rather than inhibitor adsorption. Also, elevated temperatures could induce desorption of already adsorbed inhibitors from the metal surface, indicating that 1-benzylimidazole adsorbs via a physiosorption mechanism [22].

Adsorption isotherm
Adsorption isotherms provide adequate understanding of the mode of interaction between an adsorbing corrosion inhibitor molecule and the metal substrate. They describe whether inhibitor molecules adsorb on the metal surface as a single layer of film, as multilayer, or if any repulsion exists among inhibitor molecules [23]. In this research, the Langmuir, Temkin and Freundlich isotherms were employed to test the mode of interaction of 1-benzylimidazole on the stainless steel surface. The Langmuir isotherm predicts that the inhibitor adsorbs on the stainless steel surface as a monolayer. The Temkin isotherm predicts that the inhibitor forms a multilayer with plausible inhibitor-inhibitor interaction. On the other hand, the Freundlich isotherm depicts that the inhibitor is adsorbing on a rough and heterogeneous surface. The linear forms of the Langmuir, Temkin and Freundlich isotherms are, respectively, presented as follows: θ = surface coverage = %IE 100 , K ads = adsorption-desorption equilibrium constant, C = inhibitor concentration (in ppm) and the factor, α, accounts for the lateral interaction between inhibitor and metal surface.
The value of n describes the feasibility of inhibitor adsorption; which is highly feasible when 0 < 1 n < 1 but, respectively, moderate or difficult when 1 n = 1 or > 1 [24]. A plot of versus C will yield the Langmuir isotherm curve, as shown in Figure  4a. The Temkin isotherm, Figure 4b, is described by the plot of θ against ln C whereas the plot of log θ versus log C gives  Figure 4c. Given that it displayed the closest agreement between experimental plots and fitting line, and because its correlation coefficient (R 2 ) value was closest to unity, the Freundlich isotherm most appropriately describes the adsorption of 1-benzylimidazole on the SS316L stainless steel. The adsorption equilibrium constant was calculated as 1.025 ppm mol −1 and, from this value, the Gibbs' free energy (∆G ads = −RT ln(10 6 K ads ) ) was deduced as -34.29 kJ/mol and the negative value of ∆G confirms that the inhibitor adsorption was spontaneous and feasible.

Electrochemical results
Electrochemical measurements are capable of providing mechanistic explanations to the corrosion inhibition behaviour of 1-benzylimidazole on the SS316L stainless steel in the acid solution. At the open circuit potential, a quasi-equilibrium is established between the rates of oxidation and reduction reactions taking place in the micro-cells forming anodic and cathodic active sites on the alloy surface. By displacing the alloy from its equilibrium potential, relevant electrochemical parameters could be measured and related to the corrosion phenomenon. Steady-state electrochemical techniques like EIS can provide accurate information about the dielectric and charge transfer phenomena at the alloy-acid solution interface. A more aggressive technique like the PDP is able to provide details about the corrosion potential, corrosion kinetics, passivation and passivation breakdown/pitting corrosion phenomena.
The EIS results are presented in Figure 5, as the plots of Nyquist (Figure 5a), absolute impedance ( Figure 5b) and phase angle (Figure 5c). The characteristic arcs displayed by the stainless steel appear larger in the presence of 1benzylimidazole, compared with the size in the blank acid solution. Larger arcs usually depict greater resistance to charge transfer at the alloy-solution interface, i.e. greater corrosion resistance. The result confirms the inhibitive property of 1-benzylimidazole for the stainless steel. The trend in the Nyquist arc sizes is confirmed by the values of low-frequency absolute impedance in Figure 5b. Clearer mechanism is        frequency) and at a corrosion layer-substrate interface (at low frequency) [25]. Compared with the single time constant for the uninhibited stainless steel, the two time constants in the presence of 1-benzylimidazole is due to the adsorbed corrosion inhibitor layer which isolates the stainless steel substrate from the acid solution. Consequently, the Figure 5d and 5e were adopted as electrical models to interpret the corrosion phenomena for the uninhibited and inhibited stainless steel, respectively, after fitting experimental results using the Echem Analysts software. The values of respective electrical elements are detailed in Table 2.
The elements R s , R ct and CPE dl represent the solution re-sistance, charge transfer resistance and double layer capacitance, respectively. The use of constant phase element (CPE), rather than pure capacitor, in the model is to account for surface roughness of the corroding alloy [26]. The R ct is equivalent to the corrosion resistance of the corroding metal while the CPE dl elements, Y dl and n, describe the admittance of charge into the double layer and the roughness of the alloy surface (α dl ), respectively. In Figure 5e, the characteristic of the adsorbed inhibitor layer is characterized by the R f and CPE f elements (Y f and α f ) which, respectively, describe the porosity of the adsorbed layer and the charge capacitance at the layer-solution interface. From Table 2, the lower Y dl values obtained in the presence of 1-benzylimidazole confirms that the inhibitor adsorption reduces the accumulation of corrosion-inducing aqueous species at the stainless steel surface and, thus, lowers the rate of charge transfer at the alloy surface-solution interface.
As Table shows, the inhibitor adsorption impacts a total resistance (R ct + R f ) of 765 Ωcm 2 , compared with the value of 212 Ωcm 2 recorded for the blank solution. This gives an inhibition efficiency of 72.29 %, based on the Equation (7). Furthermore, the α dl value is also higher in the presence of inhibitor, which indicates that the inhibitor adsorption minimizes the roughness of the alloy surface due to the corrosion attack.
In Figure 6, each PDP curve shows an activation-controlled cathodic arm representing the dominant H + reduction in the acid solution. The anodic arm is characterized by an active region, a region of passivation and a region of passivation breakdown. The active region is attributed to the oxidation and dissolution of surface elements like Fe and Cr, the passivation potential region depicts the formation of protective oxides and hydroxides of Fe and Cr [27], whereas the region of passivation breakdown indicates potential regions where protective passive layer is broken, especially locally by chloride ions.
Extrapolating the PDP plots around ± 10 mV of the cathodic-anodic potential transition enabled the deduction of corrosion current density (i corr ), corrosion potential (E c orr ), anodic (β a ) and cathodic (β c ) Tafel constants [28]. The E corr indicates the thermodynamic tendency to corrode in the solution and the most preferable active sites of corrosion inhibitor interaction with the corroding surface, while the i corr is a measure of the corrosion rate. 220 By Table 3, the presence of 1-benzylimidazole in the acid solution slightly shifts the E corr towards more anodic potential and lowers the i corr value from 161.30 µ A cm −2 to 45.90 µAcm −2 ; which amounted to an inhibition efficiency of 71.54 % according to Equation (8). It is reported that 1-benzylimidazole exists as a cationic species in acidic solution [17]. Therefore, the greater inclination of 1-benzylimidazole to adsorb on anodic sites of the stainless steel strongly suggests the existence of pre-adsorbed anions on the anodic sites containing Fe 2+ and Cr 3+ , which eventually attract the cationic inhibitor towards the anode. Such phenomenon was also reported elsewhere for 1benzylimidazole in CO 2 + H 2 S-saturated NaCl solution [16]. This anodic preference is a stronger inclination towards more protective passivation, as confirmed by Figure 6; where the inhibitor shifts passivation current to lower values. Although the inhibitor does not affect the passivation potential (and passivation mechanism), it strongly modifies the passive film formed into a more protective one. This means that 1-benzylimidazole adsorption encourages the formation of Fe and Cr oxides and hydroxides. Nevertheless, the inhibitor appears not too protective once the passive film is deteriorated. This is feasible because the adsorbing inhibitor molecules and nucleating corrosion products would experience some structural mismatch which are easily targeted for localized attack and collapse.

SEM results
The surface morphologies of the stainless steel, after polarization in the acid solution without and with 1000 ppm of 1-benzylimidazole, are presented in Figure 7a and 7b, respectively. In the absence of inhibitor, Figure 7a, the stainless steel surface exhibits a rough morphology with patches of passive products around the surface. In the presence of inhibitor, Figure 7b, the passive products appear to agglomerate into a more continuous and denser layer which provides more surface coverage for the alloy surface. This justifies the findings from electrochemical measurements that the adsorption of 1-benzylimidazole could facilitate the formation of passive layer. At some points, however, some shallow cracks could be observed in the Figure 7b. These are likely points where localized corrosion occurs when the alloy is polarized beyond the passivation region.

Conclusion
The corrosion characteristics of a chemical reactor alloy SS316L stainless steel, and its inhibition by 1-benzylimidazole, have been investigated in 2 % HCl + 3.5 % NaCl solution (a typical solution used for industrial acid cleaning). The addition of 1-benzylimidazole into the test solution significantly lowered the rate of corrosion of the alloy, wherein an optimum inhibitor concentration of 1000 ppm yielded inhibition efficiency greater than 70 %. The 1-benzylimidazole acted more like an anodic-type inhibitor which improved the characteristics of the passive film formed on the alloy surface. The adsorption of 1benzylimidazole followed the Temkin isotherm and protected the stainless steel surface from localized corrosion, based on SEM characterization.