Acid (HEDP) as a Corrosion Inhibitor of AISI 304 Stainless Steel in a Medium Containing Chloride and Sulfide Ions in the Presence of Different Metallic Cations

The novelty of this paper is the analysis in a medium containing sulfide ion due to the generation of this ion in petroleum industries, in the refining stage (the sulfide ion is also present on the produced water). The performance of 1-hydroxyethylidene-1,1-diphosphonic acid inhibitor (HEDP) was investigated by potentiodynamic polarization, electrochemical impedance spectroscopy, and weight loss measurements in a dissolution of AISI 304 stainless steel immersed in a solution containing chloride and sulfide ions. The protection of the stainless was increased with the addition of divalent cations (Ca 2+ , Zn 2+ , and Mg 2+ ). Potentiodynamic polarization studies have shown that the inhibitor alone has anodic protection, but the addition of Ca 2+ (10 mg∙L −1 ) favors the cathodic protection, and the addition of Zn 2+ (20 mg∙L −1 ) and Mg 2+ (10 mg∙L −1 ) mixed-type is observed. Electrochemical impedance spectroscopy was performed at three distinct potentials: −0.3 [V vs. SCE], E corr [V vs. SCE], and 0.1 [V vs. SCE]. This revealed that calcium is responsible for favoring the formation of the film and the other elements (zinc and magnesium)


Introduction
Stainless steel is utilized in aggressive environments due to the formation of a passive film [1]. The chemical composition of this steel consists of Fe and Cr. The presence of Cr guarantees the formation of the protective film composed of chromium oxide which protects the material and minimizes pitting corrosion, increasing its industrial applicability [2] [3].
Metal corrosion is defined as the destructive attack of metal material by chemical or electrochemical action on the medium, leading to consequences such as high-cost maintenance, material loss, and product contamination [4].
AISI 304 austenitic stainless steel is utilized in the chemical, petroleum, pulp and paper, aerospace, and food industries [5] [6]. The material can suffer localized corrosion, stress corrosion, and crack corrosion. These processes occur due to possible film defects that are preferred sites for nucleation [6]. Besides environmental factors such as the presence of chloride and sulfide ions, the presence of inclusions in the microstructure, especially inclusions consisting of MnS, leads to pitting corrosion [7].
Inhibitory substances are added to reduce corrosion in materials. Their main advantage is that they protect the equipment by increasing its durability [8]. The most commonly used inhibitors are organic inhibitors and their efficiency is related to the presence of heteroatoms such as N, S, P, and O, and π bonds and groups with polar functions such as -CN, -NO 2 , -OH, -OCH 3 , -COOH, -COOC 2 H 5 , -NH 2 , -CONH 2 [9] [10] [11] [12]. Inhibitor efficiency is increased using molecules with planar geometries as they have greater contact with the metal surface, ensuring greater adsorption [10]. 1-Hydroxyethylidene-1,1-diphosphonic acid (HEDP) has a P-C-P bond and is classified as an organic phosphonic acid [13]. The HEDP is a commercial inhibitor and its molecular structure consists of a central carbon atom and two phosphonic acidic groups, which are linked to the central atom, together with a hydroxyl group and a methyl group [14] (see Figure S1 in concluded that the anodic region is protected by the HEDP inhibitor and the cathodic region by Zn 2+ , due to the formation of a Zn(OH) 2 film. Other paper developed by Sekine et al. [19] with the AISI 304 stainless steel, indicated the importance of the presence of Ni and Cr on the steel composition for the formation of a protective film together with HEDP.
The pH influences the adsorption of the inhibitor on the metal surface. An investigation carried out by Awad and Turgoose [20] on mild steel, concluded that in the absence of chloride ions, a mixture consisting of HEDP-Zn showed protective characteristics at the pH range from 6.5 to 9.5. However, with a decrease of pH to 4.5, adsorption is reduced due to the increase of free phosphonate. The effect of Ca 2+ was observed by Mohammedi et al. [21], in an analysis of carbon steel immersed in a medium containing 1.7 × 10 −3 mol•L −1 HEDP, 1.7 × 10 −3 mol•L −1 NaCl and 3.0 × 10 −3 mol•L −1 CaSO 4 , and found that at pH 7.0, the addition of Ca 2+ increased inhibitor efficiency by up to 80%. At pH 11, the addition of this divalent cation did not favor the increase in inhibitor efficiency, i.e., indicating that this cation acts at low pH.
The novelty of this paper is the analysis in a medium containing sulfide ion. The reduction of sulfate to sulfide by sulfate-reducing bacteria (SRB), occurs downhole in oil reservoirs, as well as in above-ground facilities. This process is unwanted because its toxicity presents a potential danger to human health and because its presence increases corrosion of pipelines and other steel infrastructure [22] [23].
The investigation in a medium containing sulfide ion was also carried out due to the generation of this ion in petroleum industries, in the refining stage (the sulfide ion is also present on the produced water) [24] [25] [26].
In our study, potentiodynamic polarization (PDP) techniques, electrochemical impedance spectroscopy (EIS), and weight loss measurements were employed to verify the efficiency of the HEDP inhibitor on AISI 304 austenitic stainless steel immersed in a solution containing 3.5 wt% NaCl, 1 mmol•L −1 Na 2 S. EIS is one of the most important techniques for studying the strength characteristics of the film formed on the metal surface, which provides information on the corrosive process [12].
Subsequently, an evaluation was performed of the effect of the addition of divalent cations, Ca 2+ , Zn 2+ , and Mg 2+ , ranging from 10 mg•L −1 to 30 mg•L −1 , in a solution containing 50 mg•L −1 of the inhibitor under investigation. The study was carried out to verify the performance of the presence of divalent cations with HEDP, on AISI 304 austenitic stainless steel immersed in an aggressive medium containing chloride and sulfide ions. Surface analysis after applied potential was also performed by Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDS).

Samples Preparations
The samples were cut in an L-shape for electrochemical and morphological analysis. The cut was performed using a hand guillotine. The test area was 1.0 cm 2 , which was isolated with epoxy resin (Araldite ® ). For all investigations, the samples were wet sanded using silicon carbide sandpaper with grit sizes of 180, 220, 320, 500, 800, and 1200 mesh. The sanded samples were cleaned with 97% ethanol and dried in hot air. They were subsequently polished using Arotec ® 6 µm, 3 µm, 1 µm, and 1⁄4 µm diamond paste to obtain a scratch-free specular surface. The samples were sanded and polished using an Arotec ® polishing machine, the Aropol VV model.

Solutions
The solutions used in the electrochemical tests (Potentiodynamic Polarization and Electrochemical Impedance Spectroscopy) for the analysis of the efficiency of the HEDP inhibitor were composed of a mixture of 3.5 wt% NaCl PA (Dinâmica, Indaiatuba, SP, Brazil), 1 mmol•L −1 Na 2 S PA (Impex, Diadema, SP, Brazil) and 20, 30, 50, and 100 mg•L −1 HEDP (Polyorganic Technology, São Paulo, SP, Brazil). All the solutions were prepared using the ultrapure water (SARTORIUS mini Arium ® with a resistivity of 18.2 MΩ cm at 22˚C ± 3˚C).
The pH of the solution was measured using the pocket-sized pH meter Isfetcom, S2K712 model. The pH of the solution in the absence of the inhibitor is 9.5, in the presence of the inhibitor (50 mg•L −1 ) 8.3, in the presence of calcium (10 mg•L −1 ) 5.9, in the presence of zinc (20 mg•L −1 ) 5.8 and in the presence of magnesium (10 mg•L −1 ) 6.4. The electrolytes were not stirred or heated. The temperature for all the measurements was 21˚C ± 3˚C. In order to obtain the real operation conditions, the oxygen dissolved in the solution was not removed.

Optical Microstructural Characterizations
The samples were prepared in bakelite phenolic resin by means of an Arotec ® automatic mounting press, Pre Mi model. Then, the samples were electrolyzed per 30 s, using a voltage of 6 V and in the presence of oxalic acid 10 wt%, ac-cording to ASTM A262-15 [27]. Microstructural characterizations by optical microscopy were performed using a Nikon Inverted Optical Microscope, Eclipse MA 200 model.

Scanning Electron Microscopy and Energy Dispersive X-Ray Spectroscopy Characterizations
The microstructural characterizations of the samples under investigations were carried out using two different equipment: Scanning Electron Microscopy (SEM) coupled to the X-ray Dispersive Energy Spectrometer (EDS), Shimadzu ® SS550 model microscope with voltage acceleration of 25 kV; and the microscope from Zeizz ® , EVO I MA 10 model, and the Oxford Instruments model X-MaxN spectrometer. The data were obtained by AZtec 2.1a software. The voltage acceleration was 30 kV.

Electrochemical Tests
Electrochemical experiments were performed on an AUTOLAB 302 potentiostat/galvanostat equipped with Nova 2. the frequency range from 100 kHz to 10 mHz, and the signal amplitude sine wave used was E rms (root mean square) = 5 mV (p/p) with 10 points per logarithmic decade using "single sine" mode. The simulation of the obtained data was performed in EIS Spectrum Analyzer Software using the Newton algorithm and the amplitude function. The electrical equivalent circuit used in the fit was R s (CPE − R p ) (see Figure S2 in the supplementary material), where R s is the solution resistance, R p is the polarization resistance, and CPE is the constant phase element and is related to capacitive characteristics of the system. Data were adjusted with errors below 10% and chi-square at 10 −3 . Chi-square ( 2 c r ) is determined according to Equation (1) The inhibitor efficiency was calculated by Equation (2) [29], where η E is the Advances in Chemical Engineering and Science inhibitor efficiency, R p is the polarization resistance in the presence of the inhibitor, and p R′ is the polarization resistance in the absence of the inhibitor.

Weight Loss Measurements
Weight loss measurements were based on ASTM G31-72 [30]. The AISI 304 stainless steel samples were immersed in a 50 mL volume of the solutions that obtained the best result in the electrochemical tests for 3 months. After the soaking time, they were washed and treated for pickling. The steel pickling process was performed according to ASTM A380/A380M-17 [ The surface coverage (θ) and weight loss efficiency (η m ) calculations were performed according to Equations (4) and (5), respectively, where C R0 is the corrosion rate in the absence of the inhibitor and C RI is the corrosion rate in the presence of the inhibitor [32].

Microstructural Characterization before Potentiodynamic Polarization Tests
In the optical microscopy image of the AISI 304 stainless steel there is a microstructure of recrystallized austenite grains and annealing twins, which are characterized by parallel bands in contrast to the grains [33]. The inclusions are points susceptible to dissolution during corrosion and allow the formation of a corrosion cell within the metal and will act as an anode, cathode, or be inert, according to its potential [34] [35]. For these points, EDS analysis was performed and the spectra showed the presence of inclusions composed of Mn and S which favors the material corrosion process, as they are preferential sites for nucleation and pit growth [30] (see Figure S3 and Figure S4 in the supplementary material).

Potentiodynamic Polarization Tests
Potentiodynamic polarization curves for AISI 304 stainless steel in a solution containing 3.5 wt% NaCl, 1 mmol•L −1 Na 2 S in the absence and presence of the HEDP inhibitor at concentrations of 20, 30, 50, and 100 mg•L −1 are shown in Figure 1. Figure 1(a) shows that the addition of the inhibitor favored a change of corrosion potential (E corr ) to more positive potentials, indicating that the HEDP inhibitor is an anodic type controlling the oxidation reaction. The same result was observed by Kármán et al. [17], Sekine et al. [19] and Salasi and Shahrabi [36] indicating that the complex is formed on the metallic surface on the anode sites.
The potentiodynamic polarization results shown in Figure 1  stainless steel due to a slight lowering of j corr value, as indicated in Figure 1(b) and may be related to a greater blockage of active sites consisting of manganese sulfide, present in the sample before the potentiodynamic polarization tests (see the SEM-EDS analysis in the Figure S4 in the supplementary material). There is a slight decrease in the cathodic current density confirming that Ca 2+ promotes the oxygen reduction reaction, this behavior was also verified by Karmán et al. [17].
Potentiodynamic polarization curves with the addition of zinc are shown in Figure 1(c). Through analyzing the PDP curves, it was observed that the addition of Zn 2+ displaced the corrosion potential to low E corr -values and decrease on j corr when compared to the presence of 50 mg•L −1 HEDP. This behavior was also verified by Yan et al. [15] in a study performed on cold-rolled steel immersed in a medium containing 0.0082 mol•L −1 HEDP and 0.0082 mol•L −1 zinc nitrate and by Award [12] in a carbon steel study using a medium containing 0.003 mol•L −1 Cl − . Figure 1(d) shows that the addition of magnesium did not cause a significant change in the 50 mg•L −1 HEDP PDP curves. It was found that on the three Mg Advances in Chemical Engineering and Science concentrations investigated, there was no substantial variation of E corr . However, when comparing the cathodic and anodic polarization curves, one can verify a greater variation in the cathodic current density, when compared with the presence of 50 mg•L −1 HEDP, suggesting that this mixture acts mainly as a cathodic-type inhibitor [37]. The smallest j corr was identified by the addition of 10 mg•L −1 Mg 2+ , indicating a greater blocking of the active sites and consequently a lower flow of electrons from the anodic region to the cathodic region. Smaller j corr were observed with the addition of zinc and magnesium, suggesting that in the presence of these divalent cations a hydroxide film of these cations can be formed in the cathodic area and facilitates adsorption of the inhibitor in the anodic area.

Electrochemical Impedance Spectroscopy, EIS
The sample surface consists of metal and a protective film, which due to its chemical composition is generally not evenly distributed, thus resulting in dis-  Figure 2.
Complex plane impedance plots have the same shape as a single deformed semicircle for all the potentials investigated, indicating that the corrosion reaction in the AISI 304 stainless steel was controlled by the behavior of the double layer and the charge transfer process [38]. Moreover, it can see that the optimal inhibitor concentration to favor a surface containing a longer protective film was observed. This fact indicates that at those concentrations, the inhibitor acted more effectively in the region most prone to the corrosive process such as inclusions, discontinuities, and grain boundaries, creating a protective barrier and making the surface more homogeneous. Increased semicircle inclination is associated with increased resistance to the charge transfer process from metal to electrolyte [37]. The effect of calcium is verified in Figure 3   indicates that there has been a change in the kinetics of the corrosive process due to the formation of a protective film [39].
The adsorption of the inhibitor on the metal surface is influenced by pH.
Zenobi et al. [40] perform in-situ ATR-FTIR spectroscopy of HEDP in aqueous solution and observed a similar spectrum at pH 9.0 and 8.0, only the PO − species [29], however, in the pH range from 8.0 to 9.0, only the 2 3 PO − specie is deprotonated [40], making the adsorption difficult. With the decrease in pH, with the addition of Ca 2+ , in the range of 6.0 -5.0, there is a reduction of the 2 3 PO − bands and the appearance of new bands [40] it is suggested that in this concentration there was an increase of protective film, due to a greater transport of the inhibitor to the metal surface.
According to Deluchat et al. [16] HEDP has the ability to complex with Ca 2+ in a pH range of 5.5 -7.0. However, in the same pH-range, it has a greater affinity for Fe 2+ . This indicates that Ca 2+ facilitated the transport of HEDP to the metal surface, but did not favor the formation of a hydroxide film with protective characteristics. If one takes into account that the corrosion process occurs much more pronounced in an acidic environment, the change in pH observed from basic pH to acidic pH, the system became much more aggressive, however, when observing that even at this acidic pH the inhibitor acted to protect the specimen by inhibiting the corrosive process. In this way, one can infer that the inhibition efficiency is much greater in the presence of inhibitor than without the inhibitor.  (Figure 4(b)), the addition of Zn was positive for the protection of the steel surface from the attack of chloride and sulfide ions present in the solution. This was confirmed by an increase in deformed semicircle diameter at 10 mg•L −1 and 20 mg•L −1 concentrations. A study by Miao, Wang, and Hu [29] suggest that in the presence of Zn 2+ in a bulk solution, HEDP-Zn 2+ complexes are formed and diffused to the steel surface where it is converted to HEDP-Fe 2+ . HEDP-Fe 2+ is adsorbed to the anodic region according to Equation  (6). Zn 2+ reacts with OH − from the cathodic reaction (Equation (7)) and forms zinc hydroxide (equation (8)) which is adsorbed to the cathodic region and has protective characteristics. Reznik et al. [41] also found a better result for the addition of 20 mg•L −1 Zn 2+ , in a work with the 1020 carbon steel, immersed in a medium containing 30 mg•L −1 Cl − and 50 mg•L −1 HEDP and concluded that the addition of Zn 2+ was conducive to a greater formation of Zn(OH) 2 and its deposition on cathodic sites, delaying the corrosive process.
The pH found for the presence of Zn 2+ is similar to that found for Ca 2+ . ( Finally, at potential 0.1 [V vs. SCE] (Figure 4(c)), it is shown that for all Zn concentrations there was an increase in deformed semicircle diameters, demonstrating that this element favors stabilization of the protective film adsorbed to the cathodic region. Figure 5 shows the effect of magnesium addition in the complex plane impedance  (9)) and adsorbed in the anodic region. Mg 2+ reacts with OH − from the cathodic reaction (Equation (7)) and forms magnesium hydroxide (Equation (10) Mg aq 2OH aq Mg OH aq Bode plot (Figure 2(d)) shows that the addition of the inhibitor favored an increase in phase angle. At concentrations of 30, 50, and 100 mg•L −1 of HEDP, this value remained constant at approximately −68.0˚. When analyzing the Bode plots, in the absence of the inhibitor a greater frequency widening is found suggesting a surface covered by a more uniform protective film [43]. This larger widening is due to the presence of Ni found in the EDS spectra ( Figure S4(b) in the supplementary material), which guarantees greater resistance to the corrosive process in austenitic stainless steel [4]. According to Sekine et al. [19], on AISI 304 stainless steel a film is formed on the metal surface consisting of Ni, Cr and the HEDP inhibitor. In Bode plot (Figure 2(e)), the maximum phase angle achieved was −76.0˚ (30 mg•L −1 ). Phase angles closer to −90.0˚ indicate an increased inhibition of the corrosive process due to higher adsorption of inhibitor molecules on the metal surface [44].
Bode plot (Figure 2 SCE], the maximum phase angle was displaced to lower frequencies, indicating that the corrosive process was softened [39]. The impedance plots were analyzed by the equivalent electrical circuit described in Figure S2 in the supplementary material and the results are shown in Table 1. The capacitance of the electrical double layer (C dl ) was calculated according to equation 11 [46].
As shown in Table 1  and is associated to the adsorption of the molecule on the material surface [39].
There is also a decrease in C dl values, which may be related to a reduction in the   (Figure 1(c)). A decrease in C dl -values was also observed by Felhósi et al. [48], indicating formation of protective film on the metal surface. Film stabilization is supported by a more capacitive behavior due to an increase in the value of n reaching the value of 0.880 (0.1 [V vs. SCE]). In the same potential (0.1 [V vs. SCE]) but the presence of 20 mg•L −1 of Zn 2+ , a higher R p value was obtained showing that a protective film consisting of Zn(OH) 2 occurred on the metal/solution interface making to harder pit development [49]. Through analyzing the parameters obtained in the three distinct potentials under study, possible adsorption of Mg(OH) 2 in the cathodic region is observed due to a slight lowering of C dl . The characteristic of Mg to promote the stabilization of the pro-tective film is confirmed by decrease in C dl -values and an increase of parameter n for the addition of 10 mg•L −1 at 0.1 [V vs. SCE]. This fact is linked to the substitution of molecules of water adsorbed on the metal surface by inhibitory molecules, suggesting an increasing thickness of the electrical double layer and reducing the active area of the AISI 304 stainless steel [50] [51].  favors the reduction of the solution's pH providing acceleration of the corrosive process [52]. The product formed around the pit is composed of Fe(OH) 2 (Equation (14)) and the product formed inside the pit is composed of FeCl 2 [53].

SEM after Potential Measurements
However, the inhibitor adsorption is observed on the metal surface due to the presence of Fe and C ( Figure S5 and Figure S6 in the supplementary material) [54], indicating that possibly there was a decrease in the rate of oxygen reduction and/or iron dissolution reactions.
EDS analysis of the samples immersed in a solution containing the divalent cations showed that the presence of Cl − species was not verified, suggesting that the inhibitor favored a barrier for chloride ion penetrations ( Figure S6(b), Figure   S7(b), and Figure S8  in the EIS-results may be related to the presence of iron oxide on the metal surface, which it could not observe in the weight loss because the samples were subjected to pickling treatment.

Conclusions
The potentiodynamic polarization studies indicated that HEDP is an inhibitor anodic type. EIS-results showed an increase in R p -values and a decrease in C dl -values, indicating adsorption of the molecule on the material surface. showing that the presence of Zn 2+ and Mg 2+ cations favored a lower weight loss on AISI 304 stainless steel. For a better understanding corrosion process using HEDP inhibitor, we suggest making an appropriate study for each of the chloride and sulfide ions separately. In addition, studies by scanning electrochemical microscopy can be carried out to analyze as to the adsorption of the inhibitor along with the divalent cations. Advances in Chemical Engineering and Science Figure S1. Molecular structure of HEDP. Figure S2. Equivalent electrical circuit for electrochemical impedance spectroscopy data simulation. Figure S3. Micrograph of AISI 304 stainless steel before potentiodynamic polarization tests at 500x magnifications. Attack: Oxalic