Stainless steel is utilized in aggressive environments due to the formation of a passive film . 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  .
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 .
AISI 304 austenitic stainless steel is utilized in the chemical, petroleum, pulp and paper, aerospace, and food industries  . 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 . 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 .
Inhibitory substances are added to reduce corrosion in materials. Their main advantage is that they protect the equipment by increasing its durability . 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, -NO2, -OH, -OCH3, -COOH, -COOC2H5, -NH2, -CONH2    . Inhibitor efficiency is increased using molecules with planar geometries as they have greater contact with the metal surface, ensuring greater adsorption . 1-Hydroxyethylidene-1,1-diphosphonic acid (HEDP) has a P-C-P bond and is classified as an organic phosphonic acid . 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  (see FigureS1 in the supplementary material). This inhibitor forms complexes with divalent cations such as Ca2+, Zn2+, Mg2+, Cu2+, Fe2+, Ni2+, Cd2+ and Pb2+  .
The performance of the HEDP inhibitor has been studied by Kármán et al. . Using carbon steel immersed in a medium containing 0.5 mol∙L−1 NaClO4, 3 × 10−4 mol∙L−1 HEDP, and 3 × 10−4 mol∙L−1 Ca(NO3)2, research showed that isolated HEDP has anodic protection. On the other hand, the addition of Ca2+ contributes to shifting the corrosion potential to more negative values. In a study by Award  on carbon steel immersed in a solution containing 3 × 10−3 mol∙L−1 NaCl, it was observed that the presence of HEDP alone at a concentration of 70 mg∙L−1 accelerated the corrosive process. The addition of zinc at concentrations of 40 - 60 mg∙L−1 had an impact on the anodic reaction and inhibited corrosion. Recently, Yan et al.  studied the corrosion process on carbon steel immersed in a 3.5 wt% NaCl solution, 8.2 × 10−3 mol∙L−1 HEDP, and 8.2 × 10−3 mol∙L−1 of Zn(NO3)2, they concluded that the presence of Zn2+ after 2 hours of immersion favored an inhibition between 93.4% and 99.3%. In a study carried out with stainless steel, SS 41, immersed in a medium containing 0.3% NaCl by Sekine and Hirakawa , a maximum inhibitor efficiency at the concentration of 60 mg∙L−1 HEDP was verified with the addition of 40 mg∙L−1 ZnSO4∙7H2O. They concluded that the anodic region is protected by the HEDP inhibitor and the cathodic region by Zn2+, due to the formation of a Zn(OH)2 film. Other paper developed by Sekine et al.  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  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 Ca2+ was observed by Mohammedi et al. , 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 CaSO4, and found that at pH 7.0, the addition of Ca2+ 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  . 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)   .
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 Na2S. 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 .
Subsequently, an evaluation was performed of the effect of the addition of divalent cations, Ca2+, Zn2+, and Mg2+, 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).
2. Experimental Part
2.1. 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 cm2, 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.
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 Na2S 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 effects of calcium, zinc, and magnesium were investigated in a solution containing 3.5 wt% PA NaCl PA, 1 mmol∙L−1 Na2S PA, and 50 mg∙L−1 HEDP, CaCl2 PA (Dinâmica, Indaiatuba, SP, Brazil), ZnCl2 (97%, Dinâmica, Indaiatuba, SP, Brazil), and MgCl2 PA (Synth, Diadema,SP,Brazil) with concentrations ranged from 10 to 30 mg∙L−1.
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.
2.3. 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%, according to ASTM A262-15 . Microstructural characterizations by optical microscopy were performed using a Nikon Inverted Optical Microscope, Eclipse MA 200 model.
2.4. 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.
2.5. Electrochemical Tests
Electrochemical experiments were performed on an AUTOLAB 302 potentiostat/galvanostat equipped with Nova 2.1.1 software. A three-electrode cell configuration consisting of working electrode (1.0 cm2 AISI 304 stainless steel samples), a carbon counter electrode with dimensions of 6.76 mm × 22.55 mm × 59.61 mm, and a saturated calomel reference electrode (SCE-Hg/Hg2Cl2) was used.
Open circuit potential (EOC) was determined by employing a scan rate of 1 mV∙s−1 over 3600 s. Potentiodynamic Polarization (PDP) measurements were carried out over a potential range of ±240 mV around the EOC.
EIS measurements were carried out at three distinct potentials: E1 = corrosion potential (Ecorr [V vs. SCE]), E2 = −0.3 [V vs. SCE] and E3 = 0.1 [V vs. SCE] at the frequency range from 100 kHz to 10 mHz, and the signal amplitude sine wave used was Erms (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 Rs(CPE − Rp) (see FigureS2 in the supplementary material), where Rs is the solution resistance, Rp 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 ( ) is determined according to Equation (1) , where is the real and is the imaginary impedance values obtained from the EIS experimental data; is the real and is the imaginary impedance values calculated at the fitting of the curve. This free software was obtained from the website: http://www.abc.chemistry.bsu.by/vi/analyser/ .
The inhibitor efficiency was calculated by Equation (2) , where ηE is the inhibitor efficiency, Rp is the polarization resistance in the presence of the inhibitor, and is the polarization resistance in the absence of the inhibitor.
2.6. Weight Loss Measurements
Weight loss measurements were based on ASTM G31-72 . 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  using a 20% v/v nitric acid solution. Following the pickling process, the samples were washed, dried, and weighed. The corrosion rate (CR) was calculated according to Equation (3) , where K: constant (8.76 × 104 mm∙year−1); W: weight loss (g); A: sample area (cm2); T: immersion time (h); and D: sample density (8.0 g∙cm−3). The surface coverage (θ) and weight loss efficiency (ηm) calculations were performed according to Equations (4) and (5), respectively, where CR0 is the corrosion rate in the absence of the inhibitor and CRI is the corrosion rate in the presence of the inhibitor .
3. Results and Discussion
3.1. 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 . 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  . 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  (see FigureS3 and FigureS4 in the supplementary material).
3.2. Potentiodynamic Polarization Tests
Potentiodynamic polarization curves for AISI 304 stainless steel in a solution containing 3.5 wt% NaCl, 1 mmol∙L−1 Na2S 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 (Ecorr) 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. , Sekine et al.  and Salasi and Shahrabi  indicating that the complex is formed on the metallic surface on the anode sites.
The potentiodynamic polarization results shown in Figure 1 indicated that in the presence of the inhibitor there is a reduction in jcorr values up to the 50 mg∙L−1 inhibitor concentration, suggesting a lower electron flow from the anodic region to the cathodic region and favoring greater protection of the material in the aggressive medium containing chloride and sulfide ions. On the other hand, an increase in jcorr is observed at a concentration of 100 mg∙L−1, indicating an increase in the corrosion rate. According to Sekine et al.  the higher concentration of the inhibitor, culminate in an increase in the rate of corrosion, due to an excess of HEDP ions dissolved in the solution. The excess of HEDP leads to the formation of a soluble complex consisting of HEDP-iron capable of preventing passivation . Thus, one can infer that, the best inhibitor concentration that favored greater corrosion inhibition is 50 mg∙L−1.
In order to increase the efficiency of the corrosive process and favor a greater stabilization of the surface formed film, the effects of the addition of Ca2+, Zn2+, and Mg2+ in a solution containing 50 mg∙L−1 HEDP were studied.
The addition of calcium in a solution containing chloride and sulfide ions did not produce a very significant effect when compared to a solution containing only 50 mg∙L−1 HEDP, as shown in Figure1(b). The addition of this cation at a concentration of 10 mg∙L−1 provided an increased inhibitory effect on AISI 304 stainless steel due to a slight lowering of jcorr value, as indicated in Figure1(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 FigureS4 in the supplementary material). There is a slight decrease in the cathodic current density confirming that Ca2+ promotes the oxygen reduction reaction, this behavior was also verified by Karmán et al. .
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 Zn2+ displaced the corrosion potential to low Ecorr-values and decrease on jcorr when compared to the presence of 50 mg∙L−1 HEDP. This behavior was also verified by Yan et al.  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  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
Figure 1. (a) Potentiodynamic polarization curves of AISI 304 stainless steel in a medium containing 3.5 wt% NaCl, 1 mmol·L−1 Na2S, in the absence and presence of the HEDP inhibitor at 20, 30, 50 and 100 mg∙L−1 concentrations; (b) CaCl2 at 10, 20 and 30 mg∙L−1 concentrations in the presence of 50 mg∙L−1 HEDP; (c) ZnCl2 at 10, 20 and 30 mg∙L−1 concentrations in the presence of 50 mg∙L−1 HEDP; (d) MgCl2 at 10, 20 and 30 mg∙L−1 concentrations in the presence of 50 mg∙L−1 HEDP and scan rate of 1 mVs−1. Inserts show the potential for corrosion.
concentrations investigated, there was no substantial variation of Ecorr. 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 . The smallest jcorr was identified by the addition of 10 mg∙L−1 Mg2+, indicating a greater blocking of the active sites and consequently a lower flow of electrons from the anodic region to the cathodic region. Smaller jcorr 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.
3.3. 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 discontinuities. The film is the cathode area and the discontinuities are the anodic area. Thus, the EIS analysis was performed on three distinct potentials: −0.3 [V vs. SCE], Ecorr [V vs. SCE], and 0.1 [V vs. SCE]. Being that, two potentials far away to the Ecorr (−0.3 [V vs. SCE] and 0.1 [V vs. SCE]) were performed to verify at which inhibitor concentrations the samples would present a more homogeneous film formation. Complex plane impedance (a, b, c) and Bode (d, e, f) plots obtained for the three distinct potentials are shown in 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 . Moreover, it can see that the optimal inhibitor concentration to favor a surface containing a longer protective film was 100 mg∙L−1 for the −0.3 [V vs. SCE] and 30 mg∙L−1 for the others potentials (Ecorr
Figure 2. Complex plane impedance plots (a) −0.3 [V vs. SCE], (b) Ecorr [V vs. SCE] and (c) 0.1 [V vs. SCE] and Bode plots (d) −0.3 [V vs. SCE], (e) Ecorr [V vs. SCE] and (f) 0.1 [V vs. SCE], in a medium containing 3.5 wt% NaCl, 1 mmol∙L−1 Na2S, in the absence and presence of HEDP inhibitor at concentrations of 20, 30, 50 and 100 mg∙L−1 (where the line represents the fitting of the curves obtained in the data simulation).
[V vs. SCE], and 0.1 [V vs. SCE]). One could infer this due to higher Zre-values 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 .
The effect of calcium is verified in Figure 3 in the complex plane impedance plots obtained at −0.3 [V vs. SCE] (a), Ecorr [V vs. SCE] (b), and 0.1 [V vs. SCE] (c). These results show that the addition of a lower calcium concentration (10 mg∙L−1) ensures a greater Zre range and, consequently, a higher Rp value. This observation is in agreement with the results found in the PDP technique and
Figure 3. Complex plane impedance plots (a) −0.3 [V vs. SCE], (b) Ecorr [V vs. SCE] and (c) 0.1 [V vs. SCE] and Bode plots (d) −0.3 [V vs. SCE], (e) Ecorr [V vs. SCE] and (f) 0.1 [V vs. SCE], in a medium containing 3.5 wt% NaCl, 1 mmol∙L−1 Na2S, in the absence and presence of the HEDP inhibitor at a concentration of 50 mg∙L−1 and the addition of 10, 20 and 30 mg∙L −1 CaCl2 (where the line represents the curve fitting obtained in the data simulation).
indicates that there has been a change in the kinetics of the corrosive process due to the formation of a protective film .
The adsorption of the inhibitor on the metal surface is influenced by pH. Zenobi et al.  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 specie is protonated and, therefore, the phosphonic groups have different symmetry. At pH 7.0, bands of appear, in addition to the bands of . At pH 6.0 and 5.0 the spectra show the same bands found for pH 7.0, however, at pH 6.0 there is a decrease in the bands corresponding to the species and an increase in the , POH and P=O bands. At pH 5.0, there was an increase in the intensity of the bands of the protonated species.
The pH measured for the absence of the inhibitor was 9.5 and for the addition of 50 mg∙L−1 HEDP, 8.3. The addition of 10 mg∙L−1 Ca2+, which provided the best electrochemical results, shifted the pH to the value of 5.9. The adsorption of the inhibitor occurs by the union of the ligand with the divalent cation in the species , however, in the pH range from 8.0 to 9.0, only the specie is deprotonated , making the adsorption difficult. With the decrease in pH, with the addition of Ca2+, in the range of 6.0 - 5.0, there is a reduction of the bands and the appearance of new bands  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.  HEDP has the ability to complex with Ca2+ in a pH range of 5.5 - 7.0. However, in the same pH-range, it has a greater affinity for Fe2+. This indicates that Ca2+ 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 shows the effect of a zinc addition on the complex plane impedance plots at −0.3 [V vs. SCE] (a), Ecorr [V vs. SCE] (b), and 0.1 [V vs. SCE] (c). At potential −0.3 [V vs. SCE] (Figure 4(a)), it is observed that the addition of Zn did not contribute to the formation a protective film because it observed the larger deformed semicircle at 50 mg∙L−1 HEDP concentration in the absence of zinc cations. At Ecorr [V vs. SCE] (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  suggest that in the presence of Zn2+ in a bulk solution, HEDP-Zn2+ complexes are formed and diffused to the steel surface where it is converted to HEDP-Fe2+. HEDP-Fe2+ is adsorbed to the anodic region according to Equation
Figure 4. Complex plane impedance plots (a) −0.3 [V vs. SCE], (b) Ecorr [V vs. SCE] and (c) 0.1 [V vs. SCE] and Bode plots (d) −0.3 [V vs. SCE], (e) Ecorr [V vs. SCE] and (f) 0.1 [V vs. SCE], in a medium containing 3.5 wt% NaCl, 1 mmol∙L−1 Na2S, in the absence and presence of the HEDP inhibitor at a concentration of 50 mg∙L−1 and the addition of 10, 20 and 30 mg∙L−1 of ZnCl2. (Where the line represents the curve fitting obtained in the data simulation)
(6). Zn2+ 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.  also found a better result for the addition of 20 mg∙L−1 Zn2+, 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 Zn2+ 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 Zn2+ is similar to that found for Ca2+. ([Zn2+] = 20 mg∙L−1, the pH = 5.8). In the range of pH from 5.5 to 7.0, the Zn2+ ions can complex with HEDP, it already is verified by Deluchut et al.  that Zn2+ has a greater affinity to this inhibitor. This fact justifies greater protection for AISI 304 stainless steel in the presence of zinc, due to the formation and adsorption of a Zn(OH)2 film on cathodic sites.
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
Figure 5. Complex plane impedance plots (a) −0.3 [V vs. SCE], (b) Ecorr [V vs. SCE] and (c) 0.1 [V vs. SCE] and Bode plots (d) −0.3 [V vs. SCE], (e) Ecorr [V vs. SCE] and (f) 0.1 [V vs. SCE], in a medium containing 3.5 wt% NaCl, 1 mmol∙L−1 Na2S, in the absence and presence of the HEDP inhibitor at a concentration of 50 mg∙L−1 and the addition of 10, 20 and 30 mg∙L−1 of MgCl2. (Where the line represents the curve fitting obtained in the data simulation).
plots obtained at the potential −0.3 [V vs. SCE] (a), Ecorr [V vs. SCE] (b) and 0.1 [V vs. SCE] (c). It is observed that the addition of magnesium results in a behavior similar to that of the addition of zinc, since both the cations, at the same potential −0.3 [V vs. SCE], do not indicate contribute to the formation of the protective film. Besides, it was observed that magnesium at 10 mg∙L−1 and 20 mg∙L−1, favors the stabilization of the protective film due to a larger deformed semicircle at the Ecorr [V vs. SCE] and at the potential 0.1 [V vs. SCE]. The increase of the deformed semicircle indicates a greater inhibition of the corrosive process induced by the presence of the inhibitor . The increase in semicircle inclination may be linked to the formation of an HEDP-Mg2+ complex in bulk solution which is transported to the material surface where it is converted to HEDP-Fe2+ (Equation (9)) and adsorbed in the anodic region. Mg2+ reacts with OH− from the cathodic reaction (Equation (7)) and forms magnesium hydroxide (Equation (10)), which is adsorbed to the cathodic region. According to the EIS-results obtained, it suggests that the film is formed by magnesium hydroxide has protective characteristics. At 10 mg∙L−1 Mg2+ concentration, the pH of the solution was 6.4, indicating that the formation of the protective film consisting of Mg(OH)2 occurs at a pH value higher than that found for the presence of Ca2+ and Zn2+.
Bode plot (Figure2(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 . This larger widening is due to the presence of Ni found in the EDS spectra (FigureS4(b) in the supplementary material), which guarantees greater resistance to the corrosive process in austenitic stainless steel . According to Sekine et al. , on AISI 304 stainless steel a film is formed on the metal surface consisting of Ni, Cr and the HEDP inhibitor. In Bode plot (Figure2(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 .
Bode plot (Figure 2(f)) indicates that the maximum phase angle reached was −77.3˚ (30 mg∙L−1). Compared with the results obtained in the −0.3 [V vs. SCE] and on the corrosion potential, an increase of the maximum phase angle is evidence, indicating that the addition of the inhibitor contributed to a more capacitive behavior of the steel. Bode plots in the presence of calcium revealed that the addition of this element does not favor stabilization of the formed film due to a phase angle increasing from −73.0˚ (−0.3 [V vs. SCE]) to −77.04˚ (Ecorr [V vs. SCE]) and its subsequent decay to −73.2˚ (0.1 [V vs. SCE]) (see Figures 3(d)-(f)).
It was exposed that the corrosive process of steel in a medium containing HEDP, chloride, sulfide, and Zn2+ ions involved only a time constant. There was also a widening of range the frequency at intermediate frequencies. The impedance at intermediate frequencies reflects the contribution of capacitance  indicating that the formation of a protective film with greater homogeneity occurred, creating a barrier to the corrosive process. Film stabilization is confirmed by increasing the phase angle from −73.6˚ (−0.3 [V vs. SCE]—Figure 4(d)) to −77.1˚ (0.1 [V vs. SCE]—Figure 4(f)) at concentration of 20 mg∙L−1 Zn2+. Phase angles close to −90.0˚ suggests a more capacitive system and occur in response to increase inhibitor adsorption on the metal surface. At 0.1 [V vs. SCE], the maximum phase angle was displaced to lower frequencies, indicating that the corrosive process was softened .
Figure 5 also shows Bode plots at −0.3 [V vs. SCE] (d), Ecorr [V vs. SCE] (e), and 0.1 [V vs. SCE] (f) in a medium containing chloride, sulfide, and HEDP inhibitor in the presence and absence of magnesium. It was shown that for all investigated concentrations a single peak was identified, demonstrating that the corrosive process of AISI 304 stainless steel in these conditions is influenced by the charge transfer process and capacitance of the electrical double layer . As verified for the presence of Zn, the stabilization of the film on the AISI 304 stainless steel surface in the presence of Mg is confirmed by the phase angle increase from −71.9˚ (−0.3 [V vs. SCE]—Figure 5(d)) to −78.3˚ (0.1 [V vs. SCE]—Figure 5(f)), indicating an adsorption of the inhibitor molecule on the metal surface.
The impedance plots were analyzed by the equivalent electrical circuit described in FigureS2 in the supplementary material and the results are shown in Table 1. The capacitance of the electrical double layer (Cdl) was calculated according to equation 11 .
As shown in Table 1 (−0.3 [V vs. SCE]), Rp values increased with the addition of the inhibitor. This fact suggests the formation of a thicker film on the metal surface, which minimizes localized corrosion by the penetration of chloride and sulfide ions. In parameter n and the CPE values, no significant variation was observed indicating that the corrosion mechanism at this potential is controlled by the charge transfer process . Thus, it is indicated that at potentials below the corrosion potential the presence of the HEDP inhibitor provided the formation of a more uniform and adherent protective film, indicating greater protection for dissolution of the AISI 304 stainless steel at higher potentials. The maximum inhibitor efficiency (nE = 43.8%) achieved was [HEDP] = 100 mg∙L−1.
At Ecorr [V vs. SCE], the addition of the inhibitor generated an increase in Rp values. This increase was observed at concentrations of 30 mg∙L−1 and 50 mg∙L−1 and is associated to the adsorption of the molecule on the material surface . There is also a decrease in Cdl values, which may be related to a reduction in the
Table 1. Parameters obtained by simulating Electrochemical Impedance Spectroscopy data for AISI 304 stainless steel at -0.3 [V vs. SCE], Ecorr [V vs. SCE] and 0.1 [V vs. SCE], in a medium containing 3.5% wt% NaCl, 1 mmol∙L−1 Na2S and HEDP inhibitor; 50 mg∙L−1 HEDP and the addition of 10, 20 and 30 mg∙L−1 CaCl2, 50 mg∙L−1 HEDP and the addition of 10, 20 and 30 mg∙L−1 ZnCl2 and 50 mg∙L−1 HEDP and the addition of 10, 20 and 30 mg∙L−1 MgCl2
dielectric constant and/or an increase in the thickness of the electrical double layer. The approximation of parameter n close to 1.0 indicates that the electrical double layer formed on the metal surface resembles a pure capacitor .
At 0.1 [V vs. SCE], an increase in charge transfer resistance values (Rp-values) up to a concentration of 30 mg∙L−1 is observed, reaching a value of 9.06 × 103 Ω cm2 (and nE = 32.4%). This fact may be related to the presence of Ni in the chemical composition of AISI 304 stainless steel as responsible for the repassivation process of regions of the ruptured protective film. A higher adsorption of the inhibitor on the surface of the steel at concentration of 50 mg∙L−1 HEDP + 10 mg∙L−1 Ca2+ is confirmed by the decrease in CPE, Cdl, and an increase in n at Ecorr [V vs. SCE] which means a reduction of the exposed steel area and consequently a thicker electrical double layer . The destabilization of the film is sustained by the increase of the CPE and Cdl value at 0.1 [V vs. SCE].
The non-contribution of the protective film formation (−0.3 [V vs. SCE]) in the presence of zinc is confirmed by the increase in Cdl and the decrease in Rp values at Ecorr [V vs. SCE], there is a decrease in Cdl and an increase in parameter n at concentrations of 10 and 20 mg∙L−1 confirming what was observed in the PDP results (Figure 1(c)). A decrease in Cdl-values was also observed by Felhósi et al. , 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 Zn2+, a higher Rp value was obtained showing that a protective film consisting of Zn(OH)2 occurred on the metal/solution interface making to harder pit development . 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 Cdl. The characteristic of Mg to promote the stabilization of the protective film is confirmed by decrease in Cdl-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  .
3.4. SEM after Potential Measurements
Figure6 shows the influence of the addition of Ca, Zn, and Mg elements in a solution containing 50 mg∙L−1 HEDP on the AISI 304 stainless steel surface after open circuit potential measurements. It is observed that in the absence of divalent cations, the deposition of NaCl crystals on the metal surface was verified and the corrosion product is justified by the presence of Fe and Cl in the EDS spectra (FigureS5 in the supplementary material) and the elemental mapping (FigureS6 in the supplementary material). Fe2+ ions from the oxidation reaction (Equation 12) reacts with chloride ions (Cl−) present in the medium and forms FeCl2(s) as shown in equation 13. The formation of this solid in aqueous solution
Figure 6. SEM images for AISI 304 stainless steel, after an open circuit potential scan. (a) 50 mg∙L−1 HEDP; (b) 50 mg∙L−1 HEDP + 10 mg∙L−1 Ca2+; (c) 50 mg∙L−1 HEDP + 10 mg∙L−1 Zn2+; (d) 50 mg∙L−1 HEDP + 10 mg∙L−1 Mg2+; Magnification: 200×.
favors the reduction of the solution’s pH providing acceleration of the corrosive process . The product formed around the pit is composed of Fe(OH)2 (Equation (14)) and the product formed inside the pit is composed of FeCl2 . However, the inhibitor adsorption is observed on the metal surface due to the presence of Fe and C (FigureS5 and FigureS6 in the supplementary material) , 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 (FigureS6(b), FigureS7(b), and FigureS8(b) in the supplementary material). The non-stabilizing characteristic of the protective film, guaranteed by the presence of Ca2+ cations, is justified by a lower intensity signal from C in the EDS spectra (FigureS6(a) in the supplementary material). For the presence of Zn2+, a signal from C similar to the presence of 50 mg∙L−1 HEDP was found (FigureS7(a) in the supplementary material). A higher inhibitory behavior on film stabilization with the presence of Mg2+ is supported by the presence of a higher intensity signal from C (FigureS8(a) in the supplementary material), suggesting the formation of a more homogeneous barrier with greater protective features against corrosive attacks.
3.5. Weight Loss Measurements
Table 2 shows the corrosion rate (CR), surface coverage, and inhibitor efficiency (ηm). From the results obtained it is clear that the efficiency of the inhibitor is increased with the addition of divalent cations, corroborated with the results obtained by electrochemical tests (PDP and EIS data). The presence of Zn2+ favored a maximum efficiency (ηm = 85.2%) followed by Mg2+ (ηm = 70.4%). This result is in agreement with that observed in the EIS test, where at the Ecorr in the presence of Zn2+ culminated in higher resistance to the corrosive process. A higher weight loss value obtained by the addition of Ca2+ implies that this cation is not responsible for the stabilization of the protective film. Comparing the
Table 2. Results obtained in the weight loss test.
efficiency values obtained in the EIS technique (Table 1) and by weight loss (Table 2) a significant difference in the values is observed. Lower values found 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.
The potentiodynamic polarization studies indicated that HEDP is an inhibitor anodic type. EIS-results showed an increase in Rp-values and a decrease in Cdl-values, indicating adsorption of the molecule on the material surface.
The addition of calcium ions favored a cathodic behavior of the inhibitor. At the potential −0.3 [V vs. SCE], the concentration of 10 mg∙L−1 Ca2+ + 50 mg∙L−1 HEDP provided a higher Rp-value, increasing the inhibitor efficiency. However, at the potential 0.1 [V vs. SCE], when one increased cation concentrations, a larger deformed semicircle was observed suggesting that this element facilitates adsorption but does not stabilize the protective film.
At the corrosion potential and 0.1 [V vs. SCE] the addition of Zn was positive, especially for the concentration of 20 mg∙L−1. This fact indicates that the HEDP inhibitor not only favors better adsorption but also stabilizes the Zn(OH)2 film formed on the cathodic sites. The addition of magnesium had a similar effect to the addition of zinc in the corrosive medium, suggesting the formation of Mg(OH)2 on the cathodic sites.
SEM and EDS analyses corroborated with PDP and EIS results because these investigations showed that the addition of Zn2+ and Mg2+ ions favored greater inhibitor effectiveness by blocking the effect of chloride and sulfide ions on the corrosion process (inhibitor efficiency, for Zn2+ ηm = 85.2% and for Mg2+ ηm = 70.4%). The weight loss results corroborated with the electrochemical results showing that the presence of Zn2+ and Mg2+ 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.
The authors gratefully acknowledge Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), and Fundação de Amparo à Pesquisa e Inovação do Espirito Santo (FAPES) for providing ﬁnancial support, and Drs. Gontijo, L. C. and Boldrini, R. for their support in SEM and EDS analysis and Dr. Eliane D´Elia for support in the PDP and EIS discussion. The authors also acknowledge the Tribology, Corrosion, and Materials Laboratory (TRICORRMAT) at the Mechanical Engineering Department of the Federal University of Espírito Santo by the optical microscopy analyses.
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 Acid 10 wt%.
Figure S4. (a) SEM images and (b) EDS spectra for AISI 304 stainless steel, before applied potential. Magnification: 350×.
Figure S5. (a) EDS spectra and (b) Elemental mapping obtained for AISI 304 stainless steel, immersed in a solution containing 50 mg∙L−1 HEDP, 3.5 wt% NaCl and 1 mmol∙L−1 Na2S, after open circuit potential measurements.
Figure S6. (a) EDS spectra and (b) Elemental mapping obtained for AISI 304 stainless steel, immersed in a solution containing 50 mg∙L−1 HEDP, 3.5 wt% NaCl, 1 mmol∙L−1 Na2S and 10 mg∙L−1 CaCl2 after open circuit potential measurements.
Figure S7. (a) EDS spectra and (b) Elemental mapping obtained by EDS AISI 304 stainless steel, immersed in a solution containing 50 mg∙L−1 HEDP, 3.5 wt% NaCl, 1 mmol∙L−1 Na2S and 10 mg∙L−1 ZnCl2, after open circuit potential measurements.
Figure S8. (a) EDS spectra and (b) Elemental mapping obtained by EDS AISI 304 stainless steel, immersed in a solution containing 50 mg∙L−1 HEDP, 3.5 wt% NaCl, 1 mmol∙L−1 Na2S and 10 mg∙L−1 MgCl2, after open circuit potential measurements.
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