1. INTRODUCTION

Because of the accessibility and high-dissolving susceptibility of rock, HCl solution has been implemented as the central stimulation liquid in the petroleum industry like oil well acidizing [ 1 ]. Despite the frequent usage, the high rate of corrosion of HCl with steels makes several technical issues in practice [ 2 ]. The corrosion inhibitors lend themselves to delayed rates of reaction, and small corrosivity [ 3–6 ]. However, the challenge of injection issues still exists with additives because of the increasing acid viscosity [ 7 ].

The change in the structure and properties of materials during the magnetic field (MF) exposeure is a significant issue in different applications [ 8–12 ]. It has been stated that magnetized water causes several phenomena such as the reduction of the rate of carbon steel corrosion in water [ 13 ], decreasing rate of corrosion of AISI316L SS in Ringer’s solution [ 14 ], and reducing corrosion in the aqueous solution of NaC1 [ 15 , 16 ]. Test results revealed that used magnetic fields can influence the corrosion rate of metals or alloys in the aqueous solutions [ 16–21 ]. Sagawa showed that iron and copper corrosion in nitric acid decreased by a constant magnetic field [ 22 ]. A rate of corrosion reduction in the surface of the electrode with a great density of the magnetic flux was identified. This reduction rate of dissolution is considerably higher in the existence of chloride ions [ 23 ]. The rate of pure iron corrosion in chloride and sulphate solutions has also been observed to significantly reduce under the effect of the magnetic field [ 24 ]. The corrosion decreases the impact of a perpendicular magnetic field on AISI 303 stainless steel corrosion in solution with Fe3+‏, which is a corrosion agent [ 25 ]. The magnetic field decreased the steel dissolution in sulphuric acid in which the influence was higher at the lower presence of acid and in the absence of the inhibitor [ 26 ]. Also, the avoiding impact of the magnetic field on brass, zinc and aluminium corrosion has been studied [ 27 ].

To the best knowledge of the author, no investigation has ever explored the impact of magnetization on the N-80 CS rate of corrosion in HCl solution. All of the past studies faced an external magnetic field that was implemented on the reaction cell; however, the fluid magnetization before coming in contact with N-80 CS was studied. In the this study, the effect of magnetic field on the efficiency of the corrosion inhibition (η) of N-80 CS (that is one of the most conventional materials in the petroleum industry [ 28 ]) in HCl was demonstrated by the potentiodynamic polarization (PDP) and gravimetric weight loss (WL) procedures. The influence of the intensity of the magnetic field, elapsed time, and time of magnetization were confirmed in 13 tests. Tests were performed using the design of experiments method through Box-Behnken RSM (Response Surface Methodology) design. Last, the dependence of η on the variable factors was specified in the model. Because of the efficacious magnetization performance on decreasing the rate of corrosion, it was found that the magnetized HCl is an eco-friendly and cost-effective alternative for the regular HCl.

2. EXPERIMENTAL METHOD

2.1. Magnetizer procedure

Various kinds of magnetic devices can be based on the electromagnets (yoke-based and solenoids electromagnets) or permanent magnets. Though electromagnets can create high-intensity magnetic fields, their usage is not functional, since they require an electrical power supply, regular services, and cooling. The existing materials with permanent magnetization such as the rare earth alloys, ferrite, and alnico create fields with appropriate intensity. There are numerous magnetic systems which are implemented for the magnetized liquids production. The magnetic field can be perpendicular or parallel with the direction of flow (Fig. 1) [ 29 ].

Figure 1. Orientation of the magnetic field (A) using a ring magnet in open circuit (B) using two cylindrical magnets in open circuit (C) using four block magnets in closed circuit.

In this paper, the permanent MF influence on the intense HCl solution (12.5% weight in water or 3.8 M respectively) was obtained. The tests were done by the circulation system presented in Fig. 2. Plastic fittings were implemented to make a connection between the magnetic circuit and the pump, which was connected to the flow meter.

Figure 2. Schematic of experimental setup of fluid magnetizer

2.2. Electrochemical Measurement

Electrochemical and corrosion experiments were done by an IVIUM potentiostat (IVIUM Technologies, USA) which used a USB cable and a common 3-electrode cell to connect to a computer. The electrochemical cell includes a 3-electrode system. The utilized API N-80 grade electrode was one of the best conventional materials in the petroleum industry [ 28 ] with a surface area equal to 1.08 cm² and isolated in a glass tube by a thin epoxy resin layer, putting just the end part of the rod in contact with the solution. Before any inspection, this electrode was polished to a mirror-like finish. The N-80 carbon steel electrode was situated horizontally. The reference electrodes and counter were AgAgCl3 saturated calomel electrodes (SCE) and platinum rod, respectively. The potentials of the electrode were recorded during the test and presented here regarding SCE.

The working electrode was firstly submerged into the solution for 10-30 minutes until a S.S. of OCP (open circuit potential) reached (without stirring). The curves for the potentiodynamic polarization were plotted at a scan rate of 1.0 mV/s and in the potential limit from −250 to +300 mV versus the OCP. All the electrochemical experiments were conducted in room condition at 308 K. Each measurement was conducted at least 3 times and the obtained data points were presented in the figs and tables to ensure their appropriate reproducibility.

2.3. Measurement of corrosion weight loss

The magnetic emery paper was used to abrade the N-80 CS rectangular 6.28 cm × 1.80 cm × 0.37 cm samples. The samples were then weighed by digital scale (±0.1 mg), submerged in glass beakers including 100 ml 12.5 wt.% (3.8 M) magnetized and normal HCl. In each experiment, the suitable HCl was poured into a fluid magnetizer setup as presented in Fig. 2 and circulated for a specific time to prepare the magnetized acid. A water thermostat bath was utilized to keep the temperature at 21 ± 0.1 C. After 6 hours, the samples were removed, rinsed with water, dried with tissue paper, and weighed again with the high precision.

2.4. Design of Experiments

Regarding the influencing factors in the fluid magnetization, 3 parameters were chosen for this test which included the intensity of the magnetic field, time of the magnetization, and the elapsed time at 3 various stages (Table 1).

The response surface methodology was implemented as a systematic design of experiments (DOEs) procedure. The response surface procedure (RSM) is a method designed to reach the effective factors on the finest response value[ 30 ]. It was effectively implemented in the empirical studies [ 31–33 ]. The maximum number of the factorial experiments of 27 (33) can be finished in only 13 runs (see Table 2), which involve numerous experiments, that are substantial in both the experimental time and cost. Then, RSM including Box-Behnken design and the regression analysis were implemented to improve the empirical model for because of the acid magnetization. Based on this model, the response optimization (corrosion rate reduction) was performed.

2.5. SEM surface analysis

The SEM imaging was used to determine the morphologies of N-80 CS sample surfaces after submerging in 3.8 Molar magnetized and normal HCl solution for 6 hours.

3. RESULTS AND DISCUSSION

3.1. Weight loss study

The rate of corrosion (ν) was obtained from the next equation [ 34 ]:

$\nu =\frac{W}{\mathrm{St}}$ (1)

Where W= weight loss (g), S= entire area of one sample (m²), and t= time of the immersion (h). With the obtained rate of corrosion, the corrosion reduction rate (η) was obtained as below [ 34 ]:

$\eta =\frac{{\nu }_0-\nu }{{\nu }_0}\times \mathrm{100\%}$ (2)

Where ν₀ and ν are the rates of corrosion in the magnetized and normal HCl, respectively.

The iron corrosion electrochemical reaction includes the relocation of 2 electrons, then there are 2 levels (v_s = 2) which each presents the transfer of 1 electron, one of those will be regarded as the level that determines the rate [ 35 ]. The recommended iron reaction in the solutions including Cl- ions are as below [ 36 , 37 ]:

$\mathrm{Fe}+H_{2}O+{\mathrm{Cl}}^{-}\iff {\left[\mathrm{FeClOH}\right]}_{\mathrm{ads}}^{-}+H^{+}$ (3a)

${\left[\mathrm{FeClOH}\right]}_{\mathrm{ads}}^{-}\iff {\left[\mathrm{FeClOH}\right]}_{\mathrm{ads}}+e^{-}$ (3b)

${\left[\mathrm{FeClOH}\right]}_{\mathrm{ads}}+H^{+}\iff {\mathrm{Fe}}^{\mathrm{2+}}+{\mathrm{Cl}}^{-}+H_{2}O$ (3c)

Investigations of the potential of the steel corrosion (E_corr.) values in HCl, represent that the polarization happens frequently at the anode [ 35 , 38 , 39 ]. Consequently, the rate of the corrosion reduction of the magnetized HCl (as presented in Table 3) can be clarified based on the activity of Cl- ion and its influence on the metal dissolution acceleration [ 36 ].

Materials will have a different molecular form because of the magnetization [ 40 ]. Many researchers who studied the magnetic field impacts on the hydrogen-covered structure, observed an enhancement in the number of the hydrogen bonds [ 41–43 ]. It is also recommended that the hydrogen-covered intensity increased by 10 Tesla under the magnetic field in the hydrogen-covered molecules [ 44 ]. Enhancing hydrogen bond strength and their number decreases the choler molecules' activity and may decrease the corrosion rate of HCl. The results of the current study are consistent with those carried out on the rate of AISI316L SS corrosion in Ringer’s solution, rate of steel corrosion in water, and the aluminum corrosion in aqueous NaC1 solution in a magnetic field [ 13–15 ], and other related tests [ 22–27 ].

Moreover studies on the impact of the magnetic fields on the electrochemical reactions have shown that these impacts became stronger as the water conductivity is enhanced by ionic solutes [ 45 , 46 ]. They found that this would tend to decrease any ions or anomalous pH values concentrations and could therefore minimize the localized corrosion impacts [ 45–48 ].

3.2. Potentidynamic polarization study

Potentiodynamic polarization curves are documented and presented in Fig. 3 to attain information about the magnetization effect on the electrochemical corrosion behavior of N-80 CS in 3.8 M HCl solution under numerous magnetic fields. Related electrochemical kinetic parameters like the potential of corrosion (E_corr), corrosion current density (i_corr) and cathodic Tafel slope (b_c), obtained from these tests, are shown in Table 4. The efficiency of the corrosion inhibition (η) because of solutions, so the magnetization is obtained by:

$\eta =\frac{i_{\mathrm{corr}}-{i^{}}_{\mathrm{corr}}}{i_{\mathrm{corr}}}$ (4)

Figure 3. Potentiodynamic polarization curves of N-80 CS in normal and magnetized 3.8 M HCl solution (scan rate 1 mVs_1).

Where i'_corr and i_corr = densities of corrosion current for N-80 carbon steel electrode in the magnetized and normal solutions, respectively.

As it can be observed in Fig. 3, the anodic branch of the potantiodynamic polarization curve is unpretentious by various MFs that is related to studies in literature [ 47 , 49–53 ]. The influence of the magnetic fields in the earlier studies on the iron passivation revealed that the magnetic fields have slight on the iron passivation monitored by the electron-transfer level, but the passivation is monitored partially by mass transfer step (MTS). Though, MF changes the active–passive transition states [ 47 , 51 , 54 ]. The mass transport control was a central parameter in dissolution at small overpotentials in anodic and cathodic reactions (nearly 20 mV). Fe(OH)₂ formation on the surface of iron is a significant factor to the resistant corrosion and plays a key role in the reduction of dissolution [ 21–24 ]. The cathodic hydrogen ion decrease influenced by MF on potantiodynamic polarization curve enhanced that was defined by anodic shift of E_corr [ 17 ]. As observed in the Fig. 3, the MF suppressed the reduction rate of hydrogen which is the reduction rate of hydrogen in the used MF solution and led to reduce the corrosion current density. The corrosion current density reduced by enhancing the MF (Table 4). The happening might be related to enhance the Fe(OH)₂ formation as a barrier covering the surface of iron and decrease the corrosion current density an enhanced η up to 92%. Fig. 4 shows that 24 hr after exposing HCl to the MF, the effect of magnetization stay in the acid. By enhancing the MF, the impact of magnetization remains for longer time (Table 5). This way could be defined by the Lorentz force driven convection [ 55 ].

Figure 4. Potentiodynamic polarization curves of N-80 CS in normal and magnetized 3.8 M HCl solution (scan rate 1 mV s_1) after 24 hours.

3.3. Analysis of Variance (ANOVA)

The tests were carried out randomly for the minor unrestrained impact of the factors. A quadratic polynomial equation was established for forecasting the responses as independent variables including the squared terms and quadratic interactions. Eq. (6) presents the basis of creating a polynomial equation:

$Y={\beta }_0+\sum _{\mathrm{j=1}}^3{\beta }_j{X}_j+\sum _{\mathrm{j=1}}^3{\beta }_{\mathrm{jj}}{X}_j^2+\sum _{\mathrm{i<j}}^3{\beta }_{\mathrm{ij}}{X}_i{X}_j+\sum _{\mathrm{i<j<k}}^3{\beta }_{\mathrm{ijk}}{X}_i{X}_j{X}_k$ (5)

Several regression analysis methods included in the RSM were implemented to approximate the coefficients of the models. Equation (7) presents the second-order models that were used to reduce the rate of corrosion:

$\eta =\mathrm{5.90875}+\mathrm{0.047624}{X}_1+\mathrm{2.23938}{X}_2-\mathrm{0.72693}{X}_3+\mathrm{3.320}\times {\mathrm{10}}^{\mathrm{-4}}{X}_1{X}_2-5\times {\mathrm{10}}^{\mathrm{-6}}{X}_1{X}_3$

$+\mathrm{0.085677}{X}_2{X}_3-\mathrm{6.60460}\times {\mathrm{10}}^{\mathrm{-6}}{X}_1^2-\mathrm{0.42156}{X}_2^2+\mathrm{2.06380}\times {\mathrm{10}}^{\mathrm{-3}}{X}_3^2$ (6)

One statistical method is the analysis of variance (ANOVA) whose main purpose is to find the extent of variation of each factor from the results and leads to the entire variation detected in the results [ 56 ]. Fundamentally, it calculates the parameters of the sum of squares (SSQ), variance (V), degree of freedom (DF), F-value, and the contribution percentage for each factor [ 57 ]. The F value is the mean square ratio of regression to the error mean square. Provided that in a certain probability level (e.g., F_0.05(9,3)=8.81), the F-value table is smaller than the obtained F-value, a statistically important factor or relationship is achieved. The results of ANOVA presented in Table 6 show the F-value of 66.57 for the model. This value is greater than the F table of 8.81 and approves the high significance of the model. The P-value less than 0.05 displays the significance of the factor at 95% confidence limit. The error and the mean square calculations of the model are:

${\mathrm{MS}}_{\mathrm{model}}=\frac{{\mathrm{ss}}_{\mathrm{model}}}{{\mathrm{df}}_{\mathrm{model}}}$ (7)

${\mathrm{MS}}_{\mathrm{error}}=\frac{{\mathrm{ss}}_{\mathrm{error}}}{{\mathrm{df}}_{\mathrm{error}}}$ (8)

Where SS, df, and MS are the sum of squares, degree of freedom, and mean squares, respectively. The F value obtained by the next Eq:

$F=\frac{{\mathrm{MS}}_{\mathrm{model}}}{{\mathrm{MS}}_{\mathrm{error}}}$ (9)

High R² values for Eq. (6) show a suitable fitting for the test data and the anticipated values. Fig. 5a shows the detected in relation to the anticipated values. According to this figure, the greatest values of the test points are located near the straight line that are the anticipated values. The linearity of the pattern also displays that there is no need for the change in the response. Fig. 5b shows the residuals against the test run order. It shows the lurking variables that might have affected the results during the test. The scheme displays a random scatter that brings assurance against the trends ruining the analysis.

Figure 5. (a) Normal plot of residuals; (b) Residuals of each run.

3.4. Response Surface Plots and Contour Plots

Eq. (6) was implemented to set up the contour plots and the response surface. These schemes simplify a clear comparison of the dependent variable on the main procedure variables. Figs. 6-8 display response surface schemes of the η (%) as a design variable magnetized HCl. In addition to the central impacts, the interaction impacts between the parameters are clarified. These figures are on the basis of keeping the third variable at the best values. The η considerably improved by increasing the intensity of the magnetic field and reduced with the passage of time.

Figure 6. Corrosion rate reduction (η) response surface plot and contour-lines map depending on magnetic field intensity (Gauss) and magnetization time (min), holding third variable at fixed level (elapsed time=0 hr) in 3.8 M. HCl at 21℃

Figure 7. Corrosion rate reduction (η) response surface plot and contour-lines map depending on the magnetic field intensity (Gauss) and the elapsed time (hr), holding third variable at the fixed level (magnetization time=6 min) in 3.8 M % HCl at 21℃

Figure 8. Corrosion rate reduction (η) response surface plot and contour-lines map depending on magnetization time (min) and elapsed time (hr), holding third variable at fixed level (magnetic field=4000 Gauss) in 3.8 M HCl at 21℃

We see a rapid enhance in η by increasing the intensity of MF from 0.0 to 4000 Gauss that was anticipated. This value is greater on the first step of the elapsed time (=0 h). It is allocated to the magnetization that improves the hydrogen bonds, that is acid resistant and reduces the rate of corrosion [ 13–15 ]. In other words, as shown in Fig. 6, by enhancing the intensity of the magnetic field, the concentration of in HCl and as a result its corrosion rate caused by the magnetization reduces. An additional explanation is that when acid passes a magnetic field, its molecular formation will be affected and it becomes more ordered [ 40 ], so the movement of molecules will be controlled and the rate of the reaction with N-80 CS decreases.

Fig. 7 demonstrates that enhancing the time of magnetization of flowing liquid in MF from level 1 to level 3, does not highly change the η. Regarding the Lorentz force, when a particle with charge q passes from a magnetic field B with velocity v, then it experiences a force namely Lorentz force [ 58–60 ]. This force relies on time, yields legal results, and shows that the magnetic field changes the liquid at the first moments. However, the magnetization period might affect the liquid magnetic memory in which it can keep its magnetic features in a wide interval.

Figs. 8 displays the response surface diagrams and counterplots of η for the magnetized acid. The decrease in the rate of corrosion reduces slightly over the time. The results show that the magnetized acid keeps 80% of its feature in the initial 48 hr from the production. The initial 24 hr is the finest time for the inhibition effciency.

3.5. Surface morphological results and analysis

SEM was used to investigate the CS surface morphologies exposed and unexposed to the magnetized and submerged in normal HCl for 6 h at 294K. Prior to contact with the corrosive solution, the parallel properties were determined on the clean surface of the polished N80 CS that are related to polishing scratches (Fig. 9a). Micrographs analysis in Fig. 9 show that the CS sample displays a quite rough surface in normal (un-magnetized) HCl. As estimated owing to acid solution corrosive attack, normal HCl destructed the surface of CS severely (Fig. 9b). The SEM results were also implemented to the surface of CS after the magnetized HCl exposure (Fig. 9c) which shows that the surface is relatively softer than in the normal HCl and reveals the inhibiting influence of the magnetization.

Figure 9. SEM images for (a) unexposed carbon steel; (b) exposed carbon steel in 12.5 wt.% normal HCl solution, and (c) exposed carbon steel in 12.5 wt.% magnetized HCl solution after 6 h immersion at 294 K.

4. CONCLUSION

The corrosion inhibition effectiveness of the magnetized 12.5 wt.% (3.8 M) HCl with N-80 CS was studied by the potentiodynamic polarization (PDP) and weight loss (WL) methods and was modelled using RSM. The following results could be emphasised:

• The attained results show that, HCl pre-magnetization makes outstanding N-80 CS corrosion rate reducer in 12.5 wt.% (3.8 M) HCl solution.
• ANOVA results showed that the key parameter influencing the efficiency of the corrosion inhibition (η) is the intensity of the magnetic field (MF) with about 71% of participation. However, the effect of the elapsed time is also substantial. The influence of time of the magnetization is insignificant and negligible.
• The efficiency of the corrosion inhibition was observed about 94% for the magnetized HCl (12.5 wt.%) in a 2500 Gauss magnetic field. It enhances by the magnetic field intensity but reduces over time.
• The soft surfaces found in magnetized HCl SEM micrographs reveals that it efficiently keeps N-80 CS against corrosion.
• Because of the corrosion rate reduction during the magnetization process, it is suggested that it can magnetize acid and, in this way, it can be a reliable and cheap substitute for the retarder acids in acidizing the matrix of oil and gas wells.

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