The organic inhibitors whose mode of action results generally from their adsorption on metal are the most frequently used. The selection of the inhibitor depends on the type of acid, concentration, temperature and the type of metallic material exposed to acid solution. Thus, the sulfur-containing inhibitors are effective in sulfuric acid^{2, 3}, while those containing nitrogen such as nitrogen heterocyclic^4,5 and amino acids⁶. Are effective in hydrochloric media. Several phosphoric acids have been used as corrosion inhibitors due to their hydrolytic stability, ability to form complex with metal ions and scale inhibiting property⁷. Moreover, it is known that the inhibition of corrosion can be improved by the presence of several inhibitors in the corrosive medium. This phenomenon, called synergistic effect has been studied, in the case of the influence of iodide ions, by different authors ^8,9.

Mild steel is one of the most important metals in point of it’s used in various number of applications. Meanwhile to prevent metals from the corrosion firstly we need to understand the corrosion phenomenon. It’s simply, the result of interaction between a metal and environments which results in its gradual destruction, so corrosion is the deterioration of metals by chemical attack or interaction with its environment. It is a continuous problem, often cannot be eliminated completely. Prevention is more practical and achievable than complete elimination, targeted areas are specially that used acid solutions in the industrial processes we could cite for example: acid cleaning, acid descaling, acid pickling and oil well acidizing, require corrosion inhibitor to prevent the corrosion of metal^10,11.

In the present work, we have investigated the corrosion inhibitory efficacies of XC70 steel in 0.5MH₂SO₄the presence of (methoxy methyl) triphenyl phosphonium chloride (MMTP Cl). Electrochemical and thermodynamic measurements were made to carry out this study.

MATERIALS AND METHODS:

Materials:

Working electrode:

The Working electrode used in this work was XC70, This material is utilized widely in petroleum and gas industry; they were used without previous purification. Distilled and deionizer water was used for solution preparations. Stock solutions of 0.5M H₂SO₄ were employed as the blank, i.e., (methoxy- methyl) triphenyl phosphonium chloride (MMTP Cl)-free. For the experiments containing MMTP Cl, the appropriate weight was added to blank solutions to reach final concentrations of 1, 8, 10, 20 or 30 ppm.

Table 1 shows the chemical composition (as percentage) of a sample of the carbon steel (XC70) sheet utilized in this work.

Preparation of corrosive media:

The corrosive solution is a 0.5M H₂SO₄solution, obtained by diluting Prolabo branded 98% concentrated acid with distilled water. The inhibitor to use in this work is (methoxy methyl) triphenyl phosphonium chloride (C₆H₅)₃P ⁺(Cl^-)CH₂OCH₃).

Methods:

We have adopted three measurements methods to study corrosion inhibition

1. Gravimeter (Mass Loss Method):

Pre-weighed carbon steel sample A cubic material of size 2.44cm×1.86cm×1.40cm was used for the mass loss method, was immersed en100 mL of the blank/inhibitor solution 0.5M H₂SO₄ for a prearranged time period. After a predetermined test period, the material selection specimens were washed, dried and reweighed.

Mass Loss Method has the advantage of being easy to implement, of not requiring important equipment, but does not allow the approach of the mechanisms involved during corrosion. Its principle is based on the measurement of the weight loss Δm undergone by a surface sample S, during the immersion time t in a corrosive solution. The rate of corrosion is given by the following relation ^12,13.

Were:

· m₀ and m: represent the weight of the sample before and after immersion in the solution tested in (g),

· S: is the area of the sample in (cm²),

· t: the immersion time (min) and is the density of iron in (g/cm³).

The inhibitory efficacy of a compound is given by the following relation ¹⁴

Were V₀ and V the corrosion rate without and with the addition of the inhibitor MMTP Cl.

2. Electrochemical measurements:

Were performed using a Voltalab 40 model PGZ301 potentiostat/galvanostat driven by a personal computer with VoltaMaster 4 software. A typical three electrodes cell with a working electrode made of carbon steel with an active surface of 1 cm² was used. The auxiliary electrode was a platinum plate (1 cm²) and the reference electrode is a saturated calomel electrode (ESC) equipped with a Luggin capillary whose end is placed near the working electrode to minimize the influence of the ohmic fall. The working electrode is in the form of a surface disk S = 1 cm². The latter is introduced into a poly tetra fluoro ethylene sample holder disposed opposite the auxiliary electrode.

The Potentiodynamic polarization curves were obtained with the scan rate of 0.5 mV s^-1, in a potential range from -800 mV to -200 mV. The immersion time of the plates XC70 in the blank and in XC70 containing various concentrations of MMTP Cl was 30 minutes in open-circuit at room temperature 23-24°C.

Before immersion in the solutions, by abrasive paper polishing of decreasing particle size up to 1000, then degreased with acetone, they are rinsed with distilled water and dried in an oven.

The polarization resistance percentage (R%) and inhibition efficiency IE (%) values were obtained from i_corr and R_p data using the equations below ^15,16,17.

Where i⁰_corr and R⁰_p are the corrosion current density and the polarization resistance, respectively, measured in solutions without inhibitor and i_corr and R_p are the same parameters determined in solutions containing inhibitor ^18,19,20.

3. Electrochemical impedance spectroscopy (EIS):

Measurements were carried out after 30 minutes immersion time of the carbon steel plates in corrosive media, at the corrosion potential -482.9 mV (E_corr), in a frequency range from 10 KHz to 40 MHz by a parturition signal of 10 mV amplitude peak to peak, at room temperature 23-24°C. The frequency value at was calculated by the following equation²¹:

Where f_max is the frequency value at which the imaginary component of the impedance is maximal ²².

Where R'_Ctand R_Ct respectively represent the values of the charge transfer resistance in the presence and absence of the MMTP Cl.

Adsorption isotherm:

The adsorption of inhibitor is influenced by the nature and the charge of the metal, the chemical structure of the inhibitor, distribution of the charge in the molecule and the type of electrolyte, nature of interaction between the inhibitor and the metal surface²³.

Adsorption isotherm studies give the descriptive mechanism on how the organic inhibitors adsorb to the metal surface ²⁴. The various adsorption isotherms models (Langmuir, Temkin, Frendlich, and Frumkin, El Awady, and Flory Huggins adsorption isotherm) expressed in linear form as:

· The Langmuir adsorption isotherm model ^{25, 26}:

Rearrangement gives:

Temkin adsorption isotherm model^25,27,28 :

Frumkin adsorption isotherm model²⁵:

Or its linear form:

Where: is the surface coverage, K the adsorption–desorption equilibrium constant, C the inhibitor concentration and a or g the adsorbate interaction parameter.

The expression for Gibb’s free energy change of adsorption, ΔG _adspresented in Equation [15] was used to investigate the feasibility and nature of the adsorption ^{29, 30, 31,32}:

Were:

· T: thermodynamic temperature

· R: the universal gas constant

· the number 55.5: is the molar concentration of water in solution. is the concentration of water.

· K the adsorption equilibrium constant obtained from the isotherm, and:

RESULTS AND DISCUSSION:

Mass Loss Study:

The effect of MMTP Cl on the inhibition of XC70 corrosion was tested by mass loss measurement. Table 2, Figure 1 indicated the variation of corrosion rate and inhibition efficiency (IE %) with an MMTP Cl.

In the concentration (1 ppm) it is observed the best concentration which decreases the rate of corrosion is 1ppm, (R = 72.72%).

Table No 2. Inhibitory efficacy and corrosion rate of Steel in 0.5M H₂SO₄in absence and in presence of MMTP Cl at Different Concentrations.

Figure 1. Inhibitory efficacy and variation in corrosion rate with the addition of MMTP Cl

From the Figure 1 and Table 2 we can make the following remarks:

All the concentrations used a decrease in the rate of corrosion. We note that in the presence of MMTP Cl, the inhibitory efficiency increases maximum value of 72.72% at 1ppm.in this case this highly effective inhibitor.

Potentiodynamic Polarization Measurements:

The corrosion current density (i_corr) was calculated at intercept of the anodic and cathodic Tafel lines to corrosion potential, using VoltaMaster 4 software. While the Calculation of the corrosion rate was made with the Tafel method according to the 1st Stern equation. The kinetics of the anodic and cathodic reactions occurring on carbon steel electrodes in 0.5M H₂SO₄solutions with different concentrations of MMTP Cl (1ppm, 8ppm, 10 ppm, 20ppm, 30ppm) were studied through the polarization measurements.

The complete potentiodynamic polarization curves are shown in Figure 2. The electrochemical parameters, i.e. corrosion current density (i_corr), anodic (b_a) and cathodic (b_c) Tafel constants and polarization resistance (R_p), shown in Table 5, were collected from Tafel plots and polarization resistance experiments carried out separately.

Regarding the potentiodynamic polarization curves, as previously observed, it can be clearly seen that the E_corrvalues shifted to more negative potentials with an increase in the concentration of (methoxy methyl) triphenyl phosphonium chloride (MMTP Cl). This effect may be related to the adsorption of the organic compound at the active sites of the electrode surface, retarding the corrosion reaction. The increase in the concentration of (methoxy methyl)triphenyl phosphonium chloride (MMTP Cl).caused a clear decrease in the cathodic current density, but did not change significantly the cathodic Tafel slopes Table 3, indicating that the hydrogen evolution reaction is diminished exclusively by the surface blocking effect. The results obtained are given in Table 3.

Figure 2. Potentiodynamic polarization curves and Variation of the corrosion potential of steel in 0.5M H₂SO₄without and with addition of MMTP Cl at different concentrations.

From the curves obtained and the values which one concluded in the Table 03 observes:

When the inhibitor is added to the acid, the decrease in the density of the corrosion currentis observed (i_corr): (0.7928-0.2047 mA / cm²).

In the concentrations (8, 10, 20 and 30ppm) we observe that the curves are moved towards the negative values (cathodic displacement).

In the concentration (1 ppm) it is observed that the curve moves towards the positive values (anodic displacement), and the best concentration which decreases the rate of corrosion is 1ppm, (R = 74.19%).

The cathodic displacement of the curves shows that the inhibitor at (8, 10, 20 and 30ppm) is deposited on the cathodic sites of the metal surface and by inhibiting it, therefore the inhibitor behaves like a cathodic inhibitor in these concentrations.

The anodic shift of the curves shows that the 1 ppm inhibitor is deposited on the anodic sites of the metal surface and inhibiting it, therefore the inhibitor behaves as an anodic inhibitor in this concentration.

In order to evaluate the adsorption process of MMTP Cl on the carbon steel surface, Langmuir, Temkin and Frumkin adsorption isotherms were obtained according to the equations ^{10, 12} and ¹⁴.

Considering that the double-layer capacitance is proportional to express the adsorption quantitatively, different adsorption isotherms may be applied. These isotherms characterize the metal/inhibitor/environment system and fit the degree of surface coverage (θ) values.

Figure 3: Isotherms for the adsorption of MMTP Cl on the surface of carbon steel in 0.5MH₂SO₄with the method polarization measurements

We have compared the results of the three isotherms in the following table:

Table No 4. Value of coefficient of determination and slope

	Langmuir	Timken	Frumkin
R²	0.963	0.758	0.160
The slope	2.409	3.618	-0.924

According to the curves obtained and the values concluded in the Table 6: The straight lines obtained in case of Langmuir and Temkin have a slope > 1. The slope deviation of the unit (>1) can be explained in terms of repulsion or attraction of the molecules adsorbed next to each other ³³.

The negative slope in case of Frumkin (g =-0.924) indicates the existence of a lateral repulsive interaction in the adsorption layer ³⁴.

For the Langmuir isotherm the R² value obtained was 0.963, while for the Temkin and Frumkin isotherms these values were 0.758 and 0.160 respectively.

The free energy of adsorption (ΔG_0ads) was calculated from the equilibrium constant of adsorption of the Langmuir isotherm, which showed the best correlation with the experimental data, according to equation [15] ΔG⁰_ads=-42.460 Kj/mol.

The negative value of ∆G means that the adsorption process is spontaneous, while the value in the order of 30 kj/mol indicates that the inhibitor (MMTP Cl) has been chemically adsorbed on the surface of the steel ³⁵.

From the curves obtained have the straight lines obtained in the case of Langmuir and Temkin have a slope> 1. The slope deviation of the unit (> 1) can be explained in terms of repulsion or attraction of the adsorbed molecules next to each other on the other hand the negative slope in case of Frumkin (g = -0.924) indicates the existence of a repulsive lateral interaction in the adsorption layer.

For the Langmuir isotherm, the R² value obtained was 0.963, while for the Temkin and Frumkin these values were 0.758 and 0.160 respectively.

The adsorption free energy (ΔG⁰_ads) was calculated from the equilibrium constant of adsorption of the Langmuir isotherm, which showed the best correlation with the experimental data, according to the equation [11].

The negative value of ΔG means that the adsorption process is spontaneous, while the value in the order of -42.460 kJ / mol indicates that the inhibitor (MMTP Cl) was chemically adsorbed on the surface of the steel.

Electrochemical Impedance Spectroscopy Measurements:

The impedance parameters derived from EIS measurements R_s, R_ct, C_dl were calculated using VoltaMaster 4 software with an error of ±1 %, and are listed in Table 5.

Table No 5. Impedance data and surface coverage for carbon steel in 0.5MH₂SO₄and solutions containing MMTP Cl at different concentration

MMTP Cl	R_Ct (ohm.cm²)	Z_imax(ohm.cm²)	Z_Rmax(ohm.cm²)	F_max(Hz)	C_dL(µF/cm²)	R%	IE%
0 ppm	19.02	8.36186	23.32	25	334.6	_	_
1 ppm	29.96	13.5383	33.47	63.29	83.90	74.94	36,51
8 ppm	21.33	9.15880	25.4	40	186.4	44.29	10.82
10 ppm	21.45	9.28136	25.54	40	185.7	44.50	11.32
20 ppm	21.69	9.38308	26.2	40	183.4	45.18	12,30
30 ppm	23.51	10.3289	28.57	40	169.2	49.60	18,15

Electrochemical impedance spectroscopy (EIS) is a well-established and powerful tool in the study of corrosion like surface properties, electrode kinetics and mechanistic information can be obtained from the impedance diagrams.

Figure 4. Nyquist plots for carbon steel in 0.5MH₂SO₄and solutions containing MMTP Cl at different concentration

Figure 4 shows the Nyquist plot obtained at the corrosion potential -482.9 mV of carbon steel in acid medium, while summarizes the impedance data extracted from EIS experiments carried out both in the absence and presence of (methoxy methyl) triphenyl phosphonium chloride (MMTP Cl) in Table 5. For different concentration of MMTP Cl, a depressed charge transfer semicircle is observed at high frequency, which is attributed to the time constant of the charge transfer and double-layer capacitance. The intersection of this semicircle with the real axis at high frequencies furnished a value of (19.02 - 29.96) ᾩ cm² for the ohmic resistance (R_s) of the solution enclosed between the working electrode and the tip of the salt bridge containing the reference electrode.

At low frequencies, a charge transfer resistance (R_ct) of (19.02 - 29.96) ᾩ cm² was found from the difference in impedances at lower and higher frequencies. The double-layer capacitance A C_dl value of 83.90-334.6 µF cm^-2 was found for the carbon steel electrode in the blank 0.5M H₂SO₄solution and f_max was calculated from the equation [4]:

The values observed for R_s, R_ct and C_dl for carbon steel in corrosion inhibitor-free solutions are in agreement with the values reported recently ^11,35.

These results support those obtained from the Tafel experiments for MMTP Cl concentration and confirm the inhibitor adsorption onto the carbon steel surface. As can be seen in Table 5, slightly higher R_s values were obtained in the presence of the corrosion inhibitor. This was to be expected because, in general, organic inhibitor reduces the dielectric constant of aqueous solutions, increasing their resistance. More important, however, are the observations related to C_dl and R_ct.

The EIS results clearly indicate that MMTP Cl decreases the double-layer capacitance and increases the charge transfer resistance; as consequence a larger diameter of the semicircle is observed in Nyquist plots.

The corrosion inhibitory efficiency of steel is calculated from the charge transfer resistance according to the relation⁵.

Through The results shown in Table 5 we can be clearly seen that the R_ct value increases with the inhibitor concentration, R_Ct increases with the concentration of the inhibitor whereas the capacity of the double layer decreases as the quantity of the MMTP Cl. The decrease of C_dl is due to the adsorption of the inhibitor on the surface of the steel which has the effect of reducing the active surface of the electrode. Inhibitory efficacy increases with the concentration of the inhibitor to reach a maximum value of 74.94% at 1ppm. This result is in good agreement with those found by polarization resistance measurements and mass loss.

The results of the isotherms for the adsorption of MMTP Cl on the surface of carbon steel in 0.5MH₂SO₄with the method polarization measurements were listed in Table 6:

Table No 6. Value of coefficient of determination and slope

	Langmuir	Timken	Frumkin
R²	0.994	0.792	0.389
The slope	2.048	4.424	-1.804

Figure 5. Isotherms for the adsorption of MMTP Cl on the surface of carbon steel in 0.5MH₂SO₄with the method polarization measurements

According to the curves obtained (figure 5) and the values concluded in the table 6:

· The straight lines obtained in case of Langmuir and Temkin have a slope > 1. The slope deviation of the unit (>1) can be explained in terms of repulsion or attraction of the molecules adsorbed next to each other.

· The negative slope in case of Frumkin (g = -1.804) indicates the existence of a lateral repulsive interaction in the adsorption layer.

· For the Langmuir isotherm the R² value obtained was 0.994, while for the Temkin and Frumkin isotherms these values were 0.792 and 0.389 respectively.

The free energy of adsorption (ΔG_ads) was calculated from the equilibrium constant of adsorption of the Langmuir isotherm, which showed the best correlation with the experimental data, according to equation [15], the Figure 5 and table 6:

ΔG⁰_ads=-41.2978 kj/mol

The negative value of ∆G means that the adsorption process is spontaneous, while the value on the order of 30 kj/mol indicates that the inhibitor (MMTP Cl) has been chemically adsorbed on the surface of the steel.

CONCLUSION:

(Methoxy methyl) triphenyl phosphonium chloride (MMTP Cl) has been successfully studied as a corrosion inhibitor for carbon steel in 0.5M H₂SO₄. Exploration of Electrochemical studies showed that this salt is an excellent corrosion inhibitor for carbon steel under acidic conditions. Inhibition efficiency increases with inhibitor concentration and the maximum inhibition efficiency was more than 74.94% at the concentration of 1ppm. Besides, an excellent agreement between the inhibition efficiencies calculated using polarization techniques was obtained.

On the other hand, polarization studies indicated that were a mixed type of inhibiting, both cathodic as well as anodic reactions. (Methoxy methyl) triphenyl phosphonium chloride (MMTP Cl) acts as adsorption inhibitor on carbon steel surface; adsorption was spontaneous and well described by Langmuir, Temkin and Frumkin isotherms.

ACKNOWLEDGMENT:

The authors would like to thank to Algerian Ministry of Higher Education and Scientific Research for their support and providing the necessary facilities to carry out this research.

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Received on 03.07.2021 Modified on 28.09.2021

Asian J. Research Chem. 2022; 15(1):1-9.

DOI: 10.52711/0974-4150.2022.00001

MMTP Cl	Rp(ohm.cm²)	IE% (R_p)	E_corr(mV)	i_corr(mA/cm²)	IE% (i_corr)	V_corr(mm/y)
0.5M H₂SO₄	18.63	_	-482.9	0.7928	_	9.273
1ppm	63.16	70.51	-468.8	0.2047	74.19	2.394
8ppm	25,83	27,87	-494,7	0,474	40,21	5,544
10ppm	26.31	29.20	-496.3	0.5387	32.06	6.301
20ppm	36.49	48.95	-495.7	0.4084	48.49	4.776
30ppm	28.43	34.48	-487.5	0.4798	39.49	5.612

Chemical Element	C	P	S	Si	Mn	Cr	Ni	Cu	Al	Nb	V	Ti	Mo	Fe
Percentage (×10^-3)	65	2	1	245	1685	42	26	10	42	67	14	19	5	Reste

INTRODUCTION: