Performances of Alkaloid Extract from Rauvolfia macrophylla Stapf toward Corrosion Inhibition of C38 Steel in Acidic Media.

Alkaloid extract from Rauvolfia macrophylla Stapf (AERMS) was studied as the corrosion inhibitor for C38 steel in 1 M HCl and 0.5 M H2SO4 using electrochemistry and surface analysis. The corrosion inhibition was efficient and proceeds via adsorption of AERMS on the steel surface due to the active functional groups present in the molecules. AERMS acts as a mixed inhibitor in HCl and as a cathodic inhibitor in H2SO4. In H2SO4 corrosive medium, the presence of iodides improves the adsorption of the alkaloid molecules by reducing the surface charge of the electrode and thus substantially decreases the corrosion rate. Two pure alkaloids (tetrahydroalastonine (THA) and perakine (PER)) were quantitatively isolated from AERMS, and their anticorrosive properties for C38 steel in 1 M HCl and 0.5 M H2SO4 were evaluated. THA showed the highest efficiency while the performance of PER was less important compared to the extract. This confirms that the efficiency of AERMS was the result of the complementary action of the chemical compounds present in the extract.

Introduction

The pickling process, cleaning and removal of localized deposits (rust, bacterial deposits, calculus, etc.) from pipes and other steel based structures used in industries, are generally performed in concentrated acid solutions.[1−12] These aggressive milieus easily corrode metals during the cleaning process. Inhibitors are frequently added to the acidic solution before application to prevent or reduce corrosion. At relatively low concentrations, inhibitors can substantially reduce metal corrosion.[13,14] The most commonly used inhibitors in acid medium are organic compounds whose efficiency is attributed to the presence of heteroatoms (O, N, S, and P) in their structures.[8,15−20] These heteroatoms can coordinate with iron(II) resulting from steel corrosion, forming complexes at the metal surface, thereby serving as a barrier to aggressive agents.[13,16]

Nitrogen-rich compounds are attractive as corrosion inhibitors. The large amount of nitrogen-rich corrosion inhibitors are synthetic compounds.[13,21−23] However, it was shown recently that natural compounds extracted from plants also display very interesting properties.[6,7,15,24] Moreover, plant extracts have the advantage of being less expensive, easily obtained, renewable, highly biodegradable, available, and especially nontoxic to the environment.[25,26] Among these natural nitrogen-rich compounds extracted from plants and used as corrosion inhibitors, alkaloids represent the most important family. The size, functionalities, and geometry of these molecules are determining parameters that explain their efficiencies when used as corrosion inhibitors.[27]

Alkaloid extract of various origins have been shown to have excellent corrosion inhibition properties toward C38 steel in acidic media, mainly HCl.[15,16,28−30] Some other studies also reported that these inhibition properties decrease slightly when the temperature of the corrosive solution was increased.[14,31] In general, the activity of these compounds as corrosion inhibitors are explained by the formation of a passive layer at the metal surface, which reduces its accessibility to corrosion agents.[15] Although the nature and the chemical composition of this passive layer are not well elucidated yet; it is obvious that it consists mainly of a mixture of the alkaloids and corrosion products.[15,16]

Cameroon, with its large forest and exceptional biodiversity, is considered as a reservoir of natural substances with interesting pharmacologic applications. Indeed, lots of compounds have been isolated from local plants by organic chemists,[32] many of which are alkaloids that are potential corrosion inhibitors. Rauvolfia macrophylla Stapf (RMS) is a Cameroonian medicinal plant belonging to the Apocynaceae family. This plant contains huge alkaloid fraction and is highly solicited in traditional medicine, especially in the treatment of various diseases like rheumatism, hepatitis, and malaria. Despite, its high alkaloid content, the use of RMS as a corrosion inhibitor has never been reported to the best of our knowledge.

The aim of this work is to investigate the corrosion inhibitory properties of alkaloid extract from RMS on C38 steel in 1 M hydrochloric acid and 0.5 M sulfuric acid media. This included the elucidation of the protection mechanism involved and the study of the effect of some external parameters such as the experimentation temperature and the amount of the extract. The use of an additive such as iodide to improve the inhibition efficiency was also scrutinized. The comparative study of the alkaloid extract and the isolated compounds (tetrahydroalastonine (THA) and perakine (PER)) in the extract is performed. For this to be achieved, the alkaloid fraction was first extracted from the RMS bark, and the corrosion inhibition parameters were obtained from electrochemical measurements (polarization and electrochemical impedance spectroscopy curves).

Results and Discussion

Corrosion Inhibition Effect of AERMS in HCl and H2SO4 Solution

Electrochemical Impedance Spectroscopy (EIS)

EIS curves (Nyquist and Bode representations) of C38 steel in 1 M HCl and 0.5 M H2SO4 obtained in the absence on one hand and presence on the other hand of AERMS at various concentrations are presented in Figure .

Figure 1

EIS curves of C38 steel in the absence and presence of AERMS at various concentrations. (a) Nyquist plots, (b) Bode modulus, and (c) Bode phase angle plots in 1 M HCl. (d) Nyquist plots, (e) Bode modulus, and (f) Bode phase angle plots in 0.5 M H2SO4.

The Nyquist plots in 1 M HCl and 0.5 M H2SO4 solutions with the AERMS (Figure a,d, respectively) display the capacitive loop characteristic of the corrosion process controlled by a charge transfer step on a solid electrode with a heterogeneous and irregular surface. The diameters of the capacitive half-loops of the Nyquist diagrams and the impedance modulus (Figure b,e) increased gradually with the AERMS concentration in the corrosive solution. However, this increase is not proportional because as the concentration doubles, the distance between the curves remains approximately equal. The inductive loops at lower frequencies was attributed to the relaxation processes due to ion adsorption (mainly Clads–, Hads+, and SO4 ads2–) on the electrode surface.[13] The best equivalent circuit that matches the experimental data of the Nyquist plots is presented in the inset of Figure a for hydrochloric acid and Figure d for sulfuric acid, where Rs (the first intersection of the semicircle with the Zr axis) represents the resistance of the corrosive solution; Rct (the second intersection of the semicircle with the Zr axis) is the charge transfer resistance, L is the inductance, which is intimately associated with the inductive loop at low frequencies, RL is the inductive resistance, and A is the CPE (constant phase element) constant that accounts for surface inhomogeneity. After the fitting procedure, these electrochemical parameters are recorded in Table .

Table 1

Corrosion Parameters Extracted from Experimental Data and the Corresponding Corrosion Inhibition Efficiency in 1 M HCl and 0.5 M H2SO4 Containing Different Concentrations of AERMS

concentration (mg L–1)	Rct (Ω cm2)	10–3 A (Ω–1 sn cm–2)	n	Cdl (μF cm–2)	L (H cm2)	RL (Ω cm2)	χ2	IE (%)
1 M HCl
0	14.6	3.048	0.9792	1147	3.49	1.6	0.0050
5	53.2	1.413	0.9508	457	1.69	51.1	0.0081	73
10	71.1	1.027	0.9581	453	1.04	10.1	0.0071	80
25	98.0	1.118	0.9330	329	4.56	92.3	0.0090	85
50	226.7	0.394	0.9093	188	2.10	22.6	0.0050	94
100	372.0	0.177	0.9380	87	1.55	367.4	0.0089	96
200	554.0	0.142	0.8589	77	23.04	20.0	0.0054	97
0.5 M H2SO4
0	29.6	1.334	0.8425	620	250.10	425.8	0.0081
5	40.2	1.014	0.8409	605	364.40	334.2	0.0089	26
10	79.0	0.392	0.8192	175	399.50	1213.0	0.0019	63
25	123.4	0.302	0.8149	149	1088.00	1293.0	0.0058	76
50	189.0	0.323	0.8644	128	2265.00	2048.0	0.0019	84
100	290.0	0.184	0.8779	84	596.00	2900.0	0.0079	90
200	428.0	0.173	0.8537	100	2400.00	6732.0	0.0029	93

Low values of the goodness of fit (χ2) indicate that the fitted data have good agreement with the experimental data. The values of the charge transfer resistance increases with the amount of the AERMS in solution, while the double layer capacitance (Cdl) decreases at the same time. The formation of a protective layer (due to the reaction of alloy when in contact with oxygen of the solution) by adsorption on the surface of the C38 steel electrode fully explains the trend of the values of Rct and Cdl obtained. These molecules are probably associated to the corrosion products to form a protective layer, which reduces the number of active corrosion sites.[8,15] The values of deviation from the ideal behavior (n) did not vary significantly, therefore confirming the charge transfer controlled mechanism of C38 steel without and with inhibitor.[16] The thickness of the protective layer depends on the concentration of the inhibitor in solution: for high AERMS concentrations, the rather higher thickness was noticed, substantially increasing the charge resistance transfer and thus the inhibition efficiency. The adsorbed layer also affects the double layer capacitance (Cdl) at the electrode/solution interface by replacing water molecules. The best inhibition efficiency (97%) obtained in the hydrochloric acid solution compared to sulfuric acid (93%) could be due to the beneficial adsorption of chloride ions at the anode surface.[10,26]

Polarization Curves

The potentiodynamic curves (logarithmic transformation) recorded in hydrochloric acid and in sulfuric acid solutions in the absence and presence of AERMS at various concentrations are presented in Figure .

Figure 2

Potentiodynamic polarization curves for C38 steel obtain in (a) 1 M HCl and (b) 0.5 M H2SO4 in the absence and presence of different concentrations of AERMS.

The corrosion parameters (corrosion potential (Ecorr), corrosion current densities (Jcorr), cathodic Tafel slope (βc), and anodic Tafel slope (βa)) extracted from Figure a,b are presented in Table . This table also presents the inhibition efficiencies (IE) based on experimental corrosion current densities.

Table 2

Polarization Parameters and the Corresponding Inhibition for the Corrosion of C38 Steel in 1 M HCl and 0.5 M H2SO4 for Various AERMS Concentrations

concentration (mg L–1)	Ecorr (mV/SCE)	Jcorr (μA/cm2)	–βc (mV/decade)	βa (mV/decade)	IE (%)
1 M HCl
0	–469	381	134	88
5	–483	143	129	78	63
10	–479	114	126	76	70
25	–477	85	119	79	78
50	–464	47	112	84	88
100	–443	34	118	74	91
200	–474	30	119	83	92
0.5 M H2SO4
0	–481	234	53	44
5	–477	192	49	49	18
10	–482	101	39	45	57
25	–471	71	35	48	70
50	–485	36	54	47	85
100	–468	22	35	46	91
200	–473	16	36	59	93

A shift in corrosion potential is insignificant and cannot be ascribe to the act of the corrosion inhibitor. The cathodic branch (assigned to the proton reduction) and the anodic branch (assigned to the oxidation of iron) of the signal are well defined. In both acids, the current densities recorded in the cathodic part of the curves decrease gradually with the amount of the AERMS added, showing that the extract inhibits the cathodic reaction associated to the corrosion process. A similar observation was made on the anodic part of the curve recorded in HCl, that is, a decrease in the iron oxidation current with AERMS concentration.[33−35] In sulfuric acid, the decrease in the oxidation current densities was not clearly observed. AERMS can thus be considered as a cathodic inhibitor in sulfuric acid and a mixed inhibitor with predominant cathodic effectiveness in HCl. The values of the anodic and cathodic (βa and βc) slopes in both acids are respectively modified with the addition of the inhibitor. These observations suggest that inhibition is controlled by anodic and cathodic reactions.[15] One can also notice that at high oxidation potentials (> −0.25 mV in HCl and > −0.35 mV in H2SO4), almost no variation of the oxidation current densities was observed even when the extract was present in the corrosive medium.[27]

As expected, the corrosion current density greatly decreased as the amount of AERMS in the solution grows from 234 to 16 μA cm–2 in H2SO4 and from 381 to 30 μA cm–2 in HCl. Consecutively to these, substantial diminishes of Jcorr and high percentages of IE up to 90% were obtained for AERMS concentrations higher than 100 mg/L. One particularly noticeable result was the high efficiency of the inhibitor in HCl at lower concentrations (IE of 63% at 5 mg/L) compared to H2SO4 (18% at 5 mg/L). This clearly showed that the counteranion of the acid was inactive during the inhibition process.[13,26] The corrosion potential has no direct correlation with the composition of the corrosive solution as it varies randomly.[16]

Ours results clearly show that the activity of AERMS as an inhibitor proceeds via a step of its adsorption at the surface of the steel electrode, making the accessibility of the metal to protons more difficult. The adsorption of the inhibitor at the surface of the steel electrode was confirmed by FTIR analysis of the thin layer recovered at the electrode surface after its immersion in a corrosive solution containing AERMS for 3 h (Figure and Table ).

Figure 3

FTIR spectra in the (A) 4000–400 cm–1 frequency range and (B) 3000–1000 cm–1 frequency range of (a) the AERMS powder and deposits on the electrode surface of C38 steel after immersion for 3 h in (b) 1 M HCl + 100 mg/L AERMS, (c) 0.5 M H2SO4 + 100 mg/L AERMS, (d) 1 M HCl, and (e) 0.5 M H2SO4.

Table 3

Major Bands Assigned to Functional Groups Present in Samples

function	AERMS powder	deposit in HCl + AERMS	deposit in H2SO4 + AERMS	deposit in HCl	deposit in H2SO4
H2O	3360	3420	3442	3414	3430
	1630	1630	1630	1630	1630
C–H	3000–2800	2980–2800	2980–2800
C=O	1730	1736
SO4			1104		1096

During this adsorption step, the water molecules and other ionic species previously adsorbed on the metal surface are replaced by the inhibitor molecules. In order to assess the spatial distribution of the inhibitor on the metal surface, the coverage (θ) was determined at a given AERMS concentration, and these experimental data fitted with a well-known linearized Langmuir model (eq [36,37]where K is the Langmuir adsorption equilibrium constant, and Cinh (mg/L) is the concentration of the inhibitor. Figure represents the results obtained for HCl and H2SO4.

Figure 4

Langmuir adsorption plots for C38 steel corrosion inhibition in (a) 1 M HCl and (b) 0.5 M H2SO4 at different AERMS concentrations.

The correlation coefficients (quite close to 1 (R > 0.99) both in 1 M HCl and 0.5 M H2SO4) tend to show that the Langmuir adsorption isotherm was suitable to describe AERMS adsorption in a single layer on the C38 steel surface.[15]

Field Emission Scanning Electron Microscopy (FESEM) Analysis

This surface analysis technique is used to visualize very small topographic details on the surface and to produce less electrostatically distorted images. Figure shows the surface morphology of C38 steel during FESEM analysis.

Figure 5

FESEM micrographs of C38 steel: (a) freshly polished surface (b) after 3 h immersion in 1 M HCl, (c) after 3 h immersion in 1 M HCl with 100 mg/L AERMS, (d) after 3 h immersion in 0.5 M H2SO4, and (e) after 3 h immersion in 0.5 M H2SO4 with 100 mg/L AERMS.

FESEM micrograph observation of the surface obtained before immersion (Figure a) is smooth. After immersion in 1 M HCl and 0.5 M H2SO4 solutions in the absence of the inhibitor, a highly damaged steel surface was observed (Figure b,c) with the presence of clusters of iron and punctures. This is due to the oxidation of iron in the absence of the inhibitor. In the presence of AERMS (Figure c–e), the surface of steel exhibits an absence of rust. By comparing the micrographs obtained after immersion in the two solutions without inhibitor, we can conclude that the surface of steel in the presence of AERMS is almost free of corrosion. These observations can be explained by the formation of a film made up of alkaloid molecules on the steel surface preventing the access of aggressive agents.[1,6,13,15]

Effect of Temperature

Corrosion inhibitors are used in various environments submitted to noticeable temperature variations. The corrosion mechanism being essentially a surface phenomenon should then be very sensitive to temperature variations.

The polarization curves recorded at 30, 40, 50, and 60 °C in 1 M HCl and 0.5 M H2SO4 solutions without and with the optimal concentration of AERMS (100 mg/L) are presented in Figure . The electrochemical parameters and the inhibition efficiencies derived from these curves are recorded in Tables and 5.

Figure 6

Polarization curves for C38 steel at different temperatures in (a) 1 M HCl, (b) 1 M HCl + 100 mg/L AERMS, (c) 0.5 M H2SO4, (d) 0.5 M H2SO4 + 100 mg/L AERMS.

Table 4

Corrosion Parameters and the Corresponding Inhibition Efficiency at Different Temperatures in 1 M HCl without and with 100 mg/L AERMS

temperature (°C)	Ecorr (mV/SCE)	Jcorr (μA/cm2)	–βc (mV/decade)	βa (mV/decade)	IE (%)
1 M HCl
30	–469	381	134	88
40	–471	769	145	81
50	–461	2550	186	96
60	–447	4368	192	167
1 M HCl + AERMS
30	–443	33	118	74	91
40	–481	74	147	76	90
50	–519	176	192	104	93
60	–533	477	177	139	89

Table 5

Corrosion Parameters and the Corresponding Inhibition Efficiency at Different Temperatures in 0.5 M H2SO4 without and with 100 mg/L AERMS

temperature (°C)	Ecorr (mV/SCE)	Jcorr (μA/cm2)	–βc (mV/decade)	βa (mV/decade)	IE (%)
0.5 M H2SO4
30	–481	234	53	44
40	–487	460	55	48
50	–488	2066	56	58
60	–488	2800	57	56
0.5 M H2SO4 + AERMS
30	–468	22	36	47	90
40	–478	60	32	57	90
50	–490	432	46	47	88
60	–471	1069	38	74	85

As expected, the temperature increases with the current densities in the absence of the corrosion inhibitor. The corrosion current densities (Jcorr) follow the same trend, confirming the harmful effect of the temperature increase in the corrosion of C38 steel. This tendency is preserved even in the presence of the inhibitor (100 mg/L). However, in HCl + 100 mg/L AERMS, it is mainly the anodic branch that was strongly affected by the temperature increase, whereas it was the cathodic branch in the case of H2SO4. These results clearly suggest that the mechanism of action of the extract as the corrosion inhibitor of C38 steel is different in the two acid solutions. The decrease in inhibition efficiency in 0.5 M H2SO4 when the temperature was increased suggests the physical adsorption of the inhibitor molecules.[15]

Thermodynamic Activation Parameters

In order to further the elucidation of these corrosion phenomena, the activation energy and thermodynamic parameters associated to the adsorption of AERMS on the C38 steel surface were determined. This was achieved using current densities obtained from the extrapolation of plots at different temperatures. The activation energy derived from the Arrhenius equation (eq (38) was obtained by plotting the logarithm of the experimental corrosion current density versus (1/T) (Figure ).where K is the Arrhenius pre-exponential constant, Ea (J/mol) is the activation energy, T (K) is the temperature, and R = 8.314 J/(mol·K) is the gas constant.

Figure 7

Arrhenius plots of (a) ln Jcorr vs 1/T and (c) ln(Jcorr/T) vs 1/T in 1 M HCl and (b) ln Jcorr vs 1/T and (d) ln(Jcorr/T) vs 1/T in 0.5 M H2SO4 in the temperature range of 30–60 °C and in the absence and presence of 100 mg/L AERMS.

The activation energy (Ea) values calculated from the slopes of the straight lines in the absence and presence of AERMS are listed in Table .

Table 6

Thermodynamic Activation Parameters of C38 Steel in 1 M HCl and 0.5 M H2SO4 without and with 100 mg/L AERMS

solution	Ea (kJ mol–1)	ΔHa (kJ mol–1)	ΔSa (J mol–1 K–1)
1 M HCl	70.33	72.83	–70.80
1 M HCl + 100 mg/L AERMS	73.83	71.08	–96.61
0.5 M H2SO4	74.58	71.83	–77.60
0.5 M H2SO4 + 100 mg/L AERMS	113.74	111.24	32.11

It is clear that the addition of 100 mg/L AERMS increases the apparent activated energy from 74.58 to 113.74 kJ/mol in 0.5 M H2SO4 and from 70.33 to 73.83 kJ/mol in 1 M HCl. This behavior suggests that AERMS inhibits the corrosion reaction by increasing its activation energy. This could be done by adsorption of the molecule inhibitor on the C38 steel surface, making a barrier to the mass and charge transfer.[39]

Some other useful thermodynamic parameters such as the standard enthalpy of activation (ΔHa) and standard entropy of activation (ΔSa) can be derived from the Arrhenius equation (eq put in a different form:where h (6.626 068 × 10–34 J s) is the Planck constant, and N (6.022 × 1023 mol–1) is the Avogadro’s number. By plotting ln(Jcorr/T) = f(1/T) (Figure c,d), ΔHa and ΔSa were determined (Table ).

The positive values of ΔHa both in the absence and presence of AERMS confirm the endothermic nature of C38 steel dissolution in acid media.[15,31,40] By contrast, in the same condition, the values of ΔSa are negative. However, ΔSa is lower in the presence of the extract, confirming the higher stability of the protective layer when AERMS was added in HCl solution. Indeed, in the presence of the inhibitor, the Fe–inhibitor complex is formed in the place of Fe–H2O. The decrease in the entropy upon inhibitor addition implies that the protecting layer is more ordered in the presence of the inhibitor.[1]

The strong increase in the value of ΔSa in 0.5 M H2SO4 with the addition of alkaloid extract (32.11 kJ/mol) indicates that the activated complex is more disordered in the presence of the inhibitor.[31] The effect of the anion (Cl– or I–) is very crucial on the overall adsorption process.

Effect of Iodide on the Efficiency of AERMS on the Corrosion Inhibition of C38 Steel

In order to improve the adsorption of AERMS on the steel surface in 0.5 M H2SO4 medium, the halide ions may be used. Iodide is well known for its ability to enhance the efficiency of alkaloid extracts as corrosion inhibitors.[41,42] The large size of this highly polarizable anion seems to be the factor responsible for this exceptional efficiency.[43] The effect of the concentration of this anion on the efficiency of AERMS was investigated. The study was conducted exclusively in sulfuric acid because of the lower adsorption of the extract in this acid compared to HCl. Nyquist plots and polarization curves for C38 steel are presented in Figure , and corresponding data are given in Tables and 8.

Figure 8

(a) Nyquist plots and (b) polarization curves of C38 steel in 0.5 M H2SO4 solutions in the absence and presence of KI, AERMS, and KI with AERMS.

Table 7

EIS Parameters of C38 Steel in 0.5 M H2SO4 Solutions in the Absence and Presence of KI, AERMS, and KI with AERMS

concentration (mg L–1)	Rct (Ω cm2)	10–3 A (Ω–1 sn cm–2)	n	Cdl (μF cm–2)	L (H cm2)	RL (Ω cm2)	IE (%)
0.5 M H2SO4	29.6	1.921	0.8840	620	10.13	139.8
0.5 M H2SO4 + KI	90.0	1.253	0.8021	473	2.22	53.3	61
0.5 M H2SO4 + AERMS	290.0	0.216	0.8428	84	213.60	1717.0	90
0.5 M H2SO4 + KI + AERMS	735.0	0.207	0.8012	58	6.16	59.7	96

Table 8

Polarization Parameters of C38 Steel in 0.5 M H2SO4 Solutions in the Absence and Presence of KI, AERMS, and KI with AERMS

concentration (mg L–1)	Ecorr (mV/SCE)	Jcorr (μA/cm2)	βa (mV/decade)	–βc (mV/decade)	IE (%)
0.5 M H2SO4	–481	234	44	53
0.5 M H2SO4 + KI	–473	71	34	49	69
0.5 M H2SO4 + AERMS	–468	22	46	35	91
0.5 M H2SO4 + KI + AERMS	–466	10	43	50	96

Figure a compares the Nyquist plots of the effect of 1 mM iodide on the corrosion of C38 steel in H2SO4 without or with AERMS.

The curve obtained in the presence of the extract without iodide is also plotted as a reference. The IE determined from Rct and Jcorr reported in Tables and 8 shows that 1 mM iodide offers poor protection of steel against corrosion (IE of 61%). When iodide is associated to AERMS, a net decrease in current densities was observed, and IE moves from 90% without KI to 96% in the presence of the anion. These results clearly demonstrate that iodide improves the ability of the extract to protect the metal against the corrosive environment of the acidic solution. As a matter of fact, the IE is equivalent to the value obtained in HCl using identical AERMS concentrations.

The large size and the important polarizability of iodide facilitate the bonding of the anion at the metal surface through chemisorption. This is followed by the adsorption of protonated alkaloids present in AERMS on the first layer of adsorbed halide ions.[15,44] AERMS adsorption was more homogeneous as it was directed by the first layer of well-adsorbed iodide. This results to a more uniform metal surface coverage and thus good protection. Potentiodynamic analysis (Figure b) shows that, in the presence of 1 mM iodide alone in the acidic solution, anodic protection was displayed. Associated to the extract, a mixed-type inhibitor was obtained as both cathodic and anodic protection was obtained. These results confirm once again the synergetic effect of KI toward the corrosion inhibition of C38 steel when added to the alkaloid extract. The corrosion parameters (Jcorr and Ecorr) extracted from these curves confirmed this interpretation (Table ).

Isolation of THA and PER and Study of Their Inhibition Efficiency

Isolation of THA and PER

AERMS contains several alkaloids that can act as corrosion inhibitors. The effect observed with AERMS is certainly the result of the complementary action of the efficient compounds present in the extract. Only two compounds (THA and PER) were quantitatively isolated from AERMS and analyzed using high-resolution 13C and 1H NMR. Following the interpretation of these 1H and 13C NMR chemical shift spectra (see Supporting Information, Tables S1 and S2), the chemical structures of PER and THA were elucidated (Figure ). These structures correspond to some compounds recently isolated in the extract of Rauvolfia tetraphylla and Rauvolfia serpentina, respectively.[45,46]

Figure 9

Structure of isolated molecules: (a) perakine and (b) tetrahydroalastonine.

Examining the structure of these two indole alkaloids reveals that their adsorption on the metal surface can proceed via the free doublets of basic nitrogen, oxygen atoms, and aromatic rings.

Corrosion Inhibition of C38 Steel in 1 M HCl and 0.5 M H2SO4 by THA and PER

The corrosion inhibition of THA and PER on C38 steel in 1 M HCl and 0.5 M H2SO4 was evaluated and compared to the result obtained with the extract (AERMS). A concentration of 10 mg/L was chosen to compare the efficiencies of the inhibitors in the active domain of the process. Indeed, results obtained with AERMS showed that at a concentration of 10 mg/L, the inhibition efficiency was higher than 60%. The polarization curves and Nyquist plots obtained with AERMS, PER, and THA in HCl and H2SO4 are presented in Figure . The constants derived from these plots are summarized in Table S3.

Figure 10

Comparison of the polarization curves and Nyquist diagrams for C38 steel in (a, c) 1 M HCl and (b, d) 0.5 M H2SO4 in the absence and presence of different inhibitors (10 mg/L).

The addition of inhibitors decreases the corrosion current density. However, the cathodic curves are more shifted toward the lower current density, especially in HCl, with THA as the most efficient inhibitor. The Nyquist plots were more effective to distinguish the efficiencies of the three inhibitors. Indeed, the differences are well highlighted on the curves depicted in Figure c,d. Based on the Rct values, THA was confirmed as the most efficient inhibitor followed by AERMS. Surprisingly, AERMS is more efficient than PER, especially in H2SO4. The poor efficiency of PER compared to THA may be due to the more pronounced steric hindrance of the basic nitrogen atom.[15] This result confirms once again the decisive role played by the availability of the structural basic nitrogen atom on the corrosion inhibition. On the other hand, one can conclude that the constituents of the extract act concomitantly in AERMS; the more powerful compensating the less efficient.

Conclusions

This study puts a major emphasis on the three points: the investigation of the inhibitory properties of AERMS, the improvement of the adsorption of AERMS in H2SO4 medium by KI, and the comparative study of the inhibitory activity of AERMS and the two isolated compounds. Several interesting points can be highlighted from the obtained results. AERMS strongly adsorbs on the steel surface and thus prevents its corrosion. These surface phenomenon were confirmed by analyses of the metal surface before and after corrosion experiments (FESEM and FTIR). The inhibition of AERMS was improved in the presence of iodide in corrosive solution where it acts as a mixed inhibitor, with the efficiency increasing from 90 to 96% in H2SO4 medium. The experiment with the two isolated compounds of the extract (PER and THA) confirmed the important role played by the basic nitrogen atom in the process and the variable efficiencies of compounds present in the alkaloid extract. More investigations are still needed to determine whether the action of the compounds is synergetic or just the superimposition of the individual effect.

Materials and Experimental Methods

Materials and Solutions

Corrosion tests were performed on a cylindrical C38 steel sample from Tacinas Company (France). Its chemical composition (wt %) was 0.360 C, 0.230 Si, 0.680 Mn, 0.016 S, 0.077 Cr, 0.011 Ti, 0.059 Ni, 0.009 Co, 0,160 Cu, and 98.388 Fe according to the French standard. The C38 steel surface was coated in a polytetrafluoroethylene (PTFE) ribbon, leaving a free circular basal working surface of 0.785 cm2. To obtain reproducible results, the working electrode was polished with silicon carbide abrasive papers of different grades (100, 1200, 2500, and 4000) from the most abrasive to the least abrasive. Corrosive solutions (1 M HCl and 0.5 M H2SO4) were prepared from commercial solutions of 37% HCl (Scharlau) and 98% H2SO4 (Scharlau) and distilled water.

Extraction of Total Alkaloids from Rauvolfia macrophylla Stapf Bark

RMS barks was harvested at the So’o locality, of Nyong and So’o division, in the center region of Cameroon. The samples were cut into tiny pieces, air dried for 15 days, and finely grounded. The resulting powder was macerated at room temperature for 3 days in a dichloromethane/methanol (1:1) mixture. The crude extract was obtained by evaporating the solvent, and the total alkaloid extract was obtained following Pandian’s procedure.[3] The Dragendorff’s test confirmed the high content of alkaloids in the extract.

Electrochemical Measurements

The electrochemical experiments were performed in a thermostated electrochemical cell with three electrodes: the working electrode was the C38 steel rod, the auxiliary electrode was a platinum wire, and the reference electrode was a saturated calomel electrode (SCE). These electrodes were connected to an Autolab PGSTAT 12 (EcoChemie) potentiostat/galvanostat controlled by the software FRA (Frequency Response Analysis) for electrochemical impedance measurements, and GPES (General Purpose Electrochemical System) for the open circuit and potentiodynamic polarization experiments.

Before recording the polarization and EIS curves, the working electrode was first immersed in the test solution (acidic solution with or without AERMS) for 3 hours (experimental optimum immersion time required to reach constant potential at a steady-state open circuit).

The polarization curves were recorded by scanning the potential from −800 to −200 mV/SCE at a scan rate of 0.5 mV/s. The inhibitory efficiency (IE) was calculated using eq where Jcorr (A cm–2) and Jcorr(inh) (A cm–2) are the corrosion current density in the absence and presence of the inhibitor, respectively.

A voltage amplitude of 10 mV around the open-circuit corrosion potential and a frequency range of 10 kHz to 100 mHz were used for the EIS measurements. The inhibition efficiencies were determined using eq where Rct (Ω cm2) and Rct(inh) (Ω cm2) are the charge transfer resistance in the absence and presence of the inhibitor, respectively.

The capacity of double layer was determined using eq where ωmax = 2πfmax, and fmax (Hz) is the frequency corresponding to the maximum value of the imaginary component of the Nyquist curve. Following the model of Helmholtz,[33,47,48] we havewhere εo is the dielectric constant of the adsorbed water, and ε is the permittivity constant and the thickness of the layer formed.

The study of the influence of the concentration of AERMS was performed in 1 M HCl or 0.5 M H2SO4, while the influence of the temperature was achieved by varying the temperature of the electrolytic solution at the optimum AERMS concentration.

Fourier Transform Infrared (FTIR) Spectroscopy

The recording of the FTIR spectra of the deposits on the electrode surface after immersion in the corrosive solutions proceeded as follows: the electrode previously polished was immersed for 3 h in the acidic solution in the presence of (or without) the corrosion inhibitor, and the electrode was then removed from the solution and dried for 1 h in open air. The passive layer was then gently removed with a spatula, and the resulted powder used for the preparation of the KBr pellets used for the recording of the FTIR spectra.

Field Emission Scanning Electron Microscopy (FESEM) Analysis

FESEM analysis was used to investigate the surface morphology of C38 steel after immersion in 1 M HCl and 0.5 M H2SO4 solutions in the absence and presence of AERMS at the optimum concentration for 3 h. FESEM analysis was performed with a Sopra 55 VP field emission scanning microscope.