1 Introduction

Artemisia herba-alba (AHA) greenish-silver perennial herb grows 20–40 cm in height and belongs to the daisy family Asteraceae [1]. This plant usually renowned in Morocco as “Chih” and in France as “Armoise balnche” [2]. It has been utilized in popular medicine by many cultures since antique times, to treat colds, coughing, bronchitis, intestinal disturbances, diarrhea, neuralgias arterial hypertension, and/or diabetes [3,4,5]. Over the last decades, studies on AHA were focused on its essential oils. Their content through the world indicated a high level of polymorphism and guided to the definition of various chemotypes [6]. Many investigators have reported miscellaneous biological and/or pharmacological activities of AHA essential oil as an antimicrobial, antidiabetic, antioxidant, anthelmintic, Antileishmanial, and antispasmodic agent [1, 7,8,9,10].

Mild steel has widespread industrial applications due to their availability and low cost. In processes such as acid cleaning, pickling, and descaling operations in oil and gas exploration, acidic solutions are widely used. Mild steel surfaces exert in service in these environments which undergo considerable corrosion. Significant reduction in corrosion rates has been attained by various means inclosed reduction of the mild steel impurity content, application of several surface modification techniques as well as insertion of desirable alloying elements [11]. However, the preventions used to reduce the corrosion of materials include those related to the use of corrosion inhibitors, and the choice of appropriate inhibitors is dependent primarily on the structure. Most of these inhibitors are organic molecules which generally include heteroatoms, π-electron, and aromatic rings, which allow an adsorption on the metal surface [12,13,14,15,16,17]. These inhibitors are adsorbed on the metal surface and block active corrosion sites, and the majority of them are toxic to humans and their environment. Hence, the use of natural products as eco-friendly and harmless corrosion inhibitors has become popular [18,19,20,21,22,23]. These inhibitory substances can beget temporary or permanent damage to organs such as the kidneys or liver, or disrupt the enzymatic system in the body. The toxicity can occur either during the synthesis of these compounds or during their applications. As a result, natural substances are increasingly seen as an alternative to these synthetic inhibitors, which are environmentally friendly and safe. Inhibitors of natural origin are utilized for the protection of metals in the acid environment, in order to replace the toxic chemicals currently utilized [24, 25].

In the present work, the chemical composition of Artemisia herba-alba essential oil (AHAO) is established using GC and GC–MS. Corrosion inhibition efficiency has been experimentally evaluated using gravimetric and polarization methods. The effect of temperature on the corrosion behavior of mild steel in 0.5 M H2SO4 with the addition of AHAO was studied in the temperature range of 298–343 K. The adsorption and kinetic parameters for mild steel/AHAO/0.5 M H2SO4 system were calculated from experimental gravimetric data and the interpretation of the results is given. Scanning electron microscopy (SEM) has been applied to study the mechanism of mild steel corrosion inhibition of this essential oil in acidic medium.

2 Experimental Procedures

2.1 Plant Collection and Essential Oil Extraction

Artemisia herba-alba was harvested in Mars 2008 from the garden of the reserve area of Boulemane locality in Morocco. The collection of the plant and the extraction of the essential oil was made according to the same protocol described in a previously published work [26].

2.2 Gas Chromatography Analysis (GC–FID) and Gas Chromatography Mass Spectrometry (GC–MS)

GC–FID, GC–MS, and the identification of the essential oil constituents were made according to previously published works [26].

2.3 Components Identification

The identification of the essential oil constituents was based on (i) comparison with the mass spectra of authentic reference compounds where possible and by reference to WILEY275, NIST 02, and Adams mass spectral libraries [27], (ii) comparison of their retention index (RI), calculated relative to the retention times of a series of C-5 to C-30 n-alkanes, with linear interpolation, with those of our own library of authentic compounds or literature data [27, 28].

2.4 Corrosion Tests

Mild steel (C: 0.21, Mn: 0.05, Si: 0.38, S: 0.05, P: 0.09, Al: 0.01, and the remainder iron) was utilized. Prior to each experiment, the mild steel specimen was abraded with a series of emery paper from 400 to 1200 grades. The specimen was washed several times with distilled water, then with ethanol, and finally dried using a stream of air.

The acidic solution 0.5 M H2SO4 was prepared by dilution of Analytical Grade H2SO4 with distilled water. The concentration range of AHAO in these dilute solutions was from 0.65 to 2.76 g L−1.

Gravimetric methods are carried out in a double-walled glass cell equipped with a thermostated cooling condenser. The solution volume is 50 mL. The steel specimens utilized have rectangular shapes 3 cm × 1 cm × 0.1 cm. For each sample, three tests are realized and the corresponding average value is calculated. The samples were weighted with an uncertainty of 10−4 g. The immersion time for weight loss amounts is 6 h for all the temperatures.

2.5 Polarization Measurements

Polarization curves were conducted using an electrochemical measurement system Voltalab 40 potentiostat–galvanostat (Radiometer Analytical PGZ 301) and controlled with corrosion analysis software VoltMaster 4.0. The working electrode (WE) in the form of disc cut from steel has a geometric area of 1 cm2 and is embedded in polytetrafluoroethylene (PTFE). A saturated calomel electrode (SCE) and a platinum electrode were utilized, as reference and auxiliary electrodes, respectively. The WE was immersed in test solutions for 30 min to establish steady-state open circuit potential (Eocp) by applying 10 mV ac voltage peak-to-peak. After measuring the Eocp, the electrochemical measurements were performed. All electrochemical tests have been performed in aerated solutions at 298 K. The potential was swept to anodic potentials by a constant sweep rate of 0.5 mV s−1 and potential was scanned in the range of − 800 to 0 mV/SCE relative to the corrosion potential.

3 Results and Discussion

3.1 Analysis of Artemisia herba-alba Essential Oil

The analysis of essential oil from AHA was carried out by CG/FID and CG/MS. The chemical composition of essential oil was characterized by 24 compounds, which accounted for 98.4% of the total oil. The retention time of volatile compounds (RIa and RIp) and their percentage are summarized in Table 1. The oil was dominated by oxygenated monoterpenes (70.2%) followed by monoterpenic hydrocarbons (26.6%). The sesquiterpene hydrocarbons and oxygenated sesquiterpenes accounted only for 1.6%. The essential oil was characterized by high amounts of 1,8-Cineole (35.6%). The other major components were camphor (24.1%), α-Pinene (11.6%), and Camphene (4.9%). The 24 other compounds are reported in low amounts in AHA essential oil from Morocco. It should be noted that several studies have been published on chemical composition of AHA in Mediterranean basin. The study made by Boutkhil et al. [29] on the regions of Oujda and Errachidia showed that the composition of the AHAO from Oujda was rich in α-thujone (46.75%), β-thujone (21.17%), and camphor (8.41%), the essential oil from Errachidia was rich in camphor (17.79%), α-thujone (17.27%), the cis-chrysanthenyl acetate (10.9%), β-thujone (9.91%), davanone (6.65%), the chrysanthenone (6.37%), and the eucalyptol (4.98%), and the difference observed in the chemical composition of AHA for the same country can be explained by the techniques utilized for the extraction.

Table 1 Chemical composition (%) of the Essential oils from Artemisia herba-alba
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3.2 Gravimetric Measurements

3.2.1 Effect of AHAO Concentration

The effect of addition of AHAO tested at different concentrations on the corrosion of mild steel in 0.5 M H2SO4 solution was studied by weight loss measurements at 298 K after 6 h of immersion period. The corrosion rate (CR) and inhibition efficacy ⎜(%) were calculated according to the Eqs. 1 and 2 [30], respectively:

$${C_{\text{R}}}=\frac{{{W_{\text{b}}} - {W_{\text{a}}}}}{{At}}$$
(1)
$${\eta _{{\text{WL}}}}(\% )=\left( {1 - \frac{{{w_{\text{i}}}}}{{w_{{\text{0}}}^{{}}}}} \right) \times 100,$$
(2)

where Wb and Wa are the specimen weight before and after immersion in the tested solution, w0 and wi are the values of corrosion weight losses of mild steel in uninhibited and inhibited solutions, respectively, A the area of the mild steel specimen and t is the exposure time (h).

The weight loss data are given in Table 2.

Table 2 Gravimetric results of mild steel in 0.5 M H2SO4 at different concentrations of AHAO during 6 h at 298 K
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It is very clear from this table that the corrosion rate increased with the decreased with inhibitor concentration. This phenomenon is caused by the adsorption of active molecules of Artemisia herba-alba oil (AHAO) on the mild steel surface. The highest value of inhibition efficacy is obtained at 2.76 g L−1 (by weight) inhibitor concentration.

3.2.2 Effect of the Temperature

The effect of temperature on the inhibited acid–metal reaction is very complex, because many changes occur on the metal surface such as rapid etching, desorption of inhibitor, and the inhibitor itself may undergo decomposition. The change of the corrosion rate at selected concentrations of the AHAO during 6 h of immersion with the temperature was studied in 0.5 M H2SO4, both in the absence and presence of AHAO. For this purpose, gravimetric experiments were performed at different temperatures (303–343 K) and the results are given in Table 3.

Table 3 The effect of AHAO concentration on the weight loss of mild steel in 0.5 M H2SO4 solution
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The obtained data in Table 3 reveal that the inhibition efficacy increased with an increase in the AHAO concentration. This suggests that the AHAO species are adsorbed on the mild steel/solution interface where the adsorbed species mechanically screen the coated part of the metal surface from the action of the corrosive medium.

As observed from Table 3, the effect of temperature on the inhibition efficacy of the studied AHAO at all concentrations and temperatures shows that a remarkable decrease in the AHAO efficacy was observed with increasing temperature up to 343 K. This variation of the efficacy versus temperature shows that physisorption mechanism is effectively enhanced with rising temperature.

The effect of temperature on the inhibited acid-metal reaction is highly complex, this is due to the change that occurs on the metal surface such as rapid etching and desorption of AHAO. Also the inhibitor itself may undergo decomposition and/or rearrangement [31]. However, it was found that few inhibitors with acid-metal systems have specific reactions which are effective at high temperature as (or more) they are at low temperature [32, 33].

To calculate activation thermodynamic parameters of the corrosion process, Arrhenius Eq. (3) and transition state Eq. (4) were utilized [34]:

$${C_R}=A\,\exp \left( { - \frac{{{E_{\text{a}}}}}{{RT}}} \right)$$
(3)
$${C_R}=\frac{{RT}}{{Nh}}\exp \left( {\frac{{\Delta S_{{\text{a}}}^{{}}}}{R}} \right)\exp \left( { - \frac{{\Delta H_{{\text{a}}}^{{}}}}{{RT}}} \right),$$
(4)

where Ea is the apparent activation corrosion energy, R is the universal gas constant, A is the Arrhenius pre-exponential factor, h is Plank’s constant, N is Avogadro’s number, ΔSa is the entropy of activation, and ΔHa is the enthalpy of activation.

Arrhenius plots for the corrosion rate of mild steel [Ln(CR) vs. 1/T] are given in Fig. 1. Values of apparent activation energy of corrosion (Ea) for mild steel in 0.5 M H2SO4 with the absence and presence of various concentrations of AHAO are calculated by linear regression between Ln(CR) and 1/T and the results are given in Table 4.

Fig. 1
figure 1

Arrhenius plots for mild steel corrosion rates (CR) in 0.5 M H2SO4 in the absence and in presence of different concentrations of AHAO

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Table 4 Kinetic-thermodynamic corrosion parameters for mild steel corrosion in the absence and presence of various concentrations of AHAO
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All the linear regression coefficients are close to 1, indicating that the mild steel corrosion in 0.5 M H2SO4 can be elucidated using the kinetic model. As observed from the Table 4, the Ea increased with increasing concentration of AHAO, but all values of Ea in the range of the studied concentration were higher than that of the uninhibited solution (blank). The increase in Ea in the presence of AHAO may be interpreted as physical adsorption. Indeed, a higher energy barrier for the corrosion process in the inhibited solution is associated with physical adsorption or weak chemical bonding between the inhibitor species and the steel surface [13, 35]. Szauer et al. explained that the increase in activation energy can be attributed to an appreciable decrease in the adsorption of the inhibitor on the mild steel surface with the increase in temperature. A corresponding increase in the corrosion rate occurs because of the greater area of metal that is consequently exposed to the acid environment [36].

Figure 2 shows a plot of Ln (CR/T) against 1/T. Straight lines are obtained with a slope of (− ΔHa/R) and an intercept of (Ln R/Nh + ΔSa/R) from which the values of ΔHa and ΔSa are calculated, and are listed in Table 4.

Fig. 2
figure 2

Transition state plots for mild steel corrosion rates (CR) in 0.5 M H2SO4 in the absence and presence of different concentrations of AHAO

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Inspection of these data reveals that the ΔHa values for dissolution reaction of mild steel in 0.5 M H2SO4 in the presence of AHAO are higher (62.7–74.5 kJ mol−1) than that of in the absence of AHAO (36.5 kJ mol−1). The positive signs of ΔHa values reflect the endothermic nature of the mild steel dissolution process suggesting that the dissolution of mild steel is slow [37] in the presence of inhibitor. All values of Ea are larger than the analogous values of ΔHa indicating that the corrosion process must involve a gaseous reaction, simply the hydrogen evolution reaction, associated with a decrease in the total reaction volume [24].

From Table 4, it is clear that the increase in AHAO concentration leads to an increase in entropy. The increase of ΔSa is generally interpreted as an increase in disorder as the reactants are converted to the activated complexes [30]. This observation is in agreement with the findings of other workers [38,39,40,41]. This behavior can also be explained as a result of the replacement process of water molecules during adsorption of AHAO molecules on the steel surface and therefore the increase in entropy of activation was attributed to the increasing in solvent entropy [42].

3.3 Adsorption Isotherms

The extent of inhibitive actions of the studied compounds has been expanded in terms of the adsorption mode of the inhibitor. An efficient organic corrosion inhibitor is expected to adhere onto the metal surface immersed in aqueous solution via a quasi-substitution mode [43, 44]. A correlation between surface coverage (θ) defined by η%/100 obtained from weight loss and the concentrations of AHAO (Cinh) was fitted to Langmuir and Temkin adsorption isotherms. Both isotherms are presented in Figs. 3 and 4, and the mathematical forms of the isotherms employed are

Fig. 3
figure 3

Langmuir’s isotherm adsorption model of AHAO on the mild steel surface in 0.5 M H2SO4 at different temperatures

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Fig. 4
figure 4

Temkin isotherm adsorption model of AHAO on the mild steel surface in 0.5 M H2SO4 at different temperatures

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$${\text{Langmuir}}:\,\frac{C}{\theta }=\frac{1}{K}+C$$
(5)
$${\text{Temkin}}:\,Ln\left( {\frac{\theta }{C}} \right)=LnK - g\theta,$$
(6)

where θ is the surface coverage, K is the adsorption–desorption equilibrium constant, C is the concentration of AHAO inhibitor, and g is the adsorbate parameter.

Ultimately, the accuracy of the fit was examined using the correlation coefficient (R2) given in Table 5.

Table 5 Adsorption parameters for AHAO in 0.5M H2SO4 obtained from Langmuir and Temkin adsorption isotherms at different temperatures
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As can be observed in Table 5, the R2 values clearly show that data obtained for the Langmuir isotherms are closer to unity than the Langmuir isotherm. Consequently, the set of AHAO inhibitor under study was found to prefer the Langmuir adsorption isotherm.

In our study, it is very important to note that discussion of the adsorption isotherm behavior, using natural product as inhibitors, in terms of the standard free energy of adsorption value, is not possible because the molecular mass of the AHAO constituents is not known. Some authors [45,46,47], in their study on acid corrosion with natural products, noted the same limitation.

3.4 Polarization Measurements

Polarization measurements have been carried out in order to gain knowledge concerning the kinetics of the anodic and cathodic reactions. Figure 5 depicts the typical potentiodynamic cathodic and anodic curves of carbon steel electrode after immersion in 0.5 M H2SO4 in the absence and presence of various concentrations of AHAO at 298 K. The addition of AHAO to acid solutions shifts both the anodic and cathodic branches of the Tafel plot of the pure acid solution to lower values of current density at all investigated concentrations.

Fig. 5
figure 5

Polarization curves of steel in 0.5 M H2SO4 in the presence of different concentrations of AHAO at 298 K of the corrosive medium

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This indicates that AHAO inhibits both hydrogen evolution and metal dissolution and suggests it to act as a mixed type inhibitor. Electrochemical parameters determined from these experiments by extrapolation method [48], as corrosion potential (Ecorr), corrosion current density (icorr), and cathodic Tafel slopes (βc), are listed in Table 6. The icorr was determined by Tafel extrapolation of only the cathodic polarization curve alone, which usually produces a longer and better defined Tafel region [48]. The icorr values were used to calculate the inhibition efficiency, ηTafel(%), using the following equation:

Table 6 Polarization parameters and ηTafel (%) for steel corrosion in 0.5 M H2SO4 without and with various concentrations of AHAO at 298 K
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$${\eta _{{\text{Tafel}}}}\left( \% \right)=\left( {\frac{{i_{0}^{{{\text{corr}}}} - {i^{{\text{corr}}}}}}{{i_{0}^{{{\text{corr}}}}}}} \right) \times 100,$$
(7)

where icorr and icorr(i) are the corrosion current densities for steel electrode in the uninhibited and inhibited solutions, respectively.

Inspection of the results in Table 6 shows that the values of corrosion current density (icorr) noticeably decrease in the presence of inhibitor (AHAO), which suggests that rate of electrochemical reaction was retarded due to the formation of a barrier layer on mild surface by adsorption of the AHAO molecules. Moreover, the maximum shift in Ecorr, compared to that of uninhibited solution, was 150 mV towards cathodic direction indicating therefore that AHAO acts as cathodic-type inhibitor (Fig. 5; Table 6). The cathodic curves (Fig. 5) give rise to parallel lines suggesting that the addition of inhibitor to corrosive environment does not modify the hydrogen evolution mechanism and reduction of H+ ions at the carbon steel surface follows charge transfer mechanism. The values of βc in Table 6 show a slight change with increasing inhibitor concentration, indicating the influence of the AHAO inhibitor on the kinetics of hydrogen evolution. The adsorbed protective film of inhibitor on steel surface impedes by blocking the reaction sites of the metal resulted in the significant change of the iron dissolution mechanism [49,50,51]. In this way, actual surface area available for H+ ions is decreased while the actual reaction mechanism remains unaffected [51]. The organic compounds existed in the AHAO include functional groups containing large number of heteroatoms like oxygen. As a result, the vacant d-orbital of metal can be interacted with lone pair of electrons of oxygen atoms or π-electron cloud of the donor atoms. The inhibition efficiency values, ηTafel (%), calculated by potentiodynamic polarization method are given in Table 6. The polarization data also confirm the gravimetric results, i.e., AHAO is a good inhibitor and its inhibition efficiency depends on the inhibitor concentration.

3.5 Scanning Electron Microscopy (SEM)

The scanning electron micrographs images were recorded in order to confirm the protective film formation during the corrosion process. Figure 6 shows the SEM images of the mild steel surface before and after immersion in 0.5 M H2SO4 with and without corrosion AHAO. Figure 6a signifies the SEM image of the polished mild steel surface, except the presence of polishing scratches; the surface shows the absence of noticeable defects such as pits and cracks. Figure 6b, c shows the steel surface after 6 h of immersion in 0.5 M H2SO4 without and with AHAO. The resulting of the SEM micrograph shows that the surface was rough and harshly corroded for the reason that the violent attack by 0.5 M H2SO4 in the absence of the AHAO. However, there are less pits and cracks observed in the micrographs in the presence of AHAO (Fig. 6b), which suggests a formation of protective film on steel surface which was responsible for the corrosion inhibition. Indeed, AHAO has a strong tendency to adhere to the steel surface and can be regarded as good inhibitor for steel corrosion in sulfuric acid medium. Figure reveals the formation of a protective film of the inhibitor on the mild steel surface which inhibits the corrosion considerably in acidic medium.

Fig. 6
figure 6

SEM micrographs of the mild steel surface: a metallic surface after being polished, b metallic surface after 6 h of immersion in 0.5 M H2SO4 with AHAO and c metallic surface after 6 h of immersion in 0.5 M H2SO4

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4 Conclusions

  • The inhibition efficacy of AHAO increases with increasing inhibitor concentration. It reaches a maximal value of 88% at 2.76 g L−1 at 298 K.

  • The adsorption of the inhibitor on the mild steel surface is correctly described using the Langmuir isotherm.

  • Thermodynamic activation parameters were calculated and discussed.