Adsorption and thermodynamic studies of the corrosion inhibition effect of Rosmarinus officinalis L. leaves on aluminium alloy in 0.25 M HCl and effect of an external magnetic field

The corrosion inhibition of Aluminium alloy AA8011 in 0.25 M hydrochloric acid solution by Rosmarinus officinalis L. (rosemary) leaves extract was studied using the gravimetric technique at 303, 313, 323 and 333K, and in the presence of an external magnetic field. The study reveals that the methanolic extract of rosemary leaves inhibits corrosion of Al in 0.25 M HCl. The inhibition efficiency was found to increase with increase in concentration of the extract, but decreased with increase in temperature. Thermodynamic activation parameters like the activation energy ( E a ), ranged from 15.29 to 35.06 kJ/mol, thereby suggesting the mix mechanism of physichemisorption; calculated values of the standard adsorption enthalpies ( ) oads H  were positive indicating an endothermic process, while negative values of entropies ( ) oads S  implied an associative interaction between the inhibitor molecules and the Al surface. The corrosion data was found to be a good fit for the Langmuir (R 2 > 0.98) and Villamil (R 2 > 0.99) isotherm models. Values of the adsorption free energy ( ) o ads G  obtained were negative, thereby describing a spontaneous adsorption process. The effect of an external magnetic field was found to have a mixed behaviour in the presence of the R. offinalis L. extract at room temperature. Nonetheless, the corrosion rate was generally found to decrease with increase in strength of the magnetic field.


INTRODUCTION
Aluminium makes up about 8.3% of the earth's crust and is the most widely used non-ferrous alloy Millberg (2018). Aluminium and its alloys are extensively used in the area of transportation, construction of bridges and buildings, fabrication of electrical machinery/equipment, and is the choice material for packaging food and beverages *Corresponding author. E-mail: paulandoor@gmail.com.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License (Maghraby, 2009;Chaubey et al., 2018;Popoola et al., 2012). Its wide range of applications is due to its low price, high strength-to-weight ratio, high electrical and thermal conductivities (Sukiman et al., 2012). In addition, aluminium is generally non-toxic, paramagnetic, and possesses high resistance to atmospheric or environmental corrosion Davis (1993). The corrosion resistance of aluminium is due to its ability to readily passivate, to form a tough, inert thin oxide film on its surface almost immediately when exposed to the atmosphere (Noor, 2009;Hansson 2011;Sangeetha et al, 2013) thereby reducing its susceptibility to corrosion (Abdallah et al. (2013).
However, this protective film is known to deteriorate in the presence of very aggressive acidic media containing Clions (El-Awady et al., 1993;Jegdić et al., 2016;Lowson, 1978) like seawater, as well as solutions of hydrochloric acid (HCl) used for pickling, chemical, and electrochemical etching of aluminium (Khan et al., 2015;Philip and Schweitzer, 2003). Several factors like temperature, and the presence of a magnetic field can greatly affect the corrosion rate of Al in HCl media (Davis, 1993;Philip and Schweitzer, 2003) The corrosion of aluminium and its alloy is destructive and poses an extremely costly problem (Gilbert, 1978). There are several methods of corrosion control, however, focus has shifted to the use of green inhibitors, that is, plant extracts (either from the leaves, seeds, fruits, or even roots) as a low-cost, eco-friendly alternative to synthesized organic inhibitors (Velázquez-González et al, (2014); Kesavan et al. (2012) Manimegalai and Manjula, 2015). Green inhibitors contain neither toxic compounds nor heavy metals. When introduced in very small concentrations, certain heteroatoms like O, P, S, and N (Khan et al., 2015) present in organic compounds like tannins, amino acids, dyes and alkaloids found in the plants extracts help adsorb these compounds onto the surface of the metal by displacing water molecules, thereby forming a protective film (Patni et al., 2013) which inhibits further corrosion.
The rosemary plant (Rosmarinus officinalis L.) is the green inhibitor of choice studied in this paper. It is an aromatic plant of Mediterranean origin (Velázquez-González et al, (2014) and contains rosmarinic acid as its active ingredient, in addition to antioxidants like carnosic acid and carnosol (Rani and Basu, 2012;Fouda et al., 2015;Francis et al., 2019;Barbut et al., 1985;Nakatani, 2000), therefore making it one of the most widely used and commercialized green inhibitors (Fouda et al., 2015;Deyab, 2016;Catic et al., 2016). The structure of its active constituents is as shown in Figure 1 (Velázquez-González et al., 2014).
The presence of the heteroatom (O) in their aromatic rings suggests that R. officinalis L. can be a possible corrosion inhibitor for Al. Materials are classified as ferromagnetic, anti-ferromagnetic paramagnetic (like in the case of Al) or diamagnetic based on their behavior in the presence of an induced magnetic field (Myers, 1997). It is therefore expected that the presence of external magnetic field can have an influence in a chemical reaction involving such materials (Catic et al., 2016). Hence, this study seeks to investigate the corrosion inhibitive potential of R. officinalis L. on aluminium alloy in 0.25 M HCl at various temperatures, and the effect of the presence of an external magnetic field at room temperature (303K).

Rosemary leaves
Freshly harvested rosemary herb separated from it stem and room dried.

Chemicals
All chemicals used were of analytical grade: (a) Hydrochloric acid (HCl) solution by JHD ® with a purity of (35.5-37.5% wt) used as the corroding media, (b) Nitric acid (HNO 3 ) (65% purity) solution used as quenching media, (c) Methanol (purity 99.5%) by LOBAL Chemie used for the cold extraction of the rosemary herb, (d) Acetone Extra Pure (purity 99.0%) by LOBAL Chemie was used for rinsing.

The gravimetric technique
The gravimetric method is simple and reliable, and forms the baseline method for many corrosion inhibition assessment programs (Beda et al, 2017). To study the corrosion behavior of Al in an aggressive media like HCl, attempt was made to quantify the degree of dissolution of the Al coupons in a solution of the corrodant for a given amount of time by computing the mass loss. This was achieved by first finding the initial weight of the Al specimens. Prior to this, the Al specimens were hand-polished using SiC emery paper, washed with detergent, degreased using acetone, then dried at room temperature. The initial weight 0 w was obtained for each coupon using an electronic balance having an accuracy of ±0.001 g. A Vernier caliper was used to measure the full dimension , l , b and t of each coupon. The total exposed surface area, c A for each coupon was computed using the formula: where r is the radius of the hole on each coupon which was ~1 mm.
The total volume V, for each coupon was computed using: The density (  ) of each coupon was obtained from the ratio of the initial weight, 0 w of the coupons to the volume V according to the following equation: The average experimental density of the Al alloy coupons was obtained to be ~2.724 g/cm 3 which is close to the theoretical value of 2.71 g/cm 3 . The 0.25 M of HCl solution was prepared using the dilution equation. The resulting solution recorded a pH of 0.4 on a pocketsized pH tester, while the preparation of the rosemary leaves extract was carried via the cold extraction method using methanol. For the preparation of the herb, 300 ml of methanol was introduced into the container containing 25.0 g of ground rosemary herbs. The resulting mixture was tightly sealed and then left for 24 h, after which it was sieved using a clean white handkerchief. The resulting filtrate served as the stock solution from which concentrations of The mass loss experiment was carried out by totally immersing the Al coupons into glass vessels containing about 250 ml of 0.25 M HCl solution both in the absence (blank experiment) and presence of the R. officinalis L. extract (inhibited experiment). A thermometer was used to regulate the bath temperature for both the blank and inhibited experiments at 303K, 313K, 323K and 333K. After an exposure time of 3 h, the coupons were undipped from their solutions, quenched in 2.0 M HNO 3 solution, rinsed in ethanol and degreased in acetone, and then left to air-dry. The final weight, f w obtained from the analytical weighing balance was used to compute the weight loss w in grams (g): In corrosion science, there is a fundamental quantity, the corrosion rate quantifies this degree of dissolution (Abdallah et al, 2013 ..
where 3 8.75 10  k ,  is the average density of all the coupons in g/mm 3 , A is the area of the Al coupon in mm 2 , and t is the exposure time in hours.
The inhibitor efficiency of the R. officinalis L. extract was computed using the equation: where 0 CR and inh CR are the corrosion rates in the absence and presence of inhibitor, respectively.
To study the effect of an external magnetic field on the inhibition behaviour of R. officinalis L. extract on the Al alloy, a magnetic field source with a variable voltage of up to 26.0 volts was used. The electromagnets were placed on either side of the glass vessel housing the Al coupons and HCl solution. The coupons were positioned transversally to the direction of the magnetic field as shown in Figure 2.
This experiment was performed at ambient temperature with input voltages of (Khan et al., 2015); Manimegalai and Manjula, 2015;and Barbut et al, 1985) volts in the presence of the inhibitor for 3 h, with the strength of a magnetic field, B directly proportional to the applied voltage, V according to the relation: where l R is the resistance per unit length of the solenoid wire, and l is the length of a turn.

Gravimetric measurements
The variation in the corrosion rate of Al at room temperature for an exposure time of up to 10 days is as shown in Figure 3. The figure shows a steady decrease in corrosion rate of Al in 0.25 M HCl and tends to take on a more linear relationship with increase in exposure time. This is in agreement with the results obtained by (Nwosu et al., 2018).

Effect of temperature
Temperature has a significant influence in the corrosion rate of metals (Ibisi and Uwakwue, 2015). This was verified for Al in 0.25 M HCl as shown in Figure 4 which shows a plot of values for the weight loss (g) obtained via Equation 4 for the blank experiment against an exposure time of 3 h. Figure 4 shows that the values of weight loss (g) increase with increase in temperature. The corrosion rates and efficiencies at all temperatures for both the blank and inhibited experiments in 0.25 M HCl solution were calculated and listed in Table 1.
On inspection of Table 1 it is observed that the rate of degradation of Al in 0.25 M HCl increases with increase in temperature, but decreases with increase in concentration of the rosemary extract. The highest efficiency (68.68%) was obtained at 1000 mg/L concentration of inhibitor at 303K. This suggests that ambient temperature is the optimal operating temperature for the R. officinalis L. extract in 0.25 M HCl solution, and a decrease in the inhibitor efficiency with increase in temperature may be due to the increased rate of desorption of molecules of the R. officinalis L. extract from the adsorbent surface. Figure 5 shows a plot of the corrosion rate at 0.25 M HCl solution against temperature.   On inspection of the Figure 5, a general decrease in corrosion rate was observed with increase in concentration of the R. officinalis L. leaves extract. However, the effect of temperature is such that it tends to increase the corrosion rate with the highest recorded corrosion rates being observed at T=333K. For the thermodynamic studies, the activation energy () a E is an important parameter in adsorption thermodynamic studies as it determines the temperaturedependence of the corrosion rate. It gives the minimum amount of energy required for the onset of an adsorbateadsorbent interaction (Saha and Chowdhury, 2011   is as shown in Figure 6.
Straight lines were obtained from the Arrhenius plots in Figure 6 with generally negative values for the gradient.
This means values of the activation energy a E obtained were generally positive as listed in Table 2. To obtain the standard activation entropies (  o S ) and enthalpies (  o H ) of the corrosion process, the Eyring transitionstate equation was employed (Abdallah et al., 2013;Manimegalai and Manjula, 2015):   found to be higher in the presence of R. officinalis L. leaves extract than in its absence. This suggests that the Aluminium dissolution process is endothermic (Lyubchik et al., 2011) and agrees with results obtained by Abdallah et al. (2013) and Ating et al. (2010).
Al surface is associative rather than Small values of Table 3. Some Adsorption isotherms and their parameters.

Model Isotherm Linear form Equation Reference
Langmuir With only a single activated complex exists in the slowest step of the reaction, that is, the rate determining step, this means the interaction between molecules of the inhibitor and that of the absorbent dissociative owing to the decrease in disorder. This result is similar to that reported by Abdallah et al. (2013).

Adsorption isotherms
Adsorption isotherms are simply graphs used for studying the nature and mechanism behind an adsorption process. The amount of adsorbate (R. officinalis L.) on the adsorbent is expressed as function of its pressure or concentration at constant temperature (Wilson et al., 1968). When molecules of the R. officinalis L. and the adsorbent Al metal are in contact long enough, an equilibrium is established between the adsorbed molecules on the adsorbent Al surface and the unadsorbed molecules of R. officinalis L. still left in the solution. This is the equilibrium relationship described by these isotherms (Lyubchik et al., 2011). The adsorption isotherm models are generally of the form (Abdallah et al., 2013): where ( , )  g is the configurational term parameter that characterizes the given isotherm model and its assumption(s) (Beda et al, 2017;Manimegalai and Manjula, 2015). The experimental data was fitted to the empirical models of the Lagmuir, Villamil, Freundlich, Temkin, and the kinetic-thermodynamic model of El-Awady. The equation for these models and all other parameters are defined in Table 3.

The Langmuir isotherm
According to literature by (Foo and Hameed, 2010)   where the value 55.5 is the concentration of water in the solution in mol/L. Values of the correlation coefficient was in the range 0.98 < R 2 < 0.99 as shown in Table 4 suggesting that the corrosion data for 0.25 M HCl was a good fit for the Langmuir isotherm model. A similar result was reported by (Deyab, 2016) using ethanolic extract of rosemary on Al in biodiesel, and (Zhao et al, 2017) and (Chaubey et al, 2018) using triazinedithiol inhibitors for Al in 1.0 M HCl and papaya peel extract on Al in 1.0 M HCl solution, respectively.
From Table 4, values of the slopes obtained were very close to unity suggesting that the molecules of R. officinalis L. form a monolayer on the Al alloy surface which is in agreement with the Langmuir model theory. Values of the equilibrium constant ( k ) were also observe to decrease with increase in temperature. This implies that the inhibitor efficiency decreases with temperature. negative, thereby signifying a spontaneous adsorption process (Ramachandran et al., 2011). Values for  o ads G ranged from -10.38 to -11.74 kJ/mol. This is indicative of physisorption. According to literature reported by (Ituen et al., 2017;Stango and Vijayalakshmi, 2018) values of 20kJ/mol  o ads G have typically been attributed to a weak Van der Waals or electrostatic forces of interaction between the molecules of the R. officinalis L. extract and active sites on the Al alloy surface.

The Villamil isotherm
This Villamil isotherm is a hybrid of the Langmiur isotherm model. It helps account for the shortcomings of the Langmuir model in that some isothermal slopes may deviate markedly from unity (Ituen et al, 2017) thereby indicative of a multilayer adsorption. Using the (Villamil et al., 1999) isotherm of Eqn (14), values of  C plotted against C are shown in Figure 9 below.
On observation of the plot in Figure 9, it was observed that the corrosion data for 0.25 M is a good fit for the Villamil model. This is supported by coefficient of correlation of correlation values (R 2 > 0.95) listed in Table  5.
According to literature reported by (Beda et al., 2017), the number of displaced water molecules from the adsorbent surface ( w n ) is the constant introduced to account for the limitations of the Langmuir model. Values of w n obtained from the slopes of the isotherms in Figure   10 were observed to increase with increase in temperature. This suggests that a multilayer adsorption predominates at elevated temperatures. The values for the equilibrium constant ( k ) obtained from the intercepts of the isotherms were observed to increase with increase in temperature. This could mean that the inhibitor efficiency increased with increase in temperature for the 0.25 M HCl solution.

The Freundlich, Temkin and El-Awady isotherms
It is clear from Table 5 that empirical models like the Freundlich and Temkin and the kinetic-thermodynamic isotherm of the El-Awady were not really a good fit for the experimental data with values of the correlation coefficient in the range (0.16 < R 2 < 0.98) for the Freundlich isotherm, (0.42 < R 2 < 0.95) for Temkin, and (0.44 < R 2 < 0.94) for the isotherm of El-Awady. Based on the results summarized in Table 6, the conclusion that values of the Gibb's free energy (   ads G ) obtained at elevated temperatures were low and more negative than the values at room temperature can be arrived at. This suggests that an increase in temperature increases the spontaneity of the adsorption process. from the Freundlich model were very close to zero. This means the adsorption process was favourable, relatively easy, and suggests that the adsorbent Al alloy surface becomes more heterogeneous with increase in temperature (Quinlan, 2015). Moreover, negative values for the interaction parameter () f suggest lateral repulsion taking place in the adsorbed layer (Bastidas et al., 2005). From the kinetic-thermodynamic model of El-Awady shows that values of y decreased with increase in temperature which means the number of R. officinalis Table 6. Calculated values of the Adsorption and Linear regression parameters of isotherm plots for the adsorption of Rosmarinus offinalis L. on to Al alloy.

Isotherm
Temperature ( Furthermore, values of 1 1  y ranged from 2 to about 11, thus signifying that a given molecule of R. officinalis L. occupies more than one active site. As depicted in Table 6, values of the adsorption constant (  k ) increased with increase in temperature indicating that the adsorption of the R. officinalis L. extract molecules unto the Al surface is favourable at elevated temperatures in the 0.25 M HCl solution.

The Dubinin-Radushkevich isotherm
In order to discriminate between physical and chemical adsorption (Beda et al., 2017;Foo and Hameed, 2010;Ayawei et al., 2017) at the adsorbate-adsorbent interface the Dubinin-Radushkevich isotherm model is used. From Table 3 Figure 10, values for the isotherm constant ( ), and the maximum surface coverage ( max  ) can be obtained from the slopes and intercepts of the isotherms, respectively. The experimental data for 0.25 M HCl was moderately good fit with values for the correlation coefficient in the range (0.58 < R 2 < 0.87) as listed in Table 7. Calculated values for the mean free energy per molecule ( m E ) of the R. officinalis L. extract increased with increase in temperature. The maximum surface coverage decreased with increase in temperature implying that the adsorption process was less favourable at elevated temperatures. Furthermore, values of m E plotted against T help discriminate between physical and chemical adsorption; according to the literature reported by (Beda et al., 2017), values of m E less than and greater than 8 kJ/mol suggest physisorption and chemisorption, respectively. Figure 11 shows that physisorption predominates at T=303K, while the adsorption of molecules of the inhibitor unto the adsorbent Al alloy surface solution takes a more chemical nature as the temperature approaches 313K and above. A similar result was reported in the literature by (Beda et al., 2017). From Table 7, values of max  generally decrease with increase in temperature. This confirms a decrease in the rate of adsorption with increasing temperature.

Effect of an external magnetic field
The aim of this experiment was to study the effects of an externally applied magnetic field on the corrosion Al alloy AA8011 by HCl both in the presence and absence of the R. officinalis L. extract. The corrosion data obtained under the influence of a magnetic field (B) in the 0.25 M HCl solution for the blank and inhibited experiments at 303K are listed in Table 8. Figure 12 shows that the results obtained in at lower concentrations of the R. officinalis L. extract below 250 mg/L reveal that the corrosion rate of Al increases in the presence of a magnetic field than in the absence of it. It appears that the presence of a magnetic field enhances the dissolution of Al in the 0.25 M HCl solution.  A similar result was reported in the literature by (Rousan and Al-Rawashdeh, 2006). However, above the 250 mg/L limit, the corrosion rate is suppressed in the presence of the magnetic field with increasing concentrations of the R. officinalis L. extract, this could mean that the presence of the magnetic field and an increase in the concentration of the inhibitor above 250 mg/L enhanced the passivation of Al. At concentrations of the inhibitor it was observed that an increase in the strength of the magnetic field (with increase in voltage) also suppresses the corrosion rate of Al. The effect of an external magnetic field was found to generally suppress the corrosion rate of the Al alloy at room temperature only for concentrations of the extract above 250 mg/L. Moreover, the corrosion rate was found to decrease with increase in strength of the magnetic field.