Milk Thistle Leaves Aqueous Extract as a New Corrosion Inhibitor for Aluminum Alloys in Alkaline Medium

The aqueous extract of milk thistle (Silybum marianum) leaves as a green corrosion inhibitor for AA7051 aluminum alloy in sodium hydroxide solution was investigated at a range of temperatures. Potentiodynamic polarization findings exhibit a mixed–type inhibitor with directly increased inhibition efficiency with the concentration of inhibitor. The adsorption of the inhibitor on aluminum alloy obeys Langmuir isotherm and the kinetic as well as thermodynamic parameters were measured and discussed.


Introduction
In the late 19th century, demand for aluminum was increased in the manufacturing of boats and cars, while the real revolution began in the early 20th century when it was used in aviation industry. However, aluminum applications in the second half of 20th century were not limited to its light weight but rather to corrosion resistance, electrical conductivity, thermal conductivity, easy fabrication, variable strength, and comparably cheap prices. Hence, the applications of aluminum in packaging, architecture, petroleum, and transportation were increased dramatically [1]. The contact of aluminum and aluminum alloy parts with alkaline solutions causes dissolution of the oxide layer that is naturally formed on aluminum surface and decreases corrosion resistance [2]. The corrosion can be eliminated be one of the following techniques: (i) development of new alloys that have more corrosion resistance in alkaline media; (ii) coating the aluminum with organic or inorganic coatings; (iii) inhibiting the corrosion with organic or inorganic inhibitors.

ISSN: 0067-2904
Al-Rudaini and Al-Saadie Iraqi Journal of Science, 2021, Vol. 62, No. 2, pp: 363-372 364 The applications of aluminum and its alloys restricted the type of techniques that can be used. First, the development of new alloy produces new properties that may not be suitable for certain applications; second, the coating is occasionally not ecofriendly or dissolved in concentrated alkaline; third, organic and inorganic inhibitors are either carcinogenic or expensive; and finally, the cost is an important factor, especially in industrial applications. These factors prompted researchers to find other methods that are appropriate to the industry's need.
The present article investigated the water extract of milk thistle (Silybum marianum) as a novel green corrosion inhibitor for AA7051 aluminum alloy in sodium hydroxide solution and studied the electrochemical behavior of corrosion at a range of temperatures. In addition, kinetic and thermodynamic parameters were investigated in order to gain insight to the mechanism of inhibition.

Materials and methods 2.1. Sample preparation
The locally produced AA7051 aluminum alloy was cut to small pieces with 2.5cm diameter and polished with emery paper of 400 to 4000 mesh grid. The alloy was then washed by distilled water, put in ultrasonic bath with acetone, rinsed in distilled water, dried, and finally kept in a desiccator. The alloy type was confirmed by measuring X-ray fluorescence (XRF, XEPOS spectrometer, Germany) and the results in Figure-1 show the percentages of AA7051 aluminum alloy elements [11].

Extraction method
The leaves of milk thistle (ML) were collected from a house garden in Baghdad city, cut to small pieces, rinsed in tap water, dried for three days, ground in food processer, and sieved. The fine powder was placed in an extraction thimble of 500ml soxhlet extractor with distilled water and heated to 140 o C for 7 hours. The extraction was transferred to a beaker and heated in an oven to evaporate the water, then the solid extraction was ground and kept in a closed flask for later use.

Electrochemical measurements
This electrochemical measurements were conducted by using potentiostat (Germany, MLab-2000) at 293, 303, 313, 323 and 333K with three-electrodes system of platinum (auxiliary sample), saturated calomel (reference sample), and aluminum alloy (working sample). The corrosion cell was filled with 1000mL of 0.01M sodium hydroxide with and without different amounts of extract (0.1, 0.2, 0.3, 0.4, 0.5g). After immersion in corrosive media for 15min, the open circuit potential (OCP) was measured

Surface characterization
The surface was characterized by Fourier transform infrared spectroscopy (FTIR) for the extract and the aluminum alloy sample immersed in alkaline solution containing the extract, after performing the corrosion test in the range of 400 to 4000 by Shimadzu 8400S, Japan, spectrophotometer.

Results and discussion 3.1. Corrosion parameters
The corrosion parameters were calculated from Tafel plot by MLabSci software associated with the potentiostat and by Excel using the following equations [12,13]: where IE% is the inhibition efficiency, and of inhibitor, respectively, is polarization resistance (Ω. ) and are anodic and cathodic slopes, respectively. The data obtained are listed in Table-1. Table 1-Electrochemical corrosion parameters for aluminum alloy in basic medium (pH=12) containing different concentrations of milk thistle extract at a range of temperatures. The data demonstrate a shift in open circuit potential (OCP) to more negative values with the addition of milk thistle extract. However, the open circuit potential was roughly independent upon the extract concentration. Furthermore, the addition of milk thistle extract considerably reduced the current of corrosion. This behavior shows its ability to prevent corrosion of aluminum alloy in NaOH medium [14]. The marginally dropped corrosion potential and dramatically increased corrosion current with increasing temperature are due to the increase in the rate of chemical dissolution of aluminum surface with temperature. In the same way, the weight loss (W.L) and penetration loss (P.L) were increased. After adding the inhibitor, the rate of corrosion was decreased and attained high inhibiting efficiency at 0.5g/L concentration of inhibitor.

Adsorption studies
In order to understand the mechanism of interaction of the inhibitor with metal surface and to calculate the thermodynamics adsorption parameters, surface coverage was calculated from the current of corrosion in the presence ( ) and absence ( ) of inhibitor, using the following equation [15]: The surface coverage Ɵ and inhibitor concentrations were substituted in Langmuir, Freundlich, and Temkin isotherms and the best fit with Langmuir isotherm was calculated by the following equation [16]: where is the equilibrium constant of adsorption obtained from the plot of against . As shown in Figure-2, the linear correlation ( > 0.98) assumes that the adsorption of milk thistle on the aluminum alloy agreed with this isotherm. Langmuir model suggests a uniform surface with no interactions between adsorbed molecules [17]. The thermodynamic parameters of adsorption were calculated by using the following equations [15,18]: = -2.303RT log (55.5 ) (5) Log = (6) where T (K) is the absolute temperature, R is the gas constant (8.314 J. . ), 55.5 is the concentration of water in the molar unit, and is the free energy of adsorption, is the standard adsorption enthalpy calculated from the slope of log versus 1000/T (Figure-3) and from the intercept standard entropy of adsorption calculated. The thermodynamic data represented in Table-2

Kinetics of corrosion
The influence of temperature upon corrosion behaviors of the 7051 aluminum alloy in 0.01M NaOH, in presence and absence of various concentrations of milk thistle extract was examined at a range of temperatures of 293-333K, using polarization measurements (Figure-4). The relative kinetic parameters are listed in Table-3. The findings indicate that the rates of the 7051 aluminum alloy corrosion, in non-inhibited and inhibited alkaline solutions, rose linearly with temperature due to the increase in passive film dissolution [20]. Nevertheless, Figure-5 exhibits the increase in inhibition efficiency with increasing temperature, which evidently confirms the effectiveness of milk thistle extract as corrosion inhibitor in the range of temperature of this investigation. The energy of activation for the corrosion process was calculated by using the following natural logarithm form of Arrhenius equation [21]: Ln = lnA -(7) where is the current density of corrosion, A is the Arrhenius pre-exponential factor, and is the apparent activation energy of the corrosion reaction. The apparent activation energy of the noninhibited and inhibited corrosion reactions was determined from the slope of the linear plot of ln versus 1/T (Figure-6). The values obtained are listed in Table-3 and exhibit that the values of aluminum alloy in alkaline solutions with milk thistle extract are lower than those of the alkaline solutions without milk thistle extract. The decline in the apparent activation energy values for alkaline solutions containing milk thistle extract compared with pure alkaline solutions, in addition to the noticeable increase in the efficiency of inhibition with the increase of temperature, assume that few milk thistle components would be chemisorbed on the aluminum surface [22]. The activation parameters were calculated using Eyring transition state equation . ), h is Planck constant, is the activation entropy, and is the activation enthalpy [24]. From the plot of ln( /T) versus 1/T in Figure-7, the and values were calculated from the slope (-/R) and the intercept (ln(R/ h) + ( )), respectively, and the observed data are listed in Table-

Al-Rudaini and Al-Saadie
Iraqi Journal of Science, 2021, Vol. 62, No. 2, pp: 363-372 368 positive signs, referring to mild endothermic dissolution reaction of aluminum under the conditions of this investigation [25]. The large negative values of activation entropy in the presence and absence of milk thistle extract exhibited that the activated complex in the rate-determining step denotes association instead of dissociation and that the disorder of the activated complex is reduced [26].

. Mechanism of inhibition
The milk thistle extract components adsorbed on the aluminum surface, presumably in monolayer thickness, which blocks the aluminum dissolution and discharge. Hence, the addition of extract to base solution does not considerably alter the corrosion potential, although the rate of corrosion dramatically decreased.
The main components of milk thistle extract are silybin, isosilybin, silychristin and silydianin, which contain a large and favorable polar group (OH-group), the size, shape, orientation and electric charge of which all participate in inhibition efficiency [16,27]. The increase of inhibition efficiency with temperature is probably because the adsorption increases or the film structure becomes more favorable at high temperature. FTIR measurement (Figure-8)  disappears, confirming the participation of the OH-group, which is supported by the absence of the N-H group. Table-4 shows the chemical structures of main milk thistle extract compounds. Table 4-Chemical structure of main milk thistle extract compounds [26,28].

Conclusions
1. The water extract of milk thistle acts as an admirable inhibitor for corrosion of 7051 aluminum alloy in 0.01 M NaOH media. The raise of the extract quantity present in the alkali solution increased the inhibition efficiency. 2. The inhibition process occurred as a result of adsorption of the milk thistle extract components onto the surface of the 7051 aluminum alloy. 3. This study assumes chemical adsorption on the surface of 7051 aluminum alloy based on increasing the efficiency of inhibition with temperature. 4. The FTIR measurements supported by the chemical structure of the milk thistle extract components showed that the hydroxide group is the main factor in the adsorption process.