¹ Department of Chemistry, Ahmadu Bello University, Zaria, Kaduna State, Nigeria

² Department of Chemistry, Akwa Ibom State University, Ikot Akpaeden. Mkpat Enin, Akwa Ibom State, Nigeria

Abstract

Gas chromatography mass spectrophotometer (GCMS) analysis of Ficus tricopoda gum indicated the presence of 4.75, 56.15, 32.10 and 7.00 % of camphene, sucrose, 2-methylene cholestan-3-ol and 7-hexadecenal, respectively. Several stretching and bending vibrations were observed in the Fourier transformed infra-red (FTIR) spectrum of the gum. Physicochemical examinations of the gum revealed that it is pale yellow in colour, mildly acidic, ionic and display characteristics of sour taste. The solubility of the gum in water was found to increase with increase in temperature. Knowledge of the chemical constitution of the gum (hence chemical structures of its constituents) was useful in predicting the corrosion inhibition potential of Ficus tricopoda (FT) gum. Consequently, the gum was found to be a good adsorption inhibitor for the corrosion of aluminum in solution of H₂SO₄. The adsorption of the gum (which, followed first order kinetic) was found to be endothermic at FT gum critical concentration of 0.3 g/L and exothermic at concentrations above the critical limit. The short coming of the Langmuir adsorption model in describing the existent of interaction between the molecules of the gum was complimented by the Frumkin and Dubinin-Radushkevich adsorption models. Calculated values of activation and free energies of activation indicated that the adsorption of Ficus tricopoda gum on Al surface exhibited both physical and chemical adsorption mechanism.

Keywords: corrosion, aluminum, inhibition, Ficus tricopoda gum.

Introduction

Corrosion inhibition is essential in combating the negative environmental and industrial impact of corrosion; being one of the most effective methods of preventing metal in petroleum, fertilizers, metallurgical and other industries, corrosion inhibition is concern with the use of chemical substances that can retard the rate of corrosion of a metal in contact with an aggressive medium [12]. Several researches have been successful in the discovery of broad ranges of inhibitors for the control of corrosion [3-5]. However, current challenges are based on utilization of inhibitors that are environmentally friendly, easy available, cost effective and biodegradable [6]. These conditions are hardly fulfilled by most inorganic inhibitors such as chromate and heavy metal rich compounds, hence current researches are directed towards the use of green inhibitors for the inhibition of the corrosion of metals [7-9]. On the list there are extracts of plant and animal and some gum exudates [10-17].

Available literature revealed that most corrosion inhibitors are chosen through knowledge of their chemical structures [18]. Generally, organic compounds that contain either hetero atoms, aromatic ring, π-electron, or long carbon chain are found to be effective corrosion inhibitors [19]. Therefore, the present study is aimed at investigating the physicochemical parameters of FT gum and its potential as a green inhibitor for the corrosion of aluminum in HCl medium.

Materials and methods

Samples of Ficus tricopoda (FT) gum were obtained as dried exudates from their parent trees grown at Samaru, Zaria in Sabon Gari LGA of Kaduna State of Nigeria. The outer bark of the tree was broken using a small axe. The cut was widened upward and downward and the gum formed was collected. The crude gum was purified through dissolution in cold distilled water. The solution was strained through muslin and centrifuged to obtain a small quantity of a dense gel. The straw coloured supernatant liquor obtained was separated and acidified to a pH of 2 with dilute hydrochloric acid. Ethyl alcohol was added until the liquor was 80 percent alcohol. The gum precipitated out was removed by centrifugation at a rate of 2000 revolution per minutes, washed with alcohol, ether and dried in a desiccator [20].

Determination of physicochemical properties of the gum

The pH of the gum was determined using a pre calibrated Oklon pH meter. The solubility of the gum was determined in cooled and hot distilled water, acetone and chloroform using the method reported by Carter [21].

Corrosion studies

Aluminum alloy sheet of composition (wt. %, as determined by quantiometric method) Mn (1.28), Pb (0.064), Zn (0.006), Ti (0.029), Cu (0.81), Si (0.381), Fe (0.57) and Al (96.65%) was used. The sheets were mechanically pressed and cut into different coupons, each of dimensions, 5 × 4 × 0.11 cm. Each coupon was degreased by washing with ethanol, cleaned with acetone and allowed to dry in the air before preservation in a desiccator. All reagents used for the study were analar grade and double distilled water was used for their preparation.

Weight loss and hydrogen evolution experiments were carried out as reported elsewhere [18]. From weight loss measurements, inhibition efficiency, corrosion rate and degree of surface coverage were calculated using the following equations

where W₁ and W₂ are the weight losses (g) for aluminum in the presence and absence of the inhibitor, θ is the degree of surface coverage of the inhibitor, ΔW = W₂ - W₁, A is the area of the aluminum coupon (in cm²), t is the period of immersion (in hours) and ΔW is the weight loss of aluminum after time, t. In gasometric experiment, the test solution was poured into the reaction vessel.

Upon the introduction of mild steel, the flask was quickly corked and the rise in volume of the paraffin due to hydrogen evolution was noted after every minute until a steady volume was observed. Inhibition efficiency was calculated as the quotient of the difference between the volume of hydrogen evolved by the blank and that of the test solution to the volume of hydrogen evolved by the blank.

FTIR analysis

FTIR analysis of the gum was carried out and that of the corrosion products (in the absence and presence of the gum) were carried out using a Scimadzu FTIR8400S Fourier transform infra-red spectrophotometer. The sample was prepared in KBr and the analysis was carried out by scanning the sample through a wave number range of 400 to 4000 cm^-1.

GCMS analysis

GCMS analysis was carried out on a GC Clarus 500 Perkin Elmer system comprising of an AOC-20i auto-sampler and gas chromatograph interfaced to a mass spectrometer (GCMS) instrument employing the following conditions: column Elite-1 fused silica capillary column (30 × 0.25 mm ID × 1 μM df, composed of 100 % dimethylpoly diloxane), operating in electron impact mode at 70 eV; helium (99.999 %) was used as carrier gas at a constant flow of 1 mL/min and an injection volume of 0.5 μL was employed (split ratio of 10:1) at an injector temperature of 250 °C; ion-source temperature of 280 °C. The oven temperature was programmed from 110 °C (isothermal for 2 min), with an increase of 10 °C/min, to 200 °C, then 5 °C/min to 280 °C, ending with a 9 min isothermal at 280 °C. Mass spectra were taken at 70 eV; a scan interval of 0.5 seconds and fragments from 40 to 450 Da. Total GC running time was 36 min. Interpretation on mass spectrum GCMS was conducted using the database of the National Institute Standard and Technology (NIST) Abuja, having more than 62,000 patterns. The spectrum of the unknown component was compared with the spectra of the known components stored in the NIST library. The name, molecular weight and structure of the components of the test materials were ascertained. The concentrations of the identified compounds were determined through area and height normalization.

Results and discussions

Physiochemical parameters of FT gum

The measured pH of the gum (4.10) revealed that the gum is acidic and is characterized with a sour taste. The solubility of the FT gum in water tends to increase with increase in temperature, indicating that the heat given off in dissolving the gum is less than the heat required to break the gum apart. The net dissolution reaction is endothermic (energy required). Therefore, addition of more heat facilitates the dissolution of the gum by providing energy to break bonds within the gums. The gum was also found to be slightly soluble in ethanol but insoluble in acetone and chloroform. The solubility of the gum in ethanol may be due to the presence of polar and non-polar ends in ethanol, which made it to dissolve some polar and non-polar compounds. On the other hand, chloroform and acetone are non-polar solvents and as expected, non-polar compounds are soluble in non-polar solvent and vice versa. Hence FT gum is ionic and it is expected to be insoluble in chloroform and acetone.

GCMS study of FT gum

A close examination of the spectra shows that there are various peaks to the right of 300 cm^-1 indicating the presence of alkyl groups (in most organic compounds). A peak was also found to the left of 300 cm^-1. This may be attributed to C=C bond due to camphene or 7-hexadecenal. The presence of sucrose in FT gum is correlated with the presence of cyclopentanone at 1745 cm^-1 and several OH vibrations. OH stretch due to alcohol was also found at 3509.60 cm^-1. This may be due to the presence of 2-methylen cholestan-3-ol in the gum. C-H stretches due to alkanes were also found at 2856 and 2923 cm^-1. Also at 1589 cm^-1, C-C stretch due to ring was also significant in the spectrum. At peak frequencies of 1454.38 and 1377.22 cm^-1 , C-H bend and C-H rock due to alkanes were observed. Several C-O vibrations were also found between the wave number range of 1071 to 1281 cm^-1. Finally, between the wave number range of 647 and 985 cm^-1, C-H bend, C-H rock and C-H out of plane vibrations were eminent.

Corrosion study

Effect of FT gum on the corrosion of aluminum

From the plots, it was deduced that weight loss of Al increases with increase in the period of contact but decreases with increasing concentration of FT gum. These imply that FT gum retarded the corrosion of Al in solution of HCl and that FT gum is an adsorption inhibitor for the corrosion of Al.

The results obtained revealed that the inhibition efficiency of FT gum increases with increasing concentration and with increase in temperature at FT gum concentrations above 0.3 g/L. Based on literature, it can be inferred from the pattern of variation of inhibition efficiency with temperature, that two mechanisms of adsorption are possible. A physical adsorption mechanism applied when the inhibition efficiency decreases with increase in temperature, while a chemical adsorption mechanism applied when the inhibition efficiency decreases with increase in temperature [8]. From the calculated values of inhibition efficiency, it can be seen that the mechanisms of physical and chemical adsorption are clearly indicative at lower and higher concentrations of the inhibitors. Hence the mechanism of adsorption of FT gum on Al surface is concentration and temperature dependent.

Kinetic study

Corrosion of most metals including aluminum has been confirmed to be a first order reaction [22]. Hence the rate of corrosion can be represented as follows [23],

The half-life of a first order reaction (t_1/2) is related to the rate constant according to the following equation [22],

The activation energies are within the range of values expected for the mechanism of physical adsorption and tend to increase with increase in the concentration of FT gum. This implies that the mechanism of inhibition of Al corrosion by FT gum is concentration dependent.

It is also significant to note that the activation energy calculated for the blank is higher than those calculated at FT gum concentrations of 0.1 to 0.3 g/L and vice versa. This implies that less energy is needed for the adsorption of the inhibitor compared to the corrosion of the metal. On the other hand, above FT gum concentration of 0.3 g/L, calculated values of the activation energy were higher than the corresponding value for the blank but lower than the threshold value required for chemisorption. This suggests that at this concentration, the diffusion of the inhibitor's molecules (and not the energy of activation) may be the rate limiting process.

Thermodynamics and adsorption considerations

Adsorption of FT gum on Al surface can be exothermic or endothermic depending on the reaction condition, the nature of FT gum's molecules and the mechanism of reaction. In order to calculate the heat of adsorption of FT gum on Al surface, an established thermodynamic equation was used [25]:

where f(θ, x) is the configurational factor which depends upon the physical model and the assumptions underlying the derivation of the isotherm, θ, the surface coverage, C, the inhibitor concentration in the electrolyte, 'x' is the size factor ratio, 'a' is the molecular interaction parameter and 'b' is the equilibrium constant of the adsorption process. In this study, calculated values of θ at various concentrations of the inhibitor were fitted into various adsorption isotherms including Langmuir, Temkin, Freundlich, Flory-Huggins, Bockris-Swinkels and Frumkin isotherms. The test indicated that the adsorption of FT gum best fitted the Langmuir and Frumkin adsorption isotherms.

The Langmuir adsorption isotherm relates the degree of surface coverage to the concentration of the inhibitor according to the following equation [27],

Also values of R² were very close to unity but slope values were not equal to unity as expected in the Langmuir model indicating the existence of interaction between the inhibitor's molecules.

In order to account for the existence of molecular interaction, the Frumkin adsorption model was found to be applicable to the adsorption of FT gum on Al surface. The Frumkin adsorption isotherm can be expressed as follows [28],

where q_max is the maximum surface coverage and σ is the polany potential and can be estimated from the following equation,

Straight lines were obtained from plots of ln(q) versus σ² indicating the application of the D-RIM isotherm to the adsorption of FT gum on Al surface.

R² values for the plots were 0.8450 and 0.9636, while E values were 11.18 and 8.55 kJ/mol. From the calculated results, chemisorption mechanism is significant in the inhibition of the corrosion of Al by FT gum.

The free energy of adsorption of FT gum on Al surface was estimated using the Gibb-Helmoltz equation, which relates the adsorption equilibrium constant with the free energy of adsorption as follows [30 - 31],

where ΔG⁰_ads is the standard free energy of adsorption of FT gum on Al surface.