Corrosion is degradation of materials’ properties due to interactions with their environments, and corrosion of most metals (and many materials for that matter) is inevitable. While primarily associated with metallic materials, all material types are susceptible to degradation [1] . Degradation of polymeric insulating coatings on wiring has been a concern in aging aircraft. Even ceramics can undergo degradation by selective dissolution. Like death and taxes, corrosion is something we hope to avoid; but ultimately it is something we must learn to deal with [2] . The fundamental cause or driving force for all corrosion is the lowering of a system’s Gibbs energy [3] .

There are basically five methods of corrosion control, these are: material selection, coating, inhibitors, design and cathodic protection. For the sake of this research, the method of corrosion control employed is the use of inhibitors. Corrosion inhibitors are substances, which when added to a solution, reduces the rate at which a material mostly metals deteriorate [4] . Corrosion inhibition is often attained by the formation of a coating of the inhibiting material on the surface of the metal. For example, Benzotriazole inhibits corrosion of copper by forming an inert layer of this polymer on the metal’s surface [5] .

The study of the mechanism of corrosion and corrosion inhibition of Tin in aqueous solutions containing tartaric acid was conducted by Rabab and Waheed [6] . They discovered that with only the solution of tartaric acid, the dissolution of the tin was enhanced, but an introduction of about 0.02 mol∙dm⁻³ of a naturally occurring glycine inhibits the rate of corrosion by 30%.

Onen, et al. [7] studied the inhibition properties of titanium oxide for aluminium and mild steel using the method of absorbance difference. They found that there is an increase in percentage inhibition as the concentration of the titanium (IV) oxide also increases. They attributed the inhibition efficiency of the titanium (IV) oxide to the presence of lone pair of electrons on the oxygen which is delocalized and provides resonance stability to the compound [7] .

Organic inhibitors are the most widely used nowadays, owing to the fact that they are environmentally friendly. The organic type inhibitors studied are the azoles and their derivatives, imidazoles, amines and their derivatives, amino acids―which, are considered the most non toxic organic compounds, highly soluble in aqueous media and produced with high purity and at low cost; triphenylmethane derivatives, thiol group compounds, organic derivatives of phosphates such as inositol hexaphosphates, potassium ethyl xanthate, etc. [8] .

Dioctyl phthalate (DOP) with IUPAC nomenclature dioctyl benzene-1,2-dicarboxylate, is a clear, colourless, and viscous liquid with a slight, characteristic odor, soluble in ethanol, ether, mineral oil, and the majority of organic solvents, immiscible with water, resistant to hydrolysis and air oxygen activity. The major applications of DOP are found in plastic industry for plasticizing plastic, wood coatings, manufacture of medical and sanitary equipments such as blood bags and dialysis equipments and it is also applicable in the manufacture of capacitors as dielectric fluid, hydraulic liquid and solvent in light stick. It has a molecular weight of 390.6 gmol⁻¹, with a specific gravity of 0.98 at 25˚C [9] .

This particular compound was selected for this research because, since most of the experiments involving corrosion studies was done using plant extracts. Attributing the inhibiting effect to one compound in the extract is difficult, hence posing difficulty in studying the mechanism of the adsorption. With a pure compound, which is readily available, studying the inhibiting properties will open doors for its application in industries.

The method employed for the study of the inhibiting properties of Dioctyl phthalate in HCl is the weight loss method [10] . Coupons of mild steel of size 5 cm by 4 cm were used, and concentration of the inhibitor ranges from 0.01 mol∙dm⁻³ to 0.07 mol∙dm⁻³, in 1.0 M HCl [10] . The immersion time was 1 - 3 hrs and temperature ranges of 303 K to 313 K.

Determination of Parameters

The percentage inhibition efficiency %IE, the surface coverage θ, and the corrosion rate C_r were determined from Equations (1)-(3) respectively.

% IE = [ 1 − ( W 0 − W t W ) ] × 100 (1)

θ = 1 − ( W 0 − W t W ) (2)

where W₀ = initial weight of the mild steel before suspending in the solutions, W = weight of the mild steel without the inhibitor, that is, weight in blank solution, and; W_t = weight of the mild steel after inhibition the inhibitor

C_r (mg∙cm⁻²∙hr⁻¹) = W 0 − W t A T (3)

where A = surface area of the coupon (cm²), T = time of exposure in hrs, W₀ ? W_t = weight loss.

The inhibitor efficiency gives important information about the performance of the inhibitor in various medium [11] .

The activation energy of adsorption E_ads, Enthalpy and entropy of adsorption H_ads and S_ads, and free energy change of adsorption ΔG_ads were determined from the following Equations (4)-(6) respectively.

ln C r = ln A − ( E ads / R T ) (4)

where C_r = rate of corrosion, A = a frequency factor, R = Gas constant (Atm. L. mol⁻¹∙K⁻¹), T = temperature (K).

A plot of lnC_r against 1/T, gives a straight line, with slope equals to E_ads/R and an intercept of lnA. [12] . The values of E_a greater than 20 kJ indicate that the adsorption process is controlled by the surface reactions [13]

ln C r T = ln R N h + ( Δ S ads R ) − ( Δ H ads R T ) (5)

where R = gas constant, N = Avogadro’s number 6.03 × 10²³ mol⁻¹, h = the Planck’s constant 6.62 × 10⁻³⁴ Js

A plot of ln C_r/T against 1/T is a straight line with a slope equal to Δ H ads R T and the intercept of the graph is equal to ln R N h + ( Δ S ads R ) . The values of ∆H_ads indicates whether the adsorption process absorbed heat (+∆H) or releases heat (−∆H) [14]

∆G_ads = −RT (ln55.5 K_ads) (6)

where K_ads is the equilibrium constant for the adsorption of the inhibitor on mild steel surface [15]

Other parameters determined are the rate constant for the adsorption process k, and the half life of the metal in the presence of the inhibitor according to Equation (7) and Equation (8).

lnw_f = lnw_i − kt (7)

where w_i and w_f are respectively the weights of the mild steel in blank acid solution and after immersion in the inhibitor solution. This value tells how fast the adsorption process is taking place. A plot of ln(w_i/w_f) against t, gives a straight line with a slope which is equal to k.

t 1 / 2 = ln 2 k (8)

where t_1/2 = half-life of the mild steel.

Freundlich and Langmuir adsorption isotherms (Equation (9), and Equation (10)) were used to model thee adsorption process.

Freundlich log θ = log k + n log C (9)

Langmuir ( C θ ) = 1 k + C (10)

where k is the equilibrium constant, C is the concentration of the inhibitor, n is a constant which tells the intensity of the adsorption process, and have a typical value of 0.6 [16] .

The variation of the rate of corrosion of mild steel at different concentration of inhibitor solution in 1.0 M HCl solution as the corrodant and at different temperatures is shown in Figures 1-3. The Figures show the effect of exposure time, and the concentration of the inhibitor on the corrosion rate of the mild steel. From the Figures, it is clear that corrosion rate decreases with increase in concentration of the inhibitor and decrease with time of exposure to the inhibitor solution [17] .

The effect of temperature is shown in Figure 4. Corrosion rate increases with increasing temperature from 303 K to 313 K, and the corrosion rate decreases with increase in the concentration of the inhibitor. This is because as the concentration is increasing, more molecules of the inhibitor become available to

cover the surface of the mild steel, preventing it from coming in contact with the corrodant.

1) Thermodynamic Considerations

The activation energy E_ads of the adsorption process is represented in Table 1. Generally, higher values of E_ads, in presence of additives support physical adsorption mechanism whereas an unchanged or lower value of E_ads for inhibited systems compared to the blank is indicative of chemisorption mechanism. From Table 1, it can be seen that the values of the activation energies for the inhibited systems increases from 82.96 kJ for the uninhibited system to a maximum of 162.54 kJ for the inhibited system. This is an indication of a physisorption (a process in which the electronic structure of the adsorbed molecule is barely disturbed on adsorption) mechanism of adsorption [18] .

The dissolution process of the mild steel is considered difficult if the values of the enthalpy of adsorption ∆H_ads for the corrosion process in the blank and inhibited solution is found to be positive, indicating an endothermic process [19] . From Table 1, it can be seen that all the values of the ∆H_ads are positive, hence the dissolution of mild steel in the solution of the inhibitor is difficult. The entropy of the inhibition process ∆S_ads, as shown in Table 1, a higher value of the entropy obtained in the presence of the inhibitor, showed that the recombination step is a more orderly arrangement compared to the initial state without the inhibitor [14] . It can be seen from Table 1 that the values of Q_ads are negative throughout and decreases as the concentration of the inhibitor decreases. This indicates that the adsorption of the inhibitor to the surface of the mild steel is spontaneous, and this is indicative of a strong interaction between the inhibitor and the mild steel [20] .

It was also discovered that the values of E_ads increases with increase in the concentration of the inhibitor. The values of E_ads and those of ∆H_ads varies in a similar manner, and the average values of E_ads − ∆H_ads equal to 2.41 kJmol⁻¹, which is very close to the value of RT (2.51 kJmol⁻¹) indicating that the adsorption process is unimolecular in nature [21] .

2) Adsorption Studies

The isotherms selected for the adsorption study are Langmuir and Freundlich. The isotherm parameters were calculated from Equation (9) and Equation (10) for Freundlich and Langmuir isotherm respectively. The values obtained are represented in Table 2.

The adsorption equilibrium constant K_ads decreases with increase in experimental temperature in the corrosion media (Table 2), indicating that the interactions between the adsorbed molecules and the metal surface are weakened and consequently, the adsorbed molecules could become easily removable. Such data explains the decrease in the inhibition efficiency with increasing temperature [22] . The positive adsorption equilibrium constant K_F values are indication of favourable adsorption [23] . The parameter n_F in the Freundlich isotherm relates to intensity of adsorption and it varies with heterogeneity of the material [24] ,

Key: K_L=equilibrium constant for Langmuir isotherm, K_F = equilibriumconstant for Freundlich isotherm, n_F= adsorption intensity.

and the value is always positive, but not an integer, with typical value of 0.6 [25] . The average value of n_F is 0.717 for the inhibitor in 1.0 M HCl, a value not far from 0.6, an indication that this adsorption process can reasonably be modelled by Freundlich isotherm and is heterogeneous [23] .

3) Standard Free Energy of Adsorption Δ G ads o

The standard free energy of adsorption, Δ G ads o , which can characterize the interaction of adsorbed molecules and metal surface, was calculated using Equation 11. The values of K_ads were determined from the different isotherm plots, and are represented in Table 3. The negative values of Δ G ads o ensure the spontaneity of adsorption process and stability of the adsorbed layer on the aluminium surface. Generally, the values of Δ G ads o below −40 kJ/mol are consistent with physisorption, while those above −40 kJ/mol involve chemisorptions mechanism [26] . From Table 3 all the values of the standard free energy of adsorption are negative. This indicates that the adsorption process is spontaneous. The values also, were found to be below −40 kJ/mol; which concur with the physisorption mechanism earlier proposed for the adsorption process in 1.0 M HCl/inhibitor solutions [27] .

4) Kinetic Studies

The corrosion of mild steel follows a first order rate law with respect to the concentration of the inhibitor [28] . A decrease in the rate constant k, with

increase in temperature indicates that the rate of the mild steel corrosion is greatly reduced in the presence of the inhibitor (dioctyl phthalate) in 1.0 M HCl medium. The half-life calculated from Equation (8), and presented in Table 4, indicated that the life span of the mild steel is increased as the temperature increased.

The inhibiting effect of dioctyl phthalate on the corrosion of mild steel was carried out in 1.0 M HCl solution and at a temperature range of 303 to 313 K using the weight loss method. The results obtained, showed that dioctyl phthalate is a good corrosion inhibitor of mild steel. The efficiency was found to decrease with increasing temperature and increase with increase in the concentration of the inhibitor in the media. The adsorption of the inhibitor on mild steel was found to be spontaneous. The most suitable isotherm to be used for modelling the corrosion inhibition studies of the dioctyl phthalate is the Freundlich adsorption isotherm. Physisorption mechanism was proposed for the adsorption of dioctyl phthalate in 1.0 M HCl solution on mild steel. It can therefore be recommended that more research should be carried out on the properties of dioctyl phthalate in corrosion inhibition using other methods, other than the weight loss method.

We wish to thank Gombe State University for providing the reagents for the research, and University of Jos for providing the Laboratory space and equipments required for the research work.

The authors declare no conflicts of interest regarding the publication of this paper.

Emmanuel, M., Nwufo, B.T. and Ekwenchi, M.M. (2018) Inhibiting Effect of Dioctyl Phthalate on the Corrosion of Mild Steel in 1.0 M Hydrochloric Acid Solution. Open Access Library Journal, 5: e4927. https://doi.org/10.4236/oalib.1104927