Inhibition of Pipeline Steel Corrosion in 0.5 M H2SO4 Using Cotyledon of Chrysophyllum Albidum

The corrosion inhibition characteristics of Chrysophyllum albidum cotyledon extract was studied as a green and sustainable corrosion inhibitors for pipeline steel in acidic environment attemperatures, 303,313 and 323K using gravimetric technique.The results obtained showed that the inhibition efficiency increased with increase in extract concentration but decrease with increase in temperature. The extract attained an inhibition efficiency of 94 % with 5 g/L at 303 K and 52.2 % with 1 g/L at 333K. Also increase in concentration of the extract lead to increase in activation energydepicting an exothermic process.Fourier transform infrared spectroscopy (FT-IR) studies showed the mode of inhibition as adsorption of phytochemicals from the extract on pipeline steel surface. Langmuir adsorption isotherm best fitted into the adsorption process which is spontaneous and physical. DOI: https://dx.doi.org/10.4314/jasem.v24i4.22 Copyright: Copyright © 2020 Othaki and Ngobiri. This is an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dates: Received: 01 February 2020; Revised: 14 March 2020; Accepted: 22 March 2020

Corrosion of metallic structures has persistently been a source of down turn to the economy. However, the use of metallic materials for construction presently seems inevitable due to its physical properties and cost. Importantly, iron and its alloy, used for construction poses a great challenge for corrosion scientists and manufacturing industries. More so as a result of regular contact with mineral acids such as HCl, H2SO4, CH3COOH, etc. The exposed part of a metallic structure is more affected by external corrosion due to acidic effect of most industrial environment (Ejekeme et al., 2015). While the inner metallic surfaces are affected by the corrosive industrial fluids they convey (Ngobiri et al., 2013).
However, several methods of corrosion mitigation have been employed. The use of inhibitor to mitigate the metallic degradation process is a veritable one because of the ease of application in some difficult processes and areas. (Akalezi et al., 2015, Rajaet al., 2013, Loto and Mohammed, 2000.
The present work evaluates the corrosion inhibiting properties of the Cotyledon of Chrysophyllum albidum (CA) extract by gravimetric technique. The fruit is seasonal from December to April and has immerse economic potential, especially following the report that jams obtained from the fruit-pulp could compete with raspberry jams and jellies. Also the oil from the seed has been used for diverse purposes. In Nigeria, Chrysophyllum albidum is known as ''agbalumo' in South Western and ''Udara'' in South Eastern, Nigeria. It rich sources of natural antioxidants have been established to promote health by acting against oxidative stress related diseases such as diabetes, cancer and coronary heart diseases (Abiodun and Oladapo 2011,Akbariet al., 2012,Akaleziet al.2015. The GC-Mass spectra of the crude extract of the cotyledon was used to identify the various pytochemical components. The Chrysophyllum albidum cotyledon was used because of its unique phytochemical constituents which have bonding sites to metallic surface. Importantly the cotyledon is usually discarded arbitrarily as waste constituting environmental nuisance. The success of this project is anticipated to not only to aid in waste management but convert waste to wealth.

Materials:
The Pipeline steel was procured from System Metals Industries Limited, Port-Harcourt, Nigeria. The pipeline coupons were cut to a dimension of 2cm by 3cm used for weight loss measurement studies. All other reagents were of analytical grade.
Weight Loss Measurements:The pipeline coupons were cut to a dimension of 2cm by 3cm for weight loss measurement studies.The serial dilutions of the 0.5 M H2SO4 and extract concentrations were 1g/dm 3 , 2g/dm 3 , 3g/dm 3 , 4g/dm 3 , 5g/dm 3 and blank/control with no additive. The experiment was repeated at higher temperatures in a J.P. SELECTA 6001197 thermo stated water bath at 313 K and 323 K respectively.The tests were conducted in triplicate and the mean value of theweight loss obtained at each temperaturewas used to determine the following parameters.

=
(1) Where, W = weight loss of pipeline steel coupon, wi = Initial weight of the pipeline steel coupon, wf = Final weight loss of pipeline steel coupon, and all unit in grams.
The surface coverage (θ) CA-C in 0.5M H2SO4at different concentrations was obtained by using the equation below.

= 1 − (2)
Where; wo = corrosion rates in the absence of inhibitor; wf = corrosion rates in the presence of inhibitor.
The inhibition efficiencies were calculated using the formula % = 1 − X 100 (3) Where wo and wi is weight loss in grams of metal coupon in the presence and absence of various concentration of the extract respectively.
The total surface area of pipeline steel coupon immersed in the solution was calculated asfollows: A=2KM + Kt +2Mt +2∏rt -2∏r 2 (4) Where A = Total surface area of the coupon immersed in solution, K = length of coupon, M = Width of coupon, T=Thickness of coupon, R = Radius of the hole drilled on coupon.
The corrosion rate, half-life and rate constant were obtained as showed below.
Where; CR = Corrosion rate (mpy); ∆W = change in weight loss, D = density of specimen (g/cm 3 ), A =surface area of specimen cm 2 ) and T = exposure time (hours).  The figures clearly shows a reduction in weight loss of the metal coupons in the presence of extract compared to its blank. The figure further revealed decreases in weight loss of the pipeline steel as the concentration of the inhibitors increases. This trend reversed at higher temperature. The least weight loss was recorded at 5 g/L extract concentration and inhibition efficiency of 94 %. This behaviour of flora extracts in steel corrosion in acid environment has been previously reported (Madufor et al., 2013, Ejikeme et al., 2015. Notably, is the decrease in corrosion inhibiting capacity of the extract with increase in temperature from 303 to 333 K.This trend has been reported by researchers (Iloamaeke et al., 2013, Li et al., 2014. Thermodynamically, increase in temperature increases the kinetic energy of reacting entities. Therefore, it is expected to increase the rate of oxidation of the steel in acidic environment, reducing the efficacy of the extract in inhibiting corrosion. However, there is a balance of the increased oxidation of the iron in acid environment and the inhibitive action of the CA cotyledon extract at higher temperature.The constituents of CA extract contains phytochemicals such as alkaloids, terpenes, flavonoids, tannins, etc. These have fused benzene rings and heteroatoms in the rings, the chemical complexity of the extracts makes it difficult to assign the inhibiting action to a particular constituent. (Awizar et al., 2013, Li et al. 2014Nnanna et al., 2014). Increase in corrosion rate as the temperature increase has been attributed to desorption of the inhibitive molecules at higher temperature (Ngobiri et al., 2019). This corroborated with the surface coverage data in Table 1. The overall consequence is increased corrosion rate at higher temperature though the extract still shows active inhibitive presence. Adsorption Study: Table 1 indicates high degree of surface coverage of the corrosion inhibiting molecules. The mode of their action has been severally attributed adsorption on the metallic surface (Kalpana et al., 2003, Li et al., 2014. In order to determine a suitable mode of adsorption, the corrosion data was fitted into various adsorption isotherms. This is to determine the best fit.The linearity of the plot and good correlation coefficient may suggest that the experimental data for the studied extract fits better a particular adsorption isotherm but consideration of the deviation of the slope from unity shows that the isotherm may not be strictly applied. Several adsorption isotherms were tested, the corrosion of pipeline steel in 0.5 M H2SO4 in the presence of CA-cotyledon extract fitted more to Temkin and Langmuir isotherms.

RESULTS AND DISCUSSION
Temkin adsorption isotherm which assumed a uniform distribution of adsorption energy with increase in surface coverage is given by equation (Bolaji, 2012).
Where: a, is the molecular interactions of the adsorption layer and heterogeneity of the surface of the coupons; Kads is the equilibrium constant, C is the concentration of the extracts, θ is the degree of surface coverage. Plots of surface coverage (θ) against InC at 30 O C,40 O Cand 60 O C are shown in Figure 6and coefficients (R 2 )value greater than 0.835 in H2SO4 are

OTHAKI, EP; NGOBIRI, NC
shown in Table 2. These suggest some measure of conformity of the data to the Temkin adsorption isotherm model. The values of molecular interactions are all greater than zero (›0) which suggest lateral interaction between the pipeline steel and the extract.   Where: K is the equilibrium constant, C is the concentration of the extracts and θ is the degree of surface coverage. The CA cotyledon extract fitted into the Langmuir adsorption isotherm with correlation coefficient (R 2 ) value of 0.999 which shows a better fit than Temkin isotherm, as shown in Table 3. The values of∆G o adsare all negative ranging from -10.533 and -10.797kJmol -1 in H2SO4 medium. The values of ∆G o ads are often used to suggest either chemisorptions, physisorption or both adsorption modes. Physisorption is linked to electrostatic interactions between charged extracts molecules and charged metal surface, while chemisorptions has to do with charge sharing between the metal surface and the extracts molecules resulting in special kind of coordinate bond (Li et al.,2012). A value of ∆G o ads around -20kJmol -1 or less implies physisorption while around -40kJmol -1 or greater suggest chemisorptions.
However the values of ∆G o adsobtained as showed in Table 4are less negative than -20kJmol -1 , suggesting physiosorption mechanism. Plots of ∆G o ads against T are shown in figure 10    were obtained from the equation below:

12
Where Ea represent activation energy, A, frequency factor, R gas constant and T, absolute temperature, ∆H, * enthalpy of activation, ∆S * entropy of activation.
The value of Ea were obtained from the plot of logCR against 1/T as shown in figure 10, while ∆S * and ∆H * where gotten from the intercept and slope of the plot of log CR/T against I/T shown in figure 11. Generally, physisorption is noted when Ea and ∆H * value in the presence of extract is greater than in the absence of extract while opposite is for chemisorption.
From Table 5, it is observed that Ea and ∆H * values in the presence of extract are greater than in the absence validating that physiosorption occur against chemisorption. The activation energy calculated show an increase with increased concentration compared to the blank. The value of ∆S * were noted to be negative both in the absence and presence of the extract. The negative value of energy of activation for inhibited system has already being reported with hydrogen evolution reaction by Zhang QB et al. 2009.With increase in negative value of the activation energy in the presence of extract, which increased as the concentration increased (Table 5).The negative value of ∆S * signified association rather than dissociation step of activated complex in the rate determining step, signifying a reduction in the degree of disorderliness. (Edidiong et al., 2018).    Conclusion: Chrysophyllum abldum cotyledon extracts was tested for inhibition of pipeline steel corrosion in 0.5M H2SO4 acid solution. The results obtained showed good corrosion inhibition activity of the extract. The inhibition values increases with increasing concentration of Cotyledon extractbut decreased as the temperature increase. The FT-IR studies showed the presence of functional groups such as O-H, N-H and C=O was adsorbed on the surface of pipeline steel. The adsorption of inhibitor molecules on pipeline steel obeys the Langmuir adsorption isotherm. The CA cotyledon extract acted as a good inhibitor of pipeline steel corrosion in 0.5 M H2SO4 solution.