ewemen journal of chemical kinetics

© 2016 Ewemen Resources Limited / EJCK. All rights reserved

2016 | Vol. 1 | Issue 1 | Pg. 21 - 30

Ewemen Journal of Chemical Kinetics ISSN: 2504-8635

Available online at http://ewemen.com/category/ejck/

Full Length Research

CORROSION INHIBITION AND ADSORPTION BEHAVIOR OF LONCHOCARPUS LAXIFLORUS EXTRACT ON MILD STEEL IN HYDROCHLORIC ACID

*Ijuo G.A., Chahul H.F., Eneji I.S.

Department of Chemistry, University of Agriculture, P.M.B. 2373 Makurdi, Nigeria

ABSTRACT

Received 13 June, 2016 Revised on the 17 June, 2016 Accepted 20 June, 2016

*Corresponding Author’s Email: [email protected], [email protected]

Corrosion inhibition and adsorption behaviour of Lonchocarpus laxiflorus (LL) extract on mild steel in 1.0 M HCl was carried out using weight loss and electrochemical linear polarization measurements. It was found that LL extract retarded the corrosion of mild steel in the acid solution. The inhibitive ability of LL extract was found to be enhanced in the presence of iodide ions (I-). Linear polarization data suggested that the extract acted as a mixed inhibitor. The values of activation energy (Ea) obtained is suggestive of physical adsorption mechanism while the values of Gibbs free energy (∆G0) obtained indicated a spontaneous adsorption of the extract components on the metal surface. The adsorption of LL extract onto the mild steel surface followed Langmuir and Fruendlich adsorption isotherm models. Kinetic treatment of the data followed a pseudo-first order reaction. FTIR results and optical micrographs also revealed that LL extract adsorbed effectively on the mild steel surface. Keywords: Mild steel, Corrosion inhibition, Adsorption isotherm, Lonchocarpus laxiflorus, Iodide ion, Synergism.

INTRODUCTION

Metals in service often give a superficial impression of permanence, but all except gold are chemically unstable in air and air-saturated water at ambient temperatures and most are also unstable in air-free water. Hence almost all of the environments in which metals serve are potentially hostile and their successful use in engineering and commercial applications depends on protective mechanisms. Corrosion occurs when protective mechanisms have been overlooked, break down, or have been exhausted, leaving the metal vulnerable to attack (Talbot and Talbot, 1998). Mild steel is one of the best preferred construction material for industries due to its availability and mechanical properties, however, it is highly prone to corrosion and expensive to replace.

Among the several methods of corrosion control and prevention such as material selection, coating, cathodic protection, etc., the use of corrosion inhibitors is very popular (Dean, 2003). Corrosion inhibitors are substances which when added in small concentrations to corrosive mediadecrease or prevent the reaction of the metal with the media (Ambrish et al., 2011). In recent times, considerable effort has been devoted to study the corrosion inhibiting efficacy of some natural products, particularly biomass extracts. The reason for this is not farfetched. The abundant phytochemical constituents of biomass extracts have considerable potentials as inexpensive, non-toxic, readily available and renewable sources of a wide range of organic chemicals of prospective industrial significance. A number of the phytochemical constituents of plant

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Ewemen Journal Chemical Kinetics 2016, 1(1): 21 - 30 Ijuo et al.

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biomass (including alkaloids, tannins, amino acids carbohydrates, etc.) have molecular and electronic structures bearing close similarities with those of conventional corrosion inhibitors and have been found to possess the ability to inhibit metal corrosion (Oguzie et al., 2012). Lonchocarpus laxiflorus is a species of legume in the Fabaceae family. It is called folahi in Hausa and uhia in Igede. The tree grows to 4–8 meters in height, has grey or yellowish bark and compound leaves. New leaves are accompanied by purple flowers on multi-branched pinnacles. The fruit is glabrous paper pod, usually containing one seed. Phytochemical investigation of the stem bark of Lonchocarpus laxiflorus as reported by Igoli et al. (2008) yielded three new cassane diterpenoids; lonchocassane A [cassa-13(14), 15-dien-18, 20-dioic acid], lonchocassane B [cassa-13 (14), 15-dien-20-oxo-18-oic acid] and lonchocassane C [cassa-13 (14), 15-dien-20-carboxyl-18-methylcarboxylate]. It has been reported that the addition of iodide ions to the inhibitor system synergistically enhances the corrosion inhibition of metals. Synergistic inhibition is an effective means to improve the inhibitive force of inhibitor, to decrease the amount of usage and to diversify the application of inhibitor in acidic media. Synergism (S) of corrosion inhibitors is either due to interaction between components of the inhibitors or due to interaction between the inhibitor and one of the ions present in aqueous solution. The greater influence of the iodide ion is often attributed to its large ionic radius, high hydrophobicity, and low electronegativity, compared to the other halide ions (Oguzie et al., 2012). The present study investigates the inhibitive potential of Lonchocarpus laxiflorus on the corrosion of mild steel in 1.0 M HCl solution using gravimetric and linear polarization techniques. The influence of iodide ions on the adsorption and corrosion-inhibitive properties of Lonchocarpus laxiflorus was also investigated. MATERIALS AND METHODS

Materials

Analytical grade reagents obtained from Emole Scientific Makurdi were used. Linear polarization measurement was carried out at Ahmedu Bello University Zaria, Nigeria, using Potentiostat /Galvanostat (Nova Autolab – PGSTAT 302N Version – 1.10.1.9), FTIR was done at National Research Institute, Zaria using FTIR-8400S Fourier Transform Infrared Spectrophotometer. Optical micrograph was taken

using Metallurgical microscope (TSVIEW Digital Metallurgical Microscope), model TUCSEN 0923502, at Mechanical Engineering Department, University of Agriculture, Makurdi.

Samples preparation

Mild steel specimen

Mild steel specimens were polished successively with emery cloth, washed in distilled water, degreased with acetone and finally dried in hot air. Weight loss experiments were conducted on mild steel specimens of dimension 3.0 × 2.0 × 0.14 cm.

Extraction of Lonchocarpus laxiflorus

The stem bark of Lonchocarpus laxiflorus (LL) was collected from Ipinu Igede forest in Oyinyi-Iyeche, Oju Local Government Area of Benue State. The plant specimens were identified in the Department of Forestry and Wildlife, Federal University of Agriculture Makurdi. The fresh stem barks were air dried for six weeks, then pulverized with a pestle and mortar. The fine powdered samples were stored in a polyethylene bag for analysis. The pulverized stem barks were extracted exhaustively with methanol at room temperature (27±2 °C) using Soxhlet extraction. From the respective stock solutions, inhibitor test solutions was prepared in the desired concentration range (0.2 g/L, 0.6 g/L and 1.0 g/L)by diluting with the respective aggressive solutions. Test Solution

The aggressive solution, 1.0 M HCl was used to carry out all weight loss and electrochemical experiments which was prepared by dilution of analytical grade 37% HCl with distilled water. Weight Loss Measurements

Gravimetric experiments were conducted on test coupons of dimension 2 cm x 3 cm x 0.14 cm. The pre-cleaned and weighed coupons were suspended in beaker containing the test solutions using glass hooks and rods. Tests were conducted under total immersion conditions in 200 mL of the aerated and unstirred test solutions. To determine weight loss with respect to time, the coupons were retrieved at 24 hrs intervals up to 168 hrs, immersed in 20% NaOH solution, scrubbed with bristle brush, washed, dried and weighed. The weight loss was taken to be the difference between the weight of the coupons at a given time and its initial weight. All tests were run in duplicate to obtain good


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reproducible data. Average values for each experiment was obtained and used in subsequent calculations. Mild steel specimens were immersed in duplicate in 200 mL of the test solutions with and without addition of inhibitors of different concentrations at room temperature (27±2 °C). The cleaned specimens were weighed before and after 3 hours of immersion in the test solution for effect of temperature. The percentage inhibition efficiency (IE exp ), the degree of surface

coverage (θ) and the corrosion rate (CR) of mild steel was calculated using equations (Momoh-Yahaya et al., 2012)

1001)0(

)1(

exp xW

WIE

1

where 𝑊 0 is the weight loss of the mild steel without

inhibitor and 𝑊 1 is the weight loss of mild steel with

inhibitor.

θ = 100

expIE 2

At

WcmghCR

21 3

where ∆W is weight (g), A is the area of the mild steel (cm2) and t is the period of immersion (hrs).

Electrochemical Linear Polarization Measurements

Electrochemical measurements were conducted in 1.0 cm long stem (isolated with epoxy resin) to provide an exposed surface area 1.0 cm2 of working electrode (WE).The coupon was sealed with epoxy resin in such a way that only one square surface area will be left uncovered. The exposed surface was degreased in acetone, rinsed with distilled water and dried in warm air. A conventional three-electrode system consisting of mild steel as working electrode, platinum (Pt) as an auxiliary electrode and saturated calomel electrode (SCE) as reference electrode, was used for the measurements. The experiment was conducted at room temperature (27±2 °C) using 200 mL of test solution. The %IE was calculated from the charge transfer resistance (Rct) values by using the equation

%𝐼𝐸 = 𝑅𝑐𝑡 1 − 𝑅𝑐𝑡 0

𝑅𝑐𝑡 1 × 100 4

where Rct(0) is the charge transfer resistance of MS without inhibitor and Rct(1) is the charge transfer resistance of MS with inhibitor.

The Tafel polarization curves were recorded by scanning the electrode potential from -300 mV to 300 mV (vs SCE) with a scanning rate of 1 mV/s. The linear Tafel segments of the anodic and cathodic curves was extrapolated to corrosion potential to obtain the corrosion current densities (Icorr). Surface Characterization

Optical micrographs of mild steel specimens in the absence and presence of the plant extract were taken. RESULTS AND DISCUSSION

Effect of time on corrosion

The inhibition efficiency and corrosion rate, (CR) obtained from weight loss method of MS in 1.0 M HCl at room temperature (27±2 °C) as a function of time in the presence of LL is shown in Figures 1(a and b, respectively).

Figure 1: (a), Effect of Immersion Time (h) on Corrosion Inhibition Efficiency (%IE) of mild steel in 1.0 M HCl in the presence of various concentrations of methanol extract of LL. (b), Effect of immersion time on the corrosion rates of mild steel in 1.0 M HCl in the absence and presence of LL.

0

20

40

60

80

24 48 72 96 120 144

%IE

(a)

0.2g/L

0.6g/L

1.0g/L

Time (h)

0

2

4

6

8

10

24 48 72 96 120 144 168

CR

X 1

0-4

(gh

-1cm

-2)

Time (h)

(b)

Blank

0.2g/L

0.6g/L

1.0g/L



It was observed that the inhibition efficiency of MS increased with increasing concentrations of inhibitors but decreased with days. This behavior could be attributed to the increase in adsorption of inhibitor on the metal or at the solution interface on increasing its concentration, (Elyn et al., 2011). High inhibition efficiency observed after 24 h of immersion suggests that LL adsorption on the mild steel surface was completed within 24 h, after which time the aggressive action of the acid chloride medium increasingly undermines the integrity of the adsorbed inhibitor, leading to reduced inhibition efficiency with prolonged exposure time. This result is consistent with the findings of (Ihebrodike et al., 2012; Elyn et al., 2011; Nnanna et al., 2010). Inhibition efficiency and Corrosion rates (X 10-4 gh-1cm-2) of mild steel in various concentrations of LL are summarized in Table 1. Table 1: Inhibition efficiency and corrosion rates(X 10-4 gh-

1cm-2) of mild steel in various concentrations of LL

Effect of temperature

Gravimetric measurements under these conditions were undertaken for 3 hr immersion periods at 303, 313, 323 and 333 K, to assess the effect of temperature change on the inhibitive effect of LL. The inhibitor concentrations for this study were 0.2 g/L, 0.6 g/L and 1.0 g/L. The results obtained are presented in Figure 2 (a) and (b). Inhibition efficiency increased with increase in concentration of the inhibitors but decreased with increase in temperature. This is again evidence that LF is effective corrosion inhibitor for mild steel in hydrochloric acid. The reduction in inhibition efficiency with increasing temperature has also been attributed to the nature of adsorption, wherein the inhibitor is physically adsorbed at lower temperature whilst chemisorption is favoured at higher temperature, (Ihebrodike et al., 2012). In this case, physicosorption was favoured. The inhibition efficiency at different temperatures is summarized in Table 2 below.

Figure 2: (a), Effect of temperature (K) on Corrosion Inhibition Efficiency (%IE) of mild steel in 1.0 M HCl in the presence of LL. (b), Effect of Immersion Time (h) on Corrosion Inhibition Efficiency (%IE) of mild steel in 1.0 M HCl in the presence of LL. Table 2: Inhibition Efficiency (%IE) of mild steel in 1.0 M HCl in the presence of LL at various temperatures. System Inhibition Efficiency at different temperatures 303 K 313 K 323 K 333 K 0.2 g/L 50.00 47.62 43.24 43.02 0.6 g/L 62.50 52.38 45.95 41.86 1.0 g/L 75.00 57.14 56.76 44.19

Electrochemical linear polarization

In the linear polarization resistance (LPR) technique, the values of change in current as a result of applied potential obtained from electrochemical measurements are used. After measuring the currents and potentials, a plot of the parameters measured for mild steel as the working electrode immersed in 1.0M HCl containing different concentrations of LL as inhibitor is presented in Figure 3. The measurement showed that the introduction of LL to the blank solution has influence on both anodic and cathodic half reactions, although cathodic influence appeared much pronounced.

0

20

40

60

80

303 313 323 333

%IE

(a)

0.2g/L

0.6g/L

1.0g/L

Temperature (K)

0

10

20

30

40

50

60

Co

rro

sio

n R

ate

X 1

0-4

(gcm

-2h

-1)

(b)

Blank (g)

0.2g/L

0.6g/L

1.0g/L

Temperature (K)

Time (h) 24 48 72 96 120 144 168

%IE

0.2g/L 58.46 57.90 55.71 47.99 43.83 40.11 38.40

0.6g/L 70.77 69.93 68.95 67.11 66.10 56.01 55.51

1.0 g/L 76.92 75.19 75.19 68.12 66.59 64.88 62.10

CR

Blank 4.51 4.62 5.07 5.17 5.74 6.26 7.83

0.2g/L 1.88 1.94 2.25 2.69 3.22 3.75 4.82

0.6g/L 1.32 1.39 1.57 1.70 1.94 2.75 3.48

1.0 g/L 1.04 1.15 1.44 1.65 1.92 2.20 2.97



Figure 3: Electrochemical Linear Polarisation plot for mild steel in 1.0 M HCl in the absence and presence of LL.

Table 3: Electrochemical Linear Polarisation parameters for mild steel in 1.0 M HCl in the absence and presence of LL. System Ecorr

(mV) jcorr

(μA/cm²) βa

(V/dec) βc

(V/dec) CR

(mm/yr) %IE

Blank -1577.20 1404.80 -163.33 63.46 16.324 -

0.2 g/L -938.40 986.99 -76.27 50.74 11.46 29.74

0.6 g/L -943.28 609.93 -102.18 60.03 7.09 56.58

1.0 g/L -935.66 607.25 -100.47 59.66 7.06 56.77

The linear polarization parameters obtained from the measurement are presented in Table 3. The results in the table indicates that the introduction of the various concentrations of LL extracts remarkably shift Ecorr /SCE. For instance, the difference between the Ecorr /SCE of the blank solution and that of the highest concentration of LL is 781.93 mV. It could be inferred that LL acted as a mixed-type inhibitor and the inhibition is due to simple geometric blocking mechanism (Eduok et al., 2012). Also, the corrosion current densities of the additives decreased significantly compared to the blank. It is clear also from the result (Table 3) that the βa and βc values for the various concentrations of the LL reduced remarkably compared to the blank indicating that the additive simultaneously modified both the anodic and cathodic reactions thus supporting the assertion that the extract is a mixed-type inhibitor (Nnanna et al., 2010). The corrosion inhibition efficiencies were calculated from the linear polarization data using the relation in equation (6):

%𝐼𝐸 = 𝑅𝑐𝑡 1 − 𝑅𝑐𝑡 0

𝑅𝑐𝑡 1 × 100 6

Where, Rct(0) is the charge transfer resistance of MS without inhibitor and Rct(1) is the charge transfer resistance of MS with the inhibitor.

The values obtained are given in Tables3 with the highest inhibition efficiency of 96.44%. The trend for the inhibition efficiencies were consistent with those obtained from gravimetric analysis. This result agreed with those reported by Reddy (2014) and Vrsalović et al. (2011). Kinetics/Thermodynamics of the Corrosion

Rate constant and half-life

The corrosion reaction is a heterogeneous reaction which is composed of anodic and cathodic reactions at the same or different rate (Olasehinde et al., 2013). It is on this basis that kinetic analysis of the data is considered necessary. The half-life was obtained from equation (7), Eddy et al., 2009).

Half life expression: 1

2/1

693.0

kt 7

Figure 4: (a), Variation of –log (weight loss) with time (days) for mild steel in the absence and presence of LL at room temperature. (b), Effect of temperature on the corrosion rate of mild steel in 1.0 M HCl in the absence and presence of LL.

Figures 4a shows the plot of –log (weight loss) against time (in days) in the absence and presence of LL. The rate constant parameters; rate constant and half-life are also recorded in Tables 4. The plots showed a linear

-2

-1.6

-1.2

-0.8

-0.4

0

-8-6-4-20

Po

ten

tial

(V

)

log i

Blank

0.2 g/l

0.6 g/l

1.0 g/l

0

0.4

0.8

1.2

1.6

2

1 2 3 4 5 6 7

-lo

g (w

eigh

t lo

ss)

Time (days)

(a)

Blank

0.2 g/L

0.6 g/L

1.0 g/L

0

2

4

6

8

10

24 48 72 96 120 144 168

C R

X 1

0-4

(gh

-1cm

-2)

Time (h)

(b)

Blank

0.2g/L

0.6g/L

1.0g/L



variation and slop, k, which confirms a pseudo-first order reaction kinetics with respect to the corrosion of mild steel in 1.0 M HCl solution in the absence and presence of LL. It is evidenced from the result that the half-life decreased with time. This further supports the fact that the interaction between the mild steel and the inhibitors is physicosorptive. This result is in line with that reported by Olasehinde et al. (2013). Table 4: Half-life parameters at various concentrations of LL

The Ea, ∆H and ∆S were evaluated from the transition state values and plots shown in Figure 5. The thermodynamic activation parameters obtained from the plots are shown in Table 5.

Figure 5: Transition state plots for the corrosion of mild steel in the absence and presence of LL.

In other to study the effect of temperature on the corrosion of metal in the presence of an inhibitor, then equation (8 and 9) below were used (Odiongenyi et al., 2009; Ogoke et al., 2009).

211

2 11

303.2log

TTR

Ea

CR

CR

8

RT

H

R

S adsads

Nh

RTCR expexp

9

where CR1 and CR2 are the corrosion rates of metal at the temperatures T1 and T2, Ea is the activation energy, R is the gas constant, N is the Avogadro’s number, h is the Planck’s constant, T is the temperature, ΔSads and ΔHads are the entropy and enthalpy of adsorption of the inhibitor on a metal, respectively.

The Ea was found to be 42.875 kJmol-1 for 1.0 M HCl and increased with increasing concentration of the

inhibitor, with the highest values of 58.201 kJmol-1 in the presence of 1.0 g/L of the inhibitor (Table 5). This showed that the adsorbed organic matter has provided a physical barrier to the change and mass transfer, leading to reduction in corrosion rate (Eddy et al., 2010 and 2011). It has been reported earlier that the value of Ea greater than 80 kJmol-1 indicates chemical adsorption, whereas, Ea less than 80 kJmol-1 infers physical adsorption (Ismail et al., 2011). On the basis of the experimentally determined Ea values that are all less than 80 KJmol-1, it is evidenced that the additives were physically adsorbed on the coupons. Therefore, it is plausible that a multilayer protective coverage on the entire mild steel surface was obtained.

Table 5: Thermodynamic activation parameters for the dissolution of mild steel in 1.0 M HCl in the absence and presence of LL at 303-333K.

System Ea (kJ mol-1) ∆H (J mol-1) ∆S (kJ mol-1) Blank 42.875 6.127 0.315 0.2 46.423 6.548 0.321 0.6 53.011 7.333 0.324 1.0 58.201 8.157 0.328

The results showed that all the enthalpy of activation for the inhibitors are positive, reflecting the endothermic nature of the mild steel dissolution process. Also, the entropies of activation energy were positive for the extract, indicating that the activation complex represents association steps and that the reaction was spontaneous and feasible. These results were in excellent agreement with the reports of previous work by Olasehinde et al. (2013). Synergistic effect of iodide ion

Synergism refers to combined total action of a compound greater than the sum of its individual effects. Synergistic inhibition is an effective means to improve the inhibitive force of inhibitor, to decrease the amount of usage and to diversify the application of inhibitor in acidic media. Synergism (S) of corrosion inhibitors is either due to interaction between components of the inhibitors or due to interaction between the inhibitor and one of the ions present in aqueous solution. The greater influence of the iodide ion is often attributed to its large ionic radius, high hydrophobicity, and low electronegativity, compared to the other halide ions (Oguzie et al., 2012). Table 6 below shows the inhibition efficiency of 5.0 mM KI and extract-iodide mixtures at 300C.

y = 0.32x - 6.125

y = 0.342x - 6.45

y = 0.383x - 6.595

y = 0.426x - 6.79

-8

-6

-4

-2

0

3.30E-03 3.20E-03 3.10E-03 3.00E-03

Log(

CR

/T)

1/T (K-1)

Blank

0.2g/L

0.6g/L

1.0g/L

System Rate constant, k, (day-1) Half-life (days) Blank 0.389 1.782 0.2g/L 0.465 1.490 0.6g/L 0.465 1.490 1.0g/L 0.474 1.462



Table 6: Inhibition efficiency of 5.0mM KI and extract-iodide mixtures at 300C.

The inhibition efficiency in the presence of the iodide is higher than those for only LL in 1.0 M HCl. This result is in agreement with the report by Chahul et al. (2015). Figure 6 illustrates the relationship between the inhibition efficiency and the respective concentration of the inhibitor in the presence of 5.0 mM KI.

Figure 6: Inhibition efficiency of mild steel in the presence of LL and KI

Adsorption Isotherm

Different adsorption isotherms were tested in order to obtain more information about the interaction between the inhibitors and the mild steel surface. The various isotherms tested includes Temkin, Frumkin, Freundlich and Langmuir adsorption isotherms and linear regression coefficients (r2) were used to determine the best fit. Langmuir and Freundlich adsorption isotherms were found to be best fit in which case all the linear regression coefficients (r2) were close to unity. Langmuir adsorption isotherm assumes that the solid surface contains a fixed number of adsorption sites and each site holds one adsorbed species (Yuce et al., 2012). Langmuir isotherm gives a straight line between 𝐶 𝜃 𝑣𝑠 𝐶 (Oguzie et al., 2007) as shown in Figure 7 (a), where C is the concentration of the inhibitors and 𝜃 the surface coverage. The results obtained for ∆G and Kads

are shown in Table 7. The values obtained for ∆G were

negative indicating that the adsorption process proceeded spontaneously.

Freundlich adsorption isotherm is based on the assumption that adsorption sites are distributed exponentially with respect to energy of adsorption and that the surface sites are subdivided into several types, each possessing a characteristic heat of adsorption (Odoemelam et al., 2009). The Freundlich equation was applied indiscriminately to isotherms, all of which were limited to adsorption from dilute solutions, showing that adsorption increased indefinitely with increasing concentration. A plot of log θ against log C are shown in Figures 7(b). The linearity shows that the adsorption of the inhibitors on mild steel surface in aqueous medium follows Feundlich isotherm (Zhang and Hua, 2009).

Figure 7: (a), Langmuir isotherm for the adsorption of LL onto mild steel surface in 1.0 M HCl at 303K and 333K respectively. (b), Freundlich isotherm for the adsorption of LL onto mild steel surface in 1.0 M HCl at 303K and 333K respectively.

0

20

40

60

80

100

5.0 mM KI 0.2 g/L LL + 5.0 mM KI

0.6 g/L LL + 5.0 mM KI

1.0 g/L LL + 5.0 mM KI

% IE

System

R² = 0.986

R² = 0.998

0

0.5

1

1.5

2

2.5

0.2 0.6 1

C/θ

C (g/L)

(a)

303K

333K

R² = 0.996

R² = 0.906

-1

-0.8

-0.6

-0.4

-0.2

0

-1.61 -0.51 0

ln θ

ln C

(b)

303K

333K

System %IE 5.0 mM KI 36.93 0.2 g/L LL + 5.0 mM KI 69.96 0.6 g/L LL + 5.0 mM KI 81.13 1.0 g/L LL + 5.0 mM KI 90.24



Generally, the value of ΔG°ads ≤ −20 kJ mol−1 signify physisorption and values more negative than −40 kJ mol−1 signify chemisorption (Eddy and Ekop, 2007; Ihebrodike et al., 2010, Saratha et al; 2011). The results are presented in Table 7. The values of ΔGads are negative and less than -40 kJmol-1. This implies that the adsorption of the inhibitor on metal surface is spontaneous and confirms physical adsorption mechanism. Table 7: Langmuir and Freundlich adsorption isotherm parameters obtained from the corrosion data for mild steel in 1.0 M HCl containing LL extract.

Surface Morphology

Surface morphology of MS was studied by optical microscopy after 24 h immersion in 1.0 M HCl at 2048 x 1536 resolution.

(a) (b)

(c) Figure 8: (a) Polished MS, (b) MS in 1.0 M HCl and (c) MS in the presence of 1.0 g/L plant extract

Figure 8(a), represent the micrograph of polished MS without being exposed to the corrosive environment while Figure 8(b) showed strongly damaged MS surface due to the formation of corrosion products after immersion in 1.0 M HCl solution. The micrograph

of MS surface after immersion in 1.0 M HCl with 1.0 g/L LL is shown in Figure 8(c). It could be seen that no pits and cracks are observed in the micrographs after immersion of MS in 1.0 M HCl in the presence of the inhibitor except polishing lines. Thus, it revealed the presence of a good protective film upon adsorption of inhibitor molecules onto the MS surface, which was responsible for the inhibition of corrosion. Fourier Transformed Infra-Red (FTIR) analysis Figure 9 (a) below is the FTIR spectrum of LL. The peak at 3407 cm-1could be due to a phenolic–OH while the peak at 2936 cm-1 corresponds to an alkyl C-H. The peak at 1639.58 cm-1 and 1499.70 cm-1is attributable to aromatic ring and aromatic C=C respectively, present in the molecule. The presence of C-O in the molecule is due to the peak at 1144.79-1043.52 cm-1while aromatic substitution is attributed to the peak at 619.17-401.21 cm-1. Figure 9a: FTIR spectra of pure LL extract

Figure 9b: FTIR spectra of pure LL extract adsorbed on MS surface

T (K) Intercept Slope Kads(M-1) R2 ∆G (kJ mol-1) Langmuir adsorption isotherm parameters 303 0.1992 1.1625 5.0209 0.998 -14.184 333 0.0442 2.2375 22.642 0.986 -19.758 Freundlich adsorption isotherm parameters 303 -0.313 0.241 4.149 0.996 -3.585 333 -0.624 0.150 6.667 0.906 -5.252



Figure 9 (b) shows the infrared spectra of LL adsorbed on the MS surface. It is evident from the spectra that the peakat 3407.34 cm-1 has disappeared, indicating that the oxygen atoms are participating in the adsorption process through its lone pair. All the peaks corresponding to the presence of aromatic rings also disappeared, showing that the aromatic rings of the phenolic group are active in the adsorption process. LL as indicated from the spectra data, results in strong adsorption due to donation of lone pair of electrons on oxygen to the vacant d-orbitals of the metal which leads to the formation of the metal complexes. Table 9: Important peaks of LL and adsorbed inhibitor to MS surface

CONCLUSION

Lonchocarpus laxiflorus (LL) extract acted as a good inhibitor against the corrosion of mild steel in 1.0 M HCl solution. Linear polarization results revealed that the extract acted as a mixed inhibitor. The adsorption mechanism followed the Langmuir and Fruendlich adsorption isotherm models. The inhibitive ability was found to be synergistically enhanced by iodide ions. The values of activation energy (Ea) obtained is suggestive of physical adsorption mechanism. The positive values obtained for enthalpy and entropy of activation is reflective of endothermic reaction nature and associative activation complex respectively. Kinetic treatment of the data followed a pseudo-first order reaction. ∆Gads

0 value indicated a strong and spontaneous adsorption of the extract components on the metal surface. The micrographs obtained from optical microscopy revealed the presence of a good protective film upon adsorption of inhibitor molecules onto the MS surface, which was responsible for the inhibition of corrosion. Thus, it can be concluded that LL inhibitor system acted as a good inhibitor in hydrochloric acid. ACKNOWLEDGEMENT

The authors sincerely appreciate the Department of Chemistry, University of Agriculture, Makurdi, for

granting us access to their laboratory where this work was carried out. CONFLICT OF INTEREST

None declared. REFERENCES

1. Ambrish S, Ebenso EE, and Quraishi MA (2011). Corrosion inhibition of carbon steel in HCl solution by some plant extracts. Int J Corros 2012 : 1-20. doi:10.1155/2012/897430.

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Functional group LL LL ON MS Phenolic –OH 3407.34 - Alkyl C-H 2936.72 2929.7 Aromatic ring 1638.58 - Aromatic C=C 1499.70 - C-O 1144.79 1017.6 Aromatic substitution 401.21 -



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Article’s citation Ijuo GA, Chahul HF and Eneji IS (2016). Corrosion inhibition and adsorption behavior of Lonchocarpus

laxiflorus extract on mild steel in hydrochloric acid. Ew

J Chem Kinet 1(1): 21-30.

ewemen journal of chemical kinetics

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