adsorption and corrosion inhibition of mild steel in

A Jamal Abdul Nasser et. al. / International Journal of Engineering Science and Technology Vol. 2(11), 2010, 6417-6426

ADSORPTION AND CORROSION INHIBITION OF MILD STEEL IN HYDROCHLORIC ACID

MEDIUM BY N-[MORPHOLIN-4-

YL(PHENYL)METHYL]BENZAMIDE

A. Jamal Abdul Nasser and M.Anwar Sathiq*

PG and Research Department of Chemistry, Jamal Mohamed College, (Autonomous), Tiruchirappalli – 20, Tamil Nadu, India.

E_mail address: anwarchemsathiq @ yahoo.co.in* ;

ABSTRACT

The influence of N-[morpholin-4-yl(phenyl)methyl]benzamide (MPB) on corrosion inhibition of mild steel in 1M HCl was studied by weight loss, effect of temperature, potentiodynamic polarization and electrochemical impedance spectroscopy. The experimental results showed that the inhibition efficiency increases with increasing of MPB concentrations but decreases with increasing temperatures. The adsorption of MPB on the mild steel surface obeyed the Temkin’s adsorption isotherm. Potentiodynamic polarization curves showed that MPB acted as a cathodic inhibition predominantly in hydrochloric acid. This was supported by the impedance measurements which showed a change in the charge transfer resistance and double layer capacitance indicating adsorption of MPB on the mild steel surface. Scanning electron microscopy (SEM) technique is used to confirm the effectiveness of inhibition of mild steel in hydrochloric acid medium. The thermodynamic functions of the adsorption processes were calculated from the weight loss and effect of temperature data and were used to analyse the inhibitor adsorption mechanism. Key word N-[morpholin-4-yl(phenyl)methyl]benzamide: mildsteel; Temkin’s adsorption isotherm; potentiodynamic polarization; free energy. 1. Introduction

The corrosion of metals remains a world –wide scientific problem as it affects the metallurgical, chemical and oil-industries. The increasing interest in the manufacture of hydrochloric acid has created the need for obtaining information on the corrosion resistance of mild steel to hydrochloric attack 1. Of particular importance also is the need to introduce certain organic compounds as inhibitors into the mild steel-corrodent system to prevent corrosion of the mild steel2-3. Most of the well known acid inhibitors are organic compounds containing nitrogen, sulphur and oxygen atoms4-6. It has been reported that many heterocyclic compounds containing N, O, S have been proved to be effective inhibitors for the corrosion of mild steel in acid media 6-15.

The corrosion inhibition property of these compounds is attributed to their molecular structure. The planarity and the lone pair electrons in the hetero atoms are important features that determine the adsorption of these molecules on the metallic surface. The effect of inhibitors adsorbed on metallic surfaces in acid solutions, is slow down the cathodic and anodic process of dissolution of the metal. This is due to the formation of a barrier of diffusion or by means of the blockage of the reaction sites and there by reducing the corrosion rate 11.

The main objective of this work is to investigate the corrosion inhibiting capability of MPB against the corrosion of mild steel in HCl solutions by various methods.

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2. Experimental

2.1 Materials

Mild steel strips with the composition C = 0.07% ; S = Nil ; P = 0.008% ; Si = Nil ; Mn = 0.34% and Fe = Reminder and size of 4 × 1 × 0.025 cm were used for weight loss and effect of temperature studies. Mild steel cylindrical rods of the same composition embedded in araldite with exposed area of 1 cm2 were used for potentiodynamic polarization and impedance measurements. The electrode was polished using a sequence of emery papers of different grades and then degreased with acetone.

2.2 Inhibitor

N-[morpholin-4-yl(phenyl)methyl]benzamide was synthesized, purified and characterized by IR and NMR spectroscopy. MPB was purified by recrystalization from ethanol to analytical purity grade and confirmed by elemental analysis. The concentration of the inhibitor is ranges from 10-2 M to 10-7 M.

N

O

NH

C6H5O

N-[morpholin-4-yl(phenyl)methyl]benzamide

Figure 1: Molecular structure of MPB

2.3 Solutions

The acid solution (1M HCl) was prepared by dilution of analytical grade 37% HCl with double distilled

water. All tests were conducted at different temperatures in magnetically stirred solutions.

2.4 Weight loss measurements

For weight loss measurements, each run was carried out in a glass vessel containing 100 mL test solution. A clean weighed mild steel rod (4x1x0.025 cm) was completely immersed at inclined angle in the solution. The temperature of the solution was maintained at 30±1ºC. After 2 hours of immersion, the electrode was withdrawn, rinsed with doubly distilled water, dried thoroughly and weighed. The weight loss used to calculate the corrosion rate (CR) in miles per year (mpy). All the experiments were carried out in duplicate.

2.5 Effect of Temperature

The procedure adopted for weight loss studies at ordinary temperature (303K + 1) was followed here. But the temperature of the study was varied from 308K to 333K. The loss in weights was calculated. Each experiment was duplicated to get good reproducibility. Weight loss measurements were performed in 1M HCl with and without the addition of the inhibitor at their best inhibiting concentrations. Percentage inhibitions of the inhibitor at various temperatures were calculated.

2.6 Potentiodynamic Polarization Measurements Both cathodic and anodic polarization curves were recorded (1mv / S) using the corrosion

measurement system BAS (Model: 100 A) computerized electrochemical analyzer and PL-10 digital Plotter. A platinum foil and Hg| Hg2Cl2| 1M HCl electrode were used as auxiliary and reference electrodes respectively.

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2.7 Electrochemical Impedance Measurements

Electrochemical measurements were carried out (same instrument used for polarization studies) in the conventional three-electrode, cylindrical Pyrex glass cell with a capacity of 1000mL. The working electrode in the form of disc cut from mild steel with geometric area of 1 cm2 was embedded in polytetrafluoroethylene (PTFE). A saturated calomel electrode (SCE) and a platinum disc electrode were used, respectively, as reference and auxiliary electrodes. The temperature was controlled at 308 K by thermostatically . The working electrode was abraded with silicon carbide paper (grade P1200), degreased with AR grade ethanol and acetone, and rinsed with double distilled water before use. All potentials are reported versus SCE.

2.8 Adsorption of free energy change (Gads)

Gads = -RT ln (K 55.5)15 KJ/mole. Where Gads = free energy of adsorption; R = Gas Constant in KJ; T = Temperature in K; K = Adsorptive equilibrium constant; and the value of 55.5 is the concentration of water in solution expressed in L-1 mol16.

From the above equation, the free energy change of adsorption in all the various concentrations of MPB at ordinary temperature (303K±1) and 10-2M concentration (best inhibition) of MPB at different temperatures (303 K to 333K) were calculated.

2.9 SEM analysis The specimens used for surface morphological examination were immersed in acid containing

various concentrations of inhibitor and blank for 2 hours. Then they were removed, rinsed quickly with rectified spirit, and dried. The analysis was performed on HITACHI-model S-3000 H SEM.

3. Results and Discussion 3.1 Weight loss measurements

Table 1 gives the values of inhibition efficiency for different concentrations of MPB in 1M HCl. It can be seen from this table that MPB efficiently inhibits the corrosion of mild steel in 1M HCl solutions. The corrosion rate decreased considerably with an increase in concentration of inhibitor. This is due to the presence of hetero atoms like nitrogen, oxygen and aromatic rings. The extent of inhibition depends upon the nature and mode of adsorption of inhibitor on the metal surface. The adsorption is assumed to be a quasi-substitution process between the water molecules on the surface and the organic molecules. MPB is adsorbed vertically with –NH as the anchoring group. The interaction of MPB with the metal surface may occur through both the –NH and the –CO group. It is well known that in MPB, the lone pair of electrons of nitrogen is responsible for much predominant adsorption on the mild steel surface.

The greater inhibitive power of MPB may be due to the presence of two nitrogen, two oxygen atoms

and two aromatic rings. Weight loss measurements revealed that corrosion inhibition efficiency of the MPB increases with increasing the concentration.

Table 1 Values of inhibition efficiencies for different concentrations of MPB in 1 M HCl at ordinary temperature.

3.2 Effect of Temperature

Concentration of Inhibitor (M)

Weight loss (g)

Corrosion Rate (mpy)

Inhibition Efficiency (%)

Blank 0.098 0.8676 - 0.0000001 0.033 0.2921 66.33 0.000001 0.024 0.2125 75.51 0.00001 0.019 0.1682 80.61 0.0001 0.015 0.1328 84.69 0.001 0.010 0.0089 89.80 0.01 0.007 0.0062 92.86

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The temperature increases the rate of all electrochemical processes and influences adsorption equilibrium and

kinetics as well. The effect of temperature on the corrosion inhibition with and without inhibitor are shown in Table 2. It can be seen that the weight loss increases with temperature in the absence and presence of inhibitor. Adsorption and desorption of inhibitor molecules continuously occur at the metal surface and an equilibrium exists between two processes at a particular temperature. With increase of temperature, the equilibrium between adsorption and desorption processes is shifted to a higher desorption rate than adsorption until equilibrium is again established at a different value of equilibrium constant. It explains the lower inhibition efficiency at higher temperature.

MPB is able to maintain it’s inhibition at about 86% up to 323K and the inhibition efficiency is slightly

decreased to 15% at 333K.

Table 2 Values of inhibition efficiencies and surface coverage for different temperature in the presence of 10-2 M concentration of MPB in 1 M HCl.

Temperature (K) 1-

log (/1-)

Inhibition Efficiency (%)

303 0.9286 0.0714 1.1141 92.86

308 0.9232 0.0768 1.0799 92.32

313 0.9168 0.0832 1.0422 91.68

318 0.9072 0.0928 0.9902 90.72

323 0.8542 0.1458 0.7678 85.42

328 0.7762 0.2238 0.5401 77.62

333 0.7025 0.2975 0.3732 70.25

3.3 Potentiodynamic Polarization Studies

It can be seen from the Table 3 that the values of bc were shifted to higher values with reference to blank in

the presence of MPB . This shows that the MPB inhibits the corrosion mechanism by controlling cathodic reactions predominantly and blocking cathodic sides of the metal surface. The Icorr value decreases with increasing the inhibitor concentration.

Table 3 corrosion kinetic parameters for the corrosion of mild steel in 1 M HCl containing different concentrations of the MPB.

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Concen

tration

of

inhibito

r (M)

Ecorr

(mV)

Tafel slope Icorr

(mA/c

m2)

Inhibition

Efficiency

(%) ba

(mV/dec)

bc

(mV/dec)

Blank -1075 45 120 5.10 -

0.000001

-1069 55 137 1.80 64.71

0.0001 -1058 64 163 1.01 80.20

0.01 -1047 75 191 0.40 92.16

3.4 Electrochemical Impedance Measurements

Table 4 gives the values of Rt, Cdl and Icorr which were derived from Nyquist Plots as shown in Figure 2. It can be seen from this Table that the charge transfer resistance (Rt) value increases with increasing MPB concentration but the double layer capacitance (Cdl) value decreases due to the decrease in local dielectric constant and increase in the thickness of the electrical double layer.

Both electrochemical impedance and potentiodynamic polarization results are very good agreement with weight loss measurements.

Table 4 Impedance parameters for the corrosion of mild steel in 1 M HCl containing different concentrations of the MPB.

Concentration

of inhibitor

(M)

Rt

( cm2)

Cdl

(

F/cm2)

Icorr

(mA/cm2)

Inhibition

Efficiency

(%)

Blank 68.18 9.9196 4.90 -

0.000001 110.97 7.645 1.68 65.71

0.0001 154.24 5.1087 0.96 80.41

0.01 289.13 1.0352 0.40 91.84

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Figure2. Nyquist plots for mild steel in 1M HCl in the presence of different concentrations of the MPB

3.5 Free energy of adsorption (Gads) The values of Gads are given in Table 5 and 6. The negative values of Gads suggest that the strong

interaction of the inhibitor molecules onto the mild steel surface. The results obtained from variations of adsorption free energies with the concentration (Table 5) reveal that the -Gads decreases in the range of 0.01-0.0000001 M. Similarly, the results obtained from variation of adsorption of free energies with the temperature (Table 6) indicate that the Gads decreases in the range of 303-333±1 K.

Table 5 Values of free energy changes for the corrosion of mild steel in 1 M HCl in the presence of different concentrations of MPB at ordinary temperature

Concentration

(M) 1-

Gads

(KJ/mole)

0.0000001 0.6633 0.3367 - 23.43

0.000001 0.7551 0.2449 - 24.56

0.00001 0.8061 0.1939 - 25.31

0.0001 0.8469 0.1531 - 26.03

0.001 0.8980 0.1020 - 27.19

0.01 0.9286 0.0714 - 28.18

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The large negative values of Gads indicate the spontaneous adsorption of the inhibitor and are usually characteristics of strong interaction with the metal surface. Generally values of Gads up to -20 kJmol-1 are consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption), while those which are more negative than -40 kJmol-1 involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption)17. The high negative value of Gads shows that in the presence of hydrochloric acid medium chemisorption of MPB may occur. The large values of Gads and its negative sign are usually characteristic of strong interaction and a highly efficient adsorption18.

Table 6 Values of free energy changes for the corrosion of mild steel in 1 M HCl in the presence of 10-2 M concentrations of MBU at different temperatures.

Temperature

(K) 1-

Gads

(KJ/mole)

303 0.9286 0.0714 -28.18

308 0.9232 0.0768 -27.99

313 0.9168 0.0832 -27.76

318 0.9072 0.0928 - 27.47

323 0.8542 0.1458 - 26.17

328 0.7762 0.2238 -24.86

333 0.7025 0.2975 -24.13

3.6 Temkin’s Adsorption isotherm The values of surface coverage () were plotted against log C for different concentrations of the MPB.

The straight line (Figure 3) thus obtained indicates that the adsorption of MPB on the mild steel surface follows Temkin’s adsorption Isotherm in acidic media16.

Temkin's Adsorption isotherm

0

0.2

0.4

0.6

0.8

1

-8 -7 -6 -5 -4 -3 -2 -1 0

log C

Su

rfac

e C

ove

rag

e

Figure 3 Temkin’s Adsorption Isotherm Plot for the adsorption of MPB on a mild steel in 1M hydrochloric acid solution.

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3.7 Mechanism of inhibition From the results obtained from different electrochemical and weight loss measurements, it was concluded that the MPB inhibit the corrosion of mild steel in 1M HCl by adsorption at mild steel solution interface. It is general assumption that the adsorption of organic inhibitors at the metal surface interface is the first step in the mechanism of the inhibitor action. Organic molecules may adsorbed on the metal surface in four types, namely (i). Electrostatic interaction between the charged molecules and the charged metal, (ii). Interaction of unshared electron pairs in the molecule with the metal, (iii). Interaction of pi-electrons with the metal and (iv). A combination of types (i-iii). MPB contain one amide type linkage (-NH-CO-), two aromatic ring and residue of morpholine ring.

The inhibition of active dissolution of the metal is due to the adsorption of the inhibitor molecules on the metal surface forming a protective film. The inhibitor molecules can be adsorbed onto the metal surface through electron transfer from the adsorbed species to the vacant electron orbital of low energy in the metal to form a co-ordinate type link. It is well known that iron has coordinate affinity toward, nitrogen, sulphur and oxygen-bearing ligands. Hence, adsorption on iron can be attributed to coordination through –NH-CO-group, hetero atom (N and O), and pi-electrons of aromatic ring. From the above cited data reveal that the MPB follows type (iv) inhibition mechanism. 3.7 SEM analysis

SEM photograph of the metal sample in the presence and absence of inhibitor are shown in Figure 4 and 5. The inhibited metal surface is smoother than the uninhibited surface indicating a protective layer of adsorbed inhibitor preventing acid attack.

Conclusions The main conclusions drawn from this study are

MPB efficiently inhibits the corrosion of mild steel in 1M HCl medium. MPB behaves as mixed type inhibitor i.e., Both cathodic and anodic control. Adsorption of MPB on the surface of mild steel from 1M HCl obeys Temkin’s adsorption isotherm. Reduction in the values of Icorr and Cdl in the presence of an inhibitor has been dealt. The inhibition efficiency of MPB increases with increasing the inhibitor concentration. On increasing the temperature, the corrosion rate increases. Protective film formation against the acid attack is confirmed by SEM.

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Figure 4: SEM image of surface of mild steel after immersion for 2 hours in 1 M HCl

Figure 5 SEM image of surface of mild steel after immersion for 2 hours in 1 M HCl in presence of 0.01 M MPB.

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Acknowledgment

The authors thanks the Director of ICP centre, CECRI, Karaikudi, for kind permission for providing the facilities of electrochemical studies, Prof. Dr. V. Muthupandi, MME, NIT, Trichy, for providing the SEM facilities and Principal, Jamal Mohamed College (Autonomous), Trichy – 20, for providing necessary facilities and encouragement.

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adsorption and corrosion inhibition of mild steel in

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