INTRODUCTION:

Mild steel which has extensive application in various industries suffers from severe corrosion when it comes in contact with acid solutions during acid cleaning, transportation of acid, de-scaling, storage of acids and other chemical processes. The heavy loss of metal as a result of its contact with acids can be minimized to a great extent by the use of corrosion inhibitors. Inorganic compounds like chromates, phosphates, molybdates etc. and a variety of organic compounds containing heteroatom like nitrogen, sulphur and oxygen are being investigated as corrosion inhibitors.^1-6

Pure synthetic chemicals are costly, some of them are not easily biodegradable and their disposal creates pollution problems. Plant extracts are environment friendly, bio-degradable, non-toxic, easily available and of potentially low cost. Most of the naturally occurring substances are safe and can be extracted by simple procedures. Recent literature is full of researches which test different extracts for corrosion inhibition applications.

The examples are numerous such as fenugreek⁷, henna^8,9, olive¹⁰, jojoba¹¹, black pepper¹², occimum viridis¹³, Andrographis paniculata¹⁴, Phyllanthus amarus¹⁵, onion, garlic¹⁶, Eugenia jambolans¹⁷, Pongamia glabra¹⁸, opuntia¹⁹, eugenol²⁰ etc. Many of these naturally occurring substances proved their ability to act as corrosion inhibitors for the corrosion of different metals and alloys in different aggressive media.

The aim of the present study is to investigate the thermodynamic and adsorption characteristics of Nyctanthes arbortristis leaves extract as corrosion inhibitor for mild steel in 1N HCl medium by weight loss method.

EXPERIMENTAL:

Sheet of mild steel of 2mm thickness and composition C-.029%, Mn- 0.212%, Si -0.098%, P- 0.012%, S- 0.013%, Cr-0.01%, Mo-0.011%, Ni-0.009% and Fe- 99.606% was mechanically cut in to coupons of 5x1 cm² size, having a hole of uniform diameter to facilitate suspension of the coupon in the test solution. The coupons were mechanically cleaned followed by polishing with emery sheet of fine quality to expose shining polished surface. To remove any oil and organic impurities coupons were degreased with acetone and finally with de-ionised water, dried and stored in a desiccator. Accurate weights of the samples were taken using electronic balance. (Shimadzu model AUW 220D).

Inhibitor Material:

5% stock solution of the inhibitor material (NAL extract) was prepared by refluxing 100 g of dry Nyctanthes arbortristis leaves powder with 2000ml of IN HCl for 3 hours. The refluxed solution was allowed to stand overnight and filtered through ordinary filter paper. The residue was repeatedly washed with small amounts of IN HCl and the filtrate made up to 2000 ml. From this solution, different concentrations of inhibitor solutions ranging from 0.01% to 1% v/v were diluted.

Weight Loss Method:

Pre-weighed mild steel specimens (in triplicate) were suspended for specified period in 1N HCl in the absence and presence of different concentrations of the inhibitor ranging from 0.01% to 2%. After the specified time the coupons were removed from test solution, thoroughly washed with NaHCO₃ solution and de-ionised water, dried well and then weighed. The percentage of inhibitor efficiency (IE %) and corrosion rate (CR) for various concentrations of the inhibitor were calculated as

I.E % =

Weight loss without inhibitor – weight. loss with inhibitor X 100

Weight loss without inhibitor

Corrosion rate C R = (534*W) /DAT mils per year (mpy)

Where,

W = loss in weight in milligrams

D = density in g/cm³ (7.9 g/cm³for mild steel)

A = area of specimen in square inch

T = immersion time in hours

Weight loss determinations were carried out at different temperatures viz.303, 313, 323, 333 and 343 K. After initial weighing, the specimens in triplicate were immersed in 100ml of 1N HCl acid in the absence and presence of different concentrations (0.01%, 0.05%, 0.1%, 0.5% and 1% v/v) of the plant extracts at the different temperatures for 1 hour duration. The thermostat was set to the appropriate temperature and after 1 hour immersion the specimens were removed, washed, dried and reweighed.

Corrosion inhibition is a surface phenomenon that takes place by adsorption of inhibitor constituents on metal surface. Since adsorption isotherms give information on the metal –inhibitor interaction and mechanism of inhibition reaction, adsorption studies are very crucial. The most frequently used adsorption isotherms are Langmuir, Temkin, Freundlich, Flory-Huggins, Frumkin and Bockris-Swinkel’s isotherms.

Ө = surface covered by the adsorbed molecule.

a = Parameter of the molecular interaction.

X = Relational parameter between the adsorbed molecule and the solvent molecule.

K = Equilibrium constant of adsorption.

C = Concentration of the substance that is adsorbed in the bulk of the solution

RESULTS AND DISCUSSION:

Effect of plant extract concentration and immersion time on IE of plant extract in 1N HCl medium:

Inhibition efficiency (IE) calculated from weight loss measurements of mild steel samples immersed for different time intervals in 1N HCl in the absence and presence of different concentrations of NAL extract are given in the Table 1. At lower concentrations the inhibition efficiency expressed as percent inhibition increases invariably with increase of inhibitor concentration. This is associated with increase of surface coverage by the constituents present in the extract, which retards the corrosion of mild steel and hence IE increases. The rate of increase of IE with concentration of the extract is appreciable at lower concentration range and at higher concentrations around 1%, the change in IE is not much. The maximum inhibition efficiency for 1 hour immersion period for NAL extract is 90.74% at 2%. Thus NAL extract is found to be good inhibitor for mild steel corrosion in HCl medium.

There is a general increase in IE with immersion time. The changes in IE with immersion time can be explained as due to changes in adsorption –desorption equilibria. The inhibitor shows maximum inhibition efficiency at 24h immersion time. Prolonged immersion of the sample in the medium with plant extract leads to greater adsorption of plant constituents.

Table 1 IE (%) of NAL in 1N HCl at different concentrations and different immersion periods

Conc. of plant extract {%v/v}	IE(%)
Conc. of plant extract {%v/v}	1h	3h	5h	7h	12h	24h
0.01	66.43	60.28	64.08	65.1	56.25	64.29
0.05	77.56	78	82.36	88.04	84.43	96
0.1	88.00	88	90	90	89	97.63
0.5	90.00	90.41	89.24	93.45	94	98
1	88.00	89	92	92	95	97.19
1.5	89.45	90.46	92.88	93.47	95.46	96.84
2	90.74	91.25	93.14	95.04	94.12	97.65

Effect of temperature on IE % of plant extract:

Corrosion rate of mild steel in plain acid as well as acid with plant extract increases with respect to increase in temperature. Increase in corrosion rate with temperature may be due to the fact that increase in temperature usually accelerates corrosive processes, particularly in media in which hydrogen gas evolution accompanies corrosion giving rise to higher dissolution rates of the metal. At all temperatures corrosion rate is found to decrease with inhibitor concentration. The extent of increase in corrosion rates is more pronounced in the inhibited solutions than in the free acid. This suggests that corrosion inhibition is by the physical adsorption of the plant constituents on to the metal surface and as the temperature increases they are easily desorbed exposing the metal surface for corrosion by the aggressive medium. Variation of CR of mild steel in 1N HCl with temperature for the extract and Arrhenius plot are shown in Fig.1 and 2 respectively.

Fig. 1 Effect of temperature on corrosion rate of mild steel in 1N HCl with

NAL extract:

IE% of the plant extract varies inversely with temperature (Table-2). At each temperature IE % increases with inhibitor concentrations. At each individual inhibitor concentration IE% decreases with increasing temperature. This observation confirms the mode of corrosion inhibition by the plant extract being attributed to physical adsorption of the constituents.

Fig 2 Arrhenius Plot NAL in HCl

Adsorption and thermodynamic parameters:

The experimental data obtained with the different inhibitor concentrations of the plant extract in HCl medium at different temperatures from 303 K to 343 K were applied to different adsorption isotherm equations. Degree of surface coverage Ө was calculated based on inhibition efficiency from weight loss measurements as Ө = (IE % /100)

It is found that the data fitted the Langmuir adsorption (Fig.-3), Temkin (Fig.-4), Freundlich (Fig.-5) and Flory-Huggins (Fig.-6) adsorption isotherms with correlation coefficients of >0.9

Fig 3 Langmuir Adsorption NAL

Fig 4 Temkin Adsorption isotherm

Fig 5 Freundlich Adsorption isotherm

Fig 6 Flory-Huggins Adsorption isotherm

Table 2 CR of mild steel and IE (%) of NAL in 1N HCl at different concentrations and different temperatures

	303 K		313 K		323 K		333 K		343 K
Conc. of plant extract {%v/v}	C.R (mpy)	IE (%)	C.R (mpy)	IE (%)	C.R (mpy)	IE (%)	C.R (mpy)	IE (%)	C.R (mpy)	IE %)
Blank	1962.4	-	2812.8	-	4775.3	-	5712.9	-	7980.6	-
0.01	667.23	66.00	2137.7	24.00	4059.0	15.00	4684.5	18.0	7266.3	9.00
0.05	451.36	77.00	1068.8	62.00	2148.8	55.00	2685.0	53.0	5829.0	27.00
0.1	235.49	88.00	675.08	76.00	1432.5	70.00	2285.1	60.0	4790.9	40.00
0.5	196.25	90.00	337.54	88.00	525.28	89.00	971.19	83.0	3034.3	62.00
1	235.49	88.00	196.90	93.00	477.53	90.00	685.55	88.0	1197.7	85.00

Table 3. Thermodynamic parameters of adsorption for mild steel in 1N HCl in presence of NAL extract

Extract Conc. % v/v	-ΔG (kJ/mol)					ΔS (kJ/mol)	-ΔH (kJ/mol)
Extract Conc. % v/v	_{303 K}	_{313 K}	_{323 K}	_{333 K}	_{343 K}	ΔS (kJ/mol)	-ΔH (kJ/mol)
_.01	23.48	19.41	18.47	19.65	17.96	0.11	54.69
_.05	20.83	19.50	19.35	19.72	17.13	0.07	42.47
_.1	20.92	19.42	19.22	18.59	16.84	0.09	48.02
_0.5	17.38	17.42	18.24	17.40	14.80	0.05	33.74
₁	17.51	17.16	16.66	16.61	16.37	0.03	25.99

Table 3 shows that the values of ΔG_ads calculated are in the range -14 to -23 kJ / mol indicating that the plant constituents are adsorbed on the metal surface by a strong physical adsorption process. Negative sign indicates that the adsorption of the plant constituents on to the metal surface is a spontaneous process. ΔG_ads values are found to decrease as the temperature increases. This may be due to the predominant desorption of the constituents in the adsorption- desorption equilibrium as the temperature increases.

ΔG_ads values at the different temperatures are plotted as ΔG vs T. From the slope and intercept of this plot ΔH_ads and ΔS_ads values have been calculated. Temperature studies in 1N HCl medium for the extract yield a negative value for enthalpy of adsorption indicating that the adsorption process is exothermic. Negative values of heat of adsorption suggest physical adsorption. ΔH_ads values range from -25 kJ / mol to -54 kJ / mol which is within the predicted and accepted range of enthalpy values. ΔS_ads values do not show any gradual increase or decrease with respect to inhibitor concentration. A similar trend is observed for ΔG_ads and ΔH_ads as well. This may be attributed to the fact that adsorption of the phytoconstituents is not only dependent on concentration but is also influenced by other factors like presence of other constituents, electronic and steric interaction of the inhibitor constituents among themselves as well as with the other constituents present in the corrosive media, orientation of the constituents etc.

CONCLUSION:

Acid extract of Nyctanthes arbortristis leaves acts as good corrosion inhibitor for mild steel in 1N HCl medium. IE% increases with inhibitor concentration, decreases with temperature, increases with immersion time in 1N HCl medium. Maximum inhibition efficiency for 1hour immersion time was 90% at the inhibitor concentration 2% v/v. ΔG indicates that corrosion inhibition may be due to the spontaneous physical adsorption of the plant constituents on the mild steel surface. Inhibitor is found to obey Langmuir, Temkin, Freundlich and Flory Huggins isotherms

REFERENCES:

Received on 03.03.2011 Modified on 03.04.2011

Asian J. Research Chem. 4(6): June, 2011; Page 1001-1004

Thermodynamic and Adsorption Studies on Nyctanthes arbortristis (Night Jasmine) Leaves Extract as Corrosion Inhibitor for Mild Steel in 1N HCl Medium

V.G. Vasudha¹ and R. Saratha²

¹Department of Chemistry, Nirmala College for Women, Coimbatore-18

²Dept. of Chemistry, Avinashilingam University for Women Coimbatore-43, Tamilnadu

EXPERIMENTAL:

Inhibitor Material:

Weight Loss Method:

RESULTS AND DISCUSSION:

CONCLUSION:

REFERENCES:

Thermodynamic and Adsorption Studies on Nyctanthes arbortristis (Night Jasmine) Leaves Extract as Corrosion Inhibitor for Mild Steel in 1N HCl Medium

V.G. Vasudha1 and R. Saratha2

1Department of Chemistry, Nirmala College for Women, Coimbatore-18

2Dept. of Chemistry, Avinashilingam University for Women Coimbatore-43, Tamilnadu

EXPERIMENTAL:

Inhibitor Material:

Weight Loss Method:

RESULTS AND DISCUSSION:

CONCLUSION:

REFERENCES:

V.G. Vasudha¹ and R. Saratha²

¹Department of Chemistry, Nirmala College for Women, Coimbatore-18

²Dept. of Chemistry, Avinashilingam University for Women Coimbatore-43, Tamilnadu