Journal of Scienti fic & Industri al Research Vol. 6 1, September 2002, pp 7 12-7 18

Surface Parameters and Corrosion Inhibition of Some Isothiouranium Derivatives

S T Keera**, N A egm#, S M Ahmed* and A M Badawi *

Egyptian Petroleu m Research Institute (EPR I). asr Ci ty. Cairo Egypt

Rece ived: 26 December 200 I: accepted: 02 April 2002

Four alkyl isothiouronium halide compounds in which the alky l groups arc cl oclccy l, hex adecy l, and oc tadccy l, respecti ve ly, are prepared. T he measured surface tens ion at vari ous concent rations as aq ueous so lutions of each of these compounds at vari ous temperatures enab le the ca lculati on of surface and thermodynamic param­eters. The criti ca l mice lle concentrati ons of these compounds decreases w ith increase in temperature. The calculated free energy changes for mice lle formati on and adsorpti on of these compounds shows that these processes are spontaneous in nature. The corros ion inhibiti on properti es of these compounds for mi ld stee l in sulphuric ac id as determined th rough potenti odynamic polarization measurements, indicates that dodecyl isothiouronium bromide exhibits better inhi bitor effi ciency than its higher homologues. For the same com­pound studied, the iodide salt gives a better inhibitor effi ciency than the bromide.

Introduction

Corrosion inhibitors are widely used in industry to reduce corrosion rate of metals and alloys which is present in contact with aggress ive environments. Organic compounds, especially those containing nitrogen or sul ­phur gave a very good inhibition for steel corros in in acids1

·3• Thiourea has been descri bed as an inhibitor and as an accelerator of the corros ion of iron and stee l in acidic solutions4

, while the presence of most of its de­rivati ves shows inhibition5

·7

. The inhibition effi ciency of these compounds was related to the polar group: I > Br > Cl and the size of the nitrogen substi tu ted groups.

Thi s study deals w ith the preparati on and eva lua­ti on of four alky l isothiouranium deri vati ves, dodecy l iso thi ouranium bromide, hexadecy l isothiouranium bro­mide, octadecy l isothi ouranium bromide and dodecy l isothi ouranium iodide as corrosion inhibitors for mild steel in 0.5moi/L H

2S0

4 solution.

# Author fo r correspondence * Analysis and Eva lu ation Department ** Petrochemical Departmen t

E- mail : nabelneg m@hotmai l.com

Experimental Procedure

Svnthesis r~f"A iky!Isothiouraniull l Halides

Thiourea (0.0 I mol) was refl uxed for 3h w ith 0.0 I mol of alkyl halide, dodecy l bromide, dodecy l iodide, and hexadecy l bromide and octadecy l bromide respec­tively using isopropanol as a sol vent. The reacti on mix­ture was cooled forming a crysta lline product. It was further filtered, dried and recrystall ized with ethanol. Melting points of the prepared con1pound<> were recorded ( 11 0, 102 and 120uc fo r dodecy l, hexadecy l and octadecy l isothi ouranium bromide, re pectively).

E l ement al mi croan alys i s for alk y l isothiouranium bromide was carried ou t via VATIO EL EL EMNTAR.

The IR-spectra showed: 2850--2950cm·1 (V C· ll SI

aliphatic), 980cm·1 (Vc), 1430ct · I (V( ~ ) and the char-acteri sti c NH+

2 at 3400cm·1.

Surface tension values were determined at 25, 40, and 55 "C, respecti ve ly, using Du-Nouy Tensiometer with a plati num ring. Surface and thermodynam ic parameters were cal culated, using Gibb 's adsorpti on and thermody­namic equations8· 9

KEERA el a/.: SURFACE PARAMETERS & CORROSION INHIBITION 713 .

C. per cent H. per cen t N. per cent S. per cen t Br per cent Compound MWt

Calc. Found Calc. Found Calc. Found Calc. Found Calc . Found

Dodccyl isothiouranium hromidc 325 48.00 48.05 8.92 8.9 1 8.62 8.63 9.85 9.80 24.77 24.7

Hcxadccy l i sothiouranium hromidc 381 53.54 53.60 9.711 9.70 7.35 7.36 8.40 8.38 21.13 21.2

Octadccyl iso thi ou ranium hromidc ~09 55.74 55.80 0.()3 10.0 6.85 6.87 7. 82 7.80 19.68 19.7

60

~ 55

e II

~ 50 .. .... .., -:: 45 . . S! j 40 ~ ~ 35 = VJ

30

25

5 4 3 2 s 4 3 2 s 4 3 2 -loge (moVI)

Figure I- Surfa<.:e tension isotherm of alky l isothiouranium bromide: (a) dodecyl. 6 25 "C. • 40 "C. D 55 "C. (b) hexadecyl. 6 25",

• 40". D 55 oc. (c) octade<.:yl. 6 25 oe, • 40 oc. D 55 oc The prepared compounds were evaluated as cor­

rosion inhibitors for mild steel in 0.5 M H2S0

4 solu­

tion . The experiments were performed with mild steel specimen having composition (wt per cent): 0.2C, 0.03Si, 0.5Mn, 0.8P, and the remai nder Fe. The specimen was machined into cylindrical form electrode of 11 .3 mm2

cross-sectional area. The electrode was covered with araldite so that only the cross-section area came in con­tact with the aggressive so lution . The corrosion rate measurements were undertaken using a standard three­electrode arrangement incorporating mild steel test elec­trode as the working electrode, platinq,rn counter elec­trode as the auxiliary electrode and a saturated calomel electrode as the reference electrode.

Electro-chemical polarization measurements were carried out using corrosion measurement (EG&G Model 273 Potentiostat/ Galvanostat) with scan rate I m V /sand at 25 ± I "C.

Results and Discussion

Surface Parameters

Figures 1-3 represents the variat ion of surface ten­sion, y against logarithm of concentration of alkyl isothiouranium bromide compounds at 25,40 and 55°C, respectively. It is evident that increase in the concentra-

·200

UJ -325 (.) (f)

~) . #.; . · : ·.":·"./

> -450 E w

-575

_ ~}n_.- ,

- --~==.... ~:::-·-~ ... ~·-_ .. / - - -·""-··· . '

'. -700

-200

.',1

UJ -325 (.) (f)

> -450 E w

-575

-700

-825 '-----'---..L~....L.,...__,...L__J._~L__J.~_J -8 -7 -6 -5 -4 -3 -2 -1 0

l(uA/cm2)

Figure 2- Polarization curves for mild steel in 0.5 moi/L H2S04 solu tion in the absem:e and presen<.:e of var ious concentrations of dodet:yl isothiouranium halides: (a) iodide, l: 0 ppm. 2: 25 ppm, 3: 200 ppm. 4: 300 ppm. (b) bromide. I : 0 ppm. 2: 25

714 J SCI IND RES VOL 61 SEPTEMBER 2002

Table I-Surface parameters of the alkyl isothiouranium bromide surfactants at 25. 40. and 55°C

T.oC -logCMC 7t eme PC20 rmax X 1011 AminSurfactant

mollL dyn/cm mol/em/ nrn'

25 0.00420 42.0 3.85 26.98 61.54

40 0.00500 36.2 4.19 20.57 80.75Dodecyl isothiouranium bromide

55 0.01000 36.0 5.90 9.03 183.96

25 0.00058 40.0 4.81 21.23 78.24

40 0.00053 37.5 5.05 20.88 79.56Hexadecyl isothiouranium bromide

55 0.00085 37.4 6.02 11.86 140.06

25 0.00100 31.5 4.67 13.07 127.09Octadecyl isothiouranium bromide

40 0.00130 34.0 5.33 12.86 129.17

55 0.00150 35.3 6.00 11.88 139.83

sion gradually till certain concentration, critical micelleconcentration - CMC, after which the surface tensionstill remains almost constant whenever the concentra-tion increases. Surface parameters have been calculatedand the results are given inTable 1. The increase in tem-perature (40-55°C) has a decreasing effect on the criti-cal micelle concentrations. This is due to the decrease inhydration of molecules in the aqueous phase, hence,micellization occurs at lower concentrations. Increasein the molecular weight of the halide ions has a decreas-ing effect on their critical micelle concentrations.

The results shown in Table 1 predict that the ef-fectiveness, 1t of the compounds decrease with increasein the hydrophobic chain length as well as the increasein the temperature, except for octadecyl isothiouraniumbromide, which increases with increase in the tempera-ture. Dodecyl isothiouranium bromide is the most effi-cient in bromide series.

Increase in the number of methylene groupsthrough the hydrophobic moieties of the alkylisothiouranium bromide decreases the maximum surfaceexcess F at 25°C (ref. 10). Increase in the tempera-, max

ture, causes a decrease in the maximum surface excessfor all the prepared compounds.

The minimum surface area, A. occupied bynun

each surfactant molecule increases with increase in thehydrophobic chain length. -That effect occurred due tothe interaction between the hydrophobic chains and wa-

ter molecules, which forced them to be adsorbed hori-zontally at the air/water interface. Similarly, increase inthe temperature decreases their hydration, which hassimilar influence on the minimum surface area.

Thermodynamic ParametersThermodynamic parameters of micellization and

adsorption of the prepared alkyl isothiouranium bromidenamely; standard free energy change, standard entropychange, and standard enthalpy change were calculatedat 25,40, and 55°C, respectively, and are shown in Table2 and 3. It is evident that the standard free energy changesof micellization ~G . always have -ve values, indicat-

, file

ing that the process of micellization is a spontaneousone. The negativity of the standerd free energy changeincreases with increase in the hydrophobic chain length.This is due to the fact that the transfer of methylenegroups from the aqueous phase (unfavoured position)into the spherical micellar phase increases the stabilityofthe hydrophobic moieties I I. This negativity increaseswith increase in the temperature, indicating that thehigher stabilization occurred inside the micelles ratherthan the single state molecules in the bulk of their solu-tions.

The standard entropy change of micellization,~S . has low values, indicating the restriction of freemo;i~n for the hydrophobes inside the micelles", While,

KEERA et al.: SURFACE PARAMETERS & CORROSION INHIBITION 715

Table 2-Thermodynamic parameters of micellization of the alkyl isothiouranium bromide surfactants at 25, 40 and 55 OC

Surfactant T,oCll.Gmin ll.Smin ll.H min

kJ I mol kJ I mol K kJ I mol

25 -29.61

Dodecyl isothiouranium bromide 40 -30.11 0.033

55 -27.42 0.179 +3l.29

25 -40.32

Hexadecyl isothiouranium bromide 40 -41.88 0.104

55 -42.10 0.015 +37.68

25 -37.37

Octadecyl isothiouranium bromide 40 -37.76 0.026

55 -38.72 0.064 -17.05

Table 3 -Thermodynamic parameters of adsorption of the alkyl isothiouranium bromide surfactants at 25, 40 and

550C

Surfactant T,oCll.G ~dd zs ~dd ll.H;dd

kJ I mol kJ I mol K kJ I mol

25 -31.27

Dodecyl isothiouranium bromide 40 -31.87 0.040

55 -31.41 0.030 -21.34

25 -42.21

Hexadecyl isothiouranium bromide 40 -43.68 0.098

55 -45.26 0.105 -10.72

25 -39.78

Octadecyl isothiouranium bromide 40 -40.41 0.042

55 -41.71 0.087 -13.17

increase in the temperature increases the kinetic ener-gies of the hydrophobic moieties which gets more free-dom inside the micelles, hence the standard entropychange increases.

The standard enthalpy change of micellization,~Hmicis, sometimes either positive or negative". Thisindicates that the process of micellization is governedmainly by the entropy gain and the factor affecting theprocess is the transfer of one-methylene group into thenonpolar bulk of the spherical micelles.

The standard free energy changes of adsorption,LlGads. have also negative values along with the standardfree energy change of micellization at the temperatureof the measurements. The slight increase in the negativ-ity of the standard free energy of adsorption indicatesthat excess energy is gained as the compound moleculetransferred into the bulk of the solution to form the ag-gregates (micelles). Increase in the temperature increasesthe free energy change of adsorption to less extent, hencerise in the temperature has much less effect on the ad-sorption of the compound molecules.

716 J SCI IND RES VOL 61 SEPTEMBER 2002

-75

-200

W -325UenS- -450'SW

-575

-700

-825

-200

W -325Uen

i-450

-575

-700

b

-825 L.-....L--"'.J.....,....J._....J-_L.........L~....L..........J-8 -7 -6 -5 -<I -3 -2 -1 0

I(uAlcm2)

Figure 3 - Polarization curves for mild steel in 0.5 rnol/L H2S04solution in the absence and presence of various concentrations ofalkyl isothiouronium bromide: (a) hexadecyl, 1: Oppm, 2: 50ppm,3: 100ppm, 4: 400 ppm, (b) octadecyl, 1: Oppm, 2: 100 ppm, 3:

400 ppm

Increase in the temperature increases the stan-dard entropy change of adsorption for hexadecyl andoctadecyl isothiouranium bromide, which could be ex-plained due to the higher values of the maximum sur-face excess. While the smaller hydrophobic chains havelower values of free entropy of adsorption at higher tem-perature, in spite of higher surface excess values. Thestandard enthalpy change of adsorption, LlHad" has nega-tive values, but the negativity of these values is not anevidence for the stabilization of adsorption process.

Potentiodynamic Polarization MeasurementsPolarization curves of the potentiodynamic

measurementse are shown in Figures 4-7. It is evidentthat the addition of the inhibitors shifts the corrosionpotential, Ecarr to better direction. This observation showsthat the corrosion inhibition is under anodic control. Itis evident that the corrosion current density, I valuescorrdecrease considerably with increase in concentrationsof the inhibitor, then increases. These values coincide

90

10

70

160

j~40

30

200 ~ 100 I~ 100 1~ 300 3~ 400

c•••••tratlo., (PPII)

Figure 4 - Inhibition efficiency vs concentration of the inhibitorsfor the dissolution of mild steel in 0.5 mol/L H2S04 solution at

25°C

with the polarization resistance, R , corrosion rate, Randpthe efficiency, PI, (Table 4).

The maximum inhibition efficiency of dodecylisothiouranium bromide and dodecyl isothiouraniumiodide reached 84 and 89 per cent, respectively, at con-centration of 200ppm then decreased. The maximuminhibition efficiency of hexadecyl and octadecylisothiouranium bromide reached 72 and 63 per cent, re-spectively, at concentration of 400 ppm, then decreased.This inversion in behaviour of efficiency is duo to theincrease in the number of adsorbed layers at the metal /solution interface, which has a strong influence on theinterfacial reaction mechanisms". There is no increasein the efficiency of inhibition over the critical micelleconcentration. The change in the hydrophobic chainlength of the studied compounds (Figures 2 and 3) showsits influence on the corrosion current density and theoctadecyl isothiouranium bromide reaches the lowestinhibitor efficiency, which is the same as that describedpreviously". Many studies have revealed that the inhi-bition efficiency of n-alkyl quaternary ammonium saltsincreases with increase in chain length up to C12• Withincrease in size of the alkyl group the electronic chargedensity on the nitrogen atom increases and the inhibit-ing properties of the positive group decrease as a resultof a less tightly held layer of positive ions IS. For a fam-ily of molecules, which differ in length of carbon chain

KEERA et al.: SURFACE PARAMETERS & CORROSION INHIBITION 717

rable 4 -Electrochemical polarization parameters for mild steel in 0.5moVI H2S04 solution in the presence of different concentrations(25-400ppm) of alkyl isothiouranium halide derivatives at 25°C

Compound Concentration, -Ecorr• mY I""" /lA/em B"mV/dec Bn mV/dec Renr" mpy Rp, Ohm PI,ppm Per cent

Blank 520.3 582.2 111.0. 206.8 236.30 56.27

25 505.8 124.30 84.60 188.3 50047 21604 79

50 480.0 91.25 80.61 17604 37.04 263.6 84Dodecyl

100 75.66487.0 77.65 172.9 30.71 297.9 87isothiouranium

200 478.1 63.56 73049 166.1 25.80 335.7 89iodide

300 479.2 106.10 93.66 199.3 43.06 263.1 82

400 486.8 119.90 80.38 172.7 48.68 203.6 79

25 516.5 217.1 97.69 197.7 88.13 134.0 63

50 514.6 190.0 89045 18104 77.11 14704 67

Dodecyl 100 498.9 139.0 84.24 16704 56040 181.6 76

isothiouranium 200 505.7 94.03 77.60 162.5 38.16 25604 84

bromide 300 497.1 120.0 83.57 189.3 48.72 210.9 79

400 49604 138.6 81.69 166.7 56.24 170.9 76

50 519.3 329.6 97.76 184.5 133.8 85.41 43

100 517.9 269.1 92.89 171.9 109.2 91.88 54Hexadecyl

200 499.4 252.9 89.64 185.8 102.7 104.1 57isothiouranium

300 519.4 230.3 90.36 198.8 93049 117.9 60bromide

400 511.6 15.9 87.63 176.6 67.35 149.1 65500 508.0 206.3 87.59 179.8 83.72 122.5 75

50 502.3 43004 100.1 201.6 174.70 69.21 26

Octadecyl 100 496.7 334.2 100.7 190.2 135.70 73.02 43

isothiouranium 200 497.6 295.6 93.28 188.5 120.0 91.32 49

bromide 400 487.3 212.0 88.37 196.6 86.03 117.0 63

500 499.6 338.7 93.68 210.4 137.5 80.64 42

the inhibitory action obeys Traube's rule" for values of face. Therefore iodide ion (radius = 1.33A.) is morehydrophobicity corresponding to chain lengths of less predisposed to adsorption than bromide ion (radius =than 11-12 methylene groups. Above these lengths, this 1.14 A.).rule is no longer observed, and the inhibitory action ei- The inhibition mechanism of the studied com-ther decreases or remains the same depending on the pounds is attributed to the adsorption of positivelytype of molecule. charged n-alkyl quaternary ammonium ions on the metal

The data indicate that the inhibition efficiency surface, accompanied by the co-adsorption of halide ionsdecreases in the order dodecyl isothiouranium iodide with the whole compound behaving as a nonionic com-(IV) > dodecyl isothiouranium bromide (III) > hexadecyl pound due to the charge neutralization. The first adsorbedisothiouranium bromide (II) > octadecyl isothiouranium ions of the inhibitors are attached to the most active an-bromide (I). odic sites, usually considered to be the corners or edges

A comparison of the structure of compound (IV) of incomplete layers of atoms on the metal surface. Thisand (III) reveals that the inhibitive effect of iodide ions interferes with the anodic reaction by hindering the es-is more than that of bromide ions. This indicates that the cape of Pe+2 ions from the metal surface into the solu-radii of halogen ions may playa role. With increasing tion and the attack on the metal then proceeds at a lowerionic radius, the electron cloud is facilitated, i.e. they rate from less active anodic sites. Upon increasing theare deformed more easily when adsorbed on a steel sur- concentration, more ions are adsorbed due to van der

7 18 J SCI IND RES VOL 61 SEPTEMBER 2002

Waa ls forces between the inhibitor molec ul es. The in­creasingly close-packed film thus built up further inhib­its the anodic reaction, both by steric hindrance to the escape of Fe+2 ions and also because of the adverse elec­tric field from the positively charged layer of the adsorbed ions. At thi s stage, there is some inhibition of the ca­thodic reacti on probably due to steric hindrance of the comb in ati on of the adsorbed hydrogen atoms to form molecules. A similar mechani sm was suggested for the inhibition of steel in acidic medium by n-alkyl quater­nary ammonium compounds 17

.

Conclusions From th e thermodynamic adso rpti o n and

micelli za ti on res ults, it can be co ncluded that alkyl isothiouranium halide molecules prefer the micellization in the bulk of the solution rather than the adsorpti on at air/water interface at lower temperatures. On the con­trary, at hi gher temperatures the molecul es have more tendency towards adsorpt ion at the interface .

Potenti odynami c Polari zat ion Measu rements indicate that the alkyl Isothiouran ium halide compounds proved their influence on the inhibition of mild steel in H

2S0,

1 solution . The compounds act as anodic typ

inhibitors.The efficiency increases with decreaese in chain length and increases with increase in polar group.

Dodecyl isothi ouranium iodide (IV)> dodecyl isothiouranium bromide (Ill)> hcxadecyl isothiouranium bromide (II)> octadecyl isothiouranium bromide (I) .

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