MODELLING THE ADSORYI10N OF ORGANIC CORROSION INHmITORSON METAL SURFACES

R. RAICHEFF), I. BETOVAI, M. BOTINOVl and E. LAZAROVAIJDepartment of Electrochemical Engineering and Co"osion ProtectionSofia University of Technology1156 Sofia'Bulgaria

2Central Laboratory of Electrochemical Power SourcesBulgarian Academy of Sciences1113 SofiaBulgaria

ABSTRACT. Modelling of the adsorption of blocking-type organic inhibitors on a co"odingmetal surface in aqueous solutions has been carried out, The method is based on theassumption ofa Frumkin-type interaction between the adsorbed species and enables evaluationof the maximum surface excess of the adsorbed species and hence the area occupied by a singleadsorbate.

This adsorption model approach is applied to study the inhibition ofiron co"osion in H~o4solutions by indenone derivatives. Combining electrochemical (a.c. impedance and d.c.polarization) methods with quantum chemical calculations of the chemical structure of thecompounds studied, a mechanism ofadsorption ofthe inhibitors on iron surface is hypothesized.The co"elation between chemical structure, adsorption capability and inhibiting efficiency ofindenone derivatives is discussed.

1. Introduction

The inhibiting action of organic compounds with respect to the corrosion of metalsin aggressive media is most often related with their specific adsorption on the metalsurface [1-8]. The latter process results in an effective blocking of the active sitesof metal dissolution and/or hydrogen evolution thus diminishing considerably theoverall corrosion rate. Hence, a correlation between the latter and the surfacecoverage with organic species is established for quite a number of cases of corrosionin inhibited acid solutions [9-12].In order to describe the mechanism of the inhibiting action of organic additives,

it is therefore needed to obtain in situ information for the main characteristics ofadsorption - degree of coverage of the metal surface with surfactant species,maximum surface excess of the adsorbed substance, adsorption equilibrium constantand constant of lateral interaction in the adsorption layer. With a readily corrodingmetal electrode, this is a somewhat difficult task and the use of both stationary andtransient methods is indispensable. The main purpose of the present paper is to

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K. R. Trethewey and P. R. Roberge (eds.), Modelling Aqueous Corrosion, 89-102.© 1994 Kluwer Academic Publishers.

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develop a modelling procedure for evaluation of adsorption characteristics ofblocking type organic inhibitors on a corroding metal surface.In a recent series of papersr12-14] the inhibiting properties and the adsorption

behaviour of severa] indenone derivatives on polycrystalline iron were investigatedusing mainly capacitance measurements. The likelihood of two-dimensionalcondensation of 3p-tolyl-2-phenyl indenone was discussedf14] and was associatedwith an effective blocking of active surface sites leading to more than an order ofmagnitude decrease in the corrosion current density. In this connection, it seemeduseful to apply the derived adsorption model approach to a comparative study of theinhibition of iron corrosion in H2S04 solutions by indenone derivatives. Thus, theultimate aim of the present work is to search for a correlation between themolecular structure of the derivatives and both their adsorption capability andinhibiting efficiency.

2. Tbeoretical Background

In this section, the framework of a simplified approach to the adsorption of organicadditives based on the Frumkin and Damaskin biparallel capacitor model [15] willbe presented. Its main assumptions are as follows:

a) In the whole investigated potential interval, the adsorption of the studiedsurfactants is described by the Frumkin isotherm:

6Bx= (1 -6) exp ( - 2 a6) ( 1 )

where x is the maximum bulk concentration of the additive, B is the potentialdependent constant of adsorption equilibrium, eis the potentia] dependent surfacecoverage in respect to the additive, and a is the potential dependent attractionconstant, characterising the adsorbate-adsorbate interactions in the adsorption layer;

b) The potentia] dependence of the constant of adsorption equilibrium is expressedas

(2 )

where

(3 )

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(4 )

(5)

Bo is the constant of adsorption equilibrium at the uncharged metal surface, Co isthe minimal capacitance of the blank solution with no additive (for 6 = 0 ), C is theminimal capacitance for 6 = 1 and EN is the potential of the uncharged surface at6 = 1.

3) For the adsorption of surfactants of a molecular type a linear relation betweenthe potential and the attraction constant is valid:

(6)

4)The charge of the electrode surface, q, as depending on the potential is given bythe equation:

q=qO(1-6) +c' (E-EN ) 6-RT r m :'.6 (1-6) (7 )

where qO is the charge versu potential dependence for the blank solution and daldE= at (equation (6».

5) The parameters C, r m, Bo, EN, am and at are assumed to be constant in thestudied potential interval and to fully characterize the adsorption state of thesurfactant species. From the value of the maximum surface excess, the projectedarea of a single adsorbate can be estimated by the expression:

(8)

where NA is Avogadro's number.

6) As the dependence of the attraction constant, a, on potential is assumed tobe linear (eqn.(6», for the uncharged metal surface (i.e. for the potential EN) thesimplified formula of Frumkin can be used to estimate the degree of surfacecoverage in respect to the organic additive:

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6= (Co-C)

(CO-C/)(9 )

where C is the capacitance at the potential EN for a definite bulk concentration ofthe surfactant. Relation (9) permits the construction of adsorption isotherms 6(C)Eallowing the validity of Frumkin's isotherm for the potential EN to be tested.

3. Experimental

3.1. ELECIRODES AND ELECIROLYTES

Cylindrical electrodes of spectroscopically pure iron (15 ppm impurity level) wereused (working area 0.6 cm2). The working electrode was polished mechanically withfiner grade emery paper and thoroughly rinsed with bidistilled water. This procedureresulted in a mean roughness factor of 1.66. A conventional three electrode cell wasused. The counter electrode was a large platinum mesh situated symetrically aroundthe working electrode and a saturated calomel electrode was applied as reference.Note, all the potentials in the present paper are recalculated to the standardhydrogen scale.The working electrolyte (1 M H2S04 ) was prepared from pure H2S04 (Merck)

and doubly distilled water. The indenone derivatives studied were synthesized bylaboratory methods and their purity was controlled by IR spectroscopy. They wereadded to the working medium as an ethanolic solution. The bulk concentration ofethanol was kept constant and equal to 5% by volume. The range of concentrationof the organic additives was 0.01 - 0.1 p.mol 1.1• The solutions were deaerated bybubbling of prepurified Argon. All the measurements were carried out at roomtemperature.

3.2. PROCEDURE FOR DATA ACQUISITION

The registration of a.c. impedance spectra both at the free corrosion potential andat cathodic bias [14] permitted the conclusion that for frequencies above 800 Hz, theiron electrode in 1 M H2S04 is electrically equivalent to a series combination of theohmic resistance of the electrolyte and the double layer capacitance. Thus afrequency of 870 Hz was chosen for the registration of capacitance versus potentialcurves. The capacitance was recalculated for a true surface area using the abovementioned roughness factor. The simultaneous potentiodynamic registration of thesecurves and the d.c. polarization ones (sweep rate 0.1 mV S·I) using a Solartron1286/1250 instrument allowed the characterization of both the high-frequencybehaviour of the electrode (equivalent to the double layer capacitance) and the zerofrequency behaviour (equivalent to the polarization resistance). In this way, it is

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possible to separate the contribution of non-faradaic (adsorption in the double layer)and faradaic (corrosion) processes. However, several implicit assumptions are made:

1) It is assumed that adsorption kinetics is fast compared to the faradaic processes.This assumption is justified by the experimental fact that no further loop wasobserved in the a.c. impedance spectra [14] except the ones for charge transfer andelectrosorption of intermediates of the corrosion process.

2) The surface coverage of the electrode surface,6, is assumed to depend only onpotential, i.e. steady state of both adsorption and faradaic processes is postulated.This assumption seems reasonable since in preliminary work no substantialdifference has been found between the potentiodynamically (0.1 mV S·I) andpotentiostatically traced d.c. polarization and a.c. capacitance curves. Thus a surfaceseparation of the adsorption and faradaic processes is believed to occur, the formertaking place on the covered surface and the latter on the uncovered one.

3.3 NUMERICAL PROCEDURES FOR EVALUATING TIIE ADSORPTIONPARAMElERS

The numerical procedure adopted throughout this investigation comprised severalstages as follows:

1) Using a spline - based quadrature routine, Jlumerical integration of theexperimental capacitance versus potential curves was accomplished for the maximumadditive concentration used. This procedure resulted in a charge versus potentialdependences q(E)exp derived from experimental data.

2) The Frumkin isotherm, eqn. (1), taking into account equations (2)-(6) for thepotential dependence of the constant of adsorption equilibrium and the attractionconstant, was solved iteratively in order to obtain the 6(E) dependence for apredefined initial estimates of the adsorption parameters C', rm, Boo EN' a., and a l ·

3) Using the resulting 6(E) dependence, the charge versus potential dependence forthis initial set of parameters q(E)""lc was computed using equation (7).

4) Accomplishing non-linear least squares regression of the function:

by the algorithm of Levenberg-Marquardt, the optimum values of the adsorptionparameters are evaluated.

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- 0.289

1

O---H+-0.14

+ 0.12 + 0.12H3C-0.23 CH3

"'N/-0.288

O-CH3+0.215

1

d) O---H+- 0.14

-0.030Br

+0.079

CH3

Figure 1. Molecular structure and distribution ofpartial charges for the studied diphenylindenonederivatives: a) 2,3-diphenylindenone; b)protonized form of 2,3-diphenylindenone; c)3-dimethyl­

aminophenyl-2-phenylindenone; d)3-metoxyphenyl-2-phenylindenone; e) 3-bromphenyl-2­phenylindenone; f)3-p-tolyl-2-phenylindenone.

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3.4. CHEMICALSTRUCTUREOFTIIE INVESTIGAlED INDENONEDERIVATIVES

The structural formulae of the investigated organic additives, together with thedistribution of partial electronic charges in the molecules as obtained by quantum ­chemical calculations using the MNDO method, are presented in Figure 1.

It was established that protonization of the carbonyl oxygen bonded to the fiveatom ring occurs in acid media; this phenomenon changes somewhat the electronicdistribution in the molecules and was taken into account when depicting thestructure of the compounds. These data offer the possibility of estimating the surfacearea of the whole molecule, as well as the partial areas of the different functionalgroups in it. When calculating the area of the molecule as a whole, it was taken intoaccount that the benzene rings are situated in planes perpendicular to the one of thecondensed ring.An overall conclusion can be drawn that the included substitute groups: -N(CH3)2(Figure 1c), -OCH3 (Figure 1d), -Br (Figure Ie) and CH3 (Figure If) at ap-positionin the 3-benzene ring of the 2,3-diphenylindenone molecule change the partialnegative charge of this ring. The other partial charges remain practically constant.

4. Results and Discussion

The differential capacitance versus potential curves for the Fe electrode in 1 MH2S04 for an additive concentration of 0.1 J1.mol P obtained by the a.c. impedancetechnique are presented in Figure 2.

In comparing the curves 1 and 1', the influence of the ethanol additive can bedetected. This influence is rather small and is exerted mainly on the cathodic branchof the curve.The addition of the diphenylindenone derivatives (curves 2-6, Figure 2) stronglyinfluences both the shape of the curve and the capacitance values of the Feelectrode. For the 3-p-tolyl-2phenylindenone and 3-bromphenyl-2-phenylindenoneadditives the relative decrease of the capacitance is the greatest and the capacitanceplateau region is the largest.The dependences of the degree of surface coverage, 6, on the bulk concentrationof the different additives for the potentials of uncharged surface (EN) as calculatedusing equation (9) are presented in Figure 3. Estimates for the C' and EN valueswere obtained by double extrapolation of the linear regions of the lIC - lIE curvefor the maximum additive concentration (16].Maximum values of 6 are reached for relatively low concentrations for the 3­bromphenyl-2-phenylindenone and 3-p-tolyl-2-phenylindenone additives (curves 4and 5), Le. for the compounds exhibiting the maximum drop and the broadestplateau region in the differential capacitance of the Fe electrode.The linear dependences obtained in coordinates In[6/(1-6)x] versus 6 (Figure 4)confirm the validity of Frumkin's isotherm for the adsorption of the investigated

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organic additives on iron in 1 M H2S04 at the potentials EN. As maximum coveragefor 3-p-tolyl-2phenylindenone is reached at quite low concentrations, such adependence can not be plotted for this compound.

80

70

60

C';' 505u.:-2'-L,Qu

30

20

10Ecorr

0-0.6 -0.1

Figure 2 - Capacitance versus potential curves of the Fe electrode in 1M H2S04 + 0,1 JUTloll'/ of thefollowing diphenylindenone derivatives. 1: blank solution; I' - blank solution + 5% CpIJH; 2: 2,3­diphenylindenone + 5% CpIJH; 3: 3-dimethylaminophenyl-2-phenylindenone + 5% CPIJH; 4: 3­

metoxyphenYI-2-phenYlindenone + 5% CPIJH; 5: 3-bromphenyl-2-phenylindenone + 5% CpIJH; 6:3-p-tolyl-2-phenylindenone + 5% CP50H.

The qO(E) and q(E) dependences for the maximum bulk concentrations of theindenone derivatives studied are plotted in Figure5. In this figure, points representdata obtained by numerical integration of the experimental C(E) curves (Le. q(E)exp;see subsection 3.3 above), and solid lines are calculated using equation (7) byinsertion of the optimum values of the adsorption parameters (Le. q(E)calc). A goodagreement between the experimental data and model predictions is obtainedsuggesting the validity of the proposed approach. The values of the best-fit values

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of the parameters characterizing the adsorption of the diphenyl indenone derivatives,namely r m , ao and ail are listed in Table 1.

1.0 . ;

0.8

0.6

cp

0.4

Figure 3. Surface coverage versus additive concentration at the potential EN for the followingdiphenylindenone derivatives. 1: 2,3- diphenylindenone; 2: 3-dimethylaminophenyl-2-phenylindenone;

3: 3-metoxyphenyl-2-phenylindenone; 4: 3-bromphenyl-2phenylindenone; 5: 3-p-tolyl-2-phenylindenone.

Typical polarization curves together with a.c. impedance spectra of the ironelectrode in 1 M H2S04 with and without the addition of bromdiphenyl indenone arepresented in Figure 6. The Figure indicates that the corrosion mechanism isprobably unaffected by the organic additive. However, the overall corrosion rate issignificantly reduced. It can be stated that the surfactant influences predominantlythe rate of hydrogen evolution.The values of the capacitance drop, (Co - C), resulting from the transition frome = 0 to e = 1, as well as the corrosion current densities icorr and the inhibitingefficiencies Z=icorro-icorr)licorro.100% calculated from the analysis of d.c. polarizationcurves, are also included in Table 1.The following conclusions can be drawn from the results presented in Table 1 and

Figure 6:

1) The indenone derivatives studied can be regarded as inhibitors of iron corrosionin H2S04 solutions.

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19.0• 4

18.5

18.02

cDIf 17.51

.5

17.0

16.5

16.00.3 0.8 0.9 1.0

Figure 4 - Dependence ofln(ff(1-8)x) on 8 according to Frumkin's isotherm for thediphenylindenone derivatives. 1: 2,3- diphenylindenone; 2: 3-dimethylaminophenyl-2-phenylindenone;

3: 3-metoxyphenyl-2-phenylindenone; 4:3-bromphenyl-2- phenylindenone.

2) A good correlation between the drop in the double layer capacitance (Co-C')and the corrosion current density (Le. the inhibiting efficiency) is observed, leadingto the suggestion that the inhibiting effect of the investigated additives in respect toiron corrosion in 1 M H2S04 is a result of their adsorption on the metal surface.

3) The values of the attraction constant ao at the potential of the uncharged surfaceEN increase with the increase of the capacitance drop (Co-C') thus indicating that thelateral (adsorbate-adsorbate) attraction forces increase, Le. the stability of theadsorption layer is directly related to the capacitance drop.

4) For the basic compound diphenyl indenone, the projected area of a singleadsorbate, S = 0.52 nm2 is close to the area of the condensed ring, 0.55 nm2, Le.most probably the adsorption is realized via this ring.

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-22

-71 -13

N c;<I

5-2 E~ 0u U

::L :::L-- -0- 0-3

8 20 0:1 0.2 03 0.4 0.5 0.6 0 0.6

-E/V

Figure 5 - Charge versus potential cUTlles of the Fe electrode in 1 M H2S04 + 0,1 lJl7Iol F of thefollowing diphenylindenone derivatives. 1: 2,3- diphenylindenone + 5% CPIJH; 2: 3­

dimethylaminophenyl-2-phenylindenone + 5% CpIJH; 33-metoxyphenyl-2-phenylindenone + 5%C2HsOH; 4: 3-bromphenyl-2-phenylindenone + 5% CpIJH; 5: 3-p-tolyl-2-phenylindenone + 5%CPsOH Points - data obtained by integrotion of capacitance versus potential cUTlles; solid lines -

best-fit calculation.

100.....-----------------.

4:E­....

Figure 6 - Potentiodynamic polarization cUTlles of iron in 1M H2S04 with and without the addition of0.1 lJl7Iol t l

. 3-bromphenyl-2phenylindenone + 5% CPIJH Inset a.c. impedance spectra of iron atthe corrosion potential in 1 M H2S04 with and without the addition of 0.1 lJl7Iol F 3-bromphenyl-2­

phenylindenone + 5% CPsOH

lOO

5) The projected area of the single adsorbate for the dimethylamino-diphenylindenone(O.25nm2), metoxy-diphenyl indenone (0.20 nm2) and brom-diphenylindenone(O.14 nm2) coincide within the error limit with the areas of the -N(CH3)2(0.24 nm2

), -OCH3(0.15 nm2) and -Br(O.12 nm2

) groups, respectively. These densitiesand the inhibiting efficiencies, Z, calculated from the analysis of dc polarizationcurves, are also included in Table 1.

6) The projected area of a single adsorbate for the tolylphenyl indenone (0.28nm2) does'not match any of the active parts of this molecule. This experimentalfinding could be tentatively explained by the possible two-dimensional condensationof this compound on the surface of iron[14] leading to more complicated adsorbatestructure (involving hydrogen bonds and/or association of more than one moleculeduring the adsorption process). The quite high value of the attraction constant (ao

= 2.25) can be taken as an indirect proof for such a behaviour.

TABLE 1Summary of the electrochemical and adsorption parameters for diphenylindenone

derivatives.

Compound C.-C i,.., Z% I'm S Inm2 a. a,no 1}£F.em·2 IrnA em·2 110'0 mol em·2 N"

c

20.4 0.18 82 3.19 0.52 0.26 0.180

2 22.4 0.11 89 6.60 0.25 0.72 0.180

3 26.5 0.09 92 8.20 0.20 1.20 0.020

4 33.2 0.06 94 12.1 0.14 1.45 0.014

5 34.1 0.04 96 5.85 0.28 2.25 0.015

Electrolyte: Fe in 1 M H2S04; icon = corrosion current density; Z = inhibition efficiency; rm=maximum surface excess; S = adsorbed molecule area; ao = Frumkin attraction constant at EN; a,= da/dE.Reference values for Fe in the blank 1 M H2S04 solution: Co = 45,25 p,F.cm·2, icon 0= 1,01 rnAcm·2•Derivatives: 1: 2,3- diphenylindenone; 2: 3-dimethylaminophenyl-2-phenylindenone; 3: 3­metoxyphenyl-2-phenylindenone; 4: 3-bromphenyl-2-phenylindenone; 5: 3-p-tolyl-2­phenylindenone.

S. Conclusions

A model approach to the adsorption of organic inhibitors of the blocking type oncorroding metal surface in aqueous solutions is developed. Based on simultaneousregistration of polarization and capacitance curves, this approach permits evaluationof the main parameters of the adsorption process. Using the maximum surface

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excess of the adsorbate as a starting point and combining the results derived fromcapacitance measurements with quantumchemical calculations of the molecularstructure of the surfactants, an orientation of the adsorbed species on the metalsurface is suggested. Correlation between chemical structure, adsorption capabilityand inhibiting efficiency is discussed for a range of indenone derivatives in respectto iron corrosion in H2S04 solutions.

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