Adsorption of thiophene derivatives on steel in sulphuric acid solutions

Corrosion Science, 1973, Vol. 13, pp. 557 to 565. Pergamon Press. Printed in Great Britain

ADSORPTION

OF IN

THIOPHENE

SULPHURIC

DERIVATIVES

ACID

ON

STEEL

SOLUTIONS*

M. KAMINSKI a n d Z. SZKLARSKA-SMIALOWSKA Institute of Physical Chemistry, Polish Academy of Science, P.O. Box 49, Warsaw 42, Poland Abstract--Measurements of the corrosion rate of mild steel in 1N HsSO4 solution at 25°C with and without addition of thiophene mono-derivatives (l.10-s-2.10 -i mole/l.) were performed. Adsorption of these compounds was elucidated. Kesults show that thiophene and its derivatives are adsorbed on the steel surface according to Parsons and Hill de Boer isotherms. This suggests that the adsorptive properties of the compounds under consideration approach to a physical type. From th eadsorption isotherms some thermodynamic data for the adsorption process (AG'd, and .f) are calculated and discussed. Rtsumt----La vitesse de corrosion de l'acier doux dans H2SO4 IN b. 25°C a 6t6 mesurte sans et avec addition de dtriv~s mono de thioph~ne (1.10-6-2.10 -I mole/L). L'adsorption de ces compos~s est r~solue: les r~ultats montrent qu'elle se fair sur racier selon les isothermes de Parsons et Hill de Boer. Ceci indique que les proprittts d'adsorptiort de ces composts s'approchent d'un caractd:re physique. A partir des isothermes d'adsorption, on calcule et commente certaines valeurs thermodynamiques du processus d'adsorption (AG°,d, et f). Zusammenfassung--Messungen der Korrosionsgeschwindigkeit von Gusstahl in 1N HsSO~ bei 25°C mit und ohne Zusatz yon Thiophenmonoderivaten (1.10-s-2.10 -a Mol/l) wurden durchgef'tihrt. Die Adsorption dieser Verbindungen wurde aufgekliirt. Die Ergebrtisse zeigen, dass Thiophen und seine Derivate an der Stahloberfliiche entsprechend den Parsons- und HiU-de-Boer-Isothermen adsorbiert werden. Dies liisst annehmen, dass sich die Adsorptionseigenschaften dieser Verbindungen dem physikalischen Typ n~ihern. Aus den Adsorptionsisothermen werden einige thermodynamische Daten fiir den Adsorptionsvorgang (AG°M und f)berechnet und dann diskutiert.

STtmms on the a d s o r p t i o n o f organic substances f r o m electrolyte solutions have been carried o u t for a n u m b e r o f years using as a rule perfectly polarizable electrodes, such as mercury. 1,2 T h e results o f these studies showed that the a d s o r p t i o n o f the investigated substances usually c o r r e s p o n d e d to isotherms having a characteristic shape, 3 i.e. the following isotherms: Frumkin 4 0 Ba

exp ( - - f 0 ) .

--

(1)

1--0 Hill de Boer 5

0 Ba

--

1-0

exp

Parsons s *Manuscript received 2 January 1973. 557

0

i--0

(-- f0).

(2)

558

M. KAMrNs~dand Z. SZKtJmSKA-SMIAI.OWSg.~ 0 Ba

--

2--0

exp - -

I -- 0

exp (-- fO).

(3)

(1 - - O) ~

In these equations a and 0 are th~ activity of the adsorbed substance in the solution and the degree of coverage of the metal surface by this substance respectively. The constant B is a modified equilibrium constant of the adsorption process, which is related to the standard free energy of adsorption according to the following equation:

B = exp

A~as~ 1 ~ 1 5~5

(4)

Constant f in equations (1)-(3) depends on intermolecular interactions in the adsorption layer and on heterogeneity of the surface. This parameter can be either positive or negative, but mathematical analysis of the equations, taking into account the physical sense of the phenomenon which they describe, indicates that parameterf cannot have arbitrary large positive values. Each of the above-mentioned isotherms must have its characteristic maximum value. When the values of B and f a r e known it is possible to determine unambiguously the changes in adsorption properties of the investigated substances resulting from the changes of their concentration, and to calculate certain thermodynamical quantities characterizing the adsorption process. Although the adsorption on mercury has been studied by many authors, only a few papers dealing with adsorption isotherms on solid metallic electrodes have been published. Thus Manna, 9 studied adsorption of various aliphatic amines on iron, Bockris 1° investigated adsorption of n-dodecylamine on iron, and Smialowska and Wieczorek studied the adsorption of straight chain aliphatic amines, acids and alcohols on steel, n Determination of the type of adsorption isotherm corresponding to the adsorption on the metal-electrolyte phase boundary gives much valuable information as to the adsorption process, since it makes it possible to determine such quantities as the standard free energy of adsorption, its dependence on the degree of surface coverage, the character of adsorption layer on the metal-electrolyte phase boundary, the magnitude and character of interactions between the molecules of the adsorbed substance or between these molecules and the surface atoms of the metal. Therefore the accurate determination of the type of adsorption isotherm corresponding to the investigated adsorption process is of primary importance. In the present work the adsorption of thiophene and its derivatives on iron from acid solutions was investigated and the adsorption properties of these compounds were compared with their efficiency as inhibitors of dissolution of steel. EXPERIMENTAL The adsorptive ability of thiophene and its derivatives was determined on the basis of the results of determinations of corrosion rates of steel in 1N H2SO4 in the presence and in the absence of the investigated compounds. Samples measuring 20 × 50 × 0.5 mm were prepared from mild steel having the following composition:

Adsorption of thiophene derivatives on steel

%,

C 0.09

Cu 0-1

Cr 0.12

Mn 0.31

Ni 0.05

559

Si 0.05

P

S traces

The compounds investigated are listed in Table 1. Before the measurements, the samples were degreased by 24 h extraction with benzene in a Soxhlet apparatus and, after drying, the samples were heated in a vacuum at 640°C at about 10-5 Tr for 3 h. The solutions used in the investigation were prepared directly before the measurements from chemically pure sulphuric acid and distilled water. The organic compounds were purified by vacuum distillation and their purity was checked by gas chromatography. The corrosion rates were determined at

TABLE 1.

No.

1 2 3 4 5 6 7 8

9 10 11

THERMODYNAMICAL DATA CALCUL&TED FROM EXPERIMENTAL ISOTHERMS OF ADSORPTION OF TH]OPHENE AND ITS DERIVATIAtF~ I N HtSO4, 2 0 ° C

Substituent and its position in thiophene ring

2-hydroxymethylthiophene 2-methylthiophene 3-methylthiophene thiophene 2-chlorothiophene 2-bromothiophene thiophene-2-carboxylicacid 3-bromothiophene 2-acetylothiophene 2-thiophenecarboxaldehyde thiophene-2-carboxylic acid methylester

f

Adsorption Standard free equilibrium energyof constant adsorption log k kcal/mole

B

2.3 --2"1 9.8 0.6 10.0 3.9 10.0 11.5 --1.8 +3.5 --1.5

2.8 3.75 9.5 3.0 1.6 7.1 7.3 2.8 1.65 1.34

x x x x x x x x x x

l0 s l0 s l0 t 108 108 I0 s 10a 104 104 104

4.22 5"20 5.33 3.71 5.25 4.94 5.56 5.63 6.20 5.99 5.83

--5.8 --7"2 ---7.4 --4.95 --7.3 --6.70 --7.6 --7.80 --8.5 --8.3 --8.0

25 4-0-2°C without stirring and without deaerating the solutions (For pH values close to zero the contribution of oxygen depolarization to the cathodic process was neglected.) The duration of the corrosion test was 5 h; during this time the dissolution rate of steel in dilute sulphuric acid was constant. The corrosion current density for mild steel in 1N HzSO4 is 7 × 10-5 A/cm 2. The degree o f coverage of the investigated surface by the adsorbed molecules was calculated from the equation:

0=l-V

_Vo-V v0

(5)

v0

where Uo is the corrosion rate o f steel in 1N H2SO4; U is the corrosion rate of steel in 1N sulphuric acid containing the investigated compound. When the values of 0 are determined from kinetic data alone, and not directly

560

M. KAM~Srd and Z. SZKLARSKA-SM~ALOWSKA

~

80 70

.

8 50

2-Br

x ~ 1 ~ x

x

x

x

o

4O

2O 10'

10~

r

i

......

I

rO"4

. . . . . . . .

log C,

O" I -a

. . . . . . . . .

I "zO-

mole/L.

FIG. 1. The dependence of the degree of coverage of steel surface in l N H=SO4 at 25°C on the concentration of adsorbing substances; x--2-bromothiophene, o--2,-cblorothiophene, e--3-methylothiophene. The full line represents the isotherm giving the best description of all the experimental points.

2-CHO

7O 6o

H

5O

3O 2O I

I° i I0"s

i

i

ii1

I

I

I

I

1

10-4

I I II

I

I0 "3 io -3

log C,

t

I

I

1

I !11

_ ,~-2 I0

-

mole/L.

F1o. 2. The dependence of the degree of coverage of steel surface with 2-formylthiophene, x, thiophene-2-carboxylic-acid, D, 3-bromothiophene, o on their concentration in IN H~SO~. The full line = plot of isotherm. f r o m the results o f m e a s u r e m e n t s o f the degree o f coverage, it should be r e m e m b e r e d that equation (5) can be used only for the a s s u m p t i o n t h a t a d s o r b e d molecules o f the chemical substance mechanically screen the c o a t e d p a r t o f the electrode surface a n d therefore p r o t e c t it f r o m the action o f the corrosive m e d i u m . O n the c o a t e d p a r t o f the surface the dissolution rate o f the m e t a l decreases to zero, whereas o n the u n c h a n g e d

Adsorption of thiophene derivatives on steel

561

2-COOCH3 .•,,,p O-E

2-CH2CH "

0'7

o.g o

o.~ 0"4 0"3 0'2 O'l [

l

I

I

I

f

]

l [

I

I

[

i

P

I

i

f I

10-a

~-4

P-

I

i

10-z C,

mole/L

isotherm. concentration

RO. 3. The dependence of the degree o f coverage of steel surface with 2-hydroxymethy]thiophene and methyl ester of thiophenc-2-carboxylic acid on their in I N H2SO~. The full line = the

il

~

0'7

2"C0CH3

~

8°:f

2-CH3

0'4 0-3 0'5il 0"2 0' i

,,i

± r

I l l l

I

10-4

i

f

i

i

i

i

i [

10-3

I

I

I

i

i

i

J ,

ii

I

I

,

i

J

t

r-

lO-Z

C, nlole/l..

The dependence of the degree of coverage of steel surface with 2-acetylthiophene, 2-methylthiophene and thiophene on their concentration in IN H, SO,. The full line -----the isotherm.

FIG. 4.

part of the surface it remains constant. The added substance has no effect on the mechanism of dissolution of iron, but causes only inactivation of a part of the surface with respect to the corrosive medium. The above-mentioned condition is fulfilled only when the slope of the Tafel line corresponding to the transition from pure sulphuric acid to solutions containing the examined compounds is constant. The results of

562

M. ~ S r d

and Z. SZrd.AgSKA-SMIALOWSKA

investigation of polarization curves obtained in the presence of the examined compounds 12 show that in the systems used in the present work this condition is fulfilled. The slope bc ----- 115 4- 5 mV observed in the case of the 1N sulphuric acid remained unchanged after the introduction of the examined compounds. Although equation (5) can be used when the above condition is fulfilled, the degree of surface coverage calculated from this equation can be very inaccurate. This is due to the fact that the adsorption on the electrode depends not only on the adsorptive power of the organic compound and the character of the metal but also on the electrode potential and ~ potential, i.e. on the potential drop in the diffuse part of the double layer. The adsorbed molecules can change the charge of the electrode and consequently also its potential, and as a result inaccurate values are obtained when 0 is calculated from equation (5). In the case of a heterogeneous surface the molecules of the organic compounds are adsorbed on the most active parts of the electrode. This can cause a pronounced decrease of the dissolution rate of the metal on the uncoated part of the surface and as a result the degree of coverage calculated from equation (5) is too high. Thus, when 0 is determined by means of equation (5) the following additional conditions must be fulfilled: (a) the electrode potential changes caused by adsorption of the exarmned substance must be small (b) the concentration of the electrolyte solution must be so high that ~ potential changes are small (c) the metal surface is not highly heterogeneous. In the present work attempts were made to fulfil these conditions by suitable mechanical treatment of the samples and by suitable choice of the electrolyte concentrations. RESULTS The observed changes of the degree of surface coverage 0, are shown in Figs. 1-4 in the form of functions of the logarithm of the concentration of the examined compounds in sulphuric acid solutions. The investigation was carried out in a wide range of concentration of the investigated compounds (from 1.10 -~ mole/1, to 2.10 -3 mole/1.). The results have the character of adsorption isotherms which can be divided into two groups. In one of them the plots of 0 ~- f ( l o g c) have the characteristic S-shape (Figs. 1 and 2), and in the other 0 changes almost linearly with logarithm of the concentration of the examined substance (Figs. 3 and 4). The full lines represent theoretical isotherms fulfilling most accurately the experimentally determined relationships. In the analysis the isotherms having an S-shape in the 0 -- log c system of coordinates, i.e. Frumkin's, Hill de Boer's and Parsons' isotherms, were considered. The choice of the most suitable type of isotherm was made by fitting the theoretical course of the isotherm to all the experimental data, using the sum of least squares of deviations of B and f values as the criterion. The calculations were carried out by means of ODRA-1204 computer. The obtained data showed that the relationship 0 -----f(log c) in the form of S is best represented by the equations of Hill de Boer's and Parsons' isotherms. In the case of linear change of the degree of coverage with logarithm of inhibitor concentration it is not possible to assign unambiguously the experimental data to any

Adsorption of thiophene derivatives on steel

563

definite type of isotherm, since in the moderate coverage region 0.2 ,~ 0 < 0-8 several isotherms have almost linear sections when parameters B and r are suitably chosen. However, Parsons' and Hill de Boer's isotherms were theoretically derived assuming the existence of a mobile adsorption layer on the surface of the metal. It is probable that other compounds having similar steric and electronic structures are also adsorbed according to this model. For this reason the linear relationship 0 = f (log c) was assigned to Hill de Boer's isotherm. Using B a n d f v a l u e s obtained from the equations of the isotherms it was possible to calculate some of the thermodynamical quantities characterizing the adsorption process. The experimental data are collected in Table 1. They indicate that thiophene has the lowest standard free energy of adsorption. It is also the least effective as the metal protecting agent. The other compounds, which are more effective corrosion inhibitors, have also higher standard free energy of adsorption values. However, this relationship is only qualitative and cannot be expressed by a general equation. This can be seen in Fig. 5, showing the relation between the protective effectiveness I and the standard free energy of adsorption. However, a more detailed analysis of the observed relationship indicates that when 2-hydroxymethylthiophene and 2-bromothiophene, which behave differently from the other compounds in all kinds of determinations 12 are disregarded, the remaining compounds obey the general rule that the effectiveness of corrosion inhibition increases with increasing standard free energy of adsorption. Thus once again the thesis that the protective etficiency increases with increasing adsorptive power has been confirmed.

II

1t

~ 4c

2Q

] 5

! 6

! T

! 8

, ,~ 9

i_

AGo°, Kcal/mole FIG. 5. The dependence of the protective effectiveness of the examined compounds on their standard free energy of adsorption (AG°d,). Inhibitor concentration 2 x 10-=

mole/l. Symbols as in Table 1.

564

M. KAMINSKIand Z. SZKLARSKA-SMIALOWSKA

No definite relationships were observed between the kind of substituent, the molecular size of compound and parameterfcharacterizing the magnitude and kind of interactions between adsorbed molecules and between these molecules and atoms or ions situated on the surface of the metal. Compounds containing identicar functional groups and differing only in their positions with respect to the ring sulphur atom, such as 2-methylthiophene and 3-methylthiophene, show not only large differences between absolute values of parameterf(see Table 1) but also the qualitative difference which is observed as the change of sign off. This indicates that adsorption of 3-methylthiophene is accompanied by mutual attraction of molecules, whereas that of 2-methylthiophene leads to interactions characterized by their reciprocal repulsion. CONCLUSIONS

The results of determinations of the dissolution rates of steel in IN sulphuric acid and their dependence on concentration of the examined substances indicate that thiophene and its examined derivatives are adsorbed on the surface of protected metal according to Parsons' and Hill de Boer's isotherms. This suggested that, in agreement with the assumptions used in the derivation of these isotherms, mobile layers are formed on the surface of the metal during the adsorption process. 13 This leads to the second conclusion that the adsorption of thiophene and its derivatives on iron is of a type approaching physical adsorption x4 which is due to electrical interactions between the double layer existing at the phase boundary and the adsorbing molecules. The bonds formed by physical adsorption are weak and consequently the adsorbed substances are readily removed from the surface of the metal. In order to confirm this hypothesis the dissolution rate of steel in pure dilute sulphuric acid was compared with that of samples of steel which were immersed for 20 rain in a solution of a given organic compound at a concentration approaching saturation, and after washing with a stream of water were transferred to the pure dilute sulphuric acid. The difference between the corrosion rates was small. The samples preliminarily treated with the solutions of such thiophene compounds showed a degree of protection varying from 0 to 8 per cent, which was probably due to accidental factors, such as heterogeneity of the material, and not to strong interactions of chemisorptive character. The standard free energy of adsorption/xr~ds values calculated from the equations of the obtained isotherms vary from 4.95 kcal/mole for thiophene, which is the weakest corrosion inhibitor, to 8.5 kcal/mole for 2-acetylothiophene, which is one of the strongest inhibitors of the thiophene derivatives group. Thus the standard free energy of adsorption values are within the limits of 5-10 kcal/mole which have been observed for the majority of organic inhibitors of various types in aqueous media, s REFERENCES 1. A. N. FRUtCUONand W . W. DAMASK/N,Modern Aspects of Electrochemistry, No. 3. London

(1964). 2. W. W. DAMASKI~,A. A. StrRVILAand L. E. RVBALKA,Electrochimia 3, 146 (1967). 3. W. W. DA~IASraN, O. A. PIEa'~J and W. W. BA'rP.AKOW,Adsorpcja organiczeskich sojedinienij na elektrodach. Moskwa (1968). 4. A. N. FrtUMKn%Z. phys. Chem, 116, 466 (1925).

Adsorption of thiophene derivatives on steel 5. 6. 7. 8. 9. 10. 11.

12. 13. 14.

J. HILL DE BOER, The Dynamical Character of Adsorption. Clarendon, Oxford (1953). R. PARSONS,J. electroanal. Chem. 7, 136 (1964). E. GILEADI,Electrosorption. Plenum, New York (1967). C. A. MANN, ]]. E. LAUERand C. T. HLrrnN, Ind. Engng Chem. 28, 159 (1936)." SHIN-JEN CIAO and C. A. MANN, h~i/. Engng Chem. 39, 910 0947). J. O'M. BOCKRIS, M. GREEN and D. A. J. SWrNKELS,J. electrochem. Soc. 111,736 (1964). Z. SZKLARSKA-SMIALOWSKAand G. WIECZOREK, Corros. Sci. 11, 843 (1971). Z.-SZKLARSKA-SMIALOWSKAand M. KAMINSKI, Corros. Sci. 13, 1 (1973). S. Ross and J. P. OLIvIEg, On Physical Adsorption. Interscience, New York (1964). D. O. HAYWARDand B. H. W. TRAPNELL, Chemisorption. Butterworth, Washington (1964).

565