observed that certain anions diminished the inhibition efficiency of sodium benzoate but, to the knowledge of the present authors, no systematic study of this ...
0
Pergamon
Corrosion Science, Vol. 40, No. 415, pp. 673-691, 1998 1998 Published by Elsevier Science Ltd. All rights reserved.
Printed in Great Britain. 001&938X/98 $19.00+0.00
PII:
SOOlO-938X(97)00170-4
EFFECT OF ANIONS ON THE EFFICIENCY OF AROMATIC CARBOXYLIC ACID CORROSION INHIBITORS IN NEAR NEUTRAL MEDIA: EXPERIMENTAL INVESTIGATION AND THEORETICAL MODELING P. AGARWAL Laboratoire
de metallurgic
and D. LANDOLT
chimique, Departement des materiaux, Ecole Polytechnique MX-C Ecublens, CH-1015 Lausanne, Switzerland
Fed&ale
de Lausanne,
Abstract-The influence of the nature and concentration of electrolyte anions on the efficiency of carboxylic acid based inhibitors for steel in neutral solution was studied using anodic and cathodic polarization at a rotating disk electrode. The N-ethyl-morpholine salts of a o-benzoyl alcanoic acid model compound and of benzoic acid were used as inhibitors. Both compounds were found to inhibit the anodic partial reaction. In the active potential region, the inhibition effect was more pronounced at low electrolyte concentration suggesting an adsorption mechanism. Both inhibitors favored passivation of the electrode, the effect being attributed to their capacity to act as a buffer in the anodic diffusion layer. For a given inhibitor concentration in different electrolytes, the stability of the passive film under anodic polarization conditions decreased in the order CIO; > Cl > SO; ~, A theoretical model has been developed that is able to describe the observed influence of anion and inhibitor concentration on the inhibition of iron in the active potential region. The inhibition effect is attributed to blocking of surface sites for anodic dissolution by the inhibitor molecule. The degree of adsorption follows a Langmuir isotherm and depends on the nature and concentration of the electrolyte anion present. The model predicts a stronger inhibition effect for low electrolyte concentration in agreement with experimental observations made with N-ethyl-morpholine salts of two aromatic carboxylic acids. $3 1998 Published by Elsevier Science Ltd. All rights reserved
INTRODUCTION Protection of mild steel against atmospheric corrosion by organic coatings has been widely studied. Organic coatings are complex formulations with corrosion inhibitors being one of their key constituents.’ Under normal atmospheric conditions, the pH at the coatingsubstrate interface is near neutral. Traditionally, inorganic compounds such as oxides, chromates, nitrites, etc. have been used as additives in coatings to inhibit corrosion. Due to stringent environment regulations, there is a strong need to replace them with environmentally friendly compounds. Salts of aromatic carboxylic acids provide viable alternatives because of their ability to inhibit corrosion of mild steel in near neutral solutions and their non toxic nature. The sodium salt of benzoic acid, which belongs to the above mentioned class of compounds, is the most widely known organic corrosion inhibitor of iron in neutral solutions’ and has been used as an additive in paints. Kuznetso$ recognized that new inhibitor molecules can be designed by changing the chemical structure of organic compounds while keeping the same reactive groups. He and his co-workers proposed a series Manuscript
received
5 September
1997; in amended
form 13 November 673
1997
674
P. Agarwal
and D. Landolt
of substituted aromatic acids4.’ and fatty acid? as corrosion inhibitors of mild steel in neutral solutions. Recently, a renewed industrial interest in this class of inhibitors emerged in view of their use as corrosion inhibitors in paints. This led to a need for a better understanding of the fundamental reaction mechanisms involved and of the role of the environment and of the molecular structure of the inhibitor. Design of new inhibitors for improved corrosion protection by organic coatings to be applied under a wide range of conditions entails the development of efficient and reliable screening tests and an improved understanding of the inhibition mechanism. Application considerations also require that inhibitors added to organic coatings for steel are effective even at low concentration and in the presence of surface impurities such as sulfates or chlorides. Several studies have been presented in the literature which correlate the activity of the inhibitor molecule with its molecular structure. Some authors proposed that the ability of the inhibitor to form complexes with dissolved ferrous or ferric ions plays a key role in the inhibition mechanism.’ ’ On the other hand, it has been stated” I2 that sodium benzoate diminishes the anodic dissolution rate of iron by facilitating the formation and retention of a passive oxide film. It has also been noted’j ” that many of these inhibitors are effective only above a critical minimum concentration and pH. In 1962, D. BrasherI observed that certain anions diminished the inhibition efficiency of sodium benzoate but, to the knowledge of the present authors, no systematic study of this effect has been published so far. An understanding of such effects may be important, however, for the estimation of the possible effect of surface contaminations on the efficiency of inhibitors added to paints. It is well known that the corrosion and passivation of iron depends strongly on the pH at the metalelectrolyte interface. In mechanistic studies of iron dissolution in near neutral environments the pH at the electrode surface must be kept constant using a buffer. In the absence of other buffers, the buffer capacity of an inhibitor may be a critical factor for the interpretation of experimental results. Sodium salts of aromatic carboxylic acids are known to have some buffering capacity, depending on their concentration. Takahashi et al.” found that sodium benzoate solutions below a concentration of 0.75 M exhibited a poor buffer capacity and corrosion experiments carried out at lower concentrations gave anomalous results. Corrosion experiments with iron in near neutral environments are frequently done using a borate buffer solution. However, the borate buffer itself is known to favor passivation of iron. To avoid such problems, no buffer other than the inhibitor system was used in the present study. Indeed, the aromatic carboxylic acids, neutralized with N-ethylmorpholine base, used as model compounds exhibit a good buffer capacity at near neutral PH. The purpose of the present study is to learn more about the role of electrolyte composition, especially the concentration and nature of anions, on the efficiency of aromatic carboxylic acid based inhibitors for steel in a near neutral aqueous solution. In this paper, experimental results obtained in sulfate, chloride and perchlorate electrolytes are presented and a theoretical model for the interpretation and numerical simulation of the experimental observations is shown.
EXPERIMENTAL
CONDITIONS
The model inhibitor considered in this work belongs to the family of w-benzoyl acid having the following general structure:
alkanoic
Effect of anions
on the efficiency of aromatic
carboxylic
acid corrosion
inhibitors
615
In this formula R is a functional group and m indicates the chain length. The molecular structure and inhibiting properties of the family of compounds can be varied by varying R and m but, for the purposes of the present study, R is always a CH, group and m = 2. The acid group was neutralized with a stoichiometric amount of N-ethyl-morpholine to obtain the corresponding salt which was used as the inhibitor. Some experiments were also done with the N-ethyl-morpholine salt of benzoic acid and with sodium benzoate. A rotating disc working electrode made from CK 45 steel rod of 5mm diameter was used for all experiments. The electrode was embedded in Araldite@, polished with 400, 600 and 1000 grit emery paper and washed in distilled water before being immersed in electrolyte solution. The experiments were conducted in a one liter jacketed glass cell. The temperature was controlled at 25 C. The pH of the solution was adjusted to 7.5 with NaOH and HzS04 prior to the start of the experiment and measured again at the end of each experiment. A platinum coil was used as the counter electrode and potentials were measured relative to a Hg/Hg,SO, reference electrode. The model inhibitor used in the experiments was synthesized by Ciba Specialty Chemicals Inc., Switzerland. Reagent grade benzoic acid was used. All solutions were prepared with doubly distilled water. The effect of the concentration of various anions on the efficiency of the inhibitor was studied in NaClO,, NaCl and Na,SO, solution, respectively, by varying the concentration. The linear sweep voltammetry (LSV)experiments were conducted using an AUTOLAB” potentiostat interfaced to a personal computer. A commercial software (GPES’O 4.2) was used to control the experiments and collect the data. The potentials were scanned from - 1.6V to + 1.2V (vs. Hg/Hg,SOJ. Beginning the experiments at cathodic potentials ensured that any oxide film present prior to the start of the experiments was reduced. A 1 mV potential step and a slow scan rate of 2 mV/s was used. The ohmic resistance of the solution was measured by impedance prior to each potential sweep in order to correct electrode potentials for ohmic drop. Such a correction was particularly important for solutions having a very low conductivity. Experiments were done both in air saturated solutions and in solutions de-aerated by bubbling NZ for 45 minutes before the start of the experiment.
EXPERIMENTAL
RESULTS
Air saturated solutions The first series of experiments were done in air saturated sulfate and perchlorate solutions where reduction of oxygen is the predominant cathodic reaction. The purpose of these experiments was to establish if the inhibitor under study was a cathodic, anodic or mixed inhibitor. Experiments were also done to explore the effect of sulfate concentration and hydrodynamics on inhibitor efficiency. Figure 1 shows polarization curves in 0.12 M Na,SO, solution with and without inhibitor (0.003 M) and in a 0.003 M inhibitor solution not containing sulfate, the pH being 7.5 throughout. For all conditions, a diffusion limited oxygen reduction current
616
P. Agarwal 1 o”
and D. Landolt
J-
10.’
1 0.*
“E
1 d3
2 g
,d4
1 o-5
1 da
- - - 0.12M
I 6’ -I-2
,‘“‘,‘““““,““““‘,““,“” - 1
0 E (vs WHg,Sd,
Na&D,
+ 0.003M
1
inhibitor
2
V
Fig. 1. Linear sweep voltammograms for low carbon steel rotating disc electrode in air saturated 3 mM inhibitor solution at pH 7.5 as a function of the concentration of Na,S04 supporting electrolyte.
plateau of same magnitude is observed in the cathodic part of the polarization curves. This indicates that the inhibitor does not appreciably influence the rate of oxygen reduction in the limiting current region. The anodic part of the polarization curve in 0.12 M Na,SO, is typical for active metal dissolution. In the presence of the model inhibitor a current instability and reversal of polarity is observed in the range of -0.G 1.05 V. The reason for this is not known and was not further investigated since it was not considered important for the purposes of present study. At more anodic potentials, active metal dissolution takes place in a similar way as in the absence of inhibitor. In 0.003 M inhibitor solution not containing sulfate, represented by dashed-dot line in Fig. 1, the corrosion potential is shifted towards a more anodic value and the anodic polarization curve exhibits a passivation plateau. The data suggest that the sulfate concentration in the electrolyte has an important influence on the effectiveness of the model inhibitor studied. It has been reported” that, in sulfate solution, iron does not passivate as readily as in other media and requires formation of a precursor salt film. The data of Fig. 1 confirm that the passive film observed in the pure inhibitor solution does not form in the presence of 0.12 M sulfate under the present experimental conditions. Figure 2 shows the effect of rotation rate of the electrode on polarization behavior in a solution of 0.002 M sodium perchlorate with 0.002 M inhibitor. Rotation rate has no apparent effect on the anodic partial reaction. The limiting current plateau in the cathodic potential region increases with rotation rate but less than predicted by the Levich equation (not shown). This suggests that the cathodic reduction of oxygen is only partially limited by mass transport. Within the precision of the data of Fig. 2, the corrosion potential is not dependent on rotation rate. The data of Figs 1 and 2 lead to the following conclusions;
Effect of anions
on the efficiency of aromatic
carboxylic
acid corrosion
677
inhibitors
-‘Z *-=I?&
1 o-3
L 5
1oe4 “E
1 o-5
0 a
ld6
r;\
ldgL--1.5
-1.0
0.002M acid + O.OlM NaC104 rpm 400 - - - rpm 800 --rpm 1200 - - mm 1800 1I -0.5 E (vs HgklgSO),
I
0.0 V
1
0.5
0.002M acid + O.OlM NaClO rpm 400 - - . rpm 800 -.rpm 1200 - - rom 1800
1.5x1o-3
1.0,
-1.5
Fig. 2.
-1.0
-0.5 0.0 E (vs HgklgSO), V
Linear sweep voltammograms for steel electrode in acid inhibitor rotation speed. a) semi-log scale b) linear scale.
0.5
solution
as a function
of
(a) the inhibitor acts primarily on the rate of the anodic partial reaction, facilitating the formation of a passive oxide film, (b) the rate of the anodic partial reaction does not significantly depend on mass transport conditions, (c) the inhibitor effectiveness decreases markedly in the presence of sulfate ions. Based on these results,
the subsequent
experiments
were aimed
at study of the influence
P. Agarwal
678
and D. Landolt
of the inhibitor concentration and of the electrolyte composition on the anodic partial reaction. In order to be able to investigate the active dissolution behavior without interference of oxygen, the experiments were performed in deaerated solution. Knowing that the rate of the anodic partial reaction did not depend on mass transport conditions, the polarization measurements were carried out at a single rotation rate of 400rpm. Behavior of the model inhibitor in de-aerated solution Figure 3 shows anodic polarization curves for steel obtained in de-aerated 3 mm inhibitor solution to which different concentrations of Na,SO, were added. In the absence of sulfate, the polarization curve exhibits the typical behavior of a passivating metal, showing an active dissolution region followed at higher potentials by a passivation plateau. The addition of a small quantity of sodium sulfate (3mm) to the solution results in an increase in the passivation current and a higher active dissolution rate. When the concentration of sulfate is increased to 0.12 M, the active dissolution current becomes still higher and approaches the value observed in the absence of inhibitor. The open circuit potential shifts in the cathodic direction. The presence of sulfate also decreases the stability of the passive film at high potentials which manifest itself among other by a decrease in the breakdown potential separating passive and transpassive behavior. On the other hand, the Tafel slope for active dissolution apparently does not depend on the prevailing sulfate or inhibitor concentration. This suggests that the reaction mechanism for active iron dissolution is always the same. The observed changes in active dissolution rate can therefore be interpreted by a simple surface blocking mechanism involving adsorption phenomena as will be detailed later in the paper.
lo.'4
,...
,,., -2.0
-1.5
II -1.0
,,.,
.,,,
..,,
. .
0.0
-0.5 E W
Wb,=,).
0.5
. .
,..,
.,,. 1.0
1.5
:
V
Fig. 3. Linear sweep voltammograms for low carbon steel rotating disc electrode in de-aerated 3 mM inhibitor solution at pH 7.5 as a function of the concentration of Na,SO, supporting electrolyte. The potential was corrected for ohmic drop.
Effect of anions
on the efficiency of aromatic
carboxylic
acid corrosion
679
inhibitors
A similar effect of the inhibitor on the active dissolution behavior of iron is observed perchlorate solution, as shown in Fig. 4. However, the passive film in perchlorate solution is more stable than in sulfate and the film breakdown potential varies less with concentration. The flat current peak observed in the passive region at O.OV (vs. Hg/Hg,SOJ could be due to meta stable pitting. The influence of Cl- ions on the effectiveness of the inhibitor was investigated in the same way and results are given in Fig. 5. As in perchlorate and sulfate solution, an increase in chloride concentration increases the anodic currents in the active region. In addition, chloride ions lead to pitting, the critical potential decreasing strongly with increasing chloride concentration. A comparison of the effect of chloride, perchlorate and sulfate ions on the anodic polarization behavior of steel in 3 mM inhibitor solution is given in Fig. 6. The results clearly demonstrate that in the active potential region all three anions increase the anodic current with respect to the pure inhibitor solution, the effect being most pronounced with sulfate. In terms of inhibitor effectiveness, this suggests that, for a given anion concentration, the inhibitor is less effective in sulfate than in perchlorate or in chloride solution. Similarly, the data of Fig. 6 show that the stability of the passive film at high potentials decreases in the order CIO; > CI- > SO;. Figure 7 shows the influence of inhibitor concentration in distilled water. An increase in concentration from 3 mM to 1OmM results in a slight decrease in the active dissolution current and in the passive current, but does not change the Tafel slope for active dissolution. This is consistent with a surface blocking mechanism. The data also permit one to conclude that the current increase due to electrolyte anions observed in the data of Figs 3-5 was in
:
3
-W-
-2.0
-1.5
-1.0
0.006M NaCIO,
0.5
-0.5 E (vs Hg&q),
1.0
+ 3mM inhibitor
1.5
2.0
v
Fig. 4. Linear sweep voltammograms for low carbon steel rotating disc electrode in de-aerated 3 mM inhibitor solution at pH 7.5 as a function of the concentration of NaClO, supporting electrolyte. The potential was corrected for ohmic drop.
P. Agarwal
680
and D. Landolt
lo’*
lo‘=
lOA
< g
1o‘5
l0.O
10.'
’I
lod
I.“‘I~.~‘I~“~I.~‘~I..~.I..‘.I’...
-2.0
-1.5
-1.0
-0.5
E (vs
0.0
1.5
1.0
0.5
Hg290,). V
for low carbon steel rotating disc electrode in de-aerated Fig. 5. Linear sweep voltammograms 3 mM inhibitor solution at pH 7.5 as a function of the concentration of NaCl supporting electrolyte. The potential was corrected for ohmic drop.
IO.'
10=
lOA
4 g IO"
lod +
0.12MNa193,
*
0.12MNacI
10“
-2
-1
0
1
E (a HgSL). V for low carbon steel rotating disc electrode Fig. 6. Linear sweep voltammograms 3 mM inhibitor solution at pH 7.5 as a function of the type of supporting electrolyte. was corrected for ohmic drop.
in de-aerated The potential
Effect of anions
on the efficiency of aromatic
carboxylic
acid corrosion
inhibitors
0.5
1.0
681
lo-*
10”
lo4
a = -
1o-5
loa
lo-'
10-s
-1.5
-1.0
-0.5
0.0
E(vsHg/Hg Fig. 7. inhibitor
,9o.J, V
Linear sweep voltammograms for low carbon steel rotating disc electrode in de-aerated solution at pH 7.5 as a function of the concentration of the inhibitor. The potential was corrected for ohmic drop.
not due to a change in electrolyte conductivity. Indeed, in the experiments conductivity was much higher in the 10 mM than in 3 mM solution.
of Fig. 7 the
Behavior of benzoate in de-aerated solution Benzoic acid is a well known corrosion inhibitor for steel and its reaction mechanism in acid media has been studied extensively.i3 In the present study, benzoic acid was used as a reference compound with which the model inhibitor could be compared. In near neutral solution, benzoic acid is present as a salt, sodium benzoate being most frequently used as inhibitor. Polarization experiments were performed in 1OmM sodium benzoate solution to which different amounts of perchlorate were added-results are shown in Fig. 8(a). The general trends are similar to those observed for the model inhibitor but the absolute values of the current densities are higher. Adding perchlorate to the solution results in a significant increase in the anodic current density in the active and the passive potential regions. Figure S(b) shows a close up of the behavior in the active potential region. A closer inspection of the data reveals that, although there is a clear increase in current when perchlorate is added to the solution, no well defined functional relationship between the measured current and the perchlorate concentration exists. In addition, a change in the Tafel slope for active dissolution is observed at higher potentials. The change in Tafel slope suggests a change in dissolution kinetics which is attributed to an increase in the pH at the electrode surface. The pH of the solution, was fixed at 7.5 prior to the start of each potential scan but it was observed that its value increased during the course of an experiment. It has been reported by other workers’* that sodium
P. Agarwal
682
a)
a 2
10”
and D. Landolt
,““t”“I’“‘I’“‘I’“’
lo4
-
10”
-\
1oa
-
lo-’
-
, d +t--&. -0 -64~
OM perchlorate 0.12M perchlorate 0.06M perchlorate 0.03M perchlorate 0.012M perchlorate
1oe8 -
E (vs Hg293J
-1.2
-1.1
-I
V
-0.9
-1.0
E (vs Hg,=J
J
-0.8
V
3g. 8. Linear sweep voltammograms for low carbon steel rotating disc electrode in de-aerated IOmM sodium benzoate solution at pH 7.5 as a function of the concentration of NaCIO, supporting electrolyte. The potential was corrected for ohmic drop. a) complete spectra b) active region.
Effect of anions on the efficiencyof aromatic carboxylic acid corrosion inhibitors
683
benzoate solutions have a weak buffer capacity at low concentration and for these reasons, previous mechanistic studies in near neutral solutions have been done for much higher concentrations of sodium benzoate (>0.75 M). For the present purposes, a concentration close to that of the model inhibitor was chosen. Under these conditions, the buffer capacity of sodium benzoate was clearly inferior to that of the model inhibitor. To investigate to what extent the conjugate base influenced the inhibitor behavior a series of experiments was performed using the N-ethyl-morpholine salt of benzoic acid as an inhibitor. The results of experiments performed with 1OmM benzoic acid solutions neutralized by an equimolar quantity of N-ethyl-morpholine are shown in Fig. 9(a) and Fig. 9(b). The pH of the solutions containing different amounts of perchlorate was adjusted with NaOH to a value of 7.5. The data shown in Fig. 9(b) indicate that in the active potential region, the dissolution current corresponding to a given potential increases with an increase in perchlorate concentration, but the anodic Tafel slope does not change. In these experiments, the pH of the solution remained constant during the potential scan, indicating that the N-ethyl-morpholine salt of benzoic acid had good buffering capacity. The data confirm that the ill defined behavior in Fig. 8 was due to a variation in pH due to a insufficient buffer capacity of the sodium benzoate in the pH range of interest. Furthermore, the data of Fig. 9 show that the effect of perchlorate on the inhibiting efficiency of benzoate is similar to that observed for the model inhibitor. Separate experiments performed with solutions of N-ethyl-morpholine had shown that this compound by itself did not inhibit active dissolution of steel under present experimental conditions. The different data suggest therefore that, for the model inhibitor as well as for the benzoate, the inhibiting species is the neutralized aromatic carboxylic acid. Furthermore, the inhibiting mechanism of the N-ethyl-morpholine salts of the two aromatic carboxylic acids studied here is the same.
THEORETICAL
MODEL
The mechanism of iron dissolution in acid media has been studied extensively and good reviews are available in the literature.20*21 Iron dissolution in neutral solutions has been studied less often because of difficulties associated with measuring low corrosion currents and lack of reproducibility of data. Drazic et ~1.~~observed two Tafel slopes in the active region of the polarization curves for borate buffer solutions for pH 7.1 in presence of sulfate anions. They concluded that a change of Tafel slope is difficult to explain by standard electrochemical kinetics. In a quartz crystal microbalance study of the dissolution of iron in 6.48 pH borate buffer solution in presence of chloride ions, Seo et ~1.~~also observed two Tafel slopes in the active region. These authors attributed the existence of a second Tafel slope to either an insufficient correction for the iR drop in solution or to an enrichment of the near surface region with a ferrous borate complex. Both groups applied the two consecutive one electron exchange reaction model proposed by Bockris et ~1.~~ for acid solution to the interpretation of results in neutral solutions. In this mechanism the second step is rate limiting. The theoretical model proposed here is designed to reproduce the main features of the experimental observations reported in the previous sections. For this a reaction scheme is proposed which takes explicitly into account the role of the electrolyte anions and of the inhibitor in the dissolution process. A two charge transfer step mechanism is assumed similar as that proposed by Bockris et al. 22for acid solutions, but the first step is assumed
P. Agdrwal
684
and D. Landolt
1o-5 C q
1o-6
10-’ --A-0
1o-8
1o‘g
-1.5
-1 .o
0.12M perchlorate 0.06M perchlorate 0.03M perchlorate
0.0
-0.5 E (vs Hg,SO,h
0.5
1.0
V
1)
-43 .-6. -0
-1.1
-1 .o
0.12M perchlorate 0.06M perchlorate 0.03M perchlorate
-0.9
-0.8
E (vs Hg2tD4), V Fig. 9. Linear sweep VOk%mmOgramS for low carbon steel rotating disc electrode in de-aerated 1OmM benzoic acid neutralized with N-Ethylmorpholine base at pH 7.5 as a function of the concentration of NaCIO, supporting electrolyte. The potential was corrected for ohmic drop. a) complete spectra b) active region.
Effect of anions
on the efficiency of aromatic
carboxylic
acid corrosion
inhibitors
to be rate determining in order to be able to account for the observed adsorption In absence of specific anion effects this yields the following reaction scheme: Fe+OH-
--&
FeOH,,,
FeOH,,,
of anions,
effects.
+ e-
G FeOH+ + e-
(1)
FeOH+oFe’++OHIn the presence
685
another
reaction
Fe+A-
&
scheme may proceed
in parallel.
FeA,,, + e-
FeAad,oFeA++eFeA+oFe2++A-
(2)
Here A- represents an electrolyte anion other than OH- (for simplicity the equations above are written for a monovalent anion but similar expressions can be written for bivalent anions). The inhibitor is assumed to adsorb at the surface of the actively dissolving metal following the Langmuir isotherm, each inhibitor molecule occupying Nactive sites (N 3 1). Znh+ N sites 2% The surface coverage
by the inhibitor e1 =
Inhad,
(3)
is thus given by
(KadsC,)“Neg
(4)
where Kads is the adsorption coefficient and ci is the inhibitor concentration in the solution. The inhibitor molecules block adsorption and dissolution sites, but do not electrochemically react themselves. This implies that in a solution containing only the inhibitor and water iron dissolution occurs according to scheme (1) on the sites not occupied by the inhibitor molecules. To develop a mathematical expression for the described reaction scheme the following assumptions are made: (i) Butler-Volmer kinetics apply for all charge transfer reactions, (ii) since, in schemes (1) and (2), the first step is rate limiting the surface coverages of FeOH,,, and FeA,,, respectively, are low and can be neglected, (iii) since the inhibitor is not consumed at the surface its concentration is uniform throughout and no mass transport limitations exist; (iv) iron dissolution by schemes (1) or (2) takes place only at surface sites not blocked by the inhibitor. (v) The rate of dissolution is first order with respect to the hydroxyl and electrolyte anion concentrations, respectively, the concentration of these species at the surface being the same as in the bulk. The latter assumption is justified because of low dissolution rates and absence of complex formation. With these assumptions the faradaic current for iron dissolution, Z,, is given by eqn (5).
686
P. Agarwal
and D. Landolt
I, = 2FA(k,e”coh
+ kJ&c’*)
(5)
Here co,, and c,,,_ are the concentration of the hydroxide ion and the electrolyte anions respectively, F is the Faraday’s constant and A is the surface area of the electrode. The rate constant of the first step of reaction scheme (I), k,, is potential dependent: k, = k,,,exp (h,E) where k,, is a constant and 6, is the Tafel constant. Similarly, k2 = k2,1exp(h,E) is the potential dependent rate constant of the first step of reaction scheme (2). The quantity 19~) represents the fraction of free surface sites available for dissolution. It is given by U,, = 1 - 0, where 8, is the surface coverage of the inhibitor which represents the fraction of dissolution sites blocked by the inhibitor.
NUMERICAL
RESULTS
The model was regressed to the experimental data for anodic dissolution of steel in the Tafel region corresponding to active dissolution, in order to determine the numerical values of the kinetic parameters k,“, k2,,, h,, and h?,, the adsorption coefficient, KadS,and the number N corresponding to the best fit. The kinetic parameters, k,,,, h,, and hZ, were assumed to have the same value for all solutions, while the value of k,, was assumed to be different for different anionic species. The adsorption coefficient K, d~ and number of sites covered by an inhibitor molecule N were assumed to be characteristic of the inhibitor but independent of the nature of the electrolyte. Figures lo-12 show experimental data and fitted curves for dissolution of steel in perchlorate, sulfate and chloride solutions containing the model inhibitor. Comparable data for perchlorate electrolytes containing the N-ethyl-morpholine salt of benzoic acid are given in Fig. 13. It follows from these figures that the model can satisfactorily describe the main features of experimental behavior. The kinetic parameters giving the best fit are shown in Table 1 and the corresponding values for the adsorption constants in Table 2. The calculated Tafel slope for the iron dissolution reaction scheme (1) involving hydroxide ions, 6, = 151 mV/decade, is in good agreement with the value of 154mV/decade observed by
Fig. IO.
Results of the regression
of the model to experimental sodium perchlorate electrolyte.
data for 3 mM model inhibitor
in
Effect of anions
Fig. Il.
on the efficiency of aromatic
Results of the regression
1o-6 -I
r
,
I
Results of the regression
Table 1.
.
*
*
,
I
I
.
I
(
I
I
I
of the mode1 to experimental sodium chloride electrolyte.
Mode1 parameters
151 109 7.65 x 10“ 2.9 x 10’
for charge
inhibitors
data for 3 mM mode1 inhibitor
.
,
I
CI- > SO; ~. A theoretical model was developed that is able to describe the observed influence of the nature and concentration of electrolyte anions and the carboxylic acid model inhibitors on the rate of anodic dissolution of iron in near neutral media. The model is based on Langmuir absorption of the inhibitor and on a dissolution mechanism which includes two parallel reaction paths involving hydroxyl ions and electrolyte anions respectively. A set of parameters were determined by fitting the model equations to experimental data in the Tafel region. From these the influence of anion concentration and inhibitor concentration of the corrosion current density was predicted. The model presented gives a rational interpretation of the observed increase in anodic dissolution rate with electrolyte concentration in presence of aromatic carboxylic acid based inhibitors. In conjunction with numerical simulation the model provides a useful basis for future studies of the effect of molecular structure on the efficiency of organic inhibitors for steel in near neutral media. Arknowledgmlmrs-The financial support for the work was provided by Commision pour la Technologie et l’lnnovation (CTI), Switzerland and Ciba Specialty Chemicals Inc., Switzerland. The authors wish to thank Dr. A. Kramer and Dr. M. Frey of Ciba Specialty Chemicals Inc., Switzerland for the helpful discussions and for providing the compounds used in this work.
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