Adsorption of Organic Anions on Low-Carbon Steel in
Saturated Ca„ OH…
2
and the HSAB Principle
S. Martinez,
a,z
L. Valek,
a
and I. Stipanović Oslaković
b
a
Faculty of Chemical Engineering and Technology, Department of Electrochemistry, and
b
Faculty of Civil
Engineering, Department of Materials, University of Zagreb, 10000 Zagreb, Croatia
This paper represents a fundamental study of corrosion inhibition behavior of selected organic anions on low-carbon steel in
saturated Ca OH
2
solution containing chlorides. The compounds studied include eight amino acids, two carboxylic acids, ascor-
bic acid and a mimosa tannin extract. All the inhibitors suppressed the reduction of O
2
, a process that is known to be strongly
catalyzed on passive iron, the essential step being O
2
reaction with FeII surface sites. The proposed mechanism of inhibition
involves formation of surface chelates which tend to stabilize iron in FeIII state and increase the Tafel slope of the actual
potential dependence of the FeII surface centers. In accordance with the hard and soft acid and base principle, the corrosion
inhibition effect was well correlated with the absolute electronegativity of the anions obtained from the semiempirical quantum
molecular modeling calculations. The lack of correlation of the inhibition effect with the absolute hardness of the inhibitor anions
has been explained by considering the charge of the anions as the companion parameter for absolute hardness. The highest degree
of inhibition was observed for ascorbic acid that also showed a clearly pronounced increase in the pitting initiation potential.
© 2007 The Electrochemical Society. DOI: 10.1149/1.2777882 All rights reserved.
Manuscript submitted September 26, 2006; revised manuscript received July 12, 2007.
Available electronically September 14, 2007.
The overwhelming majority of literature on corrosion inhibitors
deals with the effect of inhibitors on uniform corrosion, e.g., of steel
in acidic or neutral solutions while the inhibitors for pitting corro-
sion are far less studied.
1,2
Since the adsorption of an aggressive
anion on the film surface is associated with the breakdown of a
passive film, cation inhibitors may suppress pitting corrosion via
electrostatic interaction with the aggressive anion.
3
On the other
hand, the action of the anion corrosion inhibitors for bare or oxide-
covered metal surfaces can be interpreted in terms of the hard and
soft acid and base HSAB principle.
4-6
In a corrosion system con-
taining the aggressive anions and the inhibitor anions, oxidized
metal acts as a hard Lewis acid and anions act as Lewis bases.
Hence, the inhibitor anions should be harder bases than the aggres-
sive anions, since they must preferentially bind to the highly oxi-
dized metal ions on the film surface via replacement of oxide or
hydroxide groups.
In particular, compounds containing carboxylic group have been
reported to be effective inhibitors of passive film breakdown with
chloride on the surface of various metals and alloys by means of
their competitive adsorption on the oxide film, since the carboxylate
ions are harder bases than chloride.
7
The aim of the present work is to assess the adsorption behavior
of eight amino acids proline, PRO; cystine, CYS-SS-CYS; arginine,
ARG; phenylalanine, PHE; tryptophane, TRP; tyrosine, TYR; ala-
nine, ALA; and glutamic acid, GLU and two carboxylic acids ox-
alic, OXA and tartaric, TAR on steel in sat. Ca OH
2
solution in the
presence of chloride ions. Ascorbic acid AA and mimosa tannin
MT have been investigated also for comparative purposes as some
previous work has shown that these compounds effectively inhibit in
sat. Ca OH
2
solution.
8,9
The results are interpreted in terms of the
HSAB principle.
Experimental
Measurements were performed using potentiodynamic polariza-
tion, cyclic voltammetry and electrochemical impedance spectros-
copy EIS. Bars of nonalloy special prestressing steel EN 10020;
composition wt %: C = 0.80, Mn = 0.69, Si = 0.23, P = 0.012,
S = 0.009, Cr = 0.265, V = 0.060, Al = 0.004 were machined to
give the cross-sectional area of 0.126 cm
2
d = 4 mm and embed-
ded into epoxy resin, with metal/epoxy edges protected by an inert
coating. Prior to measurements, the electrode was wet-ground
gradually to the final finish of 1200 grit, degreased with ethanol and
rinsed in redistilled water. Counter electrode was carbon rod and
reference electrode was saturated calomel electrode SCE. All the
potentials reported refer to SCE. Experiments were performed in sat.
Ca OH
2
with addition of 0.14 mol dm
-3
NaCl, at room tempera-
ture. The concentration of the compounds was 10
-3
mol dm
-3
or
1 g dm
-3
in the case of MT, chosen on the basis of the observation
that intermediate inhibition efficiencies 20–80% are attained at this
particular concentration. Analytical grade chemicals purchased from
Sigma and mimosa tannin extract purchased from Tannin Sevnica
were used in this study. All the measurements were performed after
one half hour immersion at the open circuit potential OCP. Poten-
tiodynamic measurements were done at the scan rate of 1 mV s
-1
and voltammetric measurements at the scan rate of 10 mV s
-1
. Ca-
thodic and anodic polarization curves were recorded separately start-
ing from the last recorded OCP. EIS measurements were performed
at the OCP in the frequency range between 100 kHz and 10 mHz,
with the ac amplitude of 10 mV.
Semiempirical molecular modeling calculations
10
were per-
formed by the HyperChem 5.11 package. The geometry optimiza-
tion was obtained by the application of the unrestricted Hartree-
Fock method using the neglect of diatomic differential overlap
11
approach with PM3 parameterization.
12
The coordinates of a mo-
lecular structure that represents the potential energy minimum were
obtained using the Polak-Ribiere conjugate gradient method.
13
The
iteration procedure was continued until rms energy gradient became
lower than 0.001 kJ Å
-1
mol
-1
.
Results and Discussion
Polarization experiments.— Figure 1 shows typical polarization
curves recorded after 30 min of immersion under free corrosion con-
ditions in the presence and in the absence of the inhibitors. Table I
collects open circuit potential OCP, pitting initiation potentials
E
p
, and width of the passive range calculated from their differ-
ence. Pitting was accompanied by almost vertical current increase
and an accumulation of corrosion product at pitting sites permitted
visual confirmation of localized attack.
Literature data show
14,15
that for similar chloride to hydroxide
ion concentration ratio Cl
-
/ OH
-
4.4 in the present work pit-
ting initiation potentials measured on steel in the inhibitor free sat.
Ca OH
2
solution are below the oxygen evolution potential OEP.
The stochastic nature of E
p
, that has been long recognized in similar
systems,
14,15
has prompted the statistical representation of the results
in Table I. Similar values of E
p
, higher than the OEP, were observed
for most of the tested inhibitors. As explained elsewhere,
15
similar-
ity of E
p
for various inhibitors may reflect the effect of local OH
- z
E-mail: sanja.martinez@pierre.fkit.hr
Journal of The Electrochemical Society, 154 11 C671-C677 2007
0013-4651/2007/15411/C671/7/$20.00 © The Electrochemical Society
C671