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Pergamon Corrosion Science, Vol. 37, No. 6, pp. 1005-1019,
1995
Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain.
All rights reserved
001@938X/95 $9.50+0.00
0010-938x(95)00010-0
THE ROLE OF METAL CATIONS IN IMPROVING THE INHIBITIVE
PERFORMANCE OF HEXAMINE ON THE
CORROSION OF STEEL IN HYDROCHLORIC ACID SOLUTION
D. D. N. SINGH, T. B. SINGH and B. GAUR
Corrosion Protection Division, National Metallurgical
Laboratory, Jamshedpur - 831007, India
Abstract-Hexamethylenetctramine (HA) or hexamine (or urotropin)
have a moderate inhibitive effect on the corrosion of mild steel in
concentrated acid solution (3N) but have a negligible effect in
very dilute solutions (N/200) of the acid. Incorporation of Cu*,
As3+, Sd and Sn*+ with HA improves its performance, which is
synergistic in nature. These additives (except As), however,
exhibit an antagonistic effect when tested in dilute acid
solutions. Cu* and As3 have the most pronounced effect in 3N acid
solution. In N/200 HCI solution, the antagonistic effect is a
maximum in the case of St? followed by Sn*+ and CU*+ cations.
Weight-loss, electrochemical polarization and zeta potential
measurements are performed to understand the mechanism of action of
these inhibitors. The positive role played by the cations on the
inhibitive performance of HA is due to the formation of anionic
complexes with the chloride ions of the acid solution. These anions
replace the adsorbed chloride ion from the metal-electrolyte
interface owing to their higher affinity toward the interface and
help the protonized molecule of HA to bc adsorbed more strongly at
the interface. Accumulation of FeCI, in concentrated acid solution
lowers the performance of HA to a greater extent (about 100 times)
than HA blended with Cu*+. The latter composition also has a
substantially stronger inhibitive role on hydrogen absorption by
the steel than the former one.
INTRODUCTION
It is now well established that the use of hydrochloric acid in
pickling of metals, acidization of oil wells and in cleaning of
scales is more economical, efficient and trouble-free, compared to
other mineral acids. ~4 The great advantage of this acid over the
other acids in cleaning and pickling operations lies in its ability
to form metal chlorides which are extremely soluble in aqueous
phase, compared to sulfate, nitrate, phosphate, etc. This higher
rate of solubility of chloride salts causes the least polarizing
effect and does not hamper the rate of reaction. Some salts, e.g.
FeCl,, produced as a result of the reaction of scales and acid,
have a depolarizing effect on the reaction rate and make the acid
solution extremely aggressive towards the base metal. To control
this depolarizing effect of metal salts and also the attack of the
acid on the base metal surface, the use of efficient inhibitors in
hydrochloric acid solutions during the above operations is
essential. In acid solutions, the iron surface acquires a positive
charge (the zero charge potential of iron in aqueous acid solutions
varies from -0.4 V to -0.7 V).5.6 This indicates that an inhibitor
capable of providing anionic species in the solution should be a
good inhibitor for the system. The chloride ion of hydrochloric
acid is adsorbed strongly on the metal surface and makes a
Manuscript received 18 October 1994.
1005
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1006 D. D. N. Singh, T. B. Singh and B. Gaur
negatively-charged double layer. Cationic types of species in
the solution have a tendency to interact with this double layer and
provide corrosion inhibition as a result of the formation of a
compact film on the surface. A literature survey reveals that
inhibitors having an amino group perform very well for the HCI-iron
system, due to the formation of cationic species in the acid
solutions:.
R-NH2 + H+ + R-NH,:.
Amongst the different types of amines, hexamine is the most
widely studied compound for the dissolution of iron and iron-based
alloys in acid solutions.- Its molecule acquires a positive charge
in acid solutions and has a moderate inhibitive effect on the
corrosion of steel in hydrochloric acid. The theory of synergism on
corrosion inhibition indicates that the performance of hexamine can
be improved if surfactants having an anionic effect at the
interface are introduced with the former one in the acid solutions.
Organic anionic surfactants are quite effective in improving the
performance of inhibitors. but owing to their higher costs, a
higher degree of concentrations is required (to achieve
satisfactory inhibition), and their other effects, which complicate
further processing of either metal surface or the electro- lyte,
have necessitated a search for inorganic cations which can produce
anionic species in the acid solutions and could achieve synergism
with the amines. These ions are less expensive and produce
appreciable effects on corrosion inhibition even if used at very
low concentrations. This paper describes the use of copper,
arsenic, antimony and tin, in their chloride forms, as sources of
anionic species, in combi- nation with hexamine to control the
dissolution of steels in hydrochloric acid under the influence of
different parameters. The same compositions have also been studied
for their inhibitive performance in a very dilute solution of HCI
(N/200) to explore the possibilities of their use in acidic water.
which can be used in recirculating industrial cooling systems. The
low pH water (pH = 3.54) can be used in place of plain water. which
can minimize scale formation in pipelines, and its corrosive action
can be controlled by the incorporation of highly effective
inhibitors.
EXPERIMENTAL METHOD
Hot rolled mild steel strips were taken from a single lot and
small coupons of six 7.5 x 2.5 x 0.2 cm wcrc cut from it. These
wcrc then wet ground on polishing wheel to remove the mill scale.
Final polishing was carried out at 60 grit silicon carbide abrasive
paper. The samples were degreascd with acctone before exposure to
the test clcctrolytcs. A water bath, having a tcmperaturc
sensitivity of 2 1C. was used for weight-loss studies.
Electrochemical studies wcrc performed using coupons, of circular
arca 1 .O cm. of the same steel. A saturated calomel electrode
(SCE) and a couple of graphite rods were used as reference and
auxiliary electrodes, rcspcctivcly. Polarization experiments were
performed by using a potentiodyne analyzer supplied by M/S
Petrolyte Instruments Co.. U.S.A.
Hydrogen absorbed by the steel during its dissolution in acid
solution was dctermincd on samples ol size I .O x 0.5 x 0.2 cm of
the same material as used for the weight-loss and elcctrochcmical
tests. The technical details of the experimental method arc
described in earlier publications.3.
Zeta potential measurcmcnts wcrc performed on iron powder
(particle size 200 mesh) produced electrochemically. Lazer Zcc
Meter Model SO1 supplied by M/S Pen Kcm, Inc., NY. U.S.A. was used
for this purpose.
AR grade HCI acid was used for the preparation of the test
electrolytes. Dilution was made with double distilled water.
Hexaminc and metal salts. taken in their chloride forms, were also
of the AR grade.
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Improving the inhibitive performance of hexamine on corrosion
1007
EXPERIMENTAL RESULTS
Inhibition studies in concentrated acid solutions Determining
the optimum concentration of inhibitors. Figure 1 shows the
per-
formance of HA on the corrosion of steel in 3N HCl at its
different concentrations and temperatures. Here, it is noted that
the inhibitor performs quite well at 22C and the corrosion rate
comes down to ~750 mdd when used at and above concentrations of 300
ppm. At the elevated temperatures, however, the inhibitive
performance of HA deteriorates and attains values of 2500 and 5000
mdd at 40 and 55C, respect- ively. At all the three temperatures
studied, the optimum performance of the inhibitor is achieved at a
concentration of 300 ppm.
To enhance the performance of inhibitor at its optimum
concentration, some metal cations, i.e. Cu2+, As3+, Sb+ and Sn2+,
were added with the inhibitor during the corrosion of steel in acid
solution. The results are shown in Figs 2-5. It is evident from
these figures that the copper and arsenic cations achieve a
synergistic effect with hexamine in inhibiting the corrosion rate.
This is more pronounced at elevated temperatures (40 and 55C) than
at lower temperatures. These cations perform best at a
concentration of 10 ppm. The effect of antimony and tin cations is
nominal compared to copper and arsenic cations and improvement in
the inhibition efficiency of hexamine takes place between 2.5 and
5.0 ppm of these cations.
In order to test whether the optimum concentration determined
for the perform- ance of hexamine changes in the presence of metal
cations or not, experiments were performed taking different
concentrations of hexamine and a fixed concentration (10 ppm) of
cations at 55C only. The results are shown in Fig. 6. The optimum
performance of hexamine is still observed at a concentration of 300
ppm. In the presence of copper and arsenic ions, the trend is
slightly accelerated, with an increase in hexamine concentration
above 300 ppm. As3+ retards corrosion rate more effectively (about
2.5 times) than Cu2+, at all the studied concentrations of
hexamine.
The effect of the passage of time on the performance of
optimized (300 ppm HA
i- 0 100 200 300 400
HA concentration (ppm) 0
Fig. 1. The variation in corrosion rate of steel in 3N HCI with
concentration of HA
-
1008 D. D. N. Singh, T. B. Singh and B. Gaur
300ppmHA
*0 I I I
2.5 5.0 1.5 10.0 I Concentration of Cu*+ (ppm)
Fig. 2. The effect of Cu*+ on the corrosion rate of steel in 3N
HCI in the presence of 300
ppm of HA
300ppmHA
I .-.3oc._
1
2.5 5.0 I.5 10.0 Concentration of As3+ (ppm)
5
Fig. 3. The effect of As+ on the corrosion rate of steel in 3N
HCI in the presence of 300 ppm of HA.
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Improving the inhibitive performance of hexamine on corrosion
1009
300ppmHA
I l - .-. ._.E._
OO I I I I
2.5 5.0 7.5 10.0 12.5 Concentration of Sb3+ (ppm)
Fig. 4. The effect of Sb3+ on the corrosion rate of steel in 3N
HCI in the presence of 300 ppm of HA.
300ppmHA
Concentration of Sn*+ (ppm)
Fig. 5. The effect of Sn2+ on the corrosion rate of steel in 3N
HCI in the presence of 300 ppm of HA.
-
lOI D. D. N. Singh, T. B. Singh and B. tiaur
oi 0 loo 200 300 400 500
HA concentration (ppm)
Fig. 6. The effect of metal cations on the corrosion rate of
steel in 3N HCI in the prescncc of different concentrations of
HA.
23 C
I - 300ppmHA 2 300ppmHA + I Oppm Cu2+ 3 - 300ppmHA + 1 Oppm As3+
4 - 300ppmHA + 1Oppm Sb3+ 5 - 300ppmHA + IOppm Sn2+
I I I I J 0.25 I 2 3
Time (h)
Fig. 7. The cffcct of time on the corrosion rate of steel in 3N
HCI in the prcscnce of
optimized concentration of the inhibitor.
+ 10 ppm cations) concentration of inhibitors is shown in Fig.
7. In all the cases, it is observed that the inhibitive performance
of the compositions increases for times up to 2 h and then becomes
almost constant. A considerable degree of change in the performance
of the inhibitors is noted at longer durations of the exposure
for
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Improving the inhibitive performance of hexamine on corrosion
1011
I-
I-
, -
1 '0
I I I I I
0.1 0.2 0.3 0.4 0.5 0.6 0.7 FeCI, (M)
Fig. 8. The effect of ferric chloride on the corrosion rate and
hydrogen absorption of steel in 3N HCI in the presence of 300 ppm
HA.
inhibitor having cations and in their absence. At and above the
exposure period of 1 h, for example, the inhibitor having cations
inhibits the reaction rate about twice as much as in their absence.
This same factor is about 1.3 times when tested at 15 and 30
min.
The effect of ferric chloride on the performance of inhibitors.
Accumulation of iron salt (FeCls) in acid solution takes place due
to the dissolution of scale. This salt depolarizes the corrosion
reaction to a considerable extent. Experiments were, therefore,
performed to study the effect of FeCls on the corrosion rate and
hydrogen absorption by the steel during its corrosion in the
presence of the inhibitors (300 ppm HA and 300 ppm HA + 10 ppm
Cu+). Only one cation, i.e. Cu*+ , was taken up for the study with
300 ppm of HA and the results are compared with the performance of
HA. The observed data is plotted in Figs 8 and 9. A linear
relationship has been noted between the molar concentration of
FeC13 and the corrosion rate in plain hexamine, as well as in
hexamine having Cu 2+ The rate of increase in corrosion rate . (CR)
with molar concentration of FeCl,, i.e. dcR/dlFecI~l, is markedly
lower in the case of the inhibitor fortified with Cu2+ than in the
plain inhibitor (in the former case
dcnJdlreci,l = 5.24 X lo* mdd M-l, whereas for the latter, the
value is 350 x lo* mdd M-l). The hydrogen absorption values are
also considerably less in the case of the inhibitor boosted with
Cu2+.
Adsorption characteristics of the inhibitors. In order to test
the adsorption characteristics of the studied inhibitors, their
surface coverage at different concen- trations was fitted in
different adsorption isotherms. It is observed that the inhibitors
obey the Langmuir adsorption isotherm equation. Values of log 19/(
1 - 13) vs log Care
-
1012
0.35
0.30
0.5
0
D. D. N. Singh, T. B. Singh and B. Gaur
_ Inhibitor: 300ppmHA + 1Oppm Cu*+
I I I I I I
0.1 0.2 0.3 0.4 0.5 0.6 0.7
FeCI, (M)
0.8
0.2
Fig. 9. The effect of ferric chloride on the corrosion rate and
hydrogen absorption of steel in 3N HCI in the presence of optimized
concentration of the inhibitor.
IO
HA 10% HCl _
Fig. IO. Plots of adsorption isotherm for HA in 3N HCI at
different studied tcmperaturcs.
plotted in Figs 10-12. Here, 8 is the fraction of surface
covered by the inhibitor, whereas (1 - 0) is the bare surface which
acts as a site for corrosion reactions to proceed and C is the
molar concentration of inhibitors.
Electrochemicalstudies. The polarization studies were performed
in the presence of different combinations of the inhibitors in 3N
HCI solution and the plots are shown in Figs 13 and 14. It is noted
from these figures that the presence of HA in concentrated HCI
solution has no greater appreciable effect on the anodic and
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Improving the inhibitive performance of hexamine on corrosion
1013
I 1 .^
IO-
log 0
Fig. 11. Plots of adsorption isotherm for cations having 300 ppm
HA in 3N HCI at 23C
100 Temperature 40 C
300ppmHA
S L 2
5
Fig. 12. Plots of adsorption isotherm for cations having 300 ppm
HA in 3N HCI at 40C
cathodic slopes than on the control plot. The presence of Cu2+,
As3+, Sb3+ and Sn2+, however, has considerably affected the plots.
Amongst the additives studied, the presence of CL?+ with HA has the
maximum influence on the anodic curve, followed by Sb3+ and Sn+.
The cathodic curve in the presence of Cu*+, however, is not changed
at all. The cathodic plots in the presence of Sb3+, Sn*+ and As3
are considerably polarized.
Inhibition studies in dilute acid solution Weight-loss studies.
Weight-loss corrosion data of steel exposed in N/200 HCl in
the presence of different inhibitors are shown in Table 1. Due
to the very slow rate of
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1014 D. D. N. Singh, T. B. Singh and B. Gaur
0
-075 -
10-3 10-2 10-l I IO 102 IO
c.d. (mA cm-*)
Fig. 13. Electrochemical polarization plots of steel in BN HCI
in the prcscncc of 100 and 300 ppm HA with control (in blank).
-0.25 -
E
-0.50 -
9
4
-0.75 -
-l.Oo-
10-S 10-J 10-3 10-2 10-I I IO 102
c-d. (mA cm-)
Fig. 14. Electrochemical polarization plots of steel in 3N HCI
in the prescncc of 300 ppm HA having different cations.
-
Improving the inhibitive performance of hexamine on corrosion
1015
Table 1. The corrosion rate of mild steel and the zeta potential
of the iron particles in N/200 HCI solution in the absence and
presence of different
inhibitors at 29C
Solution Corrosion rate? Zeta potential S. No. composition (mdd
x 102) (mv)
1 Blank (b) 10.6 -10 2 b + 100 HA ppm 9.6 -35 3 b + 300 HA ppm
(c) 9.2 -40 4 c + 10 Cu2+ ppm 11.3 -16 5 c + 10 As+ ppm 7.2 -35 6 c
+ 10 Sn2+ ppm 16.6 -35 7 c + 10 Sb+ ppm 42.8 -35
:Derived from a 72 h exposure test.
attack of the electrolyte on the steel surface, the experiments
were performed for an exposure period of 72 h. In contrast to its
role in concentrated acid solution, HA exhibits very poor
inhibition in this case. HA, in combinations of all the cations
except As?, has a rather accelerating effect on the corrosion of
steel. Amongst the cations studied, Sb+ has the highest rate of
acceleration, followed by Sn2+ and cu2+.
Zetapotential measurement. The peculiar results observed for the
performance of different inhibitors on the corrosion of steel in
dilute HCI solution led to a study of their inhibitive nature in
detail. The zeta potential studies, which are generally useful for
study of the surface charges on the minerals suspended in a fluid,
were performed for the particles of iron (iron powder of 200 mesh)
suspended in N/200 HCI having different inhibitors. In these
studies, since the experiments were performed within l- 2 min of
exposure of the iron particle in acid solution and also due to an
extremely slow rate of attack of electrolyte on the metal surface,
the shapes of the iron particles were taken as unchanged during the
experimentation. The measured values of zeta potential are
summarized in Table 1. It is seen from this table that the values
of zeta potential on the iron particle exposed to N/200 HCI
solution (pH = 3.5) is negative (- 10 mV). A negative value of zeta
potential is expected for this system owing to the strong
adsorption of chloride ion in the ionic sphere of the particle in
the solution. On the addition of 100 ppm HA in the bath, the zeta
potential jumps to a more negative value (-35 mV). On increasing
the concentration of HA (300 ppm), the value shifted towards the
negative direction. The addition of Cu2+ with HA, however, brings
down the potential to a lower negative value (-16 mV), very close
to the blank solution where no inhibitor was present. A similar
trend is noted for Sn2+ and Sb3+ also and the values are very close
to that observed for Cu+. In the case of As3+, however, the
particles retain their potential to a more negative value as noted
for HA alone.
Electrochemical studies. The open circuit potential-time plot
for the steel exposed in different compositions of inhibitors and
in their absence are shown in Fig. 15. In all the cases, it is
noted that the potentials shift in the active direction with
the
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1016 D. D. N. Singh, T. B. Singh and B. Gaur
2
Time (minj3 4 5
Fig. 15. Potential-time plots for steel in N/200 HCI in the
presence of different compo sitions of inhibitors.
.-------- 3WppmHA+ 10ppmAs3+ -_- 300ppmHA + 1Oppm Cu2+ ._-_-
300ppmHA + 1Oppm Sb
-0.25 - . -- - 300ppmHA + 1Oppm Sn2+
- 100 and 300ppmHA
- - - - Control
-0.75 -
10-I 1 10
cd. (mA cm-)
Fig. 16. Electrochemical polarization behaviour of steel in
N/200 HCI solution having different inhibitors.
passage of time. HA shows the highest negative value of
stabilized potential followed by Cu2+, control and A?+.
The cathodic and anodic polarization curves in the presence of
different compo- sitions of the inhibitors are shown in Fig. 16.
The plots for Sb3+ and Sn2+ are not
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Improving the inhibitive performance of hexamine on corrosion
1017
included here to avoid crowding. Their trends were also observed
to be similar to Cu2+. In the presence of HA, the cathodic curves
are polarized to a greater extent than the controlled one,
especially near to the corrosion potential. The anodic curves, on
the other hand, in the presence of HA, are depolarized near the
corrosion potential. The overall corrosion, however, is found to be
decreased although not to a considerable extent.
The addition of Cu2+ with HA accelerates the attack and is
manifested with anodic as well as with cathodic curves. The
presence of As3+, on the other hand, has a strong polarizing effect
on anodic reactions.
DISCUSSION
An iron surface attains a positive charge in aqueous acid
solutions as its potential of zero charge varies between -0.4 and
-0.7 V (SCE).5*6 Owing to the strong anionic component of HCl, the
double layer acquires the negative charge and cationic type of
inhibitors are, therefore, found to provide good inhibition for
iron in acid solutions.
HA forms a quaternary type of compound when added in the acid
solution:
I N
/
\ CH2 CH,
\ \ I& N-CH2-NH
C/H, :.H2
+
[W
Due to the formation of a cationic surfactant in the solution,
HA is expected to inhibit the corrosion cathodically. This is
indeed reported by many other investigators.9--2
To understand the role of different inhibitors in inhibiting the
corrosion of steel in HCl solution, the following scheme is
considered for the adsorption of HA:
ElElElElEl- ooooo- +++++t---
protonized HA
chloride ion adsorbed
steel surface
Adsorbed chloride ion on the positively charged steel surface
acts as a bridge between the metal surface and the electrolyte for
the adsorption of the protonized HA molecule. The presence of a
more densely populated interface with anionic species is expected
to provide a higher degree of inhibition to HA according to the
above scheme. This has indeed been observed in the study where an
increase in the
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1018 D. D. N. Singh, T. B. Singh and B. Gaur
-x- HAalone _ -.- - HA + NaCl
/ I I --- J___L__-l-_-L- 0 2.5 5.0 1.5 10.0
NaCl in 3N HCl (%)
I I I I I I 0.3 2.0 4.0 6.0 8.0 10.0 I
Concentration of HCI (N) .O
Fig. 17. The variation in the performance of inhibitors for
steels at different conccn- trations of HCI and NaCI.
concentration of HCI enhances the inhibitive performance of HA
(Fig. 17). Incor- poration of NaCl in 3N HCl having 300 ppm
hexamine also enhances its perform- ance. This theory is further
supported by the observation on the performance of the inhibitor in
extremely dilute HCl solution (N/200), where an almost negligible
inhibitive effect has been noted (Table 1).
The enhanced inhibitive effect of HA in the presence of metal
cations (in 3N HCI) can be explained by considering the ionic
species formed in concentrated acid solutions. In strongly acidic
chloride environments, the copper cation forms species of the type
CuCl, . 2- Such types of anionic species are more surface active
and can cover more surface area on the interface compared to the
chloride ion alone. Such types of adsorbed species are also
expected to facilitate the adsorption of protonized HA to a greater
extent on the metal/electrolyte interface than the chloride ion.
Though there is no reference available in the literature on the
combined effect of metal cations with HA on the corrosion of steel,
some authors have reported an improvement in the inhibitive
performance of HA in combination with certain anions, such as Cl-,
Br , I-, CNS and sulfonates, on the corrosion of steel in acidic
solutions.6.7 The improved performance of HA in combination with
Asa+ may be ascribed to the formation of anionic species AsO; in
the acidic solution. This anion, like CuClzP, is expected to
enhance the adsorption of HA+ at the interface of the corroding
metal. Sb+ and Sn 2+ in the acid solution also enhance the
protection properties of HA and their beneficial effects can also
be explained as described above for Cu2+ and Asa+.
The adverse effect of the studied metal cations on the
inhibitive performance of HA in dilute acid solution is quite an
interesting phenomenon. HA has a negligible effect in diluted acid
solution (Table 1). This is probably owing to the inability of HA
to form cationic species in dilute acid solution. Due to the
presence of a weak negatively charged interface, the HA molecule
adsorbs on the metal-electrolyte interface though the lone pair of
unshared electrons available at the nitrogen atom of
-
Improving the inhibitive performance of hexamine on corrosion
1019
HA and, thus, increases the negative charge of the interface.
This is also manifested by the zeta potential value. In the absence
of any additive, the zeta potential value of the iron particle is
-10 mV, which jumps to -35 mV on the addition of HA in the
electrolyte. The addition of copper cation in the presence of HA
replaces the latter from the interface and may deposit at the metal
surface on cathodic sites. This brings down the zeta potential
value (- 16 mV) of the particle. In the case of Sn2+ and Sb3+, no
change in zeta potential takes place, indicating that they are not
reduced at the metal surface and remain in the solution to act as
strong depolarizers of the reaction. Unlike the other three
cations, As3+ shows an inhibitive effect with HA in dilute acid
solution. This is probably owing to the amphoteric nature of
arsenious acid (HAs02) which provides AsO+ and AsO; in the acidic
solutions. AsO+ is reduced at the cathode to deposit as a neutral
arsenic atom on the surface:
AsO+ + 2Hf + 3e- + As + H,O
Deposited arsenic on the surface increases the cathodic
overvoltage and provides the inhibition effect. Deposition of
neutral arsenic metal on the surface is further manifested from the
zeta potential value. The value of zeta potential of the iron
particle exposed in solution having 300 ppm HA does not change
after the addition of As3+ in the solution (in both cases the value
is -35 mV). It is to be noted here that the inhibition caused due
to the As3+ in dilute acid solution is solely due to the deposition
of arsenic metal on the surface and HA has neither an additive nor
synergistic effect on it.
Acknowledgemenr-The authors express their thanks to Prof. P.
Ramchandra Rao, the Director, National Metallurgical Laboratory,
for granting the permission to publish this work.
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