Pit initiation and growth control of Al in KSCN solutions

Article abstract of DOI:10.1016/j.crci.2010.09.006

The inhibitive action of a newly synthesized glycine derivative, namely 2-(4-(dimethylamino)benzylamino)acetic acid hydrochloride (GlyD) on pitting corrosion of Al in KSCN solutions was studied. Obtained results were compared with glycine (Gly) itself. Cy

Full text of DOI:10.1016/j.crci.2010.09.006

C. R. Chimie 14 (2011) 429–433
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Preliminary communication/Communication
Pit initiation and growth control of Al in KSCN solutions
b
Mohammed A. Amin a, , Mohamed M. Ibrahim
*
a Department of Chemistry, Faculty of Science, Ain Shams University, 11566 Abbassia, Cairo, Egypt
b Chemistry Department, Faculty of Science, Kafr El-Sheikh University, Kafr El-Sheikh 33516, Egypt
A R T I C L E I N F O
A B S T R A C T
Article history:
The inhibitive action of
a newly synthesized glycine derivative, namely 2-(4-
Received 2 March 2010
(dimethylamino)benzylamino)acetic acid hydrochloride (GlyD) on pitting corrosion of
Al in KSCN solutions was studied. Obtained results were compared with glycine (Gly)
itself. Cyclic polarization, potentiostatic and galvanostatic measurements were made,
complemented with ex situ scanning electron microscope (SEM) examinations. Cyclic
polarization measurements indicated that the presence of Gly and GlyD in aggressive SCN-
solutions shifted the pitting potentials to more noble values and decreased the anodic
currents in the passive region. Transient studies showed that nucleation of pit takes place
after an incubation time (ti). The rate of pit nucleation (tiꢀ1) and growth was found to
decrease to an extent depending on the type inhibitor. The SEM images showed that the
severity of the pitting corrosion of Al in KSCN solution decreased to an extent depending on
type of inhibitor. GlyD was much better than Gly in controlling pit initiation and growth of
Al in these solutions.
Accepted after revision 27 September 2010
Available online 13 December 2010
Keywords:
Aluminium
Pit initiation and growth
SCNꢀ anions
Amino acids
´
ß 2010 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
2. Experimental
Our research group has recently described pitting
corrosion of Al and Al-Zn alloys induced by the aggressive
attack of SCN- anions [1]. Recently, glycine (Gly) was
successfully reported as a corrosion safe inhibitor for some
metals and alloys [2–6]. In view of the inhibition
characteristics of Gly, a new Gly derivative (GlyD), namely
2-(4-(dimethylamino)benzylamino)acetic acid hydrochlo-
ride was synthesized in our laboratory to effectively inhibit
pitting corrosion of Al in KSCN solutions. Measurements
were carried out with the use of cyclic polarization and
potentiostatic and galvanostatic measurements. The
obtained results were compared with results recorded
for Gly. Some SEM examinations of the electrode surface in
absence and presence of the inhibitor were also performed.
The working electrode employed here was made of
pure Al (99.98%) provided by the Egyptian Aluminium
Company (EAC). The Al electrode was used in the as-
received condition. The investigated electrode was cut as a
cylindrical rod, welded with Cu-wire for electrical connec-
tion and mounted into glass tubes of appropriate diameter
using Araldite to offer an active flat disc shaped surface of
(0.50 cm2) geometric area for the working electrode, to
contact the test solution. Prior to each experiment, the
electrode surface was wet polished with silicon carbide
from 220 to 500 grits, degreased with acetone and, finally,
rinsed with distilled water.
The glycine derivative (GlyD) was synthesized in the
laboratory according to the procedure described below:
Glycine (0.75 g, 10 mmol) and 4-(dimethylamino)ben-
zaldehyde (1.49 g, 10 mmol) were dissolved in ethanol
(20 ml) in the presence of KOH (0.56 gm, 10 mmol) and
vigorously stirred at room temperature and refluxed for
2 h. A white precipitate was formed on cooling, filtered off,
washed with cold ethanol, and dried in vacuo. The
* Corresponding author.
E-mail address: maaismail@yahoo.com (M.A. Amin).
1631-0748/$ – see front matter ß 2010 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.crci.2010.09.006

M.A. Amin, M.M. Ibrahim / C. R. Ch[()TF$D]GI imie 14 (2011) 429–433
430
1.5
produced imine derivative was then added to a solution of
0.5 gm (14 mmol) of NaBH4 in ethanol and the reaction was
allowed to stand for 4 h at room temperature. The resultant
solution was acidified by concentrated HCl. A white
precipitate of 2-(4-(dimethylamino)benzylamino)acetic
acid.HCl (the newly synthesized glycine derivative; GlyD)
was formed, filtered, and dried up. Yield 1.98 (81%); FT-IR
1.0
Cathodic Tafel
region
0.5
0.0
jpass
(KBr):
1472, 1420, 1209, 1051, 908, 665, 607, and 516. 1H NMR
(500 MHz, DMSO-d6) (ppm): 13.56 (s, 1H, OH), 6.79 (s,
1H, NH), 7.62 (d, J = 7.8, 2H, C-Hbenzene), 7.42 (d, J = 7.8, 2H,
n
(cmꢀ1): 3428, 2947, 2360, 1751, 1635, 1564, 1539,
jcorr
-0.5
-1.0
-1.5
-2.0
d
Erp
Epit
j
corr = 0.268 mA cm-2
C-Hbenzene), 4.07 (s, 2H, -CH2ꢀCOꢀ), 3.49 (s, 2H,
CH2ꢀNHꢀ), and 2.95 (s, 6H, ꢀCH3).
-
(slope) = β
Ec
βc = -375 mV dec-1
Extrapolated
cathodic
Ecorr
-2000 -1500 -1000 -500
0
500
1000 1500 2000
E / mV (SCE)
Fig. 1. Typical cyclic polarization plot recorded for Al in 0.04 M KSCN
solution at a scan rate of 0.50 mVs-1 at 25 oC.
5.0 min in 0.04 M KSCN solutions without and with 10ꢀ3 M
Gly or GlyD at 25 8C. Measurements were performed under
potentiostatic regime at 1300 mV (> Epit recorded for Al in
0.04 M KSCN), and finally washed thoroughly and sub-
jected to 20 min of ultrasonic cleaning in order to remove
loosely adsorbed ions. The morphology of the electrode
surface was then examined by SEM using an Analytical
Scanning Electron Microscope JEOL JSM 6390 LA.
All experiments were carried out in 0.04 M KSCN
solutions without and with various concentrations (10ꢀ5
–
5 ꢁ 10ꢀ3 M) of Gly or its derivative (GlyD). For each run, a
freshly prepared solution as well as a cleaned set of
electrodes was used. Each run was carried out in aerated
stagnant solutions at 25 ꢂ 1 8C, using water thermostat.
Cyclic polarization measurements were carried out by
sweeping linearly the potential from the starting potential,
ꢀ2000 mV (SCE), into the positive direction at a scan rate of
0.50 mV sꢀ1 till the end potential, 2000 mV (SCE), and then
reversed with the same scan rate till forming a well-
defined hysteresis loop. The potentiostatic j/t transient
measurements were carried out after a two-step proce-
dure, namely: the working electrode was first held at the
3. Results and discussion
Fig. 1 shows s typical cyclic voltammogram recorded for
Al in 0.04 M KSCN solution at a scan rate of 0.5 mV sꢀ1 at
25 8C. The obtained plot had the familiar form for Al
showing a well-defined corrosion potential, Ecorr, followed
by a passive region. It is obvious that the passive region
extends up to a certain potential, designated as the critical
potential (Ec). As the electrode potential exceeds Ec, the
passive current (jpass) increases slightly with potential.
Then at a certain potential, a sudden increase in jpass is
observed without any sign for oxygen evolution, indicating
passivity breakdown, initiation and propagation of pitting
corrosion. When the sudden increase in jpass was observed
after the passive region, the potential value acquires the
name of pitting potential (Epit).
starting potential for 60 s to attain
a reproducible
electroreduced electrode surface. Then the electrode was
held at constant anodic step potential (Es.a) where the
anodic current was recorded with time. The stabilization
period prior to collecting data was 12 h. The open circuit
potential of the working electrode was measured as a
function of time during this stabilization time. This time
was quite sufficient to reach a quasi-stationary value for
the open circuit potential.
Electrochemical experiments were performed in a
100 ml volume Pyrex glass cell using Pt wire and a
saturated calomel electrode (SCE) as auxiliary and refer-
ence electrodes, respectively. The SCE was connected via a
Luggin capillary, the tip of which was very close to the
surface of the working electrode to minimize the IR drop.
All potentials given in this paper are referred to this
reference electrode. Electrochemical measurements were
performed using An Autolab Potentiostat/Galvanostat
(PGSTAT30) connected with a personal computer with
GPES and FRA (ver. 4.9) software provided by Autolab. The
stabilization period prior to collecting data was 12 h.
For surface morphology studies, some Al samples were
exposed to pitting attack; the samples were immersed for
At Epit
,
the aggressive SCNꢀ anions displace the
adsorbed passivating species at some locations and
accelerate local anodic dissolution. This mechanism is
accelerated when SCNꢀ concentration increases [1]. Pitting
growth occurs as
a
result of an increase in SCNꢀ
concentration resulting from its migration inside pits
and hydrolysis of Al3+ ions, since the high acidity required
for pitting corrosion site growth must be achieved by
hydrolysis of Al3+ ions inside pits; see more details in our
previous studies [1,7]. The reverse scan shows a wide
hysteresis cycle, this being characteristic of passivity
breakdown on the upward sweep and repassivation.
Finally, a repassivation potential, Erp, was attained.
Fig. 2 presents cyclic polarization plots recorded for Al
in 0.04 M KSCN solutions without and with 10ꢀ3 M Gly or
GlyD at a scan rate of 0.5 mV sꢀ1 at 25 oC. It is obvious that

[
M.A. Amin, M.M. Ibrahim / C. R. Ch[()TF$D]GI imie 14 (2011) 429–433
431
1.5
1.0
blank
10-3 M Gly
10-3 M GlyD
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2000 -1500 -1000 -500
0
500
1000
1500
2000
E / mV (SCE)
Fig. 4. Galvanostatic potential-time plots recorded for Al in 0.04 M KSCN
solution without and with 10-3 M Gly or GlyD at a an applied anodic
Fig. 2. Cyclic polarization plots recorded for Al in 0.04 M KSCN solution
without and with 10–3 M Gly or GlyD at a scan rate of 0.50 mVs-1 at 25 oC.
current of 50 m
A at 25 oC.
addition of both Gly and GlyD decreases the anodic and
cathodic current densities, reduces the corrosion current
density (jcorr), and shifts Ecorr to more negative values.
These results reflect, at first sight, the ability of these
organic compounds to inhibit uniform corrosion of Al in
these solutions. This point will be presented in details in a
further complementary paper. In the present work, special
attention was focused on the influence of Gly and GlyD on
the pitting corrosion phenomena, regarding pit initiation
and growth, of Al in SCNꢀ solutions
anodic potential of 1300 mV (>Epit) at 25 oC. In absence of
inhibitor, curve 1, the anodic current density initially
decreases, due to oxide film growth, to a minimum value
determining the characteristic pitting parameter, namely
the incubation time (ti). These data reveal that an
incubation time is necessary before pit initiation. The
magnitude of ti reflects the susceptibility of sample to
pitting corrosion. Beyond ti, the current density increases
rapidly and finally reaches a steady state value. The rise in
the current density (i.e., the pit growth current density
(jpit)) indicates pit growth with time, while the steady state
is due perhaps to pits blocked by corrosion products that
accumulated on the anode surface.
Referring again to Fig. 2, it is obvious that upon the
addition of both Gly and GlyD, a decrease in the current in
the passive region (jpass) results, with a positive shift in Epit
.
The negative shift in Ecorr and positive shift in Epit reflect
resistance towards pitting, Rpit (Rpit = jEcorr ꢀ Epitj). Electro-
chemical parameters derived from these measurements
(not included here) showed that all these electrochemical
events depend on the type of the introduced inhibitor, with
GlyD being more effective than Gly. The positive shift of Epit
suggests that pitting corrosion in presence of Gly and GlyD
is unlikely to occur. This is most probably due to inhibitor
adsorption and subsequent formation of a protective film
on the electrode surface, see later.
Addition of both Gly and GlyD decreases Al susceptibility
towards pitting. This is clearly seen from the decrease in jpit
and the subsequent increase in ti. This decrease in
susceptibility towards pitting depends on the type of the
introduced inhibitor, with GlyD being more effective than
Gly. These findings support results of polarization measure-
ments that the presence of these inhibitors protects Al
against the aggressive attack of SCNꢀ anions.
Fig. 4 presents the dependence of the anodic potential
on time for anodization of Al in 0.04 M KSCN solutions
without and with 10ꢀ3 M Gly or GlyD at a current density
of 50 m
A cmꢀ2 at 25 oC. The linear increase in potential at
Fig. 3 shows j/t transients recorded for Al in 0.04 M
ꢀ3 M Gly or GlyD at an
K[()TD$FIG] SCN solutions without and with 10
the commencement of anodization accompanies the initial
formation and growth of the oxide film. A potential
maximum appears at a certain time, assigned as the
incubation time, ti, which is necessary before pit growth to
occur. This maximum is thought to correspond, as will be
shown, to the pitting potential (Epit). The appearance of
which is the correspondence of competition between two
processes, namely, further oxide film growth and its
breakdown.
The decline in potential following the maximum
corresponds to breakdown of passive layer followed by
the nucleation and formation of pits as a result of the
aggressive attack of SCNꢀ anions [1]. After prevalence of
the latter reaction at Epit, or in other words, after ti, the
potential drops to a steady value, corresponding to the
repassivation potential (Erp). It obvious that the values of
Erp obtained from cyclic voltammetry [1] and galvanostatic
10
blank
Gly
8
6
4
GlyD
t
i
t
2
t
i
i
0
0
20 40
60 80 100 120 140 160 180 200
Time / s
Fig. 3. Potentiostatic current-time plots recorded for Al in 0.04 M KSCN
solution without and with 10-3 M Gly or GlyD at a an applied anodic
potential of 1300 mV (> Epit) at 25 oC.

[()TD$FIG]
432
M.A. Amin, M.M. Ibrahim / C. R. Chimie 14 (2011) 429–433
towards more positive value (i.e., longer ti values) in
presence of these inhibitors. These findings confirm results
of polarization measurements that these organic com-
pounds inhibited pitting corrosion of Al in KSCN solutions.
The formation of a protective film of inhibitor is
confirmed from SEM examinations of the electrode
surface. Fig. 5 presents SEM images recorded for Al in
0.04 M SCNꢀ solutions without and with 10ꢀ3 M Gly or
GlyD. It is quite clear that the surface morphology varies
depending on the type of inhibitor. Severe pitting attack is
observed in absence of the inhibitor (see image a); a large
number of small pits were observed. On the other hand, in
presence of both Gly and GlyD, the corroded areas
obviously diminished. Significant decrease in corroded
areas was again observed in presence of GlyD (image c)
more than in presence of Gly (image b). Pitting attack still
appears in case of Gly. Surface morphology in presence of
GlyD, image c, showed that pitting attack is prevented and
the surface is efficiently separated from the corrosive
medium.
The pH of the potential of zero charge (PZC) for
aluminium oxides/hydroxides is 9 [8,9]. Depending on
solution pH, the electrode surface can bear net negative, or
positive, or no charge. In 0.04 M KSCN solution (pH 6.8),
therefore oxide surface is positively charged. Addition of
GlyD provides KSCN solution with Clꢀ anions. These Clꢀ
anions adsorb, in competition with SCNꢀ anions [1], at the
oxide/solution interface through electrostatic attraction
force because of the excess positive charge at this interface
at the corrosion potential. This process changes the
solution side of the interface from positive to negative.
Thus, the quaternary ammonium cations in the GlyD
molecule, ꢀH-N+(CH3)2, electrostatically adsorb on the
oxide surface covered with primarily adsorbed Clꢀ and/or
SCNꢀ anions. Detection of Cl and N signals on the electrode
surface using EDX spectra (not shown here) may confirm
this type of adsorption.
The group –Ph– in the GlyD structure is a donor of
electron and it may offer itself the possibility to be a center
of adsorption beside nitrogen atoms. It is probable that, in
case of GlyD, the adsorbed nitrogen atoms are oriented
with the benzene ring structure parallel to the metal
surface (due to aromaticity of the GlyD molecule). This
mode of adsorption suggests that the adsorbed GlyD
molecule covers
a large area, thereby inhibiting Al
corrosion effectively more than Gly.
4. Conclusion
Fig. 5. SEM images of Al in 0.04 M KSCN solutions at 25 oC without (image
a) and with 10-3 M Gly (image b) or 10-3 M GlyD (image c). The electrode is
potentiostatic held at 1300 mV (> Epit recorded for Al in 0.04 M KSCN) for
5.0 min at 25 oC.
The presence of glycine and its derivative in KSCN
solutions inhibited uniform and pitting corrosion process.
Corrosion and passive currents were reduced, and the
pitting potential was increased with increase in additive
concentration. Transient measurements showed that pit
initiation and growth decreased in presence of these
organic compounds. The inhibition efficiency of the newly
synthesized glycine derivative was found to be higher than
that of glycine itself. Polarization measurements showed
that glycine and its derivative acted as mixed-type
inhibitors with cathodic predominance. SEM examinations
of the electrode surface showed that the aggressive pitting
measurements are in good agreements. These findings
confirm that the potential maximium observed in the E/t
plots corresponds to Epit
.
It follows from the data of Fig. 4 that the rate of
potential increase (i.e., the rate of oxide growth) and the
value and location of potential maximum depend on the
type of introduced inhibitor. The value of the potential
maximum (i.e., Epit) increases and its location shifts

M.A. Amin, M.M. Ibrahim / C. R. Chimie 14 (2011) 429–433
433
attack of SCN– anions decreased with increase in additive
concentration.
[3] M.A. Amin, K.F. Khaled, S.A. Fadl-Allah, Corros. Sci. 52 (2010) 140.
[4] M.A. Amin, S.S. Abd El Rehim, H.T.M. Abdel-Fatah, Corros. Sci. 51 (2009)
882.
[5] W.A. Badawy, K.M. Ismail, A.M. Fathi, Electrochim. Acta 51 (2006) 4182.
[6] H. Ashassi-Sorkhabi, M.R. Majidi, K. Seyyedi, Appl. Surf. Sci. 225 (2004)
176.
References
[7] M.A. Amin, S.S. Abd El Rehim, S.O. Moussa, A.S. Ellithy, Electrochim. Acta
53 (2008) 5644.
[8] H. Hohl, M. Stumm, J. Colloid Int. Sci. 55 (1976) 281.
[9] R. Wood, D. Fornasiero, J. Ralston, Colloids Surf. 51 (1990) 389.
[1] A.M. Amin, S.S. Abd El-Rehim, E.E.F. El-Sherbini, S.R. Mahmoud, M.N.
Abbas, Electrochim. Acta 54 (2009) 4288.
[2] M. Zerfaoui, H. Oudda, B. Hammouti, S. Kertit, M. Benkaddour, Prog. Org.
Coat 51 (2004) 134.