Food Preservatives Serving as Nonselective Metal and Alloy
Corrosion Inhibitors
Yair Ein-Eli,
*
,z
Esta Abelev,
**
and David Starosvetsky
Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
In this short communication we provide a glimpse into the unexpected, intriguing nature of 2,4-hexadienoic acid potassium salt
potassium sorbate K CH
3
CHvCHCHvCHCO
2
, namely, its high superiority over other corrosion inhibitors to protect metals
and alloys copper, carbon steel, aluminum, and stainless steel from surface degradation processes in a wide range of potentials
in the presence of corrosive environments.
© 2005 The Electrochemical Society. DOI: 10.1149/1.2116167 All rights reserved.
Manuscript submitted July 27, 2005; revised manuscript received August 18, 2005. Available electronically November 21, 2005.
Unsaturated aliphatic monocarboxylic acids and their salts were
discovered in the late 1930s and mid-1940s to be effective at inhib-
iting the growth of microorganism.
1
Following this discovery, the
use of an acid salt, e.g., potassium sorbate, in the food industry
increased rapidly. Short-chain fatty acids and their salts, such as
sorbate and propionate have been used for their antimicrobial ac-
tivity, causing cell stasis or lag phases in growth, rather than killing
microbe cells.
2
The attractive features of this type of material,
namely preserving food against yeast and fungi growth, without
traces of odor and taste, led to an extensive use of these materials in
food technology.
3-5
The growing interest in advanced materials, ca-
pable of producing efficient protection of metals and alloy surfaces
from degradation, and in addition satisfing the demand of being
nontoxic, benign, and even biodegradable is leading many research
groups to search of such materials.
6-11
Such benign materials are
being used in various applications, such as atmospheric and envi-
ronmental metal corrosion
12
and batteries.
13,14
However, most envi-
ronmentally friendly inhibitors are targeted to protect a specific
metal or alloy, while a nonspecific, highly efficient corrosion inhibi-
tor has not been reported until now.
Experimental
Materials, metals, and alloys.— A pencil-type specimen made
of 5 mm diam copper 99.995 wt %, aluminum 6061, carbon steel
1020, and stainless steel 316 L rods mounted in a room
temperature-curing-epoxy was used in the electrochemical measure-
ments. After polishing with 1200 grit, the samples were carefully
degreased with acetone and water rinsed. The studied solutions were
prepared using deionized DI water with the addition of analytical
grade chemicals Aldrich Chemicals, such as 2,4-hexadienoic acid
potassium salt K-sorbate, Na
2
SO
4
, KCl, and benzotriazole BTA.
Electrochemical measurements.— Electrochemical measure-
ments were conducted with an EG&G 273A potentiostat in a 500
mL three-electrode electrochemical cell, equipped with a reference
saturated calomel electrode SCE and Pt counter electrode. The
calomel reference electrode was introduced into the solution through
a Luggin-Haber capillary tip assembly.
X-ray photoelectron (XPS) measurements and data process-
ing.— X-ray photoelectron spectroscopy XPS measurements were
performed in a Thermo VG Scientific Sigma Probe using a mono-
chromotized X-ray Al K 1486.6 eV source. A 100 W X-ray spot
of 400 m in diameter was used for surface scans with pass ener-
gies of 100 and 50 eV for survey and individual core level lines
scans, respectively. The photoelectron peaks of Cu 2p, O 1s, and C
1s were measured in the standard, surface sensitive mode of opera-
tion. For the sorbate powder sample, charge neutralization was ac-
complished by a low-energy electron flooding. All spectra were ref-
erenced to the C 1s peak at 285.0 eV. Data analysis was performed
using Sigma Probe Advantage software.
Results and Discussion
The corrosion inhibition of aluminum, copper, stainless steel, and
carbon steel in the presence of potassium sorbate was evaluated and
studied in solutions containing 100-1000 ppm of chloride ions.
Figure 1 reveals that in the presence of 1% potassium sorbate
10 g/L in chloride-containing solutions KCl in concentrations of
100-1000 ppm all the studied metals and alloys exhibited a passiv-
ity state in a wide range of potentials. In the absence of the short-
chain fatty acid salt, all metals and alloy are actively dissolved upon
anodic polarization in the aggressive chloride solutions. The de-
crease in anodic currents observed in chloride solutions, from values
of approximately 1 mA/cm
2
at high anodic potentials 0.0 V for Al
and carbon steel, 0.1 V for Cu, and 0.6 V for stainless steel to less
than a few microamperes per centimenter squared in chloride solu-
tions containing sorbate ions, at the same anodic potentials, demon-
strates that potassium sorbate is a versatile, nonselective, and pow-
erful corrosion inhibitor, capable of providing passivity to metals in
a wide potential window of 1.2 V, without any evidence of pitting
no hysteresis in the cyclic polarization curve, Fig. 1c. The passiv-
ity potential range can be further extended by an increase in the
K-sorbate concentration for example, 50 g/L, as demonstrated in
Fig. 1d. It is important to note that prior to the introduction of a
passivity stage, one can observe in the appearance of an anodic
peak, at potentials higher by only 50-100 mV from the open-circuit
potential OCPeasily observed in Fig. 1b and d, related to oxide
film formation.
Evaluation of sorbate’s superiority over other anodic inhibitors
12
is demonstrated by a comparative study between a well-known cop-
per corrosion inhibitor, benzotriazole BTA, and sorbate. Such a
study was performed in equal-molar solutions, containing 70 mM of
each material and 1 g/L Na
2
SO
4
, as illustrated in Fig. 2.
As can be seen in Fig. 2, the anodic potentiodynamic polariza-
tions obtained from polarizing copper in both solutions reveal that
while both materials passivate the copper surface, the potential
range of copper passivation is considerably extended with the use of
sorbate, up to a potential of 1 V. Potentiostatic evaluations of copper
in BTA and K-sorbate solutions containing sulfate ions 1 g/L
Na
2
SO
4
at different anodic potentials shown in Fig. 3 reveal two
important features of sorbate inhibition: passive films obtained with
the use of sorbate at anodic potentials of 0.2 and 0.3 V are con-
structed faster than the BTA passive film Fig. 3a and b, and the
protective film produced in the K-sorbate solution is highly stable at
high anodic potentials, in contrast to Cu-BTA film destruction at a
potential of 0.5 V Fig. 3c.
X-ray photoelectron spectroscopy XPS, Fig. 4 studies indicate
that Cu-sorbate Cu
1+
OOC-CHvCH-CHvCH-CH
3
1-
is the
specie in charge of the protective features. Figure 4 presents X-ray
photoelectron spectra of a copper surface exposed to a potassium
sorbate solution at 0.2 V
SCE
for 30 min. The C 1s and O 1s core
* Electrochemical Society Active Member.
** Electrochemical Society Student Member.
z
E-mail: eineli@tx.technion.ac.il
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