Volume 3 Paper 25
Organic Corrosion Inhibitors for Aluminum in Sodium Hydroxide
Keywords: aluminum, sodium hydroxide, corrosion, inhibitors, sulfonic acid, sodium cumene sulfonate, sodium alkyl sulfate, weight loss
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JCSE Volume 3 Paper 25
Submitted 11th October 2002
Organic Corrosion Inhibitors for Aluminum in Sodium Hydroxide
Chemistry Department, Faculty of Science, Jordan University of Science and
Technology, P. O. Box: 3030, Irbid, Jordan.
Inhibition of corrosion of aluminum in aqueous solutions of sodium
hydroxide in the presence of sulfonic acid, sodium cumene sulfonate and sodium
alkyl sulfate was studied in relation to the concentration of inhibitor,
concentration of corrosive medium at various temperatures applying weight loss
method. The additives tested were found to be good inhibitors for aluminum
corrosion in aqueous solutions of sodium hydroxide in the studied concentration
range. Due to the adsorption of the additive molecules on the metal surface the
inhibition efficiency increases with increasing additive concentration,
Sulfonic acid shows the best inhibition capability for aluminum corrosion in
sodium hydroxide, probably, this is due to the planer orientation of the
adsorbed additive molecules. Inhibition efficiency of the inhibitors tested
increases with decreasing sodium hydroxide concentrations. The higher the
temperature, the lower was the inhibition efficiency, which is due to the fact,
that the rate of corrosion of aluminum is higher than the rate of adsorption.
aluminum, sodium hydroxide, corrosion, inhibitors, sulfonic acid, sodium cumene
sulfonate, sodium alkyl sulfate, weight loss.
comment()Because of their lightweight and mechanical
strength, aluminum and its alloys are very attractive materials for engineering
applications. The interest of these materials arises from their importance in
the recent civilization. Inhibition of metal corrosion by organic compounds is
a result of adsorption of organic molecules or ions at the metal surface
forming a protective layer. This layer reduces or prevents corrosion of the
metal. The extent of adsorption depends on the nature of the metal, the metal
surface condition, the mode of adsorption, the chemical structure of the
inhibitor, and the type of corrosive media . Heteroatoms in the structure of
inhibitor molecules, such as oxygen (O) nitrogen (N), phosphorous (P), sulpher
(S) and the presence of aromatic rings or triple bonds enhance the adsorption
process. It has been reported that the inhibition efficiency increases in the
order: O < N < S < P [2-5]. Corrosion inhibition of aluminum and its
alloys was the subject of numerous studies [6-13]. A literature survey showed
only limited systematic work done to the corrosion inhibition of aluminum and
its alloys in various corrosive media [14-15].
comment()The purpose of the present article is to study the application of
sulfonic acid (SA), sodium cumene sulfonate (SCS) and sodium alkyl sulfate
(SAS), as corrosion inhibitors for aluminum in aqueous solutions of sodium hydroxide
(NaOH). A literature survey revealed that the selected additives have never
been tested as corrosion inhibitors for aluminum in sodium hydroxide.
inhibition efficiency of aluminum in aqueous solutions of sodium hydroxide
(NaOH) by using sulfonic acid, sodium cumene sulfonate and sodium alkyl sulfate
as inhibitors was determined by the weight loss method. The testes were
performed with samples of aluminum in the form of rods measuring 30 mm by 3 mm diameter
that were cut from commercial pure aluminium (Al 99.5 %).
comment()The aluminum samples were first abraded with emery paper 800, degreased
with acetone, rinsed with distilled water then dried. The organic inhibitors
used were in pure form. Analar sodium hydroxide pills, dissolved in double
distilled water to the selected concentrations, were taken as corrosive media.
The testes were carried out at different concentrations of additives (25, 50,
100, 150 and 200 ppm) and concentrations of corrosive media (0.5, 1.0, 1.5 and
2.0 M) at temperatures (30, 40, 50 and 60 oC). The volume of the test
solution was 10 mL and the immersion time for each test was 30 minutes. A water
thermostat controlled to � 0.5 oC maintained the temperature.
comment()The percentage inhibition (I %) of aluminum was determined from weight
losses as follows:
Where Wo and Win are the weight losses of aluminum
specimens without and with inhibitor, respectively. The percentage inhibition
was then plotted vs concentration of additives and corrosive media at different
comment()The surface coverage (θ) was calculated from weight loss as
Result and Discussion
comment()The curves in Fig.1 represent the percentage inhibition (I %) of aluminum
corrosion for various NaOH concentration vs the concentration of sulfonic acid
(SA) in ppm at 30, 40, 50 and 60 oC, respectively.
comment()The corresponding curves for (SCS) and (SAS) showed similar behavior as
those of Fig.1 but with less inhibition efficiency. For the three tested
additives the percentage inhibitor (I %) increased with increasing inhibitor
concentrations reaching a maximum value. This issue might be explained by the
adsorption of inhibitor molecules forming monolayer . A very important
criterion to characterize the efficiency of inhibitors is their efficiency to
concentration ratio. High protection at low inhibitor concentrations is
required, not only for economic reasons, but also to maintain appropriate
inhibitor concentration and avoid insufficient inhibition . The ratio for
200 ppm and 0.5 M NaOH at 30 oC was calculated to 0.47, 0.42 and
0.33 for SA, SCS and SAS, respectively.
comment()High rate of reaction relative to the rate of adsorption led to a slight
decrease in the inhibition efficiency with increasing temperature, but the
inhibitive effect still persists even at 60 oC. This behavior
indicates that the adsorbed molecules formed a barrier film on the aluminum
comment()The effect of
sodium hydroxide strength on the inhibition efficiency of the additives tested
showed an increase in the efficiency with decreasing NaOH concentration, which
was an evident to apply NaOH as corrosive media even at low concentrations.
comment()Fig.2 represents the variation of SA, SCS and SAS on aluminum corrosion
in 0.5 M NaOH at 30 oC with the additive concentration. It is
obvious that the efficiency decreases in the order:� SA > SCS > SAS. To
explain this trend one can say that both the benzene ring and the attached
functional groups contribute to the inhibition action, which is also affected
by the substituents on (CH2)3 benzene ring. Sulfonic acid
(SA) molecules probably favor flat orientation on the metal surface to be
adsorbed, while sodium cumene sulfonate (SCS) molecules favor vertical
orientation due to steric effect. Thus sulfonic acid is better adsorbed and
also has higher inhibition efficiency than (SCS). Sodium alkyl sulfate (SAS)
was found to be less inhibitor than the others; this was due to its less
adsorption on the metal surface.
comment()Fig.3 shows the variation of the inhibition efficiency of SA, SCS and SAS
with temperature in 0.5 M NaOH for 200 ppm inhibitor concentration. Higher
efficiency was obtained for (SA) at lower temperature range. The limiting I %
values, in Fig.1 and 2, indicated the complete formation of the monolayer film
of additive molecules on the active sites of aluminum surface. Evidence for
this conclusion was obtained by plotting Langmuir adsorption isotherms
at 60 oC.
comment()Fig.4 represents the Langmuir isotherms for the effect of (SA) on
the Al corrosion in various NaOH concentrations. The resulting parallel
straight lines at different temperatures confirm that the inhibition was due to
the adsorption of (SA) on the metal surface.
corresponding isotherms for (SCS) and (SAS) showed similar parallel straight
lines. The degree of surface coverage (θ) varied linearly with the
logarithm inhibitor concentration fitting Langmuir adsorption isotherm
Log� θ� /� 1 - θ� =� Log� C� +� Log�
comment()Where k is the
adsorption constant, C is the inhibitor concentration.
shows Langmuir isotherms for (SA), for aluminum corrosion in 0.5 M NaOH.
The corresponding isotherms for (SCS) and (SAS) show similar shapes as those in
represents Langmuir isotherms for SA, SCS and SAS at 30 oC
and 0.5 M NaOH. This trend supports the obtained issue previously in Fig. 2.�
can be concluded, that:
The organic compounds tested were beneficial inhibitors for
aluminum corrosion in alkaline
media and the inhibition efficiency varies in the order:
����� SA >� SCS >� SAS
comment()2.�� Inhibition efficiency increases with increasing
inhibitor concentration and decreasing concentration of corrosive
comment()3.�� The higher
the temperature the lower is the inhibition efficiency.
comment()4.�� Inhibition was attributed to adsorption of inhibitor molecules at
the surface of aluminum, which fits Langmuir adsorption isotherm.�
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comment()Fig.1 Effect of SA
concentration on inhibition efficiency of Al corrosion
2 Variation of inhibition efficiency of SA, SCS and SAS with inhibitor
concentration in�0.5 M NaOH at 30 oC.����������������������������
comment()Fig. 3 Variation of
inhibition efficiency of SA, SCS and SAS on Al corrosion with�temperature for 200 ppm and 0.5 M NaOH
comment()Fig. 4 Langmuir
adsorption isotherms for SA in various NaOH concentrations at 6o oC
Langmuir adsorption isotherms for SA in 0.5 M NaOH
comment()Fig. 6 Langmuir adsorption
isotherms for SA, SCS and SAS at 30oC in 0.5 M NaOH