Production wells in gas and oil industries are always exposed
to the hostile fluid environments during acidization process. An acid solution like
HCl is generally introduced at 60oC temperature in order to eliminate
the dust. During this process, materials structure exposed to the hydrochloric acid
is highly corroded. Such equipment include mixing tanks, storage facilities, well
tubular, tubing, and casings. These are usually made from the aluminum because of
its readily available and cheap nature (Abdallah 2004; Diggle, et al., 1970; Deng
and Li, 2012). Corrosion of Al may lead to the rupturing of spills and pipes and
compromised integrity of the species. Al corrosion is a serious issue in many industries.
Corrosion of aluminum enhances the running costs and decreases
the product quality, availability and plant efficiency. In order to prevent the
Al dissolution process, the hydrochloric acid used in the acidification process
need to be protected by some chemical or physical methods (Desai et al., 1976, Fouda
et al., 2016; Gomma and Wahdan, 1995; Lee, and Pyun, 1995). Corrosion inhibitors
are the chemical species introduced to the hydrochloric acid solution in small quantities
to impede the Al corrosion rate. Inhibitors offer protection of surface by adsorption
of electron rich groups on the Al surface. Some organic and inorganic compounds
decrease the Al dissolution rate but enhance the noxious nature. Therefore, use
of conventional inhibitors such as arsenic and chromate and some synthesized organic
compounds have been banned in industries because of their unfriendly nature to the
environment. The eco friendly concerns of noxious inhibitors are the motivating
force to the sustained research into more eco friendly corrosion inhibitors. Hence,
many people investigated the eco friendly organic corrosion inhibitors, which consisting
the special elements such as N, O, P and S in their moieties. These special elements
participate in the adsorption process due to the presence of rich electrons (Oguzie
2007, Rosaliza et al., 2008; Zheludkevich et al., 2005). The inhibition of aluminum
corrosion in the hydrochloric acid system can be successfully achieved by employing
the corrosion inhibitors to the aggressive solution. The corrosion inhibitor species
adsorb on the aluminum surface and protecting the aluminum from the dissolution
process. Considerations of toxicity, biodegradability, cost, environmental friendliness
and availability are very important. The exploration of drug products as non-toxic
inhibitors is an important field of study.
The general mechanism of disintegration of aluminum in
the HCl system has reported as follows Priyanka Singh et al., 2017) :
Al (S) + H2O ↔ AlOH ads
+ H+ + e (1)
AlOH ads + 5 H2O + H+ ↔
Al3+.6H2O+2e (2)
Al3+ + H2O ↔ [AlOH]2+
+ H+ (3)
[AlOH] 2+ +X- ↔ [AlOHX]+
(4)
Povidone-iodine ointment is an antiseptic drug and it has many applications in the field of medicinal
chemistry. After the expired date, the water soluble Povidone-iodine ointment is
not fit for the people. Further, expired water soluble Povidone-iodine ointment
has never been examined as a corrosion inhibitor for aluminum metal in the acidic
system. Therefore, in this investigation, selected expired water soluble Povidone-iodine
ointment and studied their anticorrosion property on the Al in 3 M HCl solution
by using the weight loss, gasometric, conductometry, Tafel plot and impedance spectroscopy
techniques.
2. EXPERIMENTAL SECTION:
Concentrated HCl and double distilled water are the reagents
used in the preparation of 3 M HCl solution. 0.1 g/L, 0.2 g/L, 0.3 g/L and 0.4 g/L
are the different amounts of expired Povidone-iodine ointment used in the prevention
of Al corrosion process in the 3 M HCl solution. A standard electrode systems include
saturated calomel (reference cell), platinum (counter cell) and aluminum (working
cell) are used to run the both Tafel plot and impedance experiments. Weight loss
studies were carried out on the Al surface in the 3 M HCl solution without and with
0.1 g/L, 0.2 g/L, 0.3 g/L and 0.4 g/L of Povidone-iodine ointment for 1, 2, 3 and
4 days immersion time at 60 0 C. The weight loss studies were carried
out three times and average values are tabulated.
The protection efficiency of the non-toxic inhibitor can
be obtained by following equation,
Protection efficiency 
100,
Where, W1= Al weight loss without expired drug
and W2= Al weight loss in the presence of expired drug.
The Al corrosion inhibition property of expired Povidone-iodine
ointment was also investigated by hydrogen gas evolution (gasometric) technique
at 600 C. The protection efficiency of the expired Povidone-iodine ointment
was examined by using the following equation,
Percentage protection efficiency=
,
Where, Va= H2 gas liberated in the
free 3 M HCl system, and Vp= H2 gas liberated in the 3 M HCl
system in the presence of the inhibitor.
Scanning electron microscopy (SEM) technique is used to
study the metal surface property without and with corrosion inhibitor.
3. RESULTS AND DISCUSSION:
3.1 Tafel plot studies:
Tafel slope technique is well improved and gives significant
information on metal dissolution rate by measuring the corrosion current density
values. Presented in the Figure 1 and Table 1 are the Tafel plots and Tafel
results respectively. The data in the Table 1 show that, corrosion current
value decreases with addition of different amounts of expired Povidone-iodine ointment
to the 3 M HCl solution. The expired Povidone-iodine ointment adsorbed over the
Al surface in the 3 M HCl solution which blocks the further Al disintegration process.
The resulted table shows that, introduction of different amounts of drug to the
3 M HCl solution lowers the corrosion current density values leading to the simultaneous
enhancement in the protection efficiency values. Further, the change in the corrosion
potential value is not greater than 85 mV and not much variation in the cathodic
and anodic Tafel slope values, which shows that introduction of expired drug blocks
the both cathodic and anodic reactions at same rate, which is an indication of mixed
Al corrosion inhibition property of expired Povidone-iodine ointment in 3 M HCl
environment. Further, the cathodic and anodic Tafel slope values in the presence
of expired Povidone-iodine ointment did not show much deviation compared to the
unprotected system, indicating mixed metal corrosion inhibition property of expired
Povidone-iodine ointment via strong adsorption process (Outirite et al., 2010; Stern and Geary, 1957;
Ansari et al., 2014; Li et al., 2008).
Figure 1 Tafel plots
3.2 Impedance studies
Figure 2 shows the Nyquist plots of the Al specimens submerged in the 3 M HCl solution
possessing four different amounts of expired Povidone-iodine ointment. The results
of impedance studies were shown in the Table 2. With a rise in the concentration
of expired Povidone-iodine ointment, the arc impedance diameter, enhanced, showing
the development of more capacitive thick film over the metal surface. This clearly
explains the enhanced surface coverage. The enhanced surface coverage greatly decreases
the metal corrosion rate.
The Table 3 shows that, in the presence of four different
amounts of expired drug enhances the charge transfer process and this process is
high compared to the bare solution. The decreased in the Al corrosion rate was observed
with the enhancement in the charge transfer resistance values showing the corrosion
inhibition property of expired drug and it is directly proportional to the non toxic
inhibitor concentration. The expired Povidone-iodine drug ointment blocks the corrosion
process by increasing the electrical double layer and reducing the dielectric constant.
The trend of protection efficiency obtained by impedance studies is similar to that
of Tafel studies.
Figure 2 Nyquist plots
Table 2 Nyquist plot results.
|
Concentration (g/L)
|
Charge transfer
resistance (Ω)
|
Protection efficiency
(%)
|
|
Blank
0.1
0.2
0.3
0.4
|
9.936
121.3
156.5
296.4
1200
|
91.808
93.651
96.647
99.172
|
3.3 Weight loss studies:
The results of weight loss studies are presented in the
Table 3. The Al weight loss in the protected condition is very low compared
to the unprotected Al surface. This shows that, the active electron rich elements
in the expired Povidone-iodine ointment undergo an interaction with the Al surface
in the 3 M HCl solution. The resulted interaction leads to the formation of an invisible
thick layer on the Al surface. The created invisible protective layer blocks the
attack of concentrated HCl ions on the Al surface. Hence, the Al metal is protected
from the aggressive acid (hydrochloric) solution. The maximum protection efficiency
observed at one day immersion time. After that, the protection efficiency decreases
from 2nd day to 4 th day. This phenomenon is due to the instability
of the protected invisible film for a longer Al immersion period in the 3 M HCl
solution. Hence, protection efficiency has an inverse relationship with the immersion
time.
Table 3 Weight loss results
|
Time (days)
|
Concentration (g/L)
|
Protection efficiency (%)
|
|
1
2
3
4
|
Bare
0.1
0.2
0.3
0.4
Bare
0.1
0.2
0.3
0.4
Bare
0.1
0.2
0.3
0.4
Bare
0.1
0.2
0.3
0.4
|
88.888
94.484
95.010
96.111
88.857
89.142
89.428
89.714
86.010
86.400
87.001
87.200
85.000
85.666
86.012
86.333
|
3.4 Gasometric studies:
The gasometric studies carried out to support the weight
loss, impedance and Tafel plot results. The amount of hydrogen gas evolved in the
presence of four different amounts of expired Povidone-iodine ointment is very low
compared to the bare system. The Table 4 is a clear hint of corrosion protection
role of expired Povidone-iodine ointment. The expired drug prevents the hydrogen
evolution reaction by forming a thin invisible layer on the Al surface in 3 M HCl
medium. The maximum prevention of hydrogen gas evolution was achieved by 0.4 g/L
of expired water soluble Povidone-iodine ointment.
Table 4 Gasometric results.
|
Concentration (g/L)
|
Amount of hydrogen
gas evolved (in ml)
|
Protection efficiency
(in percentage)
|
|
Bare
0.1
0.2
0.3
0.4
|
25.0
2.7
2.5
2.1
0.7
|
89.200
90.010
91.600
97.200
|
3.5 Scanning electron microscopy (SEM) technique:
SEM is employed to examine the aluminum morphology after
10 hours without and with 0.4 g/L of inhibitor and the results are shown in the
Figure 3 (a, b). A deteriorate aluminum surface was observed in the absence of expired
Povidone-iodine ointment was due to aggressive nature of corrosive solution, whereas
smooth surface observed in the presence of 0.4 g/L of expired water soluble Povidone-iodine
ointment due to adsorption process. These observation evidences the expired Povidone-iodine
ointment on the surface of aluminum by isolating metal from corrosive solution.
Figure 3 (a, b) SEM images, (a) absence of inhibitor (b)
0.4 g/L of expired water soluble Povidone-iodine ointment
4. COMPARISON:
The comparative result revealing corrosion protection performance
of expired drug products reported in our previous investigations is tabulated in
the Table 5. The expired water soluble Povidone-iodine ointment exhibits
good corrosion inhibition property compared to the other expired products. Hence,
expired water soluble Povidone-iodine ointment can be employed for the inhibition
of aluminum corrosion with promising results. The robust corrosion protection efficiency
of expired water soluble Povidone-iodine ointment is due to the presence of electron
rich elements in the expired water soluble Povidone-iodine ointment.
5. CONCLUSION:
The corrosion of aluminum was successfully monitored in
our laboratory without and with expired Povidone-iodine ointment. From the weight
loss, gasometric, Tafel plot and impedance studies, the below conclusions can be
drawn:
a) Expired Povidone-iodine ointment act as efficacious
non toxic corrosion inhibitor for the Al in 3 M HCl solution.
b) Protection efficiency enhanced by raising the
expired Povidone-iodine ointment concentration.
c) Expired Povidone-iodine ointment behaves as
a mixed type via adsorption method.
d) Impedance studies confirm the presence of expired
Povidone-iodine ointment blocks the aluminum process by charge transfer phenomena.
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