1
Corrosion of Aluminum alloys in Citric acid,
W / without sodium chloride
Layla Abdulkareem Al Juhaiman
King Saud University
Chemistry Department, Science College, PO Box 22452
Riyadh, Saudi Arabia, 11495
Fax : + 966 1 4772245
e-mail: ljuhiman@ksu.edu.sa
Abdullah Al Mayouf and Abdulaziz Al Suhaybany
King Saud University
Chemistry Department, Science College, PO Box 2455
Riyadh, Saudi Arabia, 11451
Fax : + 966 1 4675952
e-mail : amayouf @ksu.edu.sa
Abstract
The corrosion behavior of two aluminum alloys (Al cookware, Al 6063) and pure
aluminum in aerated aqueous solutions containing food additives namely citric acid
with/without commercial salt were studied. Non electrochemical methods (weight loss
and surface study) and electrochemical measurements (Open circuit potential and
polarization) were applied. In polarization measurements the effect of pH,
temperature, concentration and time were studied. Surface study of Al using Energy
Dispersion X-Ray (EDX) showed the high depletion of Fe and Mg from Al6063
surface. In polarization measurements, the values of current density for citric acid
increased with increasing pH (except at neutral solutions) and temperature but were
not affected by concentration changes of citric acid. The addition of chloride ions to
citric acid greatly increased the corrosion rate in acidic and basic medium many folds;
the effect was influenced by the aluminum composition. The corrosion rate wasn't
affected in citric solution after 60 hours but decreased with time when chloride ions
where present.
1. Introduction
Aluminum alloys were subjects to wide spectrum of researches because of their
possible relation to human health [1-3]. Aluminum is now known to be a neurotoxin
agent since 1980 [1,2]. Due to its accumulation in brain, bones and liver, it was
associated with some diseases like dialysis encephalopathy, bone disorder and other
disorders [1-7]. Thus it is important to shed some light on its dissolution in aqueous
solution containing some food additives. The corrosion behavior of aluminum alloys
is studied widely in the literature in acidic [8-10], neutral [11-17] and basic medium
[18-20]. It is well known that aluminum exhibits a passive behavior in aqueous
solutions due to the thin compact oxide film on its surface. The aluminum oxide
solubility increases in acidic and basic medium which explain the importance of
studying the role of pH as shown by Lampease et al [14]. The object of this paper is
to study the corrosion behavior of two aluminum alloys (Al cookware and Al 6063) in
aqueous solutions containing food additives namely citric acid (Cit) with/without
2
commercial salt (CS) and compare the results with pure Aluminum in Cit
with/without pure NaCl.
2. Experimental methods
Two methods were used non electrochemical and electrochemical measurements to
study the corrosion rate (CR) of aluminum. Two alloys were used (Aluminum
cookware (AlCW) and Al 6063T4) in addition to pure aluminum (99.99% from Good
fellow). The composition of Al alloys is listed in Table (1). Non electrochemical
measurements include weight loss and surface study. In weight loss the corrosion
rate of Al alloys in citric acid (Cit) with/without CS was measured at pH2 and 60ºC
±1 for two hours. Al alloys were used as circular disks with surface area of cm2, 4.93,
5.03 ± 0.10 for AlCW and Al 6063 respectively. Citric acid was AR grade from
Merck and distilled water was used to prepare all solutions except for ALCW were
tab water was used. The concentration of all solutions was 0.10M. Pure NaCl was
added to Cit for pure Al while CS was added to Cit for AlCW and Al 6063. The
composition of CS is listed in Table (2). Before measurements the Al surface was
polished with 600 and 1200 emery paper, washed with distilled water then
ultrasonically cleaned for 1 minute in acetone. After performing weight loss the disks
were washed thoroughly with distilled water, then (2%Cr2O3 ,5 %H3PO4( then
weighed and kept dry for surface study. The surface of Al disks was scanned after 30
days of immersion using Scanning Electron Microscope (SEM) (Joel-JSM-T330A)
connected with Energy Dispersion X-Ray (EDX).
Electrochemical measurements (Open circuit potential and polarization
measurements) were done using a potentiostat /galvanostat connected to a computer.
All experiments were carried out using a three electrode cell with saturated calomel as
the reference electrode and a platinum electrode as counter electrode. The Al working
electrode was in the form of a circular rod. The exposed surface area of the three
electrodes used were (1.00 cm2, 0.783 cm2, 0.282 cm2 ± 0.10) for AlCW, Al 6063 and
Al pure respectively. The electrode was prepared directly before electrochemical
measurements then immersed in solutions to perform open circuit for one hour before
polarization measurements. The effect of pH, citric acid concentration, temperature
and time was studied.
3. Results and discussion
3.1 Non Electrochemical measurements
3.1.1 Weight loss From weight loss method the corrosion rate (CR) is
calculated using the following equation:
∆w
A*t
Where ∆ w is the weight loss of Al (g); A is the surface area (cm2) and t is the
immersion time (days)
Corrosion rate (CR) =
3
The CR of AlCW, Al 6063 in Cit w/without CS solutions are listed in Table (3) after
two hours of immersion at 60 ºC. It is clear that the CR of AlCW at 60 ºC is greater
than that of Al 6063. The addition of CS to Cit increased the CR 4.6 times for AlCW
and 23 times for Al6063. These results agree with Tomcsani et al [21] that chloride
ions act as partners in the electrochemical reaction. The CR of both alloys is greater
than that of pure Al. In addition weight loss from unpublished work [22] for Al6063
at 25 ºC Cit w/without NaCl showed that the CR of Al 6063 increased about ten times
when the temperature was raised to 60ºC.
3.1.2 Surface Study SEM images of Al surface after 30 days of
immersion in Cit and Cit + NaCl are shown in Figs (1-2) for Al 6063. Local corrosion
was clear for Cit acid. For Cit + NaCl in addition to increasing number of bigger pits,
the surface was full of cracks with variable depth which is in agreements with the
weight loss results. Surface analysis using EDX for Al surface are shown in Table
(4). Inspection of Table (4) shows the remarkable decrease in Mg and Fe after
immersion in acid solutions with/without chloride ions which explained the high
corrosion rate in these acids. This result is in agreements with Brillas et al [23] that
the local corrosion of Al alloys increased with increasing amounts of magnesium. It
may be attributed to deposition of the phase Mg5Al8 which makes an anodic phase in
the aluminium matrix making it an optimum region of dissolution [24]. The role of Fe
resembles that of Mg where the presence of the phase FeAl3 is accompanied with
local corrosion as mentioned by Seri [13]
3.2 Electrochemical measurements
3.2.1 Open circuit potential measurements They were done for AlCW,
Al6063 and pure Al electrodes at 0.10M, 25 ±1ºC and different pH values. The data
are shown in Fig (3, 4) for AlCW in Cit, Cit +CS as a representation of the other
electrodes. The values of steady state potential, Ess are listed in Table (5). It is known
that when open circuit potential goes to increasing potential values it indicates
strengthening of the pre-immersion oxide film. It was noticed that Ess values decrease
after addition of chloride ions to Cit. In addition Ess values decrease with increasing
pH values which indicate thinning of pre-immersion oxide film and agrees with
earlier results [25].
3.2.2 Polarization measurements They were studied at different
concentrations, pH, temperatures and times for the AlCW, Al 6063 electrodes in Cit
and Cit + CS and pure Al in Cit and Cit + NaCl. To study the effect of concentration
the temperature was kept at 25 ±1ºC and the concentration of Cit was changed from
0.01M-0.50M. The results are shown in Table (6) for Al 6063 only. It is clear that
corrosion current density values (Icorr) were almost constant which means that this
factor doesn't play a significant role in corrosion control.
To study the effect of pH the concentration was kept constant at 0.10M, the
temperature was kept at 25 ±1ºC and the pH values were changed from 2-9.5.
Polarization curves are shown in Figs (5, 6) for AlCW in Cit and Cit + CS. The
electrochemical parameters of the three electrodes in Cit with/without NaCl at
different pH values are listed in Table (7a ,7b). It is clear from these polarization
results that:
4

Corrosion potential, Ecorr decreased with increasing pH which is in agreements
with open circuit potential results [25].
 For Cit Icorr decreased with increasing pH until it reached its minimum at pH 6.5
then started to increase to reach its maximum at pH 9.5
 For Cit, Cit +CS the trend for Icorr values was :AlCW > Al 6063 > Al pure
The addition of NaCl to carboxylic acids increased greatly Icorr values in acidic
medium only but it didn't affect the neutral and alkaline medium. These results are
combatable with the surface isoelectric point of native air-formed oxide film
determined by McCafferty and Wightman [26]. The isoelectric point of Al of 9.5
makes the Al surface positive below pH 9.5 and allows for penetration of Cl- ions
whereas when the pH is ≥ 9.5 the Al surface would be negative which will prevent
the penetration of Cl- ions.
The removal of surface defects by incorporation of chloride ions was experimentally
verified by Lee & Pyun using Auger Analysis [27]. A previous model by McCaferty
[28] was extended to consider that penetration of Cl- ions can occur by film
dissolution or by migration through oxygen vacancies [29]. Our results agree also
with another recent study by Kolice et al [17] where they have shown using XPS
studies that the amount of Cl- ions taken up by the passive film on Al surface varied
with pH and the minimum concentration of surface chloride occurred at a pH near
pH9.5.
To study the effect of temperature the concentration was kept constant at
0.10M, and the temperature was changed from (10-60ºC). The electrochemical
parameters of the three electrodes in Cit, Cit + NaCl are listed in Table (8a, 8b). In
general increasing the temperature shift Ecorr to the negative direction which means
thinning of the aluminium oxide film and correlate with increasing Icorr values. It is
clear from Table (8a,8b) that:

In general corrosion current density values increase with increasing
temperature but not largely maybe because the dominant reduction reaction is
reduction of oxygen whose concentration decrease with increasing
temperature.
 It is clear that the effect of temperatures was less severe than the effect of pH.
 For Cit the trend for Icorr values was :AlCW > Al 6063 > Al pure


For Cit + NaCl the general trend for Icorr values was :Al 6063 >AlCW > Al pure
The high values of Icorr for Al 6063 reflect the effect of alloying elements.
The effect of time was studied at pH4.5 after 60 hour and compared to
that after one hour for Al 6063 only; the results are listed in Table (9).
 It is obvious that Icorr for Al 6063 in Cit wasn't affected by time while that of
Al 6063 in Cit +CS decreased 22% after 60 hours. This strange result seems to
be related to the presence of chloride ions. To validate this result the same
measurements were done for Al 6063 in solutions containing 0.10M Cit+NaCl
, CS and pure NaCl and the same trend was found.
The low Icorr in presence of chloride ions may be explained by the findings of Foley
[30] and Pyun [31] where they found that the presence of chloride ions in basic
medium increased the resistance of the oxide layer due to incorporation of chloride
5
ions in the defected sites where they form compounds like AlCl3, Al(OH)2Cl and
Al(OH)Cl2. Our findings are also in agreements with Taprizi et al [32] and Kolice et
al [15, 17]. This was experimentally verified by Pyun and Lee [33] using Augar
Analysis where they found that the ''number of available pit initiation sites was
noticeably reduced by pre immersion in 1M NaCl solution, which is due to occurrence
of metastable pitting'' [33]. This explains the complexity of adsorption of these ions
and proves that some adsorption reactions on Aluminum may need longer times as
some researchers prove [6, 33, and 34]. Although the previous results about the role
of chloride ions in forming defect repairing oxide films were in the neutral and
alkaline media, our results show that they may play the same role in acidic medium at
long immersion times.
4. Conclusion
The corrosion behavior of Al alloys in food additives depends on pH, chloride ion,
temperature, time and alloying elements. The dietary sources of aluminum are food
and water. The increase in CR of aluminum in carboxylic acids and chloride ions
needs more elaborate studies to determine the dissolution of aluminum from cooking
utensil and compare them with the daily allowed amount assigned from World Health
Organization (WHO).
Acknowledgment
The authors wish to thank King Abdulaziz City for Science and Technology for their
valuable
financial
support
of
this
study.
6
Table (1) Composition of Al alloys
Element
Al
Zn
Ti
Cr
Al 6063
AlCW
97.5098.35
97.62
0.10
--
0.1
0
--
0.10
--
Table (2) Composition of CS
Compound
NaCl
CaSO4
%
99.8
Si
Fe
Mg
Mn
Cu
Other
0.200.60
--
0.35
0.450.90
--
0.10
0.10
0.15
0.045
0.023
Sn=
0.95
1.366
Mg SO4
0.07
MgCl2 I2
Additives
0.03
0.05
MgCO3
Additive
s
0.0070.010
Table (3) CR (g/cm2.days) of Al from weight loss at pH 2, 60 ºC, two hours
Solution
Cit
Cit + NaCl
AlCW
Al 6063,
Al pure
6.61exp -4
2.01exp-4
1.41 exp-4
28.12 exp -4
23.92 exp-4
4.45 exp-4
Table (4) Surface analysis of Al 6063 in test solutions after 30 days [22]
Al
Si
Fe
Mg
Mn
Uncorroded
sample
97.13
1.4
0.21
0.94
0.38
Cit
98.19
1.69
0.01
0.05
0.16
Cit +NaCl
98.22
1.89
0.08
----
0.29
0.25
7
Table (5) Open circuit potential of Al electrodes in acid solutions at 0.10M, 25ºC
and different pH
pH
AlCW ,
Cit
Al 6063 ,
Cit
Al Pure ,
Cit
2.0
-503
-496
-487
4.5
-490
-692
-614
6.6
-452
-731
-739
9.5
-1396
-1365
-1745
pH
AlCW,
Cit +CS
Al 6063,
Cit +CS
Al Pure,
Cit+NaCl
2.0
-676
-760
-685
4.5
-666
-763
-712
6.6
-697
-770
-792
9.5
-1374
-1477
-1754
Table (6) Electrochemical parameters of Al 6063 in different concentrations of
Cit at 25ºC
[Cit] ,M
pH Ecorr
Icorr ,
mV
μA /cm2
0.01
2.7 -366
0.36
0.05
2.4
-364
0.39
0.10
2.0
-446
0.50
0.20
2.1
-370
0.38
0.50
1.9
-396
0.38
8
Table (7a) Electrochemical parameters of Al alloys in Cit w/without CS solutions
at 0.10M, 25ºC and different pH
Solution
pH
AlCW
Al 6063
Cit
Cit + CS
Ecorr ,
mV
Icorr
μA /cm2
Ecorr ,
mV
Icorr
μA /cm2
2.0
-483
4.84
-446
0.50
4.5
-512
1.92
-700
0.40
6.5
-446
0.67
-755
0.16
9.5
-1396
56.68
-1365
53.55
2.0
-674
13.24
-714
19.91
4.5
-662
4.64
-703
3.30
6.5
-734
0.25
-776
0.067
9.5
-1418
62.70
-1392
54.8
Table (7b) Electrochemical parameters of Al pure in Cit w/without NaCl
solutions at 0.10M, 25ºC and different pH
Pure Al
Solution
pH
Cit
Cit + NaCl
2.0
Ecorr ,
mV
-462
Icorr
μA /cm2
0.45
4.5
-584
0.33
6.5
-718
0.16
9.5
-1727
19.44
2.0
-679
6.91
4.5
-702
0.39
6.5
-786
0.20
9.5
-1768
18.83
9
Table (8a) Electrochemical parameters of Al alloys in Cit w/without CS
solutions at 0.10 M, pH 2.0, and different temperature
Solution
Cit
Cit + CS
T ,°C
AlCW
Al 6063
Ecorr ,
mV
Icorr
μA /cm2
Ecorr ,
mV
Icorr
μA /cm2
10
-461
2.77
-392
0.28
25
-483
4.84
-446
0.50
45
-443
7.65
-489
1.58
60
-455
8.42
-520
5.23
10
-660
9.63
-699
12.10
25
-674
13.24
-714
19.90
45
-687
14.93
-661
45.50
60
-695
17.50
-766
52.32
Table (8b) Electrochemical parameters of Al pure in Cit w/without NaCl
solutions at 0.10 M, pH 2.0, and different temperature
Pure Al
Solution
T ,°C
Cit
Cit + NaCl
10
Ecorr ,
mV
-495
Icorr
μA /cm2
0.10
25
-462
0.45
45
-580
1.47
60
-560
4.42
10
-687
2.38
25
-679
6.91
45
-686
11.59
60
-675
20.26
10
Table (9) Electrochemical parameters of Al 6063 in different solutions at 0.10 M,
pH4.5, 25ºC and different times
Solution
Time
Ecorr ,
Icorr ,
,hr
mV μA /cm2
1
-700
0.40
Cit
Cit +CS
CS
NaCl, [22]
Cit+NaCl ,
[22]
60
-590
0.44
1
-703
3.30
60
-651
0.72
1
-700
1.06
60
-867
0.10
1
-674
1.32
60
-729
0.18
1
-724
10.0
60
-663
3.59
11
Fig (1) SEM of Al 6063 in Cit after 30 days of immersion, [22]
Fig (2) SEM of Al6063 in Cit + NaCl after 30 days of immersion ,[22]
12
Fig(3) Open circuit potential of AlCW in Cit at 0.10M and different pH
Fig (4) Open circuit potential of AlCW in Cit + CS at 0.10M and different pH
13
Fig (5) Polarization curves of AlCW in Cit at 0.10M and different pH
Fig (6) Polarization curves of AlCW in Cit +CS at 0.10M and different pH
14
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