AA5052 Aluminum Alloy Corrosion in NaCl under Negative Pressure

Desalination 570 (2024) 117082
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Desalination
journal homepage: www.elsevier.com/locate/desal
Corrosion behavior of AA5052 aluminum alloy in the presence of heavy
metal ions in 3.5 % NaCl solution under negative pressure
Daquan Zhang a, b, *, Zixuan Yan a, b, Lixin Gao b, Zhiling Xin a, Yongxin Zhu a, Wei Wu a, b
a
b
Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, Shanghai University of Electric Power, PR China
Shanghai Engineering Research Centre of Energy-Saving in Heat Exchange Systems, Shanghai 201306, PR China
H I G H L I G H T S
• The corrosion of aluminum alloy in presence of metal ions under negative pressure has been investigated.
• The effect of negative pressure on the corrosion of aluminum alloys is studied.
• The decrease of pressure retards the anodic process of aluminum corrosion.
A R T I C L E I N F O
A B S T R A C T
Keywords:
Seawater desalination
Negative pressure
Corrosion
Aluminum alloy
Heavy metal cations
Aluminum alloy heat exchange tube is attractive for the Low Temperature-Multi Effect Distillation (LT-MED)
desalination device, for its low cost and excellent mechanical property. However, it is prone to galvanic coupling
corrosion in presence of heavy metal ions. In this paper, the effect of Cu2+ and Fe3+ ions on the corrosion of
aluminum alloy in 3.5 wt% NaCl solution under negative pressure is investigated by the weight-loss method and
the electrochemical measurements. It shows that the operation pressure affects significantly the corrosion
behavior of AA5052 aluminum alloy in presence of heavy metal ions. As the pressure decreases, the resistance of
the aluminum alloy to corrosion increases markedly. The corrosion products formed under different operation
pressure are detected by SEM, XRD analysis. It is found that the diffusion process of corrosion products from the
aluminum alloy surface into the solution is prevented as the decrease of pressure, resulting in the formation of
the passivation layer on the aluminum alloy surface. The decrease of pressure can effectively restrain the heavy
metal ions to be adsorbed on the defects of the oxide layer and mitigate the corrosion of aluminum during the LTMED desalination process.
1. Introduction
of the total device cost [4]. Traditionally, most of the heat exchange
tubes in LT-MED plants have been constructed with titanium [5,6].
Aluminum alloy is attractive in replacing titanium tubes in desalination
plants because of its high thermal conductivity, ease of fabrication and
cost effectiveness [7–9]. The main obstacle for wide use of aluminum
alloys is their susceptibility to pitting corrosion in sea water [10,11].
This is found to be particularly severe in the presence of trace amounts of
certain metal cations, such as Cu2+ and Fe3+ ions [12–14].
Many studies have been performed to investigate the corrosion effect
of metal cations on aluminum and its alloys in different electrolytes,
such as anhydrous ethanol [11], ethylene glycol [12,15], tap water [16],
HCl solution [17] and NaCl solution [18]. The Cu2+ and Fe3+ ions
accelerate the corrosion of aluminum by the displacement deposition of
The shortage of water resource has been a worldwide problem. The
seawater desalination is undergoing a fast development in the past de­
cades. The number of desalination plants in the world has increased at
an estimated average annual growth rate of 6.8 % since 2010, which
equates to an average annual increase in production capacity of about
4.6 million m3/day [1,2]. The low temperature multi effect distillation
(LT-MED) technology is one of the main large-scale thermal desalination
methods. MED plants utilize a number of falling-film evaporative con­
densers (effects) to produce a multiple quantity of distillated water at a
lower temperature and a lower pressure [3]. Heat exchange tube is the
core component of LT-MED device, which accounts for about 20 %–40 %
* Corresponding author at: Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, Shanghai University of Electric Power, PR
China.
E-mail address: [email protected] (D. Zhang).
https://doi.org/10.1016/j.desal.2023.117082
Received 1 August 2023; Received in revised form 9 October 2023; Accepted 17 October 2023
Available online 18 October 2023
0011-9164/© 2023 Published by Elsevier B.V.
D. Zhang et al.
Desalination 570 (2024) 117082
Cu or Fe metal, forming an electric couple. It is now well established that
the corrosion of aluminum alloy in a solution containing Cl− , Cu2+ and
Fe3+ ions is under cathodic control [13,19,20]. In addition to the elec­
tron availability for the cathodic reaction, the degree of cathodic po­
larization also determines the rate of aluminum dissolution, e.g. the rate
of oxygen diffusion towards the cathodic sites and/or of OH− diffusion
away from them [21]. A special design is made to eliminate heavy metal
ions by utilizing “ion traps” filled with scrap aluminum. Nevertheless,
the removal rate of Cu2+ from sea water via “ion traps” is around 50 %–
80 %, which is not as efficient as be expected [22]. The pitting corrosion
caused by heavy metal ions still remain a challenge for the application of
aluminum alloy materials in the LT-MED seawater desalination plant.
On the other hand, the LT-MED device works under high vacuum degree
(the vacuum degree means atmosphere pressure minus working pres­
sure) [23]. The negative pressure operation can reduce scaling phe­
nomenon. It also has influence on the oxygen diffusion and ions
transport. This will affect the role of heavy metal ions on aluminum alloy
corrosion. Unfortunately, seldom is reported about the effect of heavy
metal ions on the corrosion of aluminum alloy under negative pressure
condition. In this work, we investigate the corrosion of AA5052
aluminum alloy in the presence of aggressive cations under different
vacuum conditions. It is useful to understand the action of the corrosion
accelerating cations on aluminum alloy surface in the LT-MED desali­
nation process.
the surface area of the test piece, and t (h) is the immersion time.
2.3. Electrochemical measurements
Electrochemical impedance spectroscopy (EIS) was performed on an
electrochemical workstation (Solartron 1287/Solartron 1260) using a
classical three-electrode system. The AA5052 alloy was used as the
working electrode (WE), a platinum electrode as the auxiliary electrode
(CE), and a saturated glycerol electrode (SCE, 0.357 g/mL Cl− ) as the
reference electrode (RE). An experimental device as shown in Fig. 1 was
assembled, in which the negative pressure was made by vacuum pump,
the feed of seawater was controlled by a flow pump. The test solution
was always maintained at a level that will submerge CE and RE and
remain approximately 1 cm above WE.
The electrochemical experiments at different pressures were per­
formed in that device. The pressure gauges provide real-time feedback
on the pressure inside the device through piping connecting the various
components. All electrochemical measurements were performed at 25
± 1 ◦ C. To ensure that the working electrode was subjected to the same
hydrostatic pressure, the same volume of the test solution was pumped
into the device using a flow pump in all tests. Before each test, the
aluminum alloy electrode was catholically reduced at − 1.0 V (VS.OCP) for
120 s, then the open circuit potential test was performed for 15 min.
After the open circuit potential get stabilization, the electrochemical
impedance spectroscopy (EIS) and the dynamic potential polarization
were performed. The EIS was performed with a 5 mV sine wave signal,
and the scanning frequency was from 100 kHz to 0.01 Hz. The polari­
zation curve was scanned at 0.167 mV/s. The experimental results were
analyzed and fitted with ZView and CView software, respectively.
2. Experimental
2.1. Materials and chemicals
AA5052 aluminum alloy (in wt%: Si 0.06; Cu 0.1; Mg 2.46; Zn 0.1;
Mn 0.06; Cr 0.19; Fe 0.27 and balance Al) was used for the experiments.
The aluminum alloy was cut into a cylinder with a diameter of 1 cm and
embedded in the working electrode. Another part of the aluminum sheet
was used for corrosion test samples. The aluminum electrode and
aluminum sheet were polished with different roughness of abrasive
paper (#400, #800, #1200, #2000) in turn, rinsed with deionized water
and anhydrous ethanol, and dried for use. The Cu2+ ions and Fe3+ ions
are from Cu(NO3)2 and Fe(NO3)3, which are chemically pure reagents
and purchased from Shanghai Maclean's Biochemical Company.
As reported, the highest content of Cu2+, Fe3+ ion is about 20 μg/L
and 50 μg/L in the China Bohai Sea. The test solution is 3.5 wt% NaCl
corrosion solution with Cu (NO3)2 and Fe (NO3)3 as the source of Cu2+,
Fe3+ ions. It is reported that the nitrate ions have limited effect on the
corrosion of aluminum alloy in seawater [24]. The Cu2+ and Fe3+ ions in
the test solution are set at 20 μg/L and 50 μg/L, respectively [25].
2.4. Surface characteristic analysis
The aluminum alloy samples were immersed in 3.5 wt% NaCl solu­
tion at different pressures for 3 days(d), and then it was cleaned with
deionized water and acetone and dried under nitrogen. The morphology
of the specimen was observed by Scanning Electron Microscope (SEM).
The surface components of the tested samples were characterized by
Energy Dispersive Spectroscopy (EDS). The surface phases of the sam­
ples were analyzed by a DX-270 X-ray diffractometer. The intensity and
angle of the diffraction peaks for the aluminum alloy samples under
different operation pressure were checked.
3. Results and discussion
3.1. Weight-loss test
Fig. 2 shows the effect of Cu2+, Fe3+ and their mixture on the
corrosion rate of AA5052 alloy under different operation pressure in 3.5
wt% NaCl solution.
2.2. Weight loss experiment
Aluminum alloy samples (size 2.5 cm × 5 cm × 0.2 cm) were pol­
ished with sandpaper step by step and rinsed with the deionized water
and acetone and then dried in a vacuum oven. The weight of the sample
was measured by an electronic balance (m1). The aluminum alloy
samples were immersed in the test solution at 25 ◦ C. After 3 days of
immersion, corrosion products were removed from the surface of the
samples with sandpaper. And then the samples were rinsed with
deionized water and acetone, dried and weighed again. The weight loss
was calculated by taking the average of these specimens. All tests were
performed in the test solution with triplicates. Three specimens were
taken for each data to ensure their reproducibility and the accuracy of
the method was within 5 %.
The corrosion rate is calculated as follows:
V=
m1 − m2
s×t
(1)
where V (mg⋅m−⋅ 2h− 1) is the corrosion rate of the metal. The m1 and m2
(mg) are the test piece mass before and after corrosion test. The s (m2) is
Fig. 1. Experimental device diagram.
2
D. Zhang et al.
Desalination 570 (2024) 117082
the concentration of heavy metal ions in the 3.5 wt% NaCl solution
under different pressures after 3 d immersion was analyzed by atomic
absorption spectroscopy. The concentration of heavy metal ions
decreased after immersion test, indicating that heavy metal ions could
be adsorbed on the surface of the aluminum alloy. With the decrease of
the operation pressure, the concentration of heavy metal ions in the
corrosion solution gradually increased, suggesting that the lower pres­
sure results in the more difficult for heavy metal ions to be adsorbed on
the surface of aluminum alloy. Thus, the lower pressure slows down the
corrosion of aluminum alloy in 3.5 wt% NaCl solution owing to the
lower absorption of metal ions on its surface.
3.2. Electrochemical measurements
3.2.1. Potentiodynamic polarization
The potentiodynamic polarization curves for AA5052 alloy under
different operation pressure in 3.5 wt% NaCl solution without and with
metal cations are shown in Fig. 3. The values of corrosion potential
(Ecorr) and pitting potential (Epit) are shown in Table 1, where the Ecorr is
obtained from the polarization curve after the polarization measure­
ments. All anodic polarization plots show a passivation-dissolution
characteristic. The cathodic polarization curves appear a pseuo-Tafel
region. The other parameters shown in the Table 1, such as passiv­
ation current density (Ipass), Epit-Ecorr are obtained from the electro­
chemical polarization curves measurements.
For the aluminum alloy electrode, the anodic polarization curve
shows a large passivation zone, due to the formation of aluminum oxide.
There are two reactions on the aluminum surface [26–29]:
Fig. 2. Variation of corrosion rate of AA5052 alloy under different operation
pressure after 3 d immersion in 3.5 wt% NaCl solution containing metal cations.
It is obvious that the corrosion rate of aluminum alloy in the 3.5 wt%
NaCl solution containing Cu2+ or Fe3+ ions higher in comparison to the
blank solution. In presence of the mixture of Cu2+ and Fe3+ ions, the
corrosion rate of aluminum alloy is greatly accelerated. With the
decrease of the operation pressure, the corrosion rate of aluminum alloy
is decreased. The influence of the operation pressure on the corrosion
rate of aluminum is more evident in 3.5 wt% NaCl solution containing
heavy metal ions, for it has a higher slope in Fig. 2.
Since the corrosion of AA5052 aluminum alloy is dominated by
pitting corrosion, we can only evaluate the relative degree of corrosion
of the aluminum alloy by the weight loss method. As shown in Fig. S1,
Al(s.)ads. + 3OH− = Al(OH)3.ads + 3e−
(2)
Al(OH)3,ads = Al2 O3 ⋅3H2 O
(3)
The corrosion occurs due to the rupture of the surface oxide layer
Fig. 3. Polarization curves for AA5052 alloy under different operation pressure after 30 min immersion in 3.5 wt% NaCl solution without (a) and with metal cations:
20 μg/L Cu2+ (b); 50 μg/L Fe3+ (c); 20 μg/L Cu2++50 μg/L Fe3+(d).
3
D. Zhang et al.
Desalination 570 (2024) 117082
diffuse into solution. The corrosion products which formed on the sur­
face of aluminum alloy have passivation effect, retarding Cu2+ ions to be
adsorbed on the aluminum alloy substrate. The decrease of the operation
pressure does not have a large effect on the pitting potential of the
aluminum alloy. The Fe3+ ions have a similar effect on the corrosion of
aluminum alloy. The polarization curve of aluminum alloy in the
corrosion solution with the mixture of Cu2+ and Fe3+ ions is shown in
Fig. 3(d). It is clear that the slopes of cathodic and anodic polarization
decrease and the passivation current density increases compared with
that in the blank 3.5 wt% NaCl solution. It can be observed that the
passivation current density decreases with the gradual decrease of the
operation pressure. Meanwhile, at the same pressure, the passivation
current density of aluminum alloys is larger in the solution containing
metal ions, and the passivation current density shows a significant
decreasing trend as the pressure decreases. It is reported that the
corrosion of aluminum in neutral solution is under cathodic control. The
oxygen content in the solution will decrease with the reduce of the
operation pressure and the oxygen reduction process will be inhibited
[35]. However, the rate of dissolution of aluminum alloy is determined
by the rate of Al3+ ions diffusion into solution under negative pressure.
The decrease of the operation pressure makes the corrosion potential
positively shift. The anodic process has retard by the decrease of the
operation pressure. It indicates that the operation pressure mainly af­
fects the anodic corrosion process of aluminum alloy. The decrease of
the operation pressure retard both the diffusion of Al3+ into the solution
and the deposition of Cu2+ or Fe3+ ions onto the aluminum surface, thus
suppressing the anodic reaction process evidently. The decrease of the
operation pressure will inhibit the kinetics of the partial anodic reaction,
resulting in the observed corrosion inhibition.
Table 1
The fitting parameters of polarization curve of AA5052 alloy under different
operation pressure in 3.5 wt% NaCl solution without and with metal cations.
Solutions
Pressure
(kPa)
Ecorr (V
vs.SCE)
Epit (V
vs.SCE)
Epit-Ecorr
(V vs.SCE)
Ipass
(μA⋅cm− 2)
NaCl
101
80
60
20
1
101
80
60
20
1
101
80
60
20
1
101
80
60
20
1
− 0.973
− 0.965
− 0.952
− 0.905
− 0.917
− 0.992
− 0.880
− 0.876
− 0.952
− 0.910
− 0.975
− 0.946
− 0.938
− 0.845
− 0.846
− 0.966
− 0.923
− 0.93
− 0.854
− 0.840
− 0.734
− 0.733
− 0.729
− 0.719
− 0.727
− 0.714
− 0.743
− 0.743
− 0.722
− 0.745
− 0.740
− 0.731
− 0.730
− 0.737
− 0.729
− 0.733
− 0.738
− 0.731
− 0.731
− 0.726
0.239
0.232
0.223
0.186
0.19
0.278
0.137
0.133
0.23
0.165
0.235
0.215
0.208
0.108
0.117
0.233
0.185
0.199
0.123
0.114
1.5
1.4
1.2
1.0
0.9
1.6
1.4
1.3
1.2
1.0
1.9
1.7
1.3
1.2
1.0
2.1
1.8
1.5
1.2
1.0
NaCl+
20 μg/L
Cu2+
NaCl+
50 μg/L
Fe3+
NaCl+
20 μg/L
Cu2+
+50 μg/L
Fe3+
[32]. The anodic dissolution of aluminum in chloride-containing solu­
tion is controlled by the diffusion of aluminum chloride complexes into
the solution. There are multistep dissolution and ionic migration during
the anodic dissolution of aluminum alloy. The Cl- ions in solution
change the Al in the aluminum alloy matrix to Al3+ by the following
reaction [30]:
Al = Al3+ + 3e−
(4)
Al3+ + 4Cl− = AlCl−4
(5)
3.2.2. EIS measurement
Fig. 4 shows the Nyquist plots of aluminum alloys in 3.5 wt% NaCl
solution containing heavy metal ions under different operation pressure.
We have observed a significant dispersing effect according to the
Nyquist plots. All impedance spectra are similarly shaped and exhibit
depressed semicircles with centers below the real axis.
Generally, the larger radius of the semicircle is, the better anticorrosion of the surface film of aluminum has. The diameter of the
semicircle increases with decreasing operating pressure, suggesting that
negative operating pressure improves the corrosion resistance of
aluminum alloys in 3.5 wt% NaCl solution with or without heavy metal
ions. As shown in Fig. S2, there are two maximal values of the phase
angle, suggesting a two-time constant electrochemical process exists as a
result of the presence of passive and corroded regions on these surfaces
[17]. The first time constant at high-medium frequencies is generally
related to the protective film. The second time constant at low fre­
quencies comes from the resistance to charge transfer (Al ↔ Al3+).
The equivalent circuit shown in Fig. 4c is used to analyze all the
impedance plots. The fitted error values are below 10 % and the chisquare below 10− 4. The Rs represents the solution resistance, Q1 is the
outer layer capacitance including the passive film corrosion products, R1
is the resistance of the corrosion product, Q2 is the double layer
capacitance of the anodic site. Rct is the charge transfer resistance, The
admittance of Q is characterized as
The cathodic reaction can be expressed as [29,31,32]:
O2 + 2H2 O + 4e− = 4OH−
(6)
2H2 O + 2e− = H2 + 2OH−
(7)
The cathodic branch is a mixed process of oxygen absorption and
hydrogen evolution reactions. The cathodic process is mainly controlled
by the reduction of oxygen as Eq. (6). While in the high negative po­
tential area of Fig. 3, the cathodic process corresponds to the hydrogen
evolution reaction as Eq. (7).
The cathodic branch is a mixed process of oxygen absorption and
hydrogen evolution reactions. The cathodic process is mainly controlled
by the reduction of oxygen as Eq. (6). While in the high negative po­
tential area of Fig. 3, the cathodic process corresponds to the hydrogen
evolution reaction as Eq. (7). In addition, Cl− ions and OH− can also
simultaneously effect on Al3+, forming oxychloride as shown in Eq. (8),
further increasing the anodic solubility of the aluminum alloy [33].
Al3+ + 2Cl− + 2OH− = Al(OH)2 Cl−2
(8)
As shown in Fig. 3(b), (c) and (d), the Epit-Ecorr of the aluminum alloy
decreases in the presence of heavy metal cations, whereas the pitting
potential remains basically unchanged.
At the same time, Cu2+ ions will also participate in the cathodic
depolarization process, making the cathodic reaction rate faster. It is
reported that the defects of the passivation layer on the aluminum alloy
surface are more likely to adsorb metal cations. When aluminum is
contacted with a solution containing some cations, a displacement re­
action will occur [34]. The corresponding metal is precipitated from the
solution on the aluminum surface. The deposited metal forms the
galvanic corrosion couples with aluminum substrate and makes the
corrosion potential shift positively.
The corrosion products under negative pressure are more difficult to
Q = Y0 (jw)n
(9)
where j represents the imaginary root, ω represents the angular fre­
quency, Y0 represents the magnitude and n represents the exponential
term.
The polarization resistance (Rp) represents the corrosion resistance
of each of the samples. Rp can be calculated as follows:
{ }
Rp = lim Re Zf E=Ecorr
(10)
ω→0
where Re{Zf}is the real part of the complex faradaic impedance, and ϖ
4
D. Zhang et al.
Desalination 570 (2024) 117082
Fig. 4. Nyquist plots for AA5052 alloy under different operation pressure after 30 min immersion in 3.5 wt% NaCl solution without (a1) and with metal cations: 20
μg/L Cu2+(a2); 50 μg/L Fe3+(a3); 20 μg/L Cu2++50 μg/L Fe3+(a4); Corresponding equivalent circuit (b).
corresponds to the angular velocity of the AC signal (ϖ = 2πf, where f is
frequency, Hz). The sum of Rct and R1 is normally equal to the Rp values
[16].
Table 2 shows the fitting parameters of the electrochemical imped­
ance spectra of aluminum alloy and Rp value in 3.5 wt% NaCl solution
under different operation pressure.
A strong acceleration of corrosion is observed for the aluminum alloy
in the presence of heavy metal ions, especially for the mixture of Cu2+
and Fe3+ ions. This is attributed to deposition of the metals by
displacement on the surface of aluminum alloy to form galvanic corro­
sion couplings between the deposited metal and aluminum substrate.
The negative operation pressure causes the difference passivation films
formed on aluminum alloy surface, which then brings about a change in
the electrochemical behavior of corrosion. The corrosion products pro­
duced under the negative pressure have some protective action. The
passivation layer formed by corrosion products can effectively prevent
the attack of chloride ions as well as metal cations to the aluminum alloy
substrate. The polarization resistance increases with the decrease of the
operation pressure, suggesting the enhancement for corrosion resistance
of specimen under negative operation pressure. The EIS results agree to
that from the polarization curve test.
3.3. Analysis of corrosion morphology
As shown in Fig. 5, the morphology of the aluminum alloy under
different pressure after 3 d immersion test is characterized using the
scanning electron microscopy. Table 3 shows the elemental content of
the aluminum alloy surface from EDS analysis.
It can be observed from the SEM images that the aluminum alloy
specimens were significantly corroded in 3.5 wt% NaCl solution. In
Fig. 5(b), (c) and (d), the corrosion of the aluminum alloy is clearly
accelerated in the presence of heavy metal cations at atmospheric
pressure. Some white pitting corrosion products can be observed on the
surface of the aluminum alloy, accompanied by some obvious pitting
holes. We believe that the displacement deposition of the metal on the
aluminum surface is inhomogeneous, and this displacement is mainly
carried out through film defects. In solutions containing binary mixtures
of Cu2+ and Fe3+ ions, the localized corrosion areas are enlarged and
larger plugs of corrosion products appear.
As shown in Fig. 5 at 1 kPa, the corrosion process of aluminum alloys
in the presence of heavy metal cations is significantly hindered as the
pressure decreases. The surface of the AA5052 aluminum alloy samples
is relatively smooth, with few corrosion products observed.
Table 2
Impedance fitting values of AA5052 alloy under different operation pressure after 30 min immersion in 3.5 wt% NaCl solution without and with metal cations.
Solutions
Pressure (kPa)
Rs/Ω⋅cm2
R1/Ω⋅cm2
Q1
n
− 2
Y0 (S⋅s cm
NaCl
NaCl+
20 μg/L Cu2+
NaCl+
50 μg/L Fe3+
NaCl+
20 μg/LCu2+ +50 μg/L Fe3+
101
80
60
20
1
101
80
60
20
1
101
80
60
20
1
101
80
60
20
1
1.25
1.50
1.09
1.00
1.27
1.10
1.41
1.00
1.00
1.06
1.44
1.00
1.00
1.07
1.13
1.27
1.20
1.30
1.31
1.58
6.04 × 10− 5
4.65 × 10− 5
4.12 × 10− 5
2.09 × 10− 5
2.24 × 10− 5
6.68 × 10− 5
9.34 × 10− 6
1.16 × 10− 5
2.32 × 10− 5
6.31 × 10− 5
1.35 × 10− 4
7.55 × 10− 5
9.64 × 10− 5
6.58 × 10− 5
7.71 × 10− 4
1.35 × 10− 4
7.81 × 10− 5
5.82 × 10− 3
5.59 × 10− 3
5.41 × 10− 3
5
)
n
n1
1.00
1.00
0.83
0.86
0.89
1.00
0.96
1.00
0.88
0.89
0.97
0.86
1.00
1.00
0.99
0.89
1.00
1.00
1.00
0.82
Rct/Ω⋅cm2
Q2
Y0 (S⋅s cm
5142
13,270
18,390
21,250
36,390
630.4
1169
2146
2840
3050
295.9
774.1
1071
1257
1646
295.9
774.1
1071
1256
1645
− 2
)
3.545 × 10− 4
2.919 × 10− 4
2.893 × 10− 4
2.452 × 10− 4
9.431 × 10− 5
6.683 × 10− 6
9.285 × 10− 6
1.075 × 10− 5
7.313 × 10− 5
2.784 × 10− 4
5.989 × 10− 3
8.271 × 10− 4
1.104 × 10− 3
4.867 × 10− 4
6.144 × 10− 4
5.989 × 10− 5
7.566 × 10− 5
7.355 × 10− 5
4.433 × 10− 5
5.871 × 10− 5
n2
0.89
0.85
1.00
0.94
1.00
0.89
0.89
0.85
0.89
0.95
0.82
0.89
0.86
0.86
0.82
0.80
0.96
0.93
0.97
0.94
25,690
28,090
37,030
53,430
58,080
4457
11,820
12,460
33,270
36,910
2722
3490
6350
9133
11,470
272.2
4830
6350
9138
11,460
D. Zhang et al.
Desalination 570 (2024) 117082
Fig. 5. SEM images of AA5052 alloy surface under different operation pressure after 30 min immersion in 3.5 wt% NaCl solution without (a) and with metal cations:
20 μg/L Cu2+(b);50 μg/L Fe3+(c); 20 μg/L Cu2++50 μg/L Fe3+(d).
Table 3
Chemical compositions in different surface area of AA5052 alloy after 30 min immersion in 3.5 wt% NaCl solution measured by EDS analysis.
Composition
Area
(wt%)
“A”
“B”
“C”
“D”
“E”
“F”
“G”
“H”
“I”
“J”
“K”
“L”
Mg
O
Al
Cl
Fe
Cu
2.76
22.83
70.49
3.92
–
–
2.06
41.40
52.33
4.21
–
–
2.70
42.45
48.35
6.50
–
–
1.37
11.09
83.43
2.19
–
1.92
1.8
5.2
87.53
3.67
–
1.80
1.93
17.69
74.15
3.72
–
1.22
1.89
9.93
83.68
1.01
1.69
–
1.76
12.89
81.25
2.64
1.46
–
1.88
15.76
78.47
2.76
1.13
–
2.13
8.85
84.68
0.49
2.09
1.76
1.97
10.40
82.79
1.76
1.88
1.56
1.86
23.76
69.76
2.14
1.35
1.13
Table 3 shows that elements Cu and Fe can be detected on the
aluminum alloy surface in the solution containing the heavy metal ions,
confirming the happening of the displacement reaction. The adsorption
of chloride ions on the oxide surface is not retarded by the negative
operation pressure. On the contrary, the content of Cl element increases
with the decrease of the operation pressure. This is due to the rapid
evaporation of saline water on the aluminum surface under the negative
pressure. It can be found that the amount of heavy metal decreases with
the decrease of the operation pressure. It indicates that the deposition of
heavy metal is retarded by the decrease of the operation pressure. The
role of negative pressure on the corrosion of aluminum is related to the
inhibition of the deposition of heavy metal ions. The layer of the
corrosion products formed under negative pressure can hinder the
displacement process. The less operation pressure, the stronger the
blocking effect, and the better the corrosion-resistance of aluminum
alloy.
3.4. X-ray diffraction analysis
Fig. 6 is XRD patterns of AA5052 alloy surface after 3d immersion in
3.5 wt% NaCl solution under negative operation pressure without and
with metal cations.
As shown in Fig. 6, all the aluminum alloy surfaces contained the
characteristic diffraction peaks of Al, Al2O3 and AlCl3. The X-ray
diffraction patterns of Cu and Fe are not evident on the aluminum alloy
surface in the corrosive solution containing Fe3+ and Cu2+ ions,
respectively. This can be interpreted as the amount of the deposited Cu
and Fe is too small to be detected as the trace metal cations are used
[36,37]. The position of the characteristic peak of the corrosion products
is not affected by the pressure. However, the intensity of the charac­
teristic diffraction peak of AlCl3 (2θ = 45◦ , 78◦ ) is gradually enhanced as
the decrease of operation pressure, suggesting that the content of AlCl3
on the aluminum alloy surface gradually increases as the pressure is
reduced. It indicates that it is more difficult for AlCl3 to diffuse into
solution under the negative pressure, thus forming a passivation layer of
6
D. Zhang et al.
Desalination 570 (2024) 117082
Fig. 6. XRD patterns of AA5052 alloy surface after 3d immersion in 3.5 wt% NaCl solution under different operation pressure without (a) and with metal cations: 20
μg/L Cu2+(b); 50 μg/L Fe3+(c); 20 μg/L Cu2++50 μg/L Fe3+(d).
corrosion products. This is consistent with the study of EDS analysis.
3.5. Mechanistic analysis
It is proved that the heavy metal ions can accelerate the corrosion of
aluminum alloy in the simulated seawater [38]. Heavy metal ions can be
adsorbed at the defects on the aluminum alloy surface. Copper and iron
are deposited on the surface of the aluminum alloy as a result of the
replacement reaction between the aluminum alloy and the heavy metal
ions Cu2+ and Fe3+ in seawater [39]. According to the above experi­
ments, the reduction of the operation pressure can slow down the
corrosion rate of aluminum alloy in 3.5 wt% NaCl without and with
different metal cations. Although the decrease of the pressure is believed
to reduce the concentration of dissolved oxygen, it is mainly affecting
the anodic process of the corrosion reaction. Fig. 7 shows that it is
difficult for corrosion products to diffuse into the corrosion solution
under negative pressure as the seawater will evaporate rapidly from the
aluminum alloy surface under negative pressure. The corrosion product
layer generated is more compact and has some passivation effect under
the negative pressure. The passive layer prevents the attack of Cl− ions,
as well as the diffusion process of heavy metal ions to the defects of the
oxide film. Thus, the decrease of the operation pressure can slow down
the anodic reaction process and enhance the corrosion-resistance of
aluminum alloy in the simulated seawater solution.
4. Conclusions
Fig. 7. Schematic illustrations of the corrosion of AA5052 alloy in 3.5 wt%
NaCl containing metal cations under different operation pressure: atmospheric
pressure (a), and negative pressure (b).
In summary, the corrosion rate of AA5052 aluminum alloy in 3.5 wt
% NaCl solution containing heavy metal ions decreases significantly
7
Desalination 570 (2024) 117082
D. Zhang et al.
with the decrease of operation pressure. The anodic process of the
corrosion reaction is retarded significantly under the negative pressure.
As the operation pressure decreases, the diffusion process of corrosion
products into the corrosion solution is prevented. This leads to the for­
mation of a passivation layer of corrosion products on the surface of
aluminum alloy. It can effectively prevent the heavy metal ions in the
solution from adsorbing on the defects of the oxide layer on aluminum
alloy surface. Thus, the decrease of the operation pressure can enhance
the corrosion-resistance of aluminum alloy in the MED desalination
process.
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CRediT authorship contribution statement
Daquan Zhang: Resources, Funding acquisition, Formal analysis,
Writing - review & editing. ZiXuan Yan: Conceptualization, Method­
ology, Investigation, Software, Writing - original draft. Lixin Gao: Su­
pervision, Writing - review & editing. Zhiling Xin: Validation, Formal
analysis, Visualization. YongXin Zhu: assisted the tests. Wei Wu: re­
view & editing.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
The authors do not have permission to share data.
Acknowledgements
Our work was supported by NSFC project (Grant No 52071198). We
thank the grant from the Science and Technology Commission of
Shanghai Municipality (18DZ2204400, 19DZ2271100) very much.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.desal.2023.117082.
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9