Localized Corrosion Mechanism of 2024 Aluminum Alloy in a Simulated Dynamic Seawater/Air Interface
XIA Dahai(), JI Yuanyuan, MAO Yingchang, DENG Chengman, ZHU Yu, HU Wenbin()
Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300350, China
Cite this article:
XIA Dahai, JI Yuanyuan, MAO Yingchang, DENG Chengman, ZHU Yu, HU Wenbin. Localized Corrosion Mechanism of 2024 Aluminum Alloy in a Simulated Dynamic Seawater/Air Interface. Acta Metall Sin, 2023, 59(2): 297-308.
A seawater corrosion test platform to simulate the dynamic seawater/air interface is constructed, comprising an electric putter, a time relay, and four corrosion electrochemical sensors. The localized corrosion mechanism of 2024 aluminum alloy in a simulated dynamic seawater/air interface is investigated by corrosion potential monitoring, electrochemical impedance spectroscopy (EIS), electrochemical noise (EN) measurements, and the analysis of the surface and cross-section morphology. The differences in the corrosion behavior at the seawater/air interfacial region and that of full immersion region are discussed. The results showed that the corrosion products at the dynamic seawater/air interfacial region are continuously distributed, which is mainly due to the dissolved Al3+ flowing from the pits and reacting with the oxygen in the dynamic seawater/air interfacial region. The distribution of corrosion products in the entire leaching area is more dispersed. As aluminum alloy is immersed and removed from water periodically, the corrosion potential fluctuates periodically with an amplitude of 5-10 mV. Due to the high corrosion potential in the seawater/air interfacial region, the aluminum alloy above the waterline behaves as the cathode, and that below the waterline acts as the anode. However, because of the subtle difference in the corrosion potential, the galvanic corrosion effect is not obvious. The results of EIS revealed that the high-frequency capacitive arc radius of both seawater/air interfacial region and full immersion zone increased first and then decreased, and the corrosion product film in the interface zone had better corrosion resistance than that in the full immersion zone. The results of the EN test showed that the fluctuation amplitude of current noise decreased first and then increased, indicating that the local corrosion sensitivity decreased first and then increased. The slope of the high-frequency linear region of the power spectral density of current noise was less than -20 dB/dec, indicating that the corrosion type was local corrosion. The pit size at the seawater/air interface was much smaller than that in full immersion region, because the oxygen in the seawater/air interface region could be easily reduced within the pits by consuming H+, thereby increasing the pH value within the pits.
Fig.1 A self-built dynamic seawater/air corrosion measurement platform (a) overview of the corrosion platform (b) 2024 aluminum sample which is divided into three regions: atmosphere region, seawater/air interfacial region, and full immersion region (c) detail of the corrosion electrochemical sensor used in the EIS measurement (d) detail of the corrosion electrochemical sensor used in the electrochemical noise measurement
Fig.2 A periodical change of corrosion potential of 2024 aluminum alloy at the dynamic seawater/air interface (EOCP—open circuit voltage, t—time)
Fig.3 EIS of 2024 aluminum alloy in seawater/air interfacial region (a) and full immersion region (b) (Insets show the local magnifications)
Fig.4 Electrochemical equivalent circuit of 2024 aluminum alloy in seawater/air interfacial region and full immersion region (—the electrolyte resistance; —the capacitance of the double electric layer; —the Faradaic resistance of oxygen reduction at oxide/electrolyte interface; —the resistance of cation vacancies moving in the oxide; —the resistance of electrons moving in the oxide)
Fig.5 Polarization resistance () (a) and effective capacitance () (b) obtained from the Measurement Model
Fig.6 Time-domain electrochemical noise data of 2024 aluminum alloy in seawater/air interface zone (a) current noise (b) potential noise
Fig.7 Time-domain electrochemical noise data of 2024 aluminum alloy in full immersion zone (a) current noise (b) potential noise
Fig.8 Fast Fourier transformation analysis of the electrochemical noise data of 2024 aluminum alloy in seawater/air interface zone (a, c) and full immersion zone (b, d) (PSD—power spectral density) (a, b) potential noise (c, d) current noise
Fig.9 Macroscopical corrosion morphologies of 2024 aluminum alloy in atmosphere region, seawater/air interfacial region, and full immersion region
Fig.10 Comparisons of average pitting depth (a) and pitting density (b) of 2024 aluminum alloy in atmosphere zone, seawater/air interface zone, and full immersion zone for 54 d
Fig.11 Microcosmic corrosion morphologies of 2024 aluminum alloy in atmosphere region (a), seawater/air interfacial region (b), and full immersion region (c, d) for 54 d
Point
O
Mg
Al
Cl
Si
S
Fe
1
75.632
1.030
23.164
0.174
-
-
-
2
55.489
1.762
42.508
0.240
-
-
-
3
82.861
0.537
15.282
0.729
0.408
0.184
-
4
34.098
2.308
61.543
0.255
1.467
0.328
-
5
63.092
2.118
31.830
1.643
0.901
0.417
-
6
58.259
2.630
37.947
0.218
0.741
0.205
-
7
82.099
1.455
15.158
0.274
0.804
0.209
-
8
46.203
2.677
49.497
0.204
1.209
0.210
-
9
69.078
1.803
24.734
0.188
3.264
0.284
0.648
Table 1 EDS results of the 9 points marked in Fig.11
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