The deep sea represents a largely untapped strategic resource reservoir that has yet to be fully explored or exploited by both scientific and industrial sectors. In contrast to shallow marine environments, the deep-sea harbors abundant mineral deposits, natural gas hydrates, coal, petroleum, and other critical resources, as evidenced by available geological surveys. The sustainable extraction and utilization of these resources can significantly mitigate the global resource scarcity crisis. The harsh deep-sea environment characterized by high pressure, low temperature, and low dissolved oxygen accelerates the corrosion rate of deep-sea equipment primarily made of high-strength steel and titanium alloys, leading to reduced mechanical stability and service life. However, existing sacrificial anode materials are unsuitable for deep-sea applications, highlighting the urgent need for research on deep-sea sacrificial anodes. In this paper, to enhance the surface activity of aluminum (Al) alloy sacrificial anodes for deep-sea applications, five alloying elements (Ga, Mg, Ca, Mn, Ti) were selected on the basis of the commercial Al-Zn-In anode in combination with literature. An orthogonal experimental design was employed to develop octonary Al alloy anodes with gradient compositions. Metallographic and scanning electron microscopy (SEM) analyses confirmed that multi-element alloying refined the microstructure of the Al anode, but excessive Mg addition introduced defects. X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) revealed that Mg, Zn, Ga, and Mn were solid-solved in the Al matrix, while Ca, Ti, and In precipitated uniformly within grains as phases smaller than 2 μm in diameter. Scanning Kelvin probe (SKP) measurements demonstrated that high alloying content led to uneven surface potential distribution and increased potential difference, resulting in high galvanic corrosion susceptibility. Potentiodynamic polarization curves indicated that low temperature and low oxygen in the deep-sea environment expanded the passivation potential range of the anodes, while high pressure increased their self-corrosion current density. Proper alloying reduced the passivation range while decreasing both passivation and self-corrosion current densities. Electrochemical performance tests and the range analysis determined the optimal composition ranges for Mg, Ti, Ca, and Mn to be 0.9%-1%, 0.018%-0.02%, 0.05%-0.06% and 0.09%-0.1%, respectively, achieving a maximum current efficiency exceeding 92% for the octonary Al anode. Moreover, the factor range difference (Rj) indicated that the order of influence of different alloying elements on current efficiency of anodes was Mg > Ca = Mn > Ti. Therefore, the Mg content had a more significant effect on the octonary Al anode. X-ray photoelectron spectroscopy (XPS) confirmed that the corrosion layer on the octonary anode surface comprised Al2O3, Al(OH)3, ZnO, Mg(OH)2, Ga, and In, suggesting disruption of the Al2O3-dominated passivation effect. In simulated deep-sea conditions, micron-scale precipitates of Ca, In, and Ti formed micro-galvanic couples uniformly distributed across the anode surface, enhancing surface activity while mitigating self-corrosion. Activators like Zn, In, and Ga facilitated the "activation-redeposition" mechanism to reactivate the Al2O3-rich corrosion product layer, while Mg(OH)2 from Mg increased defect density via volumetric effects, further activating the anode surface. These findings provide a theoretical and experimental foundation for optimizing the activation behavior and performance of Al alloy anodes in deep sea.
Key words
Al alloy sacrificial anode /
deep-sea environmental conditions /
alloying elements /
surface activation /
electrochemical performance
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Funding
Natural Science Foundation of China (52201090); The National Key R&D Program of China (2022YFB3808800)
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