| The marine structures such as ships, tidal power stations, pipelines, bridges and offshore structures have been severely eroded in marine environment. It has been reported that paint and metallic coatings can durably protect the structures from corrosion. Being an anode coating, aluminum coating not only can separate the steel substrate from corrosion medium, but also has the effect of cathode protection. Aluminum/Zinc coating has excellent anticorrosive property of both aluminum and zinc. Specifically it has not only the electrochemical activity of zinc, but also has the protection of aluminum passive film. Different techniques have been used to deposit coatings on steels such as electroplating, hot-dipping, electro-deposition and thermal spraying. Among them, thermal spraying is the most popular technique. Cold gas dynamic spraying (CS), as a novel surface treatment technique, has a wide application prospect in anticorrosion under marine environment.In this paper, aluminum and aluminum/zinc coatings were prepared using CS and arc spraying (AS). The microstructure, organization structure and corrosion behavior in marine environment of different coatings were analyzed using modern methods for materials analysis and electrochemical test. Based on previous experiments, "in-situ-pitting corrosion" model was proposed for studying the corrosion behavior of coatings serviced in marine environment, and the relationship between aluminum/Zinc coating's ingredient and its anticorrosive property was discussed.By the optimal technological parameters, A125-Zn, A150-Zn, and A175-Zn coatings were prepared using CS, in contrast, aluminum and A150 (vol%)-Zn coatings were prepared using AS. Different types of coatings were analyzed using scanning electron microscopy (SEM), energy dispersive spectrum (EDS) and X-ray diffraction (XRD). The results showed that CS-coating was deposited through gross plastic deformation of particles; no obvious oxidation was discovered in the coating preparation process; it had a dense structure with a porosity of 2%. The AS-coating had a laminate structure, many pores existed in the coating surface and its porosity ran up to 15%.After immersion corrosion test in seawater, the morphology and component of corrosion product covering on CS and AS aluminum coating was examined by SEM and XRD. In different immersion time, corrosion behaviors of CS and AS aluminum coatings were researched by open-circuit-potential (OCP), Potentiodynamic polarization and electrochemical impedance spectroscopy (Eis). SEM and XRD analysis revealed that the corrosion product of CS aluminum was dense and essentially AlO(OH). In contrast, the corrosion product of AS aluminum was porous and loose, essentially Al(OH)3. The variation of OCP showed that CS and AS aluminum coating can both afford cathodic protection for Q235 substrate; OCP of CS aluminum coating was stable along with the immersion time, about -0.85V, but that of AS aluminum coating exhibited a larger fluctuation. The linear polarization curves revealed that the polarization resistant of CS aluminum coating increased gradually along with immersion time. However, the polarization resistant of AS aluminum coating exhibited considerable variation, which revealed that AS aluminum coating had been under active corrosion state. The Potentiodynamic polarization curves indicated that corrosion current of CS aluminum coating gradually reduced and tended to stability with immersion time, dissimilarly, that of AS aluminum coating increased along with time firstly, and then decreased. In anode polarization of AS aluminum coating, there existed a turning point which revealed that AS aluminum coating trended to generate localized corrosion. Eis results illustrated that a mass-transfer step was involved in corrosion process of both CS and AS aluminum coating; the AS aluminum coating presented a faster degradation than CS aluminum coating. In addition, limited diffusion impedance O was introduced in EIS modeling for the purpose of getting good fitting results.After immersion, the morphology and component of corrosion product covering on CS and AS aluminum/zinc coating was analyzed by SEM and XRD. The results showed that the corrosion product of aluminum/zinc coating was principally Zn5(OH)8Cl2·H2O and ZnO, and a very small amount of Al(OH)3, Al2O3 and Zn(OH)2。Potentiodynamic polarization was applied to three types of CS aluminum/zinc coating. The results indicated that A125-Zn coating had been active dissolution, whereas passivation regions were appeared in the polarization curves of A150-Zn and A175-Zn coatings. The CS aluminum coatings were also studied using Eis experiments. The data illustrated that the impedances of CS aluminum coatings trended to increase with immersion time. Moreover, the diameters of capacitive loops in Nyquist plot were larger with increase of aluminum content in coating. During immersion, being sacrificial anode of aluminum, zinc of the coating rapidly eroded and corrosion product gradually covered the coating. Subsequently, localized corrosion will generate in some positions. Combing cathodic protection of coatings and electrochemical measurement, CS A175-Zn coating has preferable anticorrosive property.Simulating the marine atmosphere environment, all the coating samples were examined by neutral salt spray test. During 4000h test, CS and AS aluminum, aluminum/zinc coatings can all protect the Q235 substrate from corrosion in salt fog cabinet. However, bubbling phenomena occurred on the AS coating surface. In contrast, CS coatings revealed low corrosion rate due to sealing effect of corrosion product. |
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