| Deep sea was rich in natural resources, and it had the great significance. Submersible was necessary equipment towards deep-ocean for human beings. However, steel submersible serviced in the harsh environment from the shallow to the deep sea, so it was easy to be corroded and fatigue of composite damages. Therefore, a special coating was designed and manufactured to protect the submersible from these composite damages, which was important for realizing these goals including large, hidden and deep. To solve these key problems, the following researches were studied in this paper.1) The anti-corrosion anti-fatigue coating with cage construction was innovatively designed, and Al-Ti-Si-RE cored wires(Ti:25%~28%,Si:1.5%,RE:0.5%,Al:balance) were made. Al-Ti-Si-RE coatings were made by high velocity arc spraying(HVAS) on the surface of L907 A steel, and the basic properties of this coating were studied. The research showed that the average porosity and adhesive strength were 1.6% and 39.0MPa, respectively. Compared with conventional organic coating, this coating had increased about 8 times in adhesive strength.2) Al-Ti-Si-RE coatings were mainly made of microcrystalline and nanocrystalline with a stable microstructure(Ti-Al intermetallics) through SEM,TEM and XRD analysis. Al element was continuous distributions in the coating surface, and Ti-Al intermetallic compound was like to island distributions. Therefore, the porous microstructures could be fixed with the Al corrosion products in order to form homogeneous coating to block the corrosion paths and inhibit the corrosion.3) The adjustable temperature and adjustable oxygen deep-sea simulation experiment device was designed to simulate working environment of submersible. Compared with the existing simulation devices, this device increased the dissolved oxygen concentration controlled system based on controlled pressures and temperatures, which could be more realistically to simulate the working environment.4) Combined with the corrosion electrochemical testing technologies, Al-Ti-Si-RE coatings were studied by simulated system. The researches showed that the Al-Ti-Si-RE coatings had good corrosion resistance in deep-sea simulation environment. The corrosion rate was stable in higher hydrostatic pressure and increased in higher dissolved oxygen. Due to the couple galvanic effect, steady corrosion was mainly in coating. Hydrostatic pressure promoted steady corrosion and the concentration of dissolved oxygen had obvious effect on the transformation from metastable corrosion to steady corrosion.5) For the condition of submersible parts affected by the alternating stress while diving and rising continuously in the deep sea, and comparative analysis of the uniform stress and stress concentration, the three-point bending fatigue test method was applied to accelerate test. the three-point bending test of the L907 A steel coated Al-Ti-Si-RE coatings was conducted in 3.5% NaCl solution. The results showed that the system was still steady after 107 times, while the L907 A steel fractured after 8.123875×106 times, which meant Al-Ti-Si-RE coatings could protect its substrate well. Through the couple galvanic element modeling analysis showed that the amount of dissolved Al anode showed the quadratic and index relationship of the maximum stress intensity factor kmax and cyclic loading time t.6) The thermal fatigue test of Al-Ti-Si-RE coatings is to characterize the heat corrosion and seawater corrosion conditions of submersible exhaust pipes. The results indicated that Al-Ti-Si-RE coatings could serve normally at 500℃. The reason was that the grain sizes of Al and Ti-Al intermetallic compound were very small, and most of them kept the coherent relation, which guaranteed that the crystal structures remained stable when the temperature changed suddenly.7) The XK-252 coating was carried out on the surface of hole sealing treatment since Al-Ti-Si-RE coating existing pores. The adhesive strength between the coating and the dope was as strong as 1 level. After sealing treatment, the composite coating got good anti-corrosion performance both in neutral salt spray and simulated deep sea environment. The hole sealing agent didn’t fall off. After 3 months the preliminary assessment of real sea, the coating performance was good and stable.Through the systematic researches, it showed that Al-Ti-Si-RE coatings had excellent corrosion resistance to the deep sea and fatigue resistance, which could meet the demand of the submersible to service in the deep sea environment. This research could provide theoretical and technical support to deep-sea equipment security service in our country. |
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