| Organic coatings can not provide long term protection for components in high temperature, humidity and salt fog environment due to its seriouly corrosion. Cold spraying Zn-Ni coating is an alternative method for this kind of environment. To overcome the shortage of existing cold spray equipment, computational simulation methods was used to study the gas solid two phase flow in the nozzle of DYMET 413 equipment. The anti-corrosion performance of the zinc nickel coating was studied by electrochemical method combining with surface observation.Computational Fluid Dynamic(CFD) can be used as a fast method for visualization of gas solid two phase flow. The final impacting velocity is determined by gas pressure, gas temperature in prechamber, spraying distance and diameter of particles. The final impact velocity increases as the temperature and pressure increase. The syphonage effect is strongly influenced by the powder feeding location. The temperature and pressure in prechamber has little effect on syphonage effect in powder feeder pipe. The syphonaged gas will decelerate the gas velocity and lower the gas temperature in nozzle. One of the disadvantages is that the particles will collide with the nozzle wall which makes the nozzle a short service life. Simulation results is consistence with experimental results, the most seriously erosion part is lying at the location as the impacting angle is 30°. For most zinc particles, the final impacting velocity can achieve its critical velocity, while only partly nickel particles can obtain its critical velocity.A new structure with downstream coaxial powder feeding nozzle was designed and tested with simulation methods. The gas flow can finally develop to its highest velocity at the exit of nozzle. The velocity of gas at the center of the nozzle is 100m/s lower than that of the conventional nozzle due to its annular shape. Powders are well restrained near the central line of the nozzle, and no collision occurred between the nozzle wall and the powders. The syphonage effect also varies little with temperature and pressure, which ranges from 2.74~2.77 kg/s. This type of nozzle with expansion ratio 3.25 can successfully deposit copper coating on steel substrate. This type of nozzle also can accelerate nickel particles to its critical velocity.To overcome the deceleration effect of compressed layer, a needle shape barrier is placed before the substrate to change the structure of compressed layer, subsequently make this region suitable for accelerating submicron particles. A compressed layer before the substrate can decelerate the impacting velocity to nearly 80m/s for submicron particles with diameter less than 0.5um. A sharp needle barrier can destroy the compressed layer before the substrate, which can change the thickness of compressed layer and direction of the gas flow ahead of the substrate, and subsequently elevate the final impacting velocity of submicron particles. The particle with 0.5μm can get a velocity of 560m/s on the substrate, but the area is significantly narrower than spraying band. The results obtained from the computational simulation indicate that it is applicable to deposit submicron(larger than 0.5um) copper particles directly using cold spraying.To verify the relationship between the spray parameters and the deposition efficiency, DYMET413 commercial low pressure cold spray system was used to prepare coating under different parameters. The deposition efficiency increases linearly as the gas temperature and pressure increases. The optimal alumina content in powders is 30%, and the optimal distance ranges from 15 mm to 25 mm. deposition efficiency of powder increases as the transverse speed of nozzle decreases. It is hard to predict the deposition efficiency by computational methods since the deposition hehaviour in a composite powder system is influenced by a number of factors. The interaction between particles, the erosion effect of alumina and under critical velocity particles all can influence the deposition behavior.Low pressure cold spray can deposit zinc nickel composite coating without intermediate process, such as agglomerating and ball-milling. The alumina content in coating is about 14-22%, which is lower than in powder 30%. The variation of nickel content in coating comparing to raw powder is ramdonly. The coating has excellent physical performance, with average bonding strength about 30 MPa. Coating thickness is uniform through all part of the coating. But the deposition efficiency is low, the highest deposition efficiency is about 30%. All kinds of coating have excellent anticorrosion performance during 90 days neutral salt spray test. The corrosion behavior of zinc nickel coating in seawater can be described as follows: zinc dissolved as the sample immersed in seawater, zinc ion enter into solution to form Zn(OH)2 layer. Zn(OH)2 layer separate substrate from seawater and low down the corrosion rate of zinc. Zn(OH)2 will converse to different corrosion products under different conditions. As the nickel content increases in coating, Zn(OH)2 has a high probability of converting to Zn Cl2?4Zn(OH)2?H2O, instead of converting to Zn O. Among the corrosion products of zinc in seawater, Zn Cl2?4Zn(OH)2?H2O has the most compact structure, and Zn O has the most loose structure. The nickel content exceeds some certain value, and the micro-cell can be formed between zinc and nickel, which will accelerate the consumption of zinc. |
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