| The steel structures of ports have been severely eroded in marine environment.As an important means，spraying metal protective coating technology can provide thelong-lasting protection for the steel structures in the marine environment. In the lowpressure cold spray technology，a coating is deposited on the substrate through plasticdeformation in a completely solid state with high velocity impact. The Zn-Al alloycoating not only can separate the steel substrate from corrosion medium, but also hasthe effect of cathode protection for the steel substrate. Therefore it has a wideapplication prospect in anti-corrosion under marine environment.The Zn-Al alloy coating was prepared by using low pressure cold spraytechnology in this paper. Many modern materials analysis and electrochemical testmethods were used to investigate surface properties of the coating and its corrosionresistance in the marine environment. The influences of the content of ceramic phaseAl2O3on the properties of the coating were discussed. By summarizing the corrosionlaw of the Zn-Al alloy coating in the marine environment，base data and theoreticalsupply and obtained to provide the preparation of low-cost and high-performanceanti-corrosion coating.With the process parameters in optimization，four kinds of Zn-Al alloy coatingwhich contain20%，30%，40%，50%volume content of alumina, were prepared bythe equipment of low pressure cold spray, called DYMET-314. The surface andcross-section characteristics of the coating were observed and studied by ScanningElectron Microscope (SEM), Energy Dispersive Spectrometer (EDS), andmetallographic microscope. It is indicated that the thickness of Zn-Al alloy coating isranged from250μm to310μm. The surface of the coating is relatively smooth with tiny pits on it. The porosity of Zn-Al alloy coating is entirely low, being about1%～5%. The inner binding force of the coating is bigger than that between the coating andthe steel substrate. The maximum value of the bonding strength is25.7MPa, whichbelongs to the coating with40%volume content of alumina. The higher the aluminacontent in the original spray powder, the more the alumina content and porositydistribute in the coating. So the embedding of alumina results in an increase ofinterface defects and porosity in the coating surface. The alumina in the originalspraying powder can produce a hardening effect of the processed coating. With theanalysis of physical properties test results of the Zn-Al alloy coating, the coating thatwas made from the original spraying powder with40%volume content of aluminahas a moderate coating thickness, a lower porosity, a higher bonding strength andmicrohardness as well.Coatings obtained by low pressure cold spray were immersed in the seawater totest their electrochemical properties by setting a wetting and drying recirculating ratioto the comparison of fully immersed in the seawater. Results show that in the earlyimmersion the coating has higher activity along with lower corrosion potential. Thecorrosion potential of the coating cyclically immersed in seawater changed in a biggerrange. After immersed in the seawater for200hours, due to the damage of the oxideretia on the surface of the coating, Cl-in seawater begun to enter into the inner part ofthe coating and made a great change to the corrosion potential. It was found that allthe coatings can provide effective and reliable protection for the steel substrate duringthe test. The analysis of potentiodynamic polarization curves shows that the coatingsunder the two different working conditions have the same processes of corrosion. Thecontent of alumina in the coating makes no difference to the corrosion law of thecoating. However, the corrosion products during the process of corrosion can preventthe corrosion medium from seeping into the inner part of the coating. Byelectrochemical impedance test, the results show that the process of theelectrochemical reaction can be divided into the following three stages. At the firststage, the oxide retia on the surface of the coating can hold back the infiltration of thecorrosion medium for a short time. At the second stage, with the damage of the oxideretia, as a result of Cl-in seawater permeating into the inner part of the coating, theelectrochemical reaction speeds up. And at the third stage the corrosion products areheaped up on the surface of the coating, preventing Cl-seeping into the inner part of the coating.According to the results of neutral salt spray test and the test in the tidal range inthe marine environment, the Zn-Al alloy coating has not only a good corrosionresistance, but also the effect of the long-lasting cathode protection for the steelsubstrate. Comparing with the widely used Zn-Al alloy coating that is made bythermal spraying, the Zn-Al alloy coating made by cold spraying has the higherbonding strength and corrosion potential, and can provide a better protection to thesteel substrate. |
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