Research On Surface Characteristic And Corrosion Behaviour Of Cold-Sprayed Aluminum Coating

The steel structures of ships and ports have been severely eroded in marine circumstance, it had been reported that the technique to deposit pure metallic coatings on steel substrates can durably protect the structures from corrosion. Aluminum coatings deposited on the steel structures have good corrosion resistance in marine circumstance, its applications are getting increasingly common because of the advantage of durative protect effect. Cold spraying is an emerging coating technology. In this process, a coating is deposited on the substrate through plastic deformation in a completely solid state via high velocity impact.In this study, uniform design was used for optimization of spraying parameters of cold-sprayed aluminum coating, and the surface characteristic and the corrosion behavior of cold-sprayed aluminum coating in marine circumstance has been analyzed using various instruments.First, uniform design was used to optimize cold spraying parameters. Based on the performance of the aluminum coatings by cold spray technique with different spray parameters, the best spray parameter has been obtained, gas temperature: 250℃, gas pressure: 2.0MPa, spray distance: 25mm, rate of feed particles: 1.6g/s.Second, the surface characteristics of the coating were observed and studied by Scanning Electron Microscopy (SEM), X-ray diffraction (XRD), Optical Microscopy, micro-hardness measurements and pull tests. With the SEM, it was found that the coating was deposited through intensive plastic deformation of the spray particles by high speed impact, and no oxidation was observed in the cold spray process. With the analysis of XRD, it was found that no phase transformation was observed in the coating, and the grain size decreased due to the intensive impact by high speed particles. With optical microscopy, it was found that the cold-sprayed aluminum coating with a low porosity could avoid medium diffusing into the coating. Due to the high speed impact of spray particles, the micro-hardness of cold-sprayed aluminum coating was obviously higher than the thermal-sprayed aluminum. Pull tests illustrated that the adhesion strength between coatings was higher than the adhesion strength between coating and substrate, and the coating combined to the substrate through the plastic deformation of spray particles.Third, the electrochemical experiments were performed to study the corrosion behavior of the coating, and SEM and Ultrasonic Microscopy characterization of the coating were also performed. With the SEM and Ultrasonic Microscopy,it was found that the surface of the coating was covered with corrosion product. With the analysis of Ecorr, it was found that the Ecorr of cold-sprayed aluminum coating which had the similar Ecorr to the bulk aluminum was above -0.8 VSCE in seawater. With the research of the liner polarization curves, it was found that the Rp of the cold-sprayed aluminum coating increased rapidly with time in seawater, and the coating performed similar corrosion resistance with the bulk aluminum. Nyquist plots illustrated that the oxide film formed on the surface of the cold-sprayed aluminum in the air could't effectively protect the coating from corrosion, and the steady corrosion product played a great role in avoiding medium diffusing into the coating, and the Nyquist plots of the cold-sprayed aluminum performed diffuse phenomena after eroded for 240 hours in seawater. With the analysis of potentiodynamic polarization curves, it was found that the Icorr of the cold-sprayed aluminum coating decreased rapidly with time in seawater, and the rise of the anodic polarization rate was dued to the steady corrosion product on the surface of the coating.At last, in order to study the corrosion resistance of the coating in marine circumstance, the neutral salt spray test and marine atmospheric corrosion test were further performed. The results indicated that the Q235 steel could be effectively protected by both thermal-sprayed aluminum coating and cold-sprayed aluminum coating, but the corrosion rate of the cold-sprayed aluminum coating was lower than the thermal-sprayed aluminum coating and declined rapidly with time in marine circumstance.