Fabrication And Corrosion Behavior Studies Of (Zn-Ni)-Al₂O₃ Nano Composite Coatings

The (Zn-Ni)-Al₂O₃ nano composite coatings with high anticorrosionperformance were fabricated using electrodeposition method with the aid ofultrasound, aiming to extend the application field of Zn-Ni alloy. Theelectrodeposition parameters were optimized by testing anticorrosion propertieswith immersion test and electrochemical technique. The effect of ultrasonicvibration on the Zn-Ni alloy deposition was investigated and the absorbance ofZn²⁺and Ni²⁺ on nano alumina particles was analyzed by ICP (inductivelycoupled plasma). Furthermore, the mechanism of composite electrodepositionunder ultrasonic vibration was studied. The anticorrosion behaviors andmechanisms of the nano composite coating and its passivated film were exploredby analyzing the structure, corrosion process and corrosion products of thecoating byusing XRD, SEM and XPS.The studies on nano alumina dispersants demonstrate that the synergeticeffect of Arabic gum and CTAB (Hexadecyl trimethyl ammonium Bromide)promotes the codeposition and uniform dispersion of nano alumina particles inthe coating.By comparing the different dispersion methods a very effective way,inserted ultrasonic vibration, has been established to de-agglomerate nanoalumina particles in electrolyte, which can promote the incorporation of nanoalumina particles in the coating. The mono-dispersion and high content of nanoalumina particles in the coating should be attributed to the action of cavitationsand ultrasonic streaming generated by the ultrasonic vibration. The cavitationscaused by the collapse of cavitation bubbles de-agglomerate the nano particles,while the presence of ultrasonic streaming promotes the movement of the nanoparticles toward the cathode. Fabrication of nano composite coatings by usingultrasonic agitation during electrodeposition process resolves the agglomerationproblem of nano alumina particles which restricts super performance of nanoparticles, andavoids the adverse effects of dispersants on coating properties.The studies on electrodeposition mechanism of (Zn-Ni)-Al₂O₃ coating underultrasonic conditions indicate that the ultrasound decreases Ni content in thecoating by increasing the mass transport, and the codeposition of nano alumina increases the Ni content due to the more absorbance content of Ni²⁺ on nanoalumina particles than Zn²⁺. The incorporation of nano alumina particles intomatrix results from the synergetic effect of ultrasonic and electric field. Nanoparticles accompanying absorbed Ni²⁺ and Zn²⁺ moved to the cathode vicinity,and then the absorbed Ni²⁺ and Zn²⁺ are induced directly on the particle surface,which lead to the imbedding of the nano particles. This imbedding of nanoalumina particles into matrix is very tight, which would be beneficial to theanticorrosion property.The analysis of the corrosion process and corrosion products demonstratesthat the presence of compact and uniformly dispersed nano alumina particlesblocks off the corrosion medium and coating, decreases the contact area,disperses the corrosion current because of the existence of insulative nanoalumina particles, and therefore substantially inhibits the corrosion of Zn-Ni-Al₂O₃ composite coating. The matrix is composed ofγphase intermetalliccompound, and its corrosion products, different from the corrosion products ofZn coating can form a compact ZnCl₂·4Zn(OH)₂ layer. This product with nanoalumina particles and Ni formsan isolative layer. The corrosion inhibition effectsof the enrichedNi and nano alumina particles increase the anticorrosion propertysignificantly.The performance studies of thechromate conversion coating on the(Zn-Ni)-Al₂O₃ composite coating indicate that the passivated film on the compositecoating containing 3.7mass%weight content nano alumina is more compact thanthat on Zn-Ni alloy, and the nano alumina particles are completely covered. Theemergence time of white rust and red rust in 5mass%NaCl,pH3.5 acetic acidsolution are 18.5h and 230h respectively, and the anticorrosion properties areincreased significantly compared with the conversion coating on Zn-Ni alloy.The reason for super anticorrosion property of the composite coating afterchromate passivation is the presence of enriched Ni and nano alumina layerbetween the conversion film and composite coating, which isolates the corrosionmedium and composite coating, andinhibit the corrosion reaction.