Study On Process And Properties Of Ni-p-nano Tio₂ Composite Coating On Magnesium Alloy

Magnesium alloys have been paid more and more attention because of low desity, high specific tenacity and rigidity, excellent damping, electromagnetism shielding proprty and recoverability. In the 3C shell, vehicle, sports products, family appliance and other daily product domains, magnesium alloys have broad prospect of application. Whereas. The magnesium alloys products have poor corrosion resistance and threaten personal health because of breeding bacteria after dust absorption due to electrostatic interaction. As the products closely related to people's daily life, the anti-fouling, anti-bacteria, anti-corrosion and other issues of magnesium alloys have becoming an important topic.Ni-P coating is an important means all along for the advantages of high hardness and corrosion resistance. The TiO₂ have well chemistry stability, super-hydrophile and excellent photocatalytic activity. Comprehensive utilizing all the merit, Ni-P-nanoTiO₂ coating could enhance corrosion resistance of magnesium alloys products and endow with the performance of anti-fouling and anti-bacteria.This article obtained the best craft formula of pretreatment and electroless plating, then prepared two types of Ni-P-nanoTiO₂ composite coating by joining particle formation and in-situ formation on base of electroless Ni-P plating. Tested the binding force, porosity and microhardness of coating. We observed and analysed the appearance, ingredient and structure of coatings using OM, SEM, EDS and XRD, discussed the influence of craft parameters. Meanwhile we carried on the comparative study on photocatalytic degradation activity, hydrophilic anti-fouling and corrosion resistance of the composite coatings.The results showed that both the structure and performance index of coating adopting no cadmium acid pickling could reach or exceed the coating adopting cadmium acid pickling. During the pretreatmen, the pickling process and roughness affected the structure and performance of coaing, the ultrasonic could accelerate plating rate and enhance bonding force to some extent. It is more difficult because of the magnesium alloys substrate corroded by bath when using MSO₄ as the main salt. Adding NH4F buffer could weaken the corrosion process for two reasons. Firstly, large fluoride concentration increased the probability to form MgF₂ film, secondly, the complexation of ammonium and fluoride reduced the nickel ion concentration and slowed down the anodic process of corrosion. Adding NH₄F could refine the structure and decrease porosity of coating when decreasing plating rate.The TiO₂ particles in the Ni-P-nanoTiO₂ composite coating prepared by in-situ formation were quite small, its were within nano-size range, the sol in the bath could obturate holes of coating, therefore decrease the porosity. For the dispersion strengthening of nano TiO₂, the microhardness of composite coatings were higher than Ni-P both as-plated and in heat treatment condition. After 400°C heat treatment, the microhardness of Ni-P coating decreased for coarsening of Ni₃P and microcrystalline nickel. For the composite coatings, the microhardness did not decrease because dispersively distributed TiO₂ particles slowed down the coarsening process of Ni₃P and microcrystalline nickel.The photocatalytic degradation property of the composite coating prepared by in-situ formation was better than the one prepared by joining particle formation, the reason was that TiO₂ particles in the in-situ formation composite coating were within nano-size range, thereby beneficial to effective separation of the electron-hole pairs and improed the photocatalytic degradation activity. After 400°C heat treatment, the photocatalytic degradation property of the in-situ formation composite coating improed due to the TiO₂ former body on the coating surface transformed to anatase TiO₂.Both the two types of composite coating taken on excellent hydrophile and anti-fouling property, the nano TiO₂ particles improved the hydrophile of Ni-P greatly. Ni doping could improve the hydrophile and photocatalytic property to some extent for Ni capturing the photo electrons and reducing combining between electrons and holes. The TiO₂ in the composite coatings hindered salt precipitation on the coating surface and improved the anti-fouling property of the Ni-P coating. The TiO₂ particles elevated the corrosion potential and depressed the corrosion current of Ni-P coating, thereby improved the corrosion resistance. The two types of composite coating owned the equivalent corrosion resistance, but both of them excelled the Ni-P coating.