Study On Pickling And Ni-P Bright Electroless Plating Technology Of ADC12 High Silicon Aluminum Alloy

The corrosion resistance of materials is directly related to the service safety and life time of products.Aluminum alloy has the advantages of light weight,high strength and excellent formability,and is widely used in various industries.However,poor weldability,corrosion resistance and decorativeness make surface treatment important in the field of forming and manufacturing of aluminum alloy products.Therefore,research on aluminum alloy surface modification is of great significance for expanding the application field of aluminum alloy.In this paper,ADC12 as-cast high-silicon aluminum alloy is used to develop a new environmentally friendly pickling and bright electroless plating technology.The main research work and achievements of the thesis are as follows:?1?Environmentally friendly pickling process:Based on the surface uniformity,the ash cleanliness,the residual Si content and the stability of the pickling solution after acid pickling,the effective components of the pickling solution are systematically screened and the best pickling and ash removal process is as follows:80g/L oxidant A,200g/L ammonium fluoride,60g/L organic acid B,100ml/L phosphoric acid,5g/L stabilizer,for 1min at room temperature.After alkali corrosion and pickling,the surface of the sample was analyzed by SEM and EDS.After alkali etching,a large number of irregular Si-containing phases were formed on the surface of the substrate.After the optimal pickling and ash removal process,the surface of the substrate was embedded.The silicon phase was almost cleaned and the surface silicon content decreased from 37.6%to 1.37%,indicating that the best pickling formula has excellent ash removal performance.?2?Screening and optimization of active components in Ni-P electroless plating bath:The effects of accelerators and brighteners on the electroless plating were investigated.The promoter glycine was effective in increasing the speed of electroless plating.Brightener sulfuric acid barium sulfate and leveling agent have a brightening effect and the coating is of good quality.Based on the screening results of the effective components of the plating solution,the plating speed,the appearance quality and the brightness of the coating were taken as the main evaluation indexes.The influence of the components of the plating solution and the plating parameters on the plating effect was systematically investigated,and the optimum plating solution was obtained.And plating parameters are as follows:30g/L nickel sulfate,40g/L sodium hypophosphite,30g/L sodium acetate,1g/L citric acid,3.5mg/L thiourea,4g/L glycine,10 mg/L brightener A,30mg/L leveling agent;plating solution pH of 5.4,plating time of 1.5h,plating temperature of 80°C.?3?Performance analysis of typical samples:The coated samples were tested for lightness,microhardness,coating/substrate bonding force,the microscopic structures and elemental component,and full immersion corrosion test and potentiodynamic polarization curve test were conducted.The best Ni-P coating is mirror-bright,with a brightness value of 497GU,a microhardness of 572HV,and a coating/substrate bond of 5B.The SEM analysis of the optimal coating surface shows that the substrate is completely covered by the coating,the coating is extremely flat and has no microscopic defects,indicating that the addition of the brightener is beneficial to refining the coating structure,making the plating unit cell size smaller and the surface flatter;polarization curve results shows that the coating is cathodic protective layer with respect to the aluminum alloy substrate.The i_corr of the optimal coating sample is reduced by nearly two orders of magnitude relative to the blank sample,and the corrosion protection rate P is 97.9%,and the corrosion resistance is obviously improved.