Study On The Characteristics And Properties Of Electroless Ni-P Coating

Electroless deposited Ni–P alloys is a very important method to improve the corrosion resistance of metal materials.Generally formulations of electroless deposited Ni–P alloys baths are a multi-component systems containing at least one or more additives such as complexing agent,surfactant,stabilizer,promoter,etc.The cost is higher and the process is more complicated,and usually the process contains a number of environmentally hazardous chemical reagents.Since polyvinylpyrrolidone(PVP)is water-soluble,biocompatible and nontoxic and it is found a wide application in pharmaceuticals and medical applications.Moreover,PVP known to possess a well-defined structure with N-vinylpyrrolidone monomers connected as a long chain,and it can be used as a surfactant,complexing agent,dispersant and stabilizer,etc.Therefore,we attempt to make use of PVP as a new,multi-functional additives,to study a green,simple polyvinylpyrrolidone system to replace multi-component system in order to reduce the cost of chemical reagents and simplify the process of electroless deposition.Then,the influences of temperature and pH value on the bath of electroless nickel-phosphorus alloy,the deposition rate,the structure and the properties of the coating in polyvinylpyrrolidone system were studied,by X-ray diffraction,scanning electron microscopy,microhardness tester,electrochemical impedance spectroscopy and polarization measurement.The main results are as follows.(1)Electroless Ni-P coatings were successfully prepared on magnesium alloys using PVP as the sole additive.Therefore,it is proved that polyvinylpyrrolidone can be used as a multi-functional additive in electroless nickel-phosphorus coating,and the concentration of the multi-functional additive PVP has a significant effect on the electroless Ni-P coating in the elelctroless bath.With the increase of the PVP concentration in the plating solution,the total deposition rate of the Ni-P coating decreases,Moreover,the structure of the Ni-P coating deposited by different concentration of PVP is composed of crystalline and amorphous mixture.The coating has good comprehensive performance,the hardness and corrosion resistance of them has been significantly improved compared to the magnesium alloy without electroless deposited Ni–P alloys.When the PVP concentration is 10 gL-1,the surface quality of coating is the best and the thickness of it is the highest,so the hardness and corrosion resistance arealso the highest.(2)The temperature has a very important impact on the electroless Ni-P coating itself,the deposition rate and the structure of nickel-phosphorus coating etc.As the temperature can affect the diffusion rate of ions during electroless plating reaction,when the temperature is changed from 50? to 70?,the deposition rate of electroless plating is increased with increasing temperature in the polyvinylpyrrolidone system.However,the stability of the electroless plating bath will be worse when the temperature is too high.The plating solution is liable to decompose,resulting in a decrease in the deposition rate.The deposition rate reached a maximum value of 18.51 ?mh-1 at a temperature of70°C in the polyvinylpyrrolidone system.The structure of the coating is composed of a mixture of Ni single crystal with good crystallinity and a small amount of nickel phosphorus alloy with poor crystallinity at different temperatures in the polyvinylpyrrolidone system.The hardness and corrosion resistance of the coatings are poor,when the temperature is 50? and 80?,but the properties of the coatings were better at 60? and 70?,and the properties of the coatings deposited at the two temperatures varied little.(3)Similarly,the pH value has a very important impact on the electroless Ni-P coating itself,the deposition rate and the structure of nickel-phosphorus coating etc.The total potential difference of the redox reaction of electroless plating will be different,the rate at which reactive hydrogen atoms are produced is different,and the reaction product of nickel ions with hypophosphite in thesolution will also be different.The solubility of different products are also different,So when the pH is changed from 6 to 9,the deposition rate of electroless plating is increased with increasing p H in the polyvinylpyrrolidone system.However,the reaction product is usually23HPO-at a high pH,its solubility is lower and more prone to precipitation,so that the deposition rate of electroless plating is lowered.The deposition rate reached a maximum of17.02?mh-1 when the pH is 8 in the polyvinylpyrrolidone system.Similarly,the structure of the coating is composed of a mixture of Ni single crystal with good crystallinity and a small amount of nickel phosphorus alloy with poor crystallinity at different value of p H in the polyvinylpyrrolidone system.The deposition rate of nickel-phosphorus coating is too low,resulting in poor quality of the coating and their thickness is relatively low,so its corrosion resistance and hardness are also low.When the value of pH is 7 and 8,the coating hardness and corrosion resistance is relatively good.