Research On Electroless High-P Ni-P Coating And High/Low-P Duplex Ni-P Coating

Acidic electroless plating of high phosphorus Ni-P coating is meticulous and enjoys lowporosity.It also has excellent corrosion resistance since boundary corrosion tendency is small,which is due to the amorphous structure. It has been widely used in many applications due toits good excellent corrosion, wear and abrasion resistance, including petroleum, chemical andaerospace fields. Studies have shown that the dual complex system of high phosphorus Ni-Pelectroless bath has a better performance, but there is still lack of systematic study andoptimization. Duplex Ni-P coating can strengthen the corrosion resistance relying on potentialdifference between two single layers. There is still a lack of research on high/low phosphorusduplex Ni-P alloy plating. Ni-P alloy phosphorus content is the result of various processparameters, accurate prediction of the phosphorus content will benefit the industrialapplications.The paper is focus on researching the effect of main complexing agent sodium citrateconcentration, auxiliary complexing agent categories and auxiliary complexing agentconcentration on Ni-P alloy.The stability of bath is characterizated by chloride palladiumwhich will make the solution unstable. The sticker method and gravimetric method are used tomeasure the porosity of Ni-P alloy and the plating rate. The morphology of Ni-P alloy isobserved by scanning electron microscopy (SEM) and metallurgical microscope. Thephosphorus content of Ni-P alloy is measured by EDS. The corrosion resistance of the Ni-Palloy coating is analysised by electrochemical method and immersion test. After a systematicstudy on the process parameters of Ni-P plating, the best parameter for single layer of highphosphorus Ni-P alloy is gained. To further enhance the Ni-P alloy properties, highphosphorus/low phosphorus double layers of Ni-P Alloy is researched. Through theexperiment, the phosphorus content of low-phosphorus layer and thickness ratio H/L ofduplex Ni-P alloy are investigated.Since electroless plating Ni-P involving many processparameters, the phosphorus content of coating is multi-factor synergy results for differentprocess conditions. In order to predict the Ni-P alloy phosphorus content of coating effectively,a three-layer BP neural network model is established in this study innovatively.The model istested after learning on25data.The results show:(1) The best parametre of high-phosphorus Ni-P includes pH=5.0, T=85℃,0.10mol/Lnickel sulfate,0.30mol/L sodium hypophosphite,0.10mol/L sodium citrate,0.025mol/Lmalicacid,0.12mol/L sodium acetate,0.02g/L potassium iodate. The plating rate is7.94μm/h, the phosphorus content of12.65wt%and the porosity is0.125per cm~2, corrosion potential is-0.318V in3.5wt%NaCl solution and after15day soaking with a loss of only4.4mg.(2) Low-phosphorus Ni-P with3.5wt%phosphorus will lower the porosity of Ni-P bydeposited on the surface of high phosphorus Ni-P. In addition, due to a sufficient potential gapbetween two Ni-P, the corrosion resistance of duplex Ni-P alloy is enhanced. When thethickness ratio H/L=0.88, the coating corrosion resistance can be the best, the corrosionpotential is-0.233V and the lost of quantity is only2.4mg after15days soaking.(3) It is feasible to use BP network to predict the phosphorus content of acidic highphosphorus Ni-P alloy.The electroless plating process parameters have good convergence andmeet the prediction accuracy requirements.