| There is a wide study at direct electroplating palladium membrane on the surface of stainless steel; due to palladium membrane could facilitate the passivation of stainless steel which improved the corrosion resistance in reducing acid at high temperature. But the palladium membrane made by direct electroplating show poorer physical properties, such as microhardness, porosity and adhesion. Fortunately, pulse plating could ameliorate the physical properties of films effectively.This paper obtained the process parameters of pulse electroplating palladium membrane on the surface of stainless steel by orthogonal:frequency100Hz; peak current0.05-1A; duty cycle0.1-0.9; on-time1-9ms; off-time1-9ms; plating temperature35-55℃; plating time3-10min. There is a exhaustive research about duty cycle, peak current, plating temperature and plating time effect on mirohardness, porosity and adhesion of pulse plating samples, as well, analysis the surface structure and grain size of palladium membrane with different pulse process parameters by X-ray diffraction and scanning electron microscopy. The results show when other conditions were same, microhardness, porosity and adhesion reduced as either duty cycle or peak current increased; but the grain size increased gradually. The higher or lower temperature had a negatively effect on the physical properties of palladium membrane, which could obtained the best microhardness, the lowest porosity, the greatest adhesion and the smallest grain size when pulse plating temperature is45℃. The pulse plating time mainly influenced the thickness of palladium membrane, which means increased the thickness of palladium membrane could improved the physical properties appropriately.The corrosion behavior of pulse electroplating samples with different pulse process parameters in25℃0.1mol·L-1chloride and sulfuric acid solution with chloride at high temperature had been discussed. The pulse electroplating samples with lower duty cycle showed significant passivation phenomenon in25℃0.1mol·L-1chloride solution; the corrosion resistance of pulse electroplating samples declined accompanied by an increase in the duty cycle at the sulfuric acid solution with chloride at high temperature,which mainly attribute the increase of open circuit potential. At the same time, the effect of passivation reduced gradually in25℃0.1mol·L-1chloride solution when the peak current increased, the plating temperature were much higher or lower, and the short plating time; owing to palladium membrane was prepared by pulse plating were more compacter, smaller porosity and smaller grain size, so that facilitated the passivation of stainless steel. By analyzed the fitting data of EIS for the samples in25℃0.1mol·L-1chloride and sulfuric acid solution with chloride at high temperature, the conclusion that palladium membranes own smaller porosity and smaller grain size could suppress the penetration of chloride in the corrosion solution could reached.The microstructure of palladium membrane with direct planting and pulse plating had been researched by X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy, results showed that composition and crystal structure of palladium membrane with direct planting and pulse plating were the same; but palladium membrane with pulse plating own smaller grain size and more uniform surface. Since membrane with smaller grain size, the samples with pulse plating showed higher microhardness, better adhesion and smaller porosity. The results obtained by weight loss tests and electrochemical polarization in20%sulfuric solution at80℃and20% H2SO4+0.001molCl-solution at80℃show that the pulse electroplated samples showed slightly better corrosion resistance than electroplated samples, which may be attributed to decreasing the grain size and decreasing in porosity may reduce chloride migration through the deposited film, which leads to decreasing the poisoning to the film by hydrogen. |
PDF Full Text Request