Effects Of Rare Earths And Conversion Coating On Electroless Ni-Cu-P Coating

As the global resources depletion and environmental emission problem is pressing, it is a inevitable trend of the development of human society to set up resource-saving and environmental-friendly society. As a kind of high efficient and clean energy, the propotion of natural gas in energy consumption structure will rapid rise. The new-style gas condensing heat exchanger is a kind of energy-saving and environmental-friendly equipment for high efficient use of natural gas. However the acid condensate can cause corrosion demage in the surface of heat exchanger, solving the corrosion problem is one of the key technologies for its research and development.Firstly, the rare earth Ce4+was added in electroless plating solution. The effects of Ce4+concentration on the deposition rate, surface morphology, composition, crystal structure and corrosion resistance of Ni-Cu-P coatings were investigated using microscopy, energy dispersive X-ray spectroscopy (EDX), inductively coupled plasma atomic emission spectrometry (ICP-AES), X-ray diffraction spectrometer (XRD), polarization curve (PC), electrochemical impedance spectroscopy (EIS) and immersion corrosion experiment. The study found that the deposition rate decreased with the increasing concentration of Ce4+. Ce was not detected and Cu fraction increased in the Ni-Cu-P coatings when Ce4+was added into the bath. Meanwhile Ce4+inhibited the growth rate of crystallites and promoted the amorphous degree of Ni-Cu-P coating, improving the corrosion resistance of coating. The Ce4+concentration remarkably influenced the surface morphology. When the added Ce4+concentration is below8mg/L, with the increase of Ce4+ions concentration, Ce4+ions tend to refine the average grain size, resulting in the deposition of uniform, smooth and corrosion resistant Ni-Cu-P coatings. Contrarily, the homogeneity of Ni-Cu-P coatings are degraded by the excessive addition of Ce4+ions into the bath, resulting in decrease of the corrosion resistance of Ni-Cu-P coating.Besides, the preparation and properties of rare earth conversion coating on the surface of Ni-Cu-P coating were studied in the paper. The nitrate cerium conversion coating with the best corrosion resitance was selected from three kinds of cerium salts (cerium nitrate, cerium sulfate and cerium chloride). The optimum techniques parameters were obtained by orthogonal experiment as follows:Ce(NO3)3-6H2O20g/L, H2O225mL/L, H3BO32.5g/L, pH3-4, immersing60min at25℃.There were many microcracks and particales of rare earth over the surface of the nitrate cerium conversion coating with golden yellow color. The convension coating was composed of cerium oxide or hydroxide. The complex conversion coating was prepared from the convension solution with the addition of BTA. There were few microcracks over it and the complex conversion coating was denser than that of the single nitrate cerium conversion coating. The electrochemical tests showed the corrosion tendency of the complex coating was low, because its corrosion potential (-0.018V) was higher than that of the single coating (-0.152V). The electrochemical reaction resistance of the complex coating was highest and the corrosion current density was the lowest value of0.66μA/cm2. It was reduced nearly93%as compared to that of Ni-Cu-P coatings and nearly50%as compared to that of the single coating. The complex coating got the best corrosion resistance.