Corrosion properties of nanocrystalline nickel and cobalt electrodeposits.

摘要

Nanocrystalline electrodeposits show tremendous improvements in many physical and mechanical properties compared to their conventional polycrystalline counterparts. While they show great potential as wear resistant coatings, one particular concern for this application is their intrinsic resistance to corrosive environments. In this thesis, the corrosion behaviour of nanocrystalline Ni and Co, produced by electrodeposition, was investigated in solutions with various pH values. Using potentiodynamic testing, it was shown that the passive region for the nanocrystalline Ni electrodeposits was greatly reduced compared to conventional polycrystalline Ni200 in sulfate solutions with pH 9.8. Characterization of the surface composition with EDS and XPS following the potentiodynamic testing, suggested that sulfur from the enhanced bulk concentration of 0.1wt.% in the nanocrystalline Ni accumulated during the metal dissolution, and subsequently formed corrosion products rich in sulfate, preventing the formation of protective surface films. In the case of cobalt, comparable active dissolution rates in the absence of passive films in 0.25 M Na2SO 4, and nearly identical potential-current density curves in a passivating solution of 0.1 M NaOH were observed for both poly and nanocrystalline Co. This indicates that the structurally disturbed interfacial regions in the nanocrystals do not diminish the corrosion performance of cobalt.; An attempt was also made to investigate the effect of grain size reduction to the nanometer range, on the localized corrosion behaviour in nickel and cobalt by using annealed nanocrystalline samples. While 0.04∼0.1wt.% sulfur induced considerable intergranular corrosion in polycrystalline Ni and Co, the same amounts of sulfur resulted in uniform corrosion in 0.25 M Na 2SO4 for the nanocrystalline counterparts. The large interfacial volume fractions associated with nanocrystalline materials allow for more uniform distribution of impurities, minimizing galvanically different solute segregation and precipitation along grain boundaries. This provides an enormous benefit for enhanced resistance to localized corrosion, even in the presence of the harmful impurity element, sulfur.