Effect of Thin Electrodeposited Nickel Coatings on the Corrosion Behavior of U-0.75 Ti

Thin electroplated coatings of nickel, formed from a sulfamate bath, were shown to affect the corrosion of U-0.75 Ti in moist air at 105 C and in a dilute (50 ppm) NaCl solution at 25 C. Weight losses were observed for all samples in the salt solution, and the corrosion process followed the linear rate law. The corrosion rate of a sample with a coating thickness of 0.25 mil was actually greater than that of the bare alloy, but an increase in the coating thickness to 0.5 mil or greater caused a marked decrease in the corrosion rate. A porous, yellow corrosion product (hydrated uranium trioxide) existed on the bare alloy and on the 0.25 mil sample, the product being located at pores in the coating on the latter samples. The increased corrosion rate for the 0.25 mil sample was attributed to the fact that the uranium alloy is anodic to nickel, and the current density of the alloy at the bottom of the pores in the coating was much greater than that of the bare alloy. The markedly reduced corrosion rates noted on alloys with thicker coatings was due to a much lower porosity, as measured by an electrochemical "porosimeter", and a much smaller exposed area was available for corrosion. Nickel coatings are not protective in salt solution unless coating porosity is very low. Weight gains, also linear with time, occurred for samples corroded in moist air at 105 C, the corrosion product being UO₂ formed in the pores. Nickel coatings offer excellent protection for corrosion in moist air, the protectiveness increasing with film thickness. The beneficial effect of the thin coatings was attributed to the high level of mechanical adherence of the coatings due to formation of a "root" system of nickel into tunnels in the substrate. The tunnels were produced by deep etching of the uranium alloy surface prior to plating. It was concluded that much thinner coatings than previously reported could be utilized when deeply etched surfaces were employed.