Abstract
Ni-P alloys with phosphorus content ranging from 0 to 16.2wt% were electro-deposited from Watts-type bath containing different amounts of phosphorous acid. The effects of the phosphorus content and the heat-induced structural change of the alloys on acid corrosion resistance were studied by electrochemical measurements and immersion tests in 1kmol/m3 HNO3, 1kmol/m3 HCl and 0.5 kmol/m3 H2SO4 solutions. Surface observation of the alloys after the immersion test was also carried out using an optical microscope. The results are summarized as follows. (1) Corrosion resistance of the deposited alloys immersed in the HNO3 solution increased with an increase in the phosphorus content. On the contrary, corrosion resistance of the alloys in the HCl and H2SO4 solutions decreased with an increase in the phosphorus content up to -5wt%, but increased when the phosphorus content exceeded -5wt%. (2) Passivation of the deposited alloys with phosphorus content of 4.1wt% or above did not occur on anodic polarization in the HNO3 and H2SO4 solutions. (3) Corrosion resistance of the alloys containing 8-12wt% P, which had been heated up to 603K, was found to be excellent when immersed in these three acid solutions. This superior corrosion resistance seems to have some important bearings on heat-induced structural change of the alloys. The surface appearance of these alloys remained bright after being immersed for 120 days in these three acid solutions which were kept at 293K.
