Surface modification of Ni/Ni₃Al two-phase foils by electrochemically selective etching

Abstract

Surface morphology after the selective etching of the γ matrix was examined in Ni(γ)/Ni₃Al(γ´) two-phase foils with various microstructures controlled by the 98 % cold rolling and subsequent heat treatment at 873, 1073 and 1273 K for 0.5 h. In the cold-rolled state, the elongated pancakeshape γ´ precipitates were distributed in the γ matrix, and this structure was almost the same after the heat treatment at 873 K though the recrystallization partly started. These γ´ pancakes were partitioned into the fine blocky particles of 10~100 nm in the edge after the heat treatment at 1073 K and the γ´ blocks significantly became larger at 1273 K. These foils were electrochemically etched in the electrolyte of distilled water with 1 wt.% (NH₄)₂SO₄ and 1 wt.% citric acid at a constant potential of 1.75 V for 5 h. In this etching, the γ matrix was selectively dissolved and the γ´ precipitates were left behind, yielding rough and irregular surface. The surface morphologies corresponded to the γ/γ´ two-phase structures, thus demonstrating that the two-phase structure can be used as a template to make the surface area larger. The foil heattreated at 1073 K had a number of the fine γ´ particles with 10~100 nm in size densely dispersed on the surface. Such fine surface structure was expected to improve the catalytic activity of Ni₃Al for the hydrogen production reaction.