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

Surface morphology after the selective etching of the gamma matrix was examined in Ni(gamma)/Ni(3)Al(gamma') 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 pancake-shape gamma' precipitates were distributed in the gamma matrix, and this structure was almost the same after the heat treatment at 873 K though the recrystallization partly started. These gamma' pancakes were partitioned into the fine blocky particles of 10 similar to 100 nm in the edge after the heat treatment at 1073 K and the gamma' blocks significantly became larger at 1273 K. These foils were electrochemically etched in the electrolyte of distilled water with 1 wt.% (NH(4))(2)SO(4) and 1 wt.% citric acid at a constant potential of 1.75 V for 5 h. In this etching, the gamma matrix was selectively dissolved and the gamma' precipitates were left behind, yielding rough and irregular surface. The surface morphologies corresponded to the gamma/gamma' two-phase structures, thus demonstrating that the two-phase structure can be used as a template to make the surface area larger. The foil heat-treated at 1073 K had a number of the fine gamma' particles with 10 similar to 100 nm in size densely dispersed on the surface. Such fine surface structure was expected to improve the catalytic activity of Ni(3)Al for the hydrogen production reaction.