Naoya Ito and Go Okamoto
The effect of potential on the etching figure was studied for 18-12Cr-Ni-Mo-Si duplex cast steel. The specimen was heat-treated either by quench-annealing or by sigmatizing and it was etched for an appropriate length of time in 1 N H2SO4 containing 0.01% NH4SCN under constant potentials controlled by a potentiostat. It was found that a certain kind of phase can be selectively etched according to the applied potentials with appreciable rate in both active and overpassive potential ranges. In the active potential range, the optimum potential for etching shifted to the less noble direction with the increase of Cr content of structural phase. Thus, differential etching for δ,γ' and γ can be performed at the potential range of -0.40∼-0.30, -0.30∼-0.21 and -0.25∼-0.21 V (v.s.S.C.E), respectively. The σ phase was not attacked but appeared in relief brightly in the active state. In the overpassive region, the etching rate increased with the increase of Cr content of phase. The σ phase can be well differentiated at the potential region of the so-called 2nd passivity which appeared above ca.1.4 V because of the remarkable difference between the rate of attack of this σ phase and that of the other phases.
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