Development of laser Enhanced Electrochemical Microscopy

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For evaluation of localized corrosion susceptibility, metallography for structure or composition, and laser etching/plating, a Scanning Laser Enhanced Electrochemical Microscopy (SLEEM) has been developed. Solid state semiconductor laser of AlGaAs which is small and easily controlled was used as a scanning laser source. In 1990, a 40 mW output power type laser was used and later 80 mW type was introduced in 1991. Laser out put was Chopped by bias voltage into 10 Hz to 100 Hz square wave and irradiated to the metal specimen which is kept at a constant potential in the electrolytic solution. Change in electrolytic current by laser irradiation is detected by a lock in amplifier and make a scanning image by using microcomputer. Current change induced by laser irradiation at the constant potential is found to be proportional to the electrolytic current without irradiation. Decrease in frequency of irradiation induces the current increase because of temperature increase at the metal surface as a theoretical model predicts. Characteristic increase in current for Fe, Cr and Ni which are constituents elements of stainfess steels was observed at a specified potential region for each element, so that current change of stainless steel by irradiation at the constant potential depends on its composition. Resolving power of the SLEEM is the same size of about 10 mum as the spot size of the laser beam. Based on the above analysis, the chromium depleted grain boundary image of sensitized stainless steel could be obtained succesfully at the potential that is active for Fe but passive for Cr. At the trans passive potential, the compositional image of Cr in two phase stainless steel could be obtained. Fine image etching can be made at the active and transpassive potential.