Abstract: An upright cylindrical vessel is formed with a plurality of fittings at its upper end to receive a thermometer, a stirrer, a reflux condenser, means for evacuating or pressurizing the vessel or the like, and is open at its other end which is provided with a circular bead. A pair of clamping rings, one of which engages behind the bead, are bridged by clamping screws to secure a disk of the workpiece material to the open end of the vessel. The corrosive medium is introduced into the vessel and attacks the workpiece surface exposed therein.