IMPROVEMENTS IN OR RELATING TO THE REMOVAL OF SCALE FROM METAL OBJECTS

1,203,980. Electrolytic pickling. DYNAMIT NOBEL A. G. Dec. 6, 1967 [Dec.10, 1966], No.55495/67. Heading C7B. Scale is removed from a scaled metal object by pickling the object in an alkali metal hydroxide melt (e.g. NaOH and/or KOH) containing sodium hydride, and thereafter subjecting the pickled object to anodic and subsequent cathodic treatment in an aqueous alkaline solution or a melt containing at least 60% by weight, based on the alkaline solution or melt of an alkali metal hydroxide or mixture of alkali metal hydroxides (e.g. NaOH and or KOH) at a temperature which is greater than, but not more than 25 centigrade degrees greater than, the solidification point of the alkaline solution or melt, whereby the scale is completely or substantially completely removed. The "solidification point is the temperature at which, when the alkaline solution or melt is being cooled, solid material is first precipitated out. The anodic and cathodic treatments may employ currents of 1000 to 1500 amps for 5 to 20 minutes. After descaling, an object may be washed in cold water and dipped in an aqueous solution of calcium hydroxide. The Fig. illustrates descaling of a coil of wire 4, suspended from an electrode 3 of one polarity, and adjacent electrodes 5, 6 of opposite polarity. Half way through each of the anodic and cathodic treatments the coil of wire 4 is rotated so that a different portion is in contact with the part 3.