. Metallographic etching reagents: I, for copper. t toetch the material in a series of stages, as some of the features arerevealed only after a rather severe attack, which at the same timeobliterates or obscures other features which are best seen after aslight etching. Fig. 2 shows a specimen etched in a series of fourstages with ammonium persulphate, the same area being used ineach case. The same specimen, though not the same spot as inFig. 2, is shown in Fig. 3, after a similar series of etchings in nitricacid. The dissolving of the surface metal occurs in a very definitemanner, and is the i
. Metallographic etching reagents: I, for copper. t toetch the material in a series of stages, as some of the features arerevealed only after a rather severe attack, which at the same timeobliterates or obscures other features which are best seen after aslight etching. Fig. 2 shows a specimen etched in a series of fourstages with ammonium persulphate, the same area being used ineach case. The same specimen, though not the same spot as inFig. 2, is shown in Fig. 3, after a similar series of etchings in nitricacid. The dissolving of the surface metal occurs in a very definitemanner, and is the inverse of the process by which the crystallinestructure was initially built up. This, as well as the fact that thecrystals do not all dissolve at the same rate, is well shown in Fig. difference in the rate of solution is due in all probability tothe orientation of sectioning plane through the internal structure 8B. S. Circular No. 66, Standard Samples for Thermometric Fixed Points, p. 8. L^eS Metallographic Etching Reagents for Copper 645. Fig. 2.—Structure of hot-rolled copper revealed by progressive etching with a 10 percent solution of ammonium persulphate. X500 c and d illustrate good practice for developing etching pits. Etching period: a, i minute; b, i minute,30 seconds; c, 2 minutes, 10 seconds; d, 3 minutes 646 Scientific Papers of the Bureau of Standards [Vol. 16
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