Etch Pit Investigation of Iron Whiskers

Abstract

Abstract : The dissolution behavior and the formation of etch-pits in iron whiskers were studied in the following solutions: 2% nitric acid in alcohol, 4% picric acid in alcohol, a solution of hydro chloric acid in ethanol containing cuprous chloride, a solution of hydrofluoric acid in water containing ferric-fluoride, copper-ammo niacal chloride solutions both by immersion and electrolytically, and one normal sulfuric acid solution containing 0.2 normal potassium persul fate. Although good etch pits were produced using nital and picral etchants in some cases, the results were not at all satisfactory with respect to reproducibility. The halide-contain ing etchants usually developed geometric pits, most probably due to the poisoning of dissolution ledges on the iron whisker faces. Most whiskers were composed of a distinct shell or overgrowth surrounding the core. The shell ranged in thick ness from less than one micron up to several microns and this shell thickness appeared to be a critical factor in the development of distinct etch-pits. Etch-pits present in the shell were usually smaller and better defined than in the core. Dissolution rates were determined using the HF-H2O-FeF3 system and the influence of such variables as crystallographic index of the sub strate surface, degree of under- saturation of the dissolving species, and poison concentration were studied.