抄録
The sputter etching of SUS304 stainless steel specimens was carried out, and the formation and growth processes of conical protrusions was investigated. In addition, the polarization behavior of the specimens in 3.5%NaCl aqueous solution and the surface morphology after the polarization test were examined. At an early stage of the sputter etching, the conical protrusions form mainly along grain boundaries. However, with increase in the sputter etching time the protrusions form also within the grains. With further increase in the sputter etching time, some protrusions grow largely and the entire surface is covered by the huge conical protrusions and the irregular small ones around them. The outer layer of the huge protrusion contains larger chromium but smaller nickel amount than matrix, and the interior has almost the same composition as the matrix. This means that the protrusion has a shell structure. For the specimen with short time sputter etching, the corrosion potential becomes nobler and the pitting corrosion potential becomes smaller, while the passive current density is smaller than those of non-sputter-etched specimen. In these specimens, the corrosion occurs preferentially along grain boundaries and around protrusions in grains. For the specimen with long time sputter etching, where the entire surface is covered by protrusions, the pitting corrosion potential increases and the corrosion progresses mainly from the interior of the specimen leaving the surface layer with many protrusions. In some cases, only the outer layer of the shell remains separately as a result of the corrosion of the interior and root of the huge protrusion.
