The Effect of CuCl on pitting corrosion of the extra high purity type 316 stainless steel

Aoyama, Takahito   ; Ogawa, Hiroaki  ; Ueno, Fumiyoshi  

In this study, extra high purity Type 316 stainless steel (316EHP) was used as specimen. The Cu content of the specimen was 6.8 10 mass %. The specimens were heat-treated at 1373 K for 3.6 ks and then quenched in water. The surfaces of the specimens were polished down to 1 m with a diamond paste. After that, specimens were ultrasonically cleaned in ethanol. Electrochemical measurements were conducted in deaerated 0.1 M NaCl or 0.098 M NaCl-1 mM CuCl. Except the electrode area (ca. 10 10 mm), the surfaces of the specimens were insulated by an epoxy resin. The potential scan rate was 3.33 10 V s (20 mV min). The reference electrode was an Ag/AgCl (Sat. KCl) electrode. All the potentials cited in this work refer to the Ag/AgCl (Sat. KCl) electrode. After the electrochemical measurements, the surfaces were analyzed by SEM/EDS and XPS. The inclusions of the specimen were characterized by SEM/EDS. The inclusions were identified as a chromium oxide. The anodic polarization curves were measured in deaerated 0.1 M NaCl or 0.098 M NaCl-1 mM CuCl. Although no pitting was observed in 0.1 M NaCl, pitting was occurred at 0.67 V in 0.098 M NaCl-1 mM CuCl. In both solutions, the dissolution of inclusions was not confirmed. Therefore, the inclusions were not thought to be the initiation site of pitting. This indicates that the pitting potential of 316EHP was decreased by Cu in the solution. Cu ions were thought to act as an accelerator of pitting corrosion. To analyze the effect of Cu ions on the forming passivation film, potentiostatic polarization measurements at 0.4 V were carried out in 0.098 M NaCl-1 mM CuCl. As a result, existence of a Cu signal on the surface was confirmed by XPS although it was not detected by SEM/EDS. This result indicatets that the few amounts of Cu compound was deposited on the surface in passive region of the 316 EHP.