The Effect of CuCl2 on Pitting Corrosion of the Extra High Purity Type 316 Stainless Steel

Abstract

The effects of Cu addition on corrosion behavior of stainless steels are known to be controversy1,2, which depends on the corrosion environment or the amount of alloying Cu. In chloride environment, alloying Cu is known to prohibit the growth of pitting corrosion. The dissolved Cu2+ acted as an inhibiter inside the pits3. On the other hand, there are few studies about the effect of Cu2+ ions in the solution on the pitting corrosion of the stainless steels. Since the commercially made stainless steels commonly contains more than 0.1 mass% Cu, the Cu2+ ions which was dissolved from stainless steels made it difficult to evaluate the effect of Cu2+ ions on the pitting behavior of the stainless steels. Although the amount of dissolved Cu2+ was quite few, the Cu2+ concentration inside the pits was expected to be much higher than the solution bulk because the pitting corrosion was very localized phenomenon. Therefore, Cu2+ in the solution and dissolved Cu2+ should be analyzed separately. To clarify the effect of Cu2+ in the solution on the pitting corrosion of the stainless steels, the amount of alloying Cu is needed to be low enough not to affect the corrosion morphology. In this study, extra high purity Type 316 stainless steel (316EHP) was used as specimen. The Cu content of the specimen was 6.8 × 10-4 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 µm 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 CuCl2. 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 × 10-4 V s-1 (20 mV min-1). 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 Cr-oxide. The anodic polarization curves were measured in deaerated 0.1 M NaCl or 0.098 M NaCl-1 mM CuCl2. Although no pitting was observed in 0.1 M NaCl, pitting was occurred in 0.098 M NaCl-1 mM CuCl2. 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 Cu2+ in the solution. Cu2+ ions were thought to act as an accelerator of pitting corrosion. To analyze the effect of Cu2+ ions on the forming passivation film, potentiostatic polarization measurements at 0.4 V were carried out in 0.098 M NaCl-1 mM CuCl2. 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.