A Methodology for Fabrication of Highly Pitting Corrosion-Resistant Type 304 Stainless Steel by Plasma Carburizing and Post-Pickling Treatment

Abstract

Low temperature plasma carburizing was performed to improve the pitting corrosion resistance of Type 304 stainless steel. The optimal composition of the plasma gas and post-treatment were then examined. Carburizing was carried out for 18 ks at 470°C in H2-CH4, Ar-H2-CH4 or Ar-CH4 plasma, and carburized layers ca. 9 μm in thickness with an average ca. 1.3 mass% carbon concentration were obtained. However, it was found that Cr23C6 was precipitated in the carburized layer and intergranular corrosion occurred under anodic polarization in 0.1 M NaCl. The precipitation behavior of Cr23C6 differed according to the plasma gas composition: H2 gas was found to suppress the size of Cr23C6. No Cr23C6 was precipitated in the inner part of the carburized layer formed in the H2-CH4 plasma, and no stable pit was initiated on the inner parts of the carburized layer under anodic polarization in 0.1 M NaCl. Pickling treatment using HF-HNO3 was applied to remove the Cr-depleted zones in the outer part of the plasma carburized layer. The Type 304 stainless steel after the plasma carburizing in the H2-CH4 atmosphere and subsequent the HF-HNO3 pickling exhibited no pitting corrosion under anodic polarization even in 3 M NaCl at 25°C.