Effect of Delta Ferrite on Corrosion Resistance of Type 316L Stainless Steel in Acidic Chloride Solution by Micro-Droplet Cell
More than 2% δ-ferrite phase is presented in type 316L stainless steels due to improvement of hot workability or welding process. In order to clarify the mechanism on how the δ-ferrite affects the corrosion resistance, electrochemical tests in a micro-droplet cell were conducted and microstructures were analyzed by TEM-EDS and SEM-EDS. Especially, the micro-droplet cell was modified at our laboratory to observe the polarization behavior of δ-ferrite phase in an acidic chloride solution. The anodic polarization behavior in both active zone and passive zone was affected by δ-ferrite. The degradation of corrosion resistance in the low potential zone with δ-ferrite phase is due to the preferential dissolution of a matrix, γ phase. It is explained with the fact that the formation of δ-ferrite lowers the concentrations of chromium and molybdenum in γ phase. It is also confirmed by potentiostatic test, EDS analyses and phase-equilibrium prediction. In addition, the micro-droplet cell and EDS analysis reveled that the pitting potential in the neutral pH solution was affected by Ca containing inclusion rather than δ-ferrite phase.
Byung Tae Ahn, Hyeongtag Jeon, Bo Young Hur, Kibae Kim and Jong Wan Park
S. Y. Kim et al., "Effect of Delta Ferrite on Corrosion Resistance of Type 316L Stainless Steel in Acidic Chloride Solution by Micro-Droplet Cell", Solid State Phenomena, Vols. 124-126, pp. 1533-1536, 2007