Microstructural and electrochemical characterization of the low temperature sensitization of AISI 321 stainless steel tube used in petroleum refining plants

Abstract

This work describes the behavior of ASTM A312 TP321 tubes heat treated at 600°C for periods of 1, 3, 10, 30, 40, 50, 60, 70, 80, 90, and 100 hours. The degree of sensitization that had occurred was assessed by Scanning Electron Microscopy (SEM) and by the Double Loop Electrochemical Potentiodynamic Reactivation test (DLEPR). The results showed that exposure at 600°C for 80 hours or longer caused severe precipitation of chromium carbides along the grain boundaries characterizing the sensitization of the material despite it being titanium stabilized. In order to minimize the sensitization process, solution annealing in the temperature range of 900–1100°C was studied. Solution annealing at 900°C was the best heat treatment in order to prevent sensitization at 600°C. The solution annealing at 900°C was also effective for samples from tubes that were sensitized after one year of operation in a refining plant.