Effect of Annealing Time on Pitting, Intergranular Corrosion, and Hardness of Stainless Steels

Stainless steels are the material that has chromium as a main component. The chromium content reacts with oxygen in air, subsequently, forms thin chromium oxide film on the surface of stainless steels. Thus, when these steels are exposed to high temperature for a long period of time in many applications, chromium carbide could precipitate along the grain boundary and reduce the corrosion resistance. This project is conducted to study annealing time effect on stainless steels when exposed to high temperature at various exposure time periods. Three different kind of stainless steels, namely, AISI 304, AISI 304D and AISI 2205 were used in this study. Stainless steels were heated at 600°C for 0, 6, 24, 48 and 96 hours, then cool down in air. Consequently, the investigations were performed by using double-loop Electrochemical Potentiokinetic Reactivation (DL-EPR) and Cyclic Potentiodynamic Polarization (CPP) to study degree of sensitization and film properties. In addition, chromic acid and oxalic acid were used as reagent of acid etching to observe microstructures. Finally, Vickers hardness test were also conducted. Percentage degree of sensitization increased from 2.93% to 62.20% in AISI 304, increased from 5.26% to 55.54% in AISI 304D and from 12.19% to 69.35% in AISI 2205. The pitting potential decreased from 0.47 mV to 0.23 mV for AISI 304 but remained relatively constant for AISI 304D and AISI 2205. The results indicated that after the specimens were exposed to high temperature for a long period of time, all specimens had more chromium depleted areas, more carbide along the grain boundaries, worse film quality and small changes in hardness value.