Correlation between microstructure and intergranular corrosion behavior of low delta-ferrite content AISI 316L aged in the range 550e700 C

Abstract

The microstructure and the phase identification of austenitic stainless steel AISI 316L with low δ-ferrite content (δ ≤ 1%) and aged for up to 80 000 h at temperatures ranging from 550 to 700 °C were investigated by using an optical microscope (OM), a scanning electron microscope (SEM) and a transmission electron microscope (TEM). Local changes of chromium content, resulting from nucleation and growth of chromium-rich phases during aging, were quantitatively assessed by energy dispersive X-ray spectroscopy (EDX) in the scanning transmission electron microscope (STEM). The intergranular corrosion behavior (IGC) of annealed and aged specimens was evaluated using the double loop electrochemical potentiokinetic reactivation (DL-EPR) and completed by IGC morphologies according to the ASTM A262 practice A standard. The results showed that δ-ferrite decomposed gradually into M23C6 at 550 °C and decomposed totally into intermetallic phases (σ, η, χ, and R) and into secondary austenite (γr) at temperatures equal to or higher than 650 °C. Similarly γ-austenite decomposed into M23C6 carbide at 550 °C and into intermetallic phases such as η and σ in addition to carbide, at higher temperatures. The time-temperature-sensitization diagram (TTS) was established and used to calculate the critical cooling rate (CCR) that prevents IGC sensitization. The analysis of IGC results leads to the conclusion that sensitization-desensitization is still controlled by the characteristics of chromium-depleted area surrounding austenite grain boundary regions. No significant effect of remained δ-ferrite and derived components on the corrosion behavior of AISI 316 L containing 1% of δ-ferrite.