# Effect of Corrosion Characteristics on Long-Term Aging of Austenitic 304 Steel
## 1 Introduction  
Austenitic stainless steels are chrome-nickel alloy steels with excellent corrosion resistance, high strength, and good weldability. This excellent corrosion resistance is due to the chromium content higher than 16% that forms an oxide protective film on the surface. Therefore, they are widely used as essential materials for vessels and pipes in power plants, chemical industries, medical applications, and structural facilities. Even though these austenitic structural materials have excellent mechanical properties and corrosion resistance, they become deteriorated and degraded due to high temperature, high pressure, and long-term use [1; 2; 3]. Although many previous studies have reported material degradation, there is a lack of reporting of long-term material degradation for 10,000 h and at high temperatures of 700 \({}^{\circ}\)C in austenitic steel [4; 5]. When the austenitic stainless steel is exposed to temperatures between 600 and 800 \({}^{\circ}\)C, it is susceptible to sensitization. During sensitization, the Cr-rich carbides generated along grain boundaries and twin boundaries, which caused depletion of Cr atoms in the vicinity of grain boundaries. In most studies, the mechanism of sensitization of stainless steel is about Cr depletion due to the generation of metastable intermetallic precipitation, which makes the Cr amount at the grain boundary fall below 12% [6]. [PERSON] et al. studied the grain boundary sensitization and desensitization during the aging of AISI 316 austenitic stainless steel at temperatures between 550 and 750 \({}^{\circ}\)C. They reported that the Cr contents dropped below 13% and more due to the Cr depletion at grain boundary [7]. [PERSON] et al. studied the intergranular corrosion behavior of AISI 316 stainless steel aged in the range between 550 and 700 \({}^{\circ}\)C. They measured the Cr concentration at the vicinity of grain boundary during aging at each temperature. They also reported that the Cr concentration decreased below 12% at grain boundary [8]. The \(\delta\)-ferrite, which is the body centered cubic (BCC) crystal structure, is formed during solidification and remains along grain boundaries at room temperature in stainless steel. It successfully acts as a strong barrier to the dislocation motion, slows grain growth, and enhances mechanical strength because the interphase boundaries act as strong barriers to the dislocation motion [9]. In addition, this \(\delta\)-ferrite can transform into the \(\sigma\)-phase in the austenite through heat treatment, in which the \(\sigma\)-phase is an intermetallic compound with a tetragonal crystal structure. The chemical composition of this \(\sigma\)-phase varies considerably. Therefore, it is difficult to define this phase in the form of unique formulas. The precipitation rate of the \(\sigma\)-phase from the \(\delta\)-ferrite is about 100 times more rapid than directly from austenite due to the Cr content and diffusion [10]. The \(\sigma\)-phase has a negative effect on the mechanical properties, such as toughness and ductility because it is hard, brittle and nonmagnetic. In general, the presence of the \(\delta\)-ferrite decreases the incubation period of the generation of the \(\sigma\)-phase. Therefore, the small fraction of \(\delta\)-ferrite, less than 1%, has been recommended in order to avoid hot cracking and brittle \(\sigma\)-phase resulting from the welding of austenitic stainless steel. It is therefore of considerable importance to identify the decomposition of ferrite and austenite during the long-term aging process and to quantify the associated chromium depletion in order to understand their specific effects on the corrosion behavior. In fact, the effect of \(\sigma\)-phase and \(\delta\)-ferrite on corrosion resistance is not clear and is not studied enough. The objective of this study is to investigate the influence of long-term aging on electrochemical corrosion characteristics of AISI 304 steel.