Influence of Si content on the intergranular corrosion of SUS 309L stainless steels
Introduction
SUS 309L stainless steel is a widely used material in different kinds of metal welding, high temperature boiler [1] and chemical process [2]. The mechanical properties of SUS 309L stainless steel such as high temperature strength and corrosion resistance are better than that of SUS 304L stainless steel. Therefore, this material is often used in the fields where tensile strength in high temperature range is demanded.
It has been pointed out that the addition of Si is beneficial to the oxidation resistance of stainless sleel [3], [4] because Si enhances the diffusion rate of Cr and promotes the formation of Cr2O3 and SiO2 [5]. It has also been pointed out that Si is beneficial to the formation of δ-ferrite and can avoid the occurrence of hot tearing of steels during solidification or welding [6]. However, the transformation of δ-ferrite to σ phase during heat treatment or welding [7], [8] is destructive to the mechanical properties and corrosion resistance of stainless steels. Barcik and Brzycka have pointed out that Si is the strongest element to stabilize σ phase [9]. They confirmed the concept of average group number (AGN) to quantify the stabilization tendency of σ phase and concluded that σ phase is most stable if the AGN value is between 5.5 and 7.4. Si accepts the compensation electron of Cr and enhances its stable range and makes the electron-atom ratio, grain size and compression ratio to reach the most stable state. Lin and Chang have also pointed out that Si accelerates the decomposition of δ-ferrite and increases the quantity of σ phase.
In this study, the effect of Si content on the intergranular corrosion of SUS 309L stainless steels was investigated by 10 wt.% oxalic acid test, 65 wt.% boiling nitric acid test and double loop electrochemical potential reactivation (DLEPR) test. Optical microscope (OM) and scanning electron microscope (SEM) were used to observe the structure and the morphology after corrosion test of stainless steels.
Section snippets
Experimental procedure
The SUS 309L stainless steels used in this study were commercial hot rolled rods with 5.5 mm size in diameter. Their chemical compositions are listed in Table 1. These test materials were according to their Si contents designated as 0.17, 0.34 and 0.73Si stainless steel, respectively. The samples were solution treated at 1050 °C for 30 min and quenched in water. These solution treated samples were then aged at 800 °C for 10 min, 1, 10, 50, 100 and 200 h, respectively and then also water quenched.
Results and discussion
Fig. 1, Fig. 2 show the microstructures of samples after 10 wt.% oxalic acid electrolytic etching test that indicates that the grain sizes of 0.17 and 0.34Si stainless steels increase obviously after aging for 1 h and differ from 0.73Si stainless steel. This phenomenon can be resulted from the retarding of grain growth of secondary phase in 0.73Si stainless steel. In the previous study, it has been pointed out that Si accelerates and promotes the formation of σ phase and α′ phase [10].
Fig. 3
Conclusions
The effect of aging on the intergranular corrosion resistance of stainless steels under different Si contents was investigated in this study. The results show that a maximum corrosion rate is obtained for 0.73Si stainless steel of 1 h aging because fine σ phase particles were formed at the δ/γ interface causing the formation of a Cr depletion zone. In microstructures of corroded samples there is obvious grain dropping and ditch structure to be found. After long-term aging, the intergranular
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