Original ArticlesEffect of Ce addition on the sensitization properties of stainless steels
Introduction
Austenitic stainless steels (SS) are the most widely used structural materials in applications which require both high strength and excellent corrosion resistance in aggressive environment. However, these stainless steels are not immune to localized corrosion. Sensitization induced by precipitation of chromium carbides and the resultant chromium depletion at the grain boundaries is the basic cause of their susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC).
Rare earth elements are characterized by significant negative free energy changes for compound formations, such as carbides, sulfides and oxides. It has been reported that the effect of the addition of cerium to the steels, on their corrosion properties, is to form a thermodynamically stable surface film composed of cerium oxide. This reduces the cathodic and anodic reactivity by blocking the reactive surface sites 1, 2 and improves the resistance to pitting (3) and crevice corrosion 1, 2. The addition of cerium is also reported to have improved 4, 5 the dry oxidation resistance of SS. It has been attributed to the larger atomic size of the rare earth elements with respect to that of iron. The vacancies in the alloy move to the stressed regions adjacent to the rare earth oxides at the metal/oxide interface where they serve as nucleation sites for chromium oxide and also affect the diffusion rate of chromium. These facts indicate that the addition of cerium to the steel affects the surface film properties and improves its resistance to localized corrosion. The effect of the cerium addition on the microstructure of SS has not been reported. In this study, a series of type 316 SS (UNS S31600) containing various amounts of cerium was prepared and the effects of the cerium addition on the sensitization properties were investigated.
Section snippets
Materials and experimental procedure
In order to study the cerium-carbon interaction effects, a type 316 SS containing relatively high content of carbon (about 0.06 wt. %) and very low levels of impurities was chosen as a base heat. Three cerium-modified alloys were prepared from this base heat with the concentration of cerium varying from 0.01 wt. % to 0.09 wt %. The designation and chemical composition of these heats are listed in Table 1. These materials were solution treated (1120°C/1h, water quenched) and then sensitized at
Results
The grain sizes measured for the four heats (containing various levels of cerium) are shown in Table 1. The reactivation ratios measured by the DL-EPR were normalized with regard to the grain size of Base Heat. The following equation (8) was used for this purpose: where Rm is the EPR ratio measured for a material with the ASTM grain size number m and Rn is the EPR ratio corrected for the ASTM grain size number n. These normalized ratios are illustrated in Figure 1 along with the
Carbide distribution
It is seen from the microstructures (Figure 3 and Figure 4) that the steels with 0.04 wt. % cerium or higher contain grain boundary carbides only, whereas the heats with no cerium or 0.01 wt. % cerium have intragranular carbides also. This difference in carbide distribution may be related to the difference in the atomic radius of the alloying elements. Cerium has an atomic radius of 1.83 A° as compared to the atomic radius of 1.24 A° of iron and 1.25 A° of both chromium and nickel. The presence
Conclusions
1. The material with 0.01 wt. % Ce showed lowest reactivation ratio for a given heat treatment in the initial and intermediate stages of sensitization.
2. Cerium addition influenced the precipitation sites of carbides and decreased the sensitization kinetics including the recovery stage.
3. Both the sensitization and the low temperature sensitization behaviors were similar for the cerium modified materials, indicating a beneficial effect for steels containing 0.01 wt. % cerium. The step which is
Acknowledgements
The authors acknowledge the financial support of Yazaki Memorial Foundation for Science and Technology. A part of this work was also supported by the ministry of Education, Science, Sport and Culture under Grant in aid (No. 10875131) for exploratory research.
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