Effect of grain refinement on corrosion of ferritic–martensitic steels in supercritical water environment

Abstract

The effects of grain refinement on the corrosion behavior of three ferritic-martensitic (F/M) steels, HT9, T91, and NF616, and two binary model alloys Fe-15%Cr and Fe-18%Cr in supercritical water (SCW) have been investigated. Grain refinement down to a size of about one micron in the surface regions, was achieved by introducing severe plastic deformation by shot peening. After exposure to SCW with 25 ppb oxygen at 500 8C for up to 3000 h, an improvement in corrosion resistance was observed in grain-refined samples because of the enhanced diffusion of chromium on the surface, through a high density of grain boundaries. The chromium content in the steels and the exposure durations in SCW were determined to be important factors influencing the efficacy of the grain refinement effects. These results are supported by both experimental evidence and theoretical predictions. Another approach for grain refinement, equal channel angular pressing (ECAP), was also investigated for T91 steel. ECAP resulted in lower weight gain due to corrosion compared to the untreated samples, but exhibited a slightly higher weight gain compared to the shot-peened samples after long-term exposures in SCW which is probably caused by different fractions of high-angle grain boundaries in grain-refined regions, introduced by different grain refinement techniques.

Oxidation behavior of an 11Cr ferritic–martensitic steel after Ar-ions irradiation in supercritical water

“…Meanwhile, the influence of shot-peening on the oxidation of alloy 800H in SCW was investigated by Tan et al [17], indicating that shot-peening was an effective approach not only for improving oxidation resistance, but also for mitigating oxide exfoliation. These results are consistent with the investigations on the corrosion behavior of F-M steels in SCW through the method of ultrasonic impact peening and shot-peening [18,19]. Besides, many other surface modification methods have been applied to different materials in order to improve their corrosion resistance [23,24], but the corrosion conditions acted on these materials are quite different from the SCW environment in the present experiment.…”

“…Meanwhile, some new methods were used to improve the corrosion resistance of different steels in SCW, such as GBE (Grain boundary engineering), grain refinement, and shot-peening [17][18][19][20][21][22]. However, only a few of studies regarding the oxidation behavior of steels after surface modification in SCW have been conducted [13,[17][18][19], but corrosion behavior in SCW is very important for materials being used as core structure for nuclear reactors. The corrosion behavior of T91 steel after oxygen ion implantation in SCW with a lower concentration of dissolved oxygen has been investigated by Chen et al [13], revealing that the thickness of oxide layer was reduced for samples implanted by oxygen ions.…”

Improving corrosion resistance of Al‐11mass%Si alloy through a large number of ECAP passes

“…By contrast, electrochemical properties of ECAPed materials with severe microstructure transformation have received only limited attention [6][7][8]. Chung et al [8], Vinogradov et al [9], Rofagha et al [10], Balyanov et al [11], and Ren et al [12] have studied the corrosion behavior of UFG Al, Cu, Ni, Ti alloy, and ferritic-martensitic steels produced by different grain refinement techniques, respectively, whereas these available investigations presented that the effect of UFG state on corrosion resistance varies among various alloy systems. Clearly, a good understanding of the corrosion behavior of ECAPed materials with UFG microstructure is required for their applications.…”

Dislocation engineering and its effect on the oxidation behaviour

“…Consequently, a more effective alloy diffusion coefficient is obviously of benefit in rapidly achieving and maintaining a steady growth of protective chromia. In recent studies, shotpeening martensitic steels show an improvement in the oxidation behaviour, but this is not yet sufficient to replace austenitic steels [7,8]. The present study reports an improvement of passification of these steels by 50%.…”