Effects of SMAT on corrosion properties of materials in steam generator

xiv, 140 pages : color illustrationsPolyU Library Call No.: [THS] LG51 .H577P ME 2017 LiNStress corrosion cracking (SCC) becomes animportant failure mechanism that jeopardizes safety and integrity of engineering structures. The surface mechanical attrition treatment (SMAT) process is one of the most promising methods for fabricating nanocrystalline (NC) structures on metal surface. However, the corrosion behavior of SMATedmetals under high temperature and high pressure has received limited attention. Hence, systematic studies are needed to deeply investigate the corrosion behavior of SMATed alloys used in SGs. To meet this objective, the microstructural evolution of surface nanocrystalline 304, 316L and Alloy 690 with SMATed experiments were studied. The mechanical properties and corrosion behavior of 304, 316L and Alloy 690 subjected to different SMAT processes (combined with a post nitriding treatment) were characterized. The results were as following: At the room temperature (25°C), XRD and potentiodynamic polarization studies of 304, 316L and Alloy 690 showed that a longer SMAT treatment time led to a reduced grain size and increased microstrain energy in the top surface layer, showing poorer corrosion resistance. Nitriding with SMAT as a pretreatment did not change the hardened layer depth of 316L; however, this combination did improve the microhardness from the surface down to a 100 µm depth. The maximum surface hardness was nearly five times that of the coarse-grained 316L. Compared with an untreated sample, the nitriding duration and temperature could be successfully reduced without losing improvements in the mechanical characteristics. The ultrafine-grained structure provided a high volume fraction of grain boundaries, which served as numerous fast diffusion "channels" for the nitrogen atoms. Thus, strongly enhanced kinetics for nitride growth and greatly decreased activation energies for the diffusion of nitrogen in the nanostructured compound layer, relative to those in the coarse-grained nitrides, were attributed to the fast grain boundary diffusion in the SMAT samples. Under simulated alkaline SG chemistries and at 300°C, the effects of SMAT on the SCC behavior of Alloy 690 under SG conditions in lead(Pb) free and Pb containing solutions were investigated. The experimental results showed that SMAT led to strong compressive residual stresses on the surface of specimens and a significant increase in the amount of Cr oxide in the passive films. SMAT-processed Alloy 690 exhibited better corrosion resistance in the Pb-free and Pb-containing alkaline solutions. The corrosion behavior of the SMATed 316L was also evaluated after immersion in simulated SG chemistries. Compared with the corrosion behavior at room temperature (25°C), SMAT processes evidently improved the SCC resistance under high temperature and high pressure. The breakdown potential of the treated 316L shifted positively, and the passive currentdensity greatly decreased, indicatingthe improvement of SCC resistance. The microstructure of the passive layer on the surface showed that the external layer was mainly composed of a discontinuous spinel structure of (Ni, Fe) Fe₂O₄ and magnetite Fe oxides with considerable pores, while the structure of the internal layer corresponded to a continuous Cr-rich layer. The Cr-rich layer was stable and enhanced the corrosion resistance.Department of Mechanical EngineeringPh.D., Department of Mechanical Engineering, The Hong Kong Polytechnic University, 20177]Doctorat