Erosion and corrosion of PSZ-zirconia and the t–m phase transformation

Abstract

Partially stabilised zirconia (PSZ) has received special attention due to its high strength, high corrosion and erosion resistance, and high resistance to fracture. The phase transformations of ZrO 2 in its application process have long been known to be important. Various physical, hydrothermal, and chemical effects can cause the phase transformations. Although many advances have been made in recent years in the understanding of the effects, foundations and mechanisms of the ZrO 2 phase transformation, however, no study has yet reported on the phase transformation from t-ZrO 2 to m-ZrO 2 in corrosion solution at room temperature. In the present study, both the corrosion resistance of ZrO 2 and the ZrO 2 phase transformation in a corrosive solution (1.5% HF+5% HCl) used in the oil industry to clean downhole pipes were investigated. The erosion behaviours of PSZ-ZrO 2 before and after corrosion have been studied using a SiC/water slurry jet impingement test rig with the aim of providing some information on the material's response to erosion attack. SEM and X-ray were used to characterise the microstructure of the corroded layer, and phase change after corrosion. After corrosion, the peaks of monoclinic structures increase dramatically; ZrCl 4 or ZrF 4 were not detected. By increasing the length of corrosion time, the monoclinic structure is markedly increased. Thus, the corroded layer is the main source contributing to the increase in the monoclinic phase. The possible reaction steps in HF+HCl solution are discussed. The mechanism of t–m phase transformation is discussed in this work. From the thermodynamic viewpoint, a phase structure would tend to transform from the metastable phase to the more stable under the attack of a corrodent, or under impact stress or tensile stress. However, the difference between the two cases is that during the transformation induced by impacting stress, part of the t-ZrO 2 transforms into m-ZrO 2 and microcracks develop, which consume crack propagation energy and release stress concentration, while t–m transformation due to the corrosion process forms a porous m-zirconia layer. Microstructural evaluation as well as chemical analysis shows that the surface structure of PSZ-ZrO 2 changes and becomes more porous during corrosion process, not only due to the t–m phase transformation, in which cracks and voids are anticipated owing to 3%–5% volume change; but also due to the effect of higher corrosion rates of additives in PSZ-ZrO 2 as well. The erosion behaviours of PSZ-ZrO 2 before and after corrosion, using a SiC/water slurry jet, are also compared with other ceramic materials.