Study Of Nodular Corrosion Of N18Alloy

Zirconium alloy, as cladding materials of nuclear fuel element in the reactoroperation, is affected by high temperature and high pressure water corrosion, thengenerates the oxide film. According to the research results explored of Zr-2andZr-4alloys, zirconium alloy exhibited nodular corrosion in the condition of highoxygen content. The N18alloy was the development of new zirconium alloy withthe independent research of China. Most of experimental researches showed that theN18alloy exhibited the good property of resistance to nodular corrosion. However,the recent results suggested that the nodular corrosion was occurred on the N18alloy after some processing and heat treatment conditions. In this study, the N18alloy were prepared to obtain the causes of nodular corrosion by systematical studyafter the abnormal heat treatment at500°C/10.3MPa in superheated steam. Themicrostructure of the zirconium matrix and oxide film was investigated. The mainconclusions are as follows:1) The depleted zone of alloy elements, especially the depleted zone of Nbelement, was presented after the high temperature heat treatment, which was theimportant reason of N18alloy on nodular corrosion.2) The cause on the depleted zone of alloy elements after the final heattreatment in α+β two phase regions was that, when the β-Zr phase formed in theboundary of grains for the N18alloy, the alloy elements, such as Nb、Fe、Cr,migrated into the β-Zr phase. When the N18alloy was processed after the heattreatment in the range of high temperature of α-Zr phase, as the second-phaseparticles (SPPs) merged and grew, and the Nb element might diffuse into SPPs, thedepleted zone of Nb element was still presented.3) The depleted zone of alloy elements for the N18alloy, which it is difficult toeliminate by the subsequent deformation or the annealing treatment at the lowtemperature, was eliminated by the solution treatment of the β phase at the condition of long time.4) Nodular corrosion was exhibited in the depleted zone of alloy elements at500°C/10.3MPa in superheated steam. With the sizes and contents of depletedzone of alloy elements increasing in zirconium matrix, nodular corrosiondeteriorated. The development of nodular corrosion was limited in enrichmentregion of alloy elements. Compared with the Zr-4alloy, the phenomenon ofabnormal growth for nodular corrosion was presented in the N18alloy.5) Nodular corrosion of N18alloy was related with the process and heattreatment in α+β two phase regions, or with high temperature of α phase region inrange of long time, and did not with the process in the hot or cold rolling or in thefinal heat treatment at the low temperature.6) The thickness of columnar crystalline grains of nodular oxide film formedfor the N18alloy was less than50%. Most of columnar crystalline grainstransformed to equiaxed grains. The thickness of columnar grains of uniform oxidefilm was up to60%~80%. The higher thickness of columnar grains can improve theproperty of corrosion resistance in zirconium alloy.7) An intermediate layer of hcp ZrO0.27was observed between the nodularoxide film and the zirconium matrix for the N18alloy. When the crystal planes ofα-Zr surface grain approximately paralleled the plane (0001), it was obvious for theforming trend of nodular corrosion.