Investigation on the failure mechanistic evolution of fretting corrosion of Alloy 690 with increasing cycles in high-temperature water

In practical applications, Alloy 690 tubes are not only susceptible to fretting wear, but also subjected to corrosion by high temperature and high pressure water. In this work, the fretting corrosion behavior of Alloy 690 tube was investigated in high temperature high pressure water, and the coupling effects of corrosion and mechanical wear on the microstructure evolution of Alloy 690 tube with increasing testing time have been revealed. Results show that fretting wear accelerates the material oxidation by material deformation, cracking and grain refinement. Moreover, the structure and the composition of the oxide layer formed on the fretting worn surface are completely different from those formed on the generally corroded area: a dual layer oxide scale with the loose and porous outer oxide layer composed of Fe2O3 and Fe3O4 and the dense inner oxide film composed of Cr2O3 and Fe-rich spinel oxides is formed on the worn surface, while an extremely thin single layer oxide scale composed of Cr2O3 is formed on the generally corroded area. As fretting cycles increase, the influence of mechanical wear on material oxidation and structural evolution becomes more pronounced. However, frictioninduced defects (e.g., cracks and voids) destroy the integrity of the oxide scale and aggravate material damage, which result in larger wear volume and higher wear depth of the Alloy 690 tube.