Failure analysis on aluminum alloy drill pipe with pits and parallel transverse cracks

Abstract

In recent years, aluminum alloy drill pipe are used more and more frequently in the exploration and development of the deep wells, ultra-deep wells and sour gas wells due to the high strength, light weight and corrosion resistance to H2S and CO2. However, the pitting and fracture failure sometimes occurred, resulting in giant economic losses and fatal disaster. In this study, the failure analysis of aluminum alloy drill pipe was addressed and the formation cause of the pits and parallel transvers cracks was investigated. Results show that the chemical composition and the regular mechanical properties (including tensile and impact properties) meet ISO 15546:2011 standard requirements. But the tensile strength of samples immersed in the mud environment is lower than that of ISO 15546:2011 standard requirements. Metallographic analysis results show intermittent ribbon Al6(MnFe) solid solution, S (CuMgAl2) phase and AlFeSi phase distributed on α solid solution. The failure cause of aluminum alloy drill pipe is mainly attributed to the brittleness sensitivity of intermittent banded second phase and inclusions to corrosive media from mud environment. Cracks and pits are prone to generate here. XRD and EDS analysis results of corrosion products and the pits bottom show that the main corrosion products are Al2O3, Al2S3, AlPO4, BaSO4 and CaCl5. S and Cl are from environment media. The microscopic morphology analysis results of the tensile and impact specimens fracture show that the fracture is the brittle fracture caused by the environmental medium.