Corrosion Failure Analysis of a Pressure-Resistant Cylinder for Measurement While Drilling Tools in Directional Drilling

During the drilling operations of a shale gas well in Central China, a severe failure occurred in the pressure-resistant cylinder of the measurement while drilling (MWD) tool, with numerous microcracks observed on the outer surface of the cylinder. This significantly compromised the safety of the MWD tool and the reliability of the logging data. To determine the cause of the failure, macroscopic morphology analysis and physicochemical performance tests were conducted on the failed pressure-resistant cylinder, which is made of Cr20Ni11 (UNS 308) austenitic stainless steel. Additionally, scanning electron microscopy, X-ray energy dispersive spectroscopy, white light interferometry, and X-ray photoelectron spectroscopy were employed to analyze the morphology and chemical composition of the corrosion products and cracks, thereby identifying the cause of the corrosion failure. It is demonstrated that the physicochemical properties of the pressure-resistant cylinder comply with the specifications of relevant standards. Nevertheless, the size of non-metallic inclusions in the material reaches 100 μm, which significantly enhances the material’s susceptibility to stress corrosion cracking (SCC). Meanwhile, solid particles and high-concentration Cl− present in the drilling fluid deteriorate the passive film formed on the substrate surface. EDS analysis reveals that the Cl− content is measured to be 4.09 wt%, which induces pitting on the substrate with a maximum pitting depth of 13.5556 μm. Under the synergistic effect of stress and corrosion, the pressure-resistant cylinder experiences SCC failure initiated by Cl−; specifically, cracks nucleate at the bottom of the pitting pits and propagate along the radial direction.