Stress Corrosion Cracking Resistance of Metallic Materials Used in Drilling Technology – Part 1: Test Results from Stainless Steels and UNS N07718 in Simulated Drilling Environments

Structural materials in drilling tools are subjected to high mechanical loads while in permanent contact with the drilling environment, which can become corrosive. The simultaneous combination of tensile stresses and a corrosive environment might lead to stress corrosion cracking (SCC), which is indeed, one of the most common failure mechanisms of drillstring components. Therefore, a good understanding of the SCC susceptibility of metallic materials used in downhole components is crucial from the reliability point of view. The susceptibility of metallic materials to SCC depends upon the corrosiveness of the downhole environment typically defined by temperature, pH, chloride concentration and oxygen content, as well as upon the magnitude of the tensile stress and time. Due to the large impact of SCC on tool integrity tensile stress thresholds to avoid SCC are in many cases more relevant in the design than the mechanical properties of the manufacturing materials. To assess the SCC behavior of some structural materials used in drilling technology, investigations were conducted considering testing environments representing real operational conditions. Three high-strength materials including two strain-hardened austenitic stainless steels and the Nickel-alloy 718 (UNS N07718) have been investigated using constant load tests. The influence of the presence of oxygen, of temperature in the range between 22 to 150 °C, and of the chloride concentration in the range from 17.713 mg/l (0.5 M) to 141.812 mg/l (4.0 M) in the test solution upon the SCC susceptibility of these materials was studied. Experimental results confirmed that SCC susceptibility of the investigated materials is strongly influenced by temperature and oxygen content.