Failure analysis of fractured valves for Ar-CO2 gas storage steel cylinders

In this study, a failure analysis was conducted on two CuZn40Pb2 valves for CO2 gas storage cylinders. The valves failed during maintenance and refilling operations performed by the operator. The investigation involved visual inspection, fractographic analysis, chemical composition determination, microstructure examination, and calculations of principal stresses and safety factors for the failed valves. The results indicated two significant issues: dezincification and stress corrosion cracking (SCC). Zinc levels near the fracture surface were found to be ten times lower than the standard value of the CuZn40Pb2 alloy, and the phenomenon was observed in both the valves and the cylinder probes. Additionally, tortuous cracks, typical of SCC, were observed near the inner threads of the valves and close to the fractured surface. These phenomena were caused by the convergence of several critical factors: moisture and/or an ion-rich environment in the system, and possible overloads due to excessive force applications. Stress calculations in two critical operational scenarios showed that tightening/loosening forces near 30 kg applied to a flowmeter could induce overload failures in the valve.