Effect of Compressed Natural Gas (CNG) on corrosion behaviour of brass valve of CNG cylinder

In this case study, failure root cause analysis of the Compressed Natural Gas (CNG) cylinder brass valve was investigated. Detailed metallurgical analysis of the cracked CNG valve was performed. Higher content of sulphur and carbon while depletion of zinc observed by Scanning Electron Microscope (SEM) -Energy Dispersive Spectroscopy (EDS) analysis at crack location and on black corrosion product. Microstructure analysis of the cross section near to fracture surface indicated dendritic structure of alpha and beta phase and depletion of zinc from beta phase at the inner diameter (ID) of valve and near to crack region by SEM-EDS analysis. The overall observation indicates that the failure of brass valve body of CNG cylinder was due to the dezincification. As per the IS 15958 standard, the CNG valves, which have failed, may be due to more than specified limit of the corrosive components (S and CO2) in the CNG. This resulted into diffusion of sulphur and carbon dioxide in beta phase at internal surface of valve. Adsorbed sulphur and CO2 from the CNG lead to the preferential dezincification from beta phase throughout the inner diameter (ID) of the valve. Electrochemical reaction during dezincification lead to weak spongy copper layer, which subsequently initiated and propagated crack to the outer diameter (OD) and resulted into leakage and failure.