On the estimation of fatigue crack growth in a contaminated H2S environment by interrupted cyclic tests

Regular and jumbo cylinders are often used for compressed natural gas (CNG) transportation and storage. CNG is a mixture of mostly methane (up to 98%) with a small amount of other minor hydrocarbons and can contain impurities such as hydrogen sulphide (H2S), carbon dioxide (CO2), and condensed water. For automotive applications, where CNG is used as fuel for vehicles, the natural gas is properly treated and filtered to remove such corrosive contaminants, however this might not be the case of CNG used for industrial applications, especially in newly fast growing countries. These impurities can promote the formation of an acid environment with low pH values and the well know hydrogen embrittlement (HE) phenomenon due to adsorption and entry of hydrogen into the steel. The present paper shows a methodology developed to estimate fatigue crack propagation rate for a low alloy steel grade in case of a contaminated CNG environment, where certain amount of impurities such as H2S, CO2 and condensed water are present. The methodology is presented together with preliminary experimental tests in a selected contaminated CNG environment. Further experimental verifications are in progress.