This question regards the burst disc material used as a PRD in 200 bar (settled pressure) cylinders. The material used is copper nickel, and stress corrosion cracking is suspected. Are there emerging materials being proposed for the above duty? Historically, in Australia, copper burst discs have been used up to 137 bar with success. In addition, a thin Teflon disc is placed on the atmospheric side of the burst disc, presumably to prevent atmospheric corrosion from the air & moisture as well as to insulate the copper disc from the galvanic effects of the brass housing.

Based on the question, it’s not clear if the reference to stress corrosion cracking (SCC) has been proven or is only suspected. It can be difficult to identify the nature of the cracks (SCC vs H2 embrittlement without analysis and microscopy). In addition, the question does not mention that the cylinders contain hydrogen gas, but it’s assumed since it was submitted to the Panel. 

In this case, it is the experience of Panel members that H2 cylinders rarely used Cu-Ni alloy discs due to concerns over embrittlement. Most discs for high pressure cylinders are copper to resist hydrogen embrittlement. Corrosion and/or SCC from the atmospheric environment are concerns and care should be taken to protect the disc if conditions for SCC might exist, such as the Teflon coating mentioned. The outlet of the vent stack should also be capped to avoid atmospheric contaminants and water/snow/ice.  It’s important that the cap does not interfere with the ability of the disc to function. If the gas is hydrogen and the inquiry is directed toward hydrogen embrittlement, then more information is needed on the alloy. Is it copper-based with nickel as an alloying element or nickel-based with copper as an alloying element? Either way, if the inquiry is about resistance to hydrogen embrittlement from contact with H2, then 300-series stainless steel burst discs are often used at lower pressures (e.g., for LH2 equipment) and copper burst discs are used for high pressure gas. Nickel alloys are not generally recommended for H2 service, but specific alloy qualification is needed. Limited available data show these alloys to be highly susceptible to hydrogen.