The direct cause of the over-pressurization of the two drums was the repackaging of the phosphoric acid into metal UN1A1 drums and the resultant hydrogen gas generation within the sealed drums. At the time of this incident (1997), 49 CFR and several MSDSs supported the selection of the UN1A1 drums. After the incident, laboratory studies conducted by facility staff indicated that the corrosion and subsequent hydrogen gas generation rates for the amount of phosphoric acid present would result in a pressure buildup and the drum failures observed. Facility staff then contacted DOT to request that changes be made to the packaging guidance listed for this material.

The lessons learned from this incident are:

1. Verify the gas that you are using.
2. Avoid using "quick-disconnect" fittings in this type of situation. If they are absolutely needed, there are sets available that ensure that every pair of matching fittings are different from all the others.

1. Ensure that equipment and materials exposed to hydrogen are compatible with a hydrogen environment (even hydrogen-service-rated equipment).

2. Equipment designs in a hydrogen environment that use the fracture toughness for 17-4 precipitation hardening stainless steel may want to consider a 90% derating in published values (see Attachment 1).

Additional discussion of materials selection for compressed gas storage vessels can be found in the Hydrogen Safety Best Practices Manual. Similar discussion of materials selection for cryogenic liquid storage vessels can be found in the Hydrogen Safety Best Practices Manual.

A web-based resource developed by Sandia National Laboratories to provide data on hydrogen embrittlement of various materials is available at Technical Reference for Hydrogen Compatibility of Materials.