DuPont determined that hydrogen-assisted stress corrosion cracking propagates with relative rapidity in materials of high hardness and will not readily propagate from the harder material into an area of lower hardness. The weld failed after 26 days of exposure to anhydrous hydrogen fluoride. DuPont determined that the weld material had a hardness exceeding Rockwell C 22 from the inside surface to the outside surface of the vessel. These planes of impurities usually run parallel to the surface near the mid-plane of the steel plate. As molecular hydrogen forms along the planes of impurities, the pressure increases, eventually becoming sufficient to expand the flaw and separate a localized area in the steel into two thinner plates, distorting one of the exterior surfaces.
Molecular hydrogen collects at small inclusions within the steel and forms microblisters.
It occurs in areas of the steel that have residual stress and is frequently discovered in the heat-affected zones adjacent to welds. Since the early 1970s, the DOT and the AAR have recognized compatibility problems between anhydrous hydrogen fluoride and certain carbon steels. The DOT provided limits in 49 CFR Part 179.102-13 that included a requirement that all valves and appurtenances be made of metal not subject to rapid deterioration by anhydrous hydrogen fluoride. In 1972, the AAR began studying the compatibility of materials used to construct tank car shells and heads for anhydrous hydrogen fluoride service. The petition also proposed that welding or welded repairs on tank cars should be subjected, after the initial heat treatment, to a localized postweld heat treatment that would reduce the maximum hardness of the weld and heat-affected zone to Rockwell C 22.