Hydrogen Embrittlement of Ceiling Clip Hangers

A suspended ceiling in a public building collapsed due to catastrophic fracture of the ceiling clip hangers. The ceilings in several public buildings, with similar ceiling clip hangers, were inspected and found to have cracked and broken hangers. One ceiling structure was noted to be sagging and in danger of imminent collapse.

The ceiling clip hangers are made from a medium steel stamping and heat treated to increase strength and toughness. The hangers are subsequently electroplated (zinc dichromate plating) and baked to remove nascent hydrogen.

Examination of the fractured surfaces via scanning electron microscopy revealed the failure mode was brittle indicating the ductility of the material was compromised.

Given the facts of this problem, hydrogen embrittlement (HE) was suspected as the root cause for catastrophic failure. HE is characterized by delay, brittle fracture of components under a constant tensile stress. This constant tensile stress is generally well below the yield the strength of the steel. HE occurs in parts that are exposed to processes that evolve nascent hydrogen, e.g., electroplating.

The plating process was reviewed. The practice of the supplier was to bake the hangers within 24 hours at 175 degreesC for 2 hours. This practice is inadequate for removing nascent hydrogen from electroplated components. The process was changed to baking within 1 hour of plating at 200 degreesC for 4 hours. Parts produced to this new process were successfully tested.

Skill set needed to analyze problem: Failure analysis of heat treated steels and, knowledge of hydrogen embrittlement phenomenon and electroplating systems.

To see the resume of the expert associated with this case study, see the link below.