The chain link was corroded.  A cross-section showed intergranular cavities that were present to some depth in the material. These cavities appear to have been due to hydrogen damage. The chain link failed due to corrosion resulting in hydrogen embrittlement.  The significant porosity observed could weaken the steel. The microstructure in the base metal was tempered martensite, typical for SAE 1330 steel. Atomic hydrogen may generate during corrosion reaction and diffuse into the steel since the size of the atomic hydrogen atom is relatively very small. These atomic hydrogen atoms combine in the voids of the metal to form hydrogen molecules that exerts pressure in the cavity. These pressures are high enough that the material loses its ductility and catastrophic failures may occur. High-strength, low-alloy steels are very susceptible to this condition. 

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