Hydrogen Embrittlement and Hydrogen Traps

Abstract

The permeation of H through iron and an iron‐nickel alloy as a function of time is reported. Below a certain critical H overpotential η, the H permeation occurs as a function of time in a simple way; and can be repeated indefinitely on the same specimen. But, when , the permeation‐time transient attains a new and characteristic shape. It is no longer repeatable on the same specimen. These characteristic differences of H permeation are seen during injection of H into the metal and after the cessation of H injection. The decay of H permeation follows simple laws before . At , the decay is complex and the permeation‐time (P‐t) relationship shows successive maxima. The critical overpotential is that at which microcracks begin to nucleate, spread, and provide traps for H2. Calculated pressures, using the amount of lattice‐dissolved H and adsorbed H at the cathodic surface at , are shown to be consistent with an H2 pressure in voids which would cause the spreading of cracks. This leads to an electrochemical condition for the beginning of H damage: the observed critical overpotential gives reasonable values for the embrittling critical pressure. Such an electrochemical condition for damage also applies during corrosion. The amount of trapped H is calculated from the permeation‐time transients at overpotentials higher than . The H trapping constant is calculated. Distribution of hydrogen in the embrittled membrane is given. In the permeation decay transients, the first maximum represents the hydrogen in cavities and the second, H bound to chemical impurities.