On the Nature of Electrochemical Reactions at a Crack Tip during Hydrogen Charging of a Metal,

The electrochemical conditions within cracks in metals during cathodic hydrogen charging are analyzed. The gradients of the electrical potential and concentrations of the various ionic species, and the current distribution are calculated for a model crack. The actual potential profiles were measured in slots in Fe, Ni, and Cu samples during hydrogen charging using an electrical potential probe. The measured potential drop in the electrolyte in the slots of all three metals is often very large, e.g., 0.5V. This is about an order of magnitude larger than that calculated, and is in general at odds with the usually implicit assumption that the electrode potential is not an important variable within a growing crack. The reason for this discrepancy is shown to be the large potential variation caused by the presence of trapped hydrogen gas bubbles in the slot. Tests of electrolyte samples taken from within slots while the current was flowing showed the presence of relatively large amounts of iron and nickel ions indicating that anodic dissolution of iron and nickel occurs within the slots, though not at the outer surface during cathodic hydrogen charging.