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

The electrochemical conditions within cracks in metals undergoing cathodic hydrogen charging in acid solutions 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.The measured electrode potentials in the slot during cathodic hydrogen charging are in the region of metal dissolution in the case of the two base metals, Fe and Ni. On the other hand, the outer surface of these metals is under perfect cathodic protection for the typical impressed cathodic currents of 5 or 10 mA cm−2. Subsequent tests of electrolyte samples taken from within slots while the current was flowing, indeed, 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. Analysis of the solution at the outer surface of the slot under conditions of impressed current showed the absence of significant concentrations of metal ions.