Measurement of the hydrogen diffusion through various steels with and without oxide films

Abstract

Hydrogen atoms are produced electrochemically when ferrous metal is exposed to high temperature water. The rate of hydrogen produced corresponds to the rate of corrosion which has been designated as Flow Accelerated Corrosion (FAC). In deaerated conditions, the atomic hydrogen enters the metallic lattice interstitially and permeates through the metal and recombines to form hydrogen gas at the outer surface of the metal. Due to the rapid diffusion of hydrogen through the metal at the temperature of interest, the total rate of hydrogen effusion from the metal is an indication of the instantaneous corrosion rate. The Hydrogen Effusion Probe (HEP) is being developed for on-line monitoring of FAC by measuring the through- wall hydrogen effusion. This study was carried out to better understand the fundamental principles of corrosion and transport of hydrogen through carbon steel associated with FAC. The coefficient of hydrogen diffusion in carbon steel was determined at various temperatures and found to obey Arrhenius Law. The effects of surface conditions on hydrogen permeation were studied by treating the surface to form an oxide film and by coating it with platinum in order to elucidate the diffusion barrier of hydrogen atoms and the kinetic barrier at the surface. The surface resistance due to an oxide layer on a carbon steel tube was determined on both the inside and outside of the tube.