ABSTRACT
Absorption Equilibria............................................................................. 119 2.3.2.3 Gibbs Free Energy of Adsorption for UPD H and OPD H ............... 119 2.3.2.4 Thermochemical Data for H Underpotential Electroadsorption
on Pt in Aqueous Electrolyte ................................................................. 120 2.3.2.5 Relation between the Energetic and Structural
Aspects of H Adsorption ....................................................................... 120 2.4 Mechanisms of the H Surface-Bulk Transfer ................................................................. 121
2.4.1 Structural Aspects ................................................................................................. 121 2.4.1.1 H Adsorption Sites .................................................................................. 121 2.4.1.2 H Bulk Interstitial Sites .......................................................................... 121 2.4.1.3 Role of the Surface Structure in H Absorption: Entry Sites .............. 122
It is well known that H entry (absorption) into the bulk may lead to embrittlement of metals and alloys. This process, known as H embrittlement or Η-induced cracking, often combined on nonnoble metals with corrosion cracking, is particularly detrimental to the resistance of metallic materials [1]. High H concentrations are produced in metals by thermal charging (H2 dissociation) or electrochemical charging (proton or water reduction); H can accumulate and combine at internal defects such as microcracks present in most commercial metals or alloys; high H2 pressures can build up within these microcavities which then grow and coalesce, leading to loss of ductility [1]. In aqueous solution, the reaction of H absorption (HAR) into an electrode proceeds in parallel to the reaction of H2 evolution (HER) at the surface at cathodic overpotentials and, at the corrosion potential (on nonnoble metals), simultaneously with the anodic dissolution or oxidation reactions in the mixed process controlling the rate of corrosion. The H entry into transition metals increases drastically in the presence of dissolved compounds of some electronegative elements, which are poisons of H adsorption. Although many explanations have been proposed to account for these effects, the detailed mechanisms of the action of these H entry promoters are not yet fully understood.
