The Permeability of Aluminum to Hydrogen

Abstract

The permeability constant for hydrogen in aluminum at 500~ was found to be sensitive to surface films. The constant varied over a four hundredfold range in normal hydrogen atmospheres and over a thousandfold range in the presence of a glow discharge. These differences are attributed to the varying ability of the respective surface films to catalyze the H2 ~ 2H equilibrium and to bar the diffusion of hydrogen. Surface films having high catalytic activity protect aluminum against attack by hydrogen generated in the reaction with water. Films of tungsten, iron, cobalt, nickel, molybdenum, chromium, and various halides reduced the permeability constants in the presence of the glow discharge. Etching with caustic reduced the permeability constant in normal unexcited hydrogen to as little as one hundredth of the value for untreated aluminum. A nickel-aluminum alloy formed an oxide film in water at 350~ that is very impermeable to hydrogen. The permeability of 99% aluminum was measured at temperatures from 400 ~ to 600~ and compared with other values in the literature. The dependence of the permeability of aluminum to hydrogen on heating time and surface conditions has been variously attributed to oxide films (1, 2), recrystallization of the metal (2), and rates of the dissociation and adsorption reactions on the surface (3). Surface films and alloy composition seem to be particularly important in controlling permeation by hydrogen generated in the corrosive reaction of water with aluminum at elevated temperatures. This reaction can lead to blistering of the surface and formation of hydrogen-filled voids within the metal (4) or, with liquid water above 200~ to catastrophic attack of the metal (5). Stroup (4) found that the attack by water vapor could be reduced if the aluminum were heated in the presence of sodium, potassium, or ammonium fluoborate. Eborall and Ransley (6) noted that gassing of an aluminum-magnesium alloy at 600~ in 16 mm pressure of water vapor was strongly retarded if the alloy was exposed initially to the vapors of concentrated hydrofluoric acid. Recently, Blackburn and Gulbransen (7) found that pre-treatment of high-purity aluminum in 1M HC1 at 55~ inhibited blister formation and prevented gassing of the metal upon heating in water vapor at 600~ Draley and Ruther (5) learned that the catastrophic attack of liquid water on aluminum at temperatures near 350~ could be reduced by addition of 1% nickel to the aluminum. They suggested that the added nickel or any added metal that has a low hydrogen overvoltage may catalyze the formation of hydrogen molecules at the oxide-metal interface. The direct relationship of hydrogen solubility to the square root of the hydrogen pressure, as observed by Ransley and Neufeld (8), establishes that hydrogen dissolves in aluminum in monatomie form in accordance with Sievert's rule. The atoms may ionize to protons on dissolving in the aluminum, but the dissolved hydrogen will be referred to as atoms in this work. The observed solution and per-meation of hydrogen in aluminum from hydrogen at 1 atm pressure at 500~ results from the extremely small equilibrium atomic hydrogen pressure of 10-1~ .atm. Nascent hydrogen from the reaction of aluminum with water can be produced at pressures above this equilibrium value and cause increased diffusion into the metal if the equilibra-tion reaction with molecular hydrogen is slow. In the metal, this high concentration of atomic hydrogen can equilibrate with molecular hydrogen at favorable sites (perhaps inclusions, shrinkage porosity, vacancies, etc.). The blisters and hydrogen filled voids that can form from this reaction are evidence that the pressure of molecular hydrogen in these voids can reach high enough values to exceed the yield strength of the metal. The upper limiting equilibrium pressures that can result from reaction of water vapor with aluminum to form alumina and hydrogen can be calculated from the following expressions.