The effect of ion implanting on hydrogen entry into metals
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INTRODUCTION
THE potential for hydrogen pickup by a metal exists whenever the metal is exposed to a hydrogen-containing environment.[1] However, more severe hydrogen-charging conditions are encountered when the source of hydrogen is an aqueous phase than when it is a gaseous phase. This is because higher hydrogen fugacities may be reached during corrosion reactions than during normal exposure to hydrogen baring gases. During hydrogen charging of steel from an aqueous phase, e.g., during a corrosion reaction, hydrogen adatoms, Hads, form on the steel’s surface according to the reaction H+ 1 e2 → Hads
[1]
The adatoms produced via Eq. [1] may combine to form hydrogen molecules that evolve as gas bubbles, Hads 1 Hads → H2
[2]
Hads 1 H+ 1 e2 → H2
[3]
or they may become absorbed into the steel surface, Hads → Habs
[4]
It is this absorbed hydrogen, Habs, which diffuses into the steel to areas of high triaxial tensile stress, embrittles the metal, and eventually leads to premature failure of steel components. Whether the hydrogen adatom, Hads, follows the path described by Eq. [2], [3], or [4] depends on the energetics of the steel surface. Surface modifications therefore may be designed to reduce the rate of Reaction [4] and enhance the rate of Reactions [2] and [3]. The rate of Reaction [4] is determined by the coverage, u, of the steel surface by adsorbed hydrogen. Bockris and Subramanyan[2] have shown that when the source of hydrogen is an aqueous phase, u is related to the hydrogen overpotential, h, by bhF
u 5 Ae2 2RT
[5]
S.C. CHOU, Graduate Student, and M.M. MAKHLOUF, Associate Professor, are with the Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609. Manuscript submitted December 1, 1997.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Where A is the rate coefficient for the hydrogen combination reaction, b is the symmetry factor of the activation barrier for hydrogen discharge, F is the Faraday constant, R is the gas constant, and T is the absolute temperature. Chattterjee et al.,[3] and later Zamanzadeh et al.,[4,5] have shown that electrodepositing a metal, with a higher exchange current density for the hydrogen evolution reaction than steel, onto the surface of the steel provides for a lower hydrogen overpotential on the composite surface. The lower hydrogen overpotential on the modified surface provides for lower hydrogen coverage, and consequently less hydrogen is transported from the surface into the bulk of the metal. Chatterjee et al.[3] have supported this model with experimental results for platinum electrodeposits on iron. Chatterjee et al. electrodeposited platinum, and also nickel, on Ferrovac E iron substrates. After testing the samples for hydrogen permeation, Chatterjee et al. found that a platinum coating of only 0.015 mm is much more effective in reducing hydrogen permeation into the iron than a much thicker nickel coating of about 6 mm. Since the diffiusivity of hydrogen in platinum is comparable to that of hydrogen in nickel, Chatterjee et al. con
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