Hydrogen penetration in ferrous materials may have severe deleterious effects by structural embrittlement. The phenomenon can result from metal corrosion, particularly during cathodic protection. This problem has been largely investigated, nevertheless the mechanism of hydrogen entry in the metal is still a matter of discussion. It is generally admitted that the rate of penetration is governed by the surface coverage θH by adsorbed atomic hydrogen, as intermediate of the hydrogen evolution reaction (1). An alternative model, which seems to apply in the case of palladium, assumes a direct entry of hydrogen (2,3).