In addition to forming Al2O3, which is known to decrease hydrogen permeation, aluminization of steels

forms aluminide intermetallics that are believed to also lower permeability. Most studies of aluminized samples either have intentionally grown an oxidized layer in order to achieve greater PRFs or at least have not attempted to suppress the formation of surface Al2O3 prior to permeation testing. To our knowledge, no permeation measurements on oxide-free alumi — nides have been performed. However, different pro­cesses lead to oxide scales of differing composition, thickness, and defect density, and the PRF may not be attributable to oxides alone. Steels have been alumi­nized by the hot dip process (described earlier), as well as by various chemical vapor deposition (CVD), spray, packed cementation bed, and hot isostatic press­ing (HIP) techniques. For those techniques that lay down a substantial amount of materials that does not react with the matrix (such as HIP), an aluminum- containing iron alloy can be used in preference to aluminum to offer a higher temperature barrier per — formance.189 Oftentimes, the aluminized layer will be made up of mixed FeAl, FeAl2, Fe2Al3, Fe2Al5, FeAl3, Fe3Al, and even Fe4Al13 intermetallics.190 Nickel, chro­mium, and mixed-aluminides are also formed.191,192 Due to the aluminum-rich intermetallics on the sur­face, aluminized material will often have a mixed oxide scale that is rich in Al2O3.193,194 PRFs are generally larger than for a pure aluminum layer, varying between 10 and 10 0 00,175,195 while barriers containing a clean Al2O3 surface often have the greatest PRF.196 It should be noted that aluminum additions can also stabilize ferrite in some austenitic stainless steels, producing a duplex microstructure and an increased permeation.197

4.16.3.3.2 Nitrides

As with oxides, either the native nitrides of the base metal or depositions of other nitrides can be made to serve as barriers. One of the most common native nitrides is Fe2N, which forms when the upstream face of steel samples are nitrided and reduces permeabil­ity by one to three orders of magnitude.198-200

After oxides and aluminide, TiN coatings are one of the most researched barriers because of their good adhesion and the ease of deposition.175 Reported PRFs for nitride barriers vary widely, from less than an order of magnitude to six orders of magnitude. TiN barriers reduce permeability the most when they are placed on the high-pressure side of samples, and much less change in permeation is observed when they are placed downstream.201,202

Boron nitride has been shown to reduce the permeability of hydrogen in 304SS by one to two orders of magnitude.175,203 It forms both cubic and hexagonal structures. Checchetto et a/204 noted that the hexagonal structures absorb a greater amount of hydrogen isotopes (because of a larger number of trapping sites; particularly dangling B and nitrogen bonds), but also display a greater diffusivity of hydro­gen. The latter effect may be due to preferential diffusion along the a direction of the hexagonal lattice.

Bazzanella et a/.205 found that a 1.7 pm (Al, Ti)N coating reduces the deuterium permeability of 0.1-mm thick 316L by two to three orders of magnitude. From permeation transients, they speculated that this reduction was primarily due to the very low diffusiv — ity of D in (Al, Ti)N.