Evaluation of oxidation and hydrogen permeation in Al-containing stainless steel alloys

As the national hydrogen economy continues to develop and evolve the need for structural materials that can resist hydrogen assisted degradation will become critical. To date austenitic stainless steel materials have been shown to be mildly susceptible to hydrogen attack which results in lower mechanical and fracture strengths. As a result, hydrogen permeation barrier coatings are typically applied to these steel to retard hydrogen ingress. The focal point of the reported work was to evaluate the potential for intentional alloying of commercial 300-series stainless steels to promote hydrogen permeation resistant oxide scales. Previous research on the Cr- and Fe-oxide scales inherent to 300-series stainless steels has proven to be inconsistent in effecting permeation resistance. The approach undertaken in this research was to add aluminum to the 300-series stainless steels in an attempt to promote a pure Al-oxide or and Al-rich oxide scale. Al-oxide had been previously demonstrated to be an effective hydrogen permeation barrier. Results for 304L and 347H alloys doped with Al in concentration from 0.5 to 3.0 wt% with respect to oxidation kinetic studies, cyclic oxidation and characterization of the oxide scale chemistry are reported herein. Gaseous hydrogen permeation testing of the Al-doped alloys in both the unoxidized and oxidized (600 °C, 30 min) conditions are reported. A critical finding from this work is that at concentration as low as 0.5 wt% Al, the Al stabilizes the ferrite phase in these steels thus producing duplex austenitc–ferritic microstructures. As the Al-content increases the amount of measured ferrite increases thus resulting in hydrogen permeabilities more closely resembling ferritic steels.