Preparation of Al2O3/Cr2O3 tritium permeation barrier with combination of pack cementation and sol-gel methods

In this paper, a layered Al2O3/Cr2O3 composite coating was prepared on the surface of super-clean reduced activation martensitic (SCRAM) steel by the combination of pack cementation and sol-gel methods as a tritium permeation barrier. First, the pack cementation processes was conducted at 980 °C for 30 min to form a chromed layer and then the sample was oxidized to form Cr2O3 layer at 760 °C for 2 h. Subsequently, the sol-gel method was conducted on the as-prepared Cr2O3 coating and Al2O3/Cr2O3 composite coating was finally prepared with subsequent thermochemical treatment. The results showed the first forming Cr2O3 coating was consisted of a 6 μm thick Cr2O3 exterior layer and a 2 μm thick interior diffusion layer due to inter-diffusion of Cr and Fe. The Al2O3/Cr2O3 composite coating was a three-layered structure with a total thickness of 10.5 μm. The outer layer was Al2O3, the middle layer was a mixing layer consisting of Al2O3 and Cr2O3, and the inner layer was a diffusion Cr,Fe layer. Well- crystallized α-Al2O3 formed in the coating at 960 °C which is below the formation temperature of α-Al2O3 resulting from the templet effect of isostructural Cr2O3. The hydrogen permeation test results revealed Al2O3/Cr2O3 composite coating exhibited a good barrier property against hydrogen penetration, thus the composite coating maybe a good choice to increase the hydrogen permeation resistance of SCRAM steel.