Preparation of Al 2 O 3 /Cr 2 O 3 tritium permeation barrier with combination of pack cementation and sol–gel methods

Abstract In this paper, a layered Al 2 O 3 /Cr 2 O 3 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 Cr 2 O 3 layer at 760 °C for 2 h. Subsequently, the sol-gel method was conducted on the as-prepared Cr 2 O 3 coating and Al 2 O 3 /Cr 2 O 3 composite coating was finally prepared with subsequent thermochemical treatment. The results showed the first forming Cr 2 O 3 coating was consisted of a 6 μm thick Cr 2 O 3 exterior layer and a 2 μm thick interior diffusion layer due to inter-diffusion of Cr and Fe. The Al 2 O 3 /Cr 2 O 3 composite coating was a three-layered structure with a total thickness of 10.5 μm. The outer layer was Al 2 O 3 , the middle layer was a mixing layer consisting of Al 2 O 3 and Cr 2 O 3 , and the inner layer was a diffusion Cr,Fe layer. Well- crystallized α-Al 2 O 3 formed in the coating at 960 °C which is below the formation temperature of α-Al 2 O 3 resulting from the templet effect of isostructural Cr 2 O 3 . The hydrogen permeation test results revealed Al 2 O 3 /Cr 2 O 3 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.