Effect of Alumina Film Prepared by Pack Cementation Aluminizing and Thermal Oxidation Treatment of Stainless Steels on Hydrogen Permeation

A technique concerned in preparing Al_2O_3 barrier film on the surface of stainless steels (00Cr17Ni14Mo2 and 1Cr18Ni9Ti) was studied. In order to test the permeation behavior, hydrogen was used to simulate the deuterium and tritium. Firstly, pack cementation aluminizing process was adopted to form a aluminum rich layer on the surface of stainless steels. The composition, structure and morphology of the layers were characterized by metallography, XRD and SEM. The aluminizing layer shows the compact structure and mainly FeAl phase. The layer shows multiplayer characteristics, which consists of inner layer (25 μm), external layer (5 μm) and transitional layer ( 30 μm). The good adherence can be seen between the sub-layers and the interface between the layer and substrate as observed by metallography. The mass fraction of the element Al in the aluminizing layer exceeds to 30%. Then the thermal oxidation processes were taken on the aluminized layer to form the Al_2O_3 film. The phase structure,the surface morphology, and three-dimension morphology of the Al_2O_3 film were characterized by the XRD, SEM/EDS and SPM. The thickness of Al_2O_3 film was tested by the ellipso-metry technique. It shows that an even and compact Al_2O_3 film was grown on the aluminizing layer. The film thickness is about 0.6 μm in case of oxidization for 2 h at 900 ℃ and 3 Pa oxygen pressure. In order to examine the hydrogen permeation property of the materials, the hydrogen permeation treatment were taken to both the treated and un-treated alloys. Then the amount of hydrogen in the surface region of the alloys was measured by elastic recoil detection (ERD) method. It is found that the atomic content of hydrogen droppes gradually with the increasing depth of the surface. And the atomic content of hydrogen at the distance of 0.2 μm from the surface is 0.007% that is about to the hydrogen composition of the stainless steel originally. This implies that the hydrogen atoms can not be diffused through the formed Al_2O_3 barrier film. The aluminization plus oxidation to form the Al_2O_3 barrier film is an effective method for resisting the hydrogen and its isotopes permeation.