| The confinement of tritium within the fuel cycling system is required for the safe application of tritium in tritium-pumping pipes of fusion engineering and design.The tritium permeation barrier(TPB)coated on steel structural surface is one of the most effective methods to minimize tritium permeation.The aluminide coating on stainless steel with Fe-Al transition layer is considered to be one of the most effective coatings as tritium permeation barrier in fusion reactors.The key to determine the properties of Fe-Al/Al2O3 TPB is the Al2O3 thin film.However,the ?-Al2O3 is formed in most researches rather than the ?-Al2O3 with better tritium resistance properties.Selective oxidation of Fe-Al layer is an important step in the process of Fe-Al/Al2O3 TPB,but the oxidation mechanism is uncertain.So in this study,the resistance performance of deuterium and oxidation behavior of Fe-Al layer on 21-6-9 stainless steel was investigated under different oxygen partial pressure and oxidation time in order to obtain the a-Al2O3 film and growth mechanism of Al2O3 film on Fe-Al diffusion layer.The Fe-Al diffusion layer on 21-6-9 stainless steel was prepared by a 2-step process of Al-electroplating from ionic liquid followed by heat treatment.The deposited Al coatings were covered with a continuous and uniform aluminum coating growing in a spherical or pyramidal shape and well adherent to the substrate without cracks or voids.After heat treatment in air,an aluminized coating with a thickness of 27 ?m was formed on the substrate,showing a double-layer structure with no voids and firm metallurgical bonding between interface.The Fe-Al transition layers prepared in such method had good repeatability,laying the foundation for the identical samples in subsequent experiment.The oxidation behavior on Fe-Al transition layer under different oxygen partial pressure at 980? for 1 hour was investigated.The coatings were formed with a two-layer Fe-Al layer and Al2O3 film.Selective oxidation of aluminum occurred under lower oxygen partial pressure of 1 Pa?1 kPa.The stable-state ?-Al2O3 was more easily to form under oxygen partial pressure of 100 Pa?1 kPa.Over oxidation occurred in higher PO2(10 kPa?20 kPa),insulting the formation of iron oxide.The oxidation behavior on Fe-Al transition layer of different oxidation time at 980? in 1 kPa was investigated.The oxidation kinetics of Fe-Al layer illustrated the oxidation process had three stages.(1)a linear oxidation stage with no oxidation resistance in the initial,(2)a transition stage from the linear oxidation stage to the next stage,(3)a parabolic oxidation stage with good oxidation resistance.Cr act as nucleation plating at the initial stage of oxidation and the Cr atom was replaced by Al atom forming(AlxCr(1-x))2O3.After 180 min of oxidation,the film was composed of ?-Al2O3 only.The Volmer-Weber model can be well used to explain the growth mechanism of alumina thin film.Firstly,the film started with an growth by nucleation of discrete islands.Secondly the coalescence process occurred.Finally,the formed film continued to increase until a predefined thickness is reached.The Al2O3 film was the key factor to determining the tritium resistance performance.With the mixture phase of a+y changing to a phase,the number of defects between different phase was decreased,resulting the increase of PRF.In this work,the deuterium permeation rate through the coated 21-6-9 stainless steel was reduced by 2-3 orders of magnitude at 450?C?650?... |
PDF Full Text Request