Study On The Preparation And Property Of The Cr₂O₃/Al₂O₃Hydrogen Permeation Barrier

Stainless steel is a common structural material in fields of hydrogen storage, vacuum solar receivers and fusion reactors etc. Stainless steel has a high hydrogen permeation rate at elevated temperatures and pressure, because hydrogen permeates through steel in the form of interstitial atom which would resulted in hydrogen embrittlement and radiological hazards. It is widely recognized that a thin ceramic coating on stainless steel could efficiently suppress the hydrogen permeation. In this study, Cr2O3/Al2O3gradient coating was prepared by MOCVD (Metal-oganic Chemical Vapor Deposition) on the surface of stainless steel to reduce the mismatch of the coefficient of thermal expansion (CTE) and improve the high temperature stability. We first optimized the technology of MOCVD and studied the hydrogen permeation resistance performance of Al2O3and Cr2O3coating respectively. Then the influences of the substrate roughness, mismatch of the CTE, precipitated phase, microstructure and the growth orientation on the hydrogen permeation resistance performance of the coatings were discussed. After that, influences of the CTE gradient, interfacial effect, lattice induced effect, crystal transition and the crystallization quality on the hydrogen permeation resistance performance of the Cr2O3/Al2O3coatings were studied. The research findings were reported below.(1) The influences of the gas composition, deposition time, deposition temperature, precursor temperature and thermal treatment on the microstructure and composition of the Al2O3coating prepared by MOCVD were studied. The results suggest that, the carbon could be effectively eliminated by using water vapor as the reaction gas. The coating microstructure was related to deposition temperature, the migration and nucleation of the precursor molecule on the substrate surface were remarkable influenced by the deposition temperature due to the adsorption and desorption of the precursor molecule. Therefore, as the deposition temperature rise (400℃-600℃), the3D island morphology on the surface of the coating was increasingly obvious. When the deposition temperature was above600℃, crack was found on the coating. In the deposition time1h-8h, the relationship of the coating thickness and deposition time exhibited a linear relationship following the equation d=-19.4+5.5t. Excessive thickness could cause a structural instability and depravation of the coating compactness. The deposition rate was accelerated as the precursor temperature increase, excessive growth rate could also cause a structural instability and depravation of the coating compactness. The crystalline phase of the coating was related to the annealing temperature. The Al2O3was amorphous after700℃annealing, amorphous and γ-Al2O3after900℃annealing, y-A12O3、θ-Al2O3and α-Al2O3after1100℃annealing. Cracks were observed on the Al2O3coating after annealing above900℃, which was attributed to the thermal stress and volume shrinkage in the crystal transition process.(2) Amorphous Al2O3coating with243nm thickness was prepared on316L substrate, the apparent hydrogen permeability of the coating was P=8.68x10-7exp(-107606/RT) mol/m·s·Pa05at600℃-700℃and the PRF (Permeation Reduction Factors) of the coating was42.0-75.8. The alumina coating has offered a certain extent hydrogen permeation suppression performance.(3) The hydrogen permeation performance could be affected by the substrate roughness and thermal shock test. The PRF of the coating was reduced from41-106to23-45as the substrate roughness raised from8.71nm to65.3nm. Meanwhile, a higher substrate roughness could lead to a more reduction extent after700℃/30time thermal shock tests.(4) The Al2O3transformed from amorphous to y-phase. Besides, spinel MnCr2O4was observed and it formed a network on the coating surface.’Short-circuiting effect’caused by the spinel MnCr2O4and coating spalling due to the CTE mismatch resulted in a sharply reduction of the hydrogen permeation performance. The PRF of the900℃annealed coating was reduced from65.3-138.7to5.6-11.1.(5) The Cr2O3coating on316L was prepared via MOCVD route. The coating has a corundum structure with strong preferred orientation of (110). The coating was corresponds well with the stoichiometric Cr2O3and was dense, crack-free. The apparent hydrogen permeability of Cr2O3coating with a thickness of366nm was P=2.66x10-6exp(-113522/RT) mol/m·s·Pa05and the PRF of the coating was24-117.(6) The PRF of the Cr2O3coating raised from13.1-36.1to42.2-165.4as the thickness raised from222nm to904nm. The hydrogen permeation performance increased as the thickness of the coating increase. However, excessive thickness causes an excessive inner stress. This could result in failure behaviors such as crack or spalling of the coating. The PRF of a1820nm thick Cr2O3coating was only3.1-4.6at650℃-700℃.(7) The PRF of the Cr2O3coating with preferred orientation was38.2,21.1and13.1at600℃,650℃and700℃， respectively, while the random orientation coating was only2.6,1.9and1.5,10times lower than the former one. The crystalline grain of the preferred orientation coating was regularly arranged, so the structure of the coating was compact. Whereas the random orientation coating possesses a crystalline grain of messy arrangement, defects like through-hole were easily generated in the coating resulted in a serious decline of the hydrogen permeation performance.(8) Cr2O3/Al2O3composit coating was prepared on316L. The PRF of the coating was 229.8-543.5at550℃-700℃, much higher than the A12O3coating and Cr2O3coating with similar thickness which PRF were94.7-246.9and24.1-116.5respectively. There were three reasons for it. Firstly, preparation of the Cr2O3buffer layer has formed a CTE gradient structure, reduced the inner stress and improved the high temperature stability of the coating. Secondly, the compactness of the coating was improved as the inner stress reduced. The relative density of the Al2O3coating and Al2O3in Cr2O3/Al2O3composit coating was88.7%and94.3respectively. Thirdly, preparation of the Cr2O3/Al2O3composit coating generated a Cr2O3-Al2O3interface. The migration rate of the hydrogen atom was low at the interface due to the low activity of the oxygen atom. For the above reasons, the hydrogen permeation performance of the Cr2O3/Al2O3composit coating has been significantly improved.(9) Preparation of the Cr2O3buffer layer has formed a CTE gradient structure, reduced the inner stress and volume shrinkage of the Al2O3coating when annealed at900℃. Therefore, invalidation behavior of the Al2O3coating was avoided when it transformed from amorphous to γ-Al2O3. Moreover, due to the’lattice induced effect’, a little α-Al2O3emerged in the interface of Cr2P3and Al2O3. However, compared with the amorphous Al2O3in Cr2O3/Al2O3coating, the hydrogen permeation performance of the crystalline Al2O3in Cr2O3/Al2O3coating has not been improved, but slightly decreased. The PRF of amorphous and crystalline Al2O3in Cr2O3/Al2O3coating was108.1-389.1and130.9-208.3respectively.The studies have shown that, the increase of the micro-crack numbers caused by the poor crystallization quality was the main reasons for the high permeability.