Fabrication and characterization of a dense Cr2O3-Al phosphate double coating as tritium permeation barrier

A dense Cr2O3-Al phosphate double coating was fabricated by electrodeposition and dip-coating. Cr layer was firstly electrodeposited on 321 steel substrates and subsequently was thermal oxidized in air to transform into Cr2O3 layer. Then an Al phosphate film was prepared by dip-coating to seal open pores of the Cr2O3 layer. By controlling the process parameters (current density, heat-treated temperature and dip-coating times) it is possible to obtain crack-free and homogeneous double coating. The thickness of Cr2O3 layer and Al phosphate layer is 5 μm and 0.6 μm, respectively, when electroplating current density is 0.3 A/cm2 and dip coating Al phosphate for one time. The Al phosphate layer can effectively close the open pores and decrease the porosity to 0% of Cr2O3 layer. The Cr2O3-Al phosphate double coating sample exhibits a good resistance to corrosion and improved tritium resistant performance because the Al phosphate coating successfully fills pores and defects allowing passage of gas of Cr2O3 coating. And this double coating shows good thermal cycling performance. No obvious cracks can be observed on the surface of the Cr2O3-AlPlO4 coating even after 190 cycles.