An advance process of aluminum rich coating as tritium permeation barrier on 321 steel workpiece

Abstract

We have proposed an advance three-step process, Al-electroplating in ionic liquid followed by heat treating and selectively oxidation, preparing aluminum rich coating as tritium permeation barrier (TPB). In present work, the advance process was applied to 321 steel workpieces. In the Al-electroplating, pieces were coated by galvanostatic electrodeposition at 20mA/cm2 in aluminum chloride (AlCl3)–1-ethyl-3-methylimidazolium chloride (EMIC) ionic liquid. The Al coating on those pieces all displayed attractive brightness and well adhered to surface of pieces. Within the aluminizing time from 1 to 30h, a series of experiments were carried out to aluminize 321 steel pieces with Al 20μm coating at 700°C. After heat treated for 8h, a 30μm thick aluminized coating on piece appeared homogeneous, free of porosity, and mainly consisted of (Fe, Cr, Ni)Al2, and then was selectively oxidized in argon gas at 700°C for 50h to form Al2O3 scale. The finally fabricated aluminum rich coating, without any visible defects, had a double-layered structure consisting of an outer γ-Al2O3 layer with thickness of 0.2μm and inner (Fe, Cr, Ni)Al/(Fe, Cr, Ni)3Al layer of 50μm thickness. The deuterium permeation reduction factor, PRF, of piece (Φ 80×2, L 150mm) with such coating increased by 2 orders of magnitude at 600–727°C. The reproducibility of the process was also showed.