Hot-dip aluminium deposit as a permeation barrier for MANET steel

A hot-dip process developed in Forschungzentrum Karlsruhe (FzK), was used to produce a hydrogen permeation barrier on MANET II steel. The hot-dip aluminising method produced an intermetallic layer of Al3Fe by immersing the specimens in molten aluminium at 1023 K for either 5 or 2 min. Subsequent heat treatment of the samples at 1023 K for 15 and 30 h, respectively, in air transformed the brittle Al3Fe layer into a softer phase and additionally produced alumina (Al2O3) on the surface. All processes were carried out at temperatures below 1053 K (above Ac1b=1053 K the austenitisation of MANET II begins) in order to avoid the repetition of the whole MANET II heat treatment (1348 K, 30 min fast cool; 1023 K, 2 h). The maximum obtained permeation reduction factor (PRF), compared with the uncoated MANEA II steel, has been obtained for the specimens oxidised for 15 h in air: yielding PRF’s of 260 at 743 K and 1000 at 573 K.