Hydrogen Permeation Behaviour of Hot-Dip Aluminized MANET Steel

A hot-dip process developed in FZK, was applied to produce a hydrogen permeation barrier on MANET steel. The formation of the alumina layer is a two step process. The hot-dip aluminizing method produced first an intermetallic layer of Fe_xAl_y by immersing the specimens in molten aluminium at 1073 K for 10 min. Secondly, by its exposure to an oxygen containing gas (1223 K, 10 and 30 h) the alumina layer is formed on the intermetallic layer. The last step is to form a fully martensitic phase (δ-ferritic free structure) by a specific heat treatment (1348 K, 30 min fast cool; 1023 K, 2 h).The oxide layer and bulk material were characterized by optical metallography, Vickers microhardness measurements, scanning electron microscope (SEM) with energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) on the surface. A permeation reduction larger than three orders of magnitude was obtained in the sample that has undergone a 30 h exposure in air at 1223 K.