Microcrystalline/amorphous iron alloy films for corrosion-resistant coatings

Abstract

Films of 304 stainless steel, some with additions of about one atomic per cent aluminum and/or silicon, were deposited by rf getter-sputtering on to aluminum, 304 stainless steel or Ebrite substrates. Transmission electron microscopy shows that the films contain two or more phases: a featureless matrix and small particles 15 – 30 nm in size. The transmission electron diffraction pattern of films with no additives consists of many lines, indexable as b.c.c. Fe18Cr8Ni. Diffraction patterns of films with additives contain fewer lines and a halo; lines have been tentatively assigned to both b.c.c. and f.c.c. Fe18Cr8Ni, a few lines remain unassigned, and the halo occurs where expected for an amorphous Fe18Cr8Ni alloy. The pitting potentials of these films, as measured with a potentiostat in near neutral 1 M NaCl aqueous solution, are markedly higher than those of bulk 304 stainless steel. Silicon is the more effective additive; no film breakdown was observed up to +1000 mV (limit of measurement), whereas bulk 304 broke down at −66 mV. Heat treatment at 450°C for one hour in vacuo lowered the pitting potential but retained improvement over bulk 304. These exploratory results indicate that microcrystalline/amorphous structures in steels based on Fe18Cr8Ni, as well as their composition, can lead to improved corrosion resistance over the usual crystalline material.