High-purity aluminium, when subjected to total immersion at 20" C to 80" C in pure water carries surface films which are either " amorphous " or crystalline hydrates. Below a critical temperature, between 60" C and 70" C, film growth proceeds in three stages :(1) " amorphous " ; (2) boehmite, yAIO(0H) ; orthorhombic, a = 3-78 A, b = 11.8 A, c = 2.85 8, ; and (3) bayerite, /3-A1203 . 3H20 ; monoclinic, a = 4.72 A, b = 8-68 A, c = 5.06& cc = 90" 11', the final film thus consisting of three layers. Above the critical temperature only boehmite films are formed on the top of the initially present amorphous film. Oriented overgrowth is favoured by both boehmite and bayerite: (001) parallel to the surface for boehmite, and (010) parallel to the surface for bayerite. Immersed aluminium is almost immune to pitting; it does, however, suffer severe intergranular attack, especially above 60" C.
Small amounts of a number of alloying materials were added to Type 304 stainless steel in an effort to improve its corrosion resistance to superheated steam at 650 C (1202 F), 42 kg/cm2. Aluminum and siliconwere valuable additions, but the amount of aluminum required to reduce the corrosion rate significantly embrittled the alloy. Type 304 containing about 3w/oSi appeared to be a practical alloy.
In a similar fashion, the addition of about 3w/oAl to Incoloy 800 greatly improved its resistance to static superheated steam.
Films stripped from samples corroded in steam with various prior surface preparations were examined by electron microscopy and chemical analysis in an attempt to clarify the reasons for differing corrosion behavior. It was concluded that the rapid diffusion of chromium into the corrosion films formed on mechanically worked surfaces is the key factor in the improved corrosion resistance of these surfaces.
The action of boiling distilled water on aluminium, including mechanically polished, electro-polished and anodized specimens, has been investigated by electron diffraction. The patterns correspond to boehmite, y-AlO(0H). Oriented diffractions, obtained from specimens immersed for approximately 5 min, were attributed to (100) planes in the boehmite lattice being parallel to the specimen surface. Boehmite is probably produced by the reaction of hydroxyl ions with aluminium ions moving outwards from the metal ; the reaction first occurs in the pores of the pre-existing oxide layer and later on the outer surface. An electron diffraction method for determination of film thickness on aluminium single crystals is described.
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