The use of aluminium alloys for spe

The use of aluminium alloys for specific applica- tions has promoted the manufacture of aluminium alloyed with diverse metals and non-metals in order to improve its mechanical and chemical properties. In many cases, alloying improves the mechanical strength, but it results in an increase in the susceptibility of the alloy to corrosion degradation. The decrease of the cor- rosion resistance is due to the formation of intermetallic compounds which develop an insignificant non-passive corrosion product film. This induces the formation of sites on the surface that act as local anodes in the aluminium matrix which remains properly protected by a passive oxide film. Some aluminium alloys are pro- tected against corrosion attack through processes of sur- face modification such as the application of chromate conversion coatings [1,2]. Aluminium alloys develop anodic oxide films on their surfaces when they are subjected to anodic polarization in aqueous solutions [3–5]. The anodizing process produces a close-packed array of porous columnar cells attached to a barrier layer that is perpendicular to the metallic substrate [4]. The development of non-toxic conversion coatings for aluminium alloys and other metal has shown that rare earth metals (REM) have an inhibiting effect on these materials [6–8]. On the other hand, aluminium–REM alloys have shown high corrosion resistance in alkaline electrolytes as demonstrated by Crosland et al. [9]. Also REMs have been added to other metals as minor alloy- ing elements to improve the corrosion resistance of new alloys [10]. Normally, when anodic polarization is applied to aluminium alloys, the second-phase particles precipitated and the intermetallic compounds may be transferred from the metal substrate into the anodic oxide films as either oxides or metallic particles, or they may be dissolved into the solution by selective dissolu- tion reducing the corrosion resistance [11,12].Recently developed Al–La alloys manufactured by directional solidification at different rates in Bridgman growth had three types of microstructures i.e. aAl den- drites plus eutectic, fully lamellar flake eutectoid, and b phase plates plus eutectic [13,14]. Hawksworth [13] determined the solidification microstructure selection map for Al–5.7 to 19 wt.%La and growth velocities be- tween 0.002 and 5.0 mm/s. This author also found on longitudinal sections of Bridgman samples an effect of segregation in Al–14.5 wt.%La in a velocity range be- tween 0.01 and 2.0 mm/s.