CORROSION EVALUATION OF AUSTEMPERED ALUMINIUM-ALLOYED DUCTILE IRONS IN WELL WATER AND 0.5M NaCl SOLUTION

The corrosion behaviour of Al-alloyed ductile irons processed through rotary melting furnace and sand casting was evaluated. This was to ascertain how viable is the applicability of the low cost ductile irons for design of components for mechanical infrastructures which could also be suitable for use in environment where corrosion damage is a dominant failure mechanism. The ductile irons which contained 1 - 4 wt.% Al were austempered, adopting both conventional and two-step processes, with optical microscopy used to characterize the microstructural parameters. The corrosion studies were carried out in well water and saline solution of 0.5 M sodium chloride (NaCl) using potentiodynamic polarization electrochemical method. The results revealed that the as-cast ductile irons containing Al, had improved corrosion resistance in both well and saline solutions. The corrosion rate decreased from 0.51936 mm/yr for the as-cast ductile iron without Al to 0.06303 mm/yr for the composition containing 3.74 wt.% Al. The austempered ductile irons generally had superior corrosion resistance compared to the as-cast ductile iron compositions, with the conventionally austempered ductile irons (treated at 400°C), having the best corrosion resistance. The presence of ausferritic structure in the austempered ductile irons was linked to the improved corrosion resistance.