Influence of graphite morphology on the corrosion behaviour of cast irons for automotive industry

Cast irons are widely used in the automotive industry due to its excellent castability, fluidity, machinability and wear resistance. Particularly, engine block, cylinder line, brake rotor, cylinder head, headbox cases and camshaft are fabricated from different shade of cast irons. Grey cast irons (flake graphite particles), compacted cast irons (worm-like graphite particles) and ductile cast irons (spheroidal graphite particles) are long available in the market but recently, a great attention is paid of a new generation of high silicon cast iron which are subject of researchers and engineer applications. Despite their widespread use, literature reflects scarce studies concerning the electrochemical corrosion behaviour of cast irons. The present research focusses on studying the influence of metallurgical parameters (chemical composition, graphite morphology and matrix microstructure) on the corrosion resistance of cast irons including the electrochemical response of high silicon ductile irons. Electrochemical measurements (current voltage curves and electrochemical impedance spectroscopy) were coupled with microstructural analyses from microscopy characterization. Corrosion test were performed in a 0,03M NaCl solution at room temperature under static conditions. Polarisation curves obtained after 90 min of immersion in the corrosive medium reveal none influence of the graphite morphology for ordinary cast irons; in contrast, current densities were higher when silicon was added. This result could be attributed to the silicon effect that changes the matrix microstructure from pearlite to ferrite. Impedance measurements are carrying on to completing this study and verify the hypothesis.The results obtained in the present investigation show that all cast irons have similar values of current density (Icorr). However, the alloy with flakes graphite had a significantly higher corrosion potential value (Ecorr) than those with nodules and chunky graphite. 

The corrosion products formed in grey cast iron present different morphologies compared to the corrosion products formed in spheroidal and chunky cast iron.