Relationships between microstructure and pitting corrosion of ADI in sodium chloride solution

Austempered ductile iron (ADI) has complex microstructure containing a multiphase matrix (called 'ausferrite'), graphite spheres and oxide inclusions. The corrosion resistance of ADI is related to its microstructure which is determined by heat treatment parameters (like austempering temperature, austempering time, austenitising temperature and austenitising time). In the present paper, the electrochemical behaviour and corrosion resistance of ADI have been investigated by means of the electrochemical microcell technique and classical electrochemical measurements in sodium chloride solution. Particular attention has been paid to the influence of austempering temperature on the microstructure and pitting corrosion. It has been shown that ADI austempered at 430 A degrees C has upper ausferritic microstructure and reveals a better corrosion resistance in sodium chloride solution than ADI austempered at 280 A degrees C. Moreover, the corrosion resistance increases as the volume fracture of ferrite increases and the carbon content of austenite decreases. The good corrosion behaviour of ADI austempered at 430 A degrees C was also related to the good coarsening of the austenite grains and broad ferrite needles (less ferrite/austenite interfaces). It has been demonstrated that silicon is the alloying element hindering the anodic dissolution of the alloy.