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The influence of the hold time of the austempering heat treatment at 280°C on the microstructure and corrosion resistance in NaCl-based media of austempered ductile iron was investigated using X-ray diffraction, micro-hardness measurements, corrosion tests and surface observations. Martensite was only found in the sample which was heat treated for a short period (10 minutes). Corrosion tests revealed that this phase does not play any role in the anodic processes. Numerous small pits were observed in the a-phase which is the precursor sites in all samples (whatever the value of the hold time of the austempering heat treatment). [ABSTRACT FROM AUTHOR]

• Oxidation of the ADI in the salt fog cause the formation of a dense inner and porous outer corrosion products layers. • Corrosion of ADI in the salt fog has three stages: pitting corrosion of ferrite, inward and mixed oxidation of the matrix. • Goethite and lepidocrocite were found in the inner and outer layers of corrosion products. The corrosion behavior of austempered ductile iron (ADI) under cyclic salt spray exposure was studied using a specific three-electrode cell located in the salt fog chamber. Three steps were identified, namely pitting corrosion of ferrite (very first cycles), followed by inward oxidation of the ausferrite matrix and then (iii) mixed oxidation of the matrix associated with the formation of a rust layer at the specimen surface. Evolution of the corrosion rate calculated from in-situ electrochemical measurements was correlated with surface changes quantified from optical and field emission scanning electron (FE-SEM/EDS) microscopies. Corrosion products were identified by means of micro-Raman spectroscopy. [ABSTRACT FROM AUTHOR]