Microstructure and corrosion properties of ultrafine-grained interstitial free steel

The microstructure and the corrosion behaviour of ultrafine-grained interstitial free (IF) ferritic steel processed by equal channel angular pressing (ECAP) at room temperature following route C were investigated. Already after the first pass of ECAP, the microstructure was refined by a factor of approximately 200. On further passes, ECAP was found to lead to continuous grain refinement, while elongated grain structure produced in the first pass was retained throughout all processing cycles. After 8 passes, the microstructure comprised bands of elongated grains of average length of 500–1000 nm and average width of 200–300 nm. Corrosion characteristics of a non-deformed coarse grained specimen and an ultrafine-grained specimen that had undergone 8 passes of ECAP were investigated using electrochemical potentiodynamic tests. ECAP induced grain refinement does not appear to influence the electrochemical characteristics in neutral NaCl solutions. This encouraging finding demonstrates that ECAP, while enhancing mechanical characteristics, does not compromise corrosion resistance of IF steel. A protective oxidic layer is formed in alkaline NaCl solution, which reduces the corrosion rate significantly.