Model of vacancy diffusion-assisted intergranular corrosion in low-alloy steel

A model is presented for intergranular corrosion (IGC) of nonsensitized low-carbon pipeline steels, in which enhanced grain boundary (GB) oxidation is enhanced by vacancies produced by oxidation of reactive Si atoms. Evidence for vacancy injection at corroding GBs had been provided by recent nanoindentation measurements. Model calculations of IGC evolution controlled by lattice vacancy diffusion are compared to experimental measurements. Realistic dihedral angles of corroded GB grooves are predicted based on a vacancy diffusivity within the range of values expected for bcc iron. The calculations rationalize observations of sharpening of GB grooves with time during corrosion exposures, and show that vacancy diffusion quantitatively accounts for enhancement of IGC by reactive solute atoms, without invoking solute segregation or precipitation at GBs.