Grain boundary nanochemistry and intergranular corrosion of Fe-0.01wt.%P alloy: Roles of elemental segregations and misorientation

The grain boundary (GB) segregation of alloying elements in an ultralow carbon Fe–P alloy was investigated at different annealing temperatures (500 °C, 600 °C, and 700 °C) using a three-dimensional atom probe. The impact of the segregated phosphorus and carbon concentrations on intergranular corrosion was also assessed in a sodium sulfate (Na₂SO₄) aqueous solution. Comparative analysis of specimens annealed at 600 °C and 700 °C revealed that intergranular corrosion occurred only in the specimen annealed at 600 °C, which exhibited a higher concentration of phosphorous segregation at the GBs. This suggests that intergranular corrosion resistance deteriorates with increased phosphorous segregation at GBs. Additionally, the progression of intergranular corrosion was found to depend on GB misorientations, increasing up to 30° and stabilizing at higher GB misorientation angles. Carbon segregation was observed only in the specimen annealed at 500 °C, which showed no intergranular corrosion, despite having the highest concentration of phosphorus at the GBs. This indicates that carbon, identified as an impurity in the alloy, can inhibit intergranular corrosion induced by high phosphorous concentration at the GBs. This study provides insights into preventing the macroscopic intergranular corrosion in iron alloys by controlling the elemental segregation of impurities at GBs.