Fig. 2
From: Integrated computational materials engineering of corrosion resistant alloys
CALPHAD-predicted Potential—pH Pourbaix diagram for Inconel 600. The composition used for calculation is 76.5Ni-15.5Cr-8Fe (wt%). Alloying elements Mn, Cu, Si, and C are ignored here due to their incidental concentrations (<1 wt%). The diagram was calculated in Thermo-Calc under the conditions of 25 °C, 1 atm, 1 kg H2O, 0.6 M NaCl, total metal ion concentration of 10−4 M with the concentration of Ni, Cr, and Fe ions being 7.394 × 10−5 M, 1.76 × 10−5 M, and 8.46 × 10−6 M. The databases employed were TCAQ3, SSUB3, and SSOL4. TCAQ3 contains most of the aqueous oxidized species for Ni, Cr, and Fe (e.g., Ni2+, Ni(OH)3-, Cr2+, Cr3+, CrO42-, Fe2+, Fe3+, etc.); SSUB3 includes all the three elements as single oxides/hydroxides and the Fe-Cr, Ni-Fe, Cr-Ni spinels; the FCC phase in the appended SSOL4 database is included to describe the Inconel 600 matrix. Gas phases in SSUB3 along with H2 and O2 in TCAQ3 are excluded from the calculation to reveal the complete stability regions of oxide phases, which otherwise would be overlaid and blocked by gas phase regions. The water stability region boundary lines were added back into the diagram afterwards manually (blue dashed lines). Various regions are highlighted in the diagram: the gray region is the immunity region where oxidation of metals is predicted to occur; the red region indicates where aqueous phase is stable (at least one of the aqueous metal species is stable, but not necessarily all alloying elements are aqueous) and corrosion might occur; the blue, green, and yellow regions represent where protective oxides are stable and passivity may register. More specifically, the blue region is where Fe–Cr spinel is stable, the green region is where Cr2O3 is stable, and the yellow region is where other oxides are stable
