Effect of Annealing on Nature of Corrosion Damages of Medium-nickel Austenitic Cast Iron

Within the presented research, effect of annealing on nature of corrosion damages of medium-nickel austenitic nodular cast iron castings, containing 5.5% to 10.3% Ni, was determined. Concentration of nickel, lower than in the Ni-Resist cast iron, was compensated with additions of other austenite-stabilising elements (manganese and copper). In consequence, raw castings with austenitic matrix structure and gravimetrically measured corrosion resistance increasing along with nickel equivalent value EquNi were obtained. Annealing of raw castings, aimed at obtaining nearly equilibrium structures, led to partial austenite-to-martensite transformation in the alloys with EquNi value of ca. 16%. However, corrosion resistance of the annealed alloys did not decrease in comparison to raw castings. Annealing of castings with EquNi value above 18% did not cause any structural changes, but resulted in higher corrosion resistance demonstrated by smaller depth of corrosion pits.