Microstructure and Corrosion Resistance in Cl^- Containing Environment of Three-Composition Medium Carbon Alloy Steel

Three test steels with composition (mass fraction/%) of 0.35C,0.9Mn, 3.5Cr, 0.6Si, 0.3V, 0.05Ti (namely N1), 0.35C, 1.2Mn, 2.0Cr, 0.8Si, 0.4V, 0.11Ti (namely N2) and 0.40C,0.6Mn, 3.5Cr, 0.8Si, 0.1V, 0.23Ti (namely N3) were designed based on 40Cr steel composition, and the microstructure, electrochemical properties and corrosion resistance in 3.5wt% NaCl solution of test steels were studied. The results show that N3 test steel had the smallest grain size and the narrowest martensite lath bundle, followed by N1 test steel and N2 test steel. N1 test steel had the highest open circuit potential, the highest free-corrosion potential, the lowest free-corrosion current density, the largest absolute value of the maximum phase angle, and the largest radius of the capacitive arc. After soaking in NaCl solution for 24 h, localized pitting corrosion occurred on the surface of N1 and N3 test steel, while uniform corrosion occurred on N2 test steel. N1 test steel had high chromium content, moderate grain size, the highest surface corrosion product compactness and the best corrosion resistance.