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The corrosion behavior of CoCrNi medium-entropy alloy (MEA) in the coastal marine environment polluted by excessive CO2 was investigated. Increasing NaHCO3 concentration expediates the active dissolution of the passive film and damages its protectiveness. The positive half-cycle of AC increases local acidity and encourages Cl− to adsorb on the passivation film, whereas H+ ions are easily adsorbed during the AC-negative half-cycle. H+ and Cl− as aggressive ions can significantly undermine the stability and denseness of passivation film. Furthermore, the interactive effect of alternating current and HCO3− significantly interferes with the passivation of the MEA, contributes to the nucleation of metastable pitting, and increases the occurrence probability of stable pitting, aggravating anti-corrosion performance. The cathodic reaction under the influence of alternating current is dominated by hydrogen evolution; the generated hydrogen atoms can tear the passivation film and deteriorate the corrosion resistance. [ABSTRACT FROM AUTHOR]

Effect of temperature on the corrosion behavior of CoCrNi medium-entropy alloy (MEA) in 3% NH4Cl solution was investigated by means of electrochemical measurements, immersion test and statistics analysis. The results show that increasing temperature makes it more difficult to form stable passive film on the MEA surface, accelerates the rupture of the film and reduces the anti-corrosion resistance of the MEA. Particularly at higher temperature, the corrosion rate of the MEA increases, as well the size and number of pits increase. In addition, NH4+ is further hydrolyzed with increasing solution temperature, which promotes the initiation and propagation of metastable pitting, slows down the re-passivation process and enhances the possibility of transition from metastable pitting to stable pitting. [ABSTRACT FROM AUTHOR]

A comparative study on the corrosion behavior of CoCrNi medium-entropy alloy (MEA) with CoCrFeMnNi high-entropy alloy (HEA) in SO2-polluted marine environment was investigated by a series of tests. Results indicate that CoCrNi MEA possesses smaller passive current density, larger polarization resistance and lower corrosion rate compared with those of CoCrFeMnNi HEA, which demonstrates that the MEA possesses better anti-corrosion performance than the HEA. This could be ascribed to the detail difference in the microstructure and passive film of the two alloys. Compared with the MEA, the smaller grain size and the larger number of oxides reduce the anti-corrosion performance of the HEA. Moreover, the oxide and matrix constitute the micro-galvanic corrosion cell, which promotes the generation of pits. [ABSTRACT FROM AUTHOR]

The corrosion behavior of CoCrFeMnNi high entropy alloy in 3.5 wt.% NaCl solution with and without 0.05 M NaHSO3 was studied by electrochemical measurement, immersion test, and atomic force microscope. The results show that with the addition of HSO3−, the state of activation indicated by the anodic reaction evolves to the passivation state. Corrosion resistance is related to the state of the film on the HEA surface. The addition of HSO3− may produce a protective composition that transforms the corrosion product film into passive film. With the presence of HSO3−, the dense and highly protective passive film is formed, which improves the corrosion resistance of the HEA. Without HSO3−, the loose, incompact and incomplete corrosion product film produced on the surface of the HEA exhibits a low protective effect. [ABSTRACT FROM AUTHOR]