Search

The corrosion behaviour and mechanism of CoCrNi medium-entropy alloy (MEA) imposed by alternating current (AC) in HCO3−/CO32− solution with various pH were systematically investigated via a series of corrosion tests. The results showed that with the augment of pH value, the polarisation resistance of the MEA decreased significantly, Ep shifted negatively, and ipI value increased, as well as corrosion rate was enhanced, revealing that the corrosion resistance was weakened. As solution pH increased, the formation of secondary passivation zone became easier, which may be attributed to the reaction conversion process of Cr(III) to Cr(VI) species. Moreover, the increase in pH led to an increase in defect density and a thinning of passivation film, facilitating the corrosion of the MEA. AC interference accelerated the ion exchange process and promoted the formation possibility of stable pitting. Higher iAC and pH inhibited the repassivation of metastable pitting, and increased the pitting susceptibility. Highlights The effect of pH on the corrosion behaviour of CoCrNi MEA with imposed AC was clarified. High pH and iAC accelerated the nucleation and growth of metastable pits. With increased pH and iAC, the stability and protective ability of passive film were weakened. The secondary passivation of the MEA was mainly due to the conversion of Cr(III) to Cr(VI) species. [ABSTRACT FROM AUTHOR]

The corrosion behavior and mechanism of CoCrFeNi HEA under AC application were systematically studied in 3.5 wt.% NaCl solution containing different concentrations of NaHSO3. The results showed that the existence of HSO3− increased the defect density within the passivation film on the surface of the HEA. With the increase of HSO3− concentration, the passivation zone became narrow, Ep shifted negatively, and ip increased, which indicated that the corrosion resistance of the HEA was weakened. When the HSO3− concentration increased to 0.1 M, a clear transition zone appeared, indicating that the increase in concentration reduced the passivity of the HEA, and its corrosion behavior changed from the typical passivation characteristic to the activation state with increasing iAC. The imposed AC promoted the nucleation of metastable pitting, which was indicated by a remarkable increase in the average pitting number density (Navg) from 5 to 17 mm−2 with the enhancement of iAC. [ABSTRACT FROM AUTHOR]

The effect of different annealing temperatures on the corrosion behavior of equiatomic CoCrFeMnNi high entropy alloy was investigated by electrochemical tests, immersion test, atomic force microscopy and X-ray photoelectron spectroscopy in alkaline soil simulation solution. The results show that with the increase of annealing temperature, the corrosion resistance of the HEA sample exhibits a trend of first increasing and then decreasing. The micro-galvanic corrosion effect between grain interior and grain boundary decreases as grain size increases. Moreover, MnO inclusions distributed in all annealed samples promote the occurrence of pitting corrosion due to the selective dissolution of Fe, Co, and Ni elements. For the HEA sample annealed at 850 °C, the presence of twins further facilitates the formation of pits. The 750°C-annealed sample possesses the optimal corrosion resistance due to passive film which has a stable structure and the highest contents of Cr oxide and H 2 O, and the lowest content of hydroxide. Therefore, the corrosion behavior of HEA samples treated at different annealing temperatures is closely related to grain size, inclusion and twins within microstructure, as well as the passive film on the sample surface. Corrosion mechanism diagram of HEA sample with MnO inclusion. [Display omitted] • 750°C-annealed HEA sample exhibits the highest anti-corrosion property. • Selective dissolution of Fe, Co, and Ni elements causes the occurrence of pits. • For HEA annealed at 850 °C, the presence of twins facilitates the formation of pits. • The existence of MnO inclusion promotes the occurrence of pitting corrosion. [ABSTRACT FROM AUTHOR]

This paper investigates the difference in the corrosion resistance and passive property of CoCrNi medium-entropy alloy and 316 L stainless steel in a simulated marine environment by using electrochemical measurements, immersion test and surface analysis technology. The results reveal that CoCrNi MEA has more superior anti-corrosion resistance than that of 316 L. The difference in the corrosion resistance is related to the microstructure and surface passive film. Due to the presence of oxide inclusion, micro-galvanic corrosion is formed between the oxide inclusion and CoCrNi matrix, which promotes the anodic dissolution of CoCrNi MEA and induces the formation of pits. The matrix as the anode mainly undergoes the selective dissolution of Co element, while the selective dissolution of Co and Ni elements is occurred in the cathodic oxide inclusion. [Display omitted] • In 3.5%NaCl solution, the corrosion resistance of CoCrNi MEA is better than that of 316 L. • Anti-corrosion property of MEA and 316 L is associated with grain size, oxide and passive film. • Micro-galvanic corrosion effect between oxide and matrix facilitates the corrosion of MEA. • Pits on MEA are mainly attributed to the dissolution of Co element. • Inclusion as cathode occurs the selective dissolution of Co and Ni elements. [ABSTRACT FROM AUTHOR]

The corrosion behavior and mechanism of CoCrFeMnNi HEA interfered by AC current was investigated in carbonate/bicarbonate solution. AC application can destroy the structure of passive film, generate a change in the content of constitutes, and decrease the stability and protective ability of passive film, especially at higher i AC. With increasing i AC , the corrosion form of HEA is evolved from pitting corrosion occurred at low i AC to intergranular corrosion generated at high i AC. At the interference of 200 A/m2, more H atoms are distributed at grain boundaries and interior of grains, resulting in the occurrence of severe corrosion. Moreover, the selective dissolution of Co/Mn occurs in the interior of corroded grains of HEA sample. However, selective dissolution of Fe, Co and Cr is occurred at pits and grain boundaries. Unlabelled Image • AC corrosion behavior of CoCrFeMnNi HEA was investigated in CO 3 2−/HCO 3 − solution. • AC application damaged the structure of passive film, and changed its composition. • An increased i AC reduced the stability and protective ability of passive film. • Corrosion form of the HEA was evolved from pitting corrosion to intergranular corrosion. • Selective dissolution of Fe, Co and Cr was occurred at pit and grain boundary. [ABSTRACT FROM AUTHOR]