Study On Corrosion And Cavitation-corrosion Behavior Of Al_0.3CoCrFeNi High Entropy Alloy

High entropy alloys（HEAs）are new type of multi-principal alloy that is different from the traditional single-principal alloy.The Al_0.3CoCrFeNi HEAs has become one of the focuses in research due to its excellent properties of high strength,superior plasticity and well fatigue resistance.Metals in fluid service（such as boilers,pipelines and valves in high pressure）face failure caused by corrosion and cavitation.The excellent comprehensive mechanical properties and high content of corrosion-resistant elements of Al_0.3CoCrFeNi HEAs make it have potential to obtain good corrosion resistance without degradation of strength,thus solving the problem of failure in structural materials.Most of the current studies on the corrosion behavior of Al_0.3CoCrFeNi HEAs have focused on the effect of alloying elements on corrosion resistance.The strength of the single-phase Al_0.3CoCrFeNi HEAs is not sufficient to fit the requirements in service,and the introduction of strengthening phases to further improve its strength is the focus of many studies,especially the introduction of B2 phase.However,the introduction of B2 phases is inevitably affecting the corrosion resistance while improving the mechanical properties.The influence of B2 phase on its corrosion resistance does not exclude the influence of other factors,such as content of alloying elements and other phases.Fewer studies have been conducted on the cavitation of Al_0.3CoCrFeNi HEAs.In this study,the B2 phases of rolled Al_0.3CoCrFeNi HEAs were regulated by annealing.The influence of B2 phase on the corrosion behavior of Al_0.3CoCrFeNi HEAs was investigated by microstructural characterization,electrochemical testing,and surface analysis.The microstructure characterization reveals that the content of（Al,Ni）-rich and Cr-poor B2 phases increases and the electrochemical measurements show that the pitting corrosion resistance is deteriorated as the annealing temperature decreases.Accordingly,the B2 phases are confirmed to be detrimental to the corrosion resistance of Al_0.3CoCrFeNi HEAs.It can be attributed to the preferential dissolution of B2 phases causing easier pit initiation,which is evidenced by the corrosion morphology observation and scanning Kelvin probe force microscopy measurements,and to the slower repassivation rate leading to easier pit propagation,as demonstrated by the quantitative analysis of potentiostatic polarization curves.The cavitation damage mechanism of Al_0.3CoCrFeNi HEAs was investigated by mass loss tests and electrochemical analysis.Both Al_0.3CoCrFeNi high-entropy alloys annealed at homogenizing temperature and 800 ^oC exhibited cavitation resistance superior to that of 304stainless steel.The cavitation damage mechanisms of the Al_0.3CoCrFeNi HEAs annealed at different temperature were different.The cavitation mass loss of the homogenized Al_0.3CoCrFeNi HEAs were mainly caused by pure cavitation.In contrast,the mass loss of the Al_0.3CoCrFeNi HEAs annealed at 800 ^oC were mainly caused by a combination of pure cavitation and the promotion of cavitation by corrosion.In the early stage of cavitation,cavitation mainly promotes oxygen transfer to make E_corr increase;in the later stage,it mainly breakdown the passivation film to make E_corr decrease and promote i_corr.The dissolution of B2phases forms pitting corrosion,which causes force concentration and thus promotes mechanical damage.The promotion of corrosion to cavitation resulted in slightly greater mass loss in the Al_0.3CoCrFeNi HEAs annealed at 800 ^oC than in the homogenized sample.