Study On Corrosion Behavior Of CoCrNi Medium Entropy Alloy In Simulated Marine Environment

CoCrNi medium entropy alloy（MEA）is a novel type of alloy material with high strength and hardness,excellent friction properties and corrosion resistance.Its unique properties make CoCrNi suitable for employment in many fields including mechanical equipment manufacturing,the automotive industry and aerospace.Conventional engineering materials are susceptible to corrosion failure in specific environments,such as marine environments and alternating current（AC）interference.Therefore,it is essential to explore whether CoCrNi MEA meets the requirements for service in the aforementioned environments.However,there have been few studies on the corrosion behavior of CoCrNi MEA in the marine environment,particularly on the correlation between the corrosion resistance of CoCrNi MEA and AC interference in the marine environment.Thus,this paper systematically investigates the corrosion behavior and mechanism of CoCrNi MEA in simulated marine solutions based on electrochemical tests,surface analysis techniques,immersion corrosion tests and other methods.The main conclusions are as follows.（1）The damaging effect of AC on the stability and protection of passive film may be caused by the increasing adsorption effect of harmful ions（H⁺,Cl^-）.As i _AC increases,the corrosion morphology evolves from pitting corrosion to general corrosion.The applied AC facilitates the initiation and propagation of metastable pitting corrosion.Oxide as a cathode for micro-galvanic corrosion promotes the appearance of pitting.Metal elements in the matrix and oxide exhibit selective dissolution,with Co dissolution observed in the matrix and Co and Ni dissolution in oxide inclusions.（2）HCO₃^-and HSO₃^-can both accelerate the active dissolution of the passive film.On the one hand,AC promotes the ionization of HCO₃^-and HSO₃^-into H⁺.On the other hand,the positive half cycle of AC increases the local acidity,causing Cl^-to adsorb onto the passive film,while H⁺is easily adsorbed during the negative half cycle of AC.H⁺and Cl^-as corrosive ions,significantly damage the stability of the passive film.Furthermore,the interaction between AC and acidic ions（HCO₃^-and HSO₃^-）significantly interferes with the passivation process of CoCrNi MEA.Under the influence of AC,the cathodic reaction is mainly hydrogen evolution,and the generated hydrogen atoms damage the passive film during the process of escaping into the solution.（3）The corrosion resistance of CoCrNi MEA improves with increasing annealing temperature.Higher annealing temperatures result in grain coarsening,reducing the volume fraction of grain boundaries.CoCrNi MEA annealed at higher temperatures exhibits better passivation performance and passivation film stability than samples annealed at lower temperatures.（4）The corrosion resistance of CoCrNi MEA is superior to that of 316L SS（stainless steel）.The denseness and structural stability of the passive film on CoCrNi MEA are outstanding to 316L SS,leading to lower dissolution rate in a seawater environment.Both CoCrNi MEA and 316L SS are mainly subject to localized corrosion in seawater environments.