Study Of Corrosion Behavior Of CoCrNi Medium Entropy Alloy In HF&HCl Mixed Medium

With the improvement of living standards,a new and more environmentally friendly refrigerant HFC-134a has been developed to deal with the irreversible damage to the atmospheric ozone layer caused by the use of traditional refrigerant Freon.However,since the raw material for the preparation of HFC-134a has HF and the reaction product contains HCl.This reaction environment with strong corrosive HF&HCl mixed medium obviously puts forward higher requirements for the corrosion resistance of the corresponding container materials.The medium entropy alloy is a new type of multi-principal element metal material developed in recent years.Compared with the single principal component of traditional alloys,the multi-principal component configuration leads to MEAs with high configuration entropy,slow atomic diffusion,large lattice distortion and"cocktail effect"in performance.These special intrinsic characteristics make some MEAs systems have excellent corrosion resistance.Studies have shown that the corrosion resistance of CoCrNi MEA in a single acidic medium is better than that of traditional alloys,and it is generally believed that the less the number of grain boundaries,the better the corrosion resistance of the metal.However,the corrosion behavior and microstructure evolution of CoCrNi MEA in mixed media and the effect of grain size on its corrosion resistance have not been reported in the literature,and relevant basic research is needed.In this paper,the corrosion behavior of CoCrNi MEA in HF&HCl mixed medium was studied by high temperature immersion test,and compared with the existing Ni-based alloy.At the same time,the effect of grain size on its corrosion resistance in this medium was discussed.The main results are as follows:（1）CoCrNi MEA was prepared by vacuum arc melting drop casting method,and its corrosion law in HF&HCl mixed medium at different concentrations and temperatures was studied and compared with Ni-based alloy.The results show that with the increase of corrosion medium concentration and environmental temperature,the corrosion will be more serious,but the sensitivity of CoCrNi MEA to temperature is weaker than that of Ni-based alloy.Under the same corrosion conditions,the weight loss rate of Ni-based alloy is at least 5 times that of CoCrNi MEA,which proves that the corrosion resistance of CoCrNi MEA is better than that of Ni-based alloy.（2）Through the analysis of the element distribution on the surface of the two alloys and the composition of the passivation film,it was found that compared with the Ni-based alloy,the CoCrNi MEA passivation film contained more Cr³⁺and Co elements not found in the Ni-based alloy,resulting in a significant reduction in the Cl^-and F^-content on the surface of the CoCrNi MEA.（3）CoCrNi MEA with three different grain sizes were obtained by cold rolling and heat treatment,and the effect of grain size on the corrosion resistance of CoCrNi MEA in HF&HCl mixed medium was studied.It is found that under the same corrosion time,the corrosion rate of FG sample is the slowest,the corrosion resistance is the best,and it can reach the stable corrosion stage quickly,and its corrosion type is intergranular corrosion.（4）Through the element distribution of three different grain size samples,it can be seen that with the increase of Co element and Cr element content,the content of F element invading the sample decreases.The reason why the corrosion rate of FG sample is the slowest is that a dense passivation film containing Cr³⁺and Co²⁺is deposited on the surface of the sample,which hinders the erosion of the corrosive medium.