Progress on the corrosion resistance of CoCrFeNi high entropy alloys

As a new type of metal material, high entropy alloy has been emphasized for its excellent comprehensive mechanical properties and corrosion resistance. The current research status of the corrosion resistance of CoCrFeNi high entropy alloys is discussed from the aspects of alloying, pre-deformation treatment, and heat treatment, and the development of CoCrFeNi high entropy alloys is proposed in the outlook.

Ye Junwei first proposed the concept of high entropy alloys in the 1990s, which is different from the traditional alloy materials prepared based on a single main element but is prepared by using arc melting, mechanical alloying, and magnetron sputtering with 5 or more elements in equimolar ratio or nearly equimolar ratio to form simple solid solution phases, which have high strength, high abrasion resistance, high plastic toughness, high corrosion resistance, and good Co, Cr, Fe, Ni atomic radius and electronegativity is similar, it is easy to form a simple structure of a single FCC structure, Co, Cr, Ni and other passivation elements can be added to improve its corrosion resistance. Therefore, this topic is reviewed from the alloying, pre-deformation treatment, and heat treatment on the CoCrFeNi system of high-entropy alloys of the influence of corrosion resistance.

1. The influence of alloying elements on the corrosion behavior of alloys

Co, Ni, Fe elements in CoCrFeNi high entropy alloy have high electrode potential, Cr element is the main corrosion resistant element, Co, Cr, Fe, Ni elements in the matrix can be good mutual solubility, there is almost no elemental bias, so it has good corrosion resistance. Wang Tongyang used non-self-consumption vacuum melting furnace to prepare CoCrFeNi high entropy alloy and found that the alloy in 3.5% (mass fraction) of NaCl solution slightly corrosion, can be seen that it has a certain seawater corrosion resistance. Their corrosion resistance changed when changing the molar ratio of Co, Cr, Fe, and Ni elements. Yang Hai-Ou et al. studied the CoCrFeNi high-entropy alloys of the constituent elements of the content of its corrosion resistance in NaCl solution. They found that when the Co, Cr content is the same, with the increase of Fe content and Ni content decreased, dimensional passivation current density decreases, the passivation film on the substrate has a better ability to protect the Cr content remains unchanged, the content of Co decreases, and at the same time improve the content of Fe, Ni, the self-corrosion When the Fe and Ni content is increased, the self-corrosion potential is increased. The tendency of alloy corrosion is reduced. Alloying elements will significantly affect the material’s corrosion resistance; in the CoCrFeNi, based on the addition of alloying elements, forming 5 or more than 5 kinds of corrosion-resistant high entropy alloy has become a research hot spot.

1.1 The effect of Al on the corrosion resistance of high entropy alloys

The addition of Al has different effects on the corrosion resistance of high entropy alloys in an aqueous solution at room temperature. Still, it makes them have excellent resistance to high temperature oxidation. It is found that, with the increase of Al content, the corrosion tendency of Al_xCoCrFeNi alloy in 3.5% NaCl solution increases, the surface passivation film is broken through earlier, and the corrosion rate increases significantly. The addition of Al makes the alloy change from a single FCC structure (x=0.3) to the composite structure of FCC and BCC (x=0.5,0.7), and the pore corrosion is concentrated in the BCC phase rich in Al and Ni elements. The pore erosion is mainly concentrated in the BCC phase region, rich in Al and Ni elements. With the increase of Al content, the passivation film formed on the surface of the alloy increases in Al content. In contrast, the Cr content decreases, and the surface protective film layer becomes thicker and thicker but more and more dispersed; this Al oxide film layer is porous and not dense, and it can’t effectively prevent Cl^– erosion. QIUY et al. found that with the increase of the Al content up to x=0.6, the corrosion potential of Al_xCoCrFeNiTiy alloy increases. Corrosion current density decreases, and the corrosion current density decreases. The corrosion current density decreases, and the alloy shows better resistance to full-scale corrosion than the alloy without Al. Still, the pitting potential decreases, the passivation interval narrows, and the resistance to pitting corrosion decreases. When the Al content is further increased to x=0.9, the corrosion potential of the alloy decreases rapidly, the corrosion current density increases and the corrosion resistance deteriorates. LINCM et al. studied the organization, hardness, and corrosion performance of Al_0.5CoCrFeNi high-entropy alloy. They found that Al has a large atomic radius and forms droplet-like Al-rich-Ni phase segregation in the FCC substrate. Cl- ions preferentially erode the Al-rich-Ni phase, forming Al-rich-Ni phases. Cl^– ions preferentially erode the Al-Ni rich phase, and the corrosion potential and pitting potential of the alloy in 3.5% NaCl solution are lower than that of 304L stainless steel. Similarly, Ocean Cao found that in 0.5 mol/L H₂SO₄ solution, Al_yCoCrFeNiV_0.3 (y=0.4, 0.8, 1.2, 1.6, 2.0) high entropy alloy system has no obvious passivation interval, with the gradual increase of Al content, the corrosion potential of the Al_yCoCrFeNiV_0.3 alloy system decreases, the corrosion current density increases and the corrosion resistance of the high entropy alloy deteriorates. The corrosion resistance of high entropy alloy becomes worse.
Adding Al is not conducive to the corrosion resistance of high entropy alloys in the Cl- and acidic environments. Still, the resistance to high-temperature oxidation becomes better. LIUYX et al. studied the oxidation behavior of Al_xCoCrFeNi high entropy alloys (x=0.15, 0.4) in supercritical water at 550 and 600 ℃ and the surface oxide film is more intense on the surface of the high entropy alloys, compared with that on the H₃C steel, the surface oxide film is more intense on the surface of the Al_0.15CrFeNi and Al_0.4CrFeNi alloys, compared with that of the H₃C steel, the surface oxide film on the surface of the high entropy alloys is more intense. Compared with H₃C steel, Al_0.15CrFeNi, and Al_0.4CrFeNi alloys have thinner oxide film on the surface, finer size of oxide particles, and excellent anti-temperature oxidation performance. Zhang Hua found that Al_0.5CoCrFeNi high entropy alloys at 800-900 ℃ no obvious oxide generation on the surface, good resistance to high-temperature oxidation, but at high temperatures and the mass ratio of 3:1 Na₂SO₄ and NaCl in the molten salt corrosion is serious, molten salt through the surface of the loose and porous Al₂O₃ film diffusion to the substrate inside the occurrence of internal oxidation and internal vulcanization, thus leading to high-temperature corrosion.

1.2 The effect of Mo on the corrosion resistance of high entropy alloys

An adequate amount of Mo can improve the localized corrosion resistance of high entropy alloys in the Cl^– environment. Still, it is unfavorable to the corrosion resistance in the acid environment. Li Fushun et al. compared and analyzed the corrosion resistance of CoCrFeMnMo_0.3Ni alloy and 304 stainless steel with a small amount of Mo in 1 mol/L H₂SO₄ solution and 3.5% NaCl solution and found that uniform corrosion of high entropy alloy occurred in acidic medium, and intergranular corrosion and pitting corrosion of 304 stainless steel appeared; there was obvious passivation interval of the high entropy alloy in Cl-environment. In the Cl^– environment, the high-entropy alloy has an obvious passivation interval, secondary passivation, and relatively low corrosion current density; CoCrFeMnMo_0.3Ni high-entropy alloy has better corrosion resistance than 304S stainless steel in both acidic medium and Cl^– environment. Wang Tongyang in the CoCrFeNi alloy based on the preparation of CoCrFeNiMo_0.2 and CoCrFeNiMo_0.5 high-entropy alloys, found with CoCrFeNi alloy compared to the CoCrFeNi alloy, CoCrFeNiMo_x alloys in 3.5% of the sea-salt solution of the self-corrosion potential is slightly reduced, but pitting corrosion potential is significantly improved (see Figure 1), and expand the passivation interval. The addition of Mo improves the stability of the passivation film of the alloy, which is conducive to the improvement of seawater corrosion resistance, and the corrosion resistance of CoCrFeNiMo_0.2 is better than that of CoCrFeNiMo_0.5. The passivation film of the alloy is mainly composed of Cr₂O₃ and MoO₃, and with the increase of the content of Mo, the content of MoO₃ is increased, and the content of Cr₂O₃ is increased firstly and then decreased. When the Mo content reaches (CoCrFeNi)₉₇Mo₃, Cl^– is easy to intrude into the matrix along the weakness of the passivation film at the interface of Cr₂O₃ and MoO₃. Wei Lin et al. believe that Mo content is too high, the dendrite internal Cr, Mo elements to the inter-dendrite migration precipitation of Cr, Mo elements enriched σ-phase, due to the existence of dendrites and dendrites and inter-dendrites potential difference, so that the Cr-poor, Mo phase for the anode, Cr-rich, Mo-rich phase for the cathode, the occurrence of galvanic coupling corrosion, resulting in a reduction in corrosion resistance.

Fig.1 Dynamic potential polarization curves of CoCrFeNiMo_x (x=0,0.2,0.5) high entropy alloy in 3.5% sea salt solution

1.3 Effect of Cu on the corrosion resistance of high entropy alloys

Cu can improve the corrosion resistance of the alloy in H₂SO₄, while serious localized corrosion occurs in Cl-. Ren Mingxing et al. prepared CrFeCoNiCu high entropy alloy by copper mold suction casting. They found that the alloy did not form a single-phase solid solution of Cr, Fe, Ni, Co, Cu₅ elements; the content of Co, Cr, Fe, and Ni in the dendrites is comparable, while the content of Cu is lower, and most of the Cu is gathered in the dendrites. The Cu is poorly soluble in the solidification of Cu and other elements and, as a solute element, is excluded from forming a Cu-rich phase in the dendrites. LINCM et al. found that Cu_0.5CoCrFeNi corroded seriously in 3.5% NaCl solution, and the Cu-rich phase between the dendrites was preferentially attacked by Cl- ions, and localized corrosion occurred. HSUYJ et al. investigated the corrosion behavior of FeCoNiCrCu_x high-entropy alloy in a 3.5% NaCl solution. They found that with the increase of Cu content, the Cu element in the dendrites was more than the solute element in the solidification. The Cu element was excluded from the solidification of the dendrites, the solute element in the dendrites, and the solvent element in the dendrites, which was the solute element in the dendrites. It was found that with the increase of Cu content, the Cu element in the inter-dendritic segregation is serious, Cu- rich dendrites, and Cu-poor (Cr-rich) dendrites have an obvious potential difference to form a corrosion microcell and are the first to be eroded. Fu Hongbo studied the effect of Cu content on the corrosion resistance of AlCoNiFeCrCu_x high entropy alloy and found that with the increase of Cu content, high entropy alloy in 3.5% NaCl solution corrosion potential rises, self-corrosion current decreases, AlCoNiFeCrCu_1.0 alloy shows the best corrosion resistance. Then the Cu content increases, the corrosion potential decreases significantly, the self-corrosion current corrosion density increases rapidly, and the corrosion rate is accelerated.

1.4 Effect of Ti on the corrosion resistance of high entropy alloys

Ti has a positive electrode potential; the pure Ti surface can easily form a dense self-healing oxide film and passivation, tens of times higher than stainless steel corrosion resistance. Ti atomic radius is large; adding Ti increases the lattice distortion, changes the crystal structure of high-entropy alloys, and improves the strength of high-entropy alloys. At the same time, adding an appropriate amount of Ti can generate dense TiO and TiO₂ protective film, improve the corrosion resistance of high entropy alloy in acidic and Cl^– containing aqueous solution, and improve its resistance to high-temperature oxidation.YUY et al. investigated the corrosion resistance of AlCoCrFeNiTi_0.5 high entropy alloy in 90% H₂O₂ water and found that with the prolongation of the immersion time, the corrosion products generated on the surface of the alloy had a good protective effect on the base, and the corrosion products generated on the surface of the alloy were very good, and the AlCoCrFeNiTi_0.5 high entropy alloy was very good. The corrosion resistance of AlCoCrFeNiTi_0.5 high-entropy alloy in 90% H₂O₂ water was found to be good; with the increase of immersion time, the corrosion products generated on the surface of the alloy had a good protective effect on the substrate, and the corrosion rate of AlCoCrFeNiTi_0.5 high entropy alloy immersed in the water for 14 days was only 0.00209mm.a^-1, with good corrosion resistance. The corrosion area mainly appears in the junction of Al-Ni-Ti dendritic crystal and ultrafine Fe-Cr mesh eutectic organization, which forms a corrosion microcell due to the potential difference between the two. QIUY et al. found that, in the 0.6 mol/L NaCl solution, the corrosion resistance of AlCrFeNiCu alloy is better than AlCoCrFeNiCu alloy in the corrosion resistance of AlCoCrFeNiTi_0.5 high-entropy alloy compared to that of CoCrFeNi alloy. CoCrFeNiTi_0.5 high-entropy alloy has lower corrosion potential and higher corrosion current density. Still, the pitting potential and passivation interval are significantly higher than that of Al_0.9CoCrFeNi high-entropy alloy, and the anti-pitting corrosion performance improves, but less than 304 stainless steel. High entropy alloy is used in boilers because of its good high temperature resistance; Li Ping et al. used Na₂SO₄ + 25% NaCl to simulate the use of coal-fired power station furnace tube environment, studied the CoCrFeNiTi_0.5 high entropy alloy high-temperature corrosion behavior in a molten salt environment, found that in the early stage of the reaction, the surface of the high entropy alloy oxidized to generate a continuous dense protective film containing Ti, Cr, with the prolongation of time, Ti, Cr and the surface oxidation of the surface of the high entropy alloy. With the extension of time, Ti and Cr elements diffuse to the surface, the oxidized layer and the interface due to Cr and Ti depletion are chlorinated with microporous, SO²_-4 enters the substrate through the pores to produce internal sulfidation, and the corrosion is aggravated. Pre-coated alkaline molten salt CoCrFeNiTi_0.5 alloy in the 0.75% SO₂ high-temperature atmosphere sulfide serious, not only with the film oxides reacted with the generation of metal sulfate, but also occurred in the alloying elements of sulfide.

1.5 The influence of Mn on the corrosion resistance of high-entropy alloys

Mn is used to expand the austenite phase area of the element, traditional steel materials often add Mn to replace part of the Ni element to maintain the austenite structure characteristics, but Mn on the corrosion resistance of stainless steel is unfavorable. The addition of Mn also reduces the corrosion resistance of CoCrFeNi-based alloys. CoCrFeMnNi high-entropy alloys after rapid solidification of the dendritic crystal formation of the Mn-Ni-rich zone in the Cl-enriched and depleted Mn-Ni zone and the Mn-rich zone and depleted Mn-Ni zone in Cl-enriched and depleted Mn-Ni zone in the Cl-enriched. SARRAFHT et al. studied the corrosion performance of CoCrFeNi and CoCrFeMnNi high-entropy alloys in 0.1 mol/L NaCl solution. They found that the corrosion potential and pitting potential of CoCrFeMnNi high-entropy alloys were lower, the corrosion current density was increased, and the size of surface pitting pits was larger and deeper than that of CoCrFeNi alloys. The addition of Mn reduces the melting point of the alloy, the atomic replacement migration is more likely to occur, the surface generates more defective Mn-containing oxides, while the Cr content in the surface oxide film is reduced by more than 50% (see Fig. 3), the oxide film is more likely to be damaged, and corrosion is likely to occur.

Fig.2 Surface corrosion crater morphology of CrFeCoNi and CrMnFeCoNi high entropy alloys in 0.1 mol/L NaCl solution at 55℃ under the applied potential of +0.2V_SCE.

Figure.3 Depth distribution of elemental composition of the corroded surface of high entropy alloys in 0.1 mol/L NaCl.

2. Effect of pre-deformation treatment on the corrosion resistance of high entropy alloys

Plastic deformation is often used to improve the mechanical properties of traditional alloys. CoCrFeNi alloy is a single FCC structure with good plasticity but low strength and hardness, so it is hoped to obtain the improvement of the comprehensive mechanical properties by deformation treatment. With the increase of deformation, the dislocation density and defects of the alloy increase, resulting in the decrease of corrosion resistance, while at the same time, the oxide film is easier to be generated on the surface of the defects during the corrosion process when the defects are increased to a certain degree, but it is conducive to the generation of a more dense protective film to improve its corrosion resistance. The current research on pre-deformation treatment mainly focuses on two aspects one is deformation treatment during the preparation process, and the other is deformation treatment after the completion of the preparation, such as rolling deformation and pre-pressure deformation. Huang Yina et al. carried out 30%, 60%, and 90% rolling deformation treatment on Al_0.5CoCrFeNi high entropy alloy; the alloy still had FCC structure after rolling, and the phase structure did not change, but the intensity of the diffraction peaks changed significantly due to the rotation of the grains in the rolling process. With the increase of deformation, the grain is elongated along the rolling direction, 90% deformation of the alloy appeared fibrous organization, the size and depth of corrosion holes in 3.5% NaCl solution increased, and corrosion resistance decreased. Wang Tongyang carried out 20%, 50%, and 80% pre-compression deformation treatment on CoCrFeNi high-entropy alloy and found that 20% and 50% pre-compression treatment is still a single FCC phase. In comparison, 80% pre-compression deformation appeared FeNi3 intermetallic compound phase. The CoCrFeNi high-entropy alloys with different pre-compression treatments showed different degrees of improvement in localized corrosion and corrosion resistance to seawater, of which 80% pre-compression deformation was the best. Yuan Ye studied the corrosion resistance of AlCoCrFeNi high entropy alloys with pre-compression deformation of 7%, 13%, 16%, and 21% in 1 mol/L HCl solution and found that the pre-compression deformation improved the corrosion resistance of the alloys. Still, with the increase of deformation, the corrosion resistance was first improved and then decreased. Pre-deformation changed the internal structure of the alloy, dislocations were proliferated, lattice distortion was further enlarged, and the grains were transformed from round to elongated; at the same time, deformation accelerated the diffusion of surface passivation element Co, which occupied a favorable position at 13% deformation, at this time the corrosion current density of the alloy was increased by 2 orders of magnitude compared with that of cast alloys. The content of Co, Cr, and Ni elements on its surface was higher than that of cast and other alloys: Pre-pressure, the best corrosion resistance. Tang Qunhua et al. prepared Al_0.3CoCrFeNi nanocrystalline high-entropy alloys by high-pressure twisting method and studied their corrosion resistance in 1 mol/L NaOH solution, and found that compared with the as-cast rough crystal alloys, the corrosion current density and the dimensional passivation current density of the nanocrystalline alloys were reduced by 42.9% and 21.6%, respectively. The high density of crystal boundaries and dislocations of the nanocrystalline alloys provided conditions for passivation film growth, and corrosion resistance was improved by two orders of magnitude. The high density of grain boundaries and dislocations of nanocrystalline alloys provides conditions for the growth of passivation film, and the corrosion resistance is significantly improved compared with that of cast rough crystal alloys.

3. Influence of heat treatment on the corrosion resistance of high entropy alloys

High-entropy alloys have more than one main alloying element; the atomic radius and electronegativity of these elements are inconsistent, and it is easy to produce the phenomenon of atomic bias aggregation. Numerous researches have found that the constituent elements, organizational structure, element polarization state, and phase composition of the alloys will affect the mechanical properties and corrosion resistance of the materials, and heat treatment can improve the degree of elemental segregation and enhance the corrosion resistance of the alloys. Jiang Shuying et al. prepared AlCoCrFeNi high entropy alloy using a non-self-consuming vacuum melting furnace and annealed it at 600, 800, and 1000℃. They found that Co, Cr, Fe, and Ni were uniformly distributed in the as-cast state’s dendrite, dendrites, and grain boundaries. At the same time, the Al element had some degree of bias aggregation within the dendrites. The diffusion capacity of Al atoms was increased by annealing at 600℃, the Al element dissolved back, and the bias aggregation existed within the dendrites. After annealing at 600℃, the diffusion ability of Al atoms increases, and the dissolution of Al elements reduces the polarization phenomenon. In contrast, Cr elements are enriched in the dendrites, resulting in Cr depletion among the dendrites. With the temperature increase to 800℃, Cr elements are further polarized, and when the temperature reaches 1000℃, the Cr-rich new phase of FCC is generated. With the change of Cr content, AlCoCrFeNi high-entropy alloys were passivated in the order of corrosion resistance in 3.5% NaCl solution at 1000℃ > cast state > 600℃ > 800℃and in the order of corrosion resistance in 0.5 mol/L H₂SO₄ solution at 1000℃ > 600℃ > cast state > 800℃. The passivation effect is not only related to the Cr element but also has a close relationship with Al, and the corrosion resistance in alkaline environments is not much different; both show excellent corrosion resistance. ZHANGXR et al. found that, after annealing the organization of the growth of crystal nuclei, the dendritic zone increased, the Cu element between the dendrites of the bias was improved, the Cu-rich phase area was reduced, the dendrites of the growth of weakening the metal solubility, so that the annealed 3.5% NaCl solution is almost the same as the annealed 3.5% NaCl solution. Still, the passivation effect of the passivation effect is not only related to the Cr element but also has a close relationship with Al. NaCl solution after annealing, almost no pitting pits appeared (see Figure 4), higher than the cast alloy corrosion resistance.

Figure.4 Local morphology of AlFeNiCoCuCr high-entropy alloy in different states after polarization corrosion in 3.5% NaCl solution (the upper right corner is the enlarged picture).

4. Conclusion

As a new type of metal material, CoCrFeNi high entropy alloy breaks the limitation of the traditional single element as the main element, and its simple FCC structure makes it have excellent plastic toughness and good corrosion resistance. The current corrosion resistance research mainly focuses on adding metal Al, Cu, Ti, and other main elements, pre-deformation treatment and heat treatment, etc. The influence of non-metallic elements on the corrosion resistance of less research, in addition to a variety of corrosion mechanisms in complex environments, needs to be further studied to develop an alloy system that has more than traditional materials with excellent mechanical properties, excellent corrosion resistance in complex environments and resistance to high-temperature oxidation performance, applied to a new type of materials, the application of FCC structure gives it excellent plastic toughness and better corrosion resistance. To develop alloy systems with better overall mechanical properties than traditional materials, excellent corrosion resistance in complex environments, and resistance to high-temperature oxidization, the alloy systems can be applied in complex environments such as offshore platforms, heat exchangers, and aerospace.
Author: Sun ChiChi