Effect Of Low Amount Of Si On Corrosion Resistance Of Low-alloyed FeAl Alloys Under Preheating Treatment

As one of the structural materials,metallic iron（Fe）is widely used in marine engineering,aerospace,machinery manufacturing,petrochemical industry,vehicle engineering,and other fields.However,Fe itself also has many disadvantages,such as failure phenomena during use,mainly including deformation failure,fatigue fracture failure,wear failure,and corrosion failure.Corrosion failure is the most common failure mode of Fe,which mainly refers to the corrosion of Fe in different use environments,including oxidation in air and electrochemical reactions in humid environments.This failure mode has resulted in incalculable economic losses and safety hazards,and greatly limited the scope of its actual use.The commonly used methods for corrosion prevention of Fe based alloys include traditional alloying,surface coating,and electroplating,but they all have certain disadvantages,such as rising costs,poor durability,and environmental pollution.In order to solve this problem,a combination of low alloying and preheating treatment processes is developed to improve the high-temperature oxidation resistance of metals.Considering that Al is often added to Fe as a corrosion resistant element to form alloys,this paper studied the high-temperature oxidation resistance and electrochemical corrosion resistance of low alloyed FeAl alloys under preheat treatment conditions,and further considered the effect of adding a small amount of Si.In this paper,firstly,FeAlx（x=0.5,1,and 2 wt.%）alloys were prepared and preheated in H₂ atmosphere at 800℃ and 24 h,respectively.The research shows that a protective film of Al₂O₃ with a thickness of about 20 nm is formed on the surface of FeAl1 alloy after heat treatment,the resistance to high-temperature oxidation and electrochemical corrosion of this alloy was tested and it was found that the oxidation weight gain of pure Fe under pureO₂ atmosphere at 400℃ for 7200 s was 5.27 mg cm^-2,while the oxidation weight gain of FeAl1 alloy after preheating treatment was0.47 mg cm^-2.On the other hand,the corrosion current of pure Fe in a 5 wt.%NaCl solution is 35.9μA cm^-2,while the corrosion current of preheated FeAl1 alloy is 9.8μA cm^-2.The results show that the high-temperature oxidation resistance and electrochemical corrosion resistance of FeAl1 alloy after preheating treatment are significantly enhanced,which is related to the formation of a complete Al₂O₃ protective layer on the outer surface of the alloy after preheating treatment.However,further research found that due to the strong activity of the Al element,during the preheating process,there was an obvious inner oxidation phenomenon inside the substrate,which should be an important reason for the excessively thin protective film thickness on the substrate surface,thereby limiting the further improvement of the alloy’s corrosion resistance.In order to suppress the inner oxidation of Al during preheating treatment,this paper added 1 wt.%Si alloy element to FeAl1,and further observed the effect of Si element on the corrosion resistance of low alloyed FeAl alloy after preheating treatment under the same conditions.It was found that an Al₂O₃ protective layer with a thickness of about 100 nm was grown on the surface of the FeSiAl based alloy after adding a small amount of Si,and a SiO₂ transition layer with a thickness of about 10 nm was formed between the protective layer and the substrate,and no inner oxidation of Al was found inside the substrate.The high-temperature oxidation resistance and electrochemical corrosion performance tests of the FeSiAl alloy after preheating treatment showed that the oxidation weight gain of the Fe based alloy after preheating treatment for 24 h under the same conditions was only 0.17 mg cm^-2,and its corrosion current in 5 wt.%NaCl solution was only 2.0μA cm^-2.Compared to pure Fe,its oxidation resistance has been improved by 30 times,while its electrochemical corrosion resistance has been improved by nearly 18 times.This indicates that the addition of Si can further improve the high-temperature oxidation resistance and electrochemical corrosion resistance of low alloyed FeAl alloys.This increase is related to a significant increase in the thickness of the Al₂O₃ protective layer,which can be attributed to the formation of the SiO₂ transition layer hindering the internal diffusion of O and effectively inhibiting the inner oxidation of Al in the matrix.