Study On Improving Corrosion Resistance Of High Chromium Ferrite Stainless Steel

The high chromium ferrite stainless steel developed in this paper is mainly used in chemical industry and low temperature heat recovery system,which requires high corrosion resistance in sulfuric acid,phosphoric acid and Cl-environment.Alloying elements copper,titanium niobium,rare earth and heat treatment process all affect the corrosion resistance of the material.The effects of alloying elements and heat treatment on the corrosion resistance of high chromium ferrite stainless steel are studied.It is desirable to expand the application of high-chromium ferritic stainless steel by these studies.In this paper,the high-chrome ferrite stainless steel with different composition was smelted by Vacuum Medium frequency induction smelting process.The effect of alloying elements Cu,Ti + Nb,rare earth and solution treatment on the corrosion resistance of the high-chrome ferrite stainless steel was investigated in H₂SO₄,H₃PO₄,NaCl and FeCl₃ solution.The tuffy curves and impedance spectra of the alloys were measured by electrochemical experiments to observe the changes of self-corrosion current,self-corrosion potential,dimensional current and impedance.The characteristics of microstructure,precipitation phase and distribution at the grain boundary were analyzed by scanning electron microscopy and Lycra optical microscope.The surface corrosion of the alloy was observed by scanning electron microscopy.The results show that the addition of alloying elements had a great effect on the passivation film and the precipitated phase.The researches show that the corrosion rate of high chromium ferrite stainless steel increases and then decreases with the increase of copper in 90% H₂SO₄ solution,40% phosphoric acid solution and 3.5% NaCl solution.The alloy is resistant to full corrosion when the copper content is 1%.When the copper content is less than 1%,with the increase of copper content,the passivation film of the surface of the stainless steel becomes thicker,the passivation film becomes thicker,the number of pits decreases and the area becomes smaller.When the copper content is more than 1.0%,with the increase of copper,the passivation film of the alloy is defective and the number of pits increases,and the area increases.In the FeCl₃ solution,the corrosion rate of high chromium ferrite stainless steel with different contents increased with the increase of chloride content.In the same concentration of FeCl₃ solution,the corrosion resistance of the alloy increases first and then decreases with the increase of copper,and the corrosion resistance of the alloy is the strongest at 1.5%.The addition of excess copper causes the copper in the steel to saturate the copper-rich phase of theε-Cu phase,and they are more likely to reduce the corrosion resistance of the alloy at the grain boundary.When the copper content of the alloy is 1.0%,titanium niobium and rare earth are added in succession.The corrosion resistance of the alloy is gradually enhanced in all three solutions.The optimum corrosion resistance of the cast high chromium ferrite stainless steel was just solution treatment at 950℃,1000℃,1050℃ and 1100℃.The results show that the corrosion resistance of the alloy is the strongest in the three solutions at 1050℃.The experimental results show that the addition of copper to the ferrite stainless steel is significantly reduced at the grain boundary and the precipitation phase is small,indicating that the addition of copper causes precipitation of precipitates such as Fe（Cr）₂₃C₆ inhibition;When the introduction of titanium niobium and rare earth,titanium niobium can generate a very stable titanium carbide to inhibit the formation of chromium-rich phase,rare earth can be added to purify the ferrite grain boundary,improve the uniformity of the organization,so that its corrosion resistance significantly enhanced.The results of this paper have a good effect on the theoretical research and practical application of high chromium ferrite stainless steel,which provides theoretical guidance for the design and development of high chromium ferrite stainless steel with excellent corrosion resistance.It is of great practical significance to further study the application of high chromium ferrite stainless steel in chemical production.