Corrosion Behavior Of 15CrMn Steel And 304 Stainless Steel In Sodium Aluminate Solution With Low Sulfur

In bayer process,high sulfur bauxite and pyrite reacted with caustic soda to produce sodium sulfide(Na₂S),sodium thiosulfate(Na₂S₂O₃),sodium sulfite(Na₂SO₃)and sodium sulfate(Na₂SO₄)and other substances,these products corrode the equipment seriously.In the previous study,our research group found that low sulfur S^2-and S₂O₃^2-lead to a significant corrosion to 20Cr,Q235,16Mn,12Cr1Mo V and other materials.In order to expand the selection of equipment materials,this research took 15Cr Mn steel and 304 stainless steel as the research objects,combining with high temperature immersion corrosion and electrochemical testing methods,studied the influence of low sulfur ions in sodium aluminate solution on their corrosion behavior,and analyzed the corrosion mechanism.1.The influence of S^2-,S₂O₃^2-,S^2-+S₂O₃^2-concentration on the corrosion was examined when the corrosion time was 5 d:(1)15Cr Mn steel:the corrosion degree increases with the increase of S^2-and S₂O₃^2-concentration.When the concentration of S^2-increases to 5 g/L,the surface products are mainly Fe₃O₄,Na Fe S₂,Fe S and Fe OOH.The morphology of surface products are white granular,lamellate and petals.When the concentration of S₂O₃^2-increases to 5g/L,more flocculent Fe₃O₄ is formed on the surface.When the two ions co-exist,increasing the concentration of S₂O₃^2-will reduce the corrosion behavior of S^2-,and the content of sulfide in the product decreases.(2)304 stainless steel:The increase of ion concentration makes the corrosion more intense.When the concentration of S^2-rises to 3 g/L,corrosion cracks appear on the surface.When the concentration of S^2-rises to 5 g/L,the products are mainly Fe₃O₄,Fe OOH and Fe S,The morphology of surface products shows granular and petal-like morphology.When the concentration of S₂O₃^2-reaches 5 g/L,there is no crack on the surface,and the products are mainly Fe₃O₄,Cr₂O₃,and Al₂O₃ which shows granular,square and massive morphology.When the two kinds of ions exist at the same time and the concentration is 3 g/L,corrosion cracks appear on the surface,and the surface products are Fe₃O₄ and Al₂O₃ which are granular and square,and the quantity increases.2.When the anion of S^2-,S₂O₃^2-and S^2-+S₂O₃^2-are all 2 g/L,the corrosion mechanism at the same position from 6 d to 15 d is as follow:(1)15Cr Mn steel:when the solution only contains S^2-,petal-like sulfide is formed on the surface,which is unstable in alkaline solution,resulting in the falling off of Na Fe S₂and Fe S after 9 d corrosion.When the solution contained only S₂O₃^2-,the particles of oxides were eroded by sulfur ions after 9 d corrosion,and the size of particles increased and flakes of Fe S formed after 12 d corrosion.When the solution contains both S₂O₃^2-and S^2-,spherical oxides appear in the corrosion pit after 6 d corrosion,and their size increases obviously after 12 d corrosion.(2)304 stainless steel:when the solution only contains S^2-,after 9 d corrosion,the surface of granular is mainly Fe₃O₄ and other products increase,and the products are stacking corroding for 15 d.When the solution only contains S₂O₃^2-,the depth of corrosion pit increases after 9 d corrosion,and it is filled with granular oxides after 15d corrosion.When the solution contains both S₂O₃^2-and S^2-,corrosion cracks appear after 6 d corrosion,and begin to fall off after 9 d corrosion,and granular oxides increase and cover the surface after 15 d corrosion.3.Electrochemical corrosion behavior and mechanism analysis:Fe is hydrolyzed to form soluble Fe(OH)₃^-,Na₂S is hydrolyzed to produce HS^-and S^2-adsorbed on the surface of the substrate,and transform to adsorption compound ion Fe(HS)^-with Fe²⁺.When Fe₃O₄,Cr₂O₃,Al₂O₃and other oxides are formed in the reaction,both steels appear obvious passivation phenomenon.4.Through the analysis of the experimental results of ion concentration and corrosion time,the instability of sulfide in the product falls off and the oxide will have a certain protective effect on the matrix.In the solution containing S^2-,304 stainless steel is prone to form corrosion cracks.Therefore,15Cr Mn steel is preferred.In the solution containing only S₂O₃^2-,304 stainless steel with lower corrosion rate is preferred.