Effect Of Sulfur On Corrosion Behavior Of 20Cr Alloy Steel In Sodium Aluminate Solution

The sulfur in high-sulfur bauxite is mainly in the form of pyrite.During the high-pressure dissolution process,S^2-and a small amount of S₂O₃^2-,SO₃^2-,and SO₄^2-and other forms enter the sodium aluminate solution,which seriously affects the decomposition of seeds,product quality and the corrosion behavior of the equipment material.The system of sulfur-containing sodium aluminate solution is complicated,which determines that the corrosion of the sulfur form to the equipment material must be a complex multi-phase reaction system,and the corrosion mechanism has not reached a unified understanding so far.For the commonly used material 20Cr steel with high pressure dissolution,the paper simulates the actual preparation of corrosive media,combined with high temperature soaking corrosion and electrochemical corrosion method,systematically studied the corrosion behavior of 20Cr steel in sodium sulphurate-containing solution,and analyzed the corrosion mechanism.The main conclusions are as follows:?1?In the presence of S^2-and S₂O₃^2-alone in sodium aluminate solution,the S^2-concentration is within 5 g/L,the corrosion rate of 20Cr steel increases with the increase of S^2-concentration,and S^2-reacts with Fe of the matrix to generate FeS,which is structurally loose and has strong adsorption to corrosive media,and the corrosion rate reaches a peak value of 25.24 g/m²·d at 5 g/L.When the S^2-concentration is higher than 5 g/L,the surface gradually forms a dense structure of spinel FeCr₂O₄,which will play a certain role in the corrosion,and reduce the corrosion rate.With the increase of S₂O₃^2-concentration,it gradually produces more dense corrosion products such as Fe₃O₄,FeCr₂O₄ and FeAlO₃,which reduces the corrosion rate of 20Cr steel.?2?In the sodium aluminate solution containing 3g/L S₂O₃^2-,when the concentration of S^2-is less than 5g/L,the surface corrosion products are mainly FeS,which is easy to fall off and reduce the protective effect of the surface layer.The radius of arc resistance decreases with increasing S^2-concentration,while the I_corr and corrosion rate increase gradually.When the concentration of S^2-is more than 5g/L,a compact FeCr₂O₄ product is formed on the surface,which reduces the corrosion rate.The corrosion rule is the same as that of S^2-individual,but the corrosion rate is lower than that in the presence of S^2-alone.?3?When S₂O₃^2-is added to the sodium aluminate solution containing 5g/L S^2-,the coupling effect between S₂O₃^2-and S^2-is generated,and the corrosion rate of 20Cr steel is lower than 5g/L S^2-in the presence of S^2-.The increase of S₂O₃^2-concentration facilitates the formation of a dense product layer of FeAlO₃.The corrosion rate and corrosion current density gradually decrease,and the arc radius of capacitive reactance gradually increases.?4?Adding different concentrations of SO₃^2-and SO₄^2-in a solution of sodium aluminate containing 5g/L S^2-and 3g/L S₂O₃^2-,coupling corrosion occurred on the surface of the 20Cr sample to form a more stable sulphite and sulfate sulphate.The film will play a deterrent role in the corrosion behavior of 20Cr steel.In general,S^2-corrosion to 20Cr steel in sodium aluminate solution is strong,the corrosion rate reaches a maximum at 5g/L,and S₂O₃^2-hinders the corrosion behavior of 20Cr steel.When S^2-and S₂O₃^2-coexist,the two kinds of valence sulfur have the coupling corrosion effect.The formation of dense corrosion products on the surface of the sample inhibits the S^2-corrosion behavior of 20Cr steel.When SO₃^2-or SO₄^2-was added to the sodium aluminate solution containing S^2-and S₂O₃^2-,the coupling effects of the three valence states of sulfur existed,and the S^2-corrosion behavior of 20Cr steel was hindered.