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Study on Seawater Desulfurization and Corrosion Behavior of Materials Used in the Equipment

Author: ZhaoWei
Tutor: PengQiao
School: Dalian University of Technology
Course: Chemical Engineering
Keywords: Sulfur Dioxide Seawater Solubility Stainless Steel Q235A Corrosion
CLC: TG171
Type: Master's thesis
Year: 2009
Downloads: 137
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Abstract


The solubility of sulfur dioxide in seawater was studied by the self-made experiment device.Besides,the corrosion behavior of 1Cr18Ni9Ti,304,316L stainless steel（SS） and Q235A carbon steel in seawater solution with different concentration of sulfur dioxide prepared by the device was studied.Measurements of the solubility of sulfur dioxide（SO2） whose partial pressures in mixtures SO2 + N2 was 540～1340ppm in seawater of different salinity at temperatures between 3℃to 33℃were taken.Different temperatures and partial pressures and salinity were of interest by actual operational conditions of seawater desulphurization unit in power plant.Results show that solubility increases with the increase of salinity and SO2 partial pressure and decrease of temperature.The solubility of SO2 in seawater at different partial pressures and the same temperature accords with linear relationship.Measurements were carried out by a saturation method and the concentration of SO2 in absorption solution was determined by iodimetry.The corrosion test results of SS were shown as follows.Corrosion potential of the three SS decreases with the increase of SO2 concentration.The corrosion potential of 316L is higher than that of 304 and 1Cr18Ni9Ti,and that of 304 and 1Cr18Ni9Ti differ little.The corrosion of the SS is controlled by anode process in SO2/seawater system.The corrosion rate of 1Cr18Ni9Ti,304 and 316L is 3.24×10-3,2.541×10-3,2.921×10-3mm/a when the concentration of SO2 is 13.30mmol/L,which is 1.5,1.7,1.9 times as much as that in pure seawater.The pitting potential of 316L is higher than 304,which is higher than 1Cr18Ni9Ti in seawater with the same SO2 concentration.The pitting potential of the three SS decreases with the increase of SO2 concentration,but the protection potential differ little with the variety of SO2 concentration.The cathodic current density is much more in SO2/seawater symtem than that in pure seawater,besides,it increases with the increase of SO2 concentration because of the effect of hydrogen evolution.The impedance value of 1Cr18Ni9Ti,304 and 316L decreases by 1～3 order of magnitude.when corrosion appears.Electrochemical noise（ECN） pitting index is 0.020 while 1Cr18NigTi is passivated,however,it turns into 0.112-0.166 when the passivation film is destroyed.The noise resistance decreases and the power spectral density（PSD） slope keeps about -17dB/decade with the evolution of the corrosion process, which indicates localized corrosion happens.The SS was corrupted serious in SO:/seawater system because of the cooperative effect of H+ ionized from H2SO3,C1- from seawater and reaction product of S during corrosion.The corrosion test results of Q235A were shown as follows.The corrosion potential increases with the increase of SO2 concentration in seawater.The corrosion of the Q235A is controled by anode process and cathode process in SO2/seawater system.The corrosion rate of Q235A is 2.259 mm/a when the concentration of SO2 is 13.30mmol/L,which is about 17 times as much as that-in pure seawater.The impedance value of Q235A decreases when the concentration of SO2 increases in seawater.The interface capacitance increases suddenly when the concentration of SO2 reachs 4.64 mmol/L because of the corrosion reaction’ product of SO2 and seawater.The corrosion product is so much because the reaction rate is very fast when the concentration of SO2 increases to 17.40 mmol/L that it prevents cathodic depolarizator reaching the electrode,when diffusion control becomes control step of the corrosion process.XRD analysis showed that the corrosion product was constituted by S, FeSO4·2H2O and FeOOH,and the product of S displayed white because of colour effect.As SS,the Q235A was corrupted serious in SO2/seawater system because of the cooperative effect of H+ ionized from H2SO3,C1- from seawater and reaction product of S during corrosion.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metal corrosion protection,metal surface treatment > Theory of metal corrosion
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