Study Of The Electrochemical Behaviour Of 316LN Austenitic Stainless Steel In Ammonium Chloride Solutions

A large amount of NH₃ and HCl formed during the refining process enters the heat exchanger with the flow of material,which under certain conditions combines to produce ammonium chloride,which is deposited and then clogs the tube bundle causing a pipe burst or absorbs water to form a highly concentrated or saturated ammonium chloride solution,which is extremely corrosive.The temperature of the heat exchanger tube bundle varies greatly in different areas,and the ammonium chloride in the tube bundle absorbs water to form a saturated ammonium chloride solution at the corresponding temperature,resulting in differences in the saturated ammonium chloride solution formed in the tube bundle in different areas.Very little research has been carried out on materials in saturated ammonium chloride solutions,so it is extremely important to clarify the electrochemical behaviour of materials in saturated ammonium chloride solutions.In this paper,the electrochemical behaviour of 316LN austenitic stainless steel in ammonium chloride solution was investigated by electrochemical means and surface analysis techniques using 316LN austenitic stainless steel as the study material and the effect of saturated ammonium chloride solution,immersion time and the content of Na₂S₂O₃ on the corrosion resistance of316LN was investigated.The results obtained from the thesis are as follows:（1）The effect of saturated NH₄Cl solutions corresponding to different temperatures on the electrochemical behaviour of 316LN was studied using electrochemical test methods such as dynamic potential polarisation and surface analysis techniques such as Scanning Electron Microscope（SEM）and X-ray photoelectron spectroscopy（XPS）.As the saturation concentration of NH₄Cl increased,the self-corrosion potential of 316LN gradually shifted positively,the pitting and protection potentials gradually decreased and the width of the passivation interval shortened;all the passivation films formed in the saturated NH₄Cl solutions showed both n-type and p-type semiconductor responses.XPS depth profiling shows that at lower saturation concentrations the inner layer of the passivation film is mainly Cr₂O₃ and the outer layer is mainly Fe OOH,Fe₂O₃,Fe Cl₂ and Cr（OH）₃,with increasing saturation concentrations the Cr element in the passivation film starts to enrich and the hydroxide and chloride on the surface of the passivation film starts to increase.（2）The long-term immersion process of 316LN in different saturated NH₄Cl solutions was monitored by the Electrochemical impedance spectroscopy（EIS）technique,and the morphology of the material surface after immersion was observed by optical camera and SEM.the EIS data showed that the corrosion resistance of the specimens increased and then decreased with the increase of the immersion time in the same saturated NH₄Cl solution;in different saturated In different saturated NH₄Cl solutions,the best corrosion resistance of the specimens gradually decreased with the increase of the saturation concentration.The surface morphology can be observed,with the increase of saturation concentration,the sample surface pitting increased,the corrosion pits became larger,and the surface corrosion products increased.（3）The effect of Na₂S₂O₃ incorporation on the electrochemical behaviour of 316LN austenitic stainless steel in NH₄Cl solution was investigated using electrochemical test methods such as cyclic polarisation,EIS,Mott-Schottky curves and surface analysis techniques such as SEM and XPS.The addition of Na₂S₂O₃ resulted in a significant increase in the self-corrosion potential of316LN,which then gradually decreased as the concentration of Na₂S₂O₃continued to increase;the addition of Na₂S₂O₃ had little effect on the pitting potential,but caused the passivation interval of 316LN to almost disappear;the EIS data indicated that the addition of Na₂S₂O₃ caused a decrease in the corrosion resistance of 316LN;The SEM results show that the surface pits on316LN become larger and more numerous with the addition of Na₂S₂O₃;XPS shows that the oxygen content of the material surface decreases and the S content increases with the addition of Na₂S₂O₃,resulting in the formation of FeS,FeS₂ and NiS₂.