基于氨法脱硫浆液F离子对不锈钢局部腐蚀自催化过程的阻抑作用及机理研究

Ammonia-based desulfurization has developed rapidly due to the merits of high efficiency, low investment, small in scale, and no pollution discharge. The selection of suitable materials for the equipment is difficult, because carbon steel encounter serious corrosion in the halogen contained acid slurry, Cl may cause localized corrosion and is harmful to stainless steel, and F is detrimental to glass in reinforced resin materials. The regular operation and promotion of this technique has been restricted by corrosion of the equipment. Recently, research have suggested that the initiation stage of localized corrosion with stainless steel can be suppressed by the insoluble fluoride metals formed in acid solution, but the influence of F and coexisted F/Cl on the development stage of stainless steel localized corrosion have not been studied yet. This project aims at clarifying the influence of F on the propagation of localized corrosion in ammonia-based desulfurization, the occluded cell will be applied to investigate the inhibition effect of fluoride to propagation of localized corrosion of stainless steel, the concentration of F and F/Cl ratio under optimal suppression effect, the expel to Cl and sedimentation of fluoride on the surface of stainless steel in occluded cell, and the mathematical model on the effect of F to the corrosion rate of stainless steel. This project intends to reveal the essence and discipline of stainless steel localized corrosion, and provides theoretical support to understand the corrosion process and selection of material. Meanwhile, the results will contribute to the theoretical system conformation of stainless steel concerned localized corrosion.

氨法脱硫因脱硫效率高、投资少、占地小、无废固液排放等优势，发展迅速。因含卤素酸性硫铵浆液对碳钢腐蚀严重、F-对增强树脂材料中的玻璃损伤大、氯离子对不锈钢的局部腐蚀倾向大怕穿孔而不选用，选材难。装置腐蚀是该工艺有效运行和推广的瓶颈。近来的研究表明，F-在偏酸性液中能生成难溶金属盐而对不锈钢局部腐蚀的萌生具有抑制作用，但未见F-和F-/Cl-共存时对不锈钢局部腐蚀发展阶段作用研究。项目基于氨法脱硫含Cl-浆液，着重研究F-对不锈钢局部腐蚀自催化反应的阻抑作用及机理，用构建的不锈钢闭塞电池自催化模型研究F-对闭塞电池反应的阻抑规律、最大阻抑作用时F-浓度及氟/氯离子比、闭塞区F-对氯离子的排挤和电极电沉积的阻抑机理、F-浓度与闭塞区不锈钢腐蚀速率的数学式等。可揭示出氟氯共存介质中不锈钢局部腐蚀的本质和规律，为把握其腐蚀过程和选材提供科学支撑，同时丰富不锈钢局部腐蚀理论.

本项目基于F-与Cl-共存的氨法脱硫浆液，首先探讨F-对304不锈钢材料均匀腐蚀和局部腐蚀萌生的作用，继而自行设计并优化模拟闭塞电池模型，着重研究F-对304不锈钢和316不锈钢局部腐蚀闭塞电池发展过程的影响，最后采用电渗析技术对Cl-进行分离并对F-和Cl-进行浓度调控，实现对不锈钢腐蚀的控制。通过电化学和挂片失重法考察了F-对304不锈钢均匀腐蚀和局部腐蚀萌生的作用。研究表明，Cl-浓度的增加会提高304不锈钢的均匀腐蚀速率，而且使得不锈钢发生小孔腐蚀的几率增加。当Cl-浓度一定时，低浓度的F-会增加不锈钢的均匀腐蚀速率，但高浓度的F-会抑制不锈钢的腐蚀，F-与Cl-摩尔比为10时抑制作用较显著。构建局部腐蚀模拟闭塞电池模型，可对闭塞区溶液中的F-、Cl-、pH和溶解氧进行在线监测。以含有相应(NH4)2SO4浓度的不同F-和Cl-浓度标准溶液对复合电极进行标定后，测定结果与真实值均较接近，相对标准偏差小且测量稳定性高，并且可以采用断电和不断电两种方式对闭塞区溶液各参数进行测定。优选的模拟锈层为混氧化铁+CaSO4（9:1），并且用0.5%琼脂固定在滤纸层上，可实现对硫酸根离子的排斥，有助于模拟真实的离子迁移方式。模拟闭塞电池可长时间连续运行，可有效模拟酸化自催化发展过程。研究发现，F-通过降低Cl-迁移量、调节闭塞区溶液pH以及与金属离子生成不可溶产物等方式，实现对闭塞区腐蚀的抑制作用。F-对316不锈钢模拟闭塞电池发展的作用与304不锈钢一致，说明对于奥氏体不锈钢而言，F-的作用具有一致性。使用打孔试片模拟小孔腐蚀发展过程，F-对钻孔内部腐蚀过程的影响与模拟闭塞电池实验结果一致，说明了模拟闭塞电池模型的有效性并验证了F-的作用。采用电渗析工艺以及优选阴离子交换膜，可将模拟氨法脱硫浆液中的Cl-优先脱除，以形成高氟低氯的介质环境。实验表明，使用均相膜并调节电压和溶液pH，能够选择性脱除Cl-，即使使用实际氨法脱硫浆液也能取得良好的分离效果。该项目的研究内容不仅阐明了F-对不锈钢腐蚀，尤其是不锈钢局部腐蚀闭塞电池阶段的具体作用，同时也提出了解决方案，对氨法脱硫装备的升级和技术推广具有重要意义。