Surface dissolution and formation and scallops

Flow-assisted corrosion (FAC) is a significant problem with carbon steel components exposed to rapidly moving water or water-steam mixtures. Such components often develop distinctive patterns of surface damage producing a dimpled surface looking like orange peel, called “Scalloping”. This roughness plays an important role in the corrosion of pipes made of carbon steel and it seems that the formation of scallops are major factors in the thinning rate of the pipes. To characterize scallops, study the mechanisms of scallop formation and investigate how the formation of scallops and scallop phenomena affect the dissolution rate, experiments on the pressure drop and flow characteristics, of pipes made of plaster of Paris (CaSO₄-1/2H₂O) were performed. Atomic Absorption Spectroscopy (AAS) was used to analyze the dissolution rate of the plaster. The surface was photographed with a digital camera to observe the initiation of scallops. Pressure transducer was used to measure pressure drop. The size decreases with increasing flow rate whereas the population of scallops increases with increasing flow rate. Scalloping is believed to initiate from defect at the surface and it was found that size and population of scallops increase with increasing initial defect size and initial defect concentration respectively. The average dissolution rate increases with increasing flow rate, particle size, particle concentration and temperature. The dissolution rate of plaster is controlled by mixed kinetics. The entrance section affected the mechanism of the gypsum dissolution. It is found that concentration of defects on the plaster surface has greater effect on the dissolution rate than effect of defect size. Pressure drop increases with increasing flow rate and temperature but decreases with increasing initial defect size and concentration. This means that the diameter of the plaster pipe has a greater effect than the surface roughness.

流动辅助腐蚀（Flow-assisted corrosion, FAC）是暴露于高速流动水或水-汽混合物中的碳钢构件面临的一类重要工程问题。此类构件常形成特征性的表面损伤形貌，产生类似橘皮的凹坑表面，该现象被称为“扇贝状腐蚀（Scalloping）”。这种表面粗糙度对碳钢管道的腐蚀过程具有显著影响，而扇贝状蚀坑的形成被认为是管道壁厚减薄速率的核心影响因素。为表征扇贝状蚀坑、研究其形成机理，并探究扇贝状蚀坑的形成过程与相关现象对溶解速率的影响，研究人员开展了针对熟石膏（plaster of Paris, CaSO₄·1/2H₂O）管道的压降与流动特性实验。实验采用原子吸收光谱法（Atomic Absorption Spectroscopy, AAS）分析熟石膏的溶解速率；通过数码相机拍摄管道表面，观察扇贝状蚀坑的萌生过程；利用压力传感器测量管道压降。实验结果显示：扇贝状蚀坑的尺寸随流速升高而减小，而其数量密度则随流速升高而增大。普遍认为，扇贝状腐蚀起源于材料表面的初始缺陷，且蚀坑尺寸与数量密度分别随初始缺陷尺寸与初始缺陷浓度的增加而增大。平均溶解速率随流速、颗粒尺寸、颗粒浓度以及温度的升高而提升。熟石膏的溶解过程由混合动力学机制控制，管道入口段会对石膏溶解的反应机制产生影响。研究发现，熟石膏表面的缺陷浓度对溶解速率的影响程度显著大于缺陷尺寸的影响。压降随流速与温度的升高而增大，但随初始缺陷尺寸与浓度的增加而减小。这表明，熟石膏管道的管径对压降的影响程度大于表面粗糙度的影响。