Investigation on the Electrochemical Corrosion-Induced CaCO3 Scaling at Steel/Water Interface
Author:Zhu Tian Zhen
Supervisor:liu gui chang
CaCO3 scaling on metal wall is an extremely widespread phenomenon.It often leads to pipe blockage and the decrease in heat exchange efficiency,etc.,along with the increase in operation and maintenance costs.So,various strategies have been applied to reduce the scaling tendency.Nonetheless,scale formation on the wall of metal equipment just can be mitigated to some extent due to the complexity of physicochemical environments.A further illustration of the scaling behavior is very necessary.Metal electrochemical corrosion plays important roles in CaCO3 scaling.It often occurs accompanied by the deposition of CaCO3,where a close relationship exists between the two interfacial processes.However,so far corrosion influences on CaCO3 scaling have not been systematically demonstrated.In fact,beside the electrochemical induced deposition at cathode,the anodic processes also exert some influences on the cathodic deposition behavior of CaCO3.Based on these interfacial processes,three main investigations were carried out in this thesis.Firstly,the roles of corrosion cathodic processes in the promotion of scaling were evaluated.Then,the influences of anodic processes on CaCO3 polymorphs were explored.Lastly,some work was conducted to illustrate the influences of anodic processes on the performance of scale inhibitors at metal/water interface.The main results are summarized as below:(1)The roles of cathodic induced nucleation in the promotion of CaCO3 scaling during electrochemical corrosion processes.CaCO3 scaling behavior during macro-and micro-corrosion processes was investigated at different medium temperatures.The macro-and micro-corrosion cells were build using carbon steel/stainless steel galvanic corrosion and the self corrosion of each steel substrate,respectively,where the driving forces of cathodic induced nucleation were tuned by introducing a corrosion inhibitor.Results reveal that the cathodic induced nucleation has a stronger promotion effect on scaling than the increase in medium temperature.Once the cathodic processes are inhibited,CaCO3 scaling is evidently retarded even at elevated temperatures.(2)The influences of anodically released Fe2+ ions on the polymorphs of CaCO3 on corroded carbon steel surfaces.Such effects were demonstrated by investigating the scaling behavior of CaCO3 on stainless steel and carbon steel during their galvanic corrosion processes.Results reveal that these Fe2+ ions influence CaCO3 polymorphs by changing interfacial chemical environments.Once anodically released Fe2+ ions diffuse to cathodic regions,they suppress pH increase by preferentially consuming cathodically produced OH-and interfacial dissolved oxygen,and by their incomplete hydrolysis,eventually favoring the deposition of aragonite.(3)The influences of anodically released Fe2+ ions ol the performance of scale inhibitors at carbon steel/water interface.The anti-scale behavior of phosphorus-containing scale inhibitors at corroded carbon steel/water interface was explored mainly through galvanostatic deposition tests.Results reveal that anodically released Fe2+ ions strongly interfere in their anti-scale performances by destroying the interfacial coordination equilibrium between Ca2+ and inhibitors.During corrosion processes,these Fe2+ ions preferentially replace the chelated Ca2+at carbon steel/water interface to form more stable coordinators.The released Ca2+ ions are forced to interact with cathodically formed CO32-at cathodic regions,resulting in the deposition of CaCO3 scale.Thus,the scale inhibitors cannot function effectively at carbon steel/water interface.This thesis has,from a viewpoint of electrochemical corrosion,demonstrated how corrosion cathodic and anodic processes affect the scaling and anti-scale behavior at steel/water interface.These studies not only provide a new perspective for scaling investigation,but also facilitate the improvement of the existing technologies for the evaluation of scale inhibitors or scale control.