Mi, Tangwei; (2021) Investigation into the Passivation and Corrosion Process of Mild Steel by Raman Spectroscopy. Doctoral thesis (Ph.D), UCL(University College London).

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Abstract

Since the reinforced Portland cement (PC) concrete was first used in 1854, it has been the most popular material in constriction industry owing to its durable properties. In particular, the high alkaline environment within PC concrete promotes the formation of passive film on reinforcing steel, which prevents the steel from corrosion. However, the passive film can be damaged by chlorides and carbonation attacks. Consequently, corrosion of reinforcing steel still happens due to chloride ingress or carbonation of concrete, which leads to significant economic losses. To solve this issue, timely repairs are essential to extend the service life of reinforced concrete structures. Monitoring the health condition of reinforcing steel is thus necessary to give an early warning. However, current monitoring techniques relying on the interactions with the physical properties, such as internal strain and cracks, are not able to provide comprehensive information about the causes and evolution of the corrosion. Optical fibre Raman system is potentially an alternative technique to develop a future sensor system, which is capable to provide detailed information of the evolution of corrosion products. However, to develop such a sensor system, the capability of Raman spectroscopy (without optical fibre) to differentiate the evolution of the passivation and corrosion products should be confirmed first. This thesis therefore aims to investigate the evolution of the products formed during the passivation and corrosion process of reinforcing steel by Raman spectroscopy, building on which the feasibility of the optical fibre Raman system was also established. Due to the weak Raman signal of the extremely thin passive film (~10 nm), the optimum parameters were first established for Raman spectroscopy to optimise the Raman signal. A tailor designed cell was also developed to achieve not only in-situ measurement to ensure the continuity of the data, but also simultaneous electrochemical measurements to verify the results from Raman spectroscopy. The extracted Portland cement pore solution was used to simulate the real concrete environment. The experimental results clearly show that the evolution of the passivation and corrosion products can be distinguished and followed by Raman spectroscopy. Benefiting from the sensitivity of Raman spectroscopy for characterising the nanoscale film, the passivation process is confirmed to be dominated by the growth of Fe3O4. Raman spectroscopy also successfully identified the evolution of the corrosion products formed during the carbonation and chloride induced corrosion processes. Specifically, when the mild steel was fully immersed, the corrosion products induced by chlorides were evolved from green rust (Cl-) to β-FeOOH, which were from Fe3O4 to γ-Fe2O3 for carbonation induced corrosion. When the mild steel is half immersed, the evolution of the corrosion products was from green rust to γ-FeOOH and α-FeOOH for both carbonation and chloride induced corrosion. More importantly, a two-layer rust structure was observed, where the inner layer was dominated by γ-FeOOH and outer layer was mainly comprised of α-FeOOH. Finally, the feasibility of optical fibre Raman system for characterising the corrosion products was also demonstrated in the current study, showing a great potential of developing an optical fibre Raman-based sensor for monitoring the durability of concrete structure in the future.

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