Chaotic Studies Of Electrochemical Noise On Corrosion Behaviors Of Carbon Steel And Zinc Electrodeposition

Electrochemical noise (EN) including electrochemical current noise (ECN) and electrochemical potential noise (EPN) describes the random fluctuations of current and potential in electrochemical processes. It was expected that characterization of EN based on chaotic theory, expecially on the Largest Lyapunov Exponent (LLE) and fractal dimension (Fr) calculations, could be related to the dissolution behaviors of carbon steel (Q235) and growth behaviors of zinc crystal in electrocrystallization.In Chapter2, in situ EN was used to investigate the ratio between the metastable and stable pitting corrosion of Q235in0.1M NaCl and0.1M Na2SO4solutions during wet-dry cycles by seeking the main waves and calculating Largest Lyapunov Exponent (LLE). Influence of macro composition and structure of rust layers on fraction and distributions of metastable pitting were also discussed. Calculations of noise resistance show the corrosion rate is higher in the wet cycles than that in the dry cycles. Above65percents of ECN series have positive LLE values, which demonstrate that massive metastable pits exist instead of other types of localized corrosion on Q235. The positive LLE of ECN in the Na2SO4solution mainly lies in first12wet-dry cycles. However, fractions of the positive LLE of ECN from NaCl solution have a constant level in wet cycles and decrease inconspicuously in dry cycles. The decrease of metastable pits fraction in Na2SO4solution is probability attributed to the severe competition processes of pitting growth with repassivation reaction of Q235. The rust of Q235formed in Na2SO4solution consists with two layers. The outer layer is identified as a mixture of γ-FeOOH, Fe2O3and FeSO4and inner layer formed by reduction reactions of preformed products. Q235exposured in NaCl solution always generate unstable oxides, such as β-FeOOH and Fe3O4. Metastable pitting may frequently occur in Na2SO4solution. However, localized corrosion in NaCl solution seems more irreversible. This indicates Cl+would facilitate the localized corrosion. Macro-pits can be observed on the steel by scanning electron microscopy in the presence of Cl-ion during initial6wet-dry cycles, while areas of brown product are seen on the specimens in Na2SO4solution.In Chapter3, fractal dimension were used to recognize distinctive-shaped spikes and irregular fluctuations in zinc electroplating on Q235and anodic aluminum. By changing the current density from3.06A/dm2to6.12A/dm2, the fractal dimension (Fr) of EPN has a negative correlation with structure Fr. Fr of EPN becomes larger with the deposition time. Zinc electrocrystallization in6.12A/dm2leads to compact surface phenomenon. The electroplating process is controlled by diffusion process and the interfacial layer is led to the cracking. Fr of EPN is smaller than other activation-controlled condition. Zinc crystal on AAO usually forms through holes first, then grows adhering the plate. Within the first204s, Fr of EPN in3.06A/dm2current density is bigger than that in4.59A/dm2and6.12A/dm2. Structural Fr has a negative correlation with EPN Fr.