Authors: Pratyush Mishra
Source: Iowa State University Digital Repository (Iowa State University) (article, 2019-01-01)
Field: Materials Science — Materials Chemistry

Experimental and modeling studies of formation of self-organized porous anodic oxide layers and intergranular corrosion of steel The first part of the thesis presents a flow instability mechanism for the self-ordering of porous anodic alumina (PAA). Anodizing of aluminum in electrolytes that dissolve the oxide produces PAA. For specific ranges of anodic potentials and solutions, uniform self-ordered hexagonally arranged PAA with diameters 10-400 nm are formed. A universal scaling ratio exists between the spacing between the pores and the anodizing voltage. The pore spacing depends on the type of the cell solution, e. g. sulfuric, oxalic or phosphoric acid. Nano porous anodic oxides have been used for a wide range of applications including biomedical sensors, dye-sensitized solar cells, and templates for secondary nanomaterials such as carbon, polymers and semiconductor nanomaterials. Despite the diverse application, the mechanism leading to the self-ordering is not clearly understood.\nRecent studies have demonstrated the importance of oxide flow and mechanical stress during the self-ordering of PAA. Mathematical modeling is presented here that includes surface stress driven oxide flow as an essential mechanism for self-ordering, along with ionic migration, electrochemical reactions, and boundary conditions for interface evolutions and surface stress at the oxide-solution interface. Stress generated during anodizing was predicted from the boundary conditions, which in good agreement with experimentally observed stress profile. Morphological stability analysis is performed at the inception of instability that leads to the formation of steady-state porous film. The analysis predicts steady-state pore spacing-voltage ratio in good agreement with the experimentally observed value. The results reveal that the instability at solution interface is determined by the competitive effects of destabilizing surface stress driven oxide flow and stabilizing oxygen ionic migration flux. Metal interface instability is due to pressure-driven flow caused by the surface stress at the solution interface.\nIn the second part, early stage corrosion damage preceding intergranular stress corrosion cracking (IGSCC) on high strength X70 pipeline steels was studied. In high-pH solution (pH 8 - 10.5), pipeline steels are susceptible to both intergranular cracking (IGC) and IGSCC in the same active-passive potential range, suggesting that IGC leads to IGSCC. Electrochemical experimentation and modeling studies were performed to investigate the corrosion damage during IGC of pipeline steel. IGC led to the formation of triangular wedges around grain boundaries owing to a preferential attack at the grain boundaries. A model for IGC propagation was developed, based on the enhancement of grain boundary (GB) attack due to a vacancy diffusion-controlled mechanism. The model was run as finite-element simulations that accurately predicted the formation and morphology evolution of the GB wedges. The model was consistent with experimental observations of the presence of a softened layer adjacent to the GBs and the elevated Si incorporation into the GB corrosion product. Electrochemical impedance spectroscopy (EIS) study was performed to characterize the corrosion processes. A model was developed based on anion diffusion-limited anodic dissolution of iron, coupled with surface blocking by passivating oxide. The analytical model was used to fit the experimental impedance spectra. The analysis revealed a carbonate ion catalyzed iron dissolution reaction, where the ion diffuses through a porous corrosion product layer. The diffusion resistance increases over time, leading to current decays observed during the corrosion experiments. The result further predicts the behavior of force per width change with charge density passed during the corrosion experiments. Impedance analysis also reveals a potential-dependent surface coverage of a passivating iron oxide species. The combined study of the corrosion propagation mechanism and the EIS characterization has led to an integrated understanding of the IGC process preceding high-pH IGSCC. Intergranular corrosion ; Metallurgy ; Materials science ; Corrosion ; Porosity ; Anode ; Oxide ; Composite material ; Chemistry ; Electrode Anodic Oxide Films and Nanostructures