Development of fiber optic chemical sensors for the characterization of species transport in occluded environments

In many proposed mechanisms for both corrosion cracking the rate of crack growth is either driven or limited by the concentration of specific chemical species in the crack tip solution. Concentrations of individual chemical species within the occluded crack tip can differ significantly from the corresponding bulk concentrations. These differences stem primarily from species generation at the crack tip due to chemical reactions, and from limited interchange between the crack tip and bulk environments. For example, the concentration of ionic hydrogen (H+) within occluded regions is a function of the local rates of metallic dissolution and cation hydrolysis, and the mass transport of H+. The rates of dissolution and hydrolysis reactions are dependent on the material, electrochemical, and thermodynamic properties of the system, while the contribution of mass transport by diffusion, convection, and/or ionic migration is dependent on crack geometry.; Concentrations of specific chemical species are difficult to measure due to the occluded nature of the crack. For this study, fiber optic chemical sensors (FOCS) have been developed to investigate the geometry-dependent transport of ionic hydrogen (H+) between the crack tip and bulk environments with minimal perturbation of the localized environment. The FOCS combines a pH-sensitive fluorescent chemical indicator attached by photopolymerization to the tip of an optical fiber to monitor pH in the occluded crack. This sensor geometry provides real-time pH measurement at specific points within the crack tip.; Artificial crack specimens have been used in order to eliminate chemical corrosion effects. Transport rates have been determined as a function of crack tip opening angle (CTOA), crack width, and crack wall roughness. These experimental results have been compared to analytical solutions and computational methods (ABAQUS, FLUENT).; Results clearly show that CTOA, transverse crack width, and crack wall roughness significantly affect transport of H+ to the crack tip. Transport rates will be a strong function of both diffusion and convection for all the geometries studied. While the relative contribution of convection will depend on crack geometry, diffusion is dependent only on the length of the diffusion path.