| Three corrosion control technologies were investigated, including the effect of nitrogen on the passivity of chromium in sulfate solutions, possible issues associated with the use of amines in steam turbine environments and the microstructure of naval advanced amorphous coatings.;Nitrogen (N) is a minor alloying element commonly used to increase the strength of steels by stabilizing the austenite phase. Physical vapor deposited chromium + nitrogen (0, 6.8 and 8.9 at.%N) coatings were investigated as a model system, to test the model. Because Cr passive films have been observed to be generally n-type semiconductors, an impedance function containing a n-type Faradaic impedance was constructed and optimized to electrochemical impedance spectra for the model system at pH 4,7 and 10 1M sulfate solution at 30°C. An apparent deviation from theory was observed, however. The n-type model predicted steady state currents which were independent of potential, while the observed current densities had a positive correlation with potential. Mott-Schottky analysis revealed that the test potentials were within the n-p transition and p-type potential range, which resolves the apparent deviation. Despite this difficulty, however, the impedance model produced reasonably accurate results, calculating current densities to within one order of magnitude of the measured steady state currents where anodic currents were available and passive film thicknesses on the order of 1-2 nm.;Various amines are commonly used to inhibit corrosion in thermal power generation systems, including steam turbines, by increasing the pH. However, during the shutdown phase of the power plant, it is possible for these inhibitors to concentrate and cause corrosion of the turbine rotor. The effect of two ammine inhibitors (monoethanolamine and dimethylamine) on the passivity of ASTM A470/471 steel is investigated in a simulated turbine environment at pH 7, and temperatures of 95°C and at 175°C. Potentiodynamic scans and potentiostatic measurements revealed that the steel depassivated with high (0.1M) concentrations of monoethanolamine, in combination with acetate. Because the steel depassivated at low potentials and at neutral pH, it is unlikely to be acid or transpassive depassivation. The proposed mechanism for this depassivation is resistive depassivation, whereby the potential drop incurred by the precipitated outer-layer robs the barrier layer of the passive film of the potential required to maintain a finite film thickness.;High velocity oxy-fuel (HFOV) coatings are employed in maritime environments to protect against corrosion and wear. The performance of such coatings is dominated by flaws in the microstructure, such as porosity, delamination and secondary phases. A nondestructive evaluation technique that is capable of determining the quality of a HVOF coating was developed, based on electrochemical impedance spectroscopy (EIS). The EIS measurement was correlated to the microstructure observed via scanning electron microscopy (SEM). Because a transmission line model was unable to provide discriminatory information, a convenient mathematical impedance function was constructed, with two separated time constants defined by constant phase elements, with time constants for a “fast” and a “slow” process.;Enabling the impedance studies above is a new software package for fitting complicated impedance functions of up to 50 parameters to complex impedance data, developed specifically for this work. The curve-fitting software utilizes differential evolution, an evolutionary algorithm which is relatively new to the field of impedance modeling, enabling the operator to obtain high quality fits without the need for excellent starting guesses, taking trial and error out of the curve-fitting process and vastly improving the man-hour efficiency involved in optimizing complicated impedance functions such as the Faradaic impedance of the Point Defect Model. (Abstract shortened by UMI.). |
