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Technological Aspects of Corrosion Control in Metallic Systems

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Technological Aspects of Corrosion Control in Metallic Systems The Pennsylvania State University The Graduate School Department of Materials Science and Engineering TECHNOLOGICAL ASPECTS OF CORROSION CONTROL IN METALLIC SYSTEMS A Dissertation in Materials Science and Engineering by Matthew Logan Taylor 2012 Matthew Taylor Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy December 2012 ii The dissertation of Matthew Taylor was reviewed and approved* by the following: Digby D. Macdonald1 Distinguished Professor of Materials Science and Engineering Dissertation Advisor Chair of Committee James H. Adair1 Professor of Materials Science and Engineering, Bioengineering and Pharmacology Michael A. Hickner1 Assistant Professor of Materials Science and Engineering Mirna Urquidi-Macdonald1 Professor of Engineering Science & Mechanics Bernard Normand2 Professor of Materials Science and Engineering Special Member Damien Feron3 Professor of Nuclear Science and Technology Special Member Gary L. Messing1 Distinguished Professor of Ceramic Science and Engineering Head, Department of Materials Science and Engineering *Signatures are on file in the Graduate School 1 The Pennsylvania State University 2 Institut National des Sciences Appliquées de Lyon 3 Institut National des Sciences et Techniques Nucléaires iii ABSTRACT 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. Nitrogen has been found to improve the corrosion- resistance of stainless steels, although the mechanism which confers this protection is under debate. An atomistic model is proposed which explains the incorporation of nitrogen into the passive films of chromium-based alloys and predicts the effects of such incorporation on the passive film behavior. The model is conceptualized through a novel use of the Hume-Rothery rules of solid solution, for which only one possible substitution of N into the Cr2O3 lattice was found to be compatible with all four “rules”; coordination number, electronegativity, ionic radius mismatch and valency. Four crystallographic defect reactions are proposed; one at the metal/film interface, where N(3-) occupies an oxygen vacancy and three subsequent reactions with H+ at the film/solution interface where the nitrogen defect evolves into ammonia. Rate constants are derived and reported for the reactions using the method of partial charge transfer. Using the Solute Vacancy Interaction Model as a guide, it is proposed that the negatively charged nitrogen defects would have electrostatic interactions with the positively charged oxygen vacancies and metal interstitials, and the resulting complexes should reduce the transport of these defects. Consequently, the Point Defect Model rate constants are affected, correctly predicting that N defects should reduce both the passive film thickness, and the steady state current density, as has been observed in nitrogen-bearing alloys and nitrided stainless steels. Physical vapor deposited chromium + nitrogen (0, 6.8 and 8.9 at.%N) coatings were investigated as a model system, to test iv 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. This effect was observed at both 95°C and 175°C and was found to destroy the metal at an alarming rate. This observation was made in tandem with modeling of the amine concentrations found in steam turbines during operation and shutdown. Monoethanolamine has a lower vapor v pressure than water during turbine dryout conditions, meaning that it will tend to concentrate, along with acetate, while dimethylamine evaporates. Because the monoethanolamine concentrations during operation were three orders of magnitude lower than the least concentrated experiment in this work, but equal to the most concentrated conditions in this work during shutdown, it is likely that the corrosion damage attributed to this mechanism occurs only during shutdown. It is recommended that monoethanolamine be evaluated for exclusion from use as an inhibitor in thermal power systems containing ASTM A470/471 steel. 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. When the parameters of this model is optimized to the impedance data, a microstructure-time constant relationship emerges. It was found that for the “fast” time constant, all values below 0.15s belonged to delaminated specimens, while all values above 0.30 belonged to hermitic samples. For samples with “fast” time constants between 0.15 and 0.30, a measure of the microstructure can be comparatively inferred, with lower values for both the “fast” and “slow” time constant corresponding to higher porosity. 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 vi 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. vii TABLE OF CONTENTS LIST OF FIGURES ......................................................................................................... ix LIST OF TABLES ........................................................................................................... xvii ACKNOWLEDGEMENTS ............................................................................................. xviii Chapter 1 History and Philosophy of Corrosion Control ....................................................... 1 The Pre-History of Metals ................................................................................................ 3 The Cost of Corrosion ...................................................................................................... 4 Technological Corrosion Control ..................................................................................... 4 Chapter 2 The Effect of Nitrogen on the passivity of pure Chromium .................................. 6 Background and Theory ................................................................................................... 6 The Point Defect Model of the Passive State (PDM)....................................................... 8 Chapter 3 Complex Function Optimization for One Independent Variable by Differential Evolution .....................................................................................................
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