Electrochemical noise refers to the spontaneous fluctuations in potential and current that can be observed on a corroding metal. The use of electrochemical noise for obtaining information on the corrosion process generates much interest in research fields. One important application is the measurement of corrosion rate. This can be achieved using the electrochemical noise of a pair of electrically coupled corroding metals to obtain an estimate of electrochemical impedance - an abstract quantity that reflects various aspects of the corrosion process.There are a number of problems associated with estimation of impedance information from the electrochemical noise data, particularly regarding data pre-treatment, accuracy and precision. In addition, the present methods are incomplete: current literature does not offer information regarding the phase of the impedance; and assumptions regarding symmetry of an electrode pair cannot be tested without additional measurements.The thesis addresses the above mentioned problems. Specifically,analysis of the impedance estimation process is given to determine how precision can be affected by various factors;a novel signal processing technique is described that is shown to yield a local optimum precision;the application of the proposed signal processing to time varying systems is demonstrated by use of a time varying, frequency dependent impedance estimate;a technique for recovering phase information, given certain conditions, is suggested so that Nyquist impedance diagrams can be constructed; anda technique for testing the symmetry of a coupled pair of corroding metals is described.An integral part of electrochemical noise analysis is the software used for numerical computation. The Matlab package from MathWorks inc. provides an extensible platform for electrochemical noise analysis. Matlab code is provided in Appendix A to implement ++ / much of the theory discussed in the thesis.Impedance analysis and many other electrochemical corrosion monitoring techniques are primarily used for uniform corrosion, where the corrosion patterns occur uniformly over the exposed surface. In order to map localised corrosion, where the corrosion is typically concentrated within a small area, a wire beam electrode can be used. A wire beam electrode is a surface that is divided into a matrix of mini-electrodes so that the corrosion rate at different points can be monitored. However, manual connection of each mini-electrode to the measurement device can prove cumbersome. The final chapter of this thesis describes the design and testing of specialised multiplexing hardware to automate the process.In general, the thesis shows that by careful conditioning of the electrochemical noise prior to analysis, many of the problems with the technique of impedance estimation from the electrochemical noise data can be overcome. It is shown that the electrochemical noise impedance estimation can be extended to encompass a time varying, frequency dependent quantity for studying dynamic systems; that phase information can be recovered from electrochemical noise for the purpose of constructing Nyquist impedance diagrams; and that asymmetric electrodes can be detected without requiring additional measurements.

In this research, corrosion behaviour of A36 carbon steel under engineered disbonded coating was investigated in sulphuric acid solutions containing sodium chloride and iron (III) sulphate. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analyses were carried out to study the morphology and phase composition of corrosion products formed on the carbon steel surface. The results of the SEM analysis showed that only general and pitting corrosion occurred on the carbon steel surface with the engineered crevice. The size of the pits increased as the sulphuric acid and sodium chloride concentrations increased. Moreover, the corrosion products had an open, irregular and loose structure at the pits mouth. The loose and open structure of the corrosion products facilitates diffusion of chloride ions, oxygen, water and contaminants into the carbon steel surface. In contrast, the corrosion products had a very compact and continuous structure outside the pits which provided a good protection against further corrosion. The x-ray diffraction analysis showed that the corrosion products layer mainly consisted of lepidocrocite (γ-FeOOH), goethite (α-FeOOH) and iron sulphide (FeS) on the crevice edges. The Pourbaix diagram of iron in sulphuric acid solution at room temperature indicates that iron sulphide is formed on the metal surface at different pH values. The akaganeite (β-FeOOH) diffraction peak was not identified in any spectrum which could be due to the low concentration of chloride ions in the solutions. Furthermore, the number of lepidocrocite peaks decreased as the sulphuric acid concentration increased from 10 g l-1 to 50 g l-1. The lepidocrocite is dissolved in the presence of sulphuric acid, and the dissolved ion acts as an oxidant to the metal and hence lower lepidocrocite peaks are identified. Electrochemical noise measurement (ECN) testing was also performed to investigate the corrosion process occurring on the carbon steel surface with the engineered crevice. The results of the ECN measurements showed that current increased during first few minutes and then decreased slightly. Also, the coupled potential did not change after an initial shift in negative direction. The low current flowing through the carbon steel electrodes and the constant potential showed that the crevice corrosion did not develop. These results imply that the crevice corrosion may not occur on the carbon steel surface in acidic solutions containing chloride ions.

Electrochemical techniques have potential for use in conservation, both to evaluate the protectiveness of existing coatings on metal artefacts and to evaluate potential new conservation coatings. Three electrochemical methods have been examined in this study for their applicability to conservation problems. Corrosion Potential Measurement is simple but provides only minimal information on the corrosion processes occurring in an electrochemical system. Electrochemical Impedance Spectroscopy provides both mechanistic and predictive information on coating performance, but the data are complex to interpret and measurements require equipment that is at present too bulky for effective on-site use and beyond the budget of most conservation laboratories. Electrochemical Noise Measurement can be performed using cheap, portable instrumentation and theoretically requires relatively simple statistical processing and interpretation, making it attractive for conservation applications. This project looks at the development of a simple, low cost electrochemical noise measurement system for conservation needs, and uses it to compare Electrochemical Noise Measurement with the other two techniques.

The corrosion behaviour of cast Al-Mg alloy A535 and its composites containing 10 wt.% and 15 wt.% fly ash, and 10 wt.% hybrid reinforcement (5 wt.% fly ash + 5 wt.% SiC) was investigated using weight-loss and electrochemical corrosion tests, optical microscopy, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). The tests were conducted in fresh water collected from the South Saskatchewan River and 3.5 wt.% NaCl solution at room temperature. The pH of the salt solution varied from 3 to 9. For comparison, two other aluminum alloys, AA2618 and AA5083-H116, were tested in the same electrolytes. The results of the weight-loss corrosion test showed that unreinforced A535 alloy had a lower corrosion rate in fresh water and seawater environments than the composites at all the tested pH values. The corrosion rate of the composites increased with increasing fly ash content. As expected, the corrosion rates of A535 alloy and the composites tested in fresh water were lower than those in salt solution. The results of the potentiodynamic and cyclic polarization electrochemical tests showed that the corrosion potential (Ecorr) and pitting potential (Epit) of the alloy were more positive than those of the composites. The corrosion and pitting potentials of the composites became more negative (active) with increasing fly ash content. The composites showed more positive (noble) repassivation or protection potential (Erp) than the matrix alloy, with the positivity increasing with fly ash content. Analysis of the electrochemical noise data showed that pitting corrosion was the dominant mode of corrosion for the alloy in 3.5 wt.% NaCl solution. Optical microscopy and SEM revealed that Mg2Si phase and Al-Mg intermetallics corroded preferentially to the matrix. The EDS data indicated that the protective oxide film formed on A535 contained Al2O3 and MgO.

The corrosion behaviour of cast Al-Mg alloy A535 and its composites containing 10 wt.% and 15 wt.% fly ash, and 10 wt.% hybrid reinforcement (5 wt.% fly ash + 5 wt.% SiC) was investigated using weight-loss and electrochemical corrosion tests, optical microscopy, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). The tests were conducted in fresh water collected from the South Saskatchewan River and 3.5 wt.% NaCl solution at room temperature. The pH of the salt solution varied from 3 to 9. For comparison, two other aluminum alloys, AA2618 and AA5083-H116, were tested in the same electrolytes. The results of the weight-loss corrosion test showed that unreinforced A535 alloy had a lower corrosion rate in fresh water and seawater environments than the composites at all the tested pH values. The corrosion rate of the composites increased with increasing fly ash content. As expected, the corrosion rates of A535 alloy and the composites tested in fresh water were lower than those in salt solution. The results of the potentiodynamic and cyclic polarization electrochemical tests showed that the corrosion potential (Ecorr) and pitting potential (Epit) of the alloy were more positive than those of the composites. The corrosion and pitting potentials of the composites became more negative (active) with increasing fly ash content. The composites showed more positive (noble) repassivation or protection potential (Erp) than the matrix alloy, with the positivity increasing with fly ash content. Analysis of the electrochemical noise data showed that pitting corrosion was the dominant mode of corrosion for the alloy in 3.5 wt.% NaCl solution. Optical microscopy and SEM revealed that Mg2Si phase and Al-Mg intermetallics corroded preferentially to the matrix. The EDS data indicated that the protective oxide film formed on A535 contained Al2O3 and MgO.