Introduction to Corrosion Science and Engineering

By the end of this lecture, learners should grasp the electrochemical nature of aqueous corrosion, understanding key processes like anodic and cathodic reactions, charge movement in the metal and solution, and the necessity of maintaining charge neutrality. Learners will comprehend the simultaneous occurrence of oxidation and reduction reactions on a corroding surface, where metal oxidation leads to corrosion. The terms 'anodic' and 'cathodic' reactions will be associated with oxidation and reduction, respectively, emphasizing wet corrosion as an electrochemical process allowing spatial separation of reactions on the metal surface. The lecture emphasizes the balance between anodic and cathodic reactions, with the slower process determining the corrosion rate.

By the end of this lecture, learners should comprehend the significance of anodic and cathodic reactions in corrosion and their influence on local pH. Anodic reactions lead to the loss of metallic material, with resulting metal ions having various outcomes, from dissolution to forming protective oxides. Cathodic reactions, including oxygen reduction and hydrogen evolution, impact pH by generating hydroxyl ions and consuming hydrogen ions. This interplay influences localized corrosion, with anodic regions becoming more acidic and cathodic regions more alkaline. The lecture highlights the spatial separation of reactions, creating a pH gradient on the corroding surface.

By the end of this lecture, learners should grasp the essentials of electrical charge movement in metals and solutions. The lecture introduces the concept of electrolytes as solutions capable of conducting electrical charge due to the movement of charged ions. While electrons are responsible for charge moveement within metals, electrolytes rely on the movement of positive and negative ions. The mechanisms differ, requiring electrochemical reactions at the metal-electrolyte interface for charge to traverse the surface. An important implication is that anodic regions serve as sources of positive ions, and cathodic regions as sources of negative ions. This ion movement, exemplified by chloride migration towards anodic regions, plays a role in triggering localized corrosion mechanisms.

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By the end of this lecture, learners should grasp Faraday's law and its applications in electrochemistry and corrosion. The main learning objectives involve the conversion of electrical current or charge into reaction rates and the mass of material reacted. Understanding the direct proportionality between substance reacted and electrical charge is important for considering corrosion in electrical current terms.

By the end of this lecture, learners should have a practical understanding of measuring potential differences between metal specimens and reference electrodes, and the significance of these measurements in electrochemistry. The lecture emphasizes practical scenarios without delving into fine or theoretical details. Key concepts include the use of voltmeters and multimeters to measure potential differences, the importance of reference electrodes, and the construction of a standard hydrogen reference electrode. Learners should also grasp the significance of standard potentials, as demonstrated by the electrochemical series, and the application of the Nernst equation to calculate equilibrium potentials under non-standard conditions.

By the end of this lecture, learners should be able to apply the Nernst equation to determine equilibrium potentials in non-standard conditions. They will understand its role in calculating oxygen and hydrogen reactions equilibria based on varying solution pH. The lecture shows how to obtain the Pourbaix diagram for water and discusses the Pourbaix diagrams of common metals like zinc, copper, aluminum, gold, and iron. Learners will discern the key regions—immunity, corrosion, and passivity—and comprehend the associated thermodynamic implications.

By the end of this lecture, learners should grasp the significance of investigating electrochemical reactions' kinetics, to assess corrosion rates. The lecture discusses experimental methodologies, employing a three-electrode cell connected to a Potentiostat, for investigations into the behavior of a metal, under different conditions. Learners will understand the roles of the working, reference, and counter electrodes and the Potentiostat in achieving this. Practical applications include anodic and cathodic potentiodynamic polarization experiments, wherein a linear variation of potential over time provides insights into the relationship between potential and current. The resulting current-potential diagrams, presented in both linear and semi-logarithmic scales, aid in understanding anodic and cathodic behaviors.

By the end of this lecture, learners should gain an understanding of Evans diagrams and their role in representing the behavior of corroding electrodes supporting simultaneous anodic and cathodic reactions. Focusing on the construction of Evans diagrams, the lecture discusses the scenario involving the corrosion of zinc in an oxygen-free acidic solution. Additionally, the lecture introduces Tafel law and its and explains how Tafel coefficients can be used to calculate the corrosion current through the Stern-Geary equation.

By the end of this lecture, learners should have a solid understanding of diffusion-limited reactions, particularly in the context of the oxygen reduction reaction and its significance in metal corrosion. The lecture discusses the process of diffusion of oxygen molecules to the metal surface, emphasizing scenarios where this process becomes the limiting step. The impact on cathodic overpotential is explained, leading to the construction of a current density-overpotential diagrams which show a region where the current density does not increase with higher overpotentials.

At the end of this lecture, learners should understand general corrosion as a common process where metals gradually corrode uniformly over time due to environmental exposure. The lecture emphasizes the factors influencing general corrosion, including pH, cathodic activity, temperature, and oxygen concentration. Mitigation measures, such as coatings, environmental control, corrosion inhibitors, and cathodic protection, are introduced as strategies to counteract general corrosion. Various assessment methods, including visual inspection, weight loss measurements, and electrochemical testing, are highlighted for evaluating corrosion extent and rate.

At the end of this lecture, learners should have gained an understanding of galvanic corrosion, a process occurring when two metals of different nobility are electrically connected and face a common environment. The lecture covers the fundamental mechanisms driving galvanic corrosion, explores the factors influencing its severity, and discusses various mitigation measures to limit or prevent its detrimental effects. Key concepts include the impact of practical nobility differences , the importance of the area ratio between dissimilar metals, as well as the role of electrolyte conductivity. Strategies for controlling galvanic corrosion, such as material selection, insulation, and cathodic protection, are also introduced.

At the end of this lecture, learners should have developed an understanding of crevice corrosion, a localized form of corrosion that occurs in confined spaces on the surfaces of passive metals, with a particular focus on stainless steel and aluminum. The lecture discusses the mechanisms of crevice corrosion, highlighting the initiation process when the metal is exposed to an electrolyte containing oxygen and chloride ions. The self-accelerating nature of crevice corrosion, driven by the differential aeration cell established within the crevice due to oxygen depletion, is emphasized. Factors influencing the severity of crevice corrosion, such as the size of the gap, electrolyte conductivity, and the presence of chlorides, are thoroughly explored. The lecture also discusses preventive measures, such as careful design and construction to avoid crevices and the selection of corrosion-resistant materials when crevice conditions are unavoidable.

At the end of this lecture, learners should have an understanding of pitting corrosion, a localized form of corrosion prevalent in passive metals exposed to chlorides. The lecture discusses the mechanisms of pitting corrosion and the factors influencing its severity. Learners gain insights into the initiation and propagation stages of pitting, considering the effects of alloy composition, inclusions, and environmental conditions. The importance of evaluating a material's performance against pitting corrosion through anodic kinetics is highlighted, focusing on parameters such as passivation potential, passivation current, passive potential, passive current, pitting potential, and repassivation potential. Preventive measures, such as material selection based on the Pitting Resistance Equivalent Number and the use of cathodic protection are discussed.

At the end of this lecture, learners should understand cathodic protection as a corrosion prevention method for metal structures, especially steel, in corrosive environments. The lecture covers the necessity of an electrolyte for cathodic protection, with examples like seawater and wet soil. It explains how external cathodic current prevents metal oxidation, detailing two methods: sacrificial anodes and impressed current systems. The lecture touches on IR drop issues, 'OFF' potential measurements, and protection criteria. The issue of overprotection, associated to excessive cathodic protection current, is considered due to its potential harm. Factors like anode number, position, mass, and applied voltage are mentioned in relation to designing effective cathodic protection systems.

At the end of this lecture, learners should have an understanding of corrosion inhibitors, their applications, and classification. Corrosion inhibitors, added in low concentrations to aggressive environments, can reduce the corrosion rate of metal surfaces. Their widespread use includes applications in the oil and gas sector, water cooling systems, construction, and paint formulation. The lecture discusses the classification of corrosion inhibitors based on chemistry, electrochemical mechanisms, and application, emphasizing the concept of safe and unsafe inhibitors. The mechanisms of anodic, cathodic, and mixed inhibitors are explored. The concept of efficiency of corrosion inhibitors is discussed, with a focus on the significant influence of inhibitor concentration. The lecture also addresses the safety considerations of inhibitors, distinguishing between those that consistently decrease corrosion rates with increasing concentration and those that may increase corrosion rates at intermediate concentrations.

At the end of this lecture, learners should have an understanding of organic coatings' significance in surface protection and aesthetics. Organic coatings, commonly referred to as paints, are composed of organic compounds like resins, binders, solvents, and solid particles such as pigments and fillers. The lecture discusses the role of each component in coating formulation. The roles of binders, solvents, pigments, fillers, and additives, are considered, highlighting their impact on coating properties. The protective mechanisms, including the barrier effect, active inhibition, and cathodic protection, are explained. Coating systems, consisting of primer, intermediate coat, and top coat, are discussed, and the importance of surface pretreatment is highlighted.

At the end of this lecture, learners should have acquired an understanding of the various mechanisms that contribute to the failure of organic coatings and the subsequent corrosion of the metal substrate. Starting from surface preparation issues, the lecture discusses how poor preparation, with residuals like oxides, oil, grease, or salts, can compromise coating performance. Osmotic blistering, caused by soluble salts beneath the coating, is explained as a critical issue arising from surface contaminants. The formation of pinholes during application, attributed to factors like air trapping, condensation, contamination, or surface roughness, is highlighted, emphasizing the role of multiple coating layers in mitigating this risk. The lecture explains how intact coatings primarily resist corrosion by slowing diffusion processes, though long-term degradation processes can still initiate corrosion over time. Furthermore, corrosion phenomena such as cathodic disbonding, anodic undermining, and filiform corrosion, are discussed, providing an understanding of the complexities involved in coatings failures.

At the end of this lecture, learners should have acquired an understanding of metallic and non-metallic inorganic coatings, distinguishing their categories and protective mechanisms. The classification of metallic coatings as anodic or cathodic relative to the substrate is discussed, elucidating how their nobility influences protective mechanism. The lecture introduces some of the methods of obtaining metallic coatings, such as electroplating, electroless plating, hot dipping, and thermal spray. Regarding non-metallic coatings, the lecture briefly discusses conversion coatings, anodic oxides, and enamels, detailing their formation processes and protective features.