Techniques for Corrosion Monitoring
Corrosion is the deterioration process of a material due to reactions with its surroundings. As defined in the Random House Unabridged Dictionary: “Corrosion is the act or process of eating or wearing away gradually as if by gnawing, especially by a chemical action.” The materials that are subject to corrosion include metals, ceramics, polymers, and even our own teeth. To most corrosion engineers, however, corrosion refers to the oxidation of metals by chemical and/or electrochemical processes.
The most common example of metal corrosion relates to its reaction with oxygen or water. Rusting of steel due to exposure to water or humid air is a well-known example of electrochemical corrosion. In this process, the metal reacts with water or oxygen through electrochemical processes and forms iron oxides, eventually causing damage to the steel. The following sections describe the importance of corrosion monitoring and the scope of this book.
General corrosion is the most common and benign form of corrosion because it is characterized by a corrosive attack that extends almost uniformly over the whole exposed surface or at least over a large area. Even though the term uniform corrosion is commonly used synonymously with general corrosion, this form of corrosion is seldom completely uniform and the morphology of the corroded surface always exhibits some sort of waviness and roughness. Nevertheless, the average penetration of the attack is practically the same at each point of the corroded surface. Any corrosion process involves at least one anodic (oxidation) reaction and one cathodic (reduction) reaction.Because charge accumulation cannot occur, the electrons generated by the oxidation reactions must be consumed by the cathodic reactions and, therefore, the total anodic current should be equal to the total cathodic current with the electrical potential at an anodic site equal to that at a cathodic site. This fundamental concept of coupled anodic and cathodic processes was proposed by Wagner and Traud [9].
The hypothesis leads to the definition of the corrosion potential as a mixed potential because the anodic reaction is the dissolution of the metal coupled to the cathodic reaction which could be the reduction of dissolved oxygen molecules, hydrogen ions, or any other reducible species in solution (e.g., Fe3+ ions), instead of the reduction of the dissolved metal cations. From an electrochemical point of view, the main characteristic of general corrosion is the fact that metal dissolution takes place without physical separation, even at the microscopic scale, of anodic and cathodic sites.
General corrosion causes by far the largest amount of material losses as a result of corrosion, mostly due to atmospheric corrosion. Leaving aside the costs involved, general corrosion is not of great concern because the corrosion rate and, hence, the expected life of equipment or structures can be accurately estimated by means of relatively simple corrosion tests. The most critical aspect of testing is, however, the correct definition of the environment in which the component will operate and its unexpected variations during operation. Such tests can be immersion tests, in which gravimetric methods are used for measuring the weight loss of a material specimen during a specific testing time to calculate the corrosion rate
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