Summary of Previous Lecture Development of; Human History → Technology → Population → Current Problems & Changes, Global warming, Urbanization, Aging/Increasing Population, Depletion of Natural Resources, Digitalization (Artificial Intelligence, Internet of Things, Industry 4.0), Energy Production & Storage Technologies, etc… These changes require new materials (metals, ceramics, composites, polymers) Service life of these new materials are directly related to Corrosion & Degradation Issues. Thus, understanding the nature, reason, mechanism of degration and methods for prevention are very important in engineering applications.

 Types of Corrosion & Degradation,  Cause and Mechanism,  Methods of Prevention. WHY we STUDY Corrosion and Degradation of Materials?

Most materials experience some type of interaction with a large number of diverse environments. Often, such interactions impair a material’s usefulness as a result of the deterioration of its; mechanical properties (e.g., ductility and strength), other physical properties, or appearance. Deteriorative mechanisms are different for the three material types. In metals, there is actual material loss either by dissolution (corrosion) or by the formation of nonmetallic scale or film (oxidation). Corrosion & Oxidation & Degradation

Corrosion is defined as the destructive and unintentional attack of a metal; it is electrochemical and ordinarily begins at the surface. The problem of metallic corrosion is one of significant proportions; in economic terms, it has been estimated that approximately 5% of an industrialized nation’s income is spent on corrosion prevention and the maintenance or replacement of products lost or contaminated as a result of corrosion reactions. rusting Familiar examples include the rusting of automotive body panels and radiator and exhaust components. Corrosion of Metals

1936 Model Deluxe Ford Sedan Unpainted stainless steel body 1936 Model Deluxe Ford Sedan Painted plain-carbon steel (paint offered limited protection for steel) What is the reason of Corrosion & Degradation in metals?

For metallic materials, the corrosion process is normally electrochemical. (Related to the transfer of electrons due to thermodynamical reasons/state of lowest energy-higher entropy.) There is a chemical reaction in which there is transfer of electrons from one chemical species to another. Metal atoms characteristically lose or give up electrons in what is called an oxidation reaction. Electrochemical Considerations Oxidation or rusting of iron in moist environment

A metal M that has a valence of n (or n valence electrons) may experience oxidation according to the reaction M = M n+ + ne - in which M becomes an n positively charged ion, and in the process it loses its n valence electrons. Examples in which metals oxidize: Fe → Fe e - Fe 2+ → Fe 3+ + e - Al → Al e - Electrochemical Considerations Electron Donation-Oxidation (losing or giving up e - ’s)

The site at which oxidation takes place is called the anode; oxidation is sometimes called an anodic reaction. The electrons generated from each metal atom that is oxidized, must be transferred to and become a part of another chemical species in what is termed a reduction reaction. Electrochemical Considerations

For example, some metals undergo corrosion in acid solutions, which have a high concentration of hydrogen (H + ) ions; the H + ions are reduced as follows: 2H + + 2e - → H 2 Other reduction reactions are possible, depending on the nature of the solution to which the metal is exposed. For acid solutions having dissolved oxygen: O 2 + 4H + + 4e - → 2H 2 O for neutral or basic aqueous solutions in which oxygen is also dissolved: O 2 + 2H 2 O + 4e - → 4(OH) - Electrochemical Considerations Electron Acceptance

Electrochemical Considerations Any metal ions present in the solution may also be reduced; for ions that can exist in more than one valence state (multivalent ions), reduction may occur by; M n+ + e - → M (n-1)+ (for example Fe 3+ + e - → Fe 2+ reduction reaction) Or a metal may be totally reduced from an ionic to a neutral metallic state; M n+ + ne - → M (for example Cu + + e - → Cu 0 reduction reaction) The location at which reduction occurs is called the cathode. Furthermore, it is possible for two or more of the preceding reduction reactions to occur simultaneously.

Electrochemical Considerations An overall electrochemical reaction must consist of; at least one oxidation and one reduction reaction, and will be the sum of them. The individual oxidation and reduction reactions are termed half reactions. There can be no net electrical charge accumulation from the electrons and ions; that is, the total rate of oxidation must equal the total rate of reduction, or all electrons generated through oxidation must be consumed by reduction.

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