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Module Three of CCE 281 Corrosion: Impact, Principles, and Practical Solutions

Anodic Processes

Let us consider in greater detail what takes place at the anode when corrosion occurs. For instance, reconsider the corrosion reaction for iron. This reaction involves the reduction of hydrogen ions to hydrogen gas, according to the model shown here. This hydrogen evolution reaction occurs with a wide variety of metals and acids, including hydrochloric, sulfuric, perchloric, hydrofluoric, formic, and other acids. The individual anodic reactions for iron and aluminum are listed as follows: (reference)

eq 7

eq 8

Examining the above equations shows that the anodic reaction occurring during corrosion can be written in the general form:

eq 9

That is, the corrosion of metal M results in the oxidation of metal M to an ion with a valence charge of n+ and the release of n electrons. The value of n, of course, depends primarily on the nature of the metal. Some metals, such as silver, are univalent, while others such as iron, titanium, and uranium are multivalent and possess positive charges as high as +6. Equation is general and applies to all corrosion reactions.

Example problem 3.6

Why are there always a minimum of two electrochemical reactions to explain even the simplest corrosion reaction?

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