Lecture #12 Corrosion

1. Lecture #12Corrosion Learning objective outcomes: 1-Corrosion in metals. 2-Distinguish between oxidation and reduction electrochemical reactions. 3. Describe the following: galvanic couple, standard half-cell, and standard hydrogen electrode. 4. Compute the cell potential and write the spontaneous electrochemical reaction direction for two pure metals that are electrically connected and also submerged in solutions of their respective ions. 5-Corrosin in ceramics. 6- Corrosion in polymers

2. CORROS ION OF METALS Corrosion is defined as the destructive of a metal; it is electrochemical and ordinarily begins at the surface. The problem of metallic corrosion is one of significant proportions; 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.. ELECTROCHEMICAL CONSIDERATIONS: An oxidation reaction:

3. Reduction reaction: A- In acidic medium: If the acidic medium containing dissolved oxygen B-In basic or neutral medium:

4. Example (1) : Reduction of Zinc in acidic medium oxidation or corrosion as: these electrons may be transferred to an adjacent region at which the H ions are reduced according to: The over all reaction is

5. Example (2) : the oxidation or rusting of iron in water(Two steps as the following reactions):

7. solution of H ions, saturated with hydrogen gas that is bubbled through the solution at a pressure of 1 atm and a temperature of 25C (77F). The platinum itself does not take part in the electrochemical reaction; it acts only as a surface on which • hydrogen atoms may be oxidized or hydrogen ions may be reduced.

11. Corrosion of Ceramic Materials • Ceramic materials are frequently utilized because of their resistance to corrosion. • Glass is often used to contain liquids for this reason. • Refractory ceramics must not only withstand high temperatures and provide thermal insulation. but, in many in-stances, must also resist high-temperature attack by molten metals, salts, slags, and glasses. • Ceramic materials are much better suited to with-stand most of these environments for reasonable time periods than are metals

12. Degradation of polymers: • Polymeric materials deterioration by means of environmental interactions. • However an undesirable interaction is specified as degradation rather than corrosion. • Polyethylene, if ex-posed to high temperatures in an oxygen atmosphere, suffers an impairment of its • mechanical properties by becoming brittle. • utility of poly(vinyl chloride) may be limited because this material may become colored when exposed to high temperatures, although such environments do not affect its mechanical characteristics.