An oxidation or anodic reaction is indicated by an increase in valence or a release of electrons. a decrease in valence charge or the consumption of electrons signifies a reduction or cathodic reaction. Equations 3 and 4 are partial reactions; both must occur simultaneously and at the same rate on the metal surface. If this were not true, the metal would spontaneously become electrically charged, which is clearly impossible The corrosion of zinc in hydrochloric acid is an electrochemical process. That is, any reaction that can be divided into two or more partial reactions of oxidation and reduction due to the transfer of electrical charge is termed electrochemical. Dividing corrosion or other electrochemical reactions into partial reactions makes them simpler to understand. Iron and aluminum, like zinc, are also rapidly corroded by hydrochloric acid Thus, the problem of hydrochloric acid corrosion is simplified because in every case the cathodic reaction is the evolution of hydrogen gas according to Eq 4. This also applies to corrosion in other acids such as sulfuric, phosphoric, hydrofluoric, and water-soluble organic acids such as formic and acetic. In each case, only the hydrogen ion is active, the other ions such as sulfate, phosphate, and acetate do not participate in the electrochemical reaction When viewed from the standpoint of partial processes of oxidation and reduction, all corrosion can be classified into a few generalized reactions. The anodic reaction in every corrosion reaction is the oxidation of a metal to its ion. This can be written in the general form where n is the number of electrons released A few examples are Ag→Ag+e Zn→Zn2++2e (Eq7) Al→Al+3e In each case the number of electrons produced equals the valence of the ion There are several different cathodic reactions that are frequently encountered in metallic corrosion. The most common cathodic reactions are: Hydrogen evolution 2H+2 H (Eq9) Oxygen reduction(acid solutions) O2+4H+4e→2H2O (Eq10) Oxygen reduction(neutral or basic solutions) O2+2H2O+4e→4OH Metal-ion reduction Metal deposition M+e→M (Eq13) Hydrogen evolution is a common cathodic reaction because acid or acidic media are frequently encountered Oxygen reduction is very common, because any aqueous solution in contact with air is capable of producing this reaction. Metal-ion reduction and metal deposition are less-common reactions and are most frequently found in chemical process streams. All of the above reactions are quite similar; they consume electrons The above partial reactions can be used to interpret virtually all electrochemical corrosion problems. Consider what happens when iron is immersed in water or seawater that is exposed to the atmosphere(an automobile fender or a steel pier piling are examples). Corrosion occurs. The anodic reaction is (Eq14) Since the medium is exposed to the atmosphere, it contains dissolved oxygen. Water and seawater are nearly neutral. and thus the cathodic reaction is: O2+2H2O+4e→4OH (Eq15) Remembering that sodium and chloride ions do not participate in the reaction, the overall reaction can be obtained by adding eq 9 and 12 Thefileisdownloadedfromwww.bzfxw.comAn oxidation or anodic reaction is indicated by an increase in valence or a release of electrons. A decrease in valence charge or the consumption of electrons signifies a reduction or cathodic reaction. Equations 3 and 4 are partial reactions; both must occur simultaneously and at the same rate on the metal surface. If this were not true, the metal would spontaneously become electrically charged, which is clearly impossible. The corrosion of zinc in hydrochloric acid is an electrochemical process. That is, any reaction that can be divided into two or more partial reactions of oxidation and reduction due to the transfer of electrical charge is termed electrochemical. Dividing corrosion or other electrochemical reactions into partial reactions makes them simpler to understand. Iron and aluminum, like zinc, are also rapidly corroded by hydrochloric acid. Thus, the problem of hydrochloric acid corrosion is simplified because in every case the cathodic reaction is the evolution of hydrogen gas according to Eq 4. This also applies to corrosion in other acids such as sulfuric, phosphoric, hydrofluoric, and water-soluble organic acids such as formic and acetic. In each case, only the hydrogen ion is active, the other ions such as sulfate, phosphate, and acetate do not participate in the electrochemical reaction. When viewed from the standpoint of partial processes of oxidation and reduction, all corrosion can be classified into a few generalized reactions. The anodic reaction in every corrosion reaction is the oxidation of a metal to its ion. This can be written in the general form: M → M n+ + ne (Eq 5) where n is the number of electrons released. A few examples are: Ag → Ag+ + e (Eq 6) Zn → Zn2+ + 2e (Eq 7) Al → Al3+ + 3e (Eq 8) In each case the number of electrons produced equals the valence of the ion. There are several different cathodic reactions that are frequently encountered in metallic corrosion. The most common cathodic reactions are: Hydrogen evolution: 2H+ + 2e → H2 (Eq 9) Oxygen reduction (acid solutions): O2 + 4H+ + 4e → 2H2O (Eq 10) Oxygen reduction (neutral or basic solutions): O2 + 2H2O + 4e → 4OH- (Eq 11) Metal-ion reduction: M 3+ + e → M 2+ (Eq 12) Metal deposition: M + + e → M (Eq 13) Hydrogen evolution is a common cathodic reaction because acid or acidic media are frequently encountered. Oxygen reduction is very common, because any aqueous solution in contact with air is capable of producing this reaction. Metal-ion reduction and metal deposition are less-common reactions and are most frequently found in chemical process streams. All of the above reactions are quite similar; they consume electrons. The above partial reactions can be used to interpret virtually all electrochemical corrosion problems. Consider what happens when iron is immersed in water or seawater that is exposed to the atmosphere (an automobile fender or a steel pier piling are examples). Corrosion occurs. The anodic reaction is: Fe → Fe2+ + 2e (Eq 14) Since the medium is exposed to the atmosphere, it contains dissolved oxygen. Water and seawater are nearly neutral, and thus the cathodic reaction is: O2 + 2H2O + 4e → 4OH- (Eq 15) Remembering that sodium and chloride ions do not participate in the reaction, the overall reaction can be obtained by adding Eq 9 and 12: The file is downloaded from www.bzfxw.com