158 9.Degradation of Materials (Corrosion) cause of governmental regulations,manufacturers are now switch- ing to bismuth-copper alloys.)"Soft"water is generally more cor- rosive (dissolves more trace elements)than unsoftened water.[To avoid lead contamination of drinking water,it is often recom- mended to run the water for a few seconds before use,particu- larly if it was standing for some time (e.g.,overnight)in the line or faucets.Moreover,it is advisable to avoid drinking or cooking with water drawn from the hot water side of the tap altogether. Dealloying Selective leaching or dealloying are terms used when one al- loy constituent is preferentially dissolved by a solution.Selective leaching of zinc from brass (called dezincification),or selective loss of graphite in buried gray cast iron gas lines (possibly lead- ing to porosity and explosions)may serve as examples. Corrosion is often more fully described by electrochemical reactions in which free electrons are interpreted to be trans- ferred from one chemical species (or from one part of the same species)to another.Specifically,the deterioration of metals and alloys is interpreted to be caused by an interplay between oxi- dation and reduction processes.This shall be explained in a few examples.During the oxidation process,electrons are transferred from,say iron,to another part of iron (or a different metal)ac- cording to the reaction equation: Fe→Fe2++2e- (9.1) or generally for a metal,M: M→Mm++ne-, (9.2) where n is the valency of the metal ion or the number of elec- trons transferred,and e-represents an electron.The site at which oxidation takes place is defined to be the anode.During a re- duction process,the free electrons which may have been gener- ated during oxidation are transferred to another portion of the sample and there become a part of a different chemical species according to: 2H++2e-→H2 (9.3) or in the case of a metal,M: Mn++e-→M. (9.4) The site where reduction takes place is called the cathode.In other words,oxidation and reduction can be considered as mir- ror-imaged processes.As an example,the dissolution of iron in an acid solution(e.g.,HCl)is represented by the above equations (9.1)and (9.3),or,in summary: Fe+2H+→Fe2++H2. (9.5)cause of governmental regulations, manufacturers are now switch￾ing to bismuth–copper alloys.) “Soft” water is generally more cor￾rosive (dissolves more trace elements) than unsoftened water. [To avoid lead contamination of drinking water, it is often recom￾mended to run the water for a few seconds before use, particu￾larly if it was standing for some time (e.g., overnight) in the line or faucets. Moreover, it is advisable to avoid drinking or cooking with water drawn from the hot water side of the tap altogether.] Selective leaching or dealloying are terms used when one al￾loy constituent is preferentially dissolved by a solution. Selective leaching of zinc from brass (called dezincification), or selective loss of graphite in buried gray cast iron gas lines (possibly lead￾ing to porosity and explosions) may serve as examples. Corrosion is often more fully described by electrochemical reactions in which free electrons are interpreted to be trans￾ferred from one chemical species (or from one part of the same species) to another. Specifically, the deterioration of metals and alloys is interpreted to be caused by an interplay between oxi￾dation and reduction processes. This shall be explained in a few examples. During the oxidation process, electrons are transferred from, say iron, to another part of iron (or a different metal) ac￾cording to the reaction equation: Fe
Fe2 2e (9.1) or generally for a metal, M: M
Mn ne, (9.2) where n is the valency of the metal ion or the number of elec￾trons transferred, and e represents an electron. The site at which oxidation takes place is defined to be the anode. During a re￾duction process, the free electrons which may have been gener￾ated during oxidation are transferred to another portion of the sample and there become a part of a different chemical species according to: 2H 2e
H2 (9.3) or in the case of a metal, M: Mn ne
M. (9.4) The site where reduction takes place is called the cathode. In other words, oxidation and reduction can be considered as mir￾ror-imaged processes. As an example, the dissolution of iron in an acid solution (e.g., HCl) is represented by the above equations (9.1) and (9.3), or, in summary: Fe 2H
Fe2 H2. (9.5) Dealloying 158 9 • Degradation of Materials (Corrosion)