9.2.Electrochemical Corrosion 159 Fe2+ H'H Acid e Fe FIGURE 9.2.Schematic representation of the dissolution (corrosion) of iron in an acid solu- Anode Cathode tion. The iron ions thus created transfer into the solution(or may,in other cases,react to form an insoluble compound).The process just described is depicted schematically in Figure 9.2. Rust forms readily when iron is exposed to damp air(relative humidity >60%)or oxygen-containing water.Rusting under these conditions occurs in two stages in which iron is,for ex- ample,at first oxidized to Fe2+and then to Fe3+according to the following reaction equations: Fe+02+H20→Fe(OH)2 (9.6) and 2Fe(OH)2+02+H20→2Fe(OH)3. (9.7) Fe (OH)3,that is,hydrated ferric oxide,is insoluble in water and relatively inert,that is,cathodic.As outlined above,Fe(OH)3 is not the only form of "rust."Indeed,other species,such as Feo, Fe2O3,FeOOH,and Fe304,generally qualify for the same name. In these cases,different reaction equations than those shown above apply. 9.2.Electrochemical Corrosion Electrochemical corrosion is often studied by making use of two electrochemical half-cells in which each metal is immersed in a one-molar (1-M)solution of its ion.(A 1-M solution contains 1 mole of the species in 1 dm3 of distilled water.One mole is the atomic mass in grams of the species.)The two half-cells are sep- arated by a semipermeable membrane which prevents interdiffu- sion of the solutions but allows unhindered electron transfer.Fig- ure 9.3 depicts an example of such an electrochemical cell or galvanic couple in which a piece of iron is immersed in a solutionThe iron ions thus created transfer into the solution (or may, in other cases, react to form an insoluble compound). The process just described is depicted schematically in Figure 9.2. Rust forms readily when iron is exposed to damp air (relative humidity ! 60%) or oxygen-containing water. Rusting under these conditions occurs in two stages in which iron is, for ex￾ample, at first oxidized to Fe2 and then to Fe3 according to the following reaction equations: Fe 1 2 O2 H2O
Fe(OH)2 (9.6) and 2Fe(OH)2 1 2 O2 H2O
2Fe(OH)3. (9.7) Fe (OH)3, that is, hydrated ferric oxide, is insoluble in water and relatively inert, that is, cathodic. As outlined above, Fe(OH)3 is not the only form of “rust.” Indeed, other species, such as FeO, Fe2O3, FeOOH, and Fe3O4, generally qualify for the same name. In these cases, different reaction equations than those shown above apply. Electrochemical corrosion is often studied by making use of two electrochemical half-cells in which each metal is immersed in a one-molar (1-M) solution of its ion. (A 1-M solution contains 1 mole of the species in 1 dm3 of distilled water. One mole is the atomic mass in grams of the species.) The two half-cells are sep￾arated by a semipermeable membrane which prevents interdiffu￾sion of the solutions but allows unhindered electron transfer. Fig￾ure 9.3 depicts an example of such an electrochemical cell or galvanic couple in which a piece of iron is immersed in a solution 9.2 • Electrochemical Corrosion 159 Fe2+ Acid Anode Cathode Fe e – e – H+ H+ H2 FIGURE 9.2. Schematic representation of the dissolution (corrosion) of iron in an acid solu￾tion. 9.2 • Electrochemical Corrosion