164 9.Degradation of Materials(Corrosion) Mg2+ Mg e Mg2+ Anode Cathode Soil (a) Solar panel e Scrap iron FIGURE 9.6.(a)Sacrificial anode which is wired to a steel pipeline to provide cathodic protection.(b) Soil A photovoltaic device supplies electrons to a steel pipe for cathodic protection.The connecting wires need to be electrically insulated! (b) Faraday The question then arises,how much metal is electrochemically Equation removed by corrosion (or deposited)in a given time interval,t, while applying a certain current,I?M.Faraday (see Chapter 10) empirically arrived at the following equation: w=9M=1·M n·F n·F3 (9.11) where W is the liberated mass,g is the electric charge,M is the atomic mass of the metal,F is the Faraday constant (see Ap- pendix II),and n is the valence of the metal ion.We shall return to this topic in Chapter 10.The question then arises, how much metal is electrochemically removed by corrosion (or deposited) in a given time interval, t, while applying a certain current, I? M. Faraday (see Chapter 10) empirically arrived at the following equation: W
q n   M F
I n  t   F M, (9.11) where W is the liberated mass, q is the electric charge, M is the atomic mass of the metal, F is the Faraday constant (see Ap￾pendix II), and n is the valence of the metal ion. We shall return to this topic in Chapter 10. FIGURE 9.6. (a) Sacrificial anode which is wired to a steel pipeline to provide cathodic protection. (b) A photovoltaic device supplies electrons to a steel pipe for cathodic protection. The connecting wires need to be electrically insulated! 164 9 • Degradation of Materials (Corrosion) e – e – (a) Mg + Mg2+ Mg2+ – Anode Cathode Soil e – e – (b) + – Scrap iron Soil￾￾￾ ￾ Solar panel Faraday Equation