The law that the amount of substance changed by an electrode reaction is proportional to the charge passed. The mass dissolved or deposited is m = ItM/(nF).
Japanese name: ファラデーの法則
Faraday's law of electrolysis appears in 1 practice question across 1 topic, including Corrosion and surface treatment.
Faraday's laws of electrolysis state that the amount of substance reacting at an electrode is proportional to the charge passed (first law), and that the amount reacting for the same charge is proportional to the substance's molar mass divided by the number of electrons in the reaction (second law). Combined, the mass dissolved or deposited when a current I flows for a time t is m = ItM/(nF). F is the Faraday constant, the charge of 1 mol of electrons, 96485 C/mol, equal to the product of the Avogadro constant and the elementary charge, F = N_A·e. Faraday found these relations by experiment in the 1830s.
Corrosion is also an electrochemical reaction, so once the corrosion current is known, Faraday's law converts it into a corrosion rate. Using the current density per unit area i, the rate of thickness loss is iM/(nFρ) (ρ is density). When iron dissolves as Fe²⁺, a corrosion current density of 1 µA/cm² corresponds to a loss of about 11.6 µm per year, or about 0.012 mm/y. Obtaining the corrosion current by polarization resistance measurement or Tafel extrapolation and converting it to annual thickness loss this way is a standard procedure in corrosion assessment. It is important for n to match the charge of the ions actually formed.
In electroplating, Faraday's law is used to estimate film thickness. Plating in a copper sulfate bath at a current density of 2 A/dm² (0.02 A/cm²) for 10 minutes deposits about 4.4 µm of copper, assuming all the current goes into copper deposition. In practice part of the current goes into hydrogen evolution and other reactions, so the result is multiplied by the fraction used for deposition, the current efficiency. Acidic copper and zinc baths have high current efficiencies, whereas chromium plating is known for low efficiency. In electrolytic smelting of aluminum, producing 1 kg of aluminum requires about 2980 Ah of charge, which is why the process uses so much electric power.
Battery capacity is also determined by Faraday's law. If an electrode material exchanges n mol of electrons per mol, its theoretical capacity per unit mass is nF/M, which converts to nF/(3.6M) mAh/g using 1 mAh = 3.6 C. The 372 mAh/g of graphite and the roughly 170 mAh/g of LiFePO₄ are values obtained from this calculation. In the same way, the charge a sacrificial anode can deliver and the charge obtained from 1 mol of hydrogen in a fuel cell (2F) can be calculated. As the basic relation linking charge and amount of substance, it appears in every area of electrochemistry.
Answer: B. About 0.91 kg
From Faraday's law, the dissolved mass is m = ItM/(nF). One year is 365 × 24 × 3600 = 3.15×10⁷ s, so the charge passed is Q = 0.10 × 3.15×10⁷ = 3.15×10⁶ C. Since iron goes to Fe²⁺, n = 2, and m = 3.15×10⁶ × 55.85/(2 × 96485) ≈ 913 g, about 0.91 kg. About 1.8 kg is the mistake of taking the number of electrons as n = 1. About 0.61 kg calculates with n = 3, as if iron went to Fe³⁺, which does not match the reaction given. Even a corrosion current of only 0.10 A loses nearly 1 kg of iron in a year, enough to perforate a pipe if concentrated locally.
It is about 96485 C/mol, the magnitude of the charge carried by 1 mol of electrons. It equals the elementary charge, 1.602×10⁻¹⁹ C, multiplied by the Avogadro constant, 6.022×10²³ mol⁻¹.
It is the number of electrons exchanged per particle of the reacting substance: 2 for deposition of Cu²⁺, 3 for Al³⁺ and 1 for Ag⁺. For metals such as iron that can form both Fe²⁺ and Fe³⁺, choose it to match the actual reaction.
Because part of the current is used in reactions other than the intended one, such as hydrogen evolution or reactions of impurities. This fraction is expressed as the current efficiency, and multiplying the calculated value by it brings the result closer to the actual amount.
From Faraday's law, the dissolved mass is m = ItM/(nF).
In a steel pipe, a corrosion current in which iron dissolve…Galvanic corrosion Local cell Passivity Pitting corrosion Sacrificial anode Standard electrode potential Stress corrosion cracking