ELECTROCHEMISTRY (NERNST, TAFEL, BUTLER-VOLMER)

Cell potential under non-standard conditions follows the Nernst equation: E = E° - (RT/nF)·ln Q where Q is the reaction quotient. Activation overpotential at an electrode follows the Butler-Volmer equation; in the high-overpotential limit it reduces to the Tafel equation η = a + b·log|i|. Corrosion rate is computed from corrosion current density i_corr by Faraday's law: rate (mm/yr) = 3.27×10⁻³ · i_corr · EW / ρ, with i_corr typically extracted from Tafel-extrapolation or LPR measurements.

• E = E° - (RT/nF) · ln Q (Nernst)
• i = i₀[exp(αaFη/RT) - exp(-αcFη/RT)] (Butler-Volmer)
• η = a + b·log|i| (Tafel)
• m = (Q·M) / (n·F) (Faraday)
• rate (mm/yr) = 3.27e-3 · i_corr · EW / ρ

References & StandardsASTM G59 (LPR), ASTM G102 (calc), Bard & Faulkner Ch. 3

Use for fuel cells, electrolyzers, batteries, corrosion analysis, electroplating, sensor design. The Nernst equation gives the open-circuit voltage of any electrochemical cell as a function of reactant activity. Tafel slopes characterize electrode kinetics; their crossover gives i_corr in a corroding system. Faraday's law converts current to mass deposited or consumed (essential for plating thickness control).