A buried, coated pipeline behaves like a leaky electrical transmission line. Current from the rectifier's anode ground bed enters the steel at one point and bleeds outward through coating defects into the soil, so both the protective current and the pipe-to-soil potential shift decay exponentially with distance from the bed:
α = √(R_L / R_C) attenuation constant (1/m)
λ = 1 / α attenuation length (m)
Z0 = √(R_L · R_C) characteristic impedance (Ω)
ΔV(x) = (I0/2)·Z0·e^(−α·x) potential shift at distance x from the bed
E(x) = E_corr − ΔV(x) actual pipe-to-soil potential (V, Cu/CuSO4)
R_L is the pipe steel's resistance per unit length (fixed here for a 20-inch, 9.5 mm wall steel line: R_L ≈ 1.1×10⁻⁵ Ω/m). R_C is the coating's resistance-length product — it falls sharply as holidays (coating defects) let more current leak to soil, so the Coating quality slider spans roughly three orders of magnitude in R_C. A larger λ means the same rectifier reaches farther before the potential decays back toward the unprotected native corrosion potential, E_corr ≈ −0.55 V.
Per NACE SP0169, a point is considered protected once E(x) is more negative than −0.85 V; pushed past about −1.20 V, excess current risks coating disbondment and hydrogen embrittlement in high-strength steel — the "overprotected" band above. The ground-bed's own resistance to earth, R_gb ≈ ρ/(2π·L_bed), sets the DC voltage the rectifier must supply for the chosen current, so raising Soil resistivity makes every ampere more expensive.
- Coating quality — sets R_C; poor coating forces the CP current to dissipate within a short stretch of pipe.
- Rectifier output current — the total DC current I0 injected at the anode bed, split to protect both directions of pipe.
- Anode ground-bed position — where along the pipeline the rectifier's current enters; move it to see coverage shift.
- Soil resistivity — raises the ground bed's resistance to earth and therefore the rectifier voltage needed.