Sacrificial anode CP: current demand and anode mass
Mean protective current from the surface area, the current density and the coating breakdown factor, and the net mass of galvanic anodes for the design life — per DNV-RP-B401; the number of anodes.
f_cm = a + b · t_f / 2 (≤ 1)I_cm = A_c · i_cm · f_cmM = I_cm · t_f · 8760 / (u · ε); N = ⌈M / m_a⌉A simplified check on the mean current. A full DNV-RP-B401 design also covers the initial and final current and the anode resistance: the chosen number of anodes must deliver the final current too. The category I–III constants (DNV-RP-B401) are for paint coatings on structures at depths of 0–30 m; pipelines with factory coatings (3LPE, FBE) have much lower factors — see DNV-RP-F103. Onshore buried pipelines are usually protected by impressed-current stations; galvanic anodes are used on short sections, casings and tank bottoms.
Source: DNV-RP-B401 "Cathodic Protection Design"
Inputs
You can change a field's unit: the value is converted to the formula's units automatically.
For a pipe, π · D · L: 1 km of 8-5/8 in pipe is 688 m².
Seawater, 0–30 m (DNV-RP-B401): 0.07 (tropical) to 0.12 A/m² (arctic); buried in sediment 0.02 A/m²; steel in soil roughly 0.01–0.03 A/m².
Bare steel: a = 1, b = 0.
DNV-RP-B401: Al-Zn-In 2 000 in seawater and 1 500 in sediment; zinc 780 and 700.
Long slender stand-off 0.90, long flush-mounted 0.85, bracelets and short flush-mounted 0.80.
Unit converter for this formulaArea · Time · Mass · Current · Fraction and percent
- m²1
- km²1·10⁻⁶
- ha0.0001
- 10³ m²0.001
- mm²1,000,000
- cm²10,000
- in²1,550
- ft²10.7639
- acre0.000247105
- N — Number of anodes–pcs
- I_cm — Mean current demand–
- f_cm — Mean coating breakdown factor–
More in Corrosion protection
Average general corrosion rate from the mass loss of a corrosion coupon over the exposure time, in mm/yr and mpy; the corrosivity rating per NACE SP0775.
Corrosion rate from two wall thickness surveys and the remaining life to the retirement thickness; the longest interval to the next inspection.
Results are engineering estimates from standard formulas; for design decisions check them against the codes, project documents and specialists' calculations. The formulas carried over from the original set are unchanged, and their errors are described in the notes.