สรุป (TL;DR)

A step-by-step guide to sizing a sacrificial-anode corrosion protection system — current density by environment, the anode mass formula (M = I·t·8760 / u·ε), anode count from current output, and the −850 mV / 100 mV decay criteria, with a worked example. References DNV-RP-B401, ISO 12696, NACE SP0169 / ISO 15589, ASTM B418.

Our earlier article on choosing among the 4 anode types answers "which metal in which environment." This article answers the next question an engineer must put into a real TOR/BOQ: "how many kilograms, how many units, and how do you prove it is enough."

Designing a Galvanic Cathodic Protection (GCP) system is not guesswork — there are fixed formulas and acceptance criteria under DNV-RP-B401 (marine / submerged steel), ISO 12696 (steel in concrete) and NACE SP0169 / ISO 15589 (buried pipe/tanks). Under-size the anode mass and the system dies before its design life; over-size it and you waste budget.

The 5-step design overview

graph TD
    A[1. Surface area to protect A m²
× coating breakdown factor] --> B[2. Current Demand
I = A × i_c
compute initial / mean / final]
    B --> C[3. Total anode mass
M = I_mean × t × 8760 / u × ε]
    C --> D[4. Number of anodes
max of: mass vs current output]
    D --> E[5. Verify protection criteria
−850 mV / 100 mV decay]
    E -->|fail| B
    E -->|pass| F[Design complete + BOQ]

Step 1 — Surface area + coating breakdown factor (fc)

Start with the metal surface area in contact with the electrolyte (m²). If coated, CP only has to protect the area where the coating has broken down — use a coating breakdown factor (fc) per DNV-RP-B401:

fc = a + b × t (t = years; a/b depend on coating type and water depth)

Bare steel: fc = 1.0
Good coating in shallow water: fc starts low (~0.02) and rises with age
Current demand = A × fc × i_c → the better the coating, the fewer anodes needed

Step 2 — Current demand (i_c by environment)

The key input is the design current density (i_c), which varies greatly by electrolyte — and you must compute three values: initial (first polarization), mean (lifetime average, used for mass) and final (end of life, used to check anode count):

Environment	Design current density i_c	Standard
Tropical seawater >20°C (bare steel)	initial 150 / mean 70 / final 100 mA/m²	DNV-RP-B401
Seabed sediment (mud)	~20 mA/m²	DNV-RP-B401
Steel reinforcement in concrete	0.2–20 mA/m² (typically 1–2; up to 20 if chloride-laden)	ISO 12696
Buried pipe/tank in soil	10–50 mA/m² (depends on resistivity + aeration)	NACE SP0169 / ISO 15589

Formula: I = A × fc × i_c (computed separately for initial / mean / final)

Step 3 — Total anode mass (the core DNV-RP-B401 equation)

M = (I_mean × t_f × 8760) / (u × ε)

M = total net anode mass (kg)
I_mean = mean current demand (A)
t_f = design life (years) · 8760 = hours/year
u = utilization factor (stand-off 0.80; bracelet/flush 0.85–0.90)
ε = electrochemical capacity (Ah/kg) of the anode material

Anode material	ε design (Ah/kg)	Closed-circuit potential	Notes
Aluminium (Al-Zn-In)	~2,000	−1.05 V (Ag/AgCl)	Most common in seawater (high capacity/kg)
Zinc	~700–780	−1.00 V	Not recommended >50°C or in certain muds
Magnesium	~1,100	−1.50 V	Strong driving voltage for fresh water / high-resistivity soil

ε and potential values reference DNV-RP-B401 Section 8 + ASTM B418 (zinc). Real projects must use values from the manufacturer's type-tested COC.

Step 4 — Number of anodes (must satisfy both "mass" and "current")

Anode count = the larger of two conditions:

(a) From mass: N_mass = M / m_a (m_a = net mass per anode)

(b) From current output: each anode delivers a limited current set by its resistance — use Dwight's formula for a slender stand-off anode:

R_a = (ρ / 2πL) × (ln(4L/r) − 1)

then I_a = (E_c - E_a) / R_a → you need enough anodes so that N × I_a ≥ I at both initial (full anode) and final (consumed anode, higher R_a).

Rule of thumb: low-resistivity seawater (~20–30 Ω·cm) → mass usually governs · soil / fresh water (high resistivity) → current output / count governs.

Step 5 — Protection criteria

Measure steel potential against a reference electrode; it must reach:

Environment	Protection criterion	Reference electrode
Steel in water/soil (aerobic)	≤ −0.80 V (Ag/AgCl) or ≤ −0.85 V (Cu/CuSO₄)	Ag/AgCl, Cu/CuSO₄
Soil with SRB bacteria (anaerobic)	≤ −0.90 V (Ag/AgCl)	Ag/AgCl
Steel reinforcement in concrete	100 mV potential decay within 24 h of disconnect	Ag/AgCl, Cu/CuSO₄ (concrete)

If the criterion is not met → loop back, increase current demand, and recompute.

Worked example — a steel sheet-pile sea wall

Problem: bare-steel sheet pile, submerged area A = 500 m², tropical seawater, design life 20 years, Al-Zn-In anodes.

Step 2 — Current demand (bare steel, fc = 1.0):

I_mean = 500 × 0.070 = 35 A
I_initial = 500 × 0.150 = 75 A · I_final = 500 × 0.100 = 50 A

Step 3 — Anode mass (ε = 2,000 Ah/kg, u = 0.80):

M = (35 × 20 × 8760) / (0.80 × 2,000) = 6,132,000 / 1,600 = ≈ 3,833 kg (total net Al mass)

Step 4 — Count (using 40 kg-net anodes):

From mass: 3,833 / 40 = 96 units
Low-resistivity seawater → high output per anode → mass governs → use 96 units

Step 5: distribute anodes so steel potential everywhere ≤ −0.80 V (Ag/AgCl) at both initial and final.

For a reinforced-concrete structure, switch to ISO 12696 current densities (1–20 mA/m²) + the 100 mV decay criterion, and select a Concrete Anode CR60/CR100 instead.

Common design mistakes

Using one i_c for the whole life — you must split initial/mean/final or mass/count will be wrong
Forgetting the utilization factor — you cannot consume 100% of anode mass (stand-off ~80%)
Not checking final current — near end of life the anode shrinks, R_a rises, and output may fall short even if total mass is adequate
Ignoring coating breakdown — on coated work, using fc = 1 over-designs and wastes budget

FAQ

Q: Can I calculate this myself, or do I need an engineer?

A: The formulas above suffice for a preliminary design, but real projects (especially government TOR work) require an engineer's sign-off, material-specific i_c/ε from actual test data, and a design margin. Our Sahawatthanakit team handles the calculation, design and standards-referenced BOQ.

Q: The TOR only says "corrosion protection per standard" — which document applies?

A: Submerged steel / offshore → DNV-RP-B401 · steel reinforcement in concrete → ISO 12696 + ACI 222R · buried pipe/tanks → NACE SP0169 / ISO 15589-1. Match the standard to the structure type.

Q: How do −850 mV and 100 mV decay differ?

A: −850 mV (Cu/CuSO₄) is an "absolute potential" criterion for steel in soil/water · 100 mV decay is a "change" criterion measured after disconnect, commonly used for steel in concrete (ISO 12696) because absolute potentials are hard to measure inside concrete.

Q: What do I check after installation?

A: Measure steel potential before/after a 24 h disconnect with the correct reference electrode, inspect every 6–12 months, log the polarization decay against the criterion, and verify the anode consumption rate against the design life plan.

Request a quote + CP system design

Our engineering team designs complete Cathodic Protection systems — current-demand calculation + anode mass/count + standards-referenced BOQ + anode supply (Concrete / Aluminium / Zinc / Magnesium) + commissioning + monitoring. Reference clients: State Railway of Thailand, BMTA, CPAC, SCG, Freyssinet, AGC.