How do you size a sacrificial anode for a buried pipeline?

Anode sizing for buried pipelines requires calculating total current demand based on pipeline surface area, coating efficiency and degradation, and required current density. The number of anodes is determined by the greater of: (1) current output capacity per anode divided into total current demand, and (2) total amp-hours needed over the design life divided by each anode's capacity. Anode resistance is calculated using the Dwight formula for soil resistivity.

What is the difference between magnesium and zinc sacrificial anodes?

Magnesium anodes have a higher driving voltage (-1.55 to -1.75V vs Cu/CuSO4) and higher capacity (1,100 Ah/lb), making them suitable for higher-resistivity soils (1,000-10,000 ohm-cm). Zinc anodes have lower driving voltage (-1.10V) and capacity (370 Ah/lb) but are more efficient in low-resistivity soils (<1,000 ohm-cm) and marine environments.

What is the NACE SP0169 protection criterion for buried pipelines?

NACE SP0169 specifies a minimum protection potential of -850 mV vs Cu/CuSO4 reference electrode (instant-off potential). Alternative criteria include 100 mV of cathodic polarization from the native corrosion potential. The pipeline-to-soil potential must be maintained more negative than -850 mV at all points along the pipeline.

How does coating degradation affect CP anode sizing?

Coating degradation increases the bare (unprotected) area over time, requiring more cathodic protection current. FBE coatings typically start at 98% efficiency and degrade 0.5-1% per year. The average bare area over the design life is used for anode sizing to ensure adequate protection throughout the system's service life.

CP Anode Sizing Calculator | Cathodic Protection for Buried Pipelines

Pipeline Parameters

Pipeline Diameter

Pipeline Length

miles

Coating Properties

Coating Type

Coating Efficiency

Auto-filled from coating type. Override if known.

Coating Degradation Rate

%/year

Design Parameters

Design Life

years

Soil Resistivity

ohm-cm

Typical: 500-5,000 (low), 5,000-10,000 (moderate), >10,000 (high)

Required Current Density

mA/ft²

Typical: 1.0-2.0 coated, 1.5-3.0 bare pipe

Safety Factor

Anode Properties

Anode Material

Mg HP: -1.75V, Mg Std: -1.55V, Zn: -1.10V (vs Cu/CuSO4)

Anode Weight

Anode Utilization Factor

Typical 0.85 for packaged anodes (NACE SP0572)

Understanding CP Anode Sizing

Galvanic CP Principle
Sacrificial anodes are metals more active (negative) than steel on the galvanic series. Connected to the pipeline, they corrode preferentially, supplying protective current to the pipe surface.

Anode Selection
Magnesium: high driving voltage, best for high-resistivity soils (>1,000 ohm-cm). Zinc: lower voltage, efficient in low-resistivity or marine environments (<1,000 ohm-cm).

Design Considerations
Anode count is governed by the greater of current output capacity (instantaneous) and total charge capacity (design life). Coating degradation increases bare area over time, requiring more current at end of life.

Learn the Theory

Understand cathodic protection theory, galvanic series, NACE standards, and soil resistivity effects

Read Engineering Guide

Key Formulas

N = I_total / I_anode

N = Number of anodes required

I_total = Total current demand (A)

I_anode = Current output per anode (A)

I_total = A_avg x i x SF

R_anode = 0.00521 x rho / L_anode

Life = (W x C x UF) / (I x 8760)

Standards & References

NACE SP0169
Control of External Corrosion on Underground Metallic Piping Systems
NACE SP0572
Design, Installation, Operation of Galvanic Anode CP Systems
NACE TM0497
Measurement Techniques for CP Criteria
DNV-RP-B401
Cathodic Protection Design
Peabody's Control of Pipeline Corrosion
Industry reference textbook

Engineering Notes

Protection criterion: -850 mV vs Cu/CuSO4 per NACE SP0169
Mg anodes: Best for soil resistivity 1,000-10,000 ohm-cm
Zinc anodes: Best for soil resistivity < 1,500 ohm-cm
Utilization: 0.85 typical for packaged anodes in backfill
Backfill: Standard Mg anode backfill is 75% gypsum, 20% bentonite, 5% sodium sulfate
Spacing: Uniform spacing preferred; max 1,500 ft for coated pipe

Quick Reference - Soil Resistivity

< 1,000 ohm-cm: Very corrosive soil
1,000-5,000 ohm-cm: Corrosive soil
5,000-10,000 ohm-cm: Moderately corrosive
10,000-20,000 ohm-cm: Mildly corrosive
> 20,000 ohm-cm: Low corrosivity

CP Anode Sizing Calculator

Pipeline Parameters

Coating Properties

Design Parameters

Anode Properties

Understanding CP Anode Sizing

Learn the Theory

Results

Key Formulas

Standards & References

Engineering Notes

Quick Reference - Soil Resistivity

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