1. Cathodic Protection Overview
Cathodic protection (CP) is an electrochemical technique that prevents corrosion of a metal surface by making it the cathode of an electrochemical cell. For buried pipelines, CP is the primary defense against external corrosion and is mandated by federal regulations (49 CFR 192 for gas, 49 CFR 195 for liquids).
Galvanic CP
Sacrificial Anodes
Uses reactive metals (Mg, Zn) that corrode preferentially. Simple, no external power needed. Best for well-coated pipelines in moderate soils.
Impressed Current CP
Rectifier Systems
Uses external DC power to drive current from inert anodes. Higher capacity, adjustable output. Required for large bare areas or high-resistivity soils.
Coating + CP
Combined Protection
Pipeline coatings reduce bare area; CP protects coating defects (holidays). Combined approach is standard practice per NACE SP0169.
Regulatory
49 CFR 192/195
Federal regulations require CP for all new buried steel pipelines within 1 year of installation, with annual monitoring.
Industry Practice: Over 95% of cathodic protection on buried gathering and transmission pipelines in the US uses galvanic (sacrificial) anodes due to simplicity of installation, low maintenance, and effectiveness on well-coated pipe.
2. Galvanic Series & Electrochemical Theory
The galvanic series ranks metals and alloys by their electrode potential in a given electrolyte. When two dissimilar metals are electrically connected in soil, the more active (negative) metal corrodes and protects the more noble metal.
Galvanic Series in Soil
| Metal/Alloy | Potential (V vs Cu/CuSO4) | Role in CP |
|---|---|---|
| Magnesium (High Potential) | -1.75 | Sacrificial anode |
| Magnesium (Standard AZ-63) | -1.55 | Sacrificial anode |
| Zinc | -1.10 | Sacrificial anode |
| Aluminum (special alloy) | -1.05 to -1.15 | Sacrificial anode (marine) |
| Carbon Steel (corroding) | -0.50 to -0.80 | Structure to protect |
| Carbon Steel (protected) | -0.85 or more negative | NACE SP0169 criterion |
| Copper | -0.20 | Noble (cathode) |
| High-Silicon Cast Iron | -0.20 | ICCP anode |
| Graphite | +0.30 | ICCP anode |
| Platinum / MMO | +0.40 | ICCP anode |
Electrochemical Fundamentals
Galvanic CP Electrochemistry:
At the anode (Mg dissolves):
Mg --> Mg2+ + 2e- (oxidation, metal is consumed)
At the cathode (steel pipeline):
O2 + 2H2O + 4e- --> 4OH- (reduction, steel is protected)
Driving voltage:
E_drive = E_anode - E_cathode
E_drive = (-1.75) - (-0.85) = -0.90 V (for Mg HP)
Current output (Ohm's Law):
I = E_drive / (R_anode + R_wire + R_coating)
Where R_anode dominates (typically 90%+ of total resistance)
Faraday's Law of Electrolysis
Metal consumption rate:
m = (M * I * t) / (n * F)
Where:
m = mass consumed (grams)
M = molar mass (24.31 g/mol for Mg, 65.38 for Zn)
I = current (Amperes)
t = time (seconds)
n = electrons transferred (2 for Mg and Zn)
F = Faraday constant = 96,485 C/mol
Theoretical capacity:
Mg: 2,205 Ah/kg (1,000 Ah/lb) theoretical
Zn: 820 Ah/kg (372 Ah/lb) theoretical
Practical capacity (with efficiency):
Mg: ~1,100 Ah/lb (50% electrochemical efficiency)
Zn: ~370 Ah/lb (90% electrochemical efficiency)
3. Anode Types & Properties
Magnesium Anodes
Magnesium is the most common sacrificial anode material for buried pipelines due to its high driving voltage and availability. Two alloy grades are used: standard (AZ-63 type) and high-potential.
| Property | Mg Standard (AZ-63) | Mg High Potential |
|---|---|---|
| Open-circuit potential | -1.55 V vs Cu/CuSO4 | -1.75 V vs Cu/CuSO4 |
| Driving voltage to steel | 0.70 V | 0.90 V |
| Practical capacity | 1,100 Ah/lb | 1,100 Ah/lb |
| Electrochemical efficiency | ~50% | ~50% |
| Soil resistivity range | 1,000-5,000 ohm-cm | 1,000-10,000+ ohm-cm |
| Common sizes | 9, 17, 32, 48 lb | 9, 17, 32, 48 lb |
Zinc Anodes
Zinc anodes are used in low-resistivity soils and marine environments. They have lower driving voltage but higher electrochemical efficiency than magnesium.
| Property | Zinc (ASTM B418 Type I) |
|---|---|
| Open-circuit potential | -1.10 V vs Cu/CuSO4 |
| Driving voltage to steel | 0.25 V |
| Practical capacity | 370 Ah/lb |
| Electrochemical efficiency | ~90% |
| Soil resistivity range | < 1,500 ohm-cm |
| Common sizes | 30, 60 lb |
Material Selection Rule: Use high-potential magnesium for soil resistivity above 1,000 ohm-cm. Use zinc only in low-resistivity environments (<1,500 ohm-cm) or marine applications where its higher efficiency provides economic advantage.
Anode Backfill
Packaged anodes are installed in a special backfill that reduces anode-to-earth resistance and ensures uniform consumption.
Standard Magnesium Anode Backfill (NACE SP0572):
Composition:
75% Gypsum (CaSO4.2H2O) - provides ionic conductivity
20% Bentonite clay - retains moisture around anode
5% Sodium sulfate (Na2SO4) - depolarizer
Effect on anode resistance:
Backfill resistivity: ~50 ohm-cm (vs. 1,000-10,000 for soil)
Reduces anode-to-earth resistance by 30-50%
Ensures uniform anode consumption
Maintains moisture contact in dry soils
Package dimensions (typical):
Mg anodes: 8" diameter x 18-48" long
Zn anodes: 10" diameter x 30-48" long
4. Design Calculations
Step 1: Pipeline Surface Area
Total pipeline surface area:
A_pipe = pi x D x L
Where:
D = pipe outside diameter (ft) = diameter(in) / 12
L = pipe length (ft) = length(miles) x 5,280
Example:
12" OD pipe, 10 miles long:
D = 12/12 = 1.0 ft
L = 10 x 5,280 = 52,800 ft
A_pipe = 3.14159 x 1.0 x 52,800 = 165,876 ft2
Step 2: Bare Area Calculation
Bare area with coating degradation:
Initial bare area:
A_bare_initial = A_pipe x (1 - coating_eff/100)
End-of-life bare area:
A_bare_EOL = A_pipe x (1 - (eff - degradation x years)/100)
Average bare area for design:
A_avg = (A_bare_initial + A_bare_EOL) / 2
Example (FBE coating, 98% eff, 0.5%/yr, 20-yr life):
A_bare_initial = 165,876 x (1 - 0.98) = 3,318 ft2
Eff at EOL = 98 - 0.5 x 20 = 88%
A_bare_EOL = 165,876 x (1 - 0.88) = 19,905 ft2
A_avg = (3,318 + 19,905) / 2 = 11,611 ft2
Step 3: Current Requirement
Total current demand:
I_total = A_avg x i x SF
Where:
A_avg = average bare area (ft2)
i = current density (mA/ft2)
SF = safety factor (typically 1.25)
Typical current densities:
Well-coated pipe: 1.0-2.0 mA/ft2
Degraded coating: 2.0-5.0 mA/ft2
Bare pipe: 1.5-3.0 mA/ft2
Example:
I_total = 11,611 x 1.5 x 1.25 = 21,770 mA = 21.77 A
Step 4: Anode Resistance (Dwight Formula)
Dwight formula for vertical rod in soil:
R = (rho / (2 x pi x L)) x [ln(8L/d) - 1]
Where:
R = anode-to-earth resistance (ohms)
rho = soil resistivity (ohm-cm)
L = anode length (cm)
d = anode/backfill diameter (cm)
Simplified form for packaged anodes:
R = 0.00521 x rho / L_ft
Example (3,000 ohm-cm soil, 17-lb Mg anode):
Anode package: 2.5 ft long x 8" (0.667 ft) diameter
L_cm = 2.5 x 30.48 = 76.2 cm
d_cm = 0.667 x 30.48 = 20.3 cm
R = (3000 / (2 x 3.14159 x 76.2)) x [ln(8 x 76.2 / 20.3) - 1]
R = (6.27) x [ln(30.0) - 1]
R = 6.27 x [3.40 - 1]
R = 6.27 x 2.40 = 15.0 ohms
Step 5: Anode Current Output
Single anode current (Ohm's law):
I_anode = E_drive / (R_anode + R_wire)
Where:
E_drive = |E_anode - E_protection|
E_anode = open-circuit potential of anode (V vs Cu/CuSO4)
E_protection = -0.85 V (NACE SP0169 criterion)
R_wire ~ 0.01 ohms (negligible for short leads)
Example (Mg HP anode, R = 15.0 ohms):
E_drive = |-1.75 - (-0.85)| = 0.90 V
I_anode = 0.90 / (15.0 + 0.01) = 0.0599 A = 59.9 mA
Step 6: Number of Anodes
Two criteria determine anode count:
Criterion 1 - Current capacity:
N_current = I_total / I_anode
N_current = 21,770 / 59.9 = 364 anodes
Criterion 2 - Design life (weight-based):
Total Ah needed = I_total(A) x 8,760 hr/yr x design_life(yr)
Ah per anode = weight(lb) x capacity(Ah/lb) x utilization
Total Ah = 21.77 x 8,760 x 20 = 3,814,104 Ah
Per anode = 17 x 1,100 x 0.85 = 15,895 Ah
N_life = 3,814,104 / 15,895 = 240 anodes
Governing: N = max(N_current, N_life) = 364 anodes
Spacing:
Spacing = 52,800 ft / 364 = 145 ft between anodes
Step 7: Anode Life Verification
Expected anode life:
Life (years) = (W x C x UF) / (I_anode x 8,760)
Where:
W = anode weight (lbs)
C = electrochemical capacity (Ah/lb)
UF = utilization factor (0.85 typical)
I_anode = current output (Amperes)
8,760 = hours per year
Example:
Life = (17 x 1,100 x 0.85) / (0.0599 x 8,760)
Life = 15,895 / 524.7
Life = 30.3 years
Check: 30.3 years > 20-year design life = ADEQUATE
5. Soil Resistivity Effects
Soil resistivity is the single most important environmental factor in galvanic CP design. It directly controls anode resistance, current output, and the choice between galvanic and impressed current systems.
Soil Resistivity Classification
| Resistivity (ohm-cm) | Classification | Corrosivity | CP Recommendation |
|---|---|---|---|
| < 500 | Very low | Very corrosive | Zinc anodes effective; consider ICCP |
| 500-1,000 | Low | Corrosive | Zinc or standard Mg anodes |
| 1,000-5,000 | Moderate | Moderate | High-potential Mg anodes (ideal range) |
| 5,000-10,000 | High | Mildly corrosive | High-potential Mg; verify current output |
| 10,000-20,000 | Very high | Low | Mg may struggle; consider ICCP |
| > 20,000 | Extremely high | Very low | ICCP recommended; galvanic marginal |
Measurement Methods
Wenner Four-Pin Method (ASTM G57):
Four equally-spaced pins driven into soil in a straight line.
Current flows between outer pins; voltage measured between inner pins.
rho = 2 x pi x a x R
Where:
rho = soil resistivity (ohm-cm)
a = pin spacing (cm) -- determines measurement depth
R = measured resistance (ohms)
Measurement depth:
Effective depth ~ 1.0 x pin spacing
For pipeline depth (3-5 ft): use a = 3-5 ft (91-152 cm)
Best practice:
Measure at multiple spacings along pipeline route
Measure in wet and dry seasons
Use minimum measured value for conservative CP design
Factors Affecting Soil Resistivity
- Moisture content: Primary factor. Resistivity drops 10-100x as soil goes from dry to saturated
- Salt content: Dissolved ions reduce resistivity. Coastal and irrigated soils have low resistivity
- Temperature: Resistivity decreases ~2% per degree C increase (above freezing)
- Soil type: Clay < loam < sand < gravel < rock (increasing resistivity)
- Compaction: More compacted soil has lower resistivity (better particle contact)
- Depth: Resistivity often decreases with depth due to higher moisture
Effect on Anode Current Output
| Soil (ohm-cm) | Mg HP Anode R (ohms) | Current Output (mA) | Mg Std Current (mA) | Zn Current (mA) |
|---|---|---|---|---|
| 500 | 2.5 | 360 | 280 | 100 |
| 1,000 | 5.0 | 180 | 140 | 50 |
| 3,000 | 15.0 | 60 | 47 | 17 |
| 5,000 | 25.0 | 36 | 28 | 10 |
| 10,000 | 50.0 | 18 | 14 | 5 |
Based on standard 17-lb Mg / 30-lb Zn packaged anodes, 2.5 ft length.
6. NACE Standards & Protection Criteria
NACE SP0169 Protection Criteria
NACE SP0169 defines three alternative criteria for determining adequate cathodic protection of buried steel pipelines.
| Criterion | Requirement | Application |
|---|---|---|
| Criterion 1 | -850 mV vs Cu/CuSO4 (with CP applied) | Most commonly used. Includes IR drop component. |
| Criterion 2 | -850 mV polarized (instant-off) | Most accurate. Eliminates IR drop error. Preferred for surveys. |
| Criterion 3 | 100 mV of polarization shift | Alternative when native potential is known. Used for pipelines near other structures. |
Over-Protection Warning: Potentials more negative than -1,200 mV vs Cu/CuSO4 can cause coating disbondment (cathodic disbondment) and hydrogen embrittlement of high-strength steels (API 5L X70 and above). Always maintain protection within the -850 to -1,200 mV window.
Key NACE and Industry Standards
| Standard | Title | Key Requirements |
|---|---|---|
| NACE SP0169 | External Corrosion Control on Underground Piping | Protection criteria, survey methods, monitoring |
| NACE SP0572 | Galvanic Anode CP System Design | Anode selection, installation, backfill, testing |
| NACE TM0497 | Measurement Techniques Related to CP Criteria | Instant-off measurement, IR compensation |
| DNV-RP-B401 | Cathodic Protection Design | Comprehensive design methodology, anode calculations |
| ASTM G57 | Soil Resistivity (Wenner 4-Pin) | Field measurement procedure for soil resistivity |
| 49 CFR 192 | Transportation of Natural Gas by Pipeline | CP required within 1 year, annual monitoring |
| 49 CFR 195 | Transportation of Hazardous Liquids by Pipeline | CP requirements for liquid pipelines |
CP System Monitoring
Annual CP Survey Requirements (49 CFR 192.465):
1. Pipe-to-soil potential at all test stations (annually)
2. Record ON and instant-OFF potentials
3. Verify -850 mV criterion at all measurement points
4. Investigate and correct any deficiencies within 12 months
Test Station Spacing:
Transmission lines: Every 1 mile (typical)
Distribution: Every 1,000-2,000 ft at changes
Road crossings: Test station at each crossing
Foreign line crossings: Test station at each crossing
Close Interval Survey (CIS):
Measurements every 2.5-5 ft along pipeline
Identifies coating defects and CP gaps
Required every 5-7 years for integrity management
Galvanic vs Impressed Current Decision Guide
| Factor | Galvanic (Sacrificial) | Impressed Current (ICCP) |
|---|---|---|
| Current capacity | Low (10-200 mA/anode) | High (0.1-50 A/rectifier) |
| Power source | None needed (self-powered) | AC power or solar required |
| Best for | Well-coated pipe, moderate soil | Bare pipe, high resistivity, long runs |
| Maintenance | Very low (check annually) | Monthly rectifier checks |
| Soil resistivity | < 10,000 ohm-cm (practical limit) | Any resistivity |
| Interference risk | None (low voltage) | Can interfere with nearby structures |
| Over-protection risk | Low (self-limiting) | Requires adjustment to prevent |
| Capital cost | Lower per anode | Higher (rectifier + groundbed) |
| Operating cost | Replacement every 15-30 years | Electricity + maintenance |
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