Cathodic Protection Basics

Overview

Cathodic protection (CP) is an electrochemical technique that prevents corrosion of buried or submerged metal structures (pipelines, storage tanks, offshore platforms, ship hulls, marine pilings, reinforced concrete bridges). By making the metal surface the cathode of an electrochemical cell, the anodic (corrosion) reactions are suppressed. Two methods exist: galvanic (sacrificial anode) and impressed current (ICCP). Both are standardized under NACE SP0169 (pipeline CP) and NACE SP0285 (tank CP).

Corrosion Basics

Corrosion is an electrochemical reaction:

• Anode (oxidation): Fe → Fe²⁺ + 2e⁻ (metal dissolves — this is corrosion)
• Cathode (reduction): O₂ + 2H₂O + 4e⁻ → 4OH⁻ (or 2H⁺ + 2e⁻ → H₂ in acidic/anaerobic conditions)
• Current flows through the electrolyte (soil/water) from anode to cathode.

Corrosion requires four elements: anode, cathode, metallic path, and electrolyte. CP works by making the entire structure a cathode, eliminating the anodic sites where metal is lost.

Two Methods of CP

Galvanic (Sacrificial Anode) CP

• Connect the structure to a more active (more negative potential) metal (zinc, magnesium, aluminum alloy), which acts as the anode and corrodes sacrificially.
• The natural potential difference (galvanic cell) drives current without external power.
• Advantages: No external power; simple installation; low risk of over-protection; no interference with other structures; low maintenance.
• Disadvantages: Limited driving voltage (~0.1-0.7 V); limited current output; anode must be replaced periodically (5-20 years).
• Best for: small structures, well-coated pipelines in low-resistivity soil, offshore applications, heat exchangers, ship hulls, short pipelines.

Impressed Current CP (ICCP)

• External DC power supply (rectifier) forces current from inert anodes (mixed metal oxide, graphite, high-silicon cast iron, scrap steel) through the electrolyte to the structure (cathode).
• Rectifier converts AC to DC; voltage adjustable to deliver required current.
• Advantages: High driving voltage (up to 50+ V); unlimited current output; can protect large bare or poorly coated structures; adjustable for changing conditions.
• Disadvantages: Requires external power; risk of interference (stray current) with adjacent pipelines; over-protection possible (causes coating disbondment and hydrogen embrittlement); higher maintenance.
• Best for: long cross-country pipelines, large storage tanks, offshore platforms, structures in high-resistivity soil.

Design Criteria

Protection Potential Criteria (NACE SP0169)

For steel in soil/natural waters:

• -0.85 V minimum (negative) relative to Cu/CuSO₄ reference electrode (CSE), with CP applied and IR drop considered
• -0.95 V minimum for anaerobic sulfate-reducing bacteria (SRB) environments
• Polarized potential minimum -0.80 V, minimum 100 mV polarization decay after current off (the "100 mV criterion" — most accurate because it eliminates IR drop)

Do not over-protect: potentials more negative than -1.10 V can cause hydrogen evolution at the cathode, leading to hydrogen embrittlement of high-strength steels and coating disbondment.

Current Requirement

Current density required for protection:

Total current required: I = current density × exposed surface area. Good coatings reduce current demand by 99% — coating + CP is the standard approach (coating is primary protection, CP backs up coating defects).

Soil Resistivity

Key factor for anode design. Measured with Wenner 4-pin method (soil resistivity in ohm·cm).

• <1000 Ω·cm: low — galvanic anodes work well
• 1000-10,000 Ω·cm: moderate — both methods possible
• >10,000 Ω·cm: high — ICCP preferred; galvanic anodes cannot produce enough current output

Anode Selection for Galvanic Systems

Anode weight W (kg) = current required × design life × consumption rate / utilization factor (0.85 typical).

Anode Bed Design for ICCP

• Anodes installed in a coke breeze backfill column (lowers ground resistance, extends anode life by distributing current)
• Typical depth: 30-150 m deep vertical groundbed, or horizontal shallow bed
• Anode spacing: 3-5 m apart
• Total ground bed resistance must be low enough to pass required current at available rectifier voltage (Ohm's law: V = I × R_total)
• Distributed anodes along the pipeline for long lines; deep anode groundbeds where right-of-way is limited

System Components

ICCP System

• Rectifier: AC input, DC output; adjustable; typically 10-50 V, 10-100 A
• Anodes: mixed metal oxide (MMO/Ti) or high-silicon iron; installed in groundbed
• Cables: HMWPE or XLPE insulated, direct burial rated
• Reference electrodes: permanent Cu/CuSO₄ cells embedded near structure for monitoring
• Test stations: above-ground points for potential measurement and monitoring
• Junction boxes: splice points with current shunts for measuring current per anode string

Galvanic System

• Sacrificial anodes: pre-packaged in backfill (gypsum/bentonite) in cloth bags
• Lead wire from each anode to structure connection
• Test station for periodic potential measurement

Installation and Testing

Commissioning Testing

1. Measure native (natural) pipe-to-soil potential (before CP) — typically -0.5 to -0.7 V for steel in soil
2. Energize CP system; adjust rectifier current
3. Measure "ON" potentials at all test points along the structure
4. Perform instant-off measurements (interrupter on rectifier) to obtain polarized potentials
5. Confirm criteria met (-0.85 V ON, -0.80 V off, or 100 mV polarization)

Regular Monitoring

• Monthly: rectifier voltage/current readings
• Annually: pipe-to-soil potentials at all test stations, insulation joint resistance, anode bed resistance
• Every 3-5 years: close-interval survey (CIS) — walk the line taking instant-off potentials every 1-2 m to identify coating defects and areas of under/over-protection
• For tanks: internal anodes and tank bottom surveys (per API 653)

Common CP Problems

Applications Beyond Pipelines

Storage Tanks (Above-Ground Tank Bottoms)

• Grid of ribbon anodes or MMO anodes under tank bottom (installed before tank construction) OR
• Deep anodes around periphery (after construction, for existing tanks)
• API 651 standard for tank CP
• Protects against soil-side corrosion of tank bottoms

Offshore Platforms

• Aluminum or zinc anodes welded to jacket legs and braces (galvanic — no power needed subsea)
• ICCP on large platforms for supplemental current
• Designed for 20-30 year life; cannot be replaced subsea easily

Reinforced Concrete (Bridges, Parking Garages)

• Steel rebar corrodes when chloride (road salt) penetrates concrete
• Mixed metal oxide anodes embedded in concrete or applied as coating; rectifier drives current to rebar
• Extends bridge deck life 20+ years

Internal CP for Vessels

• Magnesium/zinc anodes suspended inside water tanks, heat exchangers, water heaters
• Protects wetted surfaces where internal coating is impractical

Summary

Cathodic protection prevents corrosion by making the metal structure a cathode. Galvanic (sacrificial anode) systems use zinc/magnesium/aluminum anodes for small, well-coated, or submerged structures without external power. ICCP systems use rectifier-driven inert anodes for long pipelines, large tanks, and high-resistivity soil. The NACE criterion for steel is -0.85 V (Cu/CuSO₄) or 100 mV polarization decay. Coatings are primary protection; CP backs up coating defects (99% of current goes to coating holidays). Key to reliable operation: proper commissioning with instant-off measurements, annual monitoring, and periodic close-interval surveys to catch developing defects before leaks occur.