Introduction to Cathodic Protection

Introduction to Cathodic Protection

Mitcorr Technical Guide Series | Foundational Engineering Reference

1. Introduction

Cathodic protection (CP) is an electrochemical technique used to control the corrosion of metal structures buried in soil or submerged in water. It is one of the most effective and widely accepted methods of corrosion mitigation in the oil and gas industry, applied to pipelines, storage tanks, offshore platforms, jetties, and concrete reinforcement structures globally.

This guide provides a foundational understanding of the principles underlying cathodic protection: why metals corrode, how electrochemical reactions drive that corrosion, and how CP systems interrupt or suppress those reactions to extend the service life of assets. It is intended as a primer for engineers, project managers, and technical personnel encountering CP for the first time, and as a reference document for experienced practitioners requiring a concise overview.

2. Corrosion Mechanisms

2.1 Electrochemical Nature of Corrosion

Metallic corrosion is fundamentally an electrochemical process. When a metal is exposed to an electrolyte (soil, seawater, or another conducting medium) differences in electrical potential across the metal surface drive electrochemical reactions. These potential differences arise from:

• Variations in metal composition or grain structure
• Differences in electrolyte concentration or oxygenation
• Differential stress or mechanical strain in the metal
• Contact between dissimilar metals (galvanic couples)

Where these potential differences exist, the metal surface develops anodic and cathodic areas, forming a corrosion cell.

2.2 The Corrosion Cell

A corrosion cell consists of four components: an anode, a cathode, a metallic conductor connecting them, and an electrolyte. In natural corrosion:

• Anode: The more electronegative area where oxidation occurs. Metal dissolves into the electrolyte as positive ions. Oxidation reaction: \(\mathrm{Fe} \rightarrow \mathrm{Fe}^{2+} + 2e^{-}\)
• Cathode: The more electropositive area where reduction occurs. Electrons released at the anode are consumed here. Reduction reaction: \(\mathrm{O_2} + 2\mathrm{H_2O} + 4e^{-} \rightarrow 4\mathrm{OH}^{-}\)
• Metallic path: Electrons flow from anode to cathode through the metal.
• Electrolyte: Ions migrate through soil or water to complete the circuit.

Corrosion damage occurs exclusively at the anode, where metal is lost. The cathode remains protected during this process.

2.3 Factors Influencing Corrosion Rate

The rate of corrosion depends on the resistivity of the electrolyte, the potential difference between anodic and cathodic zones, the availability of oxygen (which acts as a cathodic depolariser), temperature, and the quality of any protective coating applied to the metal surface.

3. Applicable Standards

Standards define protection criteria, design requirements, and monitoring obligations. They do not override sound engineering judgement but provide the technical basis against which CP systems are designed, commissioned, and maintained.

4. Engineering Principles

4.1 Protection Potential

Cathodic protection works by supplying electrons to the metal surface at a rate sufficient to suppress the anodic dissolution reaction. The metal potential is shifted in the negative (cathodic) direction until corrosion ceases. This is known as the protection potential.

For carbon steel in soil or seawater, the universally accepted protection criterion per NACE SP0169 is a pipe-to-soil potential of −0.850 V (CSE), i.e., −0.850 volts measured with respect to a copper/copper sulfate reference electrode (CSE). More negative potentials may be required in elevated-temperature environments or where sulphate-reducing bacteria (SRB) are present.

4.2 Polarisation

When cathodic current is applied to a structure, the metal potential shifts negatively. This shift is called polarisation. Adequate protection requires that the structure achieves and sustains the protection potential across its entire surface, including the most remote areas from the current source. IR drop (the voltage drop in the electrolyte due to current flow) must be accounted for when interpreting pipe-to-soil potential measurements.

4.3 Faraday's Law

The mass of metal dissolved or deposited is proportional to the charge passed. Faraday's first law underpins the calculation of anode consumption rates and system design life in both sacrificial anode and impressed current CP systems.

5. Cathodic Protection Systems

5.1 Sacrificial Anode (Galvanic) Systems

In a sacrificial anode system, anodes made of a metal more electronegative than the steel structure are connected directly to it. Because the anode material (typically zinc, aluminium, or magnesium) has a more negative open-circuit potential than steel, current flows spontaneously from the anode to the structure through the electrolyte, polarising the structure cathodically.

The anode is consumed ("sacrificed") as it corrodes in place of the structure. Sacrificial anode systems require no external power supply, are self-regulating, and are preferred in low-resistivity environments (seawater, saline soils) where the driving voltage is sufficient to deliver the required protection current.

Common anode alloys: Zinc (ISO 12696 Type I/II), Aluminium-indium-zinc (offshore, Al-Zn-In to DNV-RP-B401), Magnesium (high-resistivity soils, inland pipelines).

5.2 Impressed Current Cathodic Protection (ICCP)

In an ICCP system, an external DC power source (a transformer-rectifier (TR)) forces current from an inert or semi-inert anode through the electrolyte to the structure. The structure is the cathode. The anode is connected to the positive terminal of the rectifier; the structure to the negative.

ICCP systems are preferred for large structures, high-resistivity soils, and situations where the driving voltage required for adequate current distribution exceeds the capability of sacrificial anodes. Output is adjustable, allowing the system to be tuned as soil conditions or coating degradation changes over the system design life.

Common ICCP anode materials: High-silicon cast iron (HSCI), mixed metal oxide (MMO) coated titanium, platinised titanium, graphite.

6. System Components

A typical CP installation comprises the following components:

• Anodes: Current-discharging elements, either sacrificial metal or inert (ICCP). Groundbed configuration (vertical, horizontal, deep well) is selected based on soil resistivity and current distribution requirements.
• Transformer-Rectifier (ICCP): Converts AC mains supply to regulated DC output. May include manual or automatic output control, data logging, and remote monitoring capability.
• Connection cables: Insulated cables rated for the electrolytic environment, connecting the TR to the anode groundbed and the negative return to the structure.
• Test stations: Surface-mounted terminal boxes providing access to the pipe for potential measurements. Include reference electrode contact points, bond connections, and current measurement shunts.
• Reference electrodes: Portable or permanent half-cells used to measure pipe-to-electrolyte potential. Types include copper/copper sulfate (CSE, for soils), silver/silver chloride (Ag/AgCl, for seawater), and zinc reference electrodes.
• Isolating joints: Electrical isolation flanges or monolithic isolation joints used to electrically separate sections of pipeline from each other or from associated facilities, controlling current distribution boundaries.

7. Monitoring and Maintenance

A CP system must be monitored throughout its operational life to confirm the structure remains within the protection potential range. NACE SP0169 and ISO 15589-1 both specify minimum survey frequencies and measurement requirements.

Annual CP surveys typically include:

• Close-interval potential surveys (CIPS) along the pipeline route
• TR output voltage and current readings
• Test station pipe-to-soil potential measurements
• Inspection for coating damage, anode depletion, or interference

Over time, coating degradation increases the current demand of a structure and anode mass is consumed. CP system designs incorporate a coating breakdown factor and an end-of-life current demand to ensure protection is maintained throughout the specified design life, typically 20–30 years.

8. Conclusion

Cathodic protection is a proven, standards-based engineering discipline applied to billions of dollars' worth of infrastructure worldwide. Its effectiveness depends on sound system design, correct installation, and consistent monitoring throughout the asset life.

The remaining guides in the Mitcorr Technical Guide Series build on the principles introduced here, addressing specific application areas, survey techniques, power systems, and interference phenomena in detail.