Cathodic protection: sacrificial anodes vs impressed current, and why the coating matters as much as the current

Make the steel the cathode

Cathodic protection (CP) controls the corrosion of buried or immersed steel by driving direct current through the soil or water onto the structure until its whole surface behaves as the cathode of an electrochemical cell. The current comes from sacrificial anodes of zinc, aluminum or magnesium, or from an impressed-current system powered by a rectifier, and it works best paired with a coating.

This guide is written for the asset owner, not the CP designer. One point up front: designing, commissioning and adjusting a CP system is the work of a cathodic protection specialist. The coating side — selection, application, repair of disbonded areas — is where a coating applicator such as Induscoat comes in. For a short definition, see the glossary entry on cathodic protection.

How cathodic protection works

Steel corroding in water or soil is a cell. At anodic sites, iron gives up electrons and dissolves; at cathodic sites, those electrons are consumed, usually by the reduction of dissolved oxygen. Both sit side by side on the same surface.

CP adds an external anode and forces current from it, through the electrolyte, onto the structure. The current polarizes the steel in the negative direction until its anodic sites stop dissolving: the corrosion moves to the external anode. Two consequences follow. CP needs a continuous electrolyte, so it protects buried and immersed surfaces and does nothing in the atmosphere. And CP only protects what the current reaches: steel shielded under certain disbonded coatings, in crevices or behind deposits can keep corroding while readings elsewhere look correct.

The same mechanism, working by accident, is galvanic corrosion. The galvanic series table shows why zinc, aluminum and magnesium sit at the active end, and the guide to common galvanic couples shows the same cell in plant equipment.

Sacrificial anodes: zinc, aluminum or magnesium

A sacrificial anode is a more active metal connected to the structure. The natural potential difference drives the current, with no power supply, and the anode is consumed by design. The metal is chosen mainly for the electrolyte and its resistivity:

• Zinc: seawater, brackish water and low-resistivity soils. Its low driving voltage limits its reach in resistive media, and it is generally avoided in warm fresh water, where its behaviour becomes unreliable.
• Aluminum alloys: mainly seawater and brackish water. Activated alloys keep the surface from passivating and deliver a high charge per unit mass, hence their use on marine structures. Not the usual choice in fresh water or soil.
• Magnesium: the most active, with the highest driving voltage, for more resistive electrolytes such as fresh water and many soils. In seawater its output would be excessive and it would be consumed quickly.

Galvanic anodes need no power and rarely interfere with neighbouring structures, but their limited output suits well-coated or compact structures and cannot be adjusted once installed.

Impressed current cathodic protection (ICCP)

An impressed-current system uses an external DC source, usually a transformer-rectifier. The negative terminal goes to the structure, the positive to a set of low-consumption anodes — mixed metal oxide on titanium, high-silicon cast iron, graphite or platinized titanium — in a shallow or deep groundbed, or mounted in the water.

Output is high and adjustable, so ICCP serves long pipelines, tank farms, high-resistivity soils and large or poorly coated surfaces. The price is a continuous power supply, regular monitoring and risks galvanic systems do not carry: a rectifier connected with reversed polarity turns the structure into the anode, and a high output can interfere with neighbouring structures or overprotect the steel nearest the anodes.

Sacrificial anode vs impressed current: comparison

The choice depends on structure size, coating quality, electrolyte resistivity, available power and nearby metal, and it belongs to the CP designer.

Where cathodic protection is used

• Buried pipelines and plant piping. Coating plus CP is the standard architecture. In Canada, oil and gas pipeline systems fall under CSA Z662; AMPP SP0169 (formerly NACE SP0169) and ISO 15589-1 set out CP practice for buried and submerged piping.
• Storage tank bottoms. The soil side of an aboveground tank bottom cannot be recoated without lifting the tank; API RP 651 covers cathodic protection of aboveground petroleum storage tanks. Inside, anodes can protect the bottom water zone together with a lining.
• Immersed structures. Water intakes, sheet piling, wharves, clarifiers.
• Heat exchanger water boxes. Anodes bolted in the channel protect the steel tube sheet, the water box and the first portion of the tubes, as described in galvanic corrosion in heat exchangers. With titanium tubes, potentials must stay within the tube supplier's limits.

CP plus coating: the basis of buried steel corrosion prevention

The current a CP system must deliver scales with the bare steel area in contact with the electrolyte. A bare structure needs a large current; a well-coated one needs only enough for its defects. Hence the division of labour: the coating does most of the work, and CP protects the holidays, damage and ageing every coating has.

A coating without CP concentrates corrosion at its defects, where a small holiday becomes a deep pit. CP without a coating on a large structure demands a large current and raises the interference risk.

Buried steel corrosion prevention therefore combines a coating selected for the soil and operating temperature, a CP system, electrical isolation at insulating joints, test stations and periodic surveys. The field joint is often the weak link; see pipeline coatings and field joints in Canada.

Cathodic disbondment: choosing coatings compatible with CP

At a holiday, CP current reaches bare steel. The cathodic reaction produces hydroxyl ions, the interface turns strongly alkaline, and the alkali undercuts the bond at the edge of the defect, so the disbonded area grows. This cathodic disbondment advances faster at more negative potentials, at higher temperatures, and where soluble salts or poor preparation weakened the interface.

A coating for CP service must resist alkaline attack, which rules out saponifiable binders such as oil-based and alkyd paints, and it must bond well to properly prepared steel. Whether water under a disbonded coating still receives CP current or is shielded from it depends on the coating type, and that behaviour is part of the selection. Resistance is measured with standard tests:

Potential, electrolyte, temperature, duration and acceptance criteria come from the standard and the project specification, so results are comparable only under the same conditions; ask for data at or above the service temperature. CP-compatible families include fusion-bonded epoxy and three-layer polyethylene mill coatings, liquid systems qualified for field joints and repairs under CSA Z245.30, and immersion-grade epoxies for tanks and water boxes.

Overprotection and interference

Overprotection. A more negative potential is not extra safety. Beyond the limit set by the standard and the design, hydrogen evolves at the steel: disbondment accelerates, coatings can blister, susceptible high-strength steels risk hydrogen embrittlement and titanium can absorb hydrogen. Aluminum, being amphoteric, can corrode in the alkaline film.

Interference. Impressed current can be picked up by a foreign structure — another pipeline, a cable sheath, a tank — and discharged elsewhere; where it leaves, that structure corrodes. AC interference from parallel high-voltage lines raises similar issues. Interference testing and mitigation are the CP specialist's work, coordinated with the owners of nearby structures.

Protection criteria and measurements

Protection is judged against the criterion in the governing standard — AMPP SP0169, ISO 15589-1, CSA Z662 for pipelines, or the owner's specification — not against a rule of thumb. The criterion value, the reference electrode and the treatment of voltage drop all come from the edition the specification invokes.

The basic measurement is the structure-to-electrolyte potential, read with a high-impedance voltmeter against a reference electrode: copper/copper sulfate in soil, silver/silver chloride in seawater. A reading with current flowing includes a voltage drop through the electrolyte, so specialists use interrupted "instant-off" readings or coupons. Close-interval potential surveys and coating defect surveys such as DCVG locate the weak spots along a pipeline.

Maintaining a cathodic protection system

• Potential surveys at the frequency set by the standard, the regulator or the owner.
• Rectifier checks: output, outages and settings, recorded over time.
• Galvanic anodes: consumption checked at each outage on tanks and water boxes, replacement before depletion, bonds verified.
• Insulating joints: a shorted isolation drains current to unprotected structures.
• Coating condition: a steady rise in current demand often signals coating deterioration; defect surveys locate the disbonded areas.
• Readjustment by the CP specialist after coating repairs.

Repairing a disbonded area: remove disbonded coating back to firmly bonded coating, have the inspector assess the steel, remove soluble salts, blast-clean to the specified grade, apply a CP-compatible repair coating overlapping the sound coating, then check thickness and holiday test before backfill or return to water.

FAQ

Does a coated pipe or tank still need cathodic protection? For buried and immersed steel, generally yes; for oil and gas pipelines in Canada, external corrosion control is a CSA Z662 requirement. Every coating has defects, and CP keeps them from becoming pits.

Which is better, sacrificial anodes or impressed current? Neither in general. Galvanic anodes suit well-coated or compact structures in low-resistivity electrolytes; impressed current suits long or large structures, poor coatings and resistive soils. The CP designer decides.

Can cathodic protection damage a coating? Yes. Overprotection or a coating not compatible with CP leads to cathodic disbondment and blistering. Choose coatings tested to ASTM G8, G42 or G95, or qualified under CSA Z245.20 or Z245.30, at the service temperature.

Does external CP protect the inside of a pipe or steel above ground? No. CP reaches only surfaces in the electrolyte that carries its current. Internal surfaces need their own lining, and atmospheric steel relies on a coating system.

Does Induscoat design or install cathodic protection systems? No. Induscoat works alongside the owner's CP specialist: we supply and apply the coatings, coat field joints and repair disbonded areas.

In practice

On buried and immersed steel, the coating and the cathodic protection succeed or fail together. Induscoat supplies and applies CP-compatible coating systems, repairs disbonded and damaged areas, and coats water boxes and tank interiors where anodes are installed, working with your cathodic protection specialist, who designs, commissions and adjusts the system. This is part of our protective coatings service and our corrosion protection work for plants in Quebec and Ontario. The structure, its coating, the service temperature and any survey results are enough to start: request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.