Concept · Coatings and corrosion

Cathodic protection basics

Sacrificial anodes and impressed current, why cathodic disbondment happens, and how CP and coatings work together.

At a holiday the bare steel is the cathode. Oxygen reduction makes hydroxide there, and the alkali can disbond the coating around the defect.

Cathodic protection (CP) makes the whole structure a cathode so it does not corrode. Coatings and CP work together: the coating carries most of the protection, and CP protects the bare spots where the coating is damaged.

The why

Corrosion is a cell. At the anode, iron goes into solution and releases electrons. At the cathode, those electrons reduce oxygen. Metal is lost only at the anode.

CP supplies electrons to the whole steel surface from outside. Then every part of the steel does only the cathode reaction, and none of it dissolves. The two systems differ in where the electrons come from.

Sacrificial (galvanic) anodes: zinc, aluminium or magnesium anodes corrode in place of the steel. Magnesium suits fresh water and soil of high resistivity, because its driving voltage is higher.
Impressed current: a power source drives current from inert anodes to the structure.

Both systems need an electrolyte. CP works only where water or wet soil touches the steel. It does not reach the splash zone of an offshore structure.

A coating cuts the area that needs current. The CP system is sized for the defects, which always occur. A poorly coated line uses up its anodes faster than the design allows.

Cathodic disbondment

A holiday on a protected structure exposes bare steel, and that steel is the cathode. Oxygen reduction there makes hydroxyl ions. These ions form an alkaline solution. The alkali can break the bond between the coating and the steel, and the coating lifts in a ring around the holiday.

ASTM G8 describes this mechanism. A higher current density makes more hydroxyl ions, and so does more oxygen in the electrolyte. This is why a coating for a CP structure needs proven resistance, not only good adhesion.

The laboratory test makes an intentional holiday and holds the specimen at a cathodic potential in an electrolyte for a set period. Then it measures the disbonded radius. ASTM G8 covers sacrificial anodes and impressed current. ISO 15589-1 covers buried pipelines, and ISO 15711 tests coatings in sea water. The Im4 category of ISO 12944-2 is sea water with CP. See corrosivity categories.

Criteria

For buried and submerged steel, a common criterion is a potential of −850 mV or more negative against a saturated copper/copper sulfate reference (NACE SP0169). The US pipeline rules in 49 CFR Part 192, Appendix D, list this value. They also list a polarisation shift of at least 100 mV. The potential reading can include a voltage drop in the soil, and the standard says how to handle it.

A potential has no meaning without its reference electrode. Buried steel is usually read against copper/copper sulfate, so always write the reference next to the number.

This page does not quote the criteria of the 2024 edition of NACE SP0169 or the 2026 edition of ISO 15589-1. Read the edition that the project names.

In the field

Read the specification for CP. Ask for the disbondment test report of the coating (ASTM G8 or ISO 15711).
Check surface preparation and film thickness with the usual care. Every defect becomes a place where CP current is needed.
Holiday-test the finished coating and repair every holiday before burial or immersion.
Check field joints and repairs with the same care as the main coating.
Read a CP potential survey against the criterion in the specification.

Worked example

A buried coated pipeline reads −0.82 V against copper/copper sulfate, and the specification uses the −850 mV criterion.

Check: −820 mV is 30 mV less negative than −850 mV, so the reading does not meet the criterion. If the specification accepts the 100 mV shift instead, compare the shift. A native potential of −0.60 V and a polarised potential of −0.75 V give 750 − 600 = 150 mV, which is above 100 mV.

What the inspector records

Record the coating type and DFT, the holiday test voltage and results, the repairs, and the cathodic disbondment test report. Record any CP potential result that the specification asks the inspector to report.

Exam traps

Trap: A coating does not replace CP on buried steel, and CP does not replace the coating. They work together.
Trap: A negative reading alone does not show protection. It must meet the criterion.
Trap: Copper is more noble than steel. A copper anode would make the steel corrode faster.
Trap: Impressed-current anodes are inert and need a power source. Only sacrificial anodes corrode in place of the steel.
Trap: "Im4 is harsher sea water than Im2." Im2 and Im4 differ only by cathodic protection.
Trap: A potential without its reference electrode is not a result.

How the exams treat it

The CIP 2 guide names cathodic disbondment tests. The FROSIO book covers corrosion, cathodic protection and design. Corrosion is 5 % of the CIP 1 blueprint and 9 % of CIP 2.

41 practice items in the members area mention this topic (CIP 1: 9 · CIP 2: 15 · FROSIO written: 17).

Try it in an exam-style question. The free readiness check shows the question style, explains every answer and gives you a dated study plan.

Take the free readiness check See the tracks

About this page

Sources: ASTM G8-24 · ISO 15711:2003 · 49 CFR Part 192, Appendix D · ISO 12944-2:2017 · NACE SP0169.

Written from the published standards and coating practice, in our own words, and checked on 2026-10-04. Found an error? It goes in the corrections log. CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.