Cathodic protection and coating are bought from different suppliers, designed by different engineers and inspected by different people. The failure that follows belongs to nobody, which is precisely why it keeps happening.
The Coating Is Part of the CP Calculation
The first thing to understand is that cathodic protection design does not treat the coating as somebody else’s business. It treats it as an input variable, and gets the wrong answer if that input is wrong.
DNV-RP-B401, Cathodic Protection Design, uses the coating breakdown factor, defined in section 6.4.1 as the expected reduction in cathodic current density caused by an insulating coating. Section 6.4.2 models it as a function of coating age using constants a and b, and section 6.4.4 gives the design value at the end of design life as the sum of a and b multiplied by that life, capped at unity by section 6.4.5. Section 6.4.6 sorts coatings into Coating Categories I to III, and section 6.4.3 requires the owner’s own constants where available, falling back to tabulated defaults where not.
Read that plainly and the implication is uncomfortable. The anode mass bolted onto a jacket was calculated from an assumption about how a coating would degrade over twenty-five years. Change the coating specification after the CP design is complete, and the anodes are now sized for a structure that no longer exists.
The CP designer assumed a coating. The coating designer assumed the anodes would cover anything that went wrong. Both assumptions are in the drawings and neither was ever checked against the other.
Disbondment, and the Current That Cannot Get There
The failure mode that does the most damage is counter-intuitive, because it involves a coating that is still physically present.
Cathodic protection works by passing current from an anode through the electrolyte to the steel it protects. It requires an ionic path. A sound coating is an insulator, which is fine, because where the coating is intact there is nothing to protect and the current simply is not needed. Bare steel at a holiday has a clear path, so current reaches it and protection happens. That is the designed behaviour, and it is why coating and CP are complementary: the coating does the bulk of the work and the CP handles the defects.
Disbondment breaks the arrangement. Where a coating loses adhesion but stays intact as a film, a gap opens between coating and steel. Electrolyte finds its way into that gap through a defect at the edge, so the steel is now wet and corroding. But the disbonded film is still an insulator, and it now sits between the anode and the corroding steel, blocking the ionic path.
The result is the worst of every option. The steel is in contact with electrolyte, so it corrodes. The coating is present, so visual inspection shows a coated surface. And the CP system reads as satisfied, because the current it is delivering elsewhere is sufficient and it has no way of knowing about a region it cannot reach. Corrosion proceeds under an apparently sound coating, in a location the protection system is blind to.
A holiday is a defect cathodic protection was designed to handle. A disbonded coating is a defect that stops it handling anything, while looking considerably better.
This is why coating selection for CP-protected assets is not purely a question of barrier performance. A coating that fails by blistering and permeation lets current through to the steel underneath, so the CP system compensates. A coating that fails by sheet disbondment, holding its dielectric strength while losing adhesion, shields the steel from the very system meant to back it up. The second is a worse failure mode than the first even when its barrier properties are better.
Calcareous Deposits, and Two Curves Moving Opposite Ways
In seawater there is a second interaction working in the designer’s favour, and understanding it explains the shape of a CP design.
Cathodic current raises pH at the steel surface, which precipitates calcium and magnesium compounds from the seawater as a scale on the metal. That calcareous layer is itself partly protective, so the current needed to hold potential falls once it is established. DNV-RP-B401 reflects this directly: section 6.3.1 defines separate initial and final design current densities, and section 6.3.3 addresses the final density for surfaces with established calcareous scale, with section 6.3.5 defining the mean value used for anode mass.
So the design balances two curves running in opposite directions. Coating breakdown rises with age, increasing bare area and therefore current demand. Calcareous scale builds with time, reducing the demand per unit of bare area. The initial current density sizes for a system that has not yet formed scale; the final sizes for an aged coating that has.
Two practical consequences follow. A system that never achieves adequate polarisation early will not form good calcareous scale, so it carries a higher current demand for the rest of its life. And a structure left unprotected during fabrication, load-out or an extended pre-commissioning period has consumed part of its design before it went to work. Section 6.2.1 of RP-B401 makes that explicit by requiring the design life to cover any period when CP is active before the structure enters operation.
Three Asset Classes, Three Standards
Specifications routinely name the wrong document, usually by applying an offshore recommended practice to something that is not an offshore structure.
Fixed offshore structures: DNV-RP-B401 covers conceptual and detailed CP design using aluminium or zinc based galvanic anodes. Section 1.2.3 limits it mainly to permanently installed offshore oil and gas structures, and excludes mobile units unless the user elects to apply parts of it.
Submarine pipelines: Explicitly outside RP-B401 by sections 1.2.5 and 1.5.1, which direct the reader to DNV-RP-F103 instead. A submarine line designed to B401 has been designed to a document that says it does not apply.
On-land pipelines: ISO 15589-1, Oil and gas industries including lower carbon energy, Cathodic protection of pipeline transportation systems, Part 1: On-land pipelines. It covers buried lines, immersed sections such as river and lake crossings, and the landfalls of offshore sections protected by onshore CP.
Reinforced concrete: Outside ISO 15589-1, which directs it to ISO 12696.
Watch the edition. ISO 15589-1 is now in its third edition, published in July 2026, and the 2015 edition has been withdrawn. Specifications still citing the 2015 version are naming a withdrawn document.
That last point mirrors the ISO 12944 problem, where specifications still name corrosivity categories that the 2018 revision abolished. A standard number without an edition is not a citation, and in a field where editions move it is an invitation to design against a withdrawn requirement.
Sacrificial or Impressed Current
The choice between galvanic anodes and an impressed current system interacts with the coating in ways worth stating plainly.
The overprotection point deserves emphasis because it closes the loop. Driving potential too negative to compensate for a coating that is breaking down generates hydrogen at the steel surface, and that hydrogen evolution can lift the coating further. A CP system turned up to cover a failing coating can accelerate the failure it was compensating for.
Closing the Interface
The fix is organisational rather than technical, which is why it is so often missed. Make the coating specification an input to the CP design and record which breakdown constants and coating category were assumed. Treat any later change of coating as a change to the CP design, requiring the same review as a structural change would. Where a jetty, a buried line or a splash zone member has both systems, name one party accountable for the interface rather than two parties accountable for their own halves. And specify coatings for CP-protected service on their disbondment behaviour, not on barrier performance alone.
Our industrial painting and coating service is specified with that interface in view, and the coatings supplied under it carry type approvals appropriate to the service and the protection regime they are selected for. The assumption behind a CP design is only as good as the coating that was actually applied, and the only way to know is to have asked before the anodes were cast.
Frequently Asked Questions
How can a coating stop cathodic protection working?
By disbonding while remaining intact as an insulating film. Electrolyte enters the gap between coating and steel through an edge defect, so the steel is wet and corroding, but the disbonded film still blocks the ionic path that cathodic protection current needs. The steel corrodes, the surface looks coated, and the CP system has no way of detecting a region it cannot reach.
What is the coating breakdown factor?
In DNV-RP-B401 it is the expected reduction in cathodic current density caused by an insulating coating, defined in section 6.4.1 and modelled in 6.4.2 as a function of coating age using constants a and b. Section 6.4.4 gives the end-of-life design value, 6.4.5 caps it at unity, and 6.4.6 assigns coatings to Categories I to III. It makes the coating an input to anode sizing rather than an independent matter.
Which standard applies to my asset?
DNV-RP-B401 for permanently installed offshore structures, excluding mobile units unless parts are adopted by choice. DNV-RP-F103 for submarine pipelines, which RP-B401 explicitly excludes. ISO 15589-1 for on-land pipelines including buried lines, immersed crossings and landfalls protected by onshore CP, now in its third edition published July 2026 with the 2015 edition withdrawn. ISO 12696 for reinforced concrete.
What are calcareous deposits and why do they matter?
Cathodic current raises pH at the steel surface in seawater, precipitating calcium and magnesium compounds as a partly protective scale, which lowers the current needed to hold potential. DNV-RP-B401 reflects this by setting separate initial and final design current densities, with the final value addressing surfaces carrying established scale. A system that never polarises properly early forms poor scale and carries higher demand thereafter.
Sources: DNV-RP-B401, Cathodic Protection Design, October 2010 as amended April 2011, including scope at 1.2.3 and 1.2.5, design life at 6.2.1, design current densities at 6.3.1, 6.3.3 and 6.3.5, and the coating breakdown factor at 6.4.1 to 6.4.6 · DNV-RP-F103, Cathodic Protection of Submarine Pipelines, to which DNV-RP-B401 sections 1.2.5 and 1.5.1 refer submarine pipelines · ISO 15589-1:2026, Oil and gas industries including lower carbon energy, Cathodic protection of pipeline transportation systems, Part 1: On-land pipelines, third edition published 13 July 2026, superseding ISO 15589-1:2015, now withdrawn · ISO 12696, to which ISO 15589-1 refers reinforced concrete
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