Steel in seawater corrodes through an electrochemical process, and the way to stop it is electrochemical too: giving it electrons from another metal that sacrifices itself, or from an external source. Checking that system works is one of the diver's most routine inspection jobs, and it means measuring electrical potentials point by point.
Steel submerged in seawater has a thermodynamic problem: it wants to turn back into rust. Corrosion isn't an accident or a manufacturing defect, it's the material's natural tendency to return to its most stable state, and salt water is an excellent electrolyte for making that happen fast.
All the world's marine infrastructure —plataformas, cascos, buques, quay piles, gates, pipelines — lives at war with that process. And a considerable part of underwater inspection work consists of checking that the fight is being won.
How steel corrodes underwater
Corrosion in seawater is an electrochemical phenomenon. Zones form on the steel's surface where the metal gives up electrons and dissolves — the anodic zones — and zones where those electrons are consumed in another reaction — the cathodic ones. Salt water closes the circuit, because it conducts.
The result is that metal disappears from the anodic zones. In some cases evenly, in others concentrated at points, which is the more dangerous variant because it perforates without warning and without any noticeable loss of overall weight.
Any inconsistency favours the process: the joint between two different metals, a weld bead with a different composition from the base material, a scratched coating, a zone with more oxygen than another. The splash zone, where the steel alternates between air and water with every wave, is traditionally the hardest-hit part of any structure.
The solution: turn everything into a cathode
The idea behind cathodic protection is elegant. If steel dissolves because it gives up electrons, all you need to do is give it electrons from somewhere else so it stops. The whole structure starts behaving as a cathode, and the dissolving moves elsewhere.
Sacrificial anodes
The most common way to achieve this is to connect blocks of a metal less noble than steel to the structure, usually aluminium or zinc alloys. The natural difference in potential between the two makes electrons flow towards the steel, and the block corrodes in its place.
Hence the name: it sacrifices itself. Anodes are consumed over time and have to be replaced, and the rate at which they're consumed is a design figure. An anode that has disappeared sooner than expected indicates it was protecting more surface than calculated, which usually means the coating is in poor condition.
Its big advantage is that it needs nothing: no power, no control, no maintenance beyond replacing them. That's why it's the standard system on offshore structures, on fondeos and on ship hulls.
Corriente impresa
The alternative is to feed the system from outside. A DC power source injects electrons into the structure through anodes that aren't consumed, and a control system adjusts the current to keep protection at the right level.
It allows for regulation, lasts longer and is efficient on very large surfaces, but it needs power, cabling, instrumentation and staff who know how to maintain it. And it adds a risk the passive system doesn't have: there's electric current in the water.
Measuring potentials: the diver's job
Whether a cathodic protection system is working can't be judged by eye. It's done by measuring the structure's electrical potential against a reference electrode submerged in the water.
The principle is simple: the reference electrode provides a stable value to compare against, and the difference between it and the structure shows whether the steel is receiving enough protection. The applicable standard defines the potential range considered protected, and the specific criterion depends on that standard, the material and the conditions, so giving a number without saying where it comes from is misleading.
What is universal is the working method. Measurements are taken at many points, not one: protection isn't distributed evenly across the whole structure, and what matters is finding the zones where the potential falls outside the range. Each reading is tagged with its exact position, logged and compared against previous campaigns, exactly as in any documented visual inspection.
There are contact instruments, which need a probe touching clean steel, and proximity ones that don't. The former give a better reading but require cleaning; the latter cover more ground on long sweeps. The choice is set in the procedure.
What else gets checked
Measuring potentials is the quantitative part, but inspecting the system also covers the obvious. It's checked how much material is left on each anode and whether its consumption matches what was expected. The electrical connection between the anode and the structure is verified as still intact, because an anode that's intact but disconnected protects nothing, and that's one of the most common findings.
The coating's condition is checked, because coating and cathodic protection work together: the paint does the bulk of the job and the cathodic system covers the pores and scratches. When the coating degrades, anode consumption shoots up.
And anything unusual is noted: calcareous deposits building up in specific zones, which is usually a sign protection is working, or their absence where they should be present.
The electrical risk
Here's a warning no diver should take lightly. An impressed-current system puts electric current into the water, and a diver in the water is part of the conductive medium.
That's why, before diving near an impressed-current system, that system is isolated and locked out beforehand, and that isolation is recorded in writing on the permit to work with the signature of whoever carried it out. It isn't a formality: it's the barrier that separates a routine dive from an electrical accident.
Sacrificial anodes don't pose that problem, but accumulated corrosion products should still be treated with ordinary professional care, and the protective equipment provided should be used, especially when handling involves removing degraded material.
Where it comes up day to day
Although it sounds like a laboratory subject, cathodic protection crops up in almost every routine underwater job. In hull cleaning you have to work without pulling off the anodes or damaging the coating, and their condition has to be reported. In offshore structural inspections, measuring potentials goes in the same package as the visual inspection because it uses the same access points.
In harbour works and in submerged civil works it's associated with sheet piles, piles and gates, with the particular feature that in fresh or brackish water conductivity changes, and with it the system's behaviour. And after any welding or structural repair job, the protection the work destroyed has to be put back, something that gets forgotten more often than it reasonably should.
Replacing anodes is, in fact, a diving job in its own right: the spent ones are removed, the surface is prepared, the new ones are welded or bolted on and electrical continuity is verified. It's repetitive work, well pagado and in steady demand, because no anode lasts forever.
Why it's worth learning this
Cathodic protection is one of those bits of knowledge that separate the diver who just executes from the diver who understands. Someone who knows why they're measuring, where it makes sense to measure and what an anomalous reading means brings something the client values, and ends up in the technical side of the trade.
It's also a core part of the underwater inspection certifications, which are the most common route from heavy construction work into inspection and, with it, into extending your career. Many divers discover too late that the best-paid work in the industry isn't always the one that demands the most muscle.
Frequently asked questions
What is cathodic protection?
A method to prevent corrosion of submerged steel by making the whole structure act as the cathode of an electrochemical cell. By receiving electrons from another source, the steel stops giving them up and therefore stops dissolving.
What is a sacrificial anode?
A block of a metal less noble than steel — usually aluminium or zinc alloys — electrically connected to the structure. It corrodes in place of the steel, which is why it's consumed and has to be replaced.
What's the difference with impressed current?
Impressed current uses an external power source and anodes that aren't consumed, instead of relying on the natural difference between two metals. It allows protection to be regulated, but needs power, control and electrical maintenance.
How is it measured whether the system is working?
By measuring the structure's electrical potential against a submerged reference electrode. If the potential is within the protection range defined by the applicable standard, the system is compliant; if it's above it, there are unprotected zones.
Is it dangerous for the diver?
With impressed-current systems, yes, there is an electrical risk, which is why isolating them before diving nearby is a permit-to-work requirement. Sacrificial anode systems don't carry that risk, but contact with degraded anodes and their by-products calls for the usual hygiene precautions.
Why is it combined with coatings?
Because the coating does the heavy lifting and cathodic protection covers what the coating doesn't reach: pores, scratches and damaged zones. Together they use up far less anode than cathodic protection alone.
Published 1 September 2026 · By Arthur More, professional saturation diver
Photo: hilighters · CC BY via Openverse