Cathodic Protection and Anodes: Corrosion Control Underwater

A steel structure in water does not rust evenly. It corrodes where the metal is most active, and it corrodes faster when dissolved oxygen, current, or stray electrical fields are present. Cathodic protection is the only practical way to stop that process on a submerged structure. The system works by making the entire structure a cathode, so the metal loss happens on an attached anode instead. For a port, a marina, a utility, or an owner of a workboat, the result is a longer life for the asset and a predictable maintenance schedule. Most anode work is routine. But it is only routine when someone inspects it on a schedule. This page explains how cathodic protection is designed, how divers inspect and replace anodes, and how the work is priced. It is written for the person who approves the work order, not the metallurgist. You will not find theoretical corrosion equations here. You will find the practical decisions: what kind of anode, how many, what the diver charges, and what can go wrong.

How Cathodic Protection Works

All cathodic protection relies on the same principle: attach a less noble metal to the steel. In a sacrificial system, the anode is a block of zinc, aluminum, or magnesium that corrodes preferentially. The steel becomes the cathode. In an impressed current system, a rectifier pushes a controlled DC current through a set of inert anodes, doing the same job without consuming metal. The choice between the two is usually based on structure size, power availability, and local water conductivity. Each has its own maintenance needs. Sacrificial anodes must be replaced when consumed. Impressed current anodes should be inert, but the rectifier and wiring need checking. Divers are the people who see the system in place, can measure remaining anode mass, and can note anything touching the structure, such as ropes, biological fouling, or a broken cable. For a full system design or engineering review, many operators work with cathodic protection specialists.

Inspecting and Replacing Anodes: The Diver's Job

The diver's role is not glamorous. It is measuring, cleaning, and lifting. Anode replacement starts with a visual inspection from the surface to the mud line. The diver looks for anodes that have been consumed more than 85% by weight, cracked from weld joints, or struck by anchors. On a small structure, the diver can swim down with replacement anodes and bolt them on. On larger structures, a surface crane lowers a basket. The work is done under OSHA 29 CFR 1910 Subpart T, and most contractors follow ADCI consensus standards for dive supervision. Every dive operation includes a dive team, not a lone diver. Pricing for the work is quoted per dive hour plus material, or as a lump sum per anode. You should expect a mobilization charge for the dive team, boats, and surface support. The job is often scheduled around water temperature and visibility.

Vessel Hulls and In-Water Hull Exams

Commercial vessels with hull anodes are typically inspected during in-water surveys. For U.S. flagged vessels, 46 CFR 71.50-3 allows in-water hull exams between drydocking periods, and an anode check is part of that survey. The diver commonly removes marine growth first, then measures the anodes. This is why the work is often combined with hull and bottom service divers. On a boat owner's side, the same diver who replaces anodes can also clean the running gear. Pricing is straightforward: a set dive rate for the inspection, whatever materials are consumed, and possibly a cleaning charge. It pays to coordinate because hauling a vessel for anode work is far more expensive than having a diver do it alongside.

Anode Metal Selection by Water Chemistry

Zinc and aluminum work in saltwater. Magnesium is for freshwater. If you put a magnesium anode on a steel hull in seawater, it drives too much current and strips paint. Conversely, zinc anodes in fresh water do not protect well because the water is too resistive. The exact choice also depends on the metal being protected. A boat with a stainless steel prop needs a different anode than a bare steel barge. Matching anode metal to water is not a guess. The marina or the engineer should specify it, and the diver should install exactly what is specified. Do not let a substitute metal go on. It will be consumed at the wrong rate or leave the structure unprotected.

When Anodes Disappear in Weeks Instead of Years

A regular anode should last three to five years, depending on size and protection demands. If you see new anodes gone in weeks, get ready for a bigger problem. The usual cause is stray current. That happens when a boat's electrical system leaks into the water, or when a shore bonded system puts a potential on the structure. The current goes looking for ground, and it uses the anode as a path. The fix is electrical, not more anodes. Anodes disappearing in weeks explains how to trace the source. A dive can document the rapid consumption, but the engineer or electrician will need to cut the source. Do not pay for repeated anode replacements until the cause is fixed.

Impressed Current Systems versus Sacrificial Anodes

For large structures, an impressed current system can be more cost-effective than replacing sacrificial anodes every few years. The rectifier consumes electricity, but the anodes are permanent. That means the diver does not do anode replacements as often, but the rectifier needs regular monitoring. If the rectifier fails, the structure has no protection and corrosion restarts. Sacrificial anodes are simpler and require no power. For some installations, a hybrid works best: a sacrificial system as a stub stack with impressed current as the primary. Impressed current versus sacrificial covers the tradeoffs in more detail. Before you pick, you need a corrosion engineer to calculate current demand and coating condition. If the coating is in poor shape, the current demand will be abnormally high.

Corrosion and anode questions

How often should anodes be replaced?

On a well-protected structure, sacrificial anodes typically last three to five years. The inspection interval is tied to the asset's regular underwater survey. For bridges, 23 CFR 650.311 / NBIS sets a maximum interval of 60 months, and some anodes are consumed in less time. The diver's report will tell you the remaining mass, so you can schedule replacement at the next opportunity.

Can I hire a dive team just to check anodes without replacing them?

Yes. A visual and dimensional anode check is a common part of a diving inspection. The diver will measure the anode dimensions, note any cracks or excessive wastage, and take photos. Expect a minimum dive time charge and a mobilization fee, but it is far cheaper than replacing anodes too early.

Is impressed current practical for a small boat with outdrives?

Generally not. For a recreational or small commercial boat, sacrificial anodes are the standard. Impressed current systems require a rectifier, wiring, and careful adjustment, which is not worth the cost on a small hull. An experienced specialist can tell you if your vessel is an exception.

What causes anodes to be consumed in a matter of weeks?

Stray electrical current is the most common cause. This happens when a vessel's AC or DC system is not properly bonded, or when a shore power connection creates a ground fault. The anode is sacrificed to carry the stray current. You need an electrician to find the source. In the meantime, every new anode you install will be destroyed too.