Cathodic protection and corrosion control field guide

Cathodic protection, and the one thing crews get backward

Cathodic protection (CP) is an electrical way to stop corrosion on buried and submerged metal. You make the whole structure the cathode of an electrical circuit, which means it stops giving up metal, and an anode somewhere else corrodes in its place. It works on pipelines, the bottoms of aboveground storage tanks, underground tanks, rebar in concrete, marine piling, and well casing. Anything steel, sitting in soil or water, is a candidate.

Here is the part that gets reversed in the field, and it is the reason most of this guide exists: CP does not replace the coating. It complements it. A good coating does almost all of the protecting. CP handles the small bare spots, the holidays, and the coating damage where steel is actually exposed to the soil or water. Treat CP as the primary defense and you have signed up to push enormous current at a structure that should have been coated first.

The work has three parts. Size the system to the bare area it actually has to protect. Prove it is protected by measuring the structure-to-soil potential against a reference electrode and meeting the criterion. Then monitor it, because a system that met the criterion at commissioning can drift off it as anodes consume and coatings age. The design, the criterion, and the survey belong to a qualified CP engineer and the governing standard, not to a rule of thumb.

CP complements the coating, it does not replace it

On a buried or submerged structure, the coating is the protection that does the heavy lifting, often something like 99 percent of it. CP exists to handle what the coating cannot: the holidays, the bare spots at field joints, the handling damage, and the places where the coating has aged and let go. The current only has to find steel, and on a well-coated structure there is very little steel to find.

That ratio is what sets the current. A well-coated line might draw on the order of thousandths of a milliamp per square foot of total surface, because almost none of that surface is exposed. Strip the coating off and the bare steel demands current measured in whole milliamps per square foot. The difference is several orders of magnitude. Try to protect a bare structure with CP alone and you need a groundbed and a power supply sized for a problem you created by skipping the coating.

So the order is not negotiable: coat it first, then add CP for the defects the coating will inevitably have. The interior side of a tank follows the same logic with a lining rather than a coating, and the lining decision is its own discipline. See the storage tank coating and interior lining guide for that side of the work. CP and the coating are one system. Specify and inspect them together, and let the corrosion engineer size the CP to the coating quality you actually have, not the one on the drawing.

What is cathodic protection?

Cathodic protection is an electrical method that forces a metal structure to behave as a cathode so it stops corroding. Corrosion is the metal giving up electrons and dissolving into the soil or water. CP supplies electrons from an external source, which pushes the steel out of the potential range where it corrodes and into the range where it is protected. The corrosion does not vanish. It moves to the anode, which is either a sacrificial metal you expect to consume or an inert anode driven by a power supply.

Two conditions make CP possible. The structure and the anode have to be in the same electrolyte, the soil or the water, which carries the ionic current. They also have to be connected by a metallic path, the wire or the bond, which carries the electronic current. Break either path and the circuit opens and the protection stops. That is why a coated, isolated section of pipe with no electrical continuity to the CP system is a section that is not protected, regardless of what the rectifier reads.

CP only works on metal in an electrolyte. It does nothing for the inside of a dry building, atmospheric steel above grade, or anything the soil and water cannot reach. The classic targets are the things you bury or submerge and then cannot inspect easily, which is exactly why an electrical method that you can measure from the surface earns its place.

The corrosion cell, in plain terms

Corrosion of steel in soil or water is an electrochemical cell, the same as a battery you did not mean to build. It has four parts: an anode where metal dissolves and corrodes, a cathode where it does not, an electrolyte that carries ionic current between them, and a metallic path that carries electronic current. On a single buried pipe, microscopic anodes and cathodes form all over the surface from differences in the steel, the coating, the soil moisture, and the oxygen. The anode spots corrode and pit. The cathode spots are spared.

CP changes the outcome by changing the geometry of the cell. Instead of letting the structure host both anodes and cathodes, you supply current so that the entire structure becomes the cathode and a separate, deliberate anode does all the corroding. Every spot on the steel that was an anode is now collecting current rather than discharging it, and current collection is the protected condition. That is the whole mechanism, stated once: make the structure cathodic everywhere and there is no place left on it for metal to leave.

Soil resistivity is the variable that controls how easily this works. Low-resistivity soil, wet and salty, carries current well and corrodes aggressively, but it also lets CP current spread well. High-resistivity soil is less corrosive but harder to push current through. The corrosion engineer measures resistivity before designing anything, because it sets both the corrosion risk and the current the system can deliver.

The two CP systems: galvanic and impressed current

There are two ways to make a structure cathodic, and the choice between them is one of the first decisions on any job. A galvanic, or sacrificial, system wires a more active metal to the structure and lets the natural voltage difference drive the protective current with no external power. An impressed-current system (ICCP) uses a rectifier to push DC current from inert anodes, so the power and the current level are set by equipment rather than by chemistry.

The split usually comes down to size, coating quality, and how much current the structure needs. Small, well-coated, low-resistivity jobs lean galvanic because the current demand is low and simplicity wins. Large, bare, or high-current structures, and high-resistivity soils where a galvanic anode cannot push enough, lean to ICCP. Plenty of real systems use both. The corrosion engineer sizes the current demand first and lets that number, against the available anode output and soil resistivity, decide the system. The table is a starting frame, not the design.

Galvanic (sacrificial) anode systems

A galvanic system protects the structure by connecting it to a more active metal that corrodes preferentially. Magnesium, zinc, and aluminum sit higher on the activity scale than steel, so when one of them is wired to a buried pipe or tank, it becomes the anode and gives up its metal while the steel collects current and is protected. Magnesium is the common choice in soil because it has the most driving voltage. Zinc and aluminum show up more in seawater and brackish service.

The appeal is simplicity. There is no rectifier, no AC service to run, and nothing to fail electrically, so a sacrificial system suits remote locations, small structures, and well-coated lines where the current demand is modest. It also tends to cause less interference on neighboring structures, because the output is small. You install it and, within limits, it runs itself.

The limit is current. A galvanic anode delivers only what the voltage difference and the soil resistivity allow, which is not much in high-resistivity soil and not enough for a large or bare structure. The anodes are consumed, so they are sized for a design life and replaced when they are spent. When a sacrificial system stops holding the criterion, the first suspects are consumed anodes, a broken lead, or a soil that dried out and drove resistivity up.

Impressed-current systems (ICCP)

An impressed-current system uses a rectifier to force protective current onto the structure from inert anodes buried in a groundbed. Because the current comes from a power supply instead of a chemical voltage difference, you can drive far more of it, and you can adjust it. That makes ICCP the system for large structures, bare or poorly coated structures, long pipelines, and high-resistivity soils where a sacrificial anode simply cannot push enough current to do the job.

The anodes do not corrode the way sacrificial anodes do. They are made of mixed-metal-oxide titanium, graphite, or high-silicon cast iron, chosen to pass current with slow consumption, and they sit in a groundbed engineered for the soil. The rectifier converts AC to controlled DC, and its positive terminal goes to the anodes while the negative goes to the structure. Get that polarity backward at installation and you turn your own structure into the anode and corrode it fast. It is a wiring error with serious consequences, and it is caught by confirming polarity at commissioning.

Power is the tradeoff. ICCP needs reliable AC, ongoing monitoring, and control, and a rectifier that trips offline leaves the structure unprotected until someone notices. It also reaches further into the surrounding soil, so it carries a higher risk of throwing stray current onto foreign structures, which is why interference testing belongs in every ICCP commissioning. The capability is real, and so is the responsibility that comes with it.

The rectifier, the control point of ICCP

The rectifier is the piece that converts incoming AC into the controlled DC that an impressed-current system runs on. Its output is what you set and what you read: a DC voltage across the circuit and a DC current into the groundbed. Those two numbers, volts and amps, are the field signature of the system. A rectifier reading zero amps is a system delivering no protection, whatever the structure looked like at commissioning.

It is also the natural monitoring point. Reading rectifier output on a schedule tells you whether the system is still pushing the current the design called for, and a sudden change, a drop to zero, a spike, or a creep, points to a fault before the next full survey would catch it. For DOT-regulated pipelines, the regulator sets the inspection cadence for these power sources, and the requirement is six inspections each calendar year at intervals not to exceed 2.5 months, per 49 CFR 192.465. Confirm the exact requirement for the asset and jurisdiction.

Reading the rectifier is necessary but not sufficient. Good output at the rectifier does not prove the structure is polarized to the criterion everywhere, because current distribution and IR drop sit between the rectifier and the steel. The rectifier tells you the system is alive. The potential survey tells you the structure is protected.

Anodes: sacrificial versus impressed, and anode life

The anode is where the corrosion is supposed to happen, and the two system types use different anodes for a reason. Sacrificial anodes are active metals chosen to be consumed: magnesium, zinc, and aluminum. They give up their own metal to protect the steel, so their consumption is the point, not a defect. Impressed-current anodes are the opposite. Mixed-metal-oxide, graphite, and high-silicon cast iron are picked to pass current with minimal loss, so a small mass can carry current for years in a groundbed.

Anode life is a design number, not a guess. The corrosion engineer sizes the anode mass to the current it must deliver over the intended service life, with a consumption rate and a utilization factor from the standard and the manufacturer. Undersize the anodes and the system meets the criterion at commissioning, then falls off it years early when the anodes run out. That is one of the more common slow failures, and it does not announce itself between surveys.

Placement matters as much as mass. Anodes are positioned, and groundbeds laid out, so the current reaches the whole structure rather than over-protecting the steel nearest the anode and starving the far end. That distribution is part of the design, and it is why anode location is engineered rather than improvised in the trench.

What is the -850 mV criterion?

The proof that a structure is protected is its potential, not the fact that a CP system is installed. You measure the structure-to-soil potential against a reference electrode, and you compare it to a criterion in the governing standard. For buried and submerged steel, the most-cited criterion is a polarized potential of -850 mV or more negative, measured against a copper/copper-sulfate reference electrode, with the IR drop removed. The standard also recognizes a 100 mV cathodic polarization criterion, the formation or decay of at least 100 mV of polarization, which is often the practical choice on structures where the -850 mV value is hard to reach cleanly.

These criteria come from NACE/AMPP SP0169 for external corrosion control of buried and submerged metallic piping systems, and equivalent practices apply to tanks and other structures. The word that does the work is polarized. A reading taken with the current flowing includes an IR-drop error and reads more negative than the steel actually sits, so the -850 mV value is meant to be the IR-free, polarized potential. Meeting -850 mV on a current-applied reading without removing IR is a common way to believe a structure is protected when it is not.

Treat the criterion as the engineer's call and the standard's number, not a field opinion. The applicable standard, the structure type, and the conditions decide which criterion applies and how it is demonstrated. What does not change is the principle: you prove protection by measuring potential against a reference electrode, and a system that cannot show it meets a recognized criterion is not yet protecting anything you can defend.

The reference electrode

Every CP potential is a measurement against a reference electrode, a stable half-cell that gives the voltmeter a fixed point to read from. In soil, the standard is the copper/copper-sulfate electrode (CSE), a copper rod in saturated copper-sulfate solution. When a survey reports a structure at -900 mV, it means -900 mV relative to that copper/copper-sulfate cell. Change the reference and the number changes, which is why the standard ties its criteria to a specific electrode.

Other references show up in other electrolytes. Silver/silver-chloride is common in seawater, and zinc references are used for permanent installations. The number that defines protection is meaningless without naming the electrode it was read against, so a potential with no reference stated is a potential you cannot use.

Placement affects accuracy. The electrode is set in good contact with the soil, as close to the structure as practical, because distance through the soil adds IR-drop error to the reading. On a close-interval survey the technician walks the electrode along the line at tight spacing for exactly this reason: a reading taken far from the steel is a reading contaminated by the soil between them.

Testing and monitoring: surveys, test stations, and CIS

A CP system is only as good as the last survey that proved it. The core measurement is the structure-to-soil potential survey, taken against a reference electrode and compared to the criterion. On buried pipelines, the routine reading happens at test stations, which are permanent above-grade points wired to the structure so a technician can read potential without excavating. Rectifier output, volts and amps, is read on its own schedule as the health check on the power source.

For DOT-regulated pipelines the cadence is set by regulation. The line under CP is tested at least once each calendar year at intervals not to exceed 15 months, and rectifiers are inspected six times each calendar year at intervals not to exceed 2.5 months, per 49 CFR 192.465. Confirm the exact requirements for the asset, the commodity, and the jurisdiction, because the regulator and the project specification control the schedule, not a generic annual.

When the question is whether the line is protected everywhere, not just at the test stations, the tool is the close-interval survey (CIS). A technician walks the route reading potential at roughly 5 ft spacing with the rectifiers cycled on and off in sync, which captures both the current-on and the polarized, instant-off potential along the whole line. CIS finds the gaps between test stations, the spots that fail the criterion, and the locations picking up or discharging stray current from foreign systems. It is the closest thing to seeing the protection profile of the entire structure.

IR drop and why the criterion is read instant-off

IR drop is the voltage error built into a CP reading taken while current is flowing. The current passing through the resistance of the soil between the structure and the reference electrode creates a voltage of its own, by Ohm's law, and that voltage adds to the reading. So a current-applied, or current-on, potential reads more negative than the steel is actually polarized to. Believe that inflated number and you can call a structure protected when the real polarized potential is short of the criterion.

The standard way around it is the instant-off reading. You interrupt the CP current, momentarily, and read the potential in the instant after the current stops but before the structure starts to depolarize. The IR drop disappears the moment current stops, so the instant-off value is the polarized, IR-free potential the -850 mV criterion is written against. On systems with several current sources, all of them have to be interrupted in sync, or the ones still running keep an IR error in the reading.

This is the single most misread part of CP. A technician who records current-on potentials and checks them against -850 mV is grading the system on the wrong number. The 100 mV polarization criterion is often used precisely because it sidesteps some of the IR-drop argument by measuring polarization formation or decay. Either way, the goal is the IR-free potential, and the engineer and the standard define how to get it.

Over-protection: too much CP causes its own damage

More current is not more protection past a point, and over-protection does real damage. Drive the potential too negative and the cathode reaction shifts to hydrogen evolution, generating hydrogen gas at the steel surface. That hydrogen does two bad things. It can lift and disbond the coating from the steel, which is cathodic disbondment, and it can drive hydrogen into the metal itself, embrittling high-strength steels and welds and setting up cracking. The thing you installed to stop corrosion starts attacking the coating and the metallurgy.

The risk is sharpest on coated structures, on high-strength steels, and on prestressed elements. There is a negative limit as well as the -850 mV protective limit, and the standard and the engineer set it for the structure and the steel. CP design is a balance between enough polarization to meet the criterion and not so much that hydrogen evolution and disbondment begin. It is not a dial you turn to maximum and walk away from.

On rebar in concrete the balance is tighter still. Concrete is alkaline and that alkalinity protects the steel, but excessive current can affect the steel-to-concrete bond and the alkali balance at the bar. CP of rebar is run at controlled, modest current densities for that reason. When in doubt, the corrosion engineer sets the upper limit, and the survey watches for potentials drifting too negative as much as not negative enough.

Stray-current interference on foreign structures

CP current does not respect property lines. Current pushed into the soil to protect your structure can be picked up by a foreign structure that happens to be in the path, a neighboring pipeline, a cable, a tank, and that is interference. The danger is not where the current is picked up. It is where it leaves the foreign structure to return through the soil toward your anode, because the metal corrodes fast at the discharge point. Your CP system can drill a hole in someone else's pipe.

ICCP is the bigger offender because it pushes more current and reaches further, but DC transit systems and other sources cause it too. The standard remedy is a bond, a deliberate metallic connection between the structures, sized and tested so the interference current returns through the wire instead of corroding the foreign structure at a soil discharge. Designing and setting those bonds is coordination work, often done jointly by the corrosion engineers for both structures.

Interference is found by survey. When a foreign-line potential moves as your rectifier cycles on and off, that movement is the fingerprint of interference, and it is exactly what a close-interval survey and joint testing are looking for. Commissioning an ICCP system without an interference check is how a brand-new, well-meaning installation creates a corrosion failure on the structure next door.

Where CP is used

CP shows up wherever steel sits in soil or water and cannot be inspected easily. The applications differ in scale, in the governing rules, and in how much they lean on galvanic versus impressed current, but the principle is identical across all of them: protect the bare steel the coating cannot, and prove it with the potential criterion. The structure type mostly changes the standard you answer to and the survey you run.

Buried pipelines are the most regulated case and the reason much of CP practice exists. Aboveground storage tank bottoms and underground tanks protect the soil-side steel you cannot see. Rebar in chloride-loaded concrete, in parking structures and bridge decks, is a fast-growing application that ties straight into concrete restoration. Marine piling and offshore structures, and downhole well casing, round out the common list.

Pipeline CP and the DOT requirement

Buried steel pipelines are the textbook CP application, and on regulated lines CP is not optional. For gas and hazardous-liquid transmission and distribution under DOT/PHMSA jurisdiction, external corrosion control is required, and 49 CFR 192 for gas and 195 for hazardous liquids set the obligations: provide CP, monitor it on the regulated cadence, and remediate where it falls short. The applicable part and edition govern, so confirm them for the specific line.

The pipeline approach is coating plus CP, never CP alone. The line is coated in the mill and the field joints are coated in the ditch, then CP protects the holidays and the joint defects. Monitoring is layered: routine potential reads at test stations, rectifier inspections for ICCP lines, and the close-interval survey to confirm protection between the test stations and to find interference. The CIS is where an operator learns the protection profile of the whole route rather than a handful of points.

The failures that hurt are the quiet ones. A consumed anode bed, a tripped rectifier nobody caught between inspections, a coating that disbonded and now shields CP current from the steel underneath it. Coating disbondment is a particular trap, because disbonded coating can block CP current from reaching the very steel that is now exposed to trapped moisture. The survey program exists to surface these before they become a leak.

Tank CP: AST bottoms and underground tanks

On an aboveground storage tank, the corrosion problem you cannot see is the underside of the steel floor where it sits on the soil or the foundation pad. CP protects that soil-side surface, typically with anodes installed under the tank bottom, either a galvanic ribbon and anode arrangement or an impressed-current grid, depending on tank size, pad design, and current demand. API 651 is the governing recommended practice for CP of aboveground petroleum storage tank bottoms, and it works alongside the tank's coating and lining program rather than instead of it.

The bottom-to-soil interface is the hard part. Current distribution under a large tank floor is uneven, the center is harder to protect than the rim, and the pad material and any secondary containment liner change how current reaches the steel. Designing for even protection across the whole floor, and proving it with reference electrodes that can actually read under the tank, is the engineering that separates a working tank CP system from one that protects the perimeter and lets the center corrode.

Underground storage tanks follow the same external-corrosion logic and often carry their own regulatory monitoring. The soil-side steel of the tank and its buried piping is protected and proven the same way: coating first, CP for the defects, potential survey against the criterion. The interior of the tank is a separate problem solved with a lining, and that side is covered in the storage tank coating and interior lining guide.

CP of rebar in concrete

Steel in concrete is normally protected by the alkalinity of the concrete, which keeps a passive film on the bar. Chlorides break that down. On parking structures and bridge decks, deicing salt and marine exposure drive chloride into the concrete until the steel starts corroding, and the expanding rust cracks and spalls the concrete from the inside. CP of rebar stops that corrosion electrically when chloride contamination is too widespread to chase with patch repair alone.

Both system types apply. Impressed-current CP uses an anode system, often a coating, mesh, or distributed anodes, installed over or into the concrete and driven by a rectifier. Galvanic CP uses sacrificial anodes embedded in the concrete, including the discrete anodes set around patch repairs to handle the incipient-anode, or ring-anode, effect. That effect is the trap in ordinary patching: a fresh, sound patch can drive corrosion to start in the still-contaminated concrete just outside it, and a sacrificial anode at the patch perimeter is one way to interrupt it.

Current densities here are modest, because over-protection can affect the steel-to-concrete bond. CP of rebar belongs inside a corrosion engineer's assessment of chloride levels, cover, and concrete condition, and it ties directly into structural restoration. For the repair, waterproofing, and assessment side of that work, see the parking structure restoration and repair guide. The two disciplines are decided together.

The design: current density, anode sizing, and the groundbed

CP design starts with one number: how much current the structure actually needs. That comes from the bare steel area, which is a function of coating quality, multiplied by a current density appropriate to the steel and the electrolyte, and adjusted for the soil resistivity the engineer measured on site. A well-coated structure needs a tiny fraction of the current a bare one does, which is the whole reason the coating decision precedes the CP design.

From the current demand, the rest follows. The anode mass is sized to deliver that current for the intended design life at a known consumption rate and utilization factor. The groundbed is laid out for the soil resistivity so it can actually pass the current without the rectifier running out of voltage. For ICCP, the rectifier is rated to the volts and amps the groundbed and circuit demand, with headroom for the structure aging and the anodes consuming over time.

None of this is a field guess. CP design is the work of a qualified corrosion engineer against NACE/AMPP and the relevant application standard, with site resistivity data, the structure's coating condition, and the required life as inputs. A system sized by habit either under-protects and lets the structure corrode, or over-protects and damages the coating and the steel. The number that drives all of it, the real current demand, deserves to be measured and calculated, not assumed.

Maintenance: keeping the system protecting

A CP system drifts. Anodes consume, coatings age and add bare area, rectifiers fail, soil dries out and changes resistivity, and a system that met the criterion at commissioning can quietly fall off it. Maintenance is the program that catches the drift, and on regulated assets the cadence is set by the regulator rather than by convenience.

The routine has a few moving parts on different clocks. Read the rectifier output on its schedule, which for DOT pipelines is six times each calendar year at intervals not to exceed 2.5 months. Run the potential survey on its schedule, at least annually for regulated pipelines at intervals not to exceed 15 months, and read the test stations against the criterion. Replace consumed anodes before they run out, not after the survey shows the structure unprotected. Re-survey after any change to the structure, the coating, or a neighboring CP system that could shift the picture.

What to record, and where it lives

CP is a measurement discipline, and a measurement nobody can find is a measurement that cannot defend the structure. The record is what proves to an inspector, a regulator, or the next technician that the structure was protected on a given date, and it is what turns a string of readings into a trend that catches drift before a survey calls a failure.

Capture the system type and design basis, the criterion the structure is held to, the survey results with the reference electrode and whether the reading was instant-off, the rectifier volts and amps on each inspection, anode installation and replacement dates, and any interference tests and bonds. Tie each reading to a location, a date, and the person who took it. A field record tool such as FieldOS keeps the survey, the rectifier log, and the anode history in one place so the trend is visible and the regulated cadence does not depend on memory or a clipboard left in a truck.

Common mistakes

• Relying on CP instead of a good coating, then chasing the huge current a bare structure demands.
• Calling a structure protected on a current-on reading without removing IR drop to get the polarized potential.
• Grading the system against -850 mV when the conditions call for the 100 mV polarization criterion, or vice versa.
• Over-protecting and causing hydrogen evolution, coating disbondment, or hydrogen embrittlement on high-strength steel.
• Commissioning an ICCP system without an interference survey, then corroding a foreign structure at its discharge point.
• Reading the rectifier and skipping the potential survey, so good output hides a structure that is not polarized.
• Undersizing the anodes so the system meets the criterion at startup and fails it years early when the anodes are spent.
• Reversing rectifier polarity at installation and driving current into your own structure instead of the anodes.

Field checklist

Standards and references

The governing practice for buried and submerged metallic structures is NACE/AMPP SP0169, the standard practice for control of external corrosion on underground or submerged metallic piping systems. It is where the protection criteria live, including the -850 mV polarized potential against a copper/copper-sulfate reference and the 100 mV cathodic polarization criterion, along with the IR-drop guidance that makes those numbers meaningful. Related AMPP practices, including SP0285 for external corrosion control of underground storage tank systems, cover specific structures.

Application standards layer on top. API 651 is the recommended practice for cathodic protection of aboveground petroleum storage tank bottoms. For DOT-regulated pipelines, 49 CFR Part 192 (gas) and Part 195 (hazardous liquids) make CP and its monitoring a legal requirement, with the inspection cadences cited earlier. CP of rebar in concrete follows its own assessment-driven practice and ties into concrete restoration standards. The exact standard, edition, and section apply to the structure and jurisdiction, so confirm them before citing on a submittal.

The through-line across all of it: CP complements the coating and does not replace it, protection is proven with the potential criterion measured against a reference electrode, and the design and survey belong to a qualified corrosion engineer working to the standard and the regulator. Over-protection and stray-current interference are the two ways a well-meaning system causes harm, and both are found by survey, not by assumption. Let the engineer set the criterion and the limits, and let the spec and regulator control the cadence.

Units and terms

CP carries its own vocabulary, and the same idea reads differently across a survey sheet, a design report, and a regulator's rule. These are the terms that show up most and the meaning behind them.

Cathodic protection (CP)

An electrical method that makes a metal structure the cathode of a circuit so it stops corroding and an anode corrodes instead

Galvanic / sacrificial system

CP driven by an active-metal anode (Mg, Zn, Al) that corrodes in place, with no external power

Impressed current (ICCP)

CP driven by a rectifier that forces DC from inert anodes, used for large, bare, or high-current structures

Anode / rectifier

The anode is where corrosion is directed; the rectifier is the AC-to-DC power source that drives an ICCP system

-850 mV criterion / reference electrode

A polarized potential of -850 mV or more negative versus a copper/copper-sulfate electrode, the standard proof of protection in soil

IR drop

The voltage error in a current-on reading from soil resistance; removed by the instant-off measurement to get the polarized potential

Over-protection / disbondment

Too much CP, causing hydrogen evolution, coating disbondment, and hydrogen embrittlement of susceptible steel

Stray-current interference

CP current picked up by a foreign structure that corrodes where the current discharges back to the soil; managed with bonds

Current density

Protective current per unit of bare steel area; set by coating quality and soil resistivity, and the basis for sizing

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