AMPP CP2 Technician key terms

The electrode or metal surface where reduction reactions consume electrons. In a corrosion cell it is the protected area; in a cathodic protection system the buried structure is made to behave as the cathode.

The electrode where oxidation releases electrons to the circuit. In a corrosion cell the anode corrodes; in a cathodic protection system it is the source of protective current.

A medium that conducts ionic current, such as moist soil, seawater, or concrete. It completes the circuit between anode and cathode in a corrosion or cathodic protection system.

The relationship E = I × R, meaning voltage equals current multiplied by resistance. It is used to size anodes, estimate groundbed output, and check circuit resistance.

Protective current per unit area of exposed metal, usually stated in milliamps per square meter. The value needed depends on coating quality, soil resistivity, and environment.

The shift of a structure's potential in the negative direction caused by protective current accumulating at the metal surface. It is the change that actually reduces the corrosion rate.

The decay of polarization after protection current is interrupted, as the structure drifts back toward its native potential. The rate of decay indicates coating quality and current requirement.

The potential a structure shows when no cathodic protection current is applied, reflecting its natural corrosion tendency in that electrolyte. It is the baseline for polarization shift criteria.

A widely used minimum structure-to-electrolyte potential of -0.85 volt measured against a copper/copper sulfate electrode, generally taken as adequate protection for steel in soil.

A criterion requiring at least 100 millivolts of cathodic polarization, measured as the difference between native and polarized potentials or as decay after current interruption.

Excessive cathodic polarization that wastes current and can disbond coatings or embrittle high-strength steel. It is often indicated by potentials much more negative than about -1.2 V CSE.

Damage to high-strength steel caused by hydrogen absorbed during excessive cathodic polarization. It is a key reason to limit protection levels on certain pipelines and tanks.

A stable electrochemical half-cell, such as copper/copper sulfate or silver/silver chloride, used as a fixed reference against which structure-to-electrolyte potentials are measured in the field.

The voltage difference between a buried or submerged structure and a reference electrode touching the electrolyte. It is the basic measurement used to judge cathodic protection effectiveness.

Voltage error in a potential measurement caused by current flowing through electrolyte and coating between the structure and reference electrode. Removing it gives the true polarized potential.

Temporarily switching off protection current so readings are taken before polarization decays, allowing IR drop to be separated from the true polarized potential.

A pipe-to-soil potential survey taken at short intervals, typically 1 to 2.5 meters along a pipeline, to reveal localized areas of low protection between test stations.

Direct current voltage gradient testing that measures voltage gradients in the soil to locate and estimate the size of coating defects on a protected pipeline.

A small metal sample electrically connected to a structure and exposed to the electrolyte, used to measure polarized potential, current density, or depolarization at a known surface area.

A surface access point with wires connected to a structure and sometimes to coupons or anodes, used for periodic potential and current measurements.

A metal such as magnesium, zinc, or aluminum alloy that corrodes preferentially to supply protective current, driven only by the potential difference between itself and the structure.

The fraction of a galvanic anode's mass that produces useful protective current instead of self-corrosion or unwanted by-products. It is used to estimate anode life and replacement intervals.

An anode connected to an external DC power source so driving voltage is not limited by galvanic differences. Common materials include mixed metal oxide, graphite, and high-silicon cast iron.

Equipment that converts AC power into controlled DC for an impressed current system. Tap settings or automatic controls adjust the output current and voltage.

An arrangement of anodes and backfill installed in the earth to discharge protective current into the electrolyte. Its resistance and location strongly affect system output and current distribution.

A carbonaceous conductive material packed around impressed current anodes to lower anode-to-earth resistance, spread current, and extend anode life by holding moisture.

The total resistance between an anode groundbed and remote earth, governed mainly by soil resistivity, anode spacing, and anode dimensions. It is often the largest resistance in the circuit.

A location far enough from the groundbed and structure that their potential fields do not measurably affect readings. It is used as a reference point for resistance and gradient tests.

Current flowing through paths other than the intended circuit, often from DC transit systems, welding equipment, or other CP systems. It causes corrosion where it discharges from a structure.

Metal loss where stray current leaves a structure and enters the electrolyte. The discharge point acts as an anode, and damage rate rises with current density.

A deliberate low-resistance connection between two structures or between a structure and a drainage point, used to control current paths and reduce interference.

A device that blocks direct current while passing alternating current or surge energy, used to isolate structures for testing while keeping safety and lightning protection.

A fitting that electrically separates sections of a pipeline or piping system, preventing current flow between them and helping to confine cathodic protection current to the intended structure.

A pinhole, crack, or other small defect in a coating that exposes bare metal to the electrolyte. Holidays concentrate protection current and are common starting points for corrosion.

A condition where coating, disbonded coating, or another barrier keeps protective current from reaching the metal, leaving it unprotected even when potential readings appear satisfactory.

Coating that has separated from the metal while remaining largely intact, often trapping electrolyte and blocking protective current from the steel beneath it.

A measure of how readily current passes through a coating or its defects. Higher conductance means more current is needed and protection is less uniform along the structure.

A larger pipe installed around a carrier pipe for mechanical protection. If the annulus is not properly isolated and filled, the casing can short the coating and shield cathodic protection.

A selectable connection on a transformer-rectifier that changes the AC input to the transformer, thereby adjusting the DC output delivered to the cathodic protection system.