AMPP Corrosion Technician key terms

A technique that makes a buried or submerged metal structure the cathode of an electrochemical cell, so it stops corroding. It is done by supplying electrons from an external source or from a sacrificial metal.

A metal, usually zinc, aluminum, or magnesium, that is deliberately made the anode of a corrosion cell so it corrodes instead of the protected structure. It is consumed over time and must be replaced.

A cathodic protection method that uses an external DC power source to push current from an anode bed through the electrolyte onto the protected structure. It can serve large or high-resistance systems.

A stable half-cell, such as copper-copper sulfate, silver-silver chloride, or a high-purity zinc electrode, used to measure the potential of a structure. Its known potential lets readings be compared and interpreted.

The voltage difference measured between a buried pipeline and a reference electrode contacting the soil. It is the basic field reading used to judge whether cathodic protection is adequate.

The structure-to-electrolyte potential at which corrosion is effectively stopped. For steel in soil, a common criterion is -850 mV versus a copper-copper sulfate electrode, measured with the IR drop considered.

The voltage lost as current flows through soil, coating, or other resistance between the structure and the reference electrode. It can make readings look more protective than the metal actually is.

A group of impressed-current anodes installed in the ground, often in backfill, to discharge protective current into the electrolyte. Its resistance and spacing strongly affect system output.

An electrical device that converts AC power to the DC needed for impressed current cathodic protection. Its voltage and current output are adjusted to keep protection at the required level.

Current that flows through paths other than the intended circuit, often from DC transit, welding, or other cathodic protection systems. Where it leaves a structure, it causes rapid localized corrosion.

A metallic connection that gives stray current a low-resistance path back to its source or to a grounding system, reducing damage where the current would otherwise leave a structure.

A pipe-to-soil potential survey taken at closely spaced intervals, typically about one meter, along a pipeline. It reveals spots where protection is weak or where stray current effects occur.

Loss of coating adhesion that spreads outward from a holiday or defect because of cathodic protection reactions at the exposed metal. Alkaline products form under the coating and lift it from the surface.

A small hole, pinhole, or bare spot in a protective coating that exposes the underlying metal. Holidays are common starting points for localized corrosion and are found with high-voltage detectors.

The bond strength between a coating film and the prepared substrate. Good adhesion keeps the coating in place and resists undercutting, blistering, and disbondment during service.

Cleaning and profiling the substrate before coating, usually by abrasive blast cleaning to a specified standard. It removes rust, mill scale, and contaminants and creates an anchor profile for adhesion.

The roughness of a blast-cleaned steel surface, measured as peak-to-valley height. The right profile gives the coating mechanical grip without being so deep that it causes thin spots.

A thin, brittle oxide layer formed on hot-rolled steel during manufacture. If it is not removed before coating, it can flake off and take the coating with it, causing early failure.

A field instrument that applies high voltage across a coating to find breaks or thin spots. A spark or signal at a defect shows where the coating does not insulate the metal.

Corrosion that spreads sideways beneath a coating from a damaged spot or edge. It lifts the film and can cause large areas of coating to fail even though most of the surface looked intact.

Formation of raised pockets under a coating film, often caused by trapped solvent, moisture, or corrosion products. Blisters can rupture and expose bare metal to the environment.

A ranking of metals and alloys by their corrosion potential in a given electrolyte, such as seawater. Metals farther apart on the list form stronger galvanic couples when joined.

Corrosion accelerated when two dissimilar metals are electrically connected in an electrolyte. The less noble metal becomes the anode and corrodes faster while the more noble one is protected.

Formation of a thin, stable oxide film on a metal surface that slows corrosion. Stainless steels and aluminum rely on this film, which can break down in certain chloride or low-oxygen conditions.

Localized corrosion that forms small, deep cavities while most of the surface stays intact. It is often caused by chloride attack on passive metals and can lead to rapid penetration and leaks.

Localized attack in narrow gaps where the electrolyte is stagnant and oxygen is depleted, such as under gaskets or lap joints. The shielded area becomes anodic and corrodes while the open surface stays cathodic.

The electrode where oxidation occurs and metal dissolves, releasing electrons into the circuit. In a corrosion cell, the anode is the area that loses material.

The electrode where reduction reactions consume electrons, such as oxygen reduction or hydrogen evolution. The cathode does not corrode; it receives electrons from the anode.

A corrosion cell created by differences in oxygen content across a metal surface. The low-oxygen area becomes anodic and corrodes, which is common under deposits and in crevices.

A chemical added in small amounts to a fluid or environment to slow corrosion. It works by adsorbing on the metal surface, forming a protective film, or changing the chemistry of the environment.

An inhibitor that slows the anodic corrosion reaction by forming a protective film on anodic areas. If it is underdosed, it can concentrate attack on unprotected spots, so dosage control is critical.

An inhibitor that slows the cathodic reaction, for example by reducing oxygen reduction or hydrogen evolution at the metal surface. It generally reduces the overall corrosion rate without shifting attack to small areas.

A long-chain organic molecule that adsorbs on metal surfaces to form a thin hydrophobic barrier. It is widely used to protect pipelines and condensate systems against carbon dioxide and oxygen corrosion.

A chemical such as a sulfite or hydrazine compound that removes dissolved oxygen from water. It protects boilers and closed systems by preventing oxygen-driven pitting and corrosion.

Uniform loss of metal across a surface at a fairly even rate, usually caused by a broad chemical attack. It is often predictable and managed by material selection, coatings, or inhibitors.

The speed at which metal is lost, often expressed as mils per year or millimeters per year. It is used to compare environments, materials, and the effectiveness of protection methods.

Variation in oxygen supply across a metal surface that creates potential differences and drives localized corrosion. Areas with less oxygen become anodic and corrode.

A specified potential or current value used to confirm that cathodic protection is adequate. Criteria vary by structure, environment, and standard, and must account for IR drop.