AMPP Corrosion Specialist key terms

Four components acting together: an anode where metal dissolves, a cathode where reduction occurs, an electrolyte that conducts ions, and a metallic path joining anode and cathode. Remove any one component and the corrosion current stops.

The half-reaction at the anode in which metal atoms lose electrons and enter the electrolyte as positive ions, leaving electrons behind in the metal. Oxidation of iron to ferrous ion is a common example.

The half-reaction at the cathode where electrons arriving from the anode are consumed by an oxidizing species. In neutral aerated water oxygen reduction dominates; in acids hydrogen evolution dominates.

A liquid or moist medium that conducts ions and completes the circuit between anode and cathode. Soil, seawater, process water and condensed films on metal surfaces all act as electrolytes.

A ranking of metals and alloys by their measured corrosion potentials in a specific environment such as seawater. Unlike the theoretical EMF series, it reflects real service behavior in that medium.

Formation of a thin, adherent, sparingly soluble oxide or other film that sharply reduces the corrosion rate, as with stainless steel or aluminum in aerated water. Breakdown of this film leads to localized attack.

Localized, self-sustaining breakdown of passivity that produces deep cavities while most of the surface stays intact. Chlorides and stagnant conditions favor it, and pits can penetrate walls quickly.

Localized attack caused by oxygen depletion or stagnant chemistry inside a tight gap, such as under a gasket, lap joint or deposit. Geometry rather than alloy is often the controlling variable.

Selective attack along grain boundaries, usually because sensitization has depleted chromium from a boundary zone or impurities have segregated there. Common in improperly welded or heat-treated stainless steel.

Accelerated attack of one metal when it is electrically coupled to a more noble metal in a shared electrolyte. The less noble member becomes the anode and corrodes faster; the exposed area ratio is critical.

Cracking produced by the combined action of a sustained tensile stress and a specific corrosive environment, at stress levels far below what pure overload would require. Cracks propagate fast and fail with little deformation.

Corrosion influenced by microbial activity, typically through corrosive metabolic products such as sulfide or organic acids, or by disruption of protective films. It often appears as localized pitting or underdeposit attack in water systems and soil.

A corrosion cell driven by differences in electrolyte composition or dissolved oxygen across a surface. The area with lower oxygen or higher ion concentration tends to become anodic and corrode.

Corrosion driven by direct current from a foreign source, such as a DC transit system, welding operation or incorrectly grounded cathodic protection system, leaving the metal where the current exits.

Loss of ductility and premature cracking when atomic hydrogen enters the metal lattice, often from cathodic protection overprotection, pickling or welding. High-strength steels are especially vulnerable.

A technique that makes the structure the cathode of a corrosion cell by supplying current from an external source, so metal dissolution stops. Used with coatings on buried pipelines, tanks and marine structures.

A more active metal, such as zinc, aluminum or magnesium, connected to the structure being protected and allowed to corrode instead of it. Limited driving voltage and a finite life constrain its use.

A protection system using an external DC power source and anodes to deliver current to a structure. Higher driving voltage than galvanic systems, so it suits large or high-resistivity applications.

An electrode with a stable, known potential, such as copper/copper sulfate or silver/silver chloride, used to measure a structure's potential and judge whether protection is adequate.

Extra wall thickness included in a design so the component can lose material at an expected rate over its service life and still meet strength and containment requirements.

Use of an insulating joint, flange kit or coupling to break the metallic path between two structures, so they do not form a galvanic couple or interfere with each other's protection systems.

An alloy chosen specifically for its ability to withstand a given environment, often through passivity from chromium, molybdenum, nickel or similar elements. Selection depends on exact chemistry, temperature and velocity.

A barrier layer applied to a surface to limit electrolyte contact and reduce the current demand on a cathodic protection system. Disbonding and holidays are its main failure modes.

An organized, documented set of policies, responsibilities, practices and resources that keeps corrosion risk within limits across an asset's life, aligned with safety and business objectives.

A method that ranks equipment for inspection by the likelihood and consequence of failure, so effort goes to the highest-risk items. It relies on credible damage mechanisms and consequence analysis.

A group of equipment and piping that shares the same process chemistry, material and operating conditions, so a single damage assessment and inspection plan applies to the whole group.

Defined limits on process variables such as temperature, pressure, pH and velocity within which the anticipated damage mechanisms stay at acceptable rates. Exceedances trigger review and corrective action.

A formal process that identifies and approves changes to design, materials, operations or chemicals, and confirms that corrosion and integrity impacts are assessed and documented before implementation.

The total cost of owning a facility or component, including design, materials, installation, inspection, maintenance, repair and disposal. Used to justify corrosion control spending against future savings.

A measurable metric used to track whether a corrosion or integrity program is delivering intended results, such as the count of overdue inspections or the percentage of protected structures meeting criteria.

A potential or potential shift value used to judge whether cathodic protection is adequate, such as -850 mV measured against a copper/copper sulfate electrode or a 100 mV polarization shift.

Measurement of pipe-to-soil potentials at close spacing, usually one to three meters along a buried pipeline, to find protection gaps and stray-current areas that distant readings would miss.

A field method that measures DC voltage gradients in the soil to locate coating holidays on buried pipelines and estimate their severity from the gradient size and shape.

A tool-based technique, often magnetic flux leakage or ultrasonic, that travels through a pipeline and measures metal loss or cracks from inside. Findings are normally validated by excavation or complementary methods.

A nondestructive method that uses sound waves to measure wall thickness at selected points, allowing corrosion rates and remaining life to be calculated between inspections.

A specimen of known material exposed in a process stream and later recovered, weighed and examined to estimate corrosion rate and identify the type of attack occurring.

A structured investigation that traces a failure back to its physical, environmental and organizational causes, so corrective actions prevent recurrence rather than addressing only symptoms.

Measurements, usually by the Wenner four-pin method or similar, of how strongly soil resists current flow. High values affect protection feasibility, anode spacing and corrosion likelihood.

Meeting legally mandated requirements for corrosion control, monitoring, recordkeeping and reporting on regulated pipelines, tanks or facilities, and being able to demonstrate that with documented evidence.

A break, pinhole or missed area in a protective coating that exposes bare metal to the electrolyte. Holidays concentrate cathodic protection current and are the usual starting point for localized corrosion.