AMPP O-CAT key terms

The degradation of a metal caused by an electrochemical reaction with its environment, in which the metal gives up electrons and is converted into a more stable chemical form such as an oxide or salt.

The electrode where oxidation occurs and metal dissolution takes place. In a corrosion cell the anode releases electrons into the circuit and is the location where material loss or pitting typically develops.

The electrode where reduction reactions consume electrons, such as oxygen reduction or hydrogen evolution. The cathode is protected from corrosion and receives the current that leaves the anode.

A conductive medium, usually water containing dissolved salts, that completes the electrical circuit between anode and cathode by carrying ions. Seawater and wet soil are common electrolytes in offshore service.

Accelerated attack that occurs when two dissimilar metals are electrically connected in a common electrolyte, causing the more active metal to corrode faster while the nobler metal is protected.

A ranking of metals and alloys by their relative potentials in a specific environment such as seawater. Metals farther apart on the list create a stronger galvanic couple when connected.

The natural voltage a metal exhibits in a given electrolyte when no external current is applied. It is measured against a reference electrode and indicates the metal's thermodynamic tendency to corrode.

A thin, adherent, self-forming oxide layer on a metal surface that greatly slows further corrosion. Stainless steels and aluminum depend on this film, which can break down locally and cause pitting.

Localized corrosion that penetrates the metal in small discrete cavities while most of the surface remains largely unaffected. Pits can perforate pipe walls or structural members with little overall weight loss.

Localized attack within narrow gaps or shielded areas where the electrolyte becomes stagnant and oxygen is depleted, such as under gaskets, bolt heads, or overlapping plates.

Corrosion promoted by the activity of microorganisms, often in biofilms or anaerobic sediments, producing aggressive species such as sulfides that accelerate metal loss on the seabed or inside piping.

Corrosion caused by direct current flowing through unintended paths, such as from welding equipment or impressed current systems, leaving the metal at the point where the current discharges into the electrolyte.

The portion of an offshore structure alternately wetted by waves and exposed to air, just above the mean waterline. It experiences severe wetting and drying and is usually protected by coatings rather than cathodic protection.

The part of an offshore structure permanently below the water surface. It is continuously immersed in seawater and is normally protected from corrosion by cathodic protection, often combined with a coating barrier.

The portion of a structure above the splash zone that is exposed to air, humidity, and airborne chloride salts. Corrosion there is controlled mainly by coatings and regular inspection for coating breakdown.

An iron-based alloy with small amounts of carbon and other elements. It is the primary structural material for offshore platforms but requires protective systems because it corrodes readily in seawater.

Extra metal thickness added to a component's design so it can corrode over its service life without compromising structural integrity. It is verified during inspection by measuring remaining wall thickness.

Loss of ductility and cracking caused by hydrogen absorbed into high-strength steels, often linked to cathodic protection overprotection or welding. It can cause sudden brittle failure of bolts and tendons.

A protective layer applied to a surface to isolate the metal from the electrolyte and reduce the current demand on cathodic protection. Offshore coatings must resist seawater, abrasion, and ultraviolet exposure.

Cleaning and roughening of a substrate before coating, typically by abrasive blasting to a specified cleanliness and profile. Adequate preparation is the most important factor in coating adhesion and service life.

Degradation of a coating through blistering, cracking, disbondment, or mechanical damage that exposes bare metal. It increases the cathodic protection current demand and can lead to localized corrosion.

A discontinuity, pinhole, or bare spot in a coating where the metal is exposed. Holidays are detected with high-voltage or low-voltage testers and become focal points for corrosion and current demand.

Corrosion control achieved by physically separating the metal from the corrosive environment with a coating, lining, or cladding. It works by blocking the electrochemical reactions rather than by supplying current.

A technique that prevents corrosion by making the metal the cathode of an electrochemical cell, either with sacrificial anodes or an impressed current source, so that oxidation of the structure stops.

A block or bracelet of a more active metal, such as aluminum or zinc alloy, connected to the structure. It corrodes preferentially and supplies protective current until it is consumed and must be replaced.

A cathodic protection system that uses an external DC power source and inert anodes to deliver protective current to a structure. Output can be adjusted, making it suitable for large or high-current-demand structures.

The structure-to-electrolyte voltage at which corrosion is effectively stopped, typically about minus 0.80 to minus 0.90 volts versus a silver/silver chloride reference in seawater. It is the main criterion for judging cathodic protection.

A stable electrode with a known potential, such as silver/silver chloride or zinc, used to measure a structure's potential. Reliable readings require good contact with the electrolyte and account for voltage drops in the circuit.

Applying more cathodic protection current than needed, driving the potential more negative than required. It wastes anode material and can generate hydrogen, risking embrittlement and disbondment of coatings.

The total cathodic protection current required to polarize a structure to its protection potential. It depends on bare metal area, coating condition, seawater resistivity, and environmental factors such as current velocity.

The rate at which a sacrificial anode's mass is used up per unit of current output, usually expressed in kilograms per ampere-year. It determines the anode's design life and replacement schedule.

A survey in which structure-to-electrolyte potentials are measured at closely spaced points to identify areas of inadequate or excessive cathodic protection. It is used to verify protection along pipelines and structural members.

Direct observation of a structure or coating for corrosion, damage, marine growth, or defects, often by divers or remotely operated vehicles. It is the first-line method for assessing integrity and prioritizing further testing.

Inspection methods such as ultrasonic testing, radiography, or magnetic particle inspection that reveal defects or thickness loss without damaging the component. They are used to quantify corrosion damage and verify fitness for service.

Determination of remaining metal thickness, usually by ultrasonic gauging, to assess corrosion loss against the design corrosion allowance. Results feed remaining-life calculations and integrity decisions.

An engineering assessment that determines whether a component with known corrosion or defects can continue to operate safely until the next inspection or repair, based on remaining strength and expected future degradation.

A planning approach that prioritizes inspection effort according to the likelihood and consequences of failure for each component, focusing resources on the highest-risk items to manage integrity efficiently.

An unmanned underwater vehicle controlled from the surface and equipped with cameras and sensors. It is widely used for inspecting offshore structures, coatings, and anodes in deep or hazardous water.