Corrosion & Coatings Glossary

Amine blush is a waxy, greasy or tacky film that forms on the surface of an amine-cured epoxy when the curing agent reacts with moisture and carbon dioxide in the air instead of the resin, producing carbamates. It is often invisible, and a topcoat applied over it can lose intercoat adhesion and delaminate.

The anodic index, also called the galvanic index, assigns each metal or plated finish a single voltage relative to gold, which is set at zero. The difference between the indices of two metals in contact screens whether the pair is galvanically compatible: the larger the difference, the stronger the drive to corrode the more active metal.

Austenitic manganese steel, or Hadfield steel, is a high-manganese, high-carbon cast steel that stays austenitic at room temperature after water quenching. It is tough and fairly soft as supplied, but its surface work-hardens under repeated impact and heavy compression, which is why it dominates crusher jaws, mantles, concaves and liners.

Cathodic disbondment is the loss of adhesion between a coating and a cathodically protected steel substrate, spreading outward from a holiday or defect. The cathodic reaction on the exposed steel generates hydroxyl ions, and the resulting alkaline environment attacks the coating bond at the edge of the defect, so the disbonded area grows over time.

Cathodic protection (CP) is an electrochemical technique that controls the corrosion of a metal structure in soil or water by supplying direct current that shifts its potential until the whole surface behaves as a cathode. The current comes either from sacrificial anodes or from an impressed-current system driven by a rectifier.

Cavitation damage is the removal of material caused by vapour bubbles that form where local liquid pressure falls below the vapour pressure, then collapse violently as pressure recovers. Repeated collapses against a surface produce rough, sponge-like pitting, typically at pump impeller eyes, valve seats and propellers.

Coating blistering is the formation of dome-shaped swellings in a paint or lining film where it has lost adhesion locally and the space beneath has filled with liquid or gas. The main causes are osmosis driven by soluble salts, alkali generated by cathodic reactions, and solvent trapped in the film during application.

A condition monitoring location (CML) is a designated point on a piping system where wall thickness and condition are measured repeatedly over time, so corrosion rates and remaining life can be calculated. CML placement is risk-based — elbows, injection points, dead legs and interfaces get priority.

A corrosion allowance is extra wall thickness added at design, beyond the thickness required for pressure and structural loads, to be consumed by corrosion over the planned life of a vessel, pipe or tank. It is sized from the expected corrosion rate and protects against general wall loss, not against localized attack or cracking.

Corrosion under insulation (CUI) is external corrosion of piping, vessels and tanks that occurs beneath thermal insulation or fireproofing when water penetrates the cladding and is held against the metal. Because the insulation hides the damage, CUI often progresses undetected until leaks or significant wall loss appear.

A corrosivity category is the ISO classification of how aggressive an environment is to metals. ISO 9223 defines atmospheric categories C1 (very low) to CX (extreme) from the first-year corrosion loss of reference metals; ISO 12944-2 adopts them for protective paint systems and adds Im1 to Im4 for immersed and buried structures.

Crevice corrosion is localized attack inside a narrow, shielded gap — under a gasket, washer, deposit or lap joint — where the trapped solution loses oxygen, turns acidic and concentrates chlorides. Passive alloys such as stainless steels are the most vulnerable, because the chemistry inside the gap breaks down the passive film that protects the open surface.

A cyclic corrosion test is an accelerated laboratory exposure that alternates corrosive and non-corrosive conditions — salt fog or salt application, drying, humidity, UV and sometimes freezing — in a repeating sequence. By reproducing the wet-dry cycling of real atmospheres, it ranks coatings and metals more realistically than continuous salt spray.

A dead leg is a section of piping with no or intermittent flow — bypasses, drains, spare pump suctions, capped future connections. Stagnation lets water, solids and bacteria accumulate, so dead legs routinely corrode at several times the rate of the flowing circuit they branch from.

Dealloying, or selective leaching, is corrosion that removes the more active element from an alloy and leaves a porous, mechanically weak residue of the more noble constituent. The part often keeps its shape and dimensions, so the damage can go unnoticed until a component cracks, leaks or breaks under normal load.

The dew point is the temperature at which airborne moisture condenses on a surface. In coating work, the standing rule is that the substrate must stay a margin above the dew point — commonly 3 °C / 5 °F — during application and initial cure, or an invisible film of condensation compromises adhesion.

Dry film thickness (DFT) is the thickness of a coating after it has fully cured, measured from the substrate surface to the top of the film. It is the single most-checked quantity in coating quality control, because both under-thickness and over-thickness shorten coating life.

Epoxy grout is a three-component material — epoxy resin, hardener and graded aggregate filler — placed between a machine baseplate or sole plate and its concrete foundation to transfer load over the full contact area. Low shrinkage, low porosity, chemical resistance and high bond strength make it the usual machine grout under rotating equipment.

Erosion-corrosion is accelerated metal loss caused by the combined action of a moving fluid and corrosion. Flow, turbulence or entrained solids remove the protective oxide or corrosion-product film faster than it can reform, so the exposed metal corrodes far faster than it would in the same fluid at rest.

Exfoliation corrosion is a layered form of intergranular corrosion that advances along grain boundaries running parallel to the surface of rolled or extruded metal, mainly high-strength aluminum alloys. Corrosion products take up more volume than the metal they replace, so they wedge thin layers apart and the surface lifts like the pages of a book.

Filiform corrosion is a type of corrosion that grows beneath organic coatings as thin, thread-like filaments, most often on coated steel and aluminum exposed to humid air. Each filament has an active head that tunnels under the film and a tail of corrosion products, and it typically starts at scratches, cut edges or coating defects.

Fitness-for-service (FFS) is a quantitative engineering assessment that determines whether pressurized equipment containing a flaw or damage can continue operating safely, under what conditions and for how long. In North America it is performed to API 579-1/ASME FFS-1, which covers vessels, piping and tanks damaged in service.

Flash rust is a thin layer of rust that forms on freshly blast-cleaned or waterjetted steel within minutes to hours, typically as water from wet surface preparation, washing or condensation dries on the bare metal. Standards grade it as light, moderate or heavy, and the coating data sheet states how much the primer tolerates.

Fusion-bonded epoxy (FBE) is a thermosetting powder coating applied to preheated, blast-cleaned steel pipe: the powder melts on contact, flows into a continuous film and cures with the heat of the pipe. It is one of the most widely used plant-applied external pipeline coatings and the base layer of three-layer polyolefin systems.

Galvanic corrosion is the accelerated corrosion of the more active metal in a galvanic couple: two dissimilar metals in electrical contact and wetted by a common electrolyte. The active metal becomes the anode and corrodes faster than it would on its own, while the more noble metal, the cathode, corrodes less.

A holiday test is an electrical inspection that locates discontinuities, such as pinholes, voids and cracks, in a nonconductive coating on a conductive substrate. A detector applies a voltage between an electrode moved over the film and the grounded substrate; where current passes, the film is breached and the spot is marked for repair.

Hydrogen-induced cracking (HIC) is internal cracking of carbon and low-alloy steels in wet hydrogen sulfide service. Atomic hydrogen from surface corrosion diffuses into the steel and recombines as gas at inclusions and laminations, building pressure that forms blisters and stepwise cracks without any applied stress.

Impressed current cathodic protection (ICCP) is a cathodic protection method in which an external DC power source, usually a transformer-rectifier, drives current from long-life, relatively inert anodes through the soil or water to the protected structure. It suits large or poorly coated structures where sacrificial anodes cannot deliver enough current.

Intergranular corrosion is selective attack along the grain boundaries of a metal while the grains themselves remain largely intact. In stainless steels it follows sensitization: chromium carbides precipitate at grain boundaries during heating, depleting the adjacent metal of the chromium it needs to stay passive.

Microbiologically influenced corrosion (MIC) is corrosion initiated or accelerated by the activity of microorganisms, usually living in biofilms on the metal surface. Microbes do not create a new corrosion chemistry; they change the local environment under deposits, producing aggressive, localized attack that bulk water analysis does not predict.

Mill scale is the thin, blue-grey layer of iron oxides that forms on steel during hot rolling and forging. It is brittle, does not expand with the steel and is cathodic to it, so wherever it cracks the exposed steel corrodes preferentially. Coating systems for demanding service require it to be removed before painting.

Near-white metal blast cleaning, defined by the joint standard SSPC-SP 10/NACE No. 2, is an abrasive blast grade in which the steel, viewed without magnification, is free of all visible oil, grease, dirt, mill scale, rust, coating and oxides, with random staining limited to 5 percent of each 9 in² unit area.

A novolac epoxy is an epoxy coating or lining based on epoxy novolac resin, which carries more reactive epoxy groups per molecule than standard bisphenol epoxy resins. The denser crosslinked film gives higher chemical and temperature resistance, at the cost of lower flexibility, making it a common choice for aggressive immersion service.

Passivation is the state — and the chemical treatment that restores it — in which stainless steel is protected by its thin chromium-oxide surface film. The treatment dissolves free iron and contamination so the film can reform uniformly; a stainless surface that loses its film locally corrodes like ordinary steel there.

Pitting is a localized corrosion mechanism in which attack concentrates on small points of a metal surface and penetrates inward, producing deep, narrow cavities while overall metal loss stays negligible. It is autocatalytic — a growing pit creates the acidic, chloride-rich local chemistry that accelerates its own growth.

Pot life is the period after mixing a multi-component coating, lining or compound during which the mixed material remains usable for application with its intended properties. The chemical reaction starts at mixing, so viscosity rises and performance declines with time; the manufacturer states pot life at a reference temperature and batch size.

The pitting resistance equivalent number (PREN) ranks stainless steels and related alloys by their intrinsic resistance to pitting corrosion, computed from composition — most commonly PREN = %Cr + 3.3×%Mo + 16×%N. Higher is more resistant; typical values run from about 19 (304) to over 40 (super duplex).

A pull-off adhesion test measures the tensile stress needed to detach a coating from its substrate by gluing a loading fixture (dolly) to the surface and pulling it off perpendicularly with a portable tester. The result is a stress value plus a failure mode, which together show where the coating system is weakest.

The recoat window is the period, stated by the coating manufacturer, between the minimum time a coat must cure before the next coat can be applied and the maximum time after which the next coat will no longer bond reliably without additional surface preparation. Both limits depend on temperature, film thickness and the product being applied over it.

A sacrificial anode is a piece of deliberately active metal — zinc, aluminum or magnesium — electrically connected to a structure so that galvanic action corrodes the anode instead of the structure. It is galvanic corrosion used on purpose, and it only works while anode material remains and the connection is sound.

A salt spray test exposes coated or bare metal specimens to a continuous fog of neutral sodium chloride solution in a closed chamber, under standardized conditions, to compare their corrosion resistance. ASTM B117 is the most cited practice for operating the apparatus; ISO 9227 is the international standard for salt spray tests.

Soft foot is a condition in which one or more feet of a machine do not sit flat and fully supported on the baseplate, so tightening the hold-down bolt distorts the machine frame. The distortion moves bearing bores and shaft centrelines, loads bearings and seals, and makes alignment readings unrepeatable until it is corrected.

Soluble salt contamination is the presence of water-soluble ionic species — mainly chlorides, sulfates and nitrates — on a steel surface prepared for coating. Invisible and often lodged in pits, these salts draw water through the coating film and drive osmotic blistering and under-film corrosion, so they are measured before coating against a specified limit.

Stress corrosion cracking (SCC) is the growth of cracks in a susceptible material under the combined action of tensile stress and a specific corrosive environment. Each factor alone is harmless; together they produce fine, often branched cracks that can cause sudden failure with little visible corrosion or wall loss.

Sulfate-reducing bacteria (SRB) are anaerobic microorganisms that respire sulfate instead of oxygen, producing hydrogen sulfide. Living in the oxygen-free zone under biofilms and deposits, they are the best-known drivers of microbiologically influenced corrosion (MIC), leaving black sulfide deposits and deep localized pits.

Surface profile — also called anchor pattern — is the peak-to-valley roughness left on a metal surface by abrasive blasting or power tooling. Coatings grip these peaks mechanically, so profile depth directly governs adhesion; a profile too shallow underperforms, one too deep leaves peaks under-covered.

Thermal spray aluminum (TSA) is a metallic coating made by melting aluminum wire or powder and propelling the molten particles onto blast-cleaned steel, where they flatten and build a layered film. It protects as a barrier and, at pores and small damage, sacrificially, and is widely specified against corrosion under insulation.

Three-layer polyethylene (3LPE) is a plant-applied external pipeline coating built from a fusion-bonded epoxy primer, a copolymer adhesive and a thick extruded polyethylene outer layer. The epoxy provides adhesion and corrosion resistance, the adhesive bonds two otherwise incompatible materials, and the polyethylene adds mechanical and moisture protection.

Uniform corrosion, also called general corrosion, is metal loss spread more or less evenly over the exposed surface, so the part thins at a roughly constant rate instead of being perforated locally. It often accounts for the largest total mass of metal lost, but it is the most predictable form and the basis for corrosion allowance.

A zinc-rich primer is an anticorrosive primer pigmented with a high proportion of metallic zinc dust, so that the zinc particles stay in electrical contact with each other and with the steel. The zinc corrodes preferentially and protects the steel galvanically, including at small scratches and defects in the coating.