Corrosion Types: General, Localized, Crevice, Flow-Assisted

Corrosion Types: General, Localized, Crevice, Flow-Assisted

Quick Answer: Domain 3 (Corrosion — Cause) expects you to classify corrosion types—especially general, localized, crevice, and flow-assisted (FAC / erosion-corrosion style mechanisms). General attack is relatively uniform; localized attack (including pitting) occurs at discrete sites; crevice attack sits in stagnant gaps; FAC combines electrochemical corrosion with flow-related mechanical damage. Morphology drives inspection priority and coating repair decisions.

Corrosion is the deterioration of a metal by reaction with its environment. For coatings inspectors, the useful skill is not writing a PhD thesis on electrochemistry—it is naming what you see, linking it to geometry and service, and documenting implications for barrier coatings, surface preparation, and maintenance. The July 2026 CIP Level 2 EPG groups classification of corrosion types under Corrosion Domain tasks; sample-style items often contrast localized attack at discrete sites with more uniform general corrosion.

Why Morphology Matters on Coated Assets

A coating system is a barrier (and sometimes an active inhibitor or sacrificial layer). When the barrier is intact and continuous, the electrolyte cannot complete a corrosion cell on the substrate. When the barrier fails—holidays, thin spots, mechanical damage, undercutting, or wrong coating selection—corrosion morphology tells you how the cell is operating:

• Uniform thinning may leave a large area still “coated looking” while DFT and remaining wall are compromised.
• Pits and crevices can hide under apparently sound coating or under deposits, causing leaks with little average metal loss.
• Flow-assisted damage often clusters at elbows, reducers, injection points, and high-velocity spool pieces where both fluid dynamics and coating wear matter.

CIP Level 2 inspectors support owner decisions by describing location, extent, depth indicators (where measurable), coating condition, and adjacent conditions (deposits, joints, CP activity, process fluid type)—not by inventing root-cause metallurgy beyond their scope.

General (Uniform) Corrosion

General corrosion (also called uniform corrosion) proceeds over a relatively large fraction of the exposed surface at comparable rates. Classic atmospheric rust on bare carbon steel, general thinning of unprotected tank interiors in mildly aggressive service, and broad mill-scale-related rust bloom after inadequate prep can all present as general attack.

Morphology clues

• Rough, relatively even metal loss or rust scale across panels, plates, or free surfaces
• Fewer isolated deep pits relative to overall area affected
• Remaining thickness often assessable with ultrasonic thickness (UT) grids rather than only pit gauges
• Coating failure may show as large-area blistering, peeling, chalking-to-failure sequences, or widespread underfilm rust

Where it appears on coated assets

• Exterior structural steel with systemic coating age-out or UV/weather breakdown
• Interior tank shells or vessel walls where the lining failed broadly (wrong product, incomplete cure, chemical incompatibility)
• Areas after abrasive blast where flash rust is general rather than freckled under residual contaminants

Inspection implications

General corrosion is often more predictable for remaining life than pitting if thickness data exist, but it can still force large repair scopes. Inspectors note percent of surface affected, DFT remaining where coating still adheres, adhesion quality, and whether underfilm corrosion is continuous. Repair strategies may include full recoat vs. spot repair based on specification and survey results—not on morphology alone.

Localized Corrosion (Including Pitting)

Localized corrosion concentrates metal loss at discrete sites rather than distributing it evenly. Pitting is the exam-critical form: small anodic sites penetrate deeply while surrounding metal remains relatively unattacked. Other localized patterns include under-deposit attack and preferential attack at welds or heat-affected zones when chemistry and microstructure allow.

Morphology clues

• Isolated pits, pinholes, or cavities with little average mass loss
• Rust “tubercles,” blisters, or coating holidays over single points of activity
• Sharp depth-to-width ratios; pit mouths may be small relative to subsurface undercutting
• Adjacent coating may look intact until probed or until blister fluid is found

Where it appears on coated assets

• Coating holidays, thin DFT edges, and mechanical damage (impacts, scaffold ties, clamp points)
• Splash zones, wet-dry cycling, and chloride-laden marine or deicing environments on carbon steel
• Stainless or higher-alloy items misapplied in chloride service (inspectors document condition; materials selection is usually engineering’s role)
• Under failed linings in process vessels where aggressive ions concentrate at defects

Inspection implications

Localized corrosion is high risk for perforation and leaks. Visual extent understates structural risk. Inspectors should map holiday locations, note blister sizes and fluid, photograph discrete sites with scale, and escalate for UT, pit depth measurement, or engineering assessment when thickness integrity is in question. Coating repairs at isolated holidays are common, but undercutting under the film means repair prep must remove all loose coating and treat edges to a sound boundary—spot paint over active underfilm corrosion is a classic failure.

EPG-style teaching point: localized corrosion occurs at discrete sites, whereas general corrosion is spread more uniformly. If a question contrasts “even thinning of a large plate” with “deep attack at a few coating defects,” choose localized for the discrete-site case.

Crevice Corrosion

Crevice corrosion is a specialized localized mechanism inside shielded, stagnant geometries. The crevice restricts mass transport: oxygen depletes inside the gap, the interior becomes anodic relative to the oxygen-rich exterior, and aggressive ions (often chlorides) and low pH can develop inside the crevice, accelerating attack.

Morphology clues

• Attack concentrated under gaskets, washers, lap joints, socket welds with gaps, faying surfaces, or deposit piles
• Coating disbondment edges and underfilm channels that act as artificial crevices
• Relatively little attack on boldly exposed surfaces next to severe loss under the crevice
• Brown/black corrosion products packed in the gap; metal may show trench-like or pocketed loss when the joint is opened

Where it appears on coated assets

• Bolted flange faces, skip-welded plates, and poorly sealed laps on structural steel
• Insulation support rings and under insulation (CUI-related geometries—document coating and insulation interface conditions per job scope)
• Under marine growth, mud, or sludge deposits on immersed structures
• Along coating cutbacks at field joints if sealing is incomplete

Inspection implications

Crevice sites are easy to miss because the worst metal loss is hidden. Inspectors look for seepage stains at joint edges, rust weeping from faying surfaces, soft or blistered coating at crevice mouths, and specification requirements for sealing, stripe coating, or crevice-filling practices. When opening joints is not allowed, report accessible evidence and recommend owner-authorized disassembly or advanced inspection. Coating application quality at edges, welds, and complex geometry is preventive control: stripe coats and adequate DFT on edges reduce crevice-initiating holidays.

Flow-Assisted Corrosion (FAC) and Erosion-Corrosion Style Attack

Flow-assisted corrosion (FAC) and related erosion-corrosion mechanisms combine electrochemical metal loss with fluid flow effects. In many plant contexts, FAC refers to accelerated thinning of carbon steel (often in high-temperature water/steam systems) where flow strips or thins protective oxide films, allowing rapid general thinning in high-velocity or high-turbulence regions. For coatings inspectors, the broader family includes:

• Erosion-corrosion: mechanical removal of corrosion products or coating plus electrochemical attack, often with directional grooves, horseshoe marks, or polished metal in high-velocity paths
• Impingement and entrained solids: sand, catalyst fines, or slurry cutting through linings at elbows and tees
• Cavitation-related damage (related but distinct): collapsing vapor bubbles pit metal or destroy coatings near pumps and control valves

You do not need nuclear-FAC calculation formulas for CIP Level 2, but you must recognize that flow assists corrosion and coating damage at predictable hydraulic features.

Morphology clues

• Preferential thinning or coating wear on the outside of elbows, downstream of orifices, at pump discharges, and at injection quills
• Smooth, flow-oriented metal surfaces or “comet tails” pointing in flow direction
• Coating thickness loss or bare steel on leading edges while low-flow dead legs remain coated
• Sudden wall thinning with relatively little pitting when oxide film is unstable (classic FAC plant pattern on uncoated carbon steel lines)

Where it appears on coated assets

• Lined slurry lines, seawater intakes, firewater systems with high velocity, and process transfer piping
• Tank mixers and agitator zones where lining abrasion and corrosion coexist
• Splash and spray zones with high shear on offshore and marine structures
• Field-joint and repair areas if the repair product lacks abrasion resistance specified for the service

Inspection implications

Prioritize geometry-based surveys: elbows, reducers, thermowell wakes, and known high-velocity spools. Compare DFT and lining condition on high-flow vs. stagnant areas. Document entrained solids history if available. Recommend lining systems rated for abrasion/erosion when surveying existing systems for maintenance painting. Distinguish pure mechanical abrasion (little corrosion product) from mixed erosion-corrosion (both metal oxidation signs and flow-aligned wear).

Comparison Table for Exam Recall

Inspection Reporting Language That Matches the Blueprint

When writing daily reports or coating surveys:

1. Name the morphology (general, localized/pitting, crevice-associated, flow-assisted) based on evidence—not guesswork.
2. Tie to geometry (panel center vs. edge, under gasket, outside of 90° elbow).
3. Describe coating state (intact, blistered, disbonded, holiday, thin DFT, mechanical damage).
4. Avoid overclaiming mechanism if chemistry data are missing—report observations and probable classification.
5. Connect to next actions within scope: hold points, additional NDE request, repair boundary marking, or Level 3/engineering referral for integrity.

Common Exam Traps

• Confusing general corrosion with localized because “lots of rust” is visible—ask whether loss is even or discrete and deep.
• Calling every joint failure “galvanic” when the geometry is classic crevice (stagnant gap without a second metal).
• Ignoring flow when damage clusters only at elbows and high-velocity fittings.
• Assuming a shiny polished look is “healthy metal”—it can be erosion-corrosion or FAC thinning.
• Treating FAC as “only power plants”—the inspector-relevant idea is flow-assisted mechanisms on industrial coated/lined equipment generally.

Bottom Line

Master the four labels the blueprint names: general, localized, crevice, and flow-assisted. Use morphology and location to classify what you see on coated assets, then document inspection implications: repair extent, holiday risk, hidden crevice attack, and flow-geometry priorities. That classification skill is the corrosion-domain foundation for later electrochemical and galvanic topics.