Mod 4 - Corrosion Mechanisms

Module from the Materials and Corrosion Curriculum curriculum.

CORROSION MECHANISMS · Learning Objectives · 1. Identify eight primary forms of corrosion (Fontana classification) 2. Understand electrochemical basis of corrosion 3. Recognize patterns in failure investigation to identify mechanism 4. Apply prevention strategies (material selection, inhibitors, coatings, CP) · Eight Forms of Corrosion

Form	Mechanism	Appearance	Common Locations	Prevention	Inspection Method
Uniform	General metal loss across surface	Even thinning, roughening	Any exposed surface in corrosive environment	Corrosion allowance, inhibitors, coatings	UT thickness survey
Pitting	Localized breakdown of passive film	Small deep holes, often under deposits	Stainless in chlorides, under-deposit areas	Higher PREN alloys, avoid stagnant, cleaning	Visual + UT + borescope
Crevice	Localized under deposits, gaskets, threads	Attack in tight spaces where access is restricted	Gasket faces, lap joints, threads, deposits	Design to avoid crevices, higher alloys	Disassembly inspection, ACFM
Galvanic	Dissimilar metals in electrolyte	Anodic metal attacked, cathodic protected	Bolt/flange joints, weld HAZ, mixed metals	Isolate dissimilar metals, compatible materials	Visual, UT at junction
Intergranular	Attack at grain boundaries	Grain boundary etching, sugar-like appearance	Sensitized SS, aluminum alloys, Ni alloys	Use L-grade SS, solution anneal, stabilized grades	Metallography, ASTM A262 test
Selective Leaching (Dezincification)	Preferential dissolution of one element	Red copper color on brass (Zn removed)	Brass fittings in water, admiralty tubes	Inhibited brass (As, Sb), cupronickel	Visual color change, UT
Erosion-Corrosion	Flow-accelerated metal loss	Grooves, waves, smooth thin areas	High-velocity pipes, elbows, nozzles, pump impellers	Velocity limits, erosion-resistant alloys, design	UT thickness survey at flow direction changes
SCC (Stress Corrosion Cracking)	Tensile stress + corrosive environment	Branched or transgranular cracks	Pressure-containing welds, cold-worked areas	Stress relief, appropriate alloy, environment control	WFMT, UT, PT
Source: NACE/AMPP corrosion handbook, FOS Chief Files — Corrosion folder (Corrosion Effects, NACE documents), IPE-EP-10 series

Source: Materials_and_Corrosion_Curriculum_v1.xlsx · Sheet: Mod 4 - Corrosion Mechanisms

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