Pitting Corrosion
Pitting corrosion is a localized form of corrosion that produces small, deep holes (pits) in an otherwise relatively unattacked surface, especially in stainless steels and aluminum alloys exposed to chloride-containing environments.
Definition
Pitting initiates at defects in the passive film and is autocatalytic — chemistry inside the pit accelerates further attack. Pits can perforate thin-wall equipment quickly and are difficult to size by UT due to their small footprint. ET and visual borescopy are common detection methods.
How this damage forms
Localised corrosion producing small, deep cavities while surrounding metal stays largely intact — driven by local breakdown of passive films, classically by chlorides on stainless steels.
How it is detected
Visual finds open pits; UT struggles because a narrow pit reflects poorly and a grid survey steps over it — which is why pitting-prone service uses scanning techniques and why MFL screening plus UT prosecution suits tank floors.
What causes it, and what prevents it
Assessment is by pit depth and density rather than average thickness: API 579 has a dedicated pitting assessment because general-metal-loss rules mis-handle it.
Codes that govern assessment
API 571 for the mechanism; API 579 Part 6 for assessment; ASTM G46 for pit characterisation.
The common reporting error
Reporting average remaining wall over a pitted area. The average is fiction — the deepest pit governs, and perforation arrives at the extreme value, not the mean.
Where Pitting Corrosion fits in an inspection programme
A term is only useful when it connects to a decision. Pitting Corrosion appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.
Related terms
- Erosion-Corrosion — Erosion-corrosion is accelerated metal loss caused by the combined action of erosive wear (from fluid impingement, particles, or cavitation) and corrosion, typically producing smooth, grooved, or directional wall loss patterns.
- Galvanic Corrosion — Galvanic corrosion is the accelerated corrosion of the less-noble metal in an electrolytically connected pair of dissimilar metals exposed to an electrolyte, governed by the galvanic series and the area ratio of the two metals.
- Corrosion Under Insulation (CUI) — Corrosion under insulation is external corrosion of carbon and low-alloy steel piping and vessels hidden beneath thermal insulation, caused by water ingress that accelerates corrosion in the 60–175 °C temperature range.
Further reading
internal corrosion mapping in carbon steel piping
More defect terms
Crack · Lack of Fusion · Lack of Penetration · Slag Inclusion · Porosity · Undercut · Overlap · Burn-Through · Weld Spatter · Lamination
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.