API 579 Part 6 Pitting Corrosion Assessment Screening

API 579 Part 6 Pitting Corrosion Assessment is used when corrosion is concentrated into pits rather than uniform thinning, and a small number of deep pits may control integrity even when average thickness appears acceptable. Part 6 provides a structured method to evaluate pitting severity using inspection data and to determine whether the equipment is acceptable for continued service at the evaluated operating conditions.

Use this screening workflow to confirm Part 6 applicability and whether your pit data is sufficient to support a defensible decision. In many cases, a complete evaluation must address both current acceptability and whether the equipment will remain acceptable to the next planned inspection based on pitting characteristics and inspection coverage.

Use the screening questions below to determine whether a formal Part 6 evaluation is recommended.

API 579 Part 6 — Pitting Corrosion Assessment Screening (Workflow)

Instruction: Answer all questions, then click “Check if FFS is needed”. If a question truly does not apply to your component or your current objective, select N/A.

1) Has inspection identified pitting corrosion (localized metal loss) on a pressure boundary surface that could affect strength or leak-tightness?
For example; a cooling-water exchanger channel head shows scattered pits with measured pit depths from 0.03 in to 0.12 in across the wetted surface.
2) Is the damage best described as “pits” (localized cavities) rather than a broad area of general thinning requiring Part 4/5 methods?
For example; the surface has many small discrete cavities, not a continuous wall-thinning band across the whole scan area.
3) Is the pitting primarily WIDESPREAD over a region (many pits spread over an area), rather than a tight cluster near one location?
For example; internal shell course has pits distributed over a 12 in × 18 in region (or larger), with no single “hot spot” cluster.
4) Is the pitting LOCALIZED (dense cluster / hot spot) such that it may behave like a Local Thin Area (LTA) / local metal loss region?
For example; near a nozzle reinforcement pad, a 3 in × 3 in footprint contains many deep pits closely spaced, creating a “thin patch.”
5) Is there ALSO a local metal loss / LTA present (separate from individual pits), where pits exist “within” the LTA or the LTA lies in a pitted field?
For example; UT grid shows a broader thinned patch (LTA) and within that patch there are additional discrete pits; or widespread pitting exists and one area meets LTA definition.
6) Are you evaluating a BLISTER ARRAY as equivalent pitting (Part 7 allows blister arrays to be assessed using Part 6 concepts)?
For example; a plate has many closely spaced blisters; you want to model the overall weakening effect like pitting damage rather than treat each blister individually.
7) Is the component “Type A” and designed to a recognized code/standard, and is internal pressure the primary load case being used for screening (i.e., no controlling external pressure / unusual supplemental loads)?
For example; an ASME VIII vessel shell course under internal pressure only; no external pressure case, no dominant nozzle loads controlling the section.
8) Is the pitting located on ONLY ONE surface (ID or OD), not on both surfaces?
For example; pitting is only internal from process fluid; external inspection confirms no CUI pitting on the OD.
9) Is the stress state reasonably uniform through-wall (membrane-dominated), not strongly bending-dominated at the pitted location?
For example; a straight shell course away from supports/nozzles; not a bracket/support attachment region where bending stress dominates.
10) For Level 1 screening intent, can you justify that the pitting is ARRESTED (not actively growing)?
For example; two inspections 3 years apart show the same maximum pit depth (0.08 in then 0.08 in) in the same mapped region, with no increase in pit density.
11) If pitting is ACTIVE (not arrested), are you prepared to assess acceptability at a specific FUTURE inspection date using assumed progression rates (depth/diameter/spacing/density changes)?
For example; max pit depth is 0.10 in today; you will run cases for 0.005–0.02 in/yr to evaluate acceptability at a 3-year future date.
12) Do you have (or can you justify) a Future Corrosion Allowance (FCA) for the pitting region, based on projected future metal loss?
For example; you project 0.06 in additional general loss until next outage in the pitted region and set FCA = 0.06 in.
13) Are you applying FCA correctly (used for future wall thickness / region loss), and NOT subtracting FCA from individual pit depth or pit diameter?
For example; you keep the measured pit depth as-is, but you reduce the future remaining ligament thickness basis using FCA as required.
14) Do you have a clear “most severely pitted” area identified (highest pit density), supported by photo (with scale) and/or rubbing to compare to standard pit charts?
For example; inspector boxed a 4 in × 6 in region with densest pitting, photographed with ruler, and kept it as the chart-comparison basis.
15) Do you have a defensible maximum pit depth for that region, and the minimum measured thickness at the deepest pit location?
For example; max pit depth measured by pit gauge/UT profile is 0.11 in, and minimum thickness at that pit is documented.
16) Does the minimum measured thickness satisfy the Level 1/2 minimum-thickness limitation intended to prevent local “pinhole leakage” type failures (piping vs vessels/tanks limits differ)?
For example; for a vessel/tank location, minimum measured thickness is well above the stated minimum criterion; for piping, you checked the piping-specific criterion.
17) Can you find an appropriate standard pit chart representation that matches the observed surface damage (black area fraction / pit density) for Level 1 screening?
For example; your photo/rubbing visually matches one of the standard pit charts closely enough to select it confidently.
18) Is the pitting more extensive than the most severe standard pit chart (i.e., beyond the chart range), requiring direct RSF calculation rather than chart lookup?
For example; the “black area” fraction of the pitted region is clearly worse than the most severe chart, so you must compute RSF directly.
19) Do any individual pits appear large/deep enough that they must be treated as (or trigger evaluation as) a Local Thin Area / local metal loss region rather than “pure pitting chart” behavior?
For example; one pit is so deep that the required thickness-averaging length criterion is not satisfied, so you must evaluate it as LTA (local metal loss) using the appropriate procedure.
20) Is a Level 2 Assessment required because: (a) no suitable pit chart exists, (b) you need a more detailed widespread pitting assessment (e.g., include pit-couple orientation), (c) pitting is not arrested, or (d) pitting is localized / within LTA / LTA within pitting field?
For example; the photo doesn’t match any chart AND pits are not arrested, so Level 2 is the appropriate next step.
21) For Level 2, do you have pit-couple measurements for the controlling pairs: pit diameter and pit depth for each pit, plus center-to-center spacing between pits?
For example; Pit A: 0.35 in dia × 0.10 in deep; Pit B: 0.30 in dia × 0.09 in deep; spacing kP = 0.60 in (center-to-center).
22) If using the optional refinement, do you know the pit-couple ORIENTATION relative to the biaxial stress field (longitudinal vs circumferential direction) at the location?
For example; the pit-couple line connecting pit centers is aligned with the longitudinal axis of a cylinder, and you include that orientation to refine the Level 2 calculation.
23) Do you need a Level 3 Assessment because Level 1/2 do not apply, fail due to limitations, or are overly conservative (won’t permit operation at current MAWP / intended conditions)?
For example; the location has significant bending stress, or pitting exists on both ID and OD, so you need a detailed analysis rather than Level 1/2 screening.
24) Is your objective to determine acceptability at current conditions using RSF and, if required, determine reduced rating (MAWPr for pressure components and/or MFHr for tanks) at the coincident temperature?
For example; you need to confirm acceptability at MAWP, or rerate a vessel from 200 psig to MAWPr to continue operating safely at the governing temperature.
25) Are you also trying to estimate REMAINING LIFE (time to reach an unacceptable MAWPr/MFHr) using a MAWP-based approach and assumed progression rates?
For example; you compute MAWPr “today” and again at future dates by increasing pit depth/diameter/spacing/density per assumed rates until MAWPr drops below required operating pressure.
26) Are your inspection techniques and sizing methods capable of reliably characterizing pit depth/diameter/spacing for the selected assessment level (and have you validated the data quality)?
For example; you used a cross-sectional UT thickness scan to capture the pitting profile and validated sizing on representative pits where possible.
27) Do you have complete documentation for the damage: location (sketch/grid), size/extent, key measurements, photos/rubbings, dates, and operating history relevant to pitting progression?
For example; you have a marked grid map showing the densest region, the deepest pit location, measurement dates, and associated process conditions during the interval.
28) If the pitting is active or uncertain, have you planned in-service monitoring / re-inspection strategy consistent with the damage mechanism and data needs?
For example; you set a shorter monitoring interval and repeat the same mapped pit region measurements to confirm whether pit depth/density is changing.
29) Have you considered remediation actions if screening indicates unacceptable damage (e.g., repair, replacement, cladding/overlay/lining/coating, process changes to mitigate pitting) and ensured the plan is compatible with your facility procedures?
For example; if Level 1 fails, you plan a repair or rerate; if active pitting is confirmed, you evaluate mitigation (coating/chemistry/inhibitor/process control) plus inspection plan.
30) Based on your answers, do you need a formal Part 6 assessment (Level 1/2/3) to justify continued operation and/or rerating?
For example; any “No” on required-data questions, any Level 2/3 triggers (active pitting, no pit chart, both-sided pitting, bending-dominated stress), or any LTA interaction means you should run the formal assessment.
Answer all questions, then click “Check if FFS is needed”.

When to Use API 579 Part 6

API 579 Part 6 is typically used when the controlling damage is pitting and the decision cannot be made using general thinning methods alone. Common triggers include:

  • Deep or clustered pits identified by visual inspection, pit gage readings, UT mapping, or scanning/C-scan
  • A case where the average thickness is acceptable, but pit depth or pit density may govern integrity
  • The need to determine whether the component is acceptable now and, when required, whether it will remain acceptable to the next planned inspection
  • Pitting in areas where inspection coverage and pit characterization quality strongly affect the confidence of the decision
  • A need to define practical integrity actions—repair now, repair at turnaround, or monitor with a defined inspection plan tied to pitting severity

If the controlling condition is one localized thin area or groove-like feature rather than pitting, route the evaluation to API 579 Part 5 (Local Metal Loss). If thinning is broad and widespread, route to API 579 Part 4 (General Metal Loss).

What to Gather if Screening Indicates FFS Is Needed

If this workflow indicates that a formal API 579 Part 6 assessment is recommended, prepare the following to support a defensible evaluation:

  • Pit characterization data (maximum pit depth, pit density/spacing, affected area dimensions, and representative pit measurements)
  • Remaining thickness data in the affected region (UT grid, scan/C-scan, or equivalent)
  • Location details (component, orientation, proximity to welds/nozzles/discontinuities, and whether pits occur on internal/external surfaces)
  • Design basis and operating conditions (pressure, temperature, material, and any operating restrictions already in place)
  • Inspection coverage details (how the pits were detected, sizing method, and confidence in maximum pit depth)
  • Planned inspection interval or run length to the next outage (for decisions tied to next inspection)

Request an API 579 Part 6 Pitting Corrosion Assessment (FFS)

If this workflow indicates that an API 579 Part 6 Pitting Corrosion Fitness-for-Service (FFS) assessment is needed, the next step is a decision-ready engineering evaluation using your pit sizing data, thickness readings, design basis, and operating conditions.

Inspection 4 Industry LLC (I4I) performs API 579-1 / ASME FFS-1 Part 6 assessments of existing equipment for pitting corrosion and delivers a complete report stating fit-for-service or not fit-for-service, whether the condition is acceptable now and to the next planned inspection when required, any rerated limits if needed, and practical integrity actions—repair now, repair at turnaround, or monitor and run with a defined inspection plan aligned to the controlling pitting mechanism.

To proceed, send your pit sizing and thickness dataset (pit depth measurements or scan/C-scan outputs), affected area details, inspection coverage information, and your operating basis and request an API 579 Part 6 Pitting Corrosion Assessment (FFS).

 

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