A pit removes almost no metal but can go most of the way through the wall. That combination defeats every method that works by measuring how much steel is left, which is most of them.
Pitting is localised attack that drills into the wall rather than spreading across it. A pit 10 mm across through 80% of a 12 mm wall has removed under a gram of steel from a surface of hundreds of square centimetres — and left 2.4 mm holding the pressure.
That ratio is the entire problem. Methods that average, screen or sample will not find it.
| Method | Why it fails |
|---|---|
| UT spot readings | Only measures under the probe. A pit beside the point is invisible. |
| Guided wave | Detects change in cross-sectional area. A pit changes almost none. |
| Pulsed eddy current | Averages over a large footprint. The pit disappears into the average. |
| Standard MFL (ILI) | Under-calls narrow deep features; sizing tolerance is worst exactly here. |
| Visual, external | Internal pitting is on the far side of the wall. |
Between them those cover the large majority of routine inspection. It is worth sitting with that for a moment: a plant can have a thorough, well-documented inspection programme that is structurally incapable of finding the mechanism most likely to perforate its lines.
Continuous scanning over an area, producing a thickness image. Fine enough resolution and pits show as discrete deep spots with the sound wall visible around them. This is the method that gives you both the depth and the distribution, and Part 6 needs the distribution.
Resolution matters. A scan pitch coarser than the pits will step over them. Discuss the pitch with the technician against the pit size you expect, rather than accepting a default.
An array probe covers a swathe and responds sharply to the discontinuity a pit represents. Faster than UT mapping over large accessible areas and it tolerates thin coating. Depth sizing is limited, so it finds pits and UT measures them.
Exchanger and boiler tubes are where under-deposit pitting and MIC do most of their damage, and they have their own methods. IRIS gives a true wall profile and is the accurate choice. RFT is faster on ferrous tubes with less accuracy. Conventional eddy current is the high-volume screen for non-ferrous bundles.
A common and sensible sequence is eddy current or RFT to find the suspect tubes, then IRIS on those to measure them.
On small-diameter piping, profile radiography images the wall in profile and pits show in the outline. Practical where a scanner will not fit and insulation is staying on.
Underrated. If a vessel is open and clean, pitting is visible, and a pit gauge measures depth directly. No instrument beats a person looking at the actual surface with a light. The limitation is that it needs entry, cleaning and light — which is exactly what a live plant does not offer.
Pitting is not distributed at random, and knowing the mechanism tells you where to scan.
| Mechanism | Where it concentrates |
|---|---|
| Under-deposit corrosion | Wherever solids settle — bottom of horizontal lines, dead legs, low points, behind baffles |
| Microbiological attack (MIC) | Stagnant and low-flow areas, water bottoms, after hydrotest water was left in |
| Chloride pitting of stainless | Crevices, under deposits, at gaskets, under wet insulation externally |
| Ammonium chloride | Where salts deposit and are wetted — overhead systems around the water dewpoint |
| Cavitation | Downstream of pumps, valves and orifices where pressure recovers |
Untreated water left in a system after testing, at ambient temperature, in the dark, with no flow, is an excellent environment for microbiological attack. MIC found a year after commissioning is frequently traceable to a hydrotest that was never properly drained and dried. It is worth knowing this before you inspect a new-ish system and find pits nobody can explain.
Pitting goes to API 579 Part 6, and the approach is different from local metal loss because the damage is a field rather than a single feature.
Level 1 works by comparing the pitting pattern against standard pit charts, which characterise density and depth, and produces a remaining strength factor for the pitted region. What the assessment needs from your inspection is therefore not just the deepest pit but how many there are, how big, and how close together — because closely spaced pits interact and the couple that matters is the pair that nearly joins.
Two situations move the assessment elsewhere:
Pit depth measured from the outside by UT is the remaining wall under that point. If the scan pitch was coarse, the deepest pit in the area is probably not the deepest pit you measured. Assessments based on a sparse scan are optimistic in a direction that matters, and the correct response to finding pits is almost always to scan finer before calculating anything.
Pitting corrosion, MIC, under-deposit attack and chloride pitting compared — what distinguishes them, and which method finds each.
Open the mechanism libraryWho writes this. A mechanical engineer with twelve years in oil and gas — in-line inspection, fired heater and furnace inspection, and pipeline integrity. What is here comes from the published standards and from what those years in the field actually looked like. It is not written by an API-certified inspector.
This is not an assessment. Nothing on this site may be used to justify a decision about real equipment. Assessing plant requires the current editions of the applicable codes, data from a licensed source, and a competent engineer who signs for the answer. · Integrity Field Guide