# What is pitting corrosion?

> Pitting corrosion is highly localized attack that bores small cavities, at least as deep as they are wide, into a metal, and ASTM G46-21 is the standard guide for examining and evaluating it. Because the rest of the surface loses little metal, pits can hole a pipe or vessel wall long before overall wall loss looks serious.

- Label: Fact
- Type: Explainer
- Published: 2026-10-25T11:49:46+09:00
- By: Jun Zhong
- Researched and drafted by: Beacon's AI newsroom
- Checked by: Jun Zhong on 2026-10-25
- Topics: Pipelines, Upstream and offshore
- Canonical: https://beacon.winda.ai/library/pitting-corrosion/

Pitting corrosion concentrates attack on a few small spots instead of spreading it across a surface. Each pit is a cavity whose depth is about equal to, or greater than, its width at the surface, and pits can stand apart or crowd together until the surface looks rough [2]. Because so little metal is removed overall, a wall can be holed through while weight loss stays small, and failures tend to arrive without warning [2].

## When does it happen?

Pitting is typical of metals that otherwise resist corrosion, such as passive alloys, and of local conditions that single out a few spots on a surface [3] [7]. The settings that matter most in oil and gas are these:

- **Passive alloys in chloride water.** Stainless steels and other metals protected by a thin film pit mostly where chloride ions are present together with oxygen or oxidizing salts [3].
- **Carbon steel in carbon dioxide (CO2) brines.** Carbon steels such as API 5L X65 are widely used for oil and gas pipelines, and in these "sweet" environments pitting is treated as the most dangerous form of localized attack because it is random and hard to forecast [6].
- **Under deposits.** In upstream lines at low flow, sand can settle along the bottom of the pipe. In laboratory tests on X65 steel in CO2-saturated brine, an imidazoline inhibitor adsorbed onto the silica grains and left the steel directly beneath each grain unprotected, which set up galvanic cells and pitting at those spots [7].
- **Next to a more noble metal.** Coupling a metal to a more noble one shifts its potential upward, and that shift can start pitting the metal would not suffer on its own; see [galvanic corrosion](/library/galvanic-corrosion/) [5].

Once a pit starts, it tends to keep itself going. The liquid inside becomes more acidic and richer in chloride than the bulk water, which focuses attack at the bottom of the pit; the same trapped chemistry develops in crevices and in stress corrosion and corrosion fatigue cracks [3].

## Which alloys resist pitting?

For stainless steels, resistance to pitting in chloride solutions rises with chromium (Cr), molybdenum (Mo) and nitrogen (N), and tungsten (W) counts about half as much as molybdenum by weight [4]. The pitting resistance equivalent number (PREN) folds a composition into one ranking figure: PREN = %Cr + 3.3(%Mo + 0.5 x %W) + 16 x %N, all in weight percent [4] [5]. Molybdenum is added to type 316 stainless steel for the same reason [3].

The table below is Beacon's own calculation. It applies that formula to the low and high ends of each grade's composition range as listed by the International Molybdenum Association (IMOA), with nitrogen taken as zero where the range gives only a maximum, and sets the result beside IMOA's typical range. Grades are identified by their Unified Numbering System (UNS) number [4].

| Grade (UNS number) | PREN, bottom of composition range | PREN, top of composition range | IMOA typical PREN |
|---|---|---|---|
| 304L austenitic (S30403) | 17.5 | 21.1 | 18 to 19 |
| 316L austenitic (S31603) | 22.6 | 29.5 | 24 to 25 |
| 2205 duplex (S32205) | 34.1 | 37.8 | 35 to 36 |
| 2507 super duplex (S32750) | 37.7 | 47.6 | 40 to 43 |

Two points follow from the arithmetic. First, the spread inside one grade can be as wide as the step between grades: 316L alone spans 22.6 to 29.5 [4]. Second, super duplex and super austenitic stainless steels are described as having a PREN above 40, yet S32750 made at the bottom of its range computes to 37.7, so the grade name alone does not fix the figure [5] [4].

PREN is an empirical ranking of relative resistance in chloride solutions, not a forecast of life in a particular service [4]. It says nothing about carbon steel, where pitting in CO2 service is studied through surface films, deposits and how well inhibitors reach the metal [6] [7].

## How is it found?

Pits are hard to find. They are small, often hidden under corrosion products, and vary in number and depth even between samples exposed side by side, which makes them hard to compare; in service they may take months or a year to show [2]. A pit can also be narrow at the mouth and much wider below the surface, a shape common on stainless steels, so the visible opening understates the damage [3].

ASTM G46-21, the standard guide for examining and evaluating pitting, covers how to choose among nondestructive and destructive procedures, for laboratory specimens and for field inspection alike. It frames the field question as whether pits are likely to grow into a leak, and whether a pitted component should be repaired or replaced [1].

In the laboratory, pit depth and volume are measured on cleaned specimens with optical surface tools. Researchers at the University of Leeds used surface profilometry and white light interferometry on X65 steel, and Ohio University's Institute for Corrosion and Multiphase Technology used infinite focus microscopy to work out a pit penetration rate under sand deposits [6] [7].

## Is pitting the same as crevice corrosion?

No, though the two are close relatives. Crevice corrosion is intense attack inside shielded gaps that hold a small volume of stagnant liquid, such as gasket faces, lap joints, holes, surface deposits and the space under bolt and rivet heads [2]. Both build up the same acidic, chloride-rich local chemistry, and the two are sometimes grouped together as occluded cell corrosion [3]. The difference is where attack starts: a crevice needs a geometry that traps liquid, while pits on passive metals form at random spots on an exposed surface [3] [2].

## Sources

1. [ASTM G46-21, Standard Guide for Examination and Evaluation of Pitting Corrosion (2021), scope and significance summary, ASTM International](https://store.astm.org/g0046-21.html)
2. [Fontana's Eight Forms of Corrosion (after Fontana and Greene, 1967), AMPP](https://ampp.org/technical-research/impact/corrosion-basics/group-1/eight-forms-of-corrosion)
3. [Forms of Corrosion, NASA Kennedy Space Center Corrosion Technology Laboratory](https://public.ksc.nasa.gov/corrosion/forms-of-corrosion/), 2023-02-15
4. [Practical Guidelines for the Fabrication of Duplex Stainless Steels, Third edition (2014), Table 1 and section on corrosion resistance, International Molybdenum Association](https://www.edelstahl-rostfrei.de/fileadmin/user_upload/ISER/downloads/Guidelines_Fabrication_Duplex_EN.pdf)
5. [Guides to Good Practice in Corrosion Control No. 5: Bimetallic Corrosion (R. Francis, A. Turnbull, G. Hinds, 2020), National Physical Laboratory](https://eprintspublications.npl.co.uk/8617/)
6. [Investigating pitting in X65 carbon steel using potentiostatic polarisation (S. Mohammed, Y. Hua, R. Barker, A. Neville, 2017), Applied Surface Science, vol. 423 (Elsevier), via University of Leeds](https://eprints.whiterose.ac.uk/id/eprint/117553/)
7. [Localized Corrosion of Mild Steel under Silica Deposits in Inhibited Aqueous CO2 Solutions, Paper No. 2144 (J. Huang, B. Brown, S. Nesic, 2013), NACE CORROSION 2013 conference, via Ohio University Institute for Corrosion and Multiphase Technology](https://www.icmt.ohio.edu/documents/NACE2013/NACE2013-Paper%202144_Huang.pdf)

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