Pitting corrosion: why the smallest attack sinks the biggest equipment

What is pitting corrosion?

Pitting is a localized form of corrosion in which attack concentrates on tiny points of a metal surface and drills inward, producing deep, narrow cavities while the surrounding surface stays essentially intact. It is dangerous precisely because of that geometry: total metal loss is negligible, visual inspection sees a clean surface with a few specks, and the wall is meanwhile being perforated. Failures from pitting tend to arrive as surprises — a through-wall leak on equipment whose average thickness was fine.

The mechanism: a pit is a chemical reactor

Pitting is autocatalytic — once started, a pit creates the local conditions that accelerate its own growth. The sequence:

1. Initiation. The passive oxide film that protects alloys like stainless steel breaks down at a weak point: an inclusion (typically a sulfide), mechanical damage, embedded iron contamination, or a site where aggressive ions — above all chlorides — concentrate.
2. Separation of chemistry. The small exposed point becomes an anode; the large passive surface around it becomes the cathode. That area ratio — tiny anode, huge cathode — is the same disaster configuration as in galvanic corrosion, and it concentrates the entire corrosion current on the pit.
3. The pit acidifies itself. Metal ions dissolving inside the pit hydrolyze, driving the local pH down; chlorides migrate in to balance the charge. The solution inside a growing pit becomes far more aggressive than the bulk fluid the equipment supposedly operates in.
4. Depth wins. The occluded geometry keeps the aggressive chemistry in and the oxygen out, so repassivation becomes impossible and the pit drills inward rather than sideways.

The practical consequence of step 3 is worth engraving: bulk fluid chemistry does not tell you what is happening inside a pit. Equipment can pit aggressively in a fluid that laboratory coupons survive comfortably.

Who gets pitted, and by what

Passive-film alloys — stainless steels, aluminum — are the classic victims, because their protection depends on a film that chlorides attack at points. Carbon steel pits too, notably under deposits and in soils, though it more often corrodes generally.

Comparing stainless grades: PREN. The pitting resistance equivalent number (PREN = %Cr + 3.3×%Mo + 16×%N) ranks alloys by composition. Indicatively: 304 ≈ 18-20, 316 ≈ 23-28 (its molybdenum is exactly why 316 outperforms 304 in chloride service), duplex 2205 ≈ 33-35, super duplex ≈ ≥40. PREN is a screening tool — a higher number buys margin, not immunity, and service factors (temperature, deposits, crevices) can defeat any grade.

What actually works against pitting

• Alloy selection with margin. Match PREN and CPT to the real chloride level and temperature — including upset conditions and evaporation points, which concentrate chlorides far beyond bulk values.
• Kill the stagnation. Design out dead legs, drain and dry equipment for lay-up, keep velocities up, and clean deposits — under-deposit chemistry is pitting's favorite incubator.
• Surface quality on stainless. Proper pickling and passivation after fabrication, removal of weld heat tint, and strict segregation from carbon-steel tooling deny the mechanism its initiation sites.
• Barrier coatings and linings. Where the environment cannot be moderated — brine tanks, wash systems, marine exposure — a properly selected and applied lining removes the electrolyte-metal contact entirely; system choice follows the exposure, as with any corrosion protection decision.
• Inspection that can see pits. Averaging UT grids miss narrow pits between points. Where pitting is the credible mechanism, inspection needs techniques and coverage chosen for localized attack — and depth of the deepest pit, not average loss, is the number that matters.

Repairing pitted equipment. Isolated pits in otherwise sound wall are a classic case for engineered cold-applied repairs: pit filling and resurfacing with polymer composites restores geometry and seals the attack sites without hot work, followed by a lining where the service warrants it. The decision logic — mechanism arrested, engineering assessment, documentation — is the same as for any coating service-life commitment.

FAQ

Why did my 316 stainless pit — it is supposed to resist chlorides? 316 resists more than 304; it is not immune. Above its critical chloride/temperature envelope, under deposits, in crevices, or with degraded surface condition (heat tint, iron contamination), 316 pits. The alloy's rating assumes a clean, passive surface in the stated bulk chemistry — pits create their own.

Is one deep pit worse than widespread shallow pitting? For pressure boundary integrity, yes — perforation and leak-before-break are governed by the deepest penetration. Widespread shallow pitting matters more for fatigue initiation and as a sign the environment is beyond the alloy.

Can pitting be stopped once started? Only by changing the conditions: removing the electrolyte or deposits, drying, coating, or repairing and resurfacing the attacked area. An active pit left in the same service continues — the mechanism is self-sustaining.

Does a higher PREN always mean better performance? It means better intrinsic pitting resistance in chloride service. It does not compensate for crevices, deposits, stagnation or contamination — a 2205 duplex under a wet deposit can lose to a clean, well-passivated 316.

How is pitting different from crevice corrosion? Same family, different trigger: pitting initiates on open surfaces at film weak points; crevice corrosion initiates in tight geometric gaps (gaskets, lap joints, under deposits) where the occluded chemistry develops without needing a film defect. Alloys resistant to one are usually — not always — resistant to the other.

Pitted equipment in your plant?

Induscoat Canada assesses pitting damage, repairs and resurfaces attacked equipment with engineered polymer systems, and applies linings selected for the chloride service that caused the problem. Send photos and thickness data through our request a quote page; we respond within 24 business hours.