Pitting corrosion is a localised form of attack in which a protective passive film breaks down at discrete points and forms small, deep cavities while the surrounding surface stays bright and apparently sound. It is most associated with chlorides attacking stainless steels, aluminium alloys and other passive metals, and it is dangerous out of proportion to the mass of metal lost.
Passive alloys such as stainless steel owe their corrosion resistance to a thin chromium-rich oxide film only a few nanometres thick. Pitting begins when that film fails locally, usually at an inclusion, scratch or a chloride-rich spot, exposing a tiny area of bare metal to the electrolyte. That bare spot becomes a small, intensely active anode surrounded by a large passive cathode. The result is a narrow pit that grows downward into the wall while most of the surface is untouched.
Inside a growing pit the chemistry becomes self-sustaining, which is why pitting is called autocatalytic. Metal dissolves at the pit base as M becomes M(n+) plus electrons. Those cations hydrolyse with water, releasing hydrogen ions and driving the local pH down toward strongly acid values. To balance the positive charge, chloride ions migrate into the pit, concentrating there. Low pH plus high chloride keeps the metal at the pit base actively dissolving and prevents the passive film from reforming, so the pit deepens itself. The occluded geometry restricts diffusion, locking in the aggressive electrolyte. This closed-loop acid chemistry is closely related to crevice corrosion, which uses the same occluded-cell principle under gaskets, deposits and lap joints.
A component can lose a negligible fraction of its total mass and still fail from pitting, because damage concentrates into a few deep points rather than spreading evenly. A single pit can perforate a tube wall or tank floor and cause a leak while a general-corrosion measurement would still read the alloy as healthy. Pits are also stress raisers and are a common initiation site for stress corrosion cracking and fatigue cracks. Because they are small at the surface and often hidden by corrosion product, they are easy to miss during casual inspection.
The Pitting Resistance Equivalent Number, or PREN, ranks the intrinsic pitting resistance of an alloy from its composition. The widely used form is PREN = %Cr + 3.3 x %Mo + 16 x %N, with all figures in weight percent. Chromium builds the passive film, molybdenum stabilises it against chloride, and nitrogen strengthens both effects. A higher PREN means the alloy tolerates more chloride and higher temperature before pitting starts. PREN is a comparative guide, not an absolute service limit, because it ignores surface finish, inclusions and the specific environment.
| Alloy (type) | Cr % | Mo % | N % | Approx. PREN |
|---|---|---|---|---|
| 304 austenitic | 18.0 | 0.0 | 0.05 | ~19 |
| 316 austenitic | 17.0 | 2.1 | 0.05 | ~25 |
| 2205 duplex | 22.0 | 3.0 | 0.17 | ~35 |
| 254 SMO (6Mo austenitic) | 20.0 | 6.1 | 0.20 | ~43 |
Whether pits initiate and propagate depends on the metal, the electrolyte and the local geometry acting together. The table below summarises the main levers and their effect on risk.
| Factor | Effect on pitting risk |
|---|---|
| Chloride concentration | Higher chloride sharply increases risk; the primary aggressive species |
| Temperature | Risk rises with temperature above the alloy critical pitting temperature |
| Alloy PREN (Cr, Mo, N) | Higher PREN raises the threshold for initiation, lowering risk |
| Stagnation / low flow | Stagnant zones let chloride and deposits concentrate, raising risk |
| Surface deposits and roughness | Crevices under scale and rough finishes trap electrolyte, raising risk |
| Oxidising potential (e.g. ferric ions, chlorine) | Raises potential above the pitting threshold, increasing risk |
| pH | Lower bulk pH reduces resistance; strong acids widen the pitting range |
Because pits are small and hidden, detection relies on close visual inspection under good light and magnification, dye penetrant testing for surface-breaking pits, and eddy current or ultrasonic wall-thickness measurement on tubes and vessels. Pit depth matters more than pit count, so gauges and replicas that quantify depth are useful. Electrochemical tests such as cyclic potentiodynamic polarisation measure a material pitting potential in the laboratory. Confirming that the installed material is the specified grade through positive material identification catches the common failure where a lower-PREN alloy was substituted by mistake.
Prevention starts with alloy selection: choose a PREN comfortably above the service chloride and temperature envelope, and specify the critical pitting temperature for demanding duties. Design out stagnation with self-draining layouts, adequate flow velocity and no dead legs. Keep surfaces clean and free of chloride deposits, iron contamination and weld heat tint, since pickling and passivation restore the film. Control water chemistry to limit chloride and oxidising species, and apply cathodic protection where a coating or buried structure justifies it. Structured inspection intervals turn these measures into a maintainable programme rather than a one-off. Fabrico helps reliability teams schedule chloride checks, thickness surveys and inspection rounds so pitting is caught early; you can Book a Fabrico demo to see the workflow.
Stainless steel resists corrosion through a passive oxide film, not because the metal is inert. Chlorides can locally break that film, and once bare metal is exposed the acid, high-chloride chemistry inside the pit stops the film reforming, so attack concentrates at that point.
It depends on the environment. Standard 316 with a PREN near 25 handles mild chloride service, duplex 2205 near 35 suits more aggressive water, and 6Mo grades above 40 are used for seawater. PREN is a comparative ranking, so always match it to the actual chloride level and temperature.
General corrosion removes metal roughly evenly and is easy to measure and predict. Pitting removes very little total metal but concentrates it into deep, narrow cavities that can perforate a wall, so a component can fail while overall thickness readings still look acceptable.
Yes. Pits act as stress concentrators and are common initiation sites for stress corrosion cracking and fatigue. Controlling pitting therefore also reduces the risk of these more sudden cracking failures.
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