Pitting Corrosion: Causes, Mechanism & How to Resist It

What is pitting corrosion?

Pitting corrosion is a localized corrosion mechanism confined to microscopic sites on an otherwise passive surface. Unlike general corrosion, which thins a whole surface evenly and warns you by visible, measurable weight loss, pitting concentrates all its attack into isolated points. The total mass lost can be trivial, yet a single pit can bore clean through a plate or tube wall. That mismatch, almost no surface damage, but a through-wall perforation, is what makes pitting one of the most deceptive and consequential forms of attack in chloride service.

Stainless steels and nickel alloys resist corrosion because chromium forms a self-repairing passive film (a few nanometers of chromium-rich oxide). Pitting is what happens when that film is broken locally and cannot re-heal, typically because chloride ions are present. Stainless steel is corrosion-resistant, not corrosion-proof: every grade has a chloride and temperature limit above which its passive film fails. The engineering task is to match the grade's limit to your real service conditions.

Pitting vs crevice vs general corrosion

Pitting, crevice and general corrosion share the same underlying electrochemistry but differ in where and how attack localizes. General (uniform) corrosion spreads across the full surface. Pitting initiates on an open, boldly exposed surface at a weak point in the passive film. Crevice corrosion is the geometric cousin of pitting, it starts inside shielded gaps (under gaskets, deposits or bolt heads) where stagnant, oxygen-depleted electrolyte forms, and it begins at lower chloride and temperature thresholds than open-surface pitting. For the full family of mechanisms and how they relate, see types of corrosion.

How pitting corrosion works: the mechanism

Pitting is autocatalytic: once a pit starts, the chemistry inside it drives its own acceleration, so a stable pit rarely stops growing on its own. The cycle runs in five stages:

1. Chloride adsorption at a weak point. Chloride ions concentrate at defects in the passive film, MnS (manganese sulfide) inclusions, scratches, embedded iron, grain boundaries or un-passivated weld heat-affected zones (HAZ).
2. Local breakdown of the passive film. Where chloride locally overwhelms the chromium oxide, the film ruptures and bare metal is exposed as a tiny anode surrounded by a large passive cathode.
3. Anodic dissolution. Metal dissolves rapidly at that exposed point, driving positive metal ions (Fe²⁺, Cr³⁺, Ni²⁺) into solution inside the forming pit.
4. Acidification by hydrolysis. The dissolved metal ions react with water and hydrolyze, generating hydrogen ions that drop the local pH sharply, the inside of a pit can become strongly acidic even when the bulk solution is near-neutral.
5. Chloride in-migration and propagation. Negative chloride ions migrate into the pit to balance the charge, concentrating the aggressive acid-chloride electrolyte. This low-pH, high-chloride pocket keeps dissolving metal and prevents re-passivation, the pit deepens, undercuts and perforates.

Breakdown of the passive film

Initiation is a competition at the metal surface: the chromium oxide tries to re-heal while chloride tries to break it. Non-metallic inclusions, especially MnS, are the most common initiation sites because the sulfide dissolves and leaves a microscopic crevice with exposed, active metal. Cleaner steelmaking (lower sulfur, controlled inclusions) and a proper passivation treatment that restores and thickens the passive film both raise the real initiation threshold.

Initiation at the pitting potential

Each grade in a given electrolyte has a pitting (breakdown) potential, the electrochemical potential above which stable pits nucleate. Below it the passive film holds; above it pits form and grow. Oxidizing conditions (dissolved oxygen, chlorine, ferric or cupric ions) push the metal's potential up toward the pitting potential, which is why bleach, chlorinated water and acid-chloride streams are so aggressive.

The autocatalytic pit: acidification and propagation

Once stage 4 and 5 lock in, the pit is self-sustaining. The acidified, chloride-rich interior cannot re-passivate, the surrounding passive surface acts as an efficient cathode, and attack drives inward rather than outward, producing the characteristic narrow, deep, sometimes undercut cavity. Pits also act as stress raisers and crack initiators, which links pitting directly to chloride stress-corrosion cracking in austenitic grades.

Factors that drive pitting corrosion

Pitting risk is governed by the environment and the alloy together. The specify-stage levers:

• Chloride concentration. The primary driver, higher chloride lowers the pitting potential. Seawater (~19,000 ppm Cl⁻), brines and evaporative concentration are the classic severe cases.
• Temperature. Raising temperature sharply increases susceptibility. Each grade has a Critical Pitting Temperature (CPT), the lowest temperature at which stable pits form in a defined chloride test. Above its CPT a grade pits; below it, it does not.
• pH / acidity. Lower pH reduces the passive film's stability and makes the acidified pit interior harder to neutralize, accelerating propagation.
• Oxidizing potential (ORP). Dissolved oxygen, chlorine dosing and oxidizing metal ions raise the potential toward the pitting potential and promote initiation.
• Stagnation and deposits. Low flow lets chloride and solids settle, creating concentration cells and crevice-like conditions under deposits. Moving, aerated fluid is generally less aggressive than stagnant fluid at the same chloride level.
• Surface condition, inclusions and welds. Rough surfaces, embedded iron, heat tint and un-passivated HAZ all lower the local initiation threshold. Pickling and passivation of welds restore resistance.
• Alloy composition → PREN. The one controllable material lever. Chromium, molybdenum and nitrogen raise the pitting resistance of the alloy itself, quantified as PREN and covered next. See the role of molybdenum and nitrogen for how each element contributes.

PREN: measuring pitting resistance

The Pitting Resistance Equivalent Number (PREN) ranks an alloy's resistance to chloride pitting from its composition. The standard formula is:

PREN = %Cr + 3.3 × %Mo + 16 × %N

A higher PREN means the alloy tolerates higher chloride and higher temperature before its passive film breaks down. PREN is a selection and ranking tool, not an absolute guarantee, it does not account for surface condition, welds or crevice geometry, but it is the single most useful number for comparing grades in chloride service. This page states the metric; for the full derivation and worked examples see how PREN is calculated and the dedicated pitting resistance equivalent number node.

Which steel grades resist pitting: the alloy ladder

Pitting resistance climbs as you add chromium, molybdenum and nitrogen and move up the alloy ladder. The table ranks the common chloride-service grades by PREN. Use it as a starting map, then confirm against your actual chloride, temperature and pH.

The jump from 304 to 316 is purchased almost entirely by molybdenum, the 2–3% Mo is what gives 316 the pitting resistance that 304 lacks. The jump from 316 to 904L super-austenitic adds more molybdenum, high nickel and copper for both chloride and acid service. Incoloy 825 (UNS N08825) extends resistance into mixed-acid and sour environments where even 904L is marginal.

When 316 is not enough

316 is the default chloride-service workhorse, but it has a ceiling. In the experience of the Nandini Steel Engineering Team, the most common specifying error is carrying 316 into hot, concentrated chloride streams where its PREN of ~24 is simply too low, the Mo content you paid for is still not enough for the duty. The passive film holds at ambient seawater but fails as temperature and chloride climb. When service exceeds that window, step up to 904L or Incoloy 825 rather than accepting pits: stepping up one grade is far cheaper than a perforated vessel and an unplanned shutdown. Conversely, don't over-buy, match the molybdenum to the real pitting risk instead of defaulting upward.

Examples: where pitting shows up

• Seawater and marine fittings. Chloride-rich, often warm and stagnant in dead legs, the textbook pitting environment. 316 suits mild marine / seawater service; duplex, 904L or super-duplex are specified where temperature or stagnation is severe.
• FGD and scrubber chloride condensate. Flue-gas desulfurization condensate concentrates chlorides at low pH; 904L and higher alloys are routine here.
• Bleach plant / pulp and paper. Chlorine-dioxide and chloride liquors in pulp & paper bleaching drive both pitting and crevice attack, a classic 904L application.
• Chloride-contaminated acid streams. Mixed acid-plus-chloride duty in chemical processing is where 904L's copper-bearing chemistry and 825's reducing/oxidizing-acid resistance earn their place.
• Offshore and brine / sour service. Oil & gas chloride service combines chlorides, temperature and H₂S. Incoloy 825's combined resistance to reducing and oxidizing acids is why it is chosen over 316L and 904L in mixed-acid and sour-gas duty; order it to NACE MR0175 / ISO 15156 so the heat is supplied in the qualified condition.

How to prevent pitting corrosion

• Specify adequate PREN for the service chloride and temperature, the alloy ladder above is the starting point; confirm against iso-corrosion data for borderline cases.
• Passivate and pickle welds and HAZ. Remove heat tint, embedded iron and free iron so the passive film is continuous. See passivation of stainless steel.
• Avoid stagnation and deposits. Design out dead legs, keep flow up and clean settled solids that create concentration cells.
• Specify solution-annealed material with a mill test certificate. Material supplied in the correct annealed condition, with an EN 10204 3.1/3.2 MTC, gives you a documented starting chemistry and microstructure. See corrosion testing & MTC.
• Verify with pitting tests. For critical duty, qualify the grade with ASTM G48 (ferric-chloride immersion, pass/fail with pit depth) or ASTM G150 (electrochemical CPT). Use the grade selector to find a grade for your chloride level.

Sourcing pitting-resistant material with traceability

Correct grade selection only solves pitting if the metal you receive actually matches the certificate. Nandini Steel supplies the full chloride-service ladder, 316/316L, 904L and Incoloy 825, ex-stock for immediate dispatch, cut from genuine mill plate produced by Aperam, Outokumpu, Jindal, POSCO, Nippon Steel and JFE. Every order ships with an EN 10204 3.1/3.2 mill test certificate, so the chemistry behind the PREN is documented and traceable, and material is supplied solution-annealed for full corrosion resistance. 904L, Alloy 28, Incoloy 800 and Hastelloy plate are a third-party-certified specialty on our ISO 9001:2015 scope. Third-party inspection by TÜV SÜD, Bureau Veritas, DNV, SGS or ABS can be arranged against the order. For sour service, cite NACE MR0175 / ISO 15156 on the purchase order so the heat is supplied in the qualified condition.

Frequently asked questions