Pitting corrosion: mechanism, pit types, testing, prevention and repair

What is pitting corrosion?

Pitting corrosion is a localized form of corrosion in which attack concentrates at small points on a metal surface and penetrates inward, forming cavities that are often deeper than they are wide while the surrounding surface stays largely intact. It is most common on passive metals, such as stainless steel and aluminum, exposed to chlorides.

The danger lies in that geometry. Total metal loss is small, pit openings hide under deposits or corrosion products, and the average wall thickness can look acceptable until a pit goes through. Pitting failures arrive as surprises: a pinhole leak on a tank, vessel or heat exchanger whose thickness survey showed nothing alarming.

ASTM G46 expresses the gap with the pitting factor, the ratio of the deepest penetration to the average penetration calculated from mass loss. A factor of 1 means uniform corrosion; the higher it is, the less an average figure says about the risk of perforation.

How pitting corrosion starts and grows: the mechanism

Pitting has two stages, initiation and propagation, and propagation is autocatalytic: an established pit creates the conditions that keep it growing.

1. Breakdown of the passive film. Stainless steels and aluminum owe their resistance to a thin oxide film. It breaks down locally at weak points: inclusions (manganese sulfides in stainless steel are the classic case), scratches, embedded iron from carbon-steel tools, weld heat tint, or sites under deposits. Chloride ions promote this local breakdown. Stable pits form when the metal's potential exceeds its pitting potential in that environment; many small pits start and repassivate, a few survive.
2. A small anode and a large cathode. Metal dissolves at the pit, while the large passive surface around it acts as the cathode where oxygen or another oxidant is reduced. That area ratio concentrates the corrosion current on the pit, the same unfavorable configuration described in the galvanic series guide.
3. Local acidification. Metal ions released in the pit hydrolyze and lower the local pH, and chloride ions migrate in to balance the charge. The pit fills with a concentrated, acidic chloride solution far more aggressive than the bulk fluid.
4. Self-sustaining growth. The occluded geometry, often capped by corrosion products, keeps that chemistry in and oxygen out. The film cannot reform, and the attack continues downward, sometimes widening beneath the surface.

Two consequences follow. Bulk fluid chemistry does not describe what happens inside a pit, so equipment can pit in a fluid that a short coupon test survives. And an active pit is harder to stop than to prevent: once grown, it only repassivates at a potential below the pitting potential, which is why arresting pitting means changing the conditions.

Which metals are affected by pitting corrosion?

• Stainless steels (304, 316 and related grades). The textbook case. 304 pits readily in chloride service; 316 contains molybdenum, which improves pitting resistance without making it immune.
• Aluminum alloys. Aluminum pits in chloride solutions and seawater, and where copper or other heavier metal ions deposit on it. White corrosion product often hides the pit depth.
• Copper and copper alloys. Copper tubing can pit in some potable and cooling waters, depending on water chemistry and on residues left by manufacturing. The symptom is a pinhole leak in otherwise sound tube.
• Carbon steel. Without a passive film in most waters, carbon steel tends toward more uniform corrosion. It pits under deposits and tubercles, in stagnant water, at breaks in mill scale (which is cathodic to the steel), through dissolved oxygen in boiler and feedwater systems, and under biofilms where microbiologically influenced corrosion (MIC) is active. The "rust pitting" found on steel structures is usually this: pits under a rust layer that appear once the surface is cleaned.

Types of pitting corrosion: ASTM G46 pit shapes

ASTM G46 illustrates typical pit cross-sections. Shape matters because it controls how visible a pit is and how well an inspection method measures it.

Assessment codes also distinguish widespread pitting over a large area from localized pitting confined to a region, a distinction used in the fitness-for-service section below.

What causes pitting: the environmental factors

PREN: comparing stainless steel grades for pitting resistance

The pitting resistance equivalent number ranks stainless steels by composition: PREN = %Cr + 3.3 %Mo + 16 %N. Chromium builds the passive film, molybdenum strengthens it against chlorides, nitrogen adds resistance. Some specifications use a different nitrogen coefficient or add a tungsten term.

These are orders of magnitude from typical composition ranges; when it matters, PREN is calculated from the mill certificate of the actual heat. PREN ignores temperature, crevices, deposits and microstructure, and a poor weld can underperform the base metal. A higher number buys margin, not immunity. For a specific service, use critical pitting temperature data and experience in comparable conditions.

Pitting corrosion tests: ASTM G48, G150 and G46

ASTM G48 tests stainless steels and related alloys in ferric chloride solution, a severe oxidizing chloride environment. Method A immerses specimens at a set temperature and time, then evaluates mass loss and pits; material specifications or purchase orders sometimes invoke it as an acceptance test, with their own temperature and criteria. Methods C and E determine a critical pitting temperature (CPT) by testing at increasing temperatures until pitting appears (C for nickel-base and chromium-bearing alloys, E for stainless steels). Methods B, D and F are the crevice counterparts.

ASTM G150 measures CPT electrochemically: the specimen is held at a constant anodic potential in sodium chloride solution while temperature rises, and the CPT is where the current rises and stays high, indicating stable pitting. It gives one temperature for comparing alloys, heats and welds.

ASTM G46 is the guide for examining and evaluating pits. After cleaning, pits are rated for density, size and depth against standard charts. Depth is measured by metallographic sectioning, machining, micrometer or depth gauge, or microscope focusing, and reported as maximum pit depth, average of the deepest pits and pitting factor.

These tests rank materials under standardized conditions. None predicts service life.

Detecting pitting corrosion on site

• Visual inspection after cleaning. Pits hide under deposits, scale and corrosion products; the surface must be washed, brushed or blasted first. Low-angle lighting reveals openings.
• Pit depth gauge. Measures accessible pits directly; the deepest pits are recorded against the surrounding wall.
• Spot UT thickness readings. Good for general wall loss, weak for pitting: grid points miss pits between them, and a narrow pit may not return a clear echo from its bottom.
• Scanning methods. Ultrasonic scanning including phased array, magnetic flux leakage on tank floors, profile radiography on small-bore piping and laser 3D scanning of exposed surfaces map depth over an area rather than at points.

Where pitting is a credible damage mechanism, the inspection plan must name it and set coverage for localized attack. The API 510 pressure vessel guide explains how that framework handles damage found on vessels.

Assessing pitted equipment: API 579-1/ASME FFS-1 Part 6

When pits are found on a pressure boundary, API 579-1/ASME FFS-1 Part 6 is the assessment procedure for pitting corrosion. It covers widespread pitting, localized pitting, and pitting combined with regions of general metal loss. Level 1 compares the damage with standard pit charts. Level 2 uses representative pit-couples, measuring the diameter, depth and spacing of neighboring pits to calculate the strength reduction. Level 3 is a detailed analysis. The outcome is acceptance as is, operation at a reduced maximum allowable working pressure, or repair, with a remaining life based on the expected pitting rate. It requires good inspection data and a qualified engineer.

How to prevent pitting corrosion

• Material selection with margin. Match PREN and CPT to the real chloride level and temperature, including upsets and evaporation points.
• Design without stagnation. Avoid dead legs and crevices, drain fully, keep velocities in the range the material tolerates.
• Chemistry control. Limit chlorides and oxidant residuals, control pH, remove oxygen in boiler systems, use inhibitors where the process allows.
• Fabrication quality on stainless. Remove weld heat tint, pickle and passivate (ASTM A380 and A967), and keep carbon-steel tools and grinding dust away.
• Coatings and linings. A continuous barrier removes contact between electrolyte and metal; holiday testing finds the defects where attack would concentrate.
• Cathodic protection. For immersed or buried carbon steel, and stainless steel in seawater, it shifts the potential to a range where pits do not propagate.
• Cleaning and lay-up. Remove deposits on a schedule, drain and dry for shutdowns, control biofilms.

How to repair pitting corrosion

Identify and arrest the cause, assess the remaining wall, then choose the repair.

Surface preparation decides whether a filled pit stays repaired. Blasting opens the pits, but soluble salts, chlorides above all, stay at their bottoms. They are measured on the prepared surface (ISO 8502-6 sampling, ISO 8502-9 conductivity) and removed by washing and re-blasting until the specified limit is met; a filler or coating over contaminated pits can fail by blistering and under-film corrosion.

The pits are then filled with metal-filled epoxy pressed in to exclude air, resurfaced to the original profile, and lined where the service warrants it. This cold-applied approach avoids hot work in classified areas and on thin or heat-sensitive parts; the trade-offs are set out in cold bonding vs welding. A pitted gasket seating surface has its own technique, covered in flange face corrosion repair without machining.

Pitting vs crevice vs galvanic corrosion

Pitting and crevice corrosion share the same occluded-cell chemistry; the crevice simply gives it a geometric head start, as explained in the crevice corrosion guide. Under deposits the two are often indistinguishable, and the remedy is the same.

FAQ

Why did my 316 stainless steel pit if it is supposed to resist chlorides? 316 resists pitting better than 304; it is not immune. Above its chloride and temperature limits, under deposits, in crevices, or with heat tint and iron contamination, 316 pits. Its rating assumes a clean, passive surface in the stated bulk chemistry, and pits create their own chemistry.

Can pitting corrosion be stopped once it has started? Only by changing the conditions: removing electrolyte or deposits, drying, lowering temperature or chlorides, applying cathodic protection, or cleaning, filling and coating the attacked area. An active pit left in the same service keeps growing.

How deep can pits get before equipment must be repaired? There is no single number. It depends on remaining wall, pit size and spacing, design pressure and future corrosion rate, and is settled by a fitness-for-service assessment such as API 579-1/ASME FFS-1 Part 6 or by the minimum thickness rules of the inspection code.

Is pitting on gears, bearings or shafts the same as pitting corrosion? Not always. On gear teeth and rolling bearings, "pitting" usually means contact fatigue: small pieces of metal break out under repeated contact stress, with no corrosion involved. Corrosion pitting on shafts does occur, for example on stainless shafts in chloride water, often where packing or sleeves form crevices. The cause decides the fix.

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, poor welds or contamination.

In practice

Induscoat assesses pitted equipment and repairs it in place for plants in Quebec and Ontario through its maintenance and rehabilitation service: surface preparation with soluble salt control, pit filling and resurfacing with metal-filled epoxy as a cold bonding repair, and a lining selected for the service that caused the attack. Where the damage calls for welding or replacement, we say so. Photos of the cleaned surface, pit depth or thickness readings and the service conditions are enough to start: request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.