Pitting Corrosion in Cooling Water: Why Tiny Pits Can Cause Sudden Equipment Failure

The Leak Is the Last Event in a Pitting Failure, Not the First

A cooling-water pipe develops a pinhole leak.

The surrounding wall still looks thick.

The corrosion coupon report is not alarming.

Average iron in the circulating water has not increased dramatically.

The treatment residual has generally remained within the operating range.

Yet one small location has penetrated the equipment wall.

This is why cooling water pitting corrosion must be treated differently from general corrosion.

Uniform corrosion removes metal across a broad area. Pitting concentrates electrochemical attack into a comparatively small anodic location while a much larger surrounding surface can remain cathodic or apparently protected.

The result is an uncomfortable engineering asymmetry:

very little total metal can be lost,

while one location loses enough thickness to fail.

This means a plant can simultaneously have:

• An acceptable average corrosion rate.
• Relatively low bulk-water metal concentration.
• Mostly clean equipment surfaces.
• And a rapidly penetrating pit.

The correct question after a pinhole failure is therefore not simply:

“What was the corrosion rate?”

The more useful questions are:

Where did the pit begin?

What made that location electrochemically different?

What allowed the local chemistry to become more aggressive than the bulk water?

What prevented the surface from repassivating?

And what allowed the pit to continue growing without being visible in the normal monitoring data?

That is the starting point for understanding localized corrosion cooling system failures.

A Pit Is Not Just a Small Rust Spot

The geometry may be small.

The electrochemical system is not.

Once a stable pit develops, the interior can behave like a miniature chemical reactor whose environment is increasingly separated from the surrounding water.

The Pit Becomes the Anode

Metal dissolves at the active location.

The surrounding surface supports the corresponding cathodic reactions.

Because the anodic area can be very small relative to the available cathodic area, current density inside the active location can become high.

Metal Ions Accumulate

Dissolving metal produces positively charged metal ions inside the pit.

Electrical neutrality must be maintained.

Aggressive anions—especially chloride in many industrial waters—can migrate toward the active location.

Hydrolysis Drives the Local pH Down

Hydrolysis reactions associated with dissolved metal ions can generate acidity.

The result is an internal pit environment that may be considerably more acidic than the bulk circulating water.

The Pit Becomes Chemically Different from the Cooling Water

A sample bottle may show acceptable pH.

The water inside the pit can be much more aggressive.

A plant may therefore conclude:

“The cooling water was in specification.”

The pit does not care about the average specification if its own microenvironment has already separated from it.

Pitting Has Two Different Engineering Stages: Initiation and Propagation

A critical mistake is treating every visible pit as though the same condition caused both its birth and its continued growth.

Those can be different problems.

Pit Initiation

Initiation requires a local weakness.

Possible starting conditions include:

• Damage or weakness in a passive film.
• Deposit coverage.
• Surface contamination.
• Weld-related heterogeneity.
• Inclusions or metallurgical discontinuities.
• Low-flow conditions.
• Crevices.
• Biological attachment.
• Local oxidant exposure.
• Chloride concentration.

Pit Propagation

Once the local cell becomes stable, the chemistry inside the pit can sustain further dissolution.

This distinction explains why removing the original initiating condition does not always immediately stop an established pit.

The equipment may have moved from:

environment-controlled initiation

to

self-reinforcing localized propagation.

This is one reason successful pitting corrosion prevention should focus heavily on preventing stable pit initiation rather than relying only on trying to stop a mature pit later.

Chloride Is Important, but “What Is the Chloride Limit?” Is Usually the Wrong Question

Chloride pitting corrosion is frequently discussed as though chloride concentration alone determines whether pitting will occur.

Industrial reality is more complicated.

The same chloride concentration can produce different outcomes depending on:

• Material grade.
• Temperature.
• pH.
• Oxidizing potential.
• Surface condition.
• Deposits.
• Flow.
• Fabrication quality.
• Other ions.
• Existing protective films.

That is why a universal chloride threshold for every material and every cooling system is usually not defensible.

Chloride Attacks Local Surface Stability

For stainless steels and other passivating alloys, chloride is particularly important because it can participate in local destabilization of the protective oxide film.

If the exposed location cannot repassivate quickly enough, a stable pit can develop.

Concentration Happens Locally

The chloride concentration measured in the tower basin is not automatically the maximum concentration experienced by every metal surface.

Water can concentrate locally:

• Inside pits.
• Under deposits.
• Inside crevices.
• Near evaporating or high-temperature surfaces.
• In stagnant branches.
• During abnormal operating conditions.

This is why evaluating chloride pitting corrosion from one bulk-water number can underestimate risk.

Temperature Changes the Boundary Between Stable Passivity and Localized Failure

Temperature is not simply another number on the cooling-water report.

For passivating alloys, increasing temperature can reduce the margin between stable operation and localized corrosion.

This is particularly important for heat exchangers.

The laboratory sample may be taken from water at one temperature.

The metal surface can operate at a higher temperature because heat is being transferred through it.

That means material selection based only on bulk-water temperature can be misleading.

Critical Pitting Temperature Is a Comparative Material Concept

Critical pitting temperature, often abbreviated CPT, is used in controlled testing to help compare the relative resistance of stainless steels and related alloys to pit initiation.

It should not be interpreted as a universal field guarantee.

A laboratory CPT is produced under defined test chemistry and procedures.

A plant contains different chloride levels, deposits, flow conditions, oxidants, welds and surface conditions.

The engineering value of CPT is therefore primarily comparative:

Which material has more margin against localized corrosion under a defined aggressive environment?

That is different from saying:

“This alloy will never pit below temperature X.”

Oxidizing Conditions Can Protect Passivity and Still Increase Pitting Risk

This apparent contradiction is important.

Stainless steel depends on an oxidized passive surface for its exceptional corrosion resistance.

Oxidizing conditions can help maintain passivity.

But under chloride-containing conditions, stronger oxidizing potential can also increase the driving force for localized attack once the passive film becomes locally unstable.

This is one reason chlorination, bromination and other oxidizing treatments need to be evaluated together with metallurgy.

The correct objective is not:

maximum oxidation.

It is:

sufficient microbiological control within a metallurgy-compatible operating envelope.

Shock Events Matter

A cooling system may operate normally for most of the month.

Then a short oxidant excursion occurs.

If the excursion coincides with:

• High chloride.
• Elevated temperature.
• Low flow.
• A vulnerable surface.

it may create a localized initiation event that the monthly average never reveals.

Flow Has a Nonlinear Relationship with Localized Corrosion

“More flow is good” and “less flow is bad” are useful first approximations, but they are not complete engineering rules.

Low Flow and Stagnation Are Major Pitting Concerns

Low-velocity or stagnant conditions can promote concentration gradients, deposit accumulation and localized oxygen differences.

This helps explain why pitting corrosion in pipes may appear disproportionately in:

• Dead legs.
• Standby equipment.
• Large shell-side volumes.
• Bypass lines.
• Intermittently operated branches.
• Low-demand heat exchangers.

High Flow Creates a Different Failure Mechanism

Very high velocity can produce erosion-corrosion, impingement or mechanical removal of protective films in susceptible systems.

That damage may look localized.

It should not automatically be classified as classical pitting.

Failure morphology matters.

A Deposit Can Initiate Pitting, but Not Every Pit Is Under-Deposit Corrosion

This distinction separates this article from the broader under-deposit corrosion problem.

A deposit can create:

• Restricted oxygen transport.
• Restricted inhibitor transport.
• Ion concentration.
• Local pH differences.
• Microbiological niches.

All of those conditions can promote localized electrochemical cells.

But pitting can also initiate on relatively clean surfaces through local passive-film instability, metallurgy, surface condition, chlorides or other environmental factors.

The diagnostic question should therefore remain:

Was the deposit the root initiator?

Or was it simply present near a pit created by another mechanism?

Cleaning the deposit without answering that question can create recurring failure.

A Pit and a Crevice Can Produce Similar Chemistry but They Are Not the Same Geometry

Crevice corrosion and pitting are closely related localized corrosion mechanisms.

Both can generate restricted, concentrated and acidic local environments.

The difference is how the shielded geometry begins.

Pitting

The localized attack develops on an otherwise exposed surface after local breakdown or instability.

Crevice Corrosion

The aggressive environment develops inside a pre-existing shielded geometry.

Examples include:

• Gaskets.
• Lap joints.
• Flanges.
• Deposits.
• Incomplete weld penetration.
• Threaded regions.

This matters because crevice corrosion can sometimes initiate under less aggressive conditions than free-surface pitting.

A material that appears acceptable on an open coupon may still perform poorly inside a real equipment crevice.

Carbon Steel and Stainless Steel Do Not Pit for Exactly the Same Reason

It is useful to speak about pitting as one failure class.

It is dangerous to assume every metallurgy follows the same mechanism.

Carbon Steel Pitting

Carbon steel pitting corrosion often develops when local environmental differences create stable anodic sites.

Common contributors include:

• Deposits.
• Differential aeration.
• Microbiological activity.
• Local treatment failure.
• Stagnation.
• Contamination.
• Corrosion-product deposits.

Carbon steel does not depend on the same highly protective chromium-rich passive film that defines stainless steel behavior.

Its localized corrosion assessment should therefore focus strongly on local water chemistry, deposits, protective inhibitor films and electrochemical differentials.

Stainless Steel Pitting

Stainless steel pitting corrosion is fundamentally tied to local failure of passivity.

Most of the surface can remain in excellent condition while a microscopic weak location becomes active.

This is why stainless steel equipment can produce a particularly misleading visual impression:

99.9% of the surface may look excellent.

The remaining fraction can determine the service life.

The Real Stainless-Steel Question Is How Much Pitting Resistance Margin the Grade Provides

Stainless steel is a family of alloys, not one material.

304, 316L, duplex 2205, super duplex and highly alloyed austenitic grades do not have equivalent resistance to chloride-induced localized corrosion.

Chromium, molybdenum and nitrogen are particularly important alloying elements in pitting resistance.

PREN Is a Ranking Tool

The Pitting Resistance Equivalent Number is commonly used to compare the relative chloride-pitting resistance associated with alloy composition.

A commonly used form is:

PREN = %Cr + 3.3 × %Mo + 16 × %N

Higher values generally indicate greater resistance to chloride-driven pit initiation when comparing appropriate stainless-steel grades.

But PREN is not a complete material-selection model.

It does not automatically account for:

• Surface finish.
• Welding quality.
• Heat tint.
• Inclusions.
• Crevice geometry.
• Actual process chemistry.
• Temperature.
• Fabrication contamination.

Use PREN to screen.

Do not use it to replace engineering judgment.

Fabrication Can Destroy Part of the Corrosion Resistance You Paid for in the Alloy

A buyer can specify a higher-alloy stainless steel and still experience localized failure if fabrication quality is poor.

Weld Heat Tint

Improperly treated heat-affected surfaces can have lower localized corrosion resistance than properly finished parent material.

Iron Contamination

Carbon-steel contamination from tools, grinding or fabrication environments can create surface problems that compromise the intended corrosion performance.

Rough Surface Finish

Surface defects can increase the number of locations where contamination or deposits accumulate.

Incomplete Cleaning and Passivation

A high-grade alloy delivered with poor surface condition may not demonstrate the resistance expected from its composition.

This is why material qualification for stainless steel pitting corrosion should include fabrication quality, not only the mill certificate.

Good Bulk Water Analysis Can Coexist with Severe Pitting

This is one of the most dangerous diagnostic traps.

An operator sees:

pH in target.

Conductivity in target.

Chloride within historical range.

Biocide residual acceptable.

Corrosion coupon acceptable.

Therefore:

“Water chemistry cannot be the problem.”

That conclusion is too strong.

Bulk-water chemistry tells you what was sampled.

Pitting is controlled by what happened at one local surface over time.

The Missing Variables May Be Spatial

The failed location may have:

• Lower flow.
• Higher temperature.
• A deposit.
• A weld.
• A crevice nearby.
• Different oxidant exposure.

The Missing Variables May Be Temporal

The important event may have occurred:

• During startup.
• During shutdown.
• During a biocide shock.
• During a process leak.
• During a temporary high-cycle excursion.
• During maintenance.

A weekly grab sample may never capture it.

Cycles of Concentration Can Move the System Toward a Pitting Threshold

Water conservation encourages plants to reduce blowdown and operate at higher cycles where technically practical.

This can reduce makeup-water demand.

It also concentrates dissolved constituents.

If makeup water contains chloride, chloride concentration increases as cycles increase.

The same is true for many other nonvolatile dissolved species.

Therefore, a water-conservation decision can also be a localized-corrosion decision.

Do Not Optimize Cycles with Scale Chemistry Alone

A plant may calculate that calcium carbonate, phosphate or silica scaling remains controlled at higher cycles.

That does not prove metallurgy remains acceptable.

The cycles decision should evaluate:

• Scale saturation.
• General corrosion.
• Localized corrosion.
• Chloride.
• Temperature.
• Metallurgy.
• Biocide exposure.

This becomes particularly important for stainless-steel exchangers and mixed-metallurgy systems.

Do Not Diagnose Every Pit as MIC

Microorganisms can absolutely participate in localized corrosion.

Biofilm can produce differential aeration, retain aggressive species and alter local electrochemistry.

But finding microorganisms near a pit does not prove that the microorganisms initiated the failure.

Industrial water contains microorganisms.

A mature failure investigation should distinguish:

microbes present

from

microbes mechanistically involved.

Our related guide on microbiologically influenced corrosion and biofilm diagnosis explains why biological evidence should be combined with metallurgy, chemistry, surface morphology and operating history before MIC is assigned as the root cause.

Pit Shape Is Evidence, Not Decoration

Once equipment is opened, morphology should be preserved before aggressive cleaning destroys information.

Ask:

Are pits isolated or clustered?

Are they hemispherical?

Are they narrow and deep?

Are they underneath deposits?

Are they aligned with flow?

Are they concentrated near welds?

Are corrosion products covering the pit mouth?

Is the internal cavity larger than the visible opening?

Does attack occur only on one metallurgy?

The geometry does not provide a complete root cause by itself.

But it helps eliminate incorrect mechanisms.

Average Weight Loss Is a Weak Metric for Severe Pitting

This is why pitting corrosion monitoring needs a different mindset from general corrosion monitoring.

Imagine two coupons.

Coupon A loses metal uniformly over the entire surface.

Coupon B remains nearly untouched except for several deep pits.

The total weight loss could be comparable.

The risk is not.

A penetration failure is controlled by maximum local depth, not average surface loss.

Pit Depth Matters

Where appropriate, coupon evaluation should include pit depth and distribution in addition to overall mass loss.

Actual Equipment Matters More

A coupon still represents one location, one geometry and one exposure condition.

Ultrasonic thickness mapping, tube inspection, borescope examination or other asset-specific methods may be necessary when real equipment is at risk.

ASTM G46 exists specifically because evaluating pitting requires more than simply calculating an average corrosion rate.

Pitting Monitoring Should Combine Probability and Consequence

Not every pit deserves the same monitoring intensity.

Risk depends on both:

the probability that localized attack occurs,

and the consequence if penetration occurs.

High-Consequence Equipment

Examples can include:

• Critical process heat exchangers.
• Hazardous process boundaries.
• Systems where cooling water can contaminate product.
• Equipment with difficult shutdown access.
• Thin-wall tubing.
• High-value production assets.

These locations deserve stronger inspection even when bulk-water corrosion indicators remain stable.

Known Localized-Corrosion History

A previous pitting event changes the prior probability.

Repeated failure at the same location suggests a persistent design, hydraulic, material or operating condition.

Do not reset the risk assessment simply because the damaged spool was replaced.

Build a Pitting Risk Map Before Choosing More Instrumentation

A useful risk map asks where several localized-corrosion factors overlap.

This creates a targeted pitting corrosion monitoring program instead of randomly increasing the number of sensors.

ASTM G48 Is a Ranking Tool, Not a Cooling-Tower Simulation

For stainless steels and related alloys, ASTM G48 is widely used to compare pitting and crevice corrosion resistance in aggressive ferric chloride environments.

The current methods include procedures for evaluating relative resistance and determining critical pitting temperature under defined test conditions.

That information can be extremely useful for material selection.

But an accelerated ferric-chloride test is not the same environment as an industrial cooling tower.

Use the Test to Rank Materials

Which alloy provides more localized-corrosion resistance?

How did welding affect performance?

Did a surface treatment improve resistance?

Do Not Convert the Laboratory Result Directly into a Field Guarantee

The real plant has:

• Different chloride concentration.
• Different oxidants.
• Different temperature.
• Flow.
• Deposits.
• Biology.
• Process contamination.

Laboratory ranking and field qualification should support each other.

ASTM G46 Changes the Question from “Is It Pitted?” to “How Serious Is the Pitting?”

Finding one pit is only the beginning.

A remaining-life decision needs stronger evidence.

Relevant evaluation can include:

• Pit density.
• Pit depth.
• Pit size.
• Pit distribution.
• Maximum penetration.
• Remaining wall.
• Location relative to stress or welds.

This is particularly important after a field failure.

The maintenance team may want to replace the leaking section immediately.

The corrosion engineer needs to know whether the leak represents:

one isolated defect

or

a population of developing pits across the system.

Changing to a Higher-Alloy Material Can Solve One Problem and Create a Procurement Problem

Material upgrade is a powerful form of pitting corrosion prevention.

It is not free.

A move from carbon steel to stainless steel—or from 304 to 316L, duplex, super duplex or another highly alloyed material—changes more than corrosion resistance.

It can affect:

• Material price.
• Availability.
• Welding procedures.
• Welder qualification.
• Filler metal.
• Fabrication controls.
• Inspection.
• Galvanic relationships.
• Spare-parts strategy.

The buyer should therefore ask whether the material upgrade addresses the actual mechanism.

If the pit was caused by extreme stagnation under a deposit, changing the alloy without fixing the hydraulic condition may simply increase the time to the next failure.

A Stronger Alloy Should Be Selected Against the Worst Credible Environment

Material selection often uses nominal operating data.

Pitting failures often occur during non-nominal conditions.

Selection should therefore consider credible extremes:

• Maximum chloride after evaporation or blending changes.
• Maximum metal temperature.
• Lowest expected pH.
• Highest oxidant excursion.
• Shutdown chemistry.
• Cleaning chemistry.
• Temporary stagnation.

The objective is not to design against every imaginary extreme.

It is to design against events the plant can realistically experience.

Corrosion Inhibitors Can Reduce Pitting Risk Only If They Reach and Maintain the Surface

An inhibitor residual in the bulk water does not automatically mean a pit-prone surface is protected.

Protection depends on transport and surface condition.

An inhibitor may struggle to reach:

• Under-deposit surfaces.
• Dead legs.
• Deep crevices.
• Poorly circulated branches.

Incomplete Inhibitor Coverage Can Be Worse Than the Average Suggests

A corrosion program can produce excellent carbon-steel coupon numbers in a clean bypass rack while a low-flow branch develops localized attack.

This is why inhibitor qualification should include hydraulic representativeness.

Passivating Inhibitors Require Particular Discipline

Where corrosion protection relies on maintaining a passive or protective film, inadequate coverage or unstable treatment conditions can allow local active sites to develop.

This is another reason passive film breakdown deserves more attention than the average inhibitor residual alone.

Shutdown Is Often More Dangerous Than Normal Operation

During normal production, water chemistry may be carefully controlled.

Shutdown changes the environment.

Flow stops.

Temperature changes.

Biocide delivery stops or changes.

Solids settle.

Oxygen distribution changes.

Partially drained surfaces remain wet.

Concentrated residual water can remain in low points.

This creates ideal conditions for localized corrosion in some systems.

Define Wet Layup and Dry Layup

The shutdown procedure should define whether equipment will:

remain flooded under controlled treatment,

or

be drained, cleaned and dried.

“Turn the pump off and leave it” is not a corrosion-control strategy.

Startup Can Produce Another Pitting Window

Startup disturbs the system again.

Old deposits can move.

Fresh oxygen enters.

Previously stagnant water mixes with the main circuit.

Chemical residuals may take time to stabilize.

Fresh metal surfaces may not yet have established protective films.

This is why repeated pitting discovered after maintenance should trigger a review of startup procedures, not only normal operating chemistry.

Design Is Often the Cheapest Form of Pitting Control

Chemical treatment receives most of the attention because operators can change it quickly.

Some localized-corrosion problems are fundamentally geometric.

Remove Dead Legs

Branches that cannot be flushed or treated reliably create persistent risk.

Improve Drainability

Equipment that retains stagnant water during shutdown may repeatedly create local corrosion cells.

Reduce Deposit Traps

Geometry that encourages sediment accumulation can undermine otherwise good treatment.

Design Crevices Out Where Possible

Gasket arrangements, lap joints and poor weld details can create shielded environments.

Provide Inspection Access

Risk cannot be managed effectively if the critical surface cannot be inspected.

The Failure Investigation Should Run Backward from the Deepest Pit

A practical forensic sequence can be organized in reverse.

Step 1: Preserve the Failure

Photograph and document before aggressive cleaning.

Step 2: Map the Damage

Determine whether the deepest pit is isolated or part of a wider population.

Step 3: Identify the Metallurgy

Confirm the actual grade rather than relying only on drawings.

Step 4: Examine Surface Condition

Look for deposits, weld heat tint, crevices, biological material or contamination.

Step 5: Analyze Deposits and Corrosion Products

Determine whether chloride, iron oxide, mineral scale, copper or biological material is present.

Step 6: Reconstruct Water Chemistry

Use historical data, not only the sample collected after the failure.

Step 7: Reconstruct Operating Events

Review shutdowns, biocide shocks, production changes, makeup-water changes and high-cycle excursions.

Step 8: Compare Similar Assets

Why did one exchanger pit while another operating on the same water did not?

The difference can reveal the mechanism.

Compare the Failed Asset with a Surviving Asset

This is one of the strongest diagnostic techniques available.

Two exchangers may share:

the same cooling tower,

the same treatment chemistry,

and the same nominal water.

Yet only one develops pitting corrosion in pipes or tubes.

Now compare:

The difference between the two assets can be more informative than another generic water analysis.

Procurement Specifications Should Include Localized-Corrosion Evidence

A B2B buyer purchasing heat exchangers, stainless piping or cooling-system equipment should not evaluate corrosion resistance only through the phrase:

“316 stainless steel.”

Confirm the Exact Grade

“Stainless” is not a complete material specification.

Request Material Certificates

Verify composition and traceability.

Review Fabrication

How are welds cleaned?

Is heat tint removed where required?

How is carbon-steel contamination controlled?

Ask for Localized-Corrosion Test Evidence Where Risk Justifies It

For demanding chloride service, relevant pitting or crevice testing may provide useful comparative qualification evidence.

Do Not Accept PREN Alone as a Performance Warranty

Composition is only one part of actual field resistance.

A Cooling-Water Treatment Supplier Should Explain How the Program Controls Pitting, Not Only General Corrosion

Supplier reports frequently emphasize a corrosion-rate target.

Ask the next question:

How is localized attack evaluated?

A strong supplier response should discuss:

• Representative metallurgy.
• Coupon appearance.
• Pit depth.
• Deposit condition.
• Flow.
• Water-chemistry excursions.
• Chloride.
• Oxidant conditions.
• Actual heat-exchanger performance.

A program that reports only average MPY may be adequate for general corrosion reporting but incomplete for localized asset risk.

A Practical Pitting-Control Hierarchy Starts Before Chemical Dosage

1. Select Appropriate Metallurgy

Give the system sufficient resistance margin for the real environment.

2. Design Out Stagnation and Crevices

Prevent persistent local cells where possible.

3. Control Deposits

Keep inhibitor and bulk water in contact with the surface.

4. Control Chloride and Concentration

Do not optimize water savings without localized-corrosion review.

5. Control Oxidant Excursions

Biological control should remain compatible with metallurgy.

6. Maintain Protective Chemistry

Ensure treatment reaches vulnerable locations.

7. Manage Shutdown and Startup

Do not leave uncontrolled stagnant water in critical equipment.

8. Inspect the Actual Asset

Use monitoring methods capable of detecting local penetration.

9. Learn from Every Pit

A pit is evidence about the real system.

Do not replace the component without preserving that information.

The Strongest Pitting-Control KPI Is Margin, Not a Single Corrosion Rate

A mature plant should think in terms of margin.

How far is the system from known localized-corrosion stress conditions?

Consider:

• Chloride margin.
• Temperature margin.
• Material resistance margin.
• Oxidant margin.
• Flow margin.
• Deposit-control margin.
• Wall-thickness margin.

This creates a much stronger asset-integrity model than asking whether one corrosion coupon is below an internal MPY limit.

The Final Lesson: Pitting Is a Local Failure Hidden Inside an Average System

The defining feature of cooling water pitting corrosion is not simply that the damage is small.

It is that the local environment controlling the damage can become fundamentally different from the environment the plant normally measures.

A pit can concentrate chloride.

Its chemistry can acidify.

Its anodic area can remain extremely small.

The surrounding metal can continue looking healthy.

Average corrosion indicators may remain acceptable.

That is why localized corrosion deserves its own engineering logic.

Carbon steel pitting corrosion may be driven by deposits, differential aeration, stagnation, microbial activity or breakdown of protective treatment films.

Stainless steel pitting corrosion centers more directly on local instability of the passive surface, with chloride, temperature, oxidizing conditions, metallurgy and fabrication all influencing resistance.

Neither can be controlled reliably through one universal water-quality number.

The strongest pitting corrosion prevention strategy therefore combines:

appropriate metallurgy,

good hydraulic design,

deposit control,

stable corrosion treatment,

controlled chlorination,

credible operating limits,

shutdown discipline,

and risk-based inspection.

And the strongest pitting corrosion monitoring program does not ask only:

“How much metal did we lose on average?”

It asks:

Where is the deepest damage?

Why there?

When did it begin?

What local chemistry sustained it?

What is the remaining wall?

And where else in the system can the same conditions exist?

Once those questions are answered, a pinhole leak stops being an isolated maintenance event.

It becomes what it really is:

a map of the system's hidden localized-corrosion risk.

Focused FAQ

What is pitting corrosion?

Pitting corrosion is a highly localized form of metal attack in which small anodic regions penetrate into the metal while much of the surrounding surface remains comparatively unaffected. Because penetration is concentrated, severe damage can occur with relatively little total metal loss.

Why is pitting corrosion dangerous in cooling-water systems?

Cooling water pitting corrosion can cause pinhole leaks and tube perforation even when average corrosion rates, bulk-water chemistry and general equipment appearance remain acceptable.

What causes pitting corrosion in cooling water?

Risk factors can include chloride, low flow, stagnation, deposits, temperature, oxidizing conditions, surface defects, metallurgical differences, poor fabrication, microbial activity and unstable protective films. The actual mechanism depends on the metallurgy and local environment.

Does chloride always cause pitting corrosion?

No. Chloride is a major contributor to chloride pitting corrosion, particularly in passivating alloys, but risk also depends on temperature, pH, oxidation potential, alloy grade, surface condition, flow and other environmental factors.

Why does the inside of a pit become more corrosive than bulk water?

Metal dissolution, ion migration and hydrolysis can create a more concentrated and acidic local environment inside an established pit. This allows pit chemistry to differ substantially from the bulk cooling-water sample.

Can good water analysis coexist with severe pitting?

Yes. Routine samples describe bulk water at a particular location and time. Pitting is controlled by local surface conditions and historical events, so an aggressive microenvironment can exist even when routine water analysis appears acceptable.

Why is stagnant water associated with pitting?

Low-flow and stagnant conditions can promote concentration differences, deposit accumulation, differential aeration and poor inhibitor transport, all of which can increase pitting corrosion in pipes and other equipment.

What is passive-film breakdown?

Passive film breakdown occurs when the thin protective oxide layer on a passivating material such as stainless steel becomes locally unstable. If the exposed location cannot repassivate, localized dissolution can develop into a pit.

Why does stainless steel suffer pitting if it is corrosion resistant?

Stainless steel pitting corrosion occurs because corrosion resistance depends on a very thin passive film. Most of the surface can remain passive while a small local region becomes active, especially in chloride-containing environments.

Can carbon steel experience pitting corrosion?

Yes. Carbon steel pitting corrosion can occur where deposits, differential aeration, stagnation, local inhibitor failure, microbial activity or other environmental differences create stable local anodic sites.

What is PREN?

PREN is the Pitting Resistance Equivalent Number, a composition-based index used to compare the relative pitting resistance of stainless steels in chloride-containing environments. A commonly used relationship considers chromium, molybdenum and nitrogen. PREN is useful for ranking materials but is not a field-performance guarantee.

What is critical pitting temperature?

Critical pitting temperature is a laboratory-derived temperature used under specified test conditions to compare the resistance of alloys to pit initiation. It is valuable for relative material ranking but should not be interpreted as a universal maximum service temperature.

What is ASTM G48?

ASTM G48 provides standardized laboratory methods for comparing the pitting and crevice corrosion resistance of stainless steels and related alloys in aggressive ferric-chloride environments, including methods for determining critical pitting and crevice temperatures.

What is ASTM G46?

ASTM G46 is a guide for examining and evaluating pitting corrosion. It supports evaluation of the severity and distribution of pitting for laboratory studies and field equipment assessments.

Can corrosion coupons detect pitting?

Coupons can provide valuable physical evidence of pitting, particularly when visual examination and pit-depth assessment are included. Average coupon weight loss alone can underestimate localized penetration.

What is the difference between pitting corrosion and under-deposit corrosion?

Under-deposit corrosion specifically develops beneath deposits that alter the local environment. Pitting corrosion is a broader localized mechanism and can initiate with or without deposits. A deposit may therefore be one pitting initiator but is not required for every pit.

Is pitting corrosion the same as MIC?

No. Microbiologically influenced corrosion can produce or accelerate localized attack, but the presence of bacteria near a pit does not by itself prove MIC. Biological, chemical, metallurgical and operating evidence should be evaluated together.

How can plants monitor pitting corrosion?

A strong pitting corrosion monitoring program can combine coupon pit-depth evaluation, visual inspection, ultrasonic thickness mapping, tube inspection, deposit analysis, water-chemistry trends and inspection of known high-risk locations.

How can pitting corrosion be prevented?

Effective pitting corrosion prevention combines appropriate material selection, hydraulic design, deposit control, chloride and concentration management, compatible biocide control, stable corrosion-inhibitor treatment, shutdown procedures and risk-based inspection.

Should a plant simply upgrade to a more corrosion-resistant stainless steel after pitting?

Not automatically. A higher-alloy material can provide greater localized-corrosion resistance, but the root cause should first be identified. Persistent stagnation, deposits, poor fabrication or extreme operating excursions can continue to create risk even after a material upgrade.

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