Internal Corrosion Review

CO2 corrosion

Conditions required:

• Requires free water or a water phase, corrosive gas chemistry, and operating conditions that support carbonic-acid attack.
• More credible where water dropout, low spots, separators, slug catchers, or changing pressure/temperature conditions allow water to remain in contact with the pipe wall.

Typical locations:

• Low points, drips, downstream of pressure reductions, slug-prone lines, and sections with intermittent water transport.

Data indicators:

• Broad internal metal loss or localized attack where water handling and CO2 service history support the mechanism.
• Ops records showing water carryover, high water cut, upset conditions, or weak inhibitor control.

H2S / sour corrosion

Conditions required:

• Requires sour service exposure with water present and chemistry that supports sulfide corrosion products or related localized attack.
• Often reviewed with extra caution when water chemistry, solids, or deposits make localized attack harder to interpret.

Typical locations:

• Wet sour service lines, low points, facilities tie-ins, and areas where water and solids settle out of the stream.

Data indicators:

• Localized internal attack in sour service together with monitoring or chemistry data that support a corrosive water phase.
• Mismatch between expected mitigation performance and observed metal-loss behavior.

MIC (microbiologically influenced corrosion)

Conditions required:

• More credible where water, nutrients, temperature, stagnation, and deposit build-up allow microbial activity to persist.
• Usually needs both environmental credibility and morphology or field evidence; do not call MIC from morphology alone.

Typical locations:

• Dead legs, low-flow branches, drips, under deposits, low spots, and areas with poor pigging effectiveness or prolonged water hold-up.

Data indicators:

• Highly localized pitting, recurring attack despite nominal chemistry control, or field evidence from deposits, coupons, or microbiological testing.
• Poor biocide distribution, inconsistent cleaning, or repeated corrosion in the same stagnant areas.

Under-deposit corrosion

Conditions required:

• Requires solids, deposits, wax, scale, black powder, or other accumulation that creates a shielded environment at the pipe wall.
• Often persists when cleaning is ineffective or inhibitor cannot reliably reach the corroding surface.

Typical locations:

• Low spots, dead legs, drips, pigging dead zones, flow-disturbance locations, and areas downstream of solids generation.

Data indicators:

• Localized or irregular attack associated with solids loading, poor pigging history, or field evidence of deposits at the dig.
• Coupon or probe data that under-represent the local condition because the worst area sits under deposits rather than in the bulk stream.

Erosion-corrosion / flow-assisted corrosion

Conditions required:

• Requires velocity, turbulence, impingement, entrained solids, or geometry that repeatedly strips protective films from the wall.
• Most credible where flow behavior is aggressive enough to keep the mechanism active rather than as a one-time historical condition.

Typical locations:

• Elbows, tees, reducers, downstream of choke points, injection points, and other flow-disturbance or high-velocity regions.

Data indicators:

• Attack concentrated near flow disturbances, high-velocity service, solids production, or repeated upset events.
• Feature orientation or repeated damage at geometry changes that aligns better with flow-assisted attack than simple low-point water hold-up.

Step 1: Is internal corrosion credible at this location?

• If the feature sits in a low point, dead leg, tee, lateral, drip, reducer, or known upset-prone area and water or solids are credible, keep internal corrosion on the table.
• If the location does not support water hold-up or disturbed flow, challenge the mechanism before escalating the feature as active internal corrosion.
• Do not let the absence of prior reported internal corrosion close the mechanism by itself; early-stage attack may have existed before it became detectable or reportable in ILI data.
• If location credibility is weak but service conditions are severe, collect more configuration or flow data before closing the mechanism call.

Step 2: Does the ILI morphology fit the proposed mechanism?

• If morphology looks localized, pitted, channeled, or clustered in an internal-corrosion trap, lean toward active internal corrosion rather than generic legacy metal loss.
• Remember that detectability is not binary: the reported ILI feature may represent only the detectable portion of a developing threat rather than the full corrosion footprint.
• If earlier runs were dominated by external calls and a later run suddenly shows a large shift to internal classification, do not treat that shift alone as proof of a new active internal mechanism.
• If morphology is broad and smooth but operating evidence supports long-term wet service, consider whether the feature may be older legacy attack rather than current rapid growth.
• If morphology and environment disagree, flag the mismatch and move the case into validation rather than forcing a mechanism label.

Step 3: Does the classification shift make engineering sense?

• Compare the current run with prior runs before accepting a major internal/external classification break as a real threat change.
• Check whether wax, debris, deposits, or eddy-current sensor lift-off could have changed signal response and made internal calls less reliable.
• Review tool type, vendor notes, feature comments, and any interpretation changes before concluding that a sudden increase in internal calls reflects a new or worsening mechanism.

Step 4: Is the pattern more consistent with MIC, CO2 attack, under-deposit, or flow-assisted corrosion?

• If localized pitting recurs in stagnant or deposit-prone areas and biocide / cleaning performance is questionable, MIC or under-deposit mechanisms move up the list.
• If wet CO2 service and water dropout are credible and attack is broader or low-point-driven, CO2 corrosion becomes more credible.
• If attack concentrates near tees, elbows, reducers, or injection points with high-velocity or solids service, flow-assisted mechanisms become more credible.

Step 5: Does the combined evidence suggest active or legacy damage?

• If prior ILI, digs, coupons, probes, or operating history show recurring attack or ineffective mitigation, treat the condition as potentially active until evidence says otherwise.
• If operating conditions, water hold-up, pigging performance, inhibitor performance, or monitoring data support internal corrosion before ILI clearly does, keep threat credibility separate from reported severity alone.
• If the classification shift is not supported by operating evidence, pigging history, wax or debris behavior, or field validation, keep confidence reduced and document why the mechanism remains uncertain.
• If the feature is stable across comparison, service conditions have changed favorably, and field history supports inactive damage, a legacy interpretation may be reasonable.
• If comparison data are poor or inconsistent, do not overstate certainty. Document that the mechanism or activity state remains only partially resolved.

Corrosion inhibitors

• Best used when the corrosive mechanism is chemistry-driven and inhibitor can actually reach the wetted surface.
• Often underperform when water is trapped, deposits shield the wall, or flow conditions prevent consistent distribution.
• If inhibitor is in place but localized attack continues, question distribution, dosage, water behavior, and deposit control rather than assuming treatment is working.

Pigging and mechanical cleaning

• Most useful when deposits, black powder, wax, scale, or solids are part of the corrosion mechanism.
• Mechanical cleaning often has to accompany chemical treatment because chemistry alone may not reach shielded metal under deposits.
• If pigging is infrequent or ineffective, under-deposit corrosion and MIC credibility usually stay higher.

Biocide treatment

• Best used when MIC is credible from environment, deposits, recurrence, and field evidence rather than from pitting morphology alone.
• Poor distribution, long stagnant areas, and inadequate cleaning can make biocide programs look better on paper than in the pipe.
• Use with location-specific thinking; dead legs and low-flow areas may not see the same treatment effectiveness as the main line.

Flow or operating changes

• Useful when slugging, low-flow operation, intermittent service, or high-velocity flow-assisted attack is part of the mechanism.
• Changing flow, water handling, or upset frequency can materially change corrosion risk when chemistry alone cannot stabilize the environment.
• Treat operating changes as part of the mitigation program, not as background information only.