AC CORROSION ISSUES PREDICTION & MITIGATION

AC CORROSION ISSUESPREDICTION & MITIGATIONByAllen Carlilewww.arkengineering.com

IntroductionAlthough not all aspects of the ACCorrosion mechanism have been fullyclarified, experience has reached a levelfrom which general guidelines have beendeveloped. AC Corrosion Risk Assessment, ACcorrosion mitigation , and AC corrosionmonitoring.

Overview NACE - Task Group 430 – Issued a documenttitled: “Alternating Current Corrosion onCathodically Protected Pipelines: RiskAssessment, Mitigation, and Monitoring” in May2018.NACE SP21424-2018This standard practice presents guidelines andprocedures for use during risk assessment,mitigation, and monitoring.

NACE Standard SP21424-2018AC Corrosion on a cathodically protectedunderground pipeline is commonly theresult of a combined action of the ACvoltage, the CP conditions, a coatingdefect, and the chemical and physicalconditions of the soil. If the AC component is removed orlimited, the corrosion will be mitigated.

NACE Standard SP21424-2018AC Corrosion is also influenced by DCcurrent. It can also be reduced by adjustingthe DC component through the CP system. An AC Corrosion evaluation processincludes an analysis to develop thefollowing strategies:

Evaluation ProcessAnalysis – Risk Assessment Mitigation Strategy Monitoring Strategy On-going monitoring to determine safe orunsafe conditions Used for new pipelines, new interferencesource, or existing pipelines

AC Interference vs. ACCorrosionInductive and conductive effects as a resultof AC current flowing in electric circuits. AC voltage and currents are induced uponthe pipeline. Where these AC currents leave thepipeline through coating defects, they cancause AC corrosion effects. The intensity is measured in A/m2

OverviewInduced AC voltage may be a cause ofcorrosion at coating defects where ACcurrent escapes the pipe. Small rather than large coating defects aresusceptible to AC corrosion since thespread resistance (in Ω-m2) associated withthe defects increases with increasing area

Spread ResistanceThe spread resistance RS is controlled byfactors relating to the resistance of the soil,porosity, and geometric factors in theinterface between the soil and the coatingdefect. Spread resistance is approximately theproportionality factor between AC voltageand AC density.

Spread Resistance

Spread Resistance A non-coated surface results in only a soilresistance value.A coated surface with a defect results in both soilresistance and spreading resistance values. Alarge IR drop develops near the vicinity of thepipe to soil interface where the coating defect ispresent.A geometrical spread effect is produced as aresult of concentrated current flux lines.

Effect of Surface AreaThe surface area of the pipe at a coatingholiday is important since the corrosionrate increases with increasing currentdensity. Large holidays would have a lower currentdensity than small holidays if both wereexposed to the same soil conditions.

AC Corrosion RisksThe following factors increase ACcorrosion risks: A low level of cathodic protection (lowDC current density) with a high level ofAC current density Small size coating defects Low soil resistivity

Signs of Coating Failure and AC Corrosion –Polyethylene CoatingPushed up coatingdue to corrosionby product

Nodule Out Side ofPolyethylene Coating

Risk AssessmentAC Corrosion Evaluation – If an ACCorrosion risk is present, then ACinterference calculations (analysis), ACsurvey, evaluation of historic CP data andabnormalities, DC interference, ILI results,other relevant data should be reviewed. New AC circuits or DC interferencesources may cause additional risk

AC Corrosion By Products CoatingBreakDown Magnetite PushedUpCoating

Elevated pH

Circular Morphology, Magnetite

Elevated pH

Coating Distortion

Risk AssessmentNew pipelines with interfering AC systems Pipeline Corrosion History Records mayrequire re-evaluation in view of ACcorrosion characteristics. AC Voltage measurements DC CP Potentials – Inadequate orExcessive Soil Resistivity Measurements

Risk AssessmentSoil resistivity surveys – the spreadresistance is influenced by the soilresistivity. The following soil resistivity parameterscan be applied as a risk guideline: Below 25 Ω-m: very high risk Between 25 Ω-m and 100 Ω-m: high risk Between 100 Ω-m and 300 Ω-m: Medium

Circular Morphology ActiveACCorrosion

Risk AssessmentDC Interference Effects Coupon or Corrosion Rate Probe Data –AC & CP Densities or SignificantCorrosion Rates

AC Corrosion - pH is Always 13-14 Previous pH13-14coating repairareas jeeped in inspectionat 12:00

CriteriaAC Voltage – mitigated to a level wherethe current densities are met. AC Density – recommended not to exceed:30 A/m2 – if DC Density exceeds 1 A/m2100 A/m2 – if DC Density less than 1 A/m2 Less than 0.025 mm/y corrosion rate usingweight loss coupons or probes

CriteriaConsecutive use of inspection tools may beused to quantify corrosion attacks AC and DC current densities are measuredat coupons installed along the pipeline The above criteria should be documentedfor a representative period of timeaccounting for variations in the influencingparameters.

Research StudiesAC Voltage required to produce a currentdensity of 100A/m2 in 1000 ohm-cm soil ata 1 cm2 holiday: iac 8 Vac / p π d Where:iac ac current density (A/m2)Vac pipe ac voltage to remote earth (V)p soil resistivity (ohm-m)

Research Studies d diameter of a circular holiday having a1 cm2 surface area 0.0113 mThen: for iac 100A/m2 and p 10 ohm-mVac 100A/m2 10 ohm-m 3.14 0.0113m8Vac 4.4 V

Results The calculation indicates that CP protectedpipelines subjected to AC voltages that arebelow the NACE recommended maximumsafe level of 15 volts (NACE SP0177) cansuffer from AC corrosion at holiday siteshaving a surface area of approximately 1cm2 in a soil resistivity of 3000 ohm-cm orless.

MitigationAC Voltage Mitigation – Reference NACEStandard SP0177-2104 CP System Condition – Create uniformpolarized potentials along the pipeline andminimize voltage drops. Minimize the riskof inadequate CP and excessive CP. DC Interference effects – ReferenceNACE Standard SP0169

Proving Effectiveness

Other Factors Mitigation wire provides a benefit in themitigation of AC corrosion. For a coatingholiday located in the vicinity of themitigation wire, the effective resistance ofthe holiday is increased due to the mutualresistance between the holiday and themitigation wire, thereby reducing the ACcurrent density at the holiday.

Monitoring StrategiesTest Stations and Monitoring Locations –Selected where the risk analysis indicates arisk of AC corrosion: High AC voltages Low Soil Resistivity Excessive CP conditions or DC intf. Previous ILI or excavation locations Coupon or Probe locations of high values

Coupon Test Station

Monitoring StrategiesCoupon Test Stations Corrosion Rate Measurements Data Loggers Remote Monitoring Units

Coupon T/S - DCD – Remote Monitor

Coupons or Probe InstallationCoupon test stations or ER Probes areinstalled on the pipeline, at specificintervals to measure the DC potentials, ACpotentials, and current densities. This test equipment provides the necessarydata to assess the likelihood that ACinterference is contributing to the observedcorrosion.

Long Term Monitoring Monitoring AC corrosion is a dynamic process,to provide AC corrosion criteria information.This monitoring will determine safe or unsafelocations. Unsafe locations are areas where theparameters have been exceeded and require arenewed risk assessment analysis or a renewedmitigation strategy, in an iterative process.

Thank You! Any Questions

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NACE Standard SP21424-2018 AC Corrosion on a cathodically protected underground pipeline is commonly the result of a combined action of the AC voltage, the CP conditions, a coating defect, and the chemical and physical co