CP Survey Fundamentals

1. Cathodic Protection Principles

Cathodic protection (CP) is an electrochemical technique that prevents corrosion of a metal surface by making it the cathode of an electrochemical cell. For buried steel pipelines, CP is the primary method for controlling external corrosion and is required by federal regulation (49 CFR 192 for gas, 49 CFR 195 for hazardous liquids).

Electrochemical Basis

Corrosion Cell on Buried Steel: Anodic reaction: Fe → Fe(2+) + 2e- (metal dissolution) Cathodic reaction: O2 + 2H2O + 4e- → 4OH- (oxygen reduction) Cathodic Protection Mechanism: External current forces the entire pipe surface cathodic: - All anodic sites are suppressed - Only cathodic (protective) reaction occurs - Corrosion rate drops to near zero Two CP Methods: 1. Galvanic (Sacrificial) Anodes: Mg, Zn, or Al - Driving voltage: 0.3-1.0 V - Self-regulating, limited current output - Typical for well-coated, small-diameter pipe 2. Impressed Current: External DC power supply - Driving voltage: 1-50 V (adjustable) - High current output capability - Required for large diameter, long pipelines

Why Both Coating and CP Are Needed

Coating and cathodic protection work synergistically. The coating reduces current demand by preventing electrolyte contact with most of the steel surface. CP protects the pipe at coating holidays (bare spots, damage) where corrosion would otherwise occur. Without coating, the CP current demand would be prohibitively high. Without CP, even small coating defects would allow localized corrosion to proceed unchecked.

Coating Only

Incomplete Protection

Coating defects, holidays, and age-related degradation leave bare steel exposed. Localized corrosion at defects can be severe.

CP Only

Impractical

Current density for bare steel: 1-5 mA/ft2. For a 12-inch, 50-mile pipeline, this requires enormous power and anode beds.

Coating + CP

Complete Protection

Good coating reduces demand to 0.01-0.1 mA/ft2. CP protects at holidays. Industry standard approach per NACE SP0169.

Regulatory requirement: 49 CFR 192.463 requires that each buried or submerged steel pipeline must have cathodic protection achieving one of the criteria in 49 CFR 192.463(d), which references NACE SP0169. Operators must test at least once per calendar year (not exceeding 15 months) per 49 CFR 192.465.

2. CP Criteria

NACE SP0169 (formerly RP0169) defines several criteria for establishing adequate cathodic protection. The most widely used is the -850 mV Cu/CuSO4 criterion. Understanding the basis and limitations of each criterion is essential for accurate interpretation of survey data.

Primary Criterion: -850 mV

NACE SP0169 Section 6.2.2.1: A negative (cathodic) potential of at least -850 mV with the cathodic protection applied, measured with respect to a saturated copper/copper sulfate (Cu/CuSO4) reference electrode contacting the electrolyte. Important qualifications: - Potential INCLUDES IR drop in the reading - SP0169 states consideration should be given to suppressing (eliminating) the IR drop - For compliance, most operators use IR-free (instant-off) readings per 49 CFR 192 Physical basis: At -850 mV Cu/CuSO4 (-780 mV SHE), the steel surface is polarized sufficiently negative that the anodic dissolution rate of iron is reduced to a negligible level (<1 mpy) in most soil environments.

Alternative Criteria

Criterion	NACE SP0169 Ref	Description	When Used
-850 mV	6.2.2.1	Negative potential of -850 mV or more	Most common, general use
100 mV Shift	6.2.2.2	100 mV polarization from native potential	When -850 criterion cannot be met, high-resistivity soils
-950 mV	6.2.2.3	More negative criterion for SRB environments	Anaerobic soils, sulfate-reducing bacteria
E-log-I	6.2.2.3	Net protective current from E-log-I curve	Research, interference analysis

Overprotection Concerns

While adequate protection requires sufficiently negative potentials, excessive cathodic polarization (overprotection) can cause several problems. Potentials more negative than -1200 mV Cu/CuSO4 can cause cathodic disbondment of coatings, generate hydrogen at the steel surface (risking hydrogen embrittlement of high-strength steels), and waste CP current. NACE SP0169 recommends limiting potentials to no more negative than -1200 mV for most steels and coatings.

Critical distinction: The -850 mV criterion with CP applied (ON potential) includes IR drop, which inflates the reading. The same criterion with IR drop eliminated (instant-OFF) is more conservative and more accurate. Most pipeline operators and regulators now require IR-free measurements for compliance assessment.

3. Measurement Techniques

Accurate pipe-to-soil potential measurement is the foundation of all CP assessment. Measurement errors from poor technique can easily exceed 100 mV, which is the difference between adequate and inadequate protection in many cases.

Basic Measurement Setup

Pipe-to-Soil Potential Measurement: V_ps = V_pipe - V_ref Equipment: - High-impedance voltmeter (10 Mohm minimum) - Cu/CuSO4 reference electrode (CSE) - Test lead connection to pipe (test station) - Reference electrode placed on ground surface directly above or as close as practical to pipe Measurement includes three components: V_ps(measured) = V_polarization + V_IR + V_junction V_polarization = true pipe-to-electrolyte potential V_IR = voltage drop through soil (error) V_junction = liquid junction potential (usually small)

Reference Electrodes

Type	Potential vs SHE	Equivalent -850 CSE	Application
Cu/CuSO4 (CSE)	+318 mV	-850 mV	Standard for buried pipelines
Ag/AgCl	+222 mV	-754 mV	Marine and brackish water
Zinc	-763 mV	+231 mV	Permanent buried reference
SHE	0 mV	-532 mV	Laboratory standard

Measurement best practice: Place the reference electrode directly above the pipe at grade level. Ensure good electrolytic contact (wet the soil if dry). Use a high-impedance voltmeter (at least 10 megohms) to prevent current draw through the reference electrode. Verify reference electrode accuracy monthly against a known standard.

4. IR Drop Correction

IR drop is the single largest source of error in pipe-to-soil potential measurements. It is the voltage gradient in the soil between the pipe surface and the reference electrode, caused by CP current flowing through the soil resistance. IR drop always makes readings appear more negative (more protected) than the pipe actually is.

IR Drop Magnitude

IR Drop Estimation: V_IR = I_cp × R_soil Where: V_IR = IR drop voltage (mV) I_cp = CP current flowing to pipe at measurement point R_soil = Soil resistance between pipe and reference electrode Factors affecting IR drop magnitude: - Higher CP current = more IR drop - Lower soil resistivity = less resistance but often more current - Reference electrode farther from pipe = more IR drop - Coating holidays near measurement point = localized current Typical IR drop values: Well-coated pipeline, remote anode: 20-50 mV Poorly coated, close anode: 50-200 mV Near rectifier ground bed: 100-500+ mV True polarized potential: V_true = V_on - V_IR

Correction Methods

Method	Technique	Accuracy	Limitations
Instant-OFF	Interrupt CP current, read within 0.1-1 s	High	Requires current interrupter at all rectifiers
Coupon	Measure potential on disconnected coupon	Very high	Point measurement only, requires coupon installation
Reference Electrode Spacing	Multiple readings at varying distances	Moderate	Time-consuming, requires clear ground
Calculation	Estimate IR from current and soil resistivity	Low	Too many unknowns for reliable correction

Industry consensus: The instant-off technique (current interruption) is the most practical and reliable method for IR drop correction in field surveys. All rectifiers and galvanic anode circuits must be simultaneously interrupted using synchronized GPS-based interrupters. The OFF reading is taken within the first second after interruption, before significant depolarization occurs.

5. Close-Interval Surveys (CIS)

A close-interval survey (CIS), also called close-interval potential survey (CIPS), measures pipe-to-soil potentials at closely spaced intervals (typically 2.5-5 feet) along the entire pipeline. It is the most comprehensive tool for evaluating the effectiveness of cathodic protection and identifying areas of concern. CIS methodology is defined in NACE SP0207.

CIS Methodology per NACE SP0207

CIS Survey Procedure: 1. Synchronize current interrupters at ALL rectifiers and foreign CP sources affecting the pipeline. Typical cycle: 3-4 seconds ON, 1 second OFF. 2. Surveyor walks above the pipeline with: - Trailing wire connected to pipe at test station - Cu/CuSO4 reference electrode on ground - Data logger recording ON and OFF potentials 3. Readings taken at regular intervals: - Standard: every 5 feet (1.5 m) - Detailed: every 2.5 feet (0.75 m) - Minimum: every pipeline diameter length 4. GPS coordinates recorded for each reading. 5. Data processed to generate potential profile plot. Key outputs: - ON potential profile (with IR drop) - OFF potential profile (IR-free) - IR drop profile (ON minus OFF) - Areas failing criterion highlighted

Interpreting CIS Data

Uniform OFF potentials: Indicate good coating and well-distributed CP. Typical of new or well-maintained systems.
Localized positive excursions: Suggest coating damage, shielding (rock shield, casings), or insufficient CP current. These are the highest priority for investigation.
High IR drop areas: Large difference between ON and OFF indicates coating holidays with high current demand. May indicate corrosion activity.
Gradual ON potential decline: Between test stations indicates current attenuation along the pipeline. May need additional anode installation.
Interference patterns: Cyclic or step-change patterns may indicate stray current from foreign pipelines, transit systems, or mining operations.

Survey frequency: NACE SP0207 does not mandate a specific frequency. However, many operators perform CIS every 5-10 years on critical pipelines, more frequently on pipelines with known integrity threats. PHMSA recommends CIS as part of external corrosion direct assessment (ECDA) per NACE SP0502.

6. Coating Assessment from CP Data

CP survey data provides indirect information about coating condition. While direct inspection (excavation) is the definitive assessment, survey data patterns can identify areas of concern and prioritize direct examination locations.

Coating Condition Indicators

Indicator	Good Coating	Degraded Coating	Failed Coating
IR Drop	<30 mV, uniform	30-80 mV, variable	>80 mV, highly variable
ON-OFF Spread	Consistent along route	Moderate variation	Large swings, localized peaks
Current Demand	<0.1 mA/ft2	0.1-1.0 mA/ft2	>1.0 mA/ft2
Coating Resistance	>50,000 ohm-ft2	5,000-50,000 ohm-ft2	<5,000 ohm-ft2

Coating Types and Expected Life

Coating Type	Expected Life	Degradation Mode	CP Compatibility
Fusion-Bonded Epoxy (FBE)	30-50 years	Cathodic disbondment at holidays	Excellent
Polyethylene (PE)	40-60 years	Mechanical damage, soil stress cracking	Good (can shield CP if disbonded)
Coal Tar Enamel	20-40 years	Embrittlement, cracking	Good
Tape Wrap	15-25 years	Adhesion loss, tenting, shielding	Poor (shields CP when disbonded)

CP shielding warning: Some coatings (tape wrap, shrink sleeves, rock shields) can create a gap between the coating and steel when they disbond. This gap prevents CP current from reaching the steel surface, creating corrosion under disbonded coating that CP surveys cannot detect. Tape wrap coatings are particularly problematic and are no longer recommended for new construction by most operators.

7. Soil Effects on CP

Soil properties significantly affect both the corrosion rate of bare steel and the performance of cathodic protection systems. Soil resistivity is the single most important soil parameter, as it affects current distribution, IR drop magnitude, and corrosion severity.

Soil Resistivity Classification

Resistivity (ohm-cm)	Corrosivity	CP Current Demand	Common Soil Types
<1,000	Very Corrosive	Very High	Salt marsh, marine clay, contaminated
1,000-2,000	Severe	High	Clay, wet silt, brackish water table
2,000-5,000	Corrosive	Moderate	Loam, moist clay, alluvial deposits
5,000-10,000	Moderate	Low	Sandy clay, dry silt, gravel
>10,000	Low	Very Low	Dry sand, rock, caliche

8. Data Analysis Methods

Statistical Analysis of CIS Data

Key Statistics for CP Survey Data: Mean potential: V_avg = (1/N) × sum(V_i) Standard deviation: sigma = sqrt(sum((V_i - V_avg)^2) / (N-1)) Compliance metrics: % Adequate = (points meeting criterion / total points) × 100 % Marginal = (points within 50 mV of criterion / total) × 100 % Inadequate = (points failing criterion / total) × 100 IR Drop analysis: IR_i = V_on_i - V_off_i (for each point) IR_avg = mean of all IR drops IR_std = standard deviation of IR drops Coating condition indicator: High IR variability (std dev > 40 mV) suggests non-uniform coating with localized holidays.

Trend Analysis

Comparing CIS data from successive surveys reveals trends in CP system performance and coating degradation. Key indicators include declining OFF potentials at specific locations (indicating increasing corrosion activity), increasing IR drop variability (coating degradation), and systematic shifts in potential profiles (rectifier output changes or anode depletion).

Year-over-year comparison: Plot OFF potentials from multiple survey years on the same distance axis. Locations showing progressive positive shift warrant investigation.
Seasonal effects: Soil moisture and temperature affect resistivity. Surveys performed in dry seasons may show more positive readings than wet-season surveys. Normalize for season when comparing data.
Interference detection: Dynamic stray current from DC transit or mining operations creates time-varying interference. Use 24-hour recordings at suspect locations to characterize interference patterns.

9. Troubleshooting CP Problems

Common Issues and Solutions

Problem	Symptom in CIS Data	Root Cause	Solution
Shielded pipe	Low OFF potential despite high ON	Disbonded coating, rock shield, or casing prevents CP current from reaching steel	Excavate, remove shielding, recoat
Insufficient current	Gradual decline in potential between test stations	Anode depletion, high coating demand, insufficient rectifier output	Increase rectifier output, add anodes, repair coating
Interference	Cyclic or step-change potential patterns	Foreign CP system or DC transit current	Interference bond, drainage, coordination with foreign operator
Broken wire	Abrupt potential change at specific location	Damaged test lead or broken pipe-to-anode connection	Locate and repair break
Overprotection	Very negative potentials (< -1200 mV OFF)	Excessive rectifier output, close proximity to anode bed	Reduce output, add resistance, relocate anodes

10. Industry Standards

Standard	Title	Relevance
NACE SP0169	Control of External Corrosion on Underground or Submerged Metallic Piping Systems	Primary CP criteria and design standard
NACE SP0207	Performing Close-Interval Potential Surveys	CIS methodology and data interpretation
NACE SP0502	Pipeline External Corrosion Direct Assessment (ECDA)	Integrates CIS data into ECDA process
NACE TM0497	Measurement Techniques Related to Criteria for CP	IR drop compensation methods
49 CFR 192	Transportation of Natural Gas by Pipeline	Federal CP requirements for gas pipelines
49 CFR 195	Transportation of Hazardous Liquids by Pipeline	Federal CP requirements for liquid pipelines
NACE SP0286	Electrical Isolation of CP Systems	Isolation joint design and testing
NACE SP0572	Design and Installation of Offshore CP Systems	Marine and offshore CP applications

-850 mV Cu/CuSO4

100 mV Polarization Shift

NACE SP0207 · SP0169

Contents

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