AMPP CP2 Technician cheat sheet

Ohm's law: V = I × R; power: P = V × I = I²R = V²/R for DC circuits.

Series resistances add directly: R_total = R1 + R2 + …; parallel resistances combine as 1/R_total = 1/R1 + 1/R2 + …

Kirchhoff's voltage law: the algebraic sum of voltages around a closed loop is zero; Kirchhoff's current law: the sum of currents entering a node equals the sum leaving.

A CP circuit consists of the DC source, anode or groundbed resistance, electrolyte resistance, and structure resistance; total circuit resistance limits protection current.

Anode-to-electrolyte resistance usually dominates the CP circuit because steel structure resistance is very low.

Current divides among parallel paths in inverse proportion to their resistance; coating holidays are low-resistance paths that carry most CP current.

Wire resistance: R = ρL/A, where ρ is resistivity, L is length, and A is cross-sectional area.

In steady-state DC CP analysis, capacitance and inductance are generally neglected; AC interference studies require impedance and coupling analysis.

NACE SP0169 criterion: a structure-to-electrolyte potential of at least −850 mV versus a copper/copper sulfate reference electrode with CP applied, accounting for IR drop.

Alternative criterion: at least 100 mV of polarization, measured as the shift from the native potential to the polarized potential.

The 850 mV current-applied criterion uses the potential measured while CP current is flowing and may include IR drop if not corrected.

Native potential for bare steel in soil is typically about −0.4 to −0.6 V versus Cu/CuSO4.

Polarization is the negative shift in potential caused by CP current; depolarization of at least 100 mV confirms adequate polarization.

IR drop arises from current flowing through soil and coating between the structure and reference electrode; instant-off readings reduce its effect.

Overprotection below about −1.2 V versus Cu/CuSO4 can cause coating disbondment, hydrogen evolution, and hydrogen embrittlement of high-strength steels.

CP reduces corrosion by making the structure the cathode; the anodic reaction occurs on the anode or groundbed.

Structure-to-electrolyte potentials are measured with a high-input-impedance voltmeter and a stable reference electrode such as Cu/CuSO4.

Instant-off potential is read immediately after interrupting all CP current; this removes IR drop and approximates the polarized potential.

Close interval survey (CIS) records pipe-to-soil potentials at closely spaced intervals, commonly 1 to 2.5 m along a pipeline.

Calibrate reference electrodes before and after surveys; a contaminated Cu/CuSO4 electrode produces biased potentials.

Wenner four-pin soil resistivity: ρ = 2πaR, where a is pin spacing and R is measured resistance; convert units to ohm-cm as needed.

DCVG detects coating defects by measuring voltage gradients in the soil near a holiday; ACVG uses an AC signal for similar defect location.

Current interruption and current mapping help identify current sources, interference, and IR-drop effects on potential readings.

Test stations provide permanent access to the structure for potential and current measurements and should be clearly identified.

Galvanic anode current: I = driving voltage / total circuit resistance, where driving voltage is the difference between structure potential and anode open-circuit potential.

Typical open-circuit potentials versus Cu/CuSO4: magnesium about −1.75 V, zinc about −1.10 V, aluminum about −1.05 V in seawater.

Magnesium anodes suit moderate-to-high resistivity soils; zinc anodes suit low-resistivity soils and seawater; aluminum anodes are mainly for seawater.

Anode backfill, such as gypsum and bentonite, lowers anode-to-soil resistance and promotes uniform anode consumption.

Typical consumption rates are about 8.8 lb/A·yr for magnesium, 25 lb/A·yr for zinc, and 7 lb/A·yr for aluminum.

Typical anode efficiencies are about 50% for magnesium and about 90% for zinc and aluminum.

Multiple galvanic anodes are connected in parallel to increase current output; spacing reduces mutual interference.

Galvanic systems are limited by low driving voltage and are generally unsuitable for high current demand or very high-resistivity environments.

A rectifier converts AC to DC using a transformer and rectifier stack; output is adjusted by tap settings or a controlled firing circuit.

Rectifier DC output current equals the driving voltage divided by total circuit resistance, dominated by groundbed resistance.

Vertical anode-to-soil resistance: R = (ρ/(2πL))[ln(8L/d) − 1], where ρ is soil resistivity, L is anode length, and d is anode diameter.

Coke breeze around impressed-current anodes lowers groundbed resistance and slows anode consumption by redistributing current.

Common impressed-current anode materials include mixed metal oxide, graphite, high-silicon cast iron, and polymer or conductive concrete anodes.

Rectifiers should have AC surge protection, lightning arrestors, and proper grounding; verify polarity before energizing.

A remote groundbed improves current distribution and reduces interference compared with an anode close to the structure.

Groundbed resistance increases as soil resistivity rises or as anodes are consumed; rectifier voltage margin allows for this increase.

Stray current is current flowing through unintended paths; corrosion occurs where stray current discharges from a metallic structure into the electrolyte.

Common DC stray-current sources include transit systems, welding operations, foreign cathodic protection systems, and HVDC systems.

A negative potential shift indicates current pickup; a positive shift indicates current discharge and possible corrosion.

Drainage bonds divert stray current from a structure to the source or to a grounded return path; bond resistance controls drained current.

Sacrificial anodes can mitigate stray-current discharge by supplying local protection at the affected area.

Coating defects and isolated structures concentrate stray-current pickup and discharge; electrical isolation can reduce interference paths.

Telluric currents from geomagnetic storms cause fluctuating potentials and can interfere with CP surveys.

Interference surveys combine structure-to-electrolyte potentials, current mapping, and current interruption to identify sources and discharge areas.

A coating is the primary corrosion barrier; CP protects coating holidays and areas of damage where electrolyte contacts steel.

Coating resistance controls CP current demand; a high-integrity coating reduces current demand and groundbed size.

Holidays are breaches in the coating that expose steel and concentrate CP current at the defect.

Cathodic disbondment is coating loss around a defect caused by cathodic reactions, especially at excessive negative potentials.

Disbonded coating can shield the steel from CP current, allowing corrosion to continue beneath the coating.

Coating conductance increases as the coating degrades, water is absorbed, or the coating ages.

Fusion-bonded epoxy, liquid epoxy, and coal-tar enamel are common pipeline coatings with different dielectric and adhesion properties.

Coating condition is assessed with DCVG, ACVG, close interval surveys, and direct examination at excavations.

Apply lockout/tagout to rectifiers and other electrical sources before inspection or repair; verify absence of voltage with a tested meter.

Rectifier cabinets contain hazardous AC and DC voltages; use arc-flash-rated PPE and follow electrical safe-work practices.

Cathodic protection can generate hydrogen at the cathode; ventilate confined spaces and monitor for flammable gas before entry.

Install anodes with specified backfill, depth, and cable protection; maintain separation from structures to limit interference.

Troubleshoot low CP current by checking rectifier output, AC supply, tap settings, groundbed resistance, cable continuity, and anode condition.

Troubleshoot unexpected potential changes by checking reference electrode calibration, test-station continuity, coating damage, and stray-current sources.

Use properly insulated cables, waterproof connections, and mechanical protection at test stations and anode headers.

Confined-space entry requires atmospheric testing, ventilation, a standby attendant, and rescue provisions before entry.