AMPP CP4 Specialist cheat sheet

Galvanic (sacrificial) anodes protect by driving the structure potential to the protection criterion; the anode's open-circuit potential must be more negative than the structure's protected potential by enough to supply the required current.

Driving voltage = anode open-circuit potential minus structure polarized potential; current capacity of an anode = (net anode mass × theoretical capacity × current efficiency) / (design life × utilization factor).

Anode resistance to remote earth controls current output: I = driving voltage / (anode resistance + structure-to-electrolyte resistance + cable resistance); anode resistance usually dominates.

Standard protection criteria in soil and water: -0.85 V vs Cu/CuSO4 for steel, 100 mV polarization shift, or -0.80 V vs Ag/AgCl for seawater; criteria are structure- and environment-specific.

Anode spacing and distribution determine current distribution; anodes are placed where coating is damaged, at coating holidays, and around shielded or complex geometry to avoid shielding.

Common anode materials: magnesium and zinc for soil/freshwater, aluminum alloys for seawater; zinc passivates in low-resistivity, high-pH, or sulfate-rich environments.

Anode consumption rate = current output × design life / (theoretical capacity × efficiency); utilization factor accounts for anode shape and end-of-life loss (e.g., ~0.5 for slender anodes, ~0.85 for compact anodes).

Galvanic systems are limited by driving voltage (typically < 1 V for Mg), so they are best for well-coated, low-current-demand structures in low-resistivity electrolytes.

Impressed current systems use an external DC power source (rectifier, solar, TEG) to drive current from an inert anode to the structure; the rectifier output voltage must exceed the total circuit resistance times design current.

Total circuit resistance = anode-to-electrolyte resistance + structure-to-electrolyte resistance + cable/connection resistance; rectifier voltage rating is selected with margin above this product.

Anode resistance formulas: Dwight formula for vertical rod anodes, and R = (ρ/2πL)[ln(4L/d) - 1] for a single vertical anode in homogeneous soil, where ρ is soil resistivity, L anode length, d diameter.

Anode materials: high-silicon cast iron, graphite, mixed metal oxide (MMO) coated titanium, and platinum-niobium; selection depends on environment, current density limits, and consumption rate.

Anode backfill (coke breeze) lowers anode resistance, extends anode life, and improves current distribution; backfill resistivity and column dimensions are design parameters.

Current demand = bare/coated area × current density × coating breakdown factor; coating quality strongly reduces required current and rectifier size.

Rectifier sizing: DC current = design current with margin; DC voltage = circuit resistance × current plus back-EMF; AC input and transformer taps must accommodate seasonal soil resistivity changes.

Design must address interference (stray current) on nearby metallic structures; drainage bonds, shielding, or anode relocation mitigate interference.

Ohm's law governs CP circuits: V = I × R; current output is set by driving potential divided by total circuit resistance.

Attenuation (potential drop along a pipeline or structure) is described by the attenuation coefficient α = sqrt(r/R), where r is the longitudinal resistance per unit length and R is the coating/leakage resistance to earth per unit length.

Attenuation formula: potential at distance x, E(x) = E0 × exp(-αx), where α = sqrt(r/R); higher coating resistance (better coating) increases α and reduces attenuation distance.

Coating conductance (leakage) per unit length = 1/R; bare or damaged coating lowers R, decreasing α and extending the effective protection distance but increasing current demand.

Current density criteria vary by environment: typically 10-25 mA/m² for coated steel in soil, higher for bare steel; seawater values differ and are specified by standards.

Polarization is the shift of structure potential from its free-corrosion (native) potential toward the protection potential; it is the measure of CP effectiveness, not current alone.

Soil resistivity (measured by Wenner four-pin method) determines anode resistance and current distribution; low resistivity favors galvanic anodes, high resistivity favors impressed current.

Structure-to-electrolyte potential measurements use a reference electrode (Cu/CuSO4, Ag/AgCl, or SCE); IR drop must be accounted for when interpreting readings (instant-off technique).