Cathodic Protection Anode Count and Life

Everything turns on how much steel is actually exposed. A well-coated line needs current only where the coating has holidays, so its demand is a small fraction of a bare line's -- and that is why coating is the primary corrosion control and cathodic protection is the secondary system that handles what the coating misses. A line assumed bare when it is well coated gets a wildly oversized rectifier and anode bed; a line assumed well coated when its coating has degraded gets a system that cannot hold potential, which is the more dangerous error and the one the degraded case here is for. The sensitivity is severe and it is worth seeing as a number: because demand is proportional to bare area, a coating falling from 99.9 to 99 percent efficiency multiplies the current requirement tenfold, and a coating most people would still call good can leave a correctly sized rectifier unable to protect the far end of the line. Anode life is then mass over consumption rate. The material choice drives it: galvanic magnesium is consumed roughly twenty times faster per ampere than an impressed-current high-silicon cast iron anode, which is why galvanic systems suit small, well-coated, low-current jobs and impressed current suits everything larger. The utilization factor accounts for an anode becoming ineffective before it is fully consumed. The current this returns is the figure a sacrificial-anode service-life calculation asks the user to supply, so it is computed here rather than assumed. Current density, coating efficiency, and the anode's own consumption rate and output are ENTERED, because they depend on soil resistivity, moisture, temperature, coating type and condition, and the anode bed design. This does not design the ground bed or its resistance, size the rectifier's voltage, evaluate interference with foreign structures, address stray current or AC corrosion, or set the protection criteria themselves. NACE / AMPP practice, a close-interval potential survey, and a qualified corrosion engineer govern.

required current = bare surface area x current density, with bare area = pi x diameter x length x (1 - coating efficiency); anode count = required current / current per anode; anode life = weight x utilization / (consumption rate x current per anode); and the mass for a target life is that relation inverted on the total demand.

The cathodic protection current balance as NACE / AMPP practice writes it. Current density, coating efficiency, and the anode's consumption rate and output are ENTERED, because they depend on soil resistivity, moisture, temperature, coating type and condition, and the bed design. It does not design the ground bed or its resistance, size the rectifier voltage, evaluate interference with foreign structures, address stray current or AC corrosion, or set the protection criteria.

An area, a current density, and a mass over a consumption rate.

Estimate. AHJ and licensed professional govern.

Field names used by the API: od_in, length_mi, coating_efficiency_pct, current_density_ma_per_ft2, anode_weight_lb, consumption_lb_per_a_yr, utilization, current_per_anode_a, degraded_efficiency_pct, target_life_years, total_area_ft2, bare_area_ft2, current_required_a, anode_life_years, degraded_current_a, current_multiple

• Current density is entered it depends on soil resistivity, moisture, temperature and coating conditiona soil survey and NACE / AMPP practice
• No ground bed design bed resistance and rectifier voltage are a separate calculationa qualified corrosion engineer
• Protection criteria are not set here the potential criteria and interference are the survey's joba close-interval potential survey