Designing a sacrificial anode system
cathodic protection designgalvanic anode sizingCP current demandanode life calculation
Sizing a sacrificial anode system: current demand from coated area, what one anode delivers into the soil, and the mass for the design life.
Cathodic Protection Current Demand
The total current a cathodic protection system has to supply: the structure's surface area times the current density its environment demands, times the fraction of that surface actually exposed through the coating.
Galvanic Driving Voltage
The potential difference between two metals coupled in the same electrolyte: the noble one's potential less the active one's. The voltage that drives every galvanic cell, wanted or otherwise.
Dwight's Equation for Anode Resistance to Earth
H. B. Dwight's 1936 result for a vertical rod in uniform soil: the resistance between a buried anode and remote earth, from the soil resistivity and the anode's length and diameter. Almost the whole resistance of a cathodic protection circuit.
Anode Current Output
What a galvanic anode actually delivers: the driving voltage between the anode and the polarized structure, divided by the total resistance of the circuit. Ohm's law doing the whole of cathodic protection design.
Sacrificial Anode Mass for a Required Life
How much anode metal a cathodic protection system has to carry to supply its current for the design life, given the anode alloy's current capacity and the fraction of it that can be consumed before the anode stops working.
How they fit together
Cathodic protection design is one of the tidiest sequences in corrosion engineering: five formulas, each output an input to the next, ending in a number of anodes and a mass of zinc or magnesium. Start with current demand — surface area times protection current density times a coating breakdown factor. That third term is the one that decides whether the system lasts, and it is the one most often left at its day-one value. A coating that is 1% bare at commissioning may be 20% bare at year twenty, so a system sized on the new-coating figure is roughly a fifth of the size it needed to be. Design for mean demand over the life, or design the replacement at the same time.
The middle three answer how much one anode can give. Driving voltage is the gap between the anode's potential and the protected structure's — and it is smaller than the galvanic series suggests, because the structure polarises toward the anode as protection takes hold. Magnesium against polarised steel is roughly 0.7 V of useful drive, not the 1.0 V the table implies. Dwight's equation gives resistance to earth, and note what dominates it: soil resistivity and anode length, with diameter buried inside a logarithm. Ten anodes in wet clay behave nothing like ten in dry sand, and lengthening an anode helps far more than fattening it. Then current output is simply ΔE divided by circuit resistance, and demand divided by output is how many anodes you order.
Anode mass closes the design, and it is where the two efficiency terms live. Current capacity is the amp-hours a kilogram actually delivers, always less than Faraday's theoretical figure — about 50% for magnesium, better for zinc and aluminium. Utilisation factor, usually 0.85, admits that an anode stops working while a sixth of it is still there, because the last of the metal loses contact with the core. Drop either term and the design comes out light. Note also that mass and count are separate answers to separate questions: mass sets the life, count sets whether the structure is protected at all, and a system can pass one while failing the other.