Cathodic Protection Calculator: Current and Anode Life

This cathodic protection calculator estimates how much current a buried or submerged steel structure needs and how long a set of galvanic anodes will last. The current is the bare steel area, which is the surface area times the coating breakdown, multiplied by a current density for bare steel in that soil or water. For one mile of 6 in. pipe with a good coating that leaves 2% of the surface bare, at 3 mA/ft², that is about 0.55 A, and fourteen 17 lb magnesium anodes last about 21 years at that current. Every value is a typical starting point that you can replace with test data.

How to use the calculator

Enter the pipe’s outside diameter and length, or the surface area of a tank or other structure. Choose the soil or water: the current density field fills with the high end of the published range, and you can type your own value. Enter the coating breakdown, the share of the surface that is bare. Then choose the anode material, the net mass per anode, the number of anodes and the design life. If you have measured the anodes’ output, choose “Measured anode output” to estimate their life from it. The form works in US or metric units.

How to calculate cathodic protection current

1. Find the surface area. For a pipe, π × outside diameter × length. For a tank, the bottom and the shell in contact with the soil or water.
2. Find the bare area. Multiply by the coating breakdown: 2% bare is a coating efficiency of 98%. Use 10% or more for an older or damaged coating.
3. Pick a current density for bare steel in that environment, from a current requirement test where possible.
4. Multiply the bare area by the current density to get the current.
5. Check the anodes. Anode life in years = anode mass × practical capacity × utilization factor ÷ (current × 8,760 hours).

Worked example

One mile (5,280 ft) of 6 in. steel pipe, 6.625 in. outside diameter, with a good coating that is 2% bare, in well-aerated neutral soil:

• Surface: π × 6.625 ÷ 12 × 5,280 = 9,157.7 ft².
• Bare area: 9,157.7 × 0.02 = 183.15 ft².
• Current at 3 mA/ft²: 183.15 × 3 = 549.5 mA, or 0.5495 A.
• Anodes: magnesium at 500 A·h/lb (50% efficiency) with a utilization factor of 0.85. For 20 years: 0.5495 × 20 × 8,760 ÷ (500 × 0.85) = 226.5 lb, so fourteen 17 lb anodes (238 lb). Their life at 0.5495 A is 238 × 500 × 0.85 ÷ (0.5495 × 8,760) = 21.0 years.

Pick the “Coated pipeline, 1 mile” quick start in the calculator above to see each step.

Typical values and where they come from

• Current density for bare steel: 0.4 to 1.5 mA/ft² in neutral soil, 2 to 3 in well-aerated neutral soil, 1 to 6 in wet soil, 3 to 15 in highly acidic soil, 6 to 42 in soil with sulfate-reducing bacteria, 3 to 25 in heated soil, 1 to 6 in stationary fresh water, 5 to 15 in moving fresh water with dissolved oxygen and 3 to 10 in seawater (Air Force Manual AFM 88-9, as tabulated in US Army TM 5-811-7). Choosing an environment in the calculator fills the high end of its range.
• Coating breakdown: no single value fits every coating. The calculator starts at 2% bare, which assumes a good modern coating such as FBE or extruded polyethylene; a well-applied one can stay well under 1% bare for years. Older or damaged coatings can reach 10% or more, and the design examples in TM 5-811-7 use coating efficiencies of 80 to 90%, that is 10 to 20% bare.
• Magnesium anodes: 1,000 A·h/lb (2,200 A·h/kg) in theory and about 50% efficient in service, so about 500 A·h/lb (1,100 A·h/kg), or 17.5 lb (8 kg) per ampere-year.
• Zinc anodes: 372 A·h/lb in theory and about 90% efficient, so about 335 A·h/lb, or 26 lb per ampere-year.
• Aluminum-zinc-indium anodes, seawater and brackish water only: 2,000 A·h/kg, the seawater design value of DNV-RP-B401. The calculator does not accept them in soil.
• Utilization factor: 0.85 is common for packaged anodes; the right value depends on the anode’s shape and core.

What the calculator does not check

It does not check whether the anodes can deliver the current. A galvanic anode’s output depends on the soil resistivity, the anode’s size and the driving voltage between the anode and the protected steel. It also does not show how the current spreads along a long pipe, interference with other structures, or whether the structure meets the protection criteria of AMPP (NACE) SP0169, which only measurements can confirm. It uses one coating breakdown, the value at the end of the design life, for both the current and the anode mass, which is conservative for the mass. With a measured output, the anode count assumes the total output stays at that value; more anodes usually put out more current.

Frequently asked questions

How many magnesium anodes do I need?

Divide the anode mass needed for the design life by the mass of one anode. The mass is current × years × 8,760 ÷ (capacity × utilization factor). At 0.5 A for 20 years with magnesium, that is about 206 lb, or thirteen 17 lb anodes. Then check that the anodes can deliver that current in your soil.

Magnesium or zinc?

Magnesium has an open-circuit potential of about −1.55 to −1.75 V to a copper-copper sulfate electrode, a driving voltage of about 0.7 to 0.9 V against steel protected at −0.85 V, so it suits soils of higher resistivity. Zinc, at about −1.1 V, has about 0.25 V of drive; it is used in low-resistivity soil and in seawater, where its high current efficiency pays off. Above about 49 °C (120 °F) zinc can passivate.

What current density should I use?

Best is a current requirement test on the structure itself: apply a temporary current and measure the potential shift. Without one, use the published range for the environment, and the high end for a design that has to work in the wettest or most corrosive season.

Is 20 years a typical design life?

We design galvanic systems for 20 years and impressed current systems for 30 years unless the owner specifies otherwise.

When to bring in an engineer

A cathodic protection design starts from soil resistivity, coating condition and a current requirement test, and is confirmed by a commissioning survey. Dreiym designs, installs, tests and commissions galvanic and impressed current systems, PE-stamped where required. Our corrosion team includes Stepan V. Eero, P.E., an AMPP (formerly NACE) CP4 Cathodic Protection Specialist. Related tool: the galvanic corrosion chart. See our cathodic protection design, soil corrosivity analysis and CP potential survey services, or contact us for a free quote. We typically respond within one business day.