Current Density Formula for Cathodic Protection

Current density is the amount of protective current applied per unit area of exposed metal. It is commonly used when estimating CP current requirements for buried or submerged structures.

In CP calculations, current density is usually applied to exposed area, not the total coated area.

Formula

D = I / A

Symbol Meaning Common Unit
D Current density mA/ft2
I Current mA
A Exposed area ft2

Rearranged Forms

Solving For Formula Use Case
Current density D = I / A Calculate current per exposed area.
Current I = D x A Estimate CP current requirement.
Area A = I / D Estimate represented exposed area from current and current density.

Worked Example 1: Calculate Current Density

A structure receives 600 mA of protective current over 300 ft2 of exposed area. Calculate the current density.

D = I / A

D = 600 / 300

D = 2 mA/ft2

The current density is 2 mA/ft2. This value can be compared with design assumptions, but field potentials are still needed to evaluate CP criteria.

Worked Example 2: Estimate Current Requirement

A coated structure has 250 ft2 of estimated exposed area. The selected current density is 3 mA/ft2. Estimate the current requirement.

I = D x A

I = 3 x 250

I = 750 mA

I = 0.75 amps

The estimated current requirement is 750 mA, or 0.75 amps.

Worked Example 3: Exposed Area from Current

A system is supplying 1,200 mA. The assumed current density is 4 mA/ft2. Estimate the exposed area represented by that current.

A = I / D

A = 1,200 / 4

A = 300 ft2

The current would represent approximately 300 ft2 of exposed area at 4 mA/ft2, assuming the selected current density is appropriate.

CP Interpretation Notes

  • Current density is a design and estimating tool, not a CP criterion by itself.
  • Use exposed area for coated structures unless the structure is bare or fully exposed.
  • Selected current density should reflect the environment, coating condition, and design basis.
  • Current distribution can still be poor even when total current appears adequate.
  • Field verification requires structure-to-electrolyte potentials and applicable criteria.

Common Mistakes

Using Total Area Incorrectly

For coated structures, using total area instead of exposed area can greatly overstate current demand.

Mixing Amps and Milliamps

Keep the units consistent. If current density is in mA/ft2, current should be in mA for the calculation.

Ignoring Field Response

A current density estimate does not prove that protective current reaches all exposed metal.

Practice Problems

  1. A structure receives 500 mA over 250 ft2. What is the current density?
  2. A structure has 400 ft2 exposed area and uses 2 mA/ft2. What current is required?
  3. A system provides 900 mA at 3 mA/ft2. What exposed area is represented?

Related Pages

Field Use Reminder

Current density is most useful when the exposed area estimate is reasonable and the selected value matches the environment, coating condition, and design basis. It should always be checked against measured structure response.