Coating Breakdown Factor

Coating breakdown factor estimates the portion of a coated metallic structure that is exposed to the electrolyte and requires cathodic protection current.

Basic Formula

Exposed Area = Total Coated Area × Coating Breakdown Factor

Why Coating Breakdown Factor Matters in CP

Coatings reduce cathodic protection current demand by limiting the amount of exposed metal in contact with the electrolyte. CP current is primarily needed at coating holidays, damaged areas, field joints, disbonded areas, and other exposed metal locations.

Coating breakdown factor is used to estimate exposed area for current requirement calculations. If the factor is too low, the CP system may be undersized. If it is too high, the design may be unnecessarily expensive or may create interference concerns.

Coating breakdown increases over time as coatings age, become damaged, disbond, or deteriorate under service conditions.

Reference Table

Quantity Symbol Common Units
Total coated surface area Atotal ft² or m²
Coating breakdown factor f decimal or percent
Exposed area Aexposed ft² or m²
Current density i mA/ft² or mA/m²

Formula Forms

To calculate exposed area:

Aexposed = Atotal × f

To calculate required current:

I = Aexposed × i

If the breakdown factor is given as a percent, convert it to a decimal before calculating. For example, 2 percent becomes 0.02.

Worked Example

A coated pipeline has 25,000 ft² of external surface area. The assumed coating breakdown factor is 3 percent.

Convert 3 percent to decimal:

3 percent = 0.03

Calculate exposed area:

Aexposed = 25,000 × 0.03 = 750 ft²

The estimated exposed area is 750 ft².

CP Field Example

A CP system was originally designed using a low coating breakdown factor. Ten years later, the rectifier current demand has increased and potentials are less negative at remote test stations.

One likely explanation is coating deterioration. As the coating breaks down, more metal is exposed, current demand increases, and the original CP system may no longer have enough capacity.

Common Mistakes

  1. Using percent values without converting to decimals.
    Why it is wrong: Using 3 instead of 0.03 overstates exposed area by 100 times.
  2. Assuming coating condition never changes.
    Why it is wrong: Coatings age, disbond, crack, and become damaged over time.
  3. Using one generic factor for every coating system.
    Why it is wrong: Coating type, age, installation quality, soil, operation, and damage history affect breakdown.
  4. Ignoring field evidence.
    Why it is wrong: Current demand trends, CIS data, coating surveys, and excavation data may show the original assumption is wrong.
  5. Confusing exposed area with total structure area.
    Why it is wrong: Coating breakdown calculations estimate the portion of total area that requires CP current.

Standards Relevance

This page is educational and does not replace applicable AMPP, NACE, ISO, API, DOT, manufacturer, owner, or project-specific requirements.

Coating breakdown assumptions should be selected from applicable design standards, owner specifications, coating data, field testing, or justified engineering assumptions.

Practice Problems

  1. A coated structure has 12,000 ft² total area and 2 percent breakdown. What exposed area is assumed?
  2. A coated pipe has 8,500 ft² total area and 5 percent breakdown. What exposed area is assumed?
  3. Convert 1.5 percent to decimal form.
  4. Why does coating breakdown affect current requirement?
  5. Why should coating breakdown assumptions be reviewed as a structure ages?

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