To calculate protection current, there are two general approaches. First, some cathodic protection (CP) engineers use the protection current density, including the coating breakdown (CB) effect. In contrast, some others consider coating breakdown as a separate coefficient. In CCPTools, we use the latter scenario, CB as a separate coefficient rather than included.
Background Theory
Technically, coating breakdown is a function of the coating type and physical properties of the environment. For example, old coating types like coal tar have high coating breakdown for pipelines, both in the first year and annually. However, as a new-generation coating, three-layer polyethylene (3LPE) has low coating breakdown throughout its design life.
When the environment is aggressive or harmful to the coating, the CB increases. In some cases, as with tank bottom plates, the construction activities may cause too much damage to the coating. For example, perhaps you apply the coating to steel plates for the bottom of a tank. However, while welding the steel plates, the coating that was in contact with sand was badly damaged. Therefore, the coating breakdown is much higher than the expected design value for the same coating in another application. This makes previous experience a critical factor for selecting the best CB: there is no universal rule or datasheet.
Recommendations for Coating Breakdown
If you have values for the first year and last year’s CB, you can enter them in the application. Moreover, in CCPTools, we recommend some values based on general categories to make the CB easier for the CP designers. The sections are:
- Pipeline: We present the default value of the initial and annual CB in the table below. You just need to enter the design life.
| Coating Type | Initial CB, 0 to 1 (0 to 100%) | Annual CB, 0 to 1 (0 to 100%) |
| FBE | 0.005 (0.5%) | 0.003 (0.3%) |
| 3LPE | 0.001 (0.1%) | 0.0003 (0.03%) |
| 3LPP | 0.001 (0.1%) | 0.0003 (0.03%) |
| Liquid epoxy | 0.008 (0.8%) | 0.01 (1%) |
| Coal tar urethane | 0.008 (0.8%) | 0.01 (1%) |
So, for example if we design for a pipeline with FBE coating for 20 years, the CBs are:
Initial CB: 0.005 = 0.5%
Final CB (at 20 year) = 0.005+0.003 X 20 = 0.005 + 0.06 = 0.065 = 6.5%
- Seawater: You can select based on “Durability and general condition” or “Thickness and depth”
- Typical Water Tank: In this section, there are only preliminary recommendations for the first and last years. Moreover, you will also need to add design life.
If the coating breakdown becomes greater than 100%, the CCPTools will still consider it 100%. In CCPTools, we try to help you find the best and closest CB for your coating system.