Coating Breakdown Factor and CP Current Growth
Grows CP current demand with coating breakdown over the design life.
Example
You enter
- Surface area (sq ft) 50000
- Bare-steel current density (mA/sq ft) 2
- Initial breakdown factor (%) 2
- Annual degradation (% per year) 2
- Design life (years) 30
- Faster degradation to compare (% per year) 4
You get
- Current at commissioning 2.00 A
- Current at end of life 62.0 A -- 31 times the commissioning current
- Mean current, which buys anode metal 32.0 A
- Growth ratio 31
- Sizing everything on the mean 48.3871
- Final over mean ratio 1.9375
- Years to fully bare 49
- Fast years to bare 24.5
- End of life at the faster rate 100.0 A -- the bare-steel ceiling of 100.0 A
Checked against published worked examples from: US Department of Defense.
Details, formula, and sources
ScopeThe worked structure draws 2 A at commissioning and 62 A at year 30; the mean buys anode metal while the final buys anode count and rectifier capacity. Sizing everything on the mean leaves it 48% short.
Formula or tablef(t) = f(0) + annual rate x t, capped at 1.0; current = area x bare-steel current density x f(t); mean at f(0) + rate x life / 2.
Source and editionLinear coating breakdown per DNV-RP-B401 and NACE practice. The CP designer governs the factors.
Free accessDNV-RP-B401 is freely downloadable from DNV.
What governsEstimate. AHJ and licensed professional govern.
API field namessurface_sqft, bare_current_density_ma_per_sqft, initial_breakdown_pct, annual_degradation_pct, design_life_years, fast_degradation_pct, initial_current_a, final_current_a, mean_current_a, growth_ratio, mean_shortfall_pct, final_over_mean_ratio, years_to_bare, fast_years_to_bare, fast_final_current_a
- Degradation linear, the customary design modelDNV-RP-B401
- Ceiling the breakdown factor caps at fully bare steelspec-v1767 method