Coating Breakdown Factor and CP Current Growth

Grows CP current demand with coating breakdown over the design life.

Run the calculator

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

Details, formula, and sources

The 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.

f(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.

Linear coating breakdown per DNV-RP-B401 and NACE practice. The CP designer governs the factors.

DNV-RP-B401 is freely downloadable from DNV.

Estimate. AHJ and licensed professional govern.

Field names used by the API: surface_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