Analysis of Failure Causes of Cathodic Protection System for Crude Oil Tank Bottoms

The failure of the cathodic protection system for crude oil tank bottoms mainly stems from three aspects: 1) Deterioration of the anti-corrosion coating leading to a surge in protection current demand, especially in the weld zone (leakage resistance < 10000 Ω·m²) forming a local corrosion cell; 2) Changes in soil resistivity (e.g., > 5000 Ω·cm during drought) or stray current interference (fluctuation amplitude > ±200mV) disrupting potential stability; 3) Aging of the anode ground bed (grounding resistance > 5 Ω) or insufficient output from the potentiostat (voltage < 12V), failing to maintain a protection potential of -0.85V (CSE).

A typical failure case shows that, for a coastal storage tank, the decrease in water content of the asphalt sand foundation caused the soil resistivity to rise from 2000 Ω·cm to 8000 Ω·cm, resulting in a magnesium anode consumption rate exceeding the design value by 3 times, and the tank bottom potential rose from -1.10V back to -0.75V (CSE), triggering pitting corrosion perforation. In addition, uninsulated lightning protection grounding devices caused a 30% loss of protection current, accelerating local corrosion.

System repair requires comprehensive measures: 1) Use DCVG to detect coating defects (accuracy ±0.1m) and reinforce the anti-corrosion coating; 2) Replace deep well anodes (depth ≥15m) or upgrade to intelligent potentiostats (anti-interference filtering function); 3) Install insulating flanges (resistance > 10⁶ Ω) and implement CIPS monitoring (spacing 1m) to ensure the potential is stable in the range of -0.85~-1.15V (CSE). After the renovation of a refinery, the corrosion rate of the tank bottom decreased from 0.8mm/a to 0.02mm/a, and the service life was extended to 20 years.