Wrong Time Unit
If K is in per amp-year, time must be in years.
Faraday's Law for Cathodic Protection
Faraday's Law connects electric current and time to metal consumption. In cathodic protection study, it supports corrosion-rate reasoning, galvanic anode life estimates, and the relationship between current flow and metal loss.
Formula
Weight loss = K × I × T
| Symbol | Meaning | Common Unit |
|---|---|---|
| K | Electrochemical equivalent or consumption rate | kg/A-year or lb/A-year |
| I | Current | amps |
| T | Time | years |
Worked Example 1: Metal Consumption
An anode material has a consumption rate of 4.0 kg/A-year. It discharges 0.25 amps for 10 years. Estimate theoretical consumption.
Weight = K × I × T
Weight = 4.0 × 0.25 × 10
Weight = 10 kg
The theoretical metal consumption is 10 kg before considering utilization and efficiency assumptions.
Worked Example 2: Why Current Direction Matters
Faraday's Law explains why discharge current is dangerous at an anodic area. If metal is discharging current into the electrolyte, metal loss can occur at that location. If the same structure area receives protective current, corrosion rate is reduced rather than increased.
Relation to Anode Life
Anode life calculations are practical applications of Faraday-type reasoning. They estimate whether the installed anode mass and output can support the intended service life after accounting for utilization and efficiency.
Interpretation Notes
- Current, time, and material type all affect predicted metal consumption.
- Higher discharge current means greater theoretical metal loss over the same time.
- Galvanic anode life estimates require realistic output assumptions.
- Faraday calculations do not replace field measurements or CP criteria.
Common Mistakes
Ignoring Utilization
An installed anode normally cannot be assumed to consume 100 percent of its weight usefully.
Missing Direction
Magnitude alone is not enough. Current pickup and current discharge have different corrosion implications.