Why AC Can Still Cause Corrosion: Understanding AC Interference Logic
1. The Misconception About AC
Alternating current reverses direction continuously. Because average current over time may equal zero, it is often assumed that AC cannot cause corrosion.
Corrosion, however, is controlled by instantaneous reaction behavior at the metal–electrolyte interface—not long-term current averages.
2. What Happens During Each Half-Cycle
During one half-cycle, current may leave the structure at a coating defect. Where current leaves the metal surface and enters the electrolyte, oxidation occurs.
During the opposite half-cycle, current reverses direction and reduction occurs. The surface alternates between anodic and cathodic behavior.
3. Instantaneous Current Density Matters
Even if average current is zero, instantaneous current density during each half-cycle can be high.
High current density at coating defects can disturb surface chemistry, damage protective films, and promote localized metal dissolution.
4. AC Does Not Behave Like DC
Direct current produces sustained polarization in one direction. AC does not create stable protective polarization because direction changes continuously.
Instead of shifting the mixed potential permanently, AC repeatedly perturbs reaction balance at the interface.
5. Interaction with Cathodic Protection
In systems already under cathodic protection, induced AC may superimpose alternating voltage on top of DC polarization.
This combined condition can increase risk of localized attack or hydrogen-related effects at coating defects.
6. The Logical Chain
Induced AC → Alternating Current Flow → High Instantaneous Current Density → Repeated Oxidation Events → Localized Corrosion Risk
AC interference is therefore a dynamic reaction problem, not simply a steady-state circuit condition.
- From Driving Voltage to Polarization
- Why Instant-Off Matters
- Understanding Corrosion Using Mixed Potential Theory
- Why AC Can Still Cause Corrosion (Current Guide)
- Current Distribution and Attenuation Explained