Corrosion does not occur uniformly. Hot spots can be discovered using a half cell method.
Copper/copper sulfate half cell is a universal standard for corrosion assessment of steel, whether it is in concrete as reinforcing bars or in soil as water pipes. A simple voltmeter connected to a half cell can measure the corrosion activity of the steel.
The standard for this test method is ASTM C876, Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete. It designates these criteria for a half cell:
if the potential reading is less negative than -0.20V there is a 90% probability of no corrosion
if the potential reading is more negative than -0.20V but is less negative than -0.35V then there is corrosion activity at the steel
if the potential is more negative than -0.35V there is a 90% probability of corrosion in the steel
This video illustrates how a half cell is loaded and operated. It also shows how the readings are interpreted. Corrosion Services can conduct this test in collaboration with The M&P Lab.