An Electrochemical Noise test in potentiostatic mode measures current noise at a fixed electrode-potential. A corrosion specimen is immersed in a solution and its potential (versus a reference electrode) is controlled by a three-electrode potentiostat.
Potentiostatic Mode ECN is useful when the system that you are modeling is not at its open-circuit potential. One example is a system under cathodic protection. ZRA Mode ECN cannot test this type of system.
Potentiostatic Mode ECN may also be superior to ZRA-mode test ECN for coated samples. In ZRA mode, the assumption is that anodic current on one specimen flows as cathodic current from the other specimen. If two coated specimens are tested in ZRA mode, both coatings have to fail for current to flow. In potentiostatic mode, every failure results in increased current noise.