There are mainly two commonly adopted criteria for controlling CP. One is the polarized potential criterion and the other one is the polarization shift criterion1. These criteria are not the true criterion for cathodic protection; they are the surrogate criteria (see below). The polarized potential criterion is to control the instant-off structure-to-electrolyte potential within a specified range. For example, the instant-off potential should be between -0.85 and -1.2 V vs Cu/CuSO₄ (V_CSE) for pipelines buried in soil. The polarization shift criterion is to control the polarization of a CP-protected structure to a given minimum value and this minimum value is usually 100 mV. The polarization is determined either by the difference between the corrosion potential of the structure measured before CP is applied and the instant-off structure-to-electrolyte potential, or by the difference between the depolarized potential of the structure and the instant-off structure-to-electrolyte potential.