Specification :

Cathodic Protection ( CP) Zinc & Aluminium Sacrificial Anodes
Galvanic anodes are designed and selected to have a more " active" voltage ( more negative electrochemical potential) than the metal of the structure ( typically steel) .
For effective CP, the potential of the steel surface is polarized ( pushed) more negative until the surface has a uniform potential. At that stage, the driving force for the corrosion reaction is removed.
The galvanic anode continues to corrode, consuming the anode material until eventually it must be replaced. The polarization is caused by the electron flow from the anode to the cathode. The driving force for the CP current is the difference in electrochemical potential between the anode and the cathode.