The principle of cathodic protection is based on reduction of corrosion current density by cathodic polarization. The potential of metals depends on anodic and cathodic electrochemical reactions in an electrolyte solution. The applied current negatively shifts the potential of the steel so the anodic reactions cease and only cathodic reactions occur at the steel surface.

Rebars in concrete are normally passive, leading to a rather positive potential. If the passivity starts to break down the potential becomes more negative. Hence in this particular system a drop in potential suggests that corrosion may be occurring. The potential of the rebar will not fall to such a negative value as would be produced by CP (unless, of course, CP is applied).

Thanks.

It is well known that a metal solid in an electrolyte solution would develop an electrochemical potential difference a cross the interface between the solid and the solution. The electrochemical potential is normally called the open circuit potential (OCP). As a result, exchange currents, cathodic (Ic) and anodic (Ia) currents, will form due to chemical reversible reactions between the metal solid and the solution,. The chemical reactions can be either cathodic or anodic reaction. If the cathodic exchange current (Ic) is more than the anodic exchange current (Ia), therefore, a net exchange current would form toward the cathodic polarization of the solid in the solution. This is also a true case during the cathodic protection of the solid in the solution, in which a cathodic potential normally would be applied toward the negative direction of the potential shift.    As a result, the cathodic protection will be achieved when the applied potential reaches the open circuit potential of the solids in the solution, at which the corrosion current becomes zero.

As far as the  probability of corrosion increases if the HCP of steel is more negative, I do agree with Professor Cottis.  

The subject of my work is application of an impressed-current cathodic protection system (ICCP) for the prevention of corrosion of steel reinforcement in concrete. When steel corrodes in concrete, the oxidation reaction (respectively anodic reaction = formation of corrosion products) usually proceeds on the steel surface (please see Iron E-pH (Pourbaix) Diagram). At potentials more positive than -600 (mV vs. SHE) and at pH values below about 9, ferrous ion (Fe2+ or Fe II) is the stable substance.

By supplying a protective current between anode and cathode using ICCP, the potential of the reinforcing steel (cathode) should be lowered below ‑600 mV (vs. SHE) in electrochemical inactive range.

Cathodic protection is an electrochemical techniques that allow controlling corrosion of a structure by lowering its potential by means of a DC cathodic current. So the negative shift of the potential is mandatory to achieve protection but it is obtain by an external current. The reduction of potential of corroding rebars (according to ASTM C876) occur in freely corroding condition, and is the consequence of the depassivation of the passive carbon steel rebar. It is not correct to compare the two potential reduction, since they are originated by two different mechanisms.