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Due to the strong electrochemical activity of the zinc plate, after connection, a large number of excess electrons from the zinc plate flow along the wire into the copper and iron plates.

In the formula, ------ the number of electrons flowing from the zinc plate into the iron plate.

When e1 is sufficiently large, it can offset the electron flow e_corrosion that originally caused the pitting corrosion of the iron plate. At this point, no more electrons flow from the iron plate into the copper plate along the wire. The current direction in the circuit is from the iron and copper plates through the wire into the zinc plate, and then from the zinc plate through seawater back to the iron and copper plates. Therefore, the pointer on the ammeter will point to zero or reverse. This indicates that the corrosion current of the iron has disappeared, the iron plate no longer undergoes pitting corrosion, and is thus protected. Due to the connection of the zinc plate, both the iron and copper plates in the corrosion cell become cathodes and are protected, hence this protection is called cathodic protection.

The principle of cathodic protection is explained below using polarization curves. To simplify the explanation, the cathodic and anodic polarization curves are approximated as straight lines.

The cathodic polarization curve is the anodic polarization curve, and the two intersect at point S. The corresponding current c is the corrosion current of the system, and the corresponding potential c is the corrosion potential (or mixed potential) of the system.

When cathodic protection is applied to the system, meaning a cathodic current is applied to polarize the cathode, the total potential of the system shifts in the negative direction, such as to 1. At this point, the total current on the cathode is 11, equivalent to the segment E1Q. Part of this current is externally applied, equivalent to segment PQ, while another part is still due to anodic corrosion, equivalent to segment E1P. It can be observed that the anodic corrosion current 1 is smaller than the corrosion current c, meaning the anodic corrosion rate is reduced, providing some protection.

When the externally applied current continues to increase, the system's potential will further shift in the negative direction. When the potential reaches the equilibrium potential of the anode, the anodic corrosion current becomes zero, achieving complete protection. At this point, the cathodic current lp (equivalent to the segment) is entirely externally applied. This externally applied current is called the minimum protection current, and the corresponding potential is called the minimum protection potential (generally, polarizing the metal from its stable potential by 200–300 mV in the negative direction in seawater can achieve complete protection).(1) Main Parameters of Cathodic Protection: Protection Potential

Protection potential refers to the potential value required to stop metal corrosion during cathodic protection. To completely halt corrosion,

the potential of the protected metal must be polarized to the active anode's [open-circuit] potential. For steel structures, this

potential is the equilibrium potential of iron in the given electrolyte solution.

The protection potential value has a certain range. For example, the protection potential of iron in seawater ranges between -0.80 to -1.0 V (silver/silver chloride electrode). When the potential is more positive than -0.80 V, iron cannot be fully protected, so this value is also

referred to as the minimum protection potential.

The protection potential value is often used as a criterion to determine whether cathodic protection is complete. By measuring the potential values at various parts of the protected structure,

the protection status can be understood. Therefore, the protection potential value is an important indicator for designing and monitoring cathodic protection.

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