Cathodic inhibitors are generally less effective than the anodic type. In contrast, they often form a visible film along the cathode surface, which polarizes the metal by restricting the access of dissolved oxygen to the metal substrate. The film also acts to block hydrogen evolution sites and prevent the resultant depolarizing effect.

The activity of a cathodic inhibitor can be illustrated to the left video. Starting at a zero inhibitor concentration, the system performs a corrosion rate of 100%, the untreated and unprotected corrosion level.

As we started to feed  the cathodic inhibitor, as the concentration remains at lo9w level, no significant effect in the genaral corrosion rate will be monitored. This is because the metal dissolution is taking place in the anodes, and the anodic reactions remain unchanged, having sufficient cathodic half sels potential, to combine with.

Of course as the concentration of the inhibitor increases, the cathodic sites are deactivated, and after a certain dose, the available cathodic potential becomes unsufficient to cover the anodes requiremets. So anodes start to "starve", their activity drops, and the overall general corrosion rate is droped, to a minimum value, which is related to the specific corrosion inhibiror and to the system's caracteristics.

When we start to lower the inhibitors concentration, as it is presented in the right video, after a delay, nacessary for the reactivation of the cathodic sites, the general corrosion rate increases again to reach the value of the corrosion rate of the untreated system.

Under the same conditions, if we examined the pitting tendency as a function of the inhibitor concentration, we obtain a similar graph.

The activity of the differential aeration cells (under deposit attack), follows the same way, because the area under the deposit is the anode, while the sourrounding fre metal aerea became the cathode.

So the cathode remains available to the cathodic inhibitor to act, and being deactivated, the under the deposit existing anode is deactivated  due to the cathodic potantial  unavailability to combine with it.

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