Anodic Inhibitors

Open recirculating cooling water treatment corrosion control

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Anodic inhibitors build a thin protective film along the anode, increasing the potential at the anode and slowing the corrosion reaction. The film is initiated at the anode although it may eventually cover the entire metal surface. Because the film is not visible to the naked eye, the appearance of the metal will be left unchanged.

As long as the inhibition is taking place at ther anodes, where metal dissolution takes place, the expected corrosion rates as a function of the inhibitor concentration differs in anodic inhibitors than the cathodic ones.

Especially, when we started to feed the inhibitor, at the beginning the corrosivity increases, as it is presented in the following video.

As the concentration increases, the corrosivity increases, reaches a maximum limit, and afterwards starts to decrease.

This maximum value can be even a hundred times higher than the value of the untreated one.

As the concentration increases, the corosivity continue to drop, comming asymptotically to the minimum equilibrium value, related to the corrosion inhibitor type and the system's characteristics.

The explanation of the phenomenon is that a corosion inhibitor is blocking the anodic sites,where corrosion takes place. The concentration of the inhibitor is below the required levels, and the protective films are not yet established, then the inhibitor start to block weak anodes first. as the stronger (and more active anodes resist to the blockage).

As the cathodic potential remains constant and the majority of the strong anodes remains active the blocking of the weak anodes liberates excessive potential to the strong anodes to corrode.

Of course as the concentration increases, then the inhibitor can react with the strong anodes, and blocking them the corrosivity is decreased.

The final value is finally the one tenth of the value that a typical cathodic inhibitor can yield.

If we are trying to distort the equilibrium, by lowering the inhibitor concentration, as it is presented in the video to the right, at the begining no significant increase of the corrosivity is taking place.

As the decrease continious, start to increase the corrosivity, reaches a maximum limit (much higher than the maximum limit achieved when we where increasing the concentration, and then drops sharply to the untreated system's value.

Due to this caracteristic performance anodic inhibitors are caracterized as "UNSAFE" and care has to be taken as to:

Start the application with an initial concentration, normally three to four times the maintainance level as to obtain rapid film formation and to avoid overcorrosivity of the system.

Observe the inhibitor concentration, especially if the system is leaking to avoid drop of the concentration which will result in excessive acceleration of the corrosion process.

As long as the anodic inhibitor is acting at the anodes, the presence of differential aeration cells creates big problems to the system. A differential aeration cell being protected from inhibitors acess due to the porous deposit over it, cannot be reached by the inhibitor.

So the differential aeration cells remain not only untouched, but they are combined to the entire cathodes of the system collecting the sum of the corrosion potential. Their activity is highly accelerated, controlled only from the diffusion of the corrosion products through the porous deposit.

As final conclusion, anodic inhibitors are giving an excellent proteciotn to the system, but care should be taken as :

Make an initialization treatment in higher dossages for rapid film formation.

Avoid the drop of the concentration under the required minimum level, because this will accelerate corrosion.

Never use anodic inhibitors alone in a dirty system.

This will accelerate differential aeration cells and excessive localized corrosion will take place.

Preclean the system, or use them in combination with strong cathodic ones

Anodic inhibitors:

Chromate, molybdate, and nitrite -- catalyze the reaction between the metal and oxygen to form a passivating film. They also become a part of the gamma iron oxide film. Chromate and nitrite are the only anodic inhibitors that function in the absence of oxygen.

Orthophosphate -- also catalyzes the reaction between steel and oxygen to form a passivating gamma iron oxide film. Oxygen must be present in water for orthophosphate to function as an anodic inhibitor.

Polyphosphate -- exhibits some anodic properties but functions primarily as a cathodic inhibitor.

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