Mechanism of Action of Corrosion Inhibitors (Part 2)

The physical and chemical mechanisms of corrosion inhibitors.

From the perspective of physical chemistry, the functions of corrosion inhibitors can be divided into three types: oxidation film, precipitation film and adsorption film. Therefore, corrosion inhibitors can also be divided into oxidation film type corrosion inhibitors, precipitation film type corrosion inhibitors and adsorption film type corrosion inhibitors.

1 oxide film inhibitor

The oxide film corrosion inhibitor is itself an oxidant and can interact with metals.Or itself does not have the oxidation, with the dissolved oxygen in the medium as the oxidant, the metal surface formed a close oxide film, causing metal ionization process blocked, so as to slow down the corrosion of the metal, this corrosion inhibitor is also known as the passivating agent.The corrosion inhibition of aluminum and magnesium by potassium dichromate, potassium chromate and potassium permanganate in oxygen-containing aqueous solution belongs to this category.Oxidation film corrosion inhibitor, high corrosion inhibition efficiency, has been widely used.However, if the amount is not enough, it is possible to form a large cathode and a small anode on the metal surface, resulting in hole corrosion.Therefore, this type of corrosion inhibitor is also called "dangerous corrosion inhibitor".

2. Precipitation film inhibitor

Precipitation film inhibitor, as the name implies, is the formation of a precipitation film on the metal surface.The deposited film can be formed by the interaction between the inhibitor molecules or by the interaction between the inhibitor and the metal ions in the corrosive medium.In most cases, the deposited film forms in the cathode area and covers the surface of the cathode, separating the metal from the corrosive medium and inhibiting the cathodic process of the electrochemical corrosion of the metal, that is, the cathodic inhibition type.Sometimes the deposited film can cover the entire surface of the metal and inhibit both the anode and cathode processes of the electrochemical corrosion of the metal. This is known as the mixed inhibition type.

Zinc sulfate, calcium bicarbonate, lime, polyphosphate, silicate and organic phosphonate are all cathodic inhibitors.In neutral oxygen-containing water, zinc ions can react with the hydroxide ions generated by the cathode reaction to form a refractory zinc hydroxide precipitation film covering the cathode and inhibit the cathode reaction.Phosphate, such as Na2HP04 or Na3P04, can react with Fe3+ in the presence of dissolved oxygen to form an insoluble mixture of r-fe203 and FeP04·2H20, which can inhibit the corrosion of iron.It is important to note that the presence of oxygen in the medium enhances the corrosion inhibitor.Only in the presence of oxygen can it act as a corrosion inhibitor.For example, sodium benzoate to iron corrosion inhibition, oxygen, the formation of insoluble trivalent iron salt, to protect the role of the metal.In the absence of oxygen, the formation of divalent soluble iron salt can not play a role in corrosion inhibition.

Scale and corrosion inhibitor for boiler are mostly organic compounds.The reactive groups on the organic corrosion inhibitor molecules and the metal ions generated in the corrosion process interact to form the precipitation membrane, which inhibits the electrochemical process between the Yin and Yang poles.For example, propargyl alcohol has a good corrosion inhibition effect on iron in acidic aqueous solution.It is found that when the propargyl alcohol ACTS, it is adsorbed on the metal surface first, and the polymerization reaction occurs due to the reduction of hydrogen precipitated from the iron, resulting in the formation of polymerized complex film.It covers the entire surface of the metal and inhibits both anode and cathode reactions of the corrosion electrochemical reaction.For another example, 8-hydroxyquinoline has a corrosion inhibition effect on the corrosion of aluminum in alkaline medium, which is due to the insoluble complex precipitation film generated by the reaction of corrosion inhibitor and aluminum ion covering the surface of aluminum and inhibiting the corrosion of aluminum in alkaline aqueous solution.The corrosion inhibition effect of benzotriazole on copper is also considered to be the formation of insoluble polymer precipitation film.