Rust inhibitors

Rust inhibitors are usually oxidizing (mineral) chemicals that, when added to a corrosive environment, noble the corrosion potential and thus cause the rusting of metals. Chromates, nitrates, molybdenums are included in this category.

Rust inhibitors are anodic inhibitors that shift the corrosion potential of a metal sharply in the positive (noble) direction. These inhibitors deactivate the anodic regions on the metal surface by increasing the local current density to a value greater than that required for rusting. Rust inhibitors were initially used in the neutral pH range for the treatment of cooling waters.

Rust inhibitors reduce corrosion through several mechanisms:

1. They stabilize the rust layer, reducing the corrosion rate.
2. If the layer is damaged, they rerust the metal.
3. They participate in the repair of the layer; some rust inhibitors form insoluble products that fill the pores of the layer.
4. They are prevented from absorbing corrosive anions, such as Cl–, by competitive adsorption of inhibitor anions.

If not used in sufficient quantities, corrosion inhibitors are dangerous because the metal, which is often completely corroded, has a large cathode-to-anode surface area ratio and all attack is concentrated in the non-corroded areas, creating cavities. The critical concentration required usually varies from 10-3 to 10-5 mol/l.

Corrosion inhibitors are divided into two categories:

Direct corrosion inhibitors:

Direct corrosion inhibitors, also called “oxidizers”, have anions that react with the metal surface and participate in the corrosion layer, stabilizing and repairing it. Chromates and nitrates are among the best oxidizers and corrode steel even in deaerated solutions. Molybdates and tungstates are less effective and require some oxygen in the solution to help increase the current intensity above a critical value, but molybdates can pass through microbial membranes and corrode the underlying metal. Unfortunately, suitable or efficient oxidizers cannot be used in environments where environmental sensitivities are important. Chromates are toxic and carcinogenic and often cause skin irritation and damage with prolonged contact.

Indirect Corrosion Agents:

Indirect corrosion agents are not oxidizers but improve the absorption of dissolved oxygen to the metal surface, allowing the oxygen to actually corrode the metal surface. These corrosion agents are useful when the environment contains oxygen or an oxidation inhibitor such as nitrite or molybdate. Indirect rust removers are alkaline compounds that produce a high concentration of OH- which reacts with the H+ ions adsorbed at the cathode sites and removes them from the surface so that oxygen can be adsorbed.

Indirect inorganic and organic rust removers are usually effective in controlling corrosion and are alkaline inorganic compounds such as NaOH, Na2HPO4, Na2SiO3, Na2B4O7 (borax). Indirect organic rust removers include sodium benzoate and sodium cyanate and have the property that they do not cause pitting if the chloride ion concentration in the environment is too high.

In recent years, the use of a mixture of rust removers has become common. For example, adding a direct rust remover with Na3PO4 (as an indirect rust remover) prevents corrosion of steel in aqueous solutions by up to 99%.