Introduction to Corrosion Inhibitors

Corrosion inhibitors are substances that, when added in small amounts to a corrosive environment, slow down the rate of corrosion. In fact, an inhibitor can be considered an inhibitory catalyst. There are many inhibitors with different compositions. Most of these substances have been found and modified through experimental tests, and many of them are sold under trade names and their chemical compositions are kept secret. For this reason, the protection process by this method is not completely clear. However, corrosion inhibitors can be classified according to mechanism and composition.

Adsorption inhibitors

These substances are the largest group of corrosion inhibitors. In general, these types of inhibitors are organic compounds that are adsorbed to the metal surface and slow down the metal dissolution and reduction reactions. In most cases, adsorption inhibitors affect both the anodic and cathodic processes, although in some cases this effect is not the same. Organic amines are an example of this group.

Hydrogen Sublimation Poisons

These substances, such as arsenic and antimony ions, prevent the hydrogen release reaction. As a result, such substances are very effective in acidic solutions, but are ineffective in environments where other cathodic reactions, such as oxygen reduction, are controlling.

Harmful Agent Absorbers

These substances remove corrosive agents from the environment. Examples of this type of inhibitor include sodium sulfite and hydrazine, which combine with dissolved oxygen in the environment according to the following relationships:

2Na2SO3 + O2 = 2Na2SO4

N2H4 + O2 = N2.H2O

It is clear that these types of corrosion inhibitors will work very well in solutions where oxygen reduction is controlling and will not be effective in strongly acidic solutions.

Oxidizing Agents

Substances such as chromates, nitrates, and ferric salts are also used in many cases as a corrosion inhibitor. Generally, these materials are used to prevent corrosion of metals and alloys that exhibit active-passive transfer, such as steel and its alloys and stainless steels.

Vapor phase inhibitors

These compounds are very similar to absorption inhibitors and have very high vapor pressures. As a result, these materials can be used to prevent atmospheric corrosion of metals without direct contact. In practice, these inhibitors are placed close to the metal to be protected, so that their evaporation and condensation on the metal surface protect the metal. Vapor phase inhibitors are usually effective when used in a confined space, such as inside the packaging of parts or inside machinery during storage and transportation.

It should be noted that inhibitors are usually unique in terms of metal, corrosive environment, temperature, and concentration. As mentioned above, the concentration and type of inhibitor to be used in a given corrosive environment is determined by testing and experience, and such information is usually available from the manufacturers of the materials.

Inhibitor Concentration

If the concentration of corrosion inhibitors is too low, corrosion may be accelerated, especially localized corrosion such as pitting. Therefore, if the inhibitor concentration is too low, the damage will be greater than if the inhibitor was not used at all. To avoid this risk, the concentration of corrosion inhibitors should always be higher than the required amount and its concentration should be determined intermittently. When two or more inhibitors are added to a corrosive system, their effect is sometimes greater than the effect of each one alone. This property is called the additive effect. The mechanism of the additive effect is not completely clear.

Limitations of Corrosion Protection

Although inhibitors can be used effectively in many applications, there are limitations to this type of corrosion protection. Adding inhibitors to a system may not be practical due to environmental contamination. In addition, many inhibitors are toxic and their use is limited to environments that are not directly or indirectly involved in the production of food or other products used by humans.

For this reason, arsenic salts, which are effective inhibitors in many strong acids, have limited use. Inhibitors are mainly used in closed systems and are not usually used in completely open systems. Finally, as the temperature and concentration of the environment increase, the effectiveness of the inhibitors rapidly decreases.

Inhibitory Tools

In his opening address to the NACE Annual Congress, Rosenfeld discusses theoretical and applied quantum chemical calculations, X-ray photoelectron spectroscopy, radioactive tracers such as (Cl 36), and chemical polarization as "tools" for studying the phenomenon of inhibition.

Inhibition can be achieved by both adsorption and phase layers on the metal surface. He shows that inhibition by adsorption prevents pitting of stainless steels. The protection is attributed to the adsorption of deactivating ions to the metal surface and their substitution for the destructive agent (Cl–). The protection is attributed to the substitution power rather than the oxidizing power, the anion NO3 being more effective than CrO4, although the latter is a stronger oxidizing agent.

Rosenfeld shows that the protection of steel against corrosion by hydrogen sulfide is due to the appearance of phase films consisting of iron, sulfur, and inhibitory components on the steel surface. He also points out that only a percentage of

A small portion (in one case 4%) of the surface must be covered by the adsorbed inhibitor to stop corrosion. Adsorption is in some cases chemical in nature. The main point of Rosenfeld's statement is that "the mechanism of inhibition, regardless of the amount adsorbed to the surface, consists in changing the electrophysical properties of the surface atoms by surface reactions to coat the metal surface and isolate it from the corrosive environment."

Corrosion Prevention by Formation of a Protective Phase or Adsorption to the Metal Surface

An international conference on inhibition was held in May 1983. Evidence has been presented in recent years that many organic and inorganic inhibitors protect by interacting with one of several corrosion products and forming a protective phase rather than by adsorption to the metal surface.

The conference emphasized the distinction between corrosion inhibitors that are adsorbed to the metal surface or that protect by forming phases. The proceedings of this conference, containing about 30 papers, were published by NACE in 1984. A color remover and inhibitor with the trade name KO-99 is of the fusible type.

An important cause of corrosion in the upper reaches of oil refineries is corrosion by hydrogen sulfide and hydrochloric acid. In these cases, film-forming inhibitors are recommended by Nathan and Progioni. Corrosion inhibitors are also useful in reducing hydrogen blistering.

Environmental considerations have led to the development of corrosion inhibitors containing light metals (e.g., the use of chromates has been banned).

An article by Breske provides information on tests and practical experiences with these corrosion inhibitors in aquatic environments. Algal growth (and fouling) is also discussed. Brown has replaced chromate with a triazole inhibitor.

Viocasvish and Sullivan studied the combined and separate effects of molybdate, phosphate, and borate with nitrate and triazole in automobile coolants and described the experimental procedure. Adding 0.2% water to ammonia hydrated it and stopped the corrosion of steel.