*11.5.1.1 Anodic inhibitors*

An anodic inhibitor increases the anodic polarization and hence moves the corrosion potential to the cathodic direction and hence also called as passivating inhibitors. Anodic inhibitors such as chromates, phosphates, tungstates and other ions of transition elements with high oxygen content are those that stifle the

**Figure 13.** *Classification of corrosion inhibitors.*

corrosion reaction occurring at the anode by forming a sparingly soluble compound with a newly produced metal ion. They are adsorbed on the metal surface forming a protective film or barrier, thereby reducing the corrosion rate. Anodic inhibitors build a thin protective film along the anode and increasing their potential and thus slow down the corrosion reaction [34].

Although, this type of control is affected, yet it may be dangerous since severe local attack can occur, if certain areas are left unprotected by depletion of the inhibitors. A number of inorganic inhibitors such as orthophosphates, silicates, etc. fall under anodic type. Even though anodic inhibitors are widely used, a few of them have some undesirable property. If such inhibitors are used in very low concentrations, they cause stimulation of corrosion such as pitting and for this reason anodic inhibitors are denoted as dangerous.

There are two types of passivating inhibitors.


In general, passivation inhibitors can actually cause pitting and accelerate corrosion when concentrations fall below minimum limits. For this reason, it is essential to monitor the inhibitor concentration.

## *11.5.2 Cathodic inhibitors*

Cathodic inhibitors reduce corrosion by slowing the reduction reaction rate of the electrochemical corrosion cell. This is done by blocking the cathodic sites by precipitation. Cathodic inhibitors are effective when they slow down the cathodic reaction. Elements As, Bi and Sb are referred to as cathodic poisons which reduce the hydrogen reduction reaction rate and lower the overall corrosion rate. Removal of oxygen from the corrosive environment will significantly decrease the corrosion rate. This can be done through

• The use of oxygen scavengers such as sodium sulfite and hydrazine which react with the oxygen and remove it from the solution

**17**

*Corrosion Inhibitors*

*DOI: http://dx.doi.org/10.5772/intechopen.80542*

• Boiling to lower the dissolved oxygen concentrations.

of cathodic inhibitors takes place by three mechanisms,

Cathodic inhibitors shift the corrosion potential to the anodic direction. [35, 36] Here the cations migrate towards the cathode surfaces where they are precipitated chemically or electrochemically and thus block these surfaces. The inhibiting action

• **Cathodic poisons**: The cathodic reduction process is suppressed by impeding the hydrogen recombination and mode of protection discharge but increase the

• **Cathodic precipitates**: Compounds such as calcium, magnesium will precipitate as oxides to form a protective layer which acts as a barrier on the metal surface.

• **Oxygen scavenger**: These compounds react with oxygen present in the system to form a product and reduce corrosion. For example, As3+ and Sb3+ on dissolu-

These inhibitors retard both the anodic and cathodic processes involved in the corrosion process and are therefore called mixed inhibitors [37]. They are typically film forming compounds that cause the formation of precipitates on the surface blocking both anodic and cathodic sites indirectly. Anodic inhibitors are, for the most part, dangerous inhibitors, especially if their concentrations are too less. But cathodic inhibitors are generally safe. Mixed inhibitors are less dangerous than pure anodic inhibitors, and in number of cases they may not increase the corrosion intensity. The most common inhibitors of this category are the silicates and the phosphates. Such inhibitors will have the more advantage that they control both the cathodic and anodic corrosion reactions.

The compounds such as As2O3, Sb2O3 have been reported as inhibitors in acid media. In this case, the protection is due to the reduction of electro positive ions and deposition on the metal surface and lowering of the over voltage of main cathodic depolarization reaction [38]. Recently it is shown that the addition of heavy metal

Organic compound containing oxygen, nitrogen, sulfur with multiple bonds have been reported as good corrosion inhibitors. Many organic inhibitors such as amines, aldehydes, alkaloids, nitro and nitroso compounds have been studied and tried as corrosion inhibitors [39]. Organic inhibitors can be anodic, cathodic and mixed type based on its reaction at the metal surface and potential. These are effective depending upon its size, carbon chain length, aromaticity, conjugation and

, Mn2+ and Cd2+ is found to inhibit corrosion of iron in acids.

tendency of the metal to be susceptible to hydrogen induced cracking.

• Vacuum de-aeration or

tion of Fe in acids.

*11.5.3 Based on environment*

*11.5.3.2 Inorganic inhibitors*

ions such as Pb2+, Ti+

*11.5.3.2.1 Organic inhibitors*

nature of bonding atoms [40].

*11.5.3.1 Acidic environment inhibitors*

*11.5.2.1 Mixed inhibitors*

• Vacuum de-aeration or

*Corrosion Inhibitors*

**Figure 13.**

slow down the corrosion reaction [34].

*Classification of corrosion inhibitors.*

anodic inhibitors are denoted as dangerous.

steel in the absence of oxygen.

to monitor the inhibitor concentration.

*11.5.2 Cathodic inhibitors*

rate. This can be done through

There are two types of passivating inhibitors.

require the presence of oxygen to passivate steel.

with the oxygen and remove it from the solution

corrosion reaction occurring at the anode by forming a sparingly soluble compound with a newly produced metal ion. They are adsorbed on the metal surface forming a protective film or barrier, thereby reducing the corrosion rate. Anodic inhibitors build a thin protective film along the anode and increasing their potential and thus

Although, this type of control is affected, yet it may be dangerous since severe local attack can occur, if certain areas are left unprotected by depletion of the inhibitors. A number of inorganic inhibitors such as orthophosphates, silicates, etc. fall under anodic type. Even though anodic inhibitors are widely used, a few of them have some undesirable property. If such inhibitors are used in very low concentrations, they cause stimulation of corrosion such as pitting and for this reason

• The oxidizing anions such as chromates, nitrites and nitrates that can passivate

• The non-oxidizing ions such as phosphates, tungstates and molybdates that

In general, passivation inhibitors can actually cause pitting and accelerate corrosion when concentrations fall below minimum limits. For this reason, it is essential

Cathodic inhibitors reduce corrosion by slowing the reduction reaction rate of the electrochemical corrosion cell. This is done by blocking the cathodic sites by precipitation. Cathodic inhibitors are effective when they slow down the cathodic reaction. Elements As, Bi and Sb are referred to as cathodic poisons which reduce the hydrogen reduction reaction rate and lower the overall corrosion rate. Removal of oxygen from the corrosive environment will significantly decrease the corrosion

• The use of oxygen scavengers such as sodium sulfite and hydrazine which react

**16**

• Boiling to lower the dissolved oxygen concentrations.

Cathodic inhibitors shift the corrosion potential to the anodic direction. [35, 36] Here the cations migrate towards the cathode surfaces where they are precipitated chemically or electrochemically and thus block these surfaces. The inhibiting action of cathodic inhibitors takes place by three mechanisms,

