Ejemplo de uso

These chemicals act by removing corrosive reagents. For example, by preventing cathodic depolarisation due to oxygen in the solution as well as reducing the current and potential, and hence the corrosion rate.

Oxygen scavengers are added to water, either alone or with another inhibitor to retard corrosion. One of the most common oxygen scavengers is sodium sulphite (Na2SO3).

At elevated temperature, hydrazine has been used to remove oxygen.

A reaction typical of a scavenger is indicated in the following equations

2 Na2SO3+ O2 → 2 Na 2SO4

N2H4+ O2 → N2+ 2H2O

Interface Inhibitors

Interface inhibitors form a diffusion barrier on the metal/environment interface to give rise to resistance of the anodic and cathodic reactions. They can be classified into liquid and vapour-phase inhibitors.

A- Liquid phase Inhibitors

These can be classified as anodic, cathodic or mixed, depending on the change in corrosion potential after the addition of the inhibitor.

Anodic Inhibitors

The anodic inhibition mechanism is illustrated in Figure 2-16 which shows an increase in the polarisation of the anode where a large potential change results in a small current flow, which causes the corrosion potential to shift in the noble direction. In the case of stainless steel, the cathodic curve may then intersect the anodic curve in the passive region. Adsorption of the inhibitor on the anodic areas also plays a part in the process because it decreases the current density required for the anode to reach the critical passive potential. The most effective and widely used anodic inhibitors are:

- Oxidising anions, such as nitrite, chromate and nitrate, which can passivate steel in the absence of oxygen.

- Non-oxidising ions such as phosphate and molybdate that require the presence of oxygen to passivate steel.

Figure 2-16: Evans type diagram showing corroding system under anodic inhibition [98]

Cathodic Inhibitors

The effects of the cathodic inhibitor on cathodic polarisation are shown in Figure 2-17. In this case the corrosion potential is shifted to more negative values. The cathodic reaction is either used for hydrogen ion reduction to form hydrogen gas, or reduction of oxygen. Both these phenomena cause the environment immediately adjacent to the cathodes to become alkaline. Therefore ions such as zinc, magnesium and calcium may be precipitated as oxides to form a protective layer on the metal.

Inhibition by polarisation of the cathodic reaction can be achieved in several ways such as oxygen scavengers, cathodic poisons and cathodic precipitates. A serious drawback of using cathodic poisons is that they sometimes cause hydrogen blistering and an increase in hydrogen embrittlement, especially in acid solutions.

Figure 2-17: Evans type diagram showing corroding system under cathodic inhibition [98]



Mixed inhibitors

This type of inhibitor controls both anodic and cathodic reactions, as illustrated in the Evans diagram, Figure 2-18. Organic inhibitors affect the entire surface of a corroding metal when present in sufficient concentration by forming an a desorbed film on the metal surface. Their effectiveness depends on chemical composition, molecular structure and their affinities for the metal surface. Inhibition of metal corrosion by organic compounds is a result of adsorption of organic molecules or ions at the metal surface forming a protective layer. This layer reduces or prevents corrosion of the metal.

The extent of adsorption depends on the nature of the metal, the metal surface condition, the mode of adsorption, the chemical structure of the inhibitor, and the type of corrosive media. Mixed inhibitors protect the metal in three possible ways: Physical adsorption, chemisorptions and film formation. Physical (electrostatic) adsorption may be due to the electrostatic attractive forces between ionic charges or dipoles of the adsorbed species and electric charges on the metal at the metal solution interface.

Organic inhibitors will be adsorbed according to the ionic charges of the inhibitors and the charge on the metal surface. Cationic inhibitors (positively charged) such as amines, or anionic inhibitors (negatively charged) such as sulfonates, will be adsorbed preferentially, depending on whether the metal is charged negatively or positively (Opposite sign charges attract). The charges on the metal can be expressed by their potential with respect to the zero-charge potential. As the potential becomes more positive, the adsorption of anions is favoured and as the potential become more negative, the adsorption of cations is favoured [96].

Besides electrostatic interaction, inhibitors can bond to metal surfaces by electron transfer to the metal to form a link. Electron transfer is from the adsorbed species by the presence of loosely bound electrons that can be found in anions and neutral organic molecules containing lone pair electrons orπ-electron system associated with multiple, triple bonds or aromatic ring. In organic compounds, suitable lone pair electrons for bonding occur in functional groups containing elements of groups V and VI from the periodic table. The tendency for stronger bond formation as well as stronger adsorption by these elements increases with decreasing electro negativity in the order oxygen (O) < nitrogen (N) < sulphur (S) < (Se) [99].

B- Vapour-Phase Inhibitors

These are similar to the organic adsorption-type of inhibitors and possess a very high vapour pressure. They are also called (VCIs), volatile corrosion inhibitors which are transported in a closed system to the site of corrosion by volatilisation from the source. When in contact with a metal surface, the inhibitor vapour condenses and is hydrolysed by any moisture present to liberate nitrite, benzoate and bicarbonate ions. They are usually effective in closed vapour spaces such as shipping containers and boilers because they would be lost rapidly through any leaks in the package or container