MICROESTRUCTURA DE LOS MATERIALES

Corrosion inhibitor selection is based on the metal and the environment. Figure 2-21 shows classification of corrosion inhibitors into two broad categories: environmental conditioners and interface inhibitors (Papavinasam, 2000).

2.4.1.1Environmental Conditioners

Environmental conditioners, also called scavengers, are corrosion inhibitors which act by scavenging the aggressive substances (species) from the environment. Examples of scavengers include sodium sulphite which removes dissolved oxygen from near-neutral and alkaline solutions, thereby decreasing oxygen reduction (cathodic reaction), as represented in Equation 2-20.

Figure 2-21: Classification of inhibitors. (Papavinasam, 2000)

2.4.1.2Interface 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.

Liquid-Phase Inhibitors

Liquid-phase inhibitors are generally categorized on the basis their chemical functionality in the electrochemical reactions concerned in the corrosion process. Hence, these inhibitors are classified as anodic, cathodic or mixed inhibitors, indicating the inhibited electrochemical reaction(s) respectfully.

Anodic Inhibitors

Anodic inhibitors also called passivating inhibitors work by causing a large anodic shift in the corrosion potential in the noble direction thereby bringing the metallic surface into the passivation range. There are two types of these effective and consequently the most widely used inhibitors, namely:

 Oxidizing anions, such as chromate, nitrite and nitrate that passivates the steel surface in the absence of oxygen.

 Non-oxidizing ions such as phosphate, tungstate and molybdate that requires the presence of oxygen to passivate the steel surface.

Passivating inhibitors are usually used in near-neutral solutions where sparingly soluble corrosion products, such as oxides, hydroxides, or salts, are formed. Papavinasam (2000) argues that the anodic inhibitors form, or facilitate the formation of passivating films that inhibit the anodic metal dissolution reaction. He is also of the view that the critical concentration of these inhibitors for effective inhibition in any corrosive medium depend on the nature and concentration of the aggressive ions. A number of authors also agree that inadequate concentration of anodic inhibitor may accelerate the rate of corrosion rather than reducing it (Roberge, 2000; Ramachandran et al., 2000; Papavinasam et al., 2003).

An inhibitor may decrease the rate of the anodic process, the cathodic process, or both processes. The change in the corrosion potential on addition of the inhibitor is often a useful indication of which process is retarded. Displacement of the corrosion potential in the positive direction indicates mainly retardation of the anodic process (anodic control), whereas displacement in the negative direction indicates mainly retardation of the cathodic process (cathodic control). Little change in the corrosion potential suggests that both anodic and cathodic processes are retarded.

An anodic inhibition mechanism is illustrated in Figure 2-22 which shows an increase in the polarisation of the anode where a positive corrosion potential displacement (shift in the noble direction) accompanied by significant reduction in the corrosion current flow. 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.

Figure 2-22: The effect of an anodic inhibitor on the dissolution rate of iron and iron oxide

(Roberge, 2000).

Cathodic Inhibitors

Cathodic inhibitors control corrosion by either slowing the cathodic reaction process or by selectively precipitating on the cathodic areas to increase the surface impedance and so limit the diffusion of cathodic species to these sites. Cathodic inhibitors can provide inhibition by making the recombination and discharge of hydrogen more difficult or by catalysing cathodic ions such as zinc, calcium to precipitates as oxides on the metal surface.

Inhibition by polarisation of the cathodic reaction can be achieved in several ways such as oxygen scavengers, cathodic poisons and cathodic precipitates. A major drawback in the use of cathodic poisons is that they sometimes cause hydrogen blistering and an increase in hydrogen embrittlement, especially in acid solutions (Papavinasam et al., 2003).

Figure 2-23: The effect of a cathodic inhibitor on the dissolution rate of iron and iron oxide (Roberge, 2000).

Figure 2-23 is a simplified illustration of cathodic inhibition mechanism (Roberge, 2000) which shows a negative corrosion potential displacement (shift in the active potential direction) of the polarization curves, accompanied by significant decrease in the cathodic reduction reaction.

Mixed (Organic) Inhibitors

Mixed (organic) inhibitors suppress the anodic and the cathodic reactions involved in the corrosion process by adsorbing on the metal surface forming a protective layer. It is essential that this inhibitor be present in sufficient concentration for effective inhibition since it is adsorbed unto both anodic and cathodic sites of the metal of concern. Their effectiveness also depends on chemical composition, molecular structure and their affinities for the metal surface (Papavinasam, 2000; Gulbrandsen and Granå, 2007).

Vapour-Phase Inhibitors

These are similar to the organic adsorption-type of inhibitors and possess a very high vapour pressure. They are also called volatile corrosion inhibitors (VCIs), 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