**2.5 Inhibition in organic molecules or natural liquids extracted from plants**

Particularly, the Mexican's oil-industry still uses tubular steel pipes for the specific purpose of transporting hydrocarbons or natural gas. Most of the lines are buried, so the national network extends over quite large distances, crossing varied terrains conditions some with rivers, others with salt-laden marshes, or polluted industrial or urban zones alike; the ambient temperatures and load pressure for the buried-pipelines network vary widely, to put it simply vulnerable to corrosion attack. Steel pipes are corroded as a result of iron oxidation during its exposure of longer service periods. Therefore, corrosion problems are directly related to everpresent economical and production losses, as well as environment affectations, though human losses also happen. Providing effective inhibiting substances that are added to processing fluids may reduce internal corrosion problems; there are a wide variety of organic substances known to act as corrosion inhibitors. **Figure 16** have demonstrated that small inhibitor quantities of organic molecules (2-Mercaptobenzimidazole, *MBI* or 5-Nitro-2-Mercaptobenzimidazole *NMBI* can be added to the media to diminish its inherent aggressiveness toward the steel surfaces [18, 60, 61]. It becomes evident that testing with the largest *2MBI* concentration, namely 200 ppm, there began to appear two-time constants, which suggests that two different processes are involved during the perturbation. One is related to a molecular adsorption mechanism of the organic compound over the polished metal surface, thus giving rise to multilayers, while the second constant is related to infiltration of the corrosive species through assorted passages formed during selfassembly and rearrangement of the organic molecules, very probably due to the diversity of interactive forces operating on the electrode system. This second time constant that operates at intermediate frequencies can be interpreted as a resistance to charge transfer. The *2MBI* inhibitor gave inhibiting efficiencies over 96% after adding only 20 ppm covering the metal surface exposed to the acid medium 1 M HCl. Therefore, the heterocyclic organic molecule *2MBI* was an efficient inhibitor in H2SO4 at 25 ppm. The plot of log Z vs. log f, shown in **Figure 17**, reveals that as the inhibitor concentration increases, so does the impedance, which is also related to the charge transfer resistance, Rct. This value was obtained through fitting a *RC* electrical circuit model #3 to the experimental data. The |*Z*| increment is explained by the excess of inhibitor's molecules in the solution, which as being bipolar it tends to adhere to the metal surface, also interacting among them thus forming a multilayered assembly, capable of blocking the electron charge transfer, refer to **Figure 17**, to appreciate more clearly the said |*Z*| increase.

values up to 90% at inhibitor concentrations ranging 0.8 to 2 g/L and immersion times of about 1 to 4 h. It has been found that the inhibition process takes place by the adsorption of the molecules on the surface of the metal (AISI 1045), by a

EIS *spectra in bode plots obtained from the pipeline steel API-5 L-X52 samples immersed in H2SO4 1 M as a function of the 2MBI or 5NMBI at different concentration [60, 61]. a) Response for 2- Mercaptobenzimidazole*

*Electrochemical Impedance Spectroscopy (EIS): A Review Study of Basic Aspects of the Corrosion…*

Other attractive uses of the *EIS* technique are its application to evaluate the integrity and coating performance during its exposure in corrosive environments as a function of time. Actually, *EIS* is used as a quality control to evaluate the process

*EIS spectra in bode plots obtained from the pipeline steel API-5 L-X52 samples immersed in H2SO4 1 M as a*

physisorption mechanism. See **Figure 18.**

*function of the different concentration of natural molecules.*

*and b) response for 5-Nitro-2-Mercaptobenzimidazole.*

*DOI: http://dx.doi.org/10.5772/intechopen.94470*

**Figure 17.**

**Figure 18.**

**25**

**2.6 Hard-coatings as protection; borided treatment**

Furthermore, Natural liquid-extracts like *Morinda citrifolia* has been used as corrosion inhibitor for steels (AISI-1045) exposed to acidic environments of HCl. Both the organic and inorganic compounds commonly used in the industry to inhibit the corrosion process of metals and its alloys are mostly composed by highly toxic chemicals, in addition to being more expensive. In this research sugarcomponents derived from the *Morinda citrifolia* (MC) leaves have been extracted in aqueous solutions to perform a natural inhibitor capable to control de corrosion damage, which can replace the traditional inhibitors, being environmentally friendly [62, 63]. The experimental results indicate that this compound has shown excellent performance as corrosion inhibitor, reaching inhibition efficiency (*EI*),

*Electrochemical Impedance Spectroscopy (EIS): A Review Study of Basic Aspects of the Corrosion… DOI: http://dx.doi.org/10.5772/intechopen.94470*

**Figure 17.**

EIS *spectra in bode plots obtained from the pipeline steel API-5 L-X52 samples immersed in H2SO4 1 M as a function of the 2MBI or 5NMBI at different concentration [60, 61]. a) Response for 2- Mercaptobenzimidazole and b) response for 5-Nitro-2-Mercaptobenzimidazole.*

values up to 90% at inhibitor concentrations ranging 0.8 to 2 g/L and immersion times of about 1 to 4 h. It has been found that the inhibition process takes place by the adsorption of the molecules on the surface of the metal (AISI 1045), by a physisorption mechanism. See **Figure 18.**
