Inhibition, corrosion current densities

Note, however, that there are conditions under which inhibitors can give rise to detrimental local corrosion, that is, pitting corrosion. This is the case when the amount of inhibitor is insufficient. Under these conditions, only part of the surface can be covered, thus giving rise to a local element. Corrosive attack is particularly extensive at the uncovered anode areas because of increased corrosion current density and deep cavities penetrating into the material. Similarly, if the inhibitor is too readily reduced at the cathodic areas of the metal surface, increased corrosion can result because compact protective films are not formed. Since there are no universally applicable inhibitors, they must be carefully selected and examined for each specific case. In doing so, inhibition of metal dissolution is not the only point to be considered—there is also hydrogen absorption. 

The results given in Figures 12.31 and 12.32 illustrate the application of these relations to the identification of an inhibition mechanism. Figure 12.31 shows the corrosion current density of iron as a function of benzotriazole concentration added to sulfate solutions of different pH [21]. From these results, the surface coverage of the... 

Table 3 lists the electrochemical kinetic parameters corrosion current density (icorr)/ corrosion potential (Ecorr.) and degree of inhibition efficiency (Z, %), characterizing the corrosion process in the presence and in the absence of cerium ions, determined on the basis of the results represented in Fig. 7. The degree of inhibition efficiency has been calculated on the basis of the equation ... 

The corresponding corrosion potential (Ecorr), corrosion current density (icon), anodic Tafel slope (ba), cathodic Tafel slope (be) and CR for uninhibited and inhibited systems from PP measurement are listed in Table 3. The data demonstrates that the Ecorr values shift to more positive values as the concentration of added studied inhibitors are increased. On the other hand, the corrosion current densities are markedly declined upon addition of the studied corrosion inhibitors. The extent of its decline increases with increasing of the corrosion inhibitor concentration. Moreover, the numerical values of both anodic and cathodic Tafel slopes decreased as the concentration of inhibitors were increased. This means that the three natural products have significant effects on retarding the anodic dissolution of aluminium alloy and inhibiting the cathodic hydrogen evolution reaction. 

The current density-potential characteristics ofcarbon steel in aerated 0.5 M NaC104 solution are shown in Figs. 9-19 and 9-20 as a function of Ca/HEDP and Zn/HEDP inhibitor molar ratio, respectively (Felhdsi et al., 1999a). The calculated polarization results, i.e., corrosion potential (E or) corrosion current density (icor) aod inhibition efficiency ( /dc) are listed in Table 9-14 (Felhosi et al., 1999a). 

Table 9-14. Corrosion potential ( cor)> corrosion current density (i cor). inhibition efficiency (i/dc). and synergism parameter (S) in different solutions .

As mentioned above, inhibitors may block anodic metal dissolution or the cathodic reduction reaction or both processes simultaneously. If the cathodic reaction is inhibited, the related Tafel line of Figure 1.45a is shifted to negative potentials. As a consequence, the rest potential Er shifts to a more negative value Er, and the related corrosion current density from z c to the smaller value i i- If the anodic metal dissolution is inhibited, the related... 

The fall in reduction of area and the occurrence of internal cracks are a measure of the corrosion damage. There exists a clear correlation with cathodic current density in which a slight inhibition due to O2 and stimulation by CO2 can be recognized. The susceptibility is very high in the range of cathodic overprotection and is independent of the composition of the medium. 

Passivating inhibitors act in two ways. First they can reduce the passivating current density by encouraging passive film formation, and second they raise the cathodic partial current density by their reduction. Inhibitors can have either both or only one of these properties. Passivating inhibitors belong to the group of so-called dangerous inhibitors because with incomplete inhibition, severe local active corrosion occurs. In this case, passivated cathodic surfaces are close to noninhibited anodic surfaces. 

This material can be used only in seawater or similar chloride-containing electrolytes. This is because the passivation of the silver at discontinuities in the platinum is dependent upon the formation of a film of silver chloride, the low solubility of which, in seawater, inhibits corrosion of the silver. This anode, consisting of Pt-lOPd on Ag, was tried as a substitute for rapidly consumed aluminium, for use as a trailing wire anode for the cathodic protection of ships hulls, and has been operated at current densities as high as 1 900 AmHowever, the use of trailing anodes has been found inconvenient with regard to ships manoeuvrability. 

The density of the corrosive current of jamesonite in NaOH solution is basically the same as that in Ca(OH)2 solution, but it is minimal in Na2C03 solution, about a fraction of the fourth of the former. There are obvious appearances of passivation and its breaking-down in strong polarization area in NaCOa solution Because COj ion is easier to form insoluble alkaline carbonate than OH ion, the carbonate salts are passive on the mineral surface to inhibit oxidation reaction. 

It should be mentioned that passive layers are not protective in all environments. In the presence of so-called aggressive anions, passive layers may break down locally, which leads to the formation of corrosion pits. They grow with a high local dissolution current density into the metal substrate with a serious damage of the metal within very short time. In this sense halides and some pseudo halides like SCN are effective. Chloride is of particular interest due to its presence in many environments. Pitting corrosion starts usually above a critical potential, the so-called pitting potential /i]>j. In the presence of inhibitors an upper limit, the inhibition potential Ej is observed for some metals. Both critical potentials define the potential range in which passivity may break down due to localized corrosion as indicated in Fig. 1. 

Since the process is cathode limited, it is possible to slow it down by inhibiting the rate of hydrogen evolution. Many commercial corrosion inhibitors function in this manner. Considering Fig. lOM, it is easy to see that decreasing the exchange current density of hydrogen evolution by the addition of a suitable corrosion inhibitor is equiva-... 

From the slope of the polarization curve and its variation with time (exposure time of the iron electrode), information on the kind of inhibition can be gained. An inhibition of anodic processes decreases the ia versus E current density and increases the corrosion potential correspondingly, an increase in cathodic inhibition causes a decrease in the i. and lowers the corrosion potential. 

All protein solutions shifted the corrosion potential (ia=ic) in the negative direction due to the adsorption process. For albumin and globulin systems, an enhancement in the anodic current density was observed at potentials positive to the passivation potentials. However, fibrinogen solutions exhibited an inhibition in the polarization profiles. 

Chemical Treatment. A wide variety of chemicals and water treatments are used for corrosion control. Corrosion inhibitors usually act by forming some type of impervious layer on the metallic surface of either the anode or cathode that impedes the reaction at the electrode and thereby slows or inhibits the corrosion reaction. For example, various alkali metal hydroxides, carbonates, silicates, borates, phosphates, chromates, and nitrites promote the formation of a stable surface oxide on metals. The presence of these chemicals in the electrolyte allows any faults in the metal surface or its oxide film to be repaired. If they are used in too small a quantity as anodic inhibitors, they may promote intense local attack because they can leave a small unprotected area on the anode where the current density will be very high. This is particularly true of chromates and polyphosphates. 

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