Inhibition, corrosion adsorption

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

Direct measurements on metals such as iron, nickel and stainless steel have shown that adsorption occurs from acid solutions of inhibitors such as iodide ions, carbon monoxide and organic compounds such as amines , thioureas , sulphoxides , sulphidesand mer-captans. These studies have shown that the efficiency of inhibition (expressed as the relative reduction in corrosion rate) can be qualitatively related to the amount of adsorbed inhibitor on the metal surface. However, no detailed quantitative correlation has yet been achieved between these parameters. There is some evidence that adsorption of inhibitor species at low surface coverage d (for complete surface coverage 0=1) may be more effective in producing inhibition than adsorption at high surface coverage. In particular, the adsorption of polyvinyl pyridine on iron in hydrochloric acid at 0 < 0 -1 monolayer has been found to produce an 80% reduction in corrosion rate . 

Electrochemical impedance spectroscopy was used to determine the effect of isomers of 2,5-bis( -pyridyl)-l,3,4-thiadiazole 36 (n 2 or 3) on the corrosion of mild steel in perchloric acid solution <2002MI197>. The inhibition efficiency was structure dependent and the 3-pyridyl gave better inhibition than the 2-pyridyl. X-ray photoelectron spectroscopy helped establish the 3-pyridyl thiadiazoles mode of action toward corrosion. Adsorption of the 3-pyridyl on the mild steel surface in 1M HCIO4 follows the Langmuir adsorption isotherm model and the surface analysis showed corrosion inhibition by the 3-pyridyl derivative is due to the formation of chemisorbed film on the steel surface. 

Figures 7.22 and 7.23 are the EIS of pyrite interaction with collector at pH = 12 modified by NaOH and lime respectively. Figure 7.22 shows that the radius of the capacitive reactance loop is, respectively, 8700 Q without collector, 9400 Q by the addition of xanthate and 10000 Q by the addition of dithiocarbamate. The small increase of the radius of the capacitive reactance loop at pH= 12 modified by NaOH when adding xanthate and dithiocarbamate, shows that the two collectors still have certain inhibiting corrosion action and hence adsorption on pyrite. The action of xanthate is some stronger than that of DDTC because the radius of the capacitive reactance loop in the presence of xanthate is slightly larger than that in the presence of DDTC. However, Fig. 7.23 demonstrates that at pH = 12 modified...

Azole compounds such as benzotriazole, benzimidazole, indazole and imidazoles are efficient anti-corrosion agents for copper and copper-base alloys [1-10]. Many experimental techniques [11-15] have been used to study the corrosion inhibition mechanisms, however, the mechanisms are still not well understood. It is believed that the complex formation between copper and nitrogen atoms would inhibit oxygen adsorption on copper surface [16-20]. 

It was indicated in the last section that most organic compounds maximize their adsorption near the potential of zero charge of the metal on which they adsorb. This suggests that organic molecules are more likely to be effective in inhibiting corrosion to the extent that the corrosion potential is near the pzc. However (see Fig. 12.41), the potential range in which the inhibitor adsorbs, around the pzc, can be large, particularly with aromatic compounds. 

P/100, surface coverage / , AG/2.303 RT AC, free energy of adsorption R, gas constant T, temperature c, bulk inhibitor concentration n, number of water molecules replaced per inhibitor molecule f, inhibitor interaction parameter (0, no interaction + attraction and repulsion) K, constant and % P = 1 inhibited corrosion rate/uninhibited corrosion rate. 

If one assumes that the blank corrosion rate dcorr) sp ssents the total number of "active sites" on the surface of a corroding metal and that the inhibited corrosion rate (fi p ) represents the total number of active sites minus the inhibited sites, then equation 15 is an expression for the fraction of the surface which is covered by inhibitor molecules. Thus, as explained previously, 0 stands for the fractional coverage of the surface with inhibitor and is, of course, equal to the percent protection as defined by equation 15. The dependence of coverage 0 on the concentration of the inhibitor, within the assumptions and restrictions made previously, is given by the adsorption isotherm of a particular inhibitor. A typical adsorption isotherm was derived by Langmuir and is given in equation 19 ... 

This last effect may be an indication of adsorption of a small impurity in the electrolyte. The inhibited corrosion rates decrease with time and become essentially constant after about two hours. These slopes are not dependent on scan rate or on corrosion rate. The most interesting effect is observed when the inhibited hydrochloric acid solution is aerated the anodic Tafel slope increases while the cathodic Tafel slope decreases dramatically. As would have been expected from the resistance probe measurement the corrosion rate in the aerated inhibitor solution increases. 

N.O. Eddy, A.O. Odiongenyi (2010). Corrosion inhibition and adsorption properties of ethanol extract of IT Heinsia crinataJYf on rmld steel in H2SO4. Pigment and Resin Technology 39(5), pp. 288-295. 

The consensus is that organic compounds inhibit corrosion by adsorbing at the metal/solu-tion interface. Three possible types of adsorption are associated with organic inhibitors n-bond orbital adsorption, electrostatic adsorption, and chemisorptions. A more simplistic view of the mechanism of corrosion inhibitors can be described as controlled precipitation of the inhibitor from its environment (water and hydrocarbons) onto metal surfaces. During the past decade, the primary improvements in inhibitor technology have been the refinement of formulations and the development of improved methods of applying inhibitors (Totlani and Athavale 2000 Farquhar et al. 1994). 

Most pickling inhibitors function by forming an adsorbed layer on the metal surface, probably no more than a monolayer in thickness, which essentially blocks discharge of H+ and dissolution of metal ions. For example, both iodide and quinoline are reported to inhibit corrosion of iron in hydrochloric acid by this mechanism [19]. Some inhibitors block the cathodic reaction (raise hydrogen overpotential) more than the anodic reaction, or vice versa but adsorption appears to be general over all the surface rather than at specific anodic or cathodic sites, and both reactions tend to be retarded. Hence, on addition of an inhibitor to an acid, the corrosion potential of steel is not greatly altered (<0.1 V), although the corrosion rate may be appreciably reduced (Fig. 17.3). 

The ability of aminated compounds to inhibit corrosion on metallic surfaces via adsorption phenomena has been already certified. Since operations taking place at interfaces are greatly affected by variations in surface tension, aminated surfactant molecules are expected to provide even better results. This has been the case, when self-assembled micellar or microemulsion systems are used as corrosion inhibitors. In that aspect, surfactants may be used as organic corrosion inhibitors, and act by forming a protective film onto surfaces which are exposed to corrosive media, like oxygen and saline or acidic solutions. When microemulsions are used, an oil film is also adsorbed onto the surface with the surfactants tails oriented towards it, in view of the usually positive character of the surface. In the petroleum industry, the oil itself may be the nonpolar component of such systems. Figure 15.10 is a schematic of these types of films. 

The anion adsorption processes that are occurring at the metal/solution interface will be competing with each other, so that the outcome of these conpeti-tions could either be enhancing or inhibiting corrosion... 

Most corrosion processes, e.g., metal dissolution, hydrogen or oxygen evolution, and passive film formation, involve at least one adsorption step as a part of the overall reaction. This step can be significantly affected by the presence on the metal surface of a monolayer of nonmetal species. As evidenced by studies described in this chapter, adsorbed species may act by loosening the metal-metal bond or changing the electric field at the metal-electrolyte interface. They can also favor or inhibit the adsorption or the recombination of adsorbed atoms normally involved in the anodic or cathodic reactions. 

Recent developments in the mechanisms of corrosion inhibition have been discussed in reviews dealing with acid solutions " and neutral solu-tions - . Novel and improved experimental techniques, e.g. surface enhanced Raman spectroscopy , infrared spectroscopy. Auger electron spectroscopyX-ray photoelectron spectroscopyand a.c. impedance analysis have been used to study the adsorption, interaction and reaction of inhibitors at metal surfaces. 

Adsorption is, of course, of major importance in the inhibition of corrosion by organic compounds (adsorption inhibitors) that have the ability to adsorb strongly on the metal surface, thus impeding the dissolution reaction and reducing the corrosion rate. It follows that the coverage of a metal surface by adsorbed inhibitor can be evaluated from the relationship... 

They also provide useful corrosion inhibition by the adsorption of calcium phosphonate onto iron oxide corrosion products, thus reducing the ferrous metal corrosion rate. Phosphonates can be described as cathanodic corrosion inhibitors. 

Electrochemical techniques have been utilized for many years to study metal corrosion. Two of these techniques, linear polarization (LP) and cyclic voltammetry (CV), complement each other, LP providing corrosion rates under conditions where the surface is minimally altered and CV furnishing information about the corrosion mechanism. With the advent of impedance spectroscopy (IS), both kinds of information can be gleaned simultaneously and more rapidly, while leaving the surface almost intact. In this paper, we discuss the application of IS to the study of rapid steel corrosion and describe a study we undertook to elucidate the roles played by adsorption and film formation in the inhibition mechanisms of the above-named compounds. For comparison, we also investigated two quaternary nitrogen salts, which appear to adsorb electrostatically and presumably do not form macroscopic films (8). 

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