Cathodic inhibition, 5.34

Copper Corrosion Inhibitors. The most effective corrosion inhibitors for copper and its alloys are the aromatic triazoles, such as benzotriazole (BZT) and tolyltriazole (TTA). These compounds bond direcdy with cuprous oxide (CU2O) at the metal surface, forming a "chemisorbed" film. The plane of the triazole Hes parallel to the metal surface, thus each molecule covers a relatively large surface area. The exact mechanism of inhibition is unknown. Various studies indicate anodic inhibition, cathodic inhibition, or a combination of the two. Other studies indicate the formation of an insulating layer between the water surface and the metal surface. A recent study supports the idea of an electronic stabilization mechanism. The protective cuprous oxide layer is prevented from oxidizing to the nonprotective cupric oxide. This is an anodic mechanism. However, the triazole film exhibits some cathodic properties as well. 

Environment Reduce kinetics of cathodic reaction Lower potential of metal Cathodic inhibition Reduce a , reduce O2 concentration or concentration of oxidising species lower temperature, velocity agitation Cathodically protect by sacrificial anodes or impressed current sacrificially protect by coatings, e.g. Zn, Al or Cd on steel Formation of calcareous scales in waters due to increase in pH additions of poisons (As, Bi, Sb) and organic inhibitors to acids... 

Cathodic inhibition by rare earth metal compounds... 

The formation of a rare earth metal oxide on the metal surface, impedes the cathodic reduction of oxygen and thus cathodic inhibition is achieved by the addition of a rare earth metal salt to a system. The surface atom concentration ratio, [Ce/Ce + M], where M is Fe, Al or Zn, is a function of cerium oxide film thickness determined by AES depth profiles as shown in Fig. 12.2. 

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. 

Figure 3. Inhibition (top) cathodic inhibition (bottom) anodic inhibition...

In general, however, these equations are not applicable since symmetrical anodic and cathodic inhibition is a rare case. In the case of dimethylquinoline which, as shown in Fig.6, does not affect the cathodic hydrogen evolution, Hoare and Holiday f12) have proposed a different approach. One can assume that two kinds of... 

Anhydrous NiO has been studied in the form of mosaic crystals [(100) orientation] doped with 0.1%-10% Li. This material is ap-type semiconductor and hence cathodic processes such as O2 reduction would be expected to be inhibited. Examination of redox couples such as ferri-ferrocyanide with the rotating disk technique do indicate cathodic inhibition in acid solutions with the anode branch under combined kinetic and diffusion control. Differential capacitance measurements in acid solutions indicate that most of the electrode potential change occurs across the space charge region within the NiO (0.1% Li) at electrode potentials cathodic to 0.8 V vs. In... 

Corrosion in neutral environments is most often due to the reaction of dissolved oxygen with the metal. The risk of attack is therefore considerably reduced if one limits the access of oxygen to the metal surface (cathodic inhibition). Alternatively, one can diminish the rate of corrosion by bringing about passivation of the metal (anodic inhibition). Corrosion can also be reduced by inhibitors that adsorb onto the surface and block the reactive sites. 

Zinc ions inhibit corrosion by a cathodic polarization mechanism based on the precipitation of a zinc hydroxide film at cathodic sites on the metal surface. Zinc in combination with phosphates will lead to a protective film containing zinc phosphate. Film formation is usually rapid due to the low solubility of the zinc compounds at an alkaline pH. The low solubility of zinc in alkaline solutions requires the incorporation of dispersants. The rate of film formation with cathodic inorganic inhibitors should be carefully controlled, as dangerous fouling may occur. Protective films caused by cathodic inhibition are macroscopic and often easily visible, whereas anodic inhibitors generally from very thin, hardly detectable passive films. 

Group IV These metals normally are not used in applications where cavitation damage may occur, except in cathodically inhibited solutions or when protected by elastomeric coatings. 

State the mechanism of cathodic inhibition by addition of poisons of Group VA elements. 

These compounds are considered to have a role in the cathodic inhibition properties of lanthanoid salts.They are strong bases and absorb CO from the auto give carbonate species. The solubilities, and basicities of the rare earth hydroxides, as well as the pH values at which they precipitate, decrease with increasing atomic number, with Y(OH)3 being similar to Ho/Er(OH)3 because of similar ionic radii for the Ln ions. Similarly, CeO. xH O is precipitated by addition of base to Ce salts, for example sulfate or nitrate or double salts with ammonium sulfate or nitrate, and by oxidation of Ce(OH)3 precipitates by air (see Section 1.5.1). Single crystals of Ln(OH)3 have been grown by hydrothermal methods. For La(OH)3 to Er(OH)3, the compounds have hexagonal structures with 9-coordination for whereas Lu(OH)3 and Sc(OH)3 have cubic... 

Nerrrst-Planck s eqrration which accormts for the acido-basic eqrritibrittms susceptible to occirr in the solution allows to validate the experimental pH profile over the cut-edge. Figttre 21 shows such an experimental profile performed 150 pm above the sample sirrface and the corresportding simulated profile with or without the cathodic inhibition on 1900 pm of steel. 

It validates the necessaiy assumption in the definition of the geometiy of the electrode boundaries when a cathodic inhibition is assumed, simulated pH in solution above the coating and the inhibited area of steel is lower. Although SVET measurements give direct information on the surface reactivity and the cathodic inhibition on a part of steel, pH measurements need the numerical model to be interpreted in term of protective inhibition. 

Anodic or cathodic passivation (introducing chemicals which change the natural oxide to make it more protective and less active). In cathodic inhibition, inhibitors predominantly form insoluble precipitates or salts due to pH rise during corrosion processes [14]. Anodically active materials [92] promote the adsorption of oxygen at the surface (e.g. chromates, molybdate), or by forming insoluble complex salts with metal ions at the anodic defect sites (e.g. phosphate, borate), known as pore plugging [14]. 

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