Cathodic inhibitors hydroxide ions

Precipita.tingInhibitors. As discussed earlier, the localized pH at the cathode of the corrosion cell is elevated due to the generation of hydroxide ions. Precipitating inhibitors form complexes that are insoluble at this high pH (1—2 pH units above bulk water), but whose deposition can be controlled at the bulk water pH (typically 7—9 pH). A good example is zinc, which can precipitate as hydroxide, carbonate, or phosphate. Calcium carbonate and calcium orthophosphate are also precipitating inhibitors. Orthophosphate thus exhibits a dual mechanism, acting as both an anodic passivator and a cathodic precipitator. 

Any chemical (such as zinc hydroxide) that suppresses the reduction of oxygen to hydroxyl ion. A cathodic inhibitor suppresses that part of the electrolytic corrosion process at the cathodic sites on a metal surface. 

It is worthwhile to mention that both the passivating film and the precipitate film, which are formed in the presence of foreign ions and molecules, usually inhibit not only the anodic metal dissolution but also the cathodic reaction of corroding metals. There are however some inhibitors, which are effective only to one of the anodic and the cathodic reactions of metallic corrosion. In the case of porous precipitate films loosely attached to the metal surface, the anodic metal dissolution may be accelerated at porous sites of the precipitate films. For instance, Zn2+ ions, Al3+ ions, Co2+ ions, and Ce3+ ions, which are hard or slightly hard Lewis acid, combine with hydroxide ions of hard base forming a porous precipitate film of metal hydroxide on metallic iron in neutral solution. The porous precipitate film thus formed effectively inhibits the cathodic oxygen reduction, but it may accelerate the anodic dissolution of metallic iron at the porous sites of precipitates [86],... 

Lanthanides, especially cerium, fulfil the basic requirements for alternative corrosion inhibitors the ions form insoluble hydroxides, which enable them to be used as cathodic inhibitors they have a low toxicity and are relatively abundant in nature. Cerium has a high afimity for oxygen and the bond between cerium and oxygen is unlikely to be broken under the potentials applied. For some aluminium alloys, cerium precipitation from aqueous solutions of cerium salts was observed on cathodic intermetalhc compounds and in some instances, the oxide covered the entire specimen surface [14-19]. 

Examples of the latter are chromates, which are reduced to Cr(III) hydroxide or oxyhydroxide on the metal surface, or polyphosphates, in which decomposition and subsequent precipitation of Ca phosphate has been suggested [8]. The precipitation reactions will depend on the local solution composition (pH, metal ion concentration) in the near-surface region of the corroding metal, which may pronouncedly deviate from that in the bulk. For instance, the production of OH in the cathodic partial reaction will raise the surface pH and thus promote the precipitation of compounds, such as Zn hydroxides, even in noticeable acidic solution. In a similar way, the pore-plugging ability of anodic inhibitors may be enhanced by reactions with local metal ion accumulations in the vicinity of active pores in a passive film. 

Seawater predominantly consists of about 3.5% of sodium chloride (NaCl) and many other ions. Chloride ions are very strong and could easily penetrate the passive film Thus, dissolution of the aluminium substrate occurs and results in corrosion. The adsorption of the corrosion inhibitor competes with anions such as chloride. By assuming that the corrosion inhibitor molecules preferentially react with Al - to form a precipitate of salt or complex on the surface of the aluminum substrate, the anodic and cathodic processes subsequently suppressed by inhibitor molecules. Thus, this result suggests that the protective film that was formed comprise aluminium hydroxide, oxide and salts or complexes of the corrosion inhibitor anions. 

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. 

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