Inhibition, corrosion passivation

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. 

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. 

Both factors are sensitive to alloy composition, which can be adjusted to produce electrodes having an acceptable cycle life. In AB5 alloys the effects of Ce, Co, Mn, and A1 upon cycle life in commercial AB5 -type electrodes are correlated with lattice expansion and charge capacity. Ce was shown to inhibit corrosion even though lattice expansion increases. Co and A1 also inhibit corrosion. XAS results indicate that Ce and Co inhibit corrosion though surface passivation. 

Anodic passivation of steel surfaces can be efficiently achieved by metal chromates. Chromates of Intermediate solubility (e.g., zinc chromate and strontium chromate) allow a compromise between mobility in the film and leaching from the film to be achieved. Chromates inhibit corrosion in aqueous systems by formation of a passivating oxide film. The effectiveness of chromate inhibitors in aqueous systems depends on the concentration of other ionic species in solution, for example, chloride. Synthetic resin composition can also significantly influence the effectiveness of chromate pigments. The effect appears to be related to the polarity of the resin (20) chromate pigments appear to be less effective in resins of low polarity. 

Corrosion and its inhibition or passivity both involve reactions between the metal surface and the solution. In the case... 

Oxidizing salts may either lead to rapid corrosion by providing an extra cathodic reaction (as do FeCls, and CuCy, or to inhibition and passivation (Na2Cr04,... 

Because paint is such a common means of combating corrosion, many paint companies add chemicals to inhibit corrosion. A professional society of industrial painters lists 20 rust inhibitors that are environmentally approved additives for paints. Although the specific chemical content may vary, most of these corrosion inhibitors contain one of the following ions phosphate, borosilicate, chromate, or phosphosilicate. The precise mode of corrosion inhibition depends on several factors, but all of these ions, when part of a paint coating, form compounds with oxidized iron that inhibit fiirther formation of rust. Much like an oxide coating on the surface of aluminum prevents ongoing corrosion, these coatings stop the corrosion of the iron after only a small amount of material has reacted. This process is called passivation. 

Oxygen reduction also plays a role in the corrosion of metals such as steel in the presence of air. The O2 reduction drives the potential sufficiently positive to produce the passivation film on ferrous metals, which in turn inhibits corrosion. 

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. 

To provide active corrosion inhibition by passivating the metallic substrate or by reducing the rate of the oxygen-reduction reaction... 

The reduction of chromate films is an important method of corrosion inhibition by passivation (section 10.2.2) ... 

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. 

---