Cathodic inhibitors precipitates

Precipitating (cathodic) inhibitors are simply chemicals that form insoluble precipitates that can coat and protect the surface precipitated films are not as tenacious as passive films and take longer to repair after a system upset. 

There are basically three main types of inorganic inhibitors used in industry anodic passivating inhibitors, cathodic inhibitors and cathodic precipitators. 

Cathodic inhibitors promote coverage of iron by a protective coating, but this need not be an iron oxide. Immersion of iron in a solution of phosphoric acid containing a suitable catalyst causes precipitation of a mixed Fe"-Fe " phosphate film which serves as a base for a coating of paint. 

Various zinc salts (usually chlorides and sulfates) have been used in cooling water formulations for many years zinc acts as a cathodic inhibitor by forming a protective precipitate of (often gelatinous) zinc hydroxide at the cathode due to a localized elevated pH. 

Cathodic inhibitors reduce corrosion by rapidly polarizing the cathode. Examples of cathodic inhibitors are filming amines, magnesium salts, and calcium salts. In waters containing carbon dioxide, calcium carbonate precipitates (as discussed above) to form a protective film. 

Zinc sulfate can be used as a cathodic inhibitor. The Zn + ions in solution will react with OH produced by the cathodic reactions or with carbonates to form sparingly soluble zinc precipitates that coat the cathode. 

In the late 1950 s, chromate-phosphate systems incorporating zinc as another cathodic inhibitor were introduced, followed by zinc-chromate inhibitors, without phosphate. Using chromate-zinc, or polyphosphate-chromate-zinc inhibitors it was possible to cut working concentrations still further. It was necessary to control pH to make this inhibitor system function effectively. At increased pH (above pH 7.5)tendency for zinc loss by precipitation increases. Furthermore, pH rise may cause heat exchangers to become fouled by zinc hydroxide slimes or zinc phosphate. 

PRECIPITATING (CATHODIC) INHIBITORS - The are chemicals which form insoluble precipitates that are able to coat and protect surfaces. They are less durable then the passivating type inhibitors. 

Cathodic inhibitors in near-neutral solutions interfere with the oxygen reduction reaction by restricting the diffusion of dissolved oxygen to the electrode surface. These substances usually form thick surface layers with poor electronic conductivity (the latter is an important prerequisite to avoid oxygen reduction on the film surface). Examples are phosphates, polyphosphates, silicates, borates, and inorganic inhibitors, such as Zn +, which precipitate as Zn hydroxide, and Ca ", which forms calcium carbonate films in the presence of CO3 [3]. These inhibitors... 

Cathodic inhibitors either selectively precipitate on cathodic areas or slow the cathodic reaction by increasing hydrogen overvoltage. Other cathodic inhibitors utilize alkalinity increase at cathodic sites to precipitate insoluble compounds on the metal surface. Hydrogen evolution causes the metal-concrete interface to become more alkaline and ions such as calcium or magnesium precipitate as oxides to form a protective layer. 

Cathodic inhibitors either slow the cathodic reaction itself or they selectively precipitate on cathodic areas to increase circuit resistance and restrict diffusion of reducible species to the cathodes. Acid inhibitors such as arsenic and antimony compounds and also oxygen scavengers are examples of cathodic inhibitors (Tumipseed 1997). 

Cathodic inhibitors Inhibitors which decrease the corrosion rate by suppression of cathodic reduction reaction, such as oxygen and hydrogen reduction. Cathodic precipitates Inhibitors, like calcium carbonate and magnesium carbonate, which precipitate on cathodic areas. 

Mishra and Balasubramanian studied the corrosion behavior of pnre A1 that had been exposed to 3.5% NaCl solution containing different concentrations of LaClj or CeClj. Polarization resistance and EIS data showed that LaClj additions provided better corrosion protection than CeClj with the maximum corrosion inhibition being observed at 1000 ppm. The REMs acted as cathodic inhibitors and caused only minor changes in the pitting potential. SEM observations revealed REM oxides/hydroxides precipitates with different morphologies on the smface ofAl. 

The various routes that have been examined to enhance the corrosion resistance of anodized aluminium using cerium compounds, such as nitrates or sulphates, include pre- and post-treatments and anodizing with cerium species added to the bath formulation (the addition of cerium to commercial alloys for corrosion protection has not been reported). Each of these approaches has revealed some benefits in short-term laboratory corrosion tests. However, the imderlying mechanisms of corrosion protection are imclear. Corrosion inhibition of aluminium alloys caused by cerium species is well-established, with cerium acting as a cathodic inhibitor at intermetallic particles (Amott et al, 1987). Cerium oxide/ Itydroxide precipitates in response to the increase of pH at cathodic particles as the alloy briefly corrodes following its immersion in the environment. The precipitates stifle further corrosion by hindering the reduction of oxygen and water. 

If the water contains dissolved salts that cm react with OH and form a dense, sparingly soluble coating on the cathode, the corrosion will stop completely or it will be reduced. Salts with this property are used as cathodic inhibitors for prevention of corrosion. As an example, mention can be made of highly soluble zinc salts, which can precipitate as zinc hydroxide... 

Cathodic Inhibitors slow down corrosion by reducing the rate of the cathodic reaction in the corrosion system. They may form precipitates in the cathodic locations to limit access of the cathodic reaction species, and they are also called precipitation inhibitors. " Zinc salts are cathodic inhibitors that form precipitates of zinc hydroxide at the cathode.Magnesium salts also work in a similar way. - Bicarbonate (HCOj ) forms insoluble metal carbonates in alkaline solution. Phosphates,... 

Chromates are very effective inhibitors of Fe, Al, Cu, Zn corrosion. The unique chemical and electronic properties of the oxo-compounds of chromium give rise to a unique ability to inhibit corrosion in ferrous and nonferrous materials. They are both anodic and cathodic inhibitors due to their abilities to form precipitates with the dissolving metal ions... 

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]. 

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. 

Inhibitors act and are classified in a variety of ways (1,3,37,38). The classifications used herein closely foUow the discussion in Reference 37. Types of inhibitors include (/) anodic, (2) cathodic, (3) organic, (4) precipitation, and (5) vapor-phase inhibitors. 

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