Inhibitors chromate

Precipitation and Vapor-Phase Inhibitors. Precipitation inhibitors are film-forming compounds that produce barrier films over the entire surface. Phosphates and siUcates, which are the most common, do not provide the degree of protection afforded by chromate inhibitors, but are useful in situations where nontoxic additives are required. Two main drawbacks to the use of phosphates and siUcates are the dependence on the water composition and the control required to achieve maximum inhibition (37,38). 

Jointing compounds such as neoprene tape or fabric strip impregnated with chromate inhibitor or other inhibited caulking compounds should be applied carefully to ensure complete exclusion of water. 

Aluminium coatings exposed as sprayed show remarkable protective properties and in many cases the only evidence of age is the appearance of small hard nodules of aluminium oxide, and these do not appear to be deleterious. However, the dead white of the sprayed surface soon becomes discoloured by dirt and so a paint covering is usual. A chromate inhibitor is not essential and two coats of a simple vinyl-based sealing paint have given remarkable results in the tests of the American Welding Society over periods of 12 and 15 years. 

BSI recommends in the table of water treatment for hot water systems that chromate inhibitors should not be used because of their undesirable impact on the environment. It should also be noted that some treatment chemicals are microbiologically degraded in storage. 

In practice, at the time, it was usual to destroy most or all of the alkalinity and reduce the pH to below 7.0 and often to as low as 6.0 because of the effectiveness of chromate inhibitor barrier films, it was not considered necessary to additionally practice a controlled calcium carbonate deposition program. 

Recently, the mechanism of corrosion protection by soluble chromate inhibitors has been the subject of active research, which has attempted to understand and replicate its inhibiting functions with less toxic chemical substances. In this section, some recent findings on chromate corrosion inhibition are reviewed, particularly as they pertain to corrosion of light metals, with a focus on the use of techniques that are useful for studying mechanisms of inhibition. 

Hunkeler and Bohni used this approach to show that pit growth in A1 foils occurred under ohmic control (24). It was also shown that nitrate and chromate inhibitors, added to the electrolyte after pit initiation, inhibited pit growth kinetics though the effect due to chromate additions was small. Several other inhibitors added to solution increased pit growth kinetics, since their primary influence was in decreasing the solution resistance. 

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. 

Knudsen, J.G., Tijitrasmoro, E. and Dun-gi X, 1984, The effect of zinc chromate inhibitors on the fouling characteristics of cooling tower water, in Suitor, J.W. and Pritchard, A.M. eds. Fouling in Heat Exchange Equipment. HID vol. 35, Am. Soc. Mech. Engrs. 11-18. 

It has been established by express tests that the introduction of chromate inhibitors at amounts below 3 wt% amounts in alkyd paint compositions does not improve their anticorrosion properties as compared to non-inhibited coatings. 

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. 

At the base of the pits where reducing action accompanies corrosion, copper deposition can be seen. Copper comes from corrosion of bronze circulating pumps, phosphor-bronze fittings and copper alloy lines. These cooling systems could have been well protected by a chromate inhibitor, or by a borate-nitrite-MBT product, but experienced accelerated attack when using the improper phosphate-silicate inhibitor. 

Sinko, J. 2001. Challenges of Chromate Inhibitor Pigments Replacement in Organic Coatings. Prog. Org. Coat., 42, 267. 

Need For environmental and health reasons, future military systems may be required to eliminate use of chromate inhibitors and heavy-metal coatings. Use of composite structures (e.g., polymer composites that contain carbon fibers) can exacerbate the corrosion attack of contacting metals. 

Strontium chromate, the most expensive chromate inhibitor, is mainly used on aluminium. It is used in the aviation and coil-coating industries because of its effectiveness at very low loadings. 

Experimental work on Al/MnOj primary or dry batteries was concentrated on the D-size cylindrical battery using a construction similar to the one used for the Mg/Mn02 battery (Fig. 9.3). The most successful anodes were made of a duplex metal sheet consisting of two different aluminum alloys. The inner, thicker layer was more electrochemically active, leaving the outer layer intact in the event of pitting of the inner layer. The cathode bobbin consisted of manganese dioxide and acetylene black, wetted with the electrolyte. Aqueous solutions of aluminum or chromium chloride, containing a chromate inhibitor, were the most satisfactory electrolytes. 

There is now a search for less toxic, greener alternative corrosion inhibitors. Chromates have combined effectiveness with relatively low cost. Sinko has reviewed the quality parameters needed for corrosion inhibitors in coatings, and generally concluded that inorganic non-chromate inhibitors such as phosphates, molybdates, borates and silicates are inferior to chromates. Rare earth salts have been proposed and show promise as alternative corrosion inhibitors, and we have been developing rare earth carboxylates as potential dual function inhibitors combining the roles of the rare earth and the aromatic carboxylates. If rare earth-based corrosion inhibitors are to be used, there is a need to understand the chemistry of the elements and of the inhibitors. 

Baldwin KR, Gibson MC, Lane PL Smith CJE (1990). The development of alternatives to chromate inhibitors for the protection of aerospace aluminium alloys . 7th European Symposium on Corrosion Inhibition. University of Ferrara, pp. 771-785. 

Srnko J. (2001), Challenges of chromate inhibitor pigments replacement in organic coatings . Prog. Org. Coat, 42,267. 

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