Chromate Free Inhibitors

Because of pollution problems that can be caused by chromates, numerous inhibitor systems have been developed as alternatives— for example, organic phosphonic acids, which are effective in alkaline waters and are biodegradable [8]. Nontoxic inhibitor formulations containing mixtures of azoles and water-soluble phosphates (e.g., disodium phosphate and sodium tripolyphosphate) have been developed [9]. Sodium molybdate, which is less toxic than sodium chromate, has also been reported to be a useful component of inhibitor formulations for use in recirculating aqueous systems [9,10]. Other nontoxic, chromate-free inhibitor formulations are based on mixtures of sorbitol, benzotriazole or tolytriazole, and water-soluble phosphates [9]. 

Chromate-Free Inhibitors and Performance of Superprimer Coatings... 

Uses Corrosion inhibitor primer for use with structural epo) film adhesives in aerospace and general industries Features Contains adhesion promoter and latent resin hardener chromate-free curable reduced VOCs... 

Naturally, when comparing the film thicknesses of the CCC and primer layer it is obvious that the replacement of the chromate in the paint layer has a more substantial effect on reducing the amount of chromate that may leach/ dissolve out into the environment than replacing the pretreatment layer. The first step in improving the conventional three layer paint system shown in scheme A was to replace the CCC layer with a Cr-free pretreatment and the Cr-containing primer with a primer loaded with Cr-free inhibitors. This resulted in scheme B [25, 26]. In recent years attempts have been made to incorporate the silane into the primer, which results in 2-in-1 primers, where the pretreatment layer is built in the primer paint layer. This idea was first introduced by van Ooij et al., who have investigated these types of silane-containing Cr-free primers on aluminum alloys, HDG steel and CRS [7-11, 27-30]. 

It is, however, not so easy to just replace the chromate in an existing paint with a Cr-free inhibitor, because none of the Cr-free inhibitors are active over the whole pH range as chromate is and they also do not possess similar optimal solubUity properties as, e.g., strontium chromate does. Therefore, for the Cr-free primers to perform equally to Cr-containing primers they need to contain a mixture of Cr-free inhibitors to cover the entire pH-range and the paint layer matrix needs to be designed so that they are able to leach out on demand to protect the base metal [6]. 

Aramaki, K (2001). The inhibition effects of chromate-free, anion inhibitors on corrosion of zinc in aerated 0.5 M NaCl. Corrosion Science, Vol. 43, No. 3, pp. 591-604, ISSN 0010938X... 

For this reason, corrosion inhibitors such as cerium salts or organic biocides are successfully added to the silane sol-gel layer. Akid and coworkers, for example, have developed a broad range of chromate-free xerogel coatings based on doped Ormosil formulations with very high corrosion resistance and with little evidence of change after extensive salt spray tests for a variety of different metal substrates (aluminum alloys, steel, magnesium, etc.). 

Hemandez-Alvarado L. A., Hernandez L. S., Miranda J. M. and Dominguez O. (2009), The protection of galvanised steel using a chromate-free organic inhibitor , Anti-Corros. MethodM, 56,114-20. 

Results from exposure corrosion testing show that aluminum surfaces prepared with a chromate conversion coating and a chromate-free primer perform much better than a chromate-free sol-gel type of conversion coating with the same chromate-free primer, " leading to the necessity for enriching the sol-gel coating with efficient inhibitors. 

There have been several strategies pursued to create organic coating systems containing chromate-free corrosion inhibitors these include simple addition of corrosion inhibitors as pigments, microencapsulation and release from molecularly engineered... 

The most effective way to prevent SCC in both stainless steel and brass systems is to keep the system clean and free of deposits. An effective deposit control treatment is imperative. A good corrosion inhibitor is also beneficial. Chromate and phosphate have each been used successfully to prevent the SCC of stainless steel in chloride solutions. 

Acid leaks result in the deactivation of some chemical treatment programs, for example, P-PO4 is hydrolyzed to O-PO4, and chromate (in the presence of ferrous ions) is reduced to trivalent chromium. Also, depassivation of the inhibitor film and corrosion occurs. The correct action is to first stop the leak, then to increase the pH by bleeding off the low pH water and making up, to the maximum extent of the system. If the leak is so severe that the pH drops below 4.3, and free mineral acidity (FMA) occurs, soda ash should be used to restore the pH to 4.3 to 4.5, then discontinued. Caustic soda should not be added, as this action will almost certainly result in heavy fouling from precipitated ferric hydroxide. Dispersants or antifoulants may be required. When the system is stabilized, it should be repassivated and the reason for the acid leak resolved (faulty pH meter ). 

Chem. Descrip. Calcium molybdate CAS 7789-82-4 EINECS/ELINCS 232-192-9 Uses Conosion inhibitor pigment for optimal package stability in resin systems destabilized by zinc compds. fungistat Eeatures Nonbasic chromate and lead-free Properties Wh. 

It is prohibited to discharge chromate-rich water, generally above 0.1 g-m" of CrVI. However, there are tried and true, relatively inexpensive, industrial processes to break down and eliminate chromates. Zinc can be eliminated at the same time, As a result, the zinc-chromate process yields deconcentration blowdown that is free of corrosion inhibitor components. 

Cathodic inhibitors slow the rate of the oxygen reduction reaction, and hence the companion oxidation reaction must also slow. This results in an overall decrease in the corrosion rate, as well as a decrease in the free corrosion potential. For the best inhibitors, the decrease in the corrosion potential is usually to a value well below the alloy s pitting or repassivation potential. Cathodic inhibitors have the advantage of being able to improve corrosion resistance at low concentrations. For example, chromate added at micromolar concentrations to an aerated dilute chloride solution is enough to significantly reduce the rate of oxygen reduction. 

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