Chromate systems

It is evident from Fig. 22.2 that only in very dilute solutions are monomeric vanadium ions found and any increase in concentrations, particularly if the solution is acidic, leads to polymerization. nmr work indicates that, starting from the alkaline side, the various ionic species are all based on 4-coordinate vanadium(V) in the form of linked VO4 tetrahedra until the decavana-dates appear. These evidently involve a higher coordination number, but whether or not it is the same in solution as in the solids which can be separated is uncertain. However, it is interesting to note that similarities between the vanadate and chromate systems cease with the appearance of the decavanadates which have no counterpart in chromate chemistry. The smaller chromium(VI) is apparently limited to tetrahedral coordination with oxygen, whereas vanadium(V) is not. 

There are essentially three main steps in a conversion coating process cleaning, conversion coating, and post-treating. These three different, but equally important, steps in the pretreatment of metal articles will be discussed in more detail for the purpose of providing a background for the main emphasis of this paper, the post-treatment part of the conversion coating process, and more specifically chromium-free polymeric post-treatments which have been developed in recent years to replace the environmentally unacceptable chromate systems. 

On zinc phosphate conversion coatings, the "Mannich" derivatives of poly-4-vinylphenol have demonstrated performance equivalent to chromic chromate systems in salt spray, humidity, and physical testing. In addition. Table III illustrates results observed with automotive body paint systems evaluated by the "scab" or "cycle" test which causes failure more typical of actual end use conditions than do salt spray evaluations. Again, results equivalent to chromic-chromate post-treatments were obtained. In addition, the humidity resistance and adhesion tests were essentially equivalent to the chromium controls. 

Zinc salts rapidly generate zinc hydroxide or salt protective films on cathodic surfaces when they are added to cooling water. They are generally used in conjunction with other corrosion inhibitors, such as organophosphates. BETZ manufacturers a combination HEDP-zinc inhibitor. They have also been used with chromate systems to inhibit the chromate concentrations required. A disadvantage of zinc is its tendency to precipitate in pH environments greater than 8.0 (Roti 1985). While the toxicity of zinc to humans is far lower than that of chromium, its toxicity to marine and aquatic life is high. 

In summary, non-chromate corrosion inhibitors can offer similar or sometimes superior corrosion protection than systems containing chromate, without some of the attendant environmental problems of chromate systems. Before substituting a new corrosion inhibitor, however, it is important to thoroughly examine the environmental implications that its use might imply. 

In dealing with metal water cooling systems, today s trend is toward the use of nonchromate-based treatment chemicals. Nonchromate applications rely on less tenacious films for corrosion protection, rather than conventional chromate systems. As such, it is extremely important that the corrosion protection film be established very early in the operation. 

The first, and so far only, experiments in the pulsed mode are those by Epperlein et al (207) who provide shielding data for the chromate ion as a function of counter-ion and concentration. The deuterium induced isotope effect for the chromate system is 1 -3 ppm with a higher... 

Unlike the chromate system there is no reason to believe that any... 

To establish the color change quality of an indicator on an objective basis, the color changes have to be characterized by chromatic parameters. The sequence of color changes of indicators can be described for both CIE and complementary chromaticity systems from the experimental values obtained in indicator solutions, by continuous measurement of the absorbance values at 10 nm intervals between 380 and 770 nm over the pH range of the color change of the indicator, and simultaneously by potentiometric measurement. 

Research is now strongly centred on prodncing primers with low volatile organic contents, by using water as the primary solvent or dispersion medium or actual water-based primers where the organic solvent content is zero. For similar reasons of Environment and the impact of adhesive technology, a move to non-chromate systems is also under way. 

Similar results have been obtained with a polymer bound ammonium chromate system [12]. The conversion of a number of bromides into carbonyl compounds is recorded in Table 11.4. 

Campazzi, E., Villatte, M., Druez, C., Senani, S., and Barbe, C. (2013) Alternatives to chromate systems for corrosion protection in aerospace study of Green -inhibitor loaded spheres in model sol-gel coatings. 9th Coatings Science International 2013, (une 24-28, 2013, Noordwijk, The Netherlands. 

The overall reaction for the calcium-calcium chromate system is dependent on the discharge parameters. A postulated cell reaction is ... 

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