Conversion coatings phosphates

Operation and Control. Control of a chromium phosphate conversion coating bath requires monitoring chromium and aluminum concentrations, active fluoride level, and temperature. Coating weight is very sensitive to active, ie, uncomplexed, fluoride. An innovative electrochemical method using a siHcon electrode (25) is employed for measuring active fluoride. A special precaution in chromium phosphate bath operation is the... 

Product Utilization. The principal appHcation for chromium phosphate coatings is as a paint base for painted aluminum extmsions and aluminum beverage can stock. In these appHcations, extremely demanding performance criteria are met by the chromium phosphate conversion coatings. As an example, the Architectural Aluminum Manufacturer s Association Voluntary Specification 605.2-92 requires humidity and salt spray testing for 3000 hours and allows only minimal incidence of paint failure after testing (26). 

Fitzpatrick et al. [41] used small-spot XPS to determine the failure mechanism of adhesively bonded, phosphated hot-dipped galvanized steel (HDGS) upon exposure to a humid environment. Substrates were prepared by applying a phosphate conversion coating and then a chromate rinse to HDGS. Lap joints were prepared from substrates having dimensions of 110 x 20 x 1.2 mm using a polybutadiene (PBD) adhesive with a bond line thickness of 250 p,m. The Joints were exposed to 95% RH at 35 C for 12 months and then pulled to failure. 

Fig. 31. Electron micrographs that compare crystal size of (top) a grain-refined microcrystalline coating and (bottom) a conventional zinc phosphate conversion coating [54].

Phosphate conversion coatings provide a highly crystalline, electrically neutral bond between a base metal and paint film. The most widespread use of phosphate coatings is to prolong the useful life of paint finishes. Phosphate coatings are primarily used on steel and galvanized surfaces but can also be applied to aluminum. Basically, there are three types of phosphate coatings ... 

Complex oxide film is formed in a basic solution, whereas the films described earlier are formed in an acidic solution. Complex oxide conversion coating reactions do not contain either hexavalent or trivalent chromium ions. However, the sealing rinse contains much greater quantities of hexavalent and trivalent chromium ions than do the sealing rinses associated with phosphate conversion coatings and chromate conversion coatings. 

Phosphate conversion coatings, 18 829 Phosphated flour, 26 283 Phosphate drilling mud thinners, 9 16t Phosphate ester additives, 11 500 Phosphate esters, manufacture of, 19 51 Phosphate fertilizers, 11 111, 117-122, 125, 127 23 590... 

In Region IV, performance again increases. Crystallographic transformations in phosphate conversion coatings at 180 C are known to adversely affect phosphate crystal adhesion (26). However, nitrile cyclization may be important at these higHer temperatures. Cyclization is believed to enhance barrier properties of high nitrile polymers (27,28.29). 

This more involved multi-step process can be used for both iron phosphate and zinc phosphate conversion coating processes. 

In the development of a reactive non-chrome post-treatment, a variety of phenolic resins were synthesized and commercial phenolic resins evaluated. It was found that phenol-formaldehyde resins, creso1-forma1dehyd e condensates, ortho-novo 1 ak resins, and phenol-formaldehyde emulsions gave positive results when employed as post-treatments over zinc and iron phosphate conversion coatings. The above materials all possessed drawbacks. The materials in general have poor water solubility at low concentrations used in post-treatment applications and had to be dried and baked in place in order to obtain good performance. The best results were obtained with poly-4-vinylphenol and derivatives thereof as shown in the following structure (8,9,10)... 

The quality observed with "Mannich" derivatives of polyvinylphenol is affected by the concentration, time of treatment, temperature, pH, and whether or not a final deionized water rinse is used. The results shown in Tables I-III below represent evaluations conducted for poly-[methy1(2-hydroxyethy1)amino]methyl-4-vinylphenol, as shown in Structure I. Post-treatments based on polyvinylphenols overcome deficiencies observed with previous chrome-free rinses, since these systems are reactive and a final water rinse actually improves performance as is illustrated in Table I where the new non-chrome system is evaluated on Bonderite 1000, an iron phosphate conversion coating, as a function of concentration with and without a final water rinse. It is also... 

The "Mannich" adduct synthesized from the condensation of formaldehyde, 2-(methylamino)ethanol and poly-4-vinylphenol as shown in Structure I, has been evaluated as a function of molecular weight versus corrosion resistance as measured by salt spray and humidity tests on Bonderite 1000, an iron phosphate conversion coating. The molecular weight of the polymer was varied from approximately = 2,900 to 60,000. The corrosion resistance results were essentially equivalent over the molecular weight range evaluated. 

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. 

Phosphate conversion coatings are formed on metal surfaces by immersion in aqueous phosphate solutions. These coatings are primarily intended to promote the adhesion of subsequently applied paints. Phosphate coatings typically consist of a compact mass of hydrated metal phosphate crystallites Mi(P04)2 H20. They differ from CCCs in that they are crystalline, do not attempt to provide... 

Lubrication of the faying surfaces with low-viscosity, high-tenacity oils and greases to exclude direct contact with air. This is an adequate but not complete remedy. Phosphate conversion coatings of steels, for example, are used in conjunction with lubricants since they are porous and provide oil reservoirs.16... 

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