Chromate coatings

Iodine is a less suitable reagent for use on moderately polar phases and RP material The chemical modification of the silica gel that such layers have undergone makes ther considerably more lipophilic, so that the contrast between substance-coated chromate gram zone and substance-free background is not very strong. The same applies t polyamide layers. 

Department of Energy Specification 9904151, Coating, Chromate on Aluminum, 1989. 

Extra treatments Conversion coatings—chromates prevent wet storage stain phosphates good on new sheet as a base for paints. Weathered coatings often painted (after 10-30 years) for longer service. Conversion coatings (e.g., chromates used to prevent wet storage stain. Frequently used as a base for paints. 

Clean zinc-coated steel is a suitable base for paint or adhesive systems, but the first treatment may be different from those used on uncoated steel. Chemical conversion coatings (chromate, phosphate, or oxide types) and primers have been specially formulated for first treatment of zinc-coated steel. Some passivated surfaces (e.g., chromated) are not suitable for phosphating... 

Nowadays, the use of the reflection electron microscope (REM) or, recently, the tunnel electron microscope, as well as secondary ion mass spectrometry (SIMS), AES, electron-dispersive X-ray spectrometry, impedance spectroscopy, and so on, are yielding substantial increases in the knowledge of corrosion reactions in coatings and at their interface with metal or other substrates. As far as zinc or zinc-coated surfaces are concerned, problems of interfacial and intercoat adhesion, differential diffusion phenomena and electrolytic cell behavior on the substrate, and interreactions of zinc with conversion coatings (chromates, phosphates, silanes, silanols, etc.) have been analyzed, leading toward spectacular improvements in, for example, paint adhesion, absorption of conversion coatings and, in general, the protective action inside films as well as on their substrates. 

Chromate conversion solution consists of chromic acid H2Cr04 or H2Cr207, chromate salts and certain activator ions such as sulfates, chlorides, fluorides, phosphates and complex cyanides with pH around 1 2. As the solutions for chromate conversion treatment are acidic which cause the dissolution of Mg into the solutions as Mg, there is a local rise in pH in the immediate vicinity of the metal-solution interface. Mg ions combine with chromate ions to form a compound that is insoluble at the locally higher pH region. This compound precipitates on the metal surface as an adherent coating. Chromate conversion treatment is a very fast process (30-60 s) and can... 

Figure 9.25 The effect of conversion coatings (chromates, phosphates) appiied to dissimiiar metai cou-pies. (Reproduced with permission from Wesfarmers Chemicais, Energy Fertilisers.)...

The second method is the use of chemical "conversion coatings" to alter the metal surface for protection. This is illustrated by the most widely used- and the most successful to date- chemical conversion coatings, chromates, for A1 protection. Etching agents such as HF are first used to remove native A1 oxides. The chromates are then applied, with the overall reaction ... 

ASM Committee on Chromate Conversion Coating, Chromate conversion coatings. Surface Engineering, vol. 5, ASM Handbook, ASM Publications, 1994, p. 457. 

Coated Chromate etch primer plus a two-compoaent... 

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