Phosphatic coatings

Parkerizing A trade name for a process for phosphate treatment of steel. See phosphate coatings. 

Paint-base phosphate coatings could be appHed in two to five minutes. In 1934, this time constraint was shortened even further when phosphate solutions were sprayed onto the metal surface. Processing times as short as 60 seconds became possible. 

A cmcial development for zinc phosphate coatings came in 1943 when it was found that more uniform and finer crystals would develop if the surface was first treated with a titanium-containing solution of disodium phosphate (6). This method of crystal modification is a prime reason for the excellent paint (qv) adhesion seen on painted metal articles. 

Modified Zinc Phosphates. Coatings on steel have been identified as hopeite, 4H2O, phosphophyUite, 4H2O, and... 

Product Utilization. Zinc phosphate coatings form the basis for paint adhesion in a variety of industries. These are used when long-term quahty is of concern in appHcations such as for automotive parts and vehicles, cod-coated products, and appHances. 

Chromium phosphate coatings can be deposited with very low (0.05—0.15 g/m ) weights to give colodess coatings for appHcations such as a paint base or very heavy (2.0—5.0 g/m ) weights for decorative appHcations. 

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). 

Lubricant carriers are applied by dippiag the workpiece ia hot solution or slurries such as lime or phosphate coatings. Details of surface treatments are available (5) (see also Metal surface TiiEATiffiNTs). 

Dichromates and chromic acid are used as sealers or after-dips to improve the corrosion resistance of various coatings on metals. Eor example, phosphate coatings on galvani2ed iron or steel as well as sulfuric acid anodic coatings on aluminum can be sealed by hexavalent chromium baths. 

A13.2 Sheet pre-treatment (phosphate coating) 13/400 A13.3 Pre-treatment of non-ferrous components 13/404... 

Where the phosphate coating is required to be more than 5 g/m an extra hot process is used, as noted later, when the use of toner (accelerator) becomes redundant, as it is ineffective above 70°C. 

Cold process 40-45°C for 20-25 minutes will provide a phosphate coating of up to 3-3.5 g/nr, 2 For the unaccelerated process ... 

An MS tank with a wall thickness of 3-4 mm, having a heating arrangement and a thermostat temperature control will be required. No protective lining is necessary as the phosphate coating itself is protective. 

This is the final neutralizing rinse after the pre-treatment to obtain a better corrosion resistance. The phosphated surfaces are treated with chromic acid-based or acidified sodium dichromate solutions which are not affected by moisture and thus protect the phosphate coating. 

A very high content of this acidic solution may dissolve the phosphate coating. 

The phosphate coating itself is not protective unless sealed with a protective coating of primer. Sealing is therefore carried out by applying a coat of primer within 12 hours of phosphating, if the atmosphere is dry, or immediately if it is humid. Otherwise the atmospheric humidity may react with the surface and form a film of rust (i.e. ferric oxide (Fe20,))-... 

Fig. 32. Micrographs of microcyslalline zinc phosphate coatings on (top) A514 and (bottom) A606 steel substrates showing the very different morphologies produced by identical processes [54],...

The usual method of applying phosphate coatings is by immersion, using a sequence of tanks which includes degreasing and phosphating stages, with... 

All conventional phosphate coating processes are based on dilute phosphoric acid solutions of iron, manganese and zinc primary phosphates either separately or in combination. The free phosphoric acid in these solutions reacts with the iron surface undergoing treatment in the following manner ... 

The relation between free phosphoric acid content and total phosphate content in a processing bath, whether based on iron, manganese or zinc, is very important this relation is generally referred to as the acid ratio. An excess of free acid will retard the dissociation of the primary and secondary phosphates and hinder the deposition of the tertiary phosphate coating sometimes excessive loss of metal takes place and the coating is loose and powdery. When the free acid content is too low, dissociation of phosphates (equations 15.2, 15.3 and 15.4) takes place in the solution as well as at the metal/solution interface and leads to precipitation of insoluble phosphates as sludge. The free acid content is usually determined by titrating with sodium... 

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