Phosphate coatings process

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 ... 

PHOSPHATE COATINGS Table 15.11 Typical phosphate coating processes... 

Blalock, T., Bai, X., and Rabiei, A. (2006) A study on microstructure and properties of calcium phosphate coatings processed using ion beam assisted deposition on heated substrates. Surf. Coat. Technol., 201 (12), 5850-5858. 

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. 

The appHcation of this type of conversion coating can be by spray or immersion and is easily tailored to the needs of the user (see Coating processes). The number of stages may vary from two, ie, clean and phosphate then rinse, to as many as five, ie, clean, rinse, phosphate, rinse, and post-treat. The performance requirements dictate the number of operation stages as well as the need for a post-treatment. 

The bath components for a nitrite—nitrate accelerated bath basic to this conversion coating process are (/) 2inc metal or 2inc oxide dissolved in acid (2) phosphate ions added as phosphoric acid (J) addition of an oxidant such as sodium nitrite and (4) addition of nitric acid. Other oxidants such as peroxide, chlorate, chlorate in combination with nitrate, or an organic nitro compound may also be used. 

Metal Treatment. After rolling, the oxide scale on sheet steel is removed by acid treatment (pickling) (see Metal surface treatments). Phosphoric acid, a good pickling agent, leaves the steel coated with a thin film of iron phosphates. This process improves mst resistance but presents a problem if the steel is to be electroplated. 

Table A13.1 Process of sheet pre-treatment (phosphate coating)...

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 ... 

The hot process is generally not recommended as it may dissolve the phosphate coating... 

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 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... 

Heavy phosphate coatings do not necessarily have better corrosion resistance than lighter coatings. Even with a single process, e.g. zinc/iron/ phosphate/nitrate, no consistent relationship has been found between corrosion resistance and either coating weight or weight of metal dissolved. 

The protective value of a phosphate coating is enhanced by a dip or rinse in an acid chromate solution. Joint Service Specification DEF-29 makes such a rinse mandatory for steel parts treated by an accelerated process, and optional after treatment by a non-accelerated process. Details of rinses are given in Section 15.2 (Table 15.10, p. 15 30). 

With regards to the conversion coated substrate, Wittel (25) observed that at temperatures greater than 140°C, tetrahydrate zinc phosphates lose part of their water of hydration. It is likely that the water of hydration liberated in the phosphate recrystallization process has a negative effect on the adhesion of the polymer matrix to the B40 panel. 

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

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