Zinc phosphate process

The Zinc Phosphating Process. The zinc phosphating reaction involves acid attack on the substrate metal at microanodes and deposition of phosphate crystals at microcathodes (8). Liberation of hydrogen and the formation of phosphate sludge also occur. The equation for the dissolution of iron together with precipitation of dissolved iron as sludge in a nitrite accelerated system is as foUows ... 

Similarly in Japan there has been a strong move towards full dip treatment and over 50% of car body lines now employ this method. In Europe, while there are some dip-only plants, the majority of recent installations have presprays prior to the dip tank. In the USA spray-only plants still predominate. Zinc phosphate processes normally operate in the range 50-60°C. Low temperature processes operating at 25-35°C are widely used in the UK and Italy but have not been extensively adopted elsewhere. 

Zinc-phosphating processes are used, for example, on car bodies, which require maximiim protection with thicker coatings (0.5 - 4.5 g/m ). 

The aluminium alloy Ac-120 samples of 50 X 200 mm gauge were produced with a EDT (electron discharge texturing) surface finish by Alusuisse Sierre AG, Sierre, Switzerland [40j. The surface topography was evaluated following each of four fabrication steps (a) AR, (b) pretreated in a film-forming zinc phosphate process (PP), (c) after cataphoretic lacquer BC and (d) after application of the TC lacquer. 

This is a process to provide a fine coat of zinc phosphate or zinc calcium phosphate on ferrous and non-ferrous... 

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],...

Cleaners containing silicate can cause problems. They should not be used prior to an alkaline process on aluminium, owing to the formation on the surface of alkali-insoluble aluminium silicate. Silicated cleaners can also cause problems before some surface-sensitive zinc phosphating solutions, especially the more modern low-zinc type. 

A zinc phosphate solution tends to produce coatings more quickly than iron or manganese phosphate solutions, and dissociation of primary zinc phosphate proceeds rapidly through reaction 15.2 to 15.3 or directly to tertiary zinc phosphate via reaction 15.4. Even so, a processing time of 30 min is usual with the solution near boiling. 

It is interesting that this cement has been known for over 100 years and yet certain features of its chemistry remain obscure. The exact nature of the matrix is still a matter for conjecture. It is known that the principal phase is amorphous, as a result of the presence of aluminium in the liquid. It is also known that after a lapse of time, crystallites sometimes form on the surface of the cement. A cement gel may be likened to a glass and this process of crystallization could be likened to the devitrification of a glass. Therefore, it is reasonable to suppose that the gel matrix is a zinc aluminophosphate and that entry of aluminium into the zinc phosphate matrix causes disorder and prevents crystallization. It is not so easy to accept the alternative explanation that there are two amorphous phases, one of aluminium phosphate and the other of zinc phosphate. This is because it is difficult to see how aluminium could act in this case to prevent zinc phosphate from crystallizing. 

On the properties of the hydraulic zinc phosphate cement during and after the setting process. Journal of the Osaka University Dental School, 3, 77-88. 

Zhuravlev, V. F., Volfson, S. L. Sheveleva, B. I. (1950). The processes that take place in the roasting of zinc-phosphate dental cement. Journal of Applied Chemistry (USSR), 23, 121-8. 

The presence of triethylenetetramine in the hydrothermal synthesis of open-framework zinc phosphates results in a number of frameworks with one- to three-dimensional structures. The structures include one-dimensional ladders, two-dimensional layer structures, and one structure where the tetramine is bound to the zinc center. The structural type was highly sensitive to the relative concentration of the amine and phosphoric acid.411 Piperazine and 2-methylpiperazine can be used as templating molecules in solvothermal syntheses of zinc phosphates. The crystallization processes of the zinc compounds were investigated by real time in situ measurements of synchrotron X-ray powder diffraction patterns.412... 

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. 

Depending upon the quality level desired, the type of paint and the application method used, and the metal mix treated, a number of different types of pretreatment processes are available. For use as a paint base, the phosphate processes are either of the "iron phosphate" or the "zinc phosphate" type. 

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

Granodixing Process, developed by the American Chemical Paint Co, Waterbury, Conn for rust prevention of iron, steel, zinc or cadmium consists in immersing the material in a hot soln of acid zinc phosphate. This operation forms an acid resistant coating. The resulting material may be painted... 

ZDDP decomposes by a number of routes involving free radical and redox processes. Film composition varies from the iron-rich bonding layer, through the zinc phosphate layer to the outer surface, which contains organic material incompletely converted to precursor species. The polyphosphate chain length may vary as a function of depth into the film and the conditions under which the film is formed. Formation of polyphosphate tribofilms from simple ZDDP solutions is promoted by self-association of ZDDP molecules, which increases the local concentration of ZDDP. 

Figure 31. However, the oxidation resistance can be improved by impregnation with inhibitors, especially zinc phosphate, which prevents weight loss to about 600°C (32,49). At higher temperatures up to 1400°C, SiC coatings are proving successful, if repeated but only short-time heating is considered (56), as in the case of a reentry space vehicle. An SiC-impregnation process using tetraethylorthosilicate (TEOS) was used to impregnate the "all-carbon parts of the space shuttle COLUMBIA (9).

Other silicophosphate cements that use cation-releasing silicates are based on wolla-stonite [33], and serpentinite [34,35]. Naturally occurring phosphate cements have also been known [36]. In these cements, silicates are sparsely soluble and release cations (Ca, and Mg ), which react with the phosphate anions to form hydrophosphates and eventually convert to phosphates. This process is similar to that involving zinc phosphate cements, in which hydrophosphates form first, then convert to phosphates during aging. 

The process of fabrication of zinc phosphate cements is very similar to that of magnesium phosphate ceramics. Direct reaction with phosphoric acid is fierce and needs to be slowed down. This is done by the following methods. 

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