Phosphate coatings crystal size

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

The texture or crystal size of phosphate coatings can conveniently be recorded by making an impression on clear cellulose tape moistened with acetone. Uniformity of crystal size is of importance for coatings which are to resist wear and assist metal working. Surface roughness may also be studied by means of a Talysurf meter. 

These relationships are illustrated in Fig. 6 in which the variation of coating weight, crystal size, the relative surface coverage, and the rate of substrate dissolution were measured as a function of time for the phosphatation of steel in a laboratory trication phosphatation bath. The partial free metal surface ( -9) decreases as the average crystal size (di) and coating mass increase. The dissolution rate of the metal rjiss (here defined as a negative value) drops toward zero at the end of the reaction. 

Fig. 6 Phosphating as an autopassivation process variation of partial free surface (1 -9), metal dissolution rate (rjiss). crystal size (dz), and coating mass (rricryst) with time during the phosphating of mild steel in a trication phosphate bath. (From Ref [15].)...

Insoluble Ammonium Polyphosphate. When ammonium phosphates are heated ia the presence of urea (qv), or by themselves under ammonia pressure, relatively water-iasoluble ammonium polyphosphate [68333-79-9] is produced (49). There are several crystal forms and the commercial products, avaUable from Monsanto, Albright WUson, or Hoechst-Celanese, differ ia molecular weight, particle size, solubUity, and surface coating. Insoluble ammonium polyphosphate consists of long chains of repeating 0P(0)(0NH units. 

In vitro a crystalline iron core can be laid down in apoferritin by the addition of an oxidant, such as O2, to an aqueous solution of a ferrous salt and apoferritin (32, 132, 140). The reconstituted core of horse ferritin prepared in the absence of phosphate and with O2 as oxidant is very similar to the native core in terms of its size and Mossbauer properties (85). Electron microscopy, however, reveals that it is less well ordered. Reconstitution in the presence of phosphate leads to smaller cores. Reconstituted A. vinelandii cores in the absence of phosphate were more ordered than were the native cores, and clearly contained ferrihydrite particles and, in some cases, crystal domains (85). Thus the nature of the core is not determined solely by the protein coat the conditions of core formation are also important. This is also indicated by Mossbauer spectroscopy studies of P. aeruginosa cells grown under conditions different than those employed for the large-scale pu-... 

The study by Lee et al. (2013b) confirmed that the morphology and size of elec-trochemically deposited calcium phosphate crystals on anodised cp-Ti substrates were strongly affected by the deposition time and electrolyte temperature. Flakelike CaP was observed at 25 °C, but needle-like CaP was present at 85 °C. A Ca/P ratio of 1.68, close to the stoichiometric ratio for HAp was found for coatings deposited at 85 °C. Crystalline OCP and HAp phases formed in CaP layers that were not heat treated, whereas TCP formed after annealing the CaP layers at 700 °C. 

The purpose of the reaction step is not only to extract the pho hate from the rock but also to ensure slow growth of gypsum crystals to a relatively large size. To attain this goal, reaction systems are designed to prevent direct contact between the two reactants, phosphate rock, and sulfuric acid. A hi concentration of free sulfuric add would result in coating the phosphate rock with calcium sulfate reaction product, thus Hocking further reaction. 

The reaction tank (Figure 11.14) is constructed of rubber-coated steel or concrete and is lined with carbon bricks. Baffles are fitted onto the walls to prevent the slurry from rotating bodily as a single mass inside the tank The cover of the reactor is constructed of polyester or ebonite-coated panels. The phosphate rock is fed by a special duct within a cylindrical shroud at one or two points, according to the size of the tank, and into the turbulence zone of the central agitator on the opposite side to the gas extraction hood. The sulfuric acid is introduced into one, or several, independent discs fixed to the drive shafts of some of the surface coolers. The proprietary equipment distributes the acid so evenly over the entire surface of the tank that 98% acid can be introduced directly without prior dilution. There is no risk of local sulfuric acid concentration excess or temperature peaks, which can adversely affect the crystallization. 

Excellent bonding can be achieved with zinc phosphate and mixed metal phosphates but the particle size and quantity of phosphate applied are very important. There are two main physical forms of phosphate that are used in commerce. One is an amorphous structure applied at a level typically below 4 g/m and the other an acicular (needle shaped) structure typically applied at a level of 15 g/m. The acicular form is used as an absorbent substrate for coatings and oils used to enhance the phosphate layer as protective coating. The acicular form is unsuitable for bonding as the crystal structure can fracture under the bonding agent primer. 

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