Grain refinement coatings

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

Phosphate pre-treatments may be either zinc phosphate (from zinc dihydrogen phosphate solutions) or an iron phosphate (fi om alkali phosphate solutions) (see Conversion coating and Pre-treatment of steel). The conversion reactions are promoted by accelerators (depolarizers), for example, bromates or molybdates in alkali phosphate baths or chlorates in zinc phosphate baths (with Ca or Ni grain-refining additions). Iron phosphate pre-treatment coatings are often described as amorphous . In practice, however, they are usually crystalline deposits of iron oxides and iron phosphate. Zinc phosphate pre-treatment coatings are always crystalline. A fine, dense crystal pattern of zinc phosphate on the metal surface is the ideal, as it improves both paint adhesion and corrosion resistance most effectively. 

FIGURE 23. Electron micrographs of (a) grain-refined microcrystalline conversion coating and (b) conventional zinc phosphate conversion coating. (From Reference 114.)... 

H. Lu, Y. Li and F. Wang (2006), Enhancement of the electrochemical behavior for Cu-70Zr alloy by grain refinement . Surf. Coat Tech., 201 3393-8. 

Mrowec S, New generation of nanocrystaUine coating materials resistant to high temperature corrosion , High Temp Mater Proc, 2003, 22(1), 1-24 Peng X, Yan J, Zhou Y, Wang F, Effect of grain refinement on the resistance of 304 stainless steel to breakaway oxidation in wet air , Acta Mater, 2005, 53(19), 5079-... 

A notable exception is a study by Hesse et al. (2008) who investigated the effect of powder grain size and grain size distribution on the spatial distribution of calcium phosphate phases in atmospheric plasma-sprayed hydroxyapatite coatings incubated in r-SBF (SBF-H, Table 7.8). Coatings were mechanically abraded under dry conditions in steps of 40 pm by abrasive SiC paper, and the newly created surfaces analysed by XRD with Rietveld refinement for their quantitative phase composition. The results of this depth profiling are shown in Figure 6.8. 

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