Titania enamel

Fig. 5. A 90° polished cross section of a production white titania enamel, with the microstructure showing the interface between steel and direct-on enamel as observed by reflected light micrography at 3500 x magnification using Nomarski Interface Contrast (oil immersion). A is a steel substrate B, complex interface phases including an iron—nickel alloy C, iron titanate crystals D, glassy matrix E, anatase, Ti02, crystals and F, quart2 particle.

Using present ideas of glass structure, frits can be formulated to have maximum durability. Zirconia enamels have a lower than optimum acid resistance, but titania enamels have superior durability at low pH values. Encouraging crystallisation in the enamel enhances mechanical strength and therefore chemical resistance by reducing the incidence of damage sites which act as foci for attack. The addition of an extra (clear) coat on to the top most coat of enamel gives superior chemical resistance". 

In the traditional enamelling methods, the adhesion is aided by the ground coat enamel and by adhesive oxides which arc responsible for galvanic corrosion of the metal and thus for coarsening the surface. More recent advances in technology allow direct application of the cover coat enamel onto the metal surface. This method is possible in the case of titania enamels and requires steel of special composition, either alloyed with titanium or carbon-free. Another possibility is based on the deposition of Ni on the metal surface. It is also necessary to adjust the slip composition and the coating thickness suitably the fired enamel thickness is only 0.1 —0.2 mm, compared with the usual 0.4 mm. 

Titania enamel frits usually contain 38 —45 wt. % Si02, 12—17%B203, 16 — 21%Ti02, 10—15% alkali oxides, smaller amounts of MgO, AI2O3, P2O5 and... 

Certain difficulties in the application of titania enamels arise as a result of their reduced colour stability. Even small amounts of Fe cause undesirable yellow colouring. Of some help is oxidation to Fe which does not form the ferrous--titanate colouring complex. Similar unfavourable efiects are also exhibited by other impurities (Cr, Ta, >Jb, W). 

Titania enamels are very sensitive to any variation in the frit batch composition as well as in the melting process (temperature, furnace atmosphere). Preparation of slips similarly requires special care because the rheological properties depend strongly on compliance with the technological procedure. 

Semi-opaque acid-resistant titania enamels and alkali-resistant frit generally have good detergent resistance whereas non-acid-resistant sign enamels and AI2O3/B2O3/P2O,-based finishes have poor resistance. 

Since the development of titania white enamels, the importance of antimony or sodium antimonate as a P/E opacifier for sheet steel has been greatly reduced. Nevertheless, there are still numerous applications where antimony-based white enamels cannot be replaced by titania enamels. 

Thus, for these enamels, nonpigmentary grade titanium dioxide pigment is preferred. Contrasted with self-opacified titania enamels in which titnaium dioxide is often 20 wt% of the batch, dry process enamels carry only 4-8 wt%. 

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