Phosphate coatings accelerators

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. 

Phosphate coating solution Accelerator Immersion lime (min) Type of coating Coating weight (g/m )... 

The most widely used accelerated tests are based on salt spray, and are covered by several Government Specifications. BS 1391 1952 (recently withdrawn) gives details of a hand-atomiser salt-spray test which employs synthetic sea-water and also of a sulphur-dioxide corrosion test. A continuous salt-spray test is described in ASTM B 117-61 and BS AU 148 Part 2(1969). Phosphate coatings are occasionally tested by continuous salt spray without a sealing oil film and are expected to withstand one or two hours spray without showing signs of rust the value of such a test in cases where sealing is normally undertaken is extremely doubtful. 

Table 15.13. This specification follows good industrial practice, with additional safeguards in rinsing to remove residues to treatment solutions. Nonaccelerated treatments must be followed by a single rinse which may contain chromate accelerated treatments must be followed by three rinses—cold water, hot water and a final chromate rinse. Table 15.14 shows the salt-spray test requirements for phosphate coatings with various finishes without formation of rust the paints and lacquer have the additional requirement that no rust shall be visible beyond 0-2 in (5 mm) from the deliberate scratches and no blistering, lifting or flaking beyond 0-05 in (1-27 mm) from the original boundaries of the scratches.

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). 

Similarly, plasma activation of PEEK surfaces with a N2/02 plasma resulted in a significant increase of its wettability, showing a decrease of the contact angle from 85° to 25°. On a plasma-treated PEEK surface, the deposition rate of carbonated calcium phosphate from SBF was much accelerated and coating thickness of up to 50 pm was achieved after 24 days of immersion (Ha et al., 1997). 

Iron phosphate coatings are amorphous coatings and very thin (0.1-lgm ). The working solution contains primary sodium or ammonium phosphates, together with other ingredients which may include an oxidizing accelerator, and surfactants to combine degreasing with the chemical treatment. The pretreatments operate between pH 3 and 5.5 at temperatures... 

Zinc phosphate coatings are crystalline and somewhat thicker (1-5 gm ). The bath or spray contains primary zinc phosphate, Zn(H2P04)2, phosphoric acid and oxidizing accelerators. The pretreatments operate between pH 1.5 and 3.3 at temperatures between 25 and 90°C. The process begins with acid attack on the steel, oxidation (assisted by the accelerator) and precipitation of some ferric phosphate as above. However, the zinc phosphate is in a finely balanced equilibrium with the other species ... 

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. 

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