Phosphatizing solution

Excellent results are obtained with warm 15 per cent, trisodium phosphate solution to which a little abrasive powder, such as pumice, has been added. This reagent is not suitable for the removal of tars. 

Orthophosphate Hquid mixtures are ineffective as micronuttient carriers because of the formation of metal ammonium phosphates such as ZnNH PO. However, micronutrients are much more soluble in ammonium phosphate solutions in which a substantial proportion of the phosphoms is polyphosphate. The greater solubiHty results from the sequestering action of the polyphosphate. The amounts of Zn, Mn, Cu, and Fe soluble in base solution with 70% of its P as polyphosphate are 10 to 60 times their solubiHties in ammonium orthophosphate solution. When a mixture of several micronutrients is added to the same solution, the solubiHty of the individual metals is lowered significantly. In such mixtures the total micronuttient content should not exceed 3% and the storage time before precipitates appear may be much shorter than when only one micronuttient is present. 

Paint-base phosphate coatings could be appHed in two to five minutes. In 1934, this time constraint was shortened even further when phosphate solutions were sprayed onto the metal surface. Processing times as short as 60 seconds became possible. 

Tricalcium Phosphate. Commercial tricalcium phosphate (TCP) is actually an amorphous basic calcium phosphate close to hydroxyapatite in composition. Because of its extremely low solubiUty in water, TCP is precipitated almost quantitatively from dilute phosphate solutions with a slurry of hydrated lime. TCP is separated by dmm-, spray-, or flash-drying the TCP slurry, with or without intermediate sedimentation or filtration steps. It is used as an industrial-grade flow conditioner and parting agent. 

Combination silver—silver salt electrodes have been used in electrochemistry. The potential of the common Ag/AgCl (saturated)—KCl (saturated) reference electrode is +0.199 V. Silver phosphate is suitable for the preparation of a reference electrode for the measurement of aqueous phosphate solutions (54). The silver—silver sulfate—sodium sulfate reference electrode has also been described (55). 

Both molybdate and orthophosphate are excellent passivators in the presence of oxygen. Molybdate can be an effective inhibitor, especially when combined with other chemicals. Orthophosphate is not really an oxidizer per se but becomes one ia the presence of oxygen. If iron is put iato a phosphate solution without oxygen present, the corrosion potential remains active and the corrosion rate is not reduced. However, if oxygen is present, the corrosion potential iacreases ia the noble direction and the corrosion rate decreases significantly. 

In one patent (31), a filtered, heated mixture of air, methane, and ammonia ia a volume ratio of 5 1 1 was passed over a 90% platinum—10% rhodium gauze catalyst at 200 kPa (2 atm). The unreacted ammonia was absorbed from the off-gas ia a phosphate solution that was subsequently stripped and refined to 90% ammonia—10% water and recycled to the converter. The yield of hydrogen cyanide from ammonia was about 80%. On the basis of these data, the converter off-gas mol % composition can be estimated nitrogen, 49.9% water, 21.7% hydrogen, 13.5% hydrogen cyanide, 8.1% carbon monoxide, 3.7% carbon dioxide, 0.2% methane, 0.6% and ammonia, 2.3%. 

The phosphate solution is acidic. Continuous human contact or splashing of bath solution must be avoided, and hands or skin washed clean with a dilute solution of I-2% of ammonium bicarbonate. 

The cleanliness of the surface produced by emulsifiable cleaners is rarely of a very high standard, and additional cleaning may well be necessary before further finishing operations. Success has been achieved, however, in the use of these products prior to some immersion phosphating operations, where the crystal growth can be quite refined due to the absence of the passivation effect often encountered with some heavy-duty alkali cleaners. The supplier of the phosphating solution should be asked to advise on the suitability of any particular cleaning/pretreatment combination. 

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. 

It has been established that in the case of electrolytes, such as boric acid or ammonium phosphate solutions, in which aluminium oxide is insoluble. 

Phosphate solutions containing fluorides are used for processing steel, zinc and aluminium when assembled together, but chromate solutions are generally preferred when aluminium is treated alone. The increasing use of cathodic electrophoretic painting on steel, however, has led to a reassessment of the basic processes and formulations that might be most effective. 

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. 

A measuie of the total of a phosphating solution, as indicated by the number of ml of 0 1 n sodium hydroxide (4-0 g/1) needed to neutralise 10 ml of the phosphating solution to phenolphihalein. t Presumably from nitrides present in the steel. 

Improved nucleation within the phosphate solution itself can produce smoother coatings without the necessity of recourse to preliminary chemical treatment. This may be accomplished by introducing into the phosphating bath the sparingly soluble phosphates of the alkaline earth metals or condensed phosphates such as sodium hexametaphosphate or sodium tripolyphosphate. Such modified phosphating baths produce smoother coatings than orthodox baths and are very much less sensitive to cleaning procedures. 

Determination of the Solubility of Ferric Phosphate in Phosphating Solutions Using Radioiron , US Department of Commerce, Office of Technical Services, Rep. No. PB 111, 399 (1953)... 

Early studies on oxide films stripped from iron showed the presence of chromium after inhibition in chromate solutionand of crystals of ferric phosphate after inhibition in phosphate solutions. More recently, radio-tracer studies using labelled anions have provided more detailed information on the uptake of anions. These measurements of irreversible uptake have shown that some inhibitive anions, e.g. chromateand phosphate are taken up to a considerable extent on the oxide film. However, other equally effective inhibitive anions, e.g. benzoate" pertechnetate and azelate , are taken up to a comparatively small extent. Anions may be adsorbed on the oxide surface by interactions similar to those described above in connection with adsorption on oxide-free metal surfaces. On the oxide surface there is the additional possibility that the adsorbed anions may undergo a process of ion exchange whereby... 

Tin alloys Dip for 10 min in boiling trisodium phosphate solution (15%). Scrub lightly with bristle brush under running water, and dry. 

Pipette 25.0 mL of the phosphate solution (approx. 0.05M) into a 250 mL beaker and dilute to 50 mL with de-ionised water add 1 mL of concentrated hydrochloric and a few drops of methyl red indicator. Treat with an excess of 1M magnesium sulphate solution (ca 2mL), heat the solution to boiling, and add concentrated ammonia solution dropwise and with vigorous stirring until the indicator turns yellow, followed by a further 2 mL. Allow to stand for several... 

---