Iron phosphate bath

POLYVINYLPHENOL POST-TREATMENT IRON PHOSPHATE BATH CONCENTRATION VS. FINAL WATER RINSE ... 

Hydrogenis prevented from forming a passivating layer on the surface by an oxidant additive which also oxidizes ferrous iron to ferric iron. Ferric phosphate then precipitates as sludge away from the metal surface. Depending on bath parameters, tertiary iron phosphate may also deposit and ferrous iron can be incorporated into the crystal lattice. When other metals are included in the bath, these are also incorporated at distinct levels to generate species that can be written as Zn2Me(P0 2> where Me can represent Ni, Mn, Ca, Mg, or Fe. 

Mention has been made of the necessity for controlling the acid ratio of phosphating baths, particularly those of iron, manganese and zinc operating... 

How far the formulation of a phosphating bath influences The Ratio is not entirely clear. Nitrite alone or in combination with chlorate has been the most widely used accelerator system for many years but more recently nitrite-free chlorate/organic systems have been increasingly favoured. Low zinc systems in which the bath is starved of zinc to promote a high iron content in the coating, originally introduced in Japan, have become widespread. 

Iron phosphate glasses, 12 585, 616 Iron phosphates, 18 839 Iron phosphating, 16 214-215 applications for, 16 215 Iron phthalocyanine, isolation of, 24 36 Iron(II) phthalocyanine, 14 547 Iron plating baths, 9 814t Iron porphyrin-catalyzed dye oxidation, 9 383-384... 

In zinc phosphating. a small amount of iron phosphate is formed initially, bul ihe bath contains primary zinc phosphate. ZnlHiP04)>, which crystallizes on Ihe metal surface as secondary and tertiary zinc phosphates, ZnHP04 and Znt(P04 j. respectively, when the pH rises at the mclal/solulion interlace. The most frequently used baths contain accelerators, preferably nitrates and nitrites, which oxidize the hydrogen lormcd hy the pickling reactions. The fundamental zinc phosphate reactions occur in three steps, all in the same hath ... 

Evaluation of Ultrafiltration to Recover Aqueous Iron Phosphating/ Degreasing Bath... 

In the presenee of oxidising aeeelerators (which are used in practice), ferric phosphate is deposited directly, together with zinc phosphate (12.39). The purpose of oxidising accelerators is not only to remove iron salts from the phosphating bath by precipitation, but also to reduce the formation of hydrogen gas bubbles which can slow down the coating process. 

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. 

Manganese-iron phosphate coatings are usually formed from high-temperature baths from 90 to 95 °C (190-200 °F). 

Dichromates and chromic acid are used as sealers or after-dips to improve the corrosion resistance of various coatings on metals. Eor example, phosphate coatings on galvani2ed iron or steel as well as sulfuric acid anodic coatings on aluminum can be sealed by hexavalent chromium baths. 

The relation between free phosphoric acid content and total phosphate content in a processing bath, whether based on iron, manganese or zinc, is very important this relation is generally referred to as the acid ratio. An excess of free acid will retard the dissociation of the primary and secondary phosphates and hinder the deposition of the tertiary phosphate coating sometimes excessive loss of metal takes place and the coating is loose and powdery. When the free acid content is too low, dissociation of phosphates (equations 15.2, 15.3 and 15.4) takes place in the solution as well as at the metal/solution interface and leads to precipitation of insoluble phosphates as sludge. The free acid content is usually determined by titrating with sodium... 

Directions for determining in Water the Nonvolatile Organic Matter—Silica, Iron, Alumina arid Phosphates Lime and Magnesia.—Evaporate slowly half a gallon of wator to about a pint, over a gas flame then transfer tire dish -to a sand-bath, and continue the evaporation till the bulk is reduced to about two fluid ounces. Now accurately weigh a dean platinum dish. Scrape the deposit carefully from the bottom of the porcelain basin, and rub off that which cannot be removed by the platinum knife with the middle finger rinse the whole into tire platinum dish, and then evaporate to dryness on a water-bath. Be careful to observe that every particle of the deposit is removed from the porcelain vessel, When the residuo Is dry place the dish in an air-hath, and heat it for half on hour or on hour at a temperature of 260°,... 

Iron buildup in the bath is objectionable in the iron-removal equations above, it is seen that dissolved Fe+ can he removed by oxidation, slowly in air or more rapidly by peroxides or nitrite, as shown in the final equation. The irnn removed becomes ferric phosphate, while iron in the coating is ferrous phosphate. 

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