Steel zinc-phosphating

Chem. Descrip. Trifunctional acid ester with 161 ppm MEHQ inhibitor Uses Adhesion promoter in glass and metal coatings incl. tin-free steel, tinned steel, zinc phosphated steel, galvanized steel, electrophoretically coated steel, brass, aluminum, ironed aluminum used in beverage cans, laminating adhesives for metalized films Features Offers weatherability, chem. resist., flexibility, hardness, abrasion and water resist. 

Modified Zinc Phosphates. Coatings on steel have been identified as hopeite, 4H2O, phosphophyUite, 4H2O, and... 

Fig. 32. Micrographs of microcyslalline zinc phosphate coatings on (top) A514 and (bottom) A606 steel substrates showing the very different morphologies produced by identical processes [54],...

These materials are now widely used for coating both steel and concrete surfaces that are subject to a particularly aggressive environment (e.g. North Sea oil platforms). There is less validity for their use under normal atmospheric conditions since they are relatively expensive and tend to chalk on exposure to sunlight. However, their use as zinc phosphate, pretreatment or blast primers for blast-cleaned steel which is subsequently overcoated by any other paint system is an extremely valuable contribution to the painting of new steel work. 

Filiform corrosion is characterised by the formation of a network of threadlike filaments of corrosion products on the surface of a metal coated with a transparent lacquer or a paint him, as a result of exposure to a humid atmosphere. This phenomenon first attracted attention because of its formation on lacquered steel, and for this reason it is sometimes referred to as underfilm corrosion, but although it is most readily observed under a transparent lacquer it can also occur under an opaque paint film or on a bare metal surface. Filiform corrosion has been observed on steel, zinc, magnesium and aluminium coated with lacquers and paints, and with aluminium foil coated with paper. Surface treatment of the metal by phosphating or chromating lessens the tendency for filiform corrosion to occur, but it is not completely... 

Etch primers are widely used. They are mostly based on polyvinyl butyral and contain chromates and phosphoric acid. They are said to act both as primers and as etching solutions because it is believed that the chromates and phosphoric acid form an inorganic him, which provides adhesion, while oxidised polyvinyl butyral provides an organic him. For direct application to new galvanised steel, the best known primers are based on calcium orthoplumbate pigment and metallic lead, but these are now less used for environmental reasons. Zinc-dust paints and zinc-phosphate pigmented paints are also used, but the trend is to use pretreatments to assure good adhesion. 

Good quality steel is used and electrozinc is preferred for washing machines. Steel is pretreated with iron phosphate for economy electrozinc with a fine crystal zinc phosphate. No primer is normally used 25-40/im of finish is applied direct to metal. The required properties are best obtained with a thermosetting acrylic or polyester/melamine-formaldehyde finish. Self-reactive acrylics are usually preferred these resins contain about 15 Vo 7V-butoxymethyl acrylamide (CH2=CH —CO —NH —CHj—O —C4H,) monomer and cure in a manner similar to butylated melamine-formaldehyde resins. Resistance or anti-corrosive properties may be upgraded by the inclusion of small amounts of epoxy resin. Application is usually by electrostatic spray application from disc or bell. Shapes are complex enough to require convected hot-air curing. Schedules of 20 min at 150-175°C are... 

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. 

It has been suggested that the zinc phosphate coating has the composition Zn3(P04)2 Zn(0H)2, but X-ray diffraction studies have given very good correlation between Znj(P04)2-4H2 0 and the zinc phosphate coatings on steel . 

X-ray Diffraction Study of Zinc Phosphate Coatings on Steel , US Department of Commerce, Office of Technical Services, Rep. No. PB 111, 486 (1954)... 

Conversion coating Conversion coatings are chemical solutions which react with the metal surface to create a corrosion-resistant layer onto which the coating can bond. For mild steel iron phosphate is used to attain good adhesion, but it does not give the underfilm corrosion resistance which can be obtained using zinc phosphate. Zinc coatings can be treated with either zinc phosphate or chromate. Aluminium is usually treated with chromate... 

In the hydraulic transport of solids through steel pipelines, inhibitors of the sodium-zinc-phosphate glass type have been shown" to be effective. In the case of coal slurries the polyphosphate type was rejected because the de-oxygenating action of the coal lowered the inhibitor effectiveness. Hexavalent chromium compounds at 20 p.p.m. were more effective". ... 

Environment Increase redox potential of solution Addition of anodic inhibitors Passivation of stainless steel by additions of O2, HNO3 or other oxidising species to a reducing acid Additions of chromates, nitrates, benzoates, etc. to neutral solutions in contact with Fe inhibitive primers for metals, e.g. red lead, zinc chromate, zinc phosphate... 

Attempts have been made to improve the mechanical properties of these cements by adding reinforcing fillers (Lawrence Smith, 1973 Brown Combe, 1973 Barton et al, 1975). Lawrence Smith (1973) examined alumina, stainless steel fibre, zinc silicate and zinc phosphate. The most effective filler was found to be alumina powder. When added to zinc oxide powder in a 3 2 ratio, compressive strength was increased by 80 % and tensile strength by 100 % (cements were mixed at a powder/liquid ratio of 2 1). Because of the dilution of the zinc oxide, setting time (at 37 °C) was increased by about 100%. As far as is known, this invention has not been exploited commercially. 

All nitrile-based coatings reported in this study we e applied to Bonderite 40 coated steel (B40) panels (150-300 mg/ft zinc phosphate pretreatment The Parker Company). A commercial high nitrile polymer (Barex 210) was employed as the base resin. N-vinyI pyrrolidone (Aldrich) and gamma-butyrolactone (Aldrich) were employed as reactive diluents. 

Figure 8. XPS analysis of elemental composition as a function of sputter depth (a) zinc phosphated steel (b) interfacial substrate surface of zinc phosphated steel after adhesion failure of epoxyester coating in cathodic polarization testing. Reproduced from Ref. copyright 1983, American Chemical Society.

Granodixing Process, developed by the American Chemical Paint Co, Waterbury, Conn for rust prevention of iron, steel, zinc or cadmium consists in immersing the material in a hot soln of acid zinc phosphate. This operation forms an acid resistant coating. The resulting material may be painted... 

The chemistry in the phosphating of zinc alloys is similar, with Ihe exception that iron does not play an important role in the coating or in the hath. The only cation involved is Zn. Zinc phosphate coalings are used widely in Ihe treatment of galvanized steel for refrigerators, air conditioners, kitchen cabinets, and house and building siding. 

Adhesion may also be improved by mechanical means which is dependent on maximizing the surface area exposed. The deposition of zinc phosphate serves to increase the surface area, and thus the data observed for the phosphatized steel are... 

Figure 8. Schematic diagram of coating on a zinc-phosphated steel substrate. Thin plasma polymer is first deposited on the phosphate crystal surface, followed by an organofunctional silane polymer coating.

One of the best finishes for firearm steel is phosphatizing (Parkerizing) but few manufacturers offer this finish other than if required for military or police markets. The process deposits a crystalline layer of phosphates on the metal surface by immersion in a bath of iron, zinc, or manganese dioxide and phosphoric acid. Of these, a manganese phosphate finish is preferred for military use. 

---