Phosphatizing produce an uniform coating on metal surface by corrosion reaction. Therefore, metal surface of electrochemical and metallurgical states prior to phosphatizing is essential.
The explanations and problems of electrochemical and metallurgical states as above for phosphatizing on steel are discussed.
Then, the chemical stability and practical quality for zinc phosphate coatings are fully explained.

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The effect of Fe-15%Zn upper coating on stone chipping resistance of galvannealed steel sheet and Zn-Ni electroplated steel sheet was investigated by using gravelometer under the temperature of -20°C after three-coat painting (phosphate, electropaint, primer surfacer and top coat) for automobile outer panel. ( 1 ) The plating/base-steel interface damage diameter by the stone chipping decreases largely by plating Fe-Zn upper coating on zinc alloy coated steel sheets. On the other hand, the electropaint/phosphate interface damage diameter increases by Fe-Zn upper coating. ( 2 ) P-ratio [Phosphophyllite/(Phosphophyllite+ Hopeite)] of phosphate film of zinc alloy plated steel sheets increases by Fe-Zn upper coating. The peeling resistance between electropaint and phosphate film against impact becomes low when P-ratio is high.
From these results, the reason why the peeling at plating/steel interface decreases when Fe-Zn upper coating is applied seems as follows: Under the condition where Fe-Zn upper coating is applied to zinc alloy plated steel sheets, the peeling resistance at electropaint/phosphate interface is weaker than that at plating/steel interface, then the peeling occured at interface between electropaint and phosphate, which consumes the kinetic energy of gravel, so peeling at plating/steel interface becomes small.

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Workability and adhesion of the plated film were studied after plastic working of Zn-Fe/Zn-Ni double layer electroplated steel sheet. The steel sheet is a newly developed one and excellent in wet adhesion of paint film and corrosion resistance as painted. After plastic working, many cracks were observed in the plated film of the double layer electroplated steel sheet. Occurrence of metal powder during working, however, was extremely slight because of the excellent adhesion of plated film to the substrate. Corrosion resistance of the specimens painted after working was also studied. Double layer electroplated steel sheet showed better corrosion resistance than galvannealed steel sheet. This is attributed to a large percentage of phosphophyllite in the phosphate coating precipitated by conversion treatment of double layer electroplated steel sheet even after plastic working.

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Before painting a vehicle, there are normally required the eight processes; pre-rinse,pre-cleaning, cleaning, water washing, surface activation, conversion treatment, water washing, and purified water washing, where the high equipment material and management costs are needed. For achieving the big cost reduction as the primary reason, an approach in designing the optimal composition of a new material was made, which enabled integration of the four processes from cleaning to conversion treatment into a single process. In this approach to efficiency, the parameter design was employed on trial. As a result, there was fbunded the optimal composition of a material in a short period and at low cost, which ensured the robust processing capacity for the liquid agitation condition or the oil content quantity that may become a deteriorative factor in processing capacity, Here, the optimal composition was designed with the velocity ratio method as the parameter design, and the test conditions were determined with use of an L18 orthogonal array, to the inside of which the control factors including the main ingredients of the material and the processing conditions were set, and to the outside of which the noise factor to be defined by mixture of the liquid agitation condition or the oil content quantity of liquid were set, and further where the signal factor to be given by response time was set, resulting in that the test would be efficiently conducted.

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By analysing the zinc phosphate film on pre-surface treated steel sheets using SEM, X-ray diffraction unit, EPMA and ESCA, the dependence of zinc phosphating quality on each steel was studied. The characterization of zinc phosphate crystal which determines not only the adhesive property but also the anti-corrosion property depends on the surface characteristics of steel sheet substrate. Especially, in case of one-side electrogalvanized steel, Zn remains on non-treated surface side too, then it affects the zinc phosphating reaction and causes less P ratio. This value is about 62%, which is smaller than that of general cold rolled surface (90-92%). Because Zn is active element, it inhibits Fe trapping into zinc phosphate crystal from steel substrate at zinc phosphating process. And also, in case of other steel sheets which P ratio become 0%, Ni component in zinc phosphating solution becomes nucleus in forming zinc phosphate crystal and contributes the adhesive property of paint film. Ni existence in the phosphate crystals was proved by EPMA. Further, Ni in the crystal may be indentified as nickel phosphate chemicals by ESCA analysis. In practice, Ni content in zinc phosphating solution improved the adhesive property of paint film on galvannealed steel after being phosphatized. This effect becomes significant by addition more than 1000ppm of Ni in the solution.

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The phosphatability of hot-rolled steel sheets has become increasingly important with application of higher strength and thinner steel sheets to automotive parts. Although V is an alloying element which is often added to high strength hot-rolled steel sheets, the effect of V on phosphatability was still unclear. This study investigated the phosphatability of V-added hot-rolled steel sheets by using V-free, 0.20% V and 0.47% V steel sheets as test specimens. After phosphate treatment, phosphate crystals covered the whole surface of the V-free and 0.20% V steel sheets, but there were no phosphate crystals on the surface of the 0.47% V steel sheets. In order to clarify the mechanism, potentiostatic electrolysis in the phosphate treatment solution was carried out. Phosphate crystals were found on the surface of the V-free steel sheet after both cathodic and anodic polarization. In contrast, no phosphate crystals were found on the surface of the 0.47% V steel sheet after anodic polarization, but similarly to the V-free steel sheet, phosphate crystals had formed after cathodic polarization. A surface analysis by XPS revealed that V oxides had precipitated on the surface of the 0.47% V steel sheet after anodic polarization. Evolution of hydrogen ions in the oxidation reaction of V inhibits the formation of phosphate crystals. Thus, oxidation of V on the steel surface in the phosphate treatment solution was identified as the factor responsible for deterioration of the phosphatability of V-added hot-rolled steel sheets.

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We have developed new powder magnetic core, which was named HDMC (High Density Magnetic Composite), with high density, high magnetic flux density and high strength. However, the HDMC, which was fabricated from iron particles coated with phosphate glass insulator, has large eddy current loss under high frequency because the resistivity was small (<10 μΩm). To solve the deficiency, new type phosphate glass insulator was developed. The phosphate glass insulator with Sr²⁺ or Y³⁺ was found to have high resistivity above 50 μΩm in keeping the high magnetic flux density and strength of the HDMC. The HDMC, whose starting material was iron particles coated with strontium phosphate glass insulator, compacted at 1176 MPa with heat treatment at 673 K had properties as follows; d = 7.70 Mg/m³, μ_m = 610, B_5k = 1.51T, B_10k = 1.71T, _bH_c = 300 A/m, resistivity = 100 μΩm, TRS = 180 MPa, iron loss (B_max = 1T, f = 400 Hz) = 400 kW/m³.

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