低熔融フリットを用いた鉄琺瑯の密着性と接着機構

In general, steel enamel is fired at a temperature higher than 820°C in order to make it sufficiently resistant to heat and corrosion. If only it has an excellent chemical durability, however, for some use being required not so much high thermal resistance, one can easily understand that firing at a temperature much lower than that for the conventional steel enamel should be economically much more profitable. Development of such a steel enamel is expected by studying on an enamel having a strong adhesiveness between the metal and the cover coat fired with some low melting frit which is anticipated to be obtained by using a phosphate frit having a high chemical durability in spite of its low fusibility.<br>In the present study a phosphate glass developed at our laboratory as a chemical resistant frit for aluminum enamel was used as a frit for such a steel enamel fired at a low temperature. And then, as the pretreatment for steel plate, the following treatments were tested, i.e., pickling in hydrochloric, sulfuric, or nitric acid, parkerizing, nickel-dip, and anodic oxidation.<br>Relation between the adhesiveness of enamel and the bonding mechanism ocurring at the glass-steel interface was investigated by discussing the experimental results obtained from bending tests for the enamel specimens under tension and surface texture diagrams of the pretreated metal which were drawn by Talysurf's roughness tester.<br>It was concluded as follows:<br>1) Adhesion of the low melting glass to steel was considered to attribute mostly to mechanical bonding rather than the other chemical causes.<br>2) The enamel with fairly high adhesiveness could be obtained even when the steel plate was only pickled in 20% nitric acid. It was likely to be not any clear relation between the roughness in “centre line average H_CLA (index number of texture)” of metal surface and the adhesiveness of enamel expressed by the angle which was measured by the bending test under tesion. It was confirmed, however, that there was a linear relation between the texture frequency and the adhesiveness. The good adhesiveness was given under the following condition of pickling, i.e., 20% nitric acid, bath temperature 16°±1°C or 25°±1°C, and treating time 30-60 sec. or 15 sec., respectively.<br>3) In case of nickel-dip treatment, the obtained enamel displayed a excellent adhesiveness on account of an effective action of nickel, that is to say, nickel depositing on the steel surface in this case effectively protected the metal surface against oxidation during firing. The optimum condition in the way of such a pretreatment was as follows: dipping in 20% nickel sulphate solution at 16°±1°C for 10-20min. In the opposite way, in both cases of anodic oxidation and parkerizing treatments, the adhesiveness of enamel lowered with prolonging time of treatment.<br>4) The optimum condition of firing at a low temperature for this kind of steel enamel was found to be fired at 550°±10°C for 15-30 sec.