The most troublesome and common defects in porcelain enamel technology sometimes appear in accidents. The enamel surface peels off like fish scales. This defect is called "crystalline scale". The main reasons are as follows:

1. Poor quality of steel plate

2. Insufficient firing

3. Poor adhesion of the ground coat

4. Too few bubbles

5. Excessive pickling causes hydrogen intrusion

6. Double-sided glazing of hot-rolled steel plates prevents hydrogen from escaping

7. The crystal water in the firing furnace is too high

8. Weather influence (especially rainy season)

Fish scale is a defect of enamel peeling. When it peels off, it is very small flakes of different sizes, and the scars left on the porcelain surface are several centimeters in maximum, sometimes as small as tiny crystal spots. This is called "Crystalline Fish Scale", and it is the most troubled and common defect in the enamel process. Sometimes it appears on the vessel immediately after the ground coat is fired, and sometimes it appears during storage and use (delayed fish scale ). Fish scale often occurs in accidents, and there is no change in the production process and technology, and may disappear again after a while. Therefore, many researches have been done in the industry on how to form and eliminate it.

The main reason for the formation of fish scales is that the enamel layer is cracked from the metal under the pressure of molecular hydrogen. At high temperatures, steel absorbs hydrogen in atomic form and dissolves in the metal. As for the solubility, it depends on the structure of the steel. Impurities contained in steel also account for an important part. For example, impurities such as sulfur, phosphorus, silicon, aluminum, and arsenic form hydrides with hydrogen to help steel absorb hydrogen. Once the temperature drops to room temperature, the solubility of hydrogen decreases 1,000 times. At the junction of metal and enamel, hydrogen is gaseous and dispersed and concentrated in the defects of the structure. The resulting pressure is as high as 110 atmospheres, which causes the enamel to be broken from the metal at the weak point of the bond and the defect of the metal. Combined with the strain in the enamel layer, a considerable difference develops in the thermal expansion coefficients of the metal and the enamel. The pressure of the hydrogen will cause the flakes of the enamel surface, and the depth and shape of the scales will also be different. The higher the compressive strain, the greater the tension in the small flakes parallel to the surface of the enamel. If the ground coat is under tension, the fish scales will explode and the surface of the enamel will show a big angle, and it will appear as a kind of "microcrystalline" scales.

If the hydrogen pressure is high, the fired ground coat utensils will immediately show fish scales burst as soon as they are cooled. If the pressure is lower, this defect will not appear immediately. In this way, after applying the enamel cover coat (heating rapidly during drying and firing), the hydrogen pressure increases, which leads to the formation of fish scales. Sometimes when the ground coat is softened in the furnace, hydrogen is precipitated to form a second boiling or bubbles. From the appearance point of view, the so-called fish scale explosion on the surface enamel is only the fish scale explosion on the surface layer, but in fact, the cracks often originate from the metal surface, or are bubbles or cracks from the ground coat of the enamel, and penetrate into the metal.