When there appear defects on the surface chemical processing section before ppgi steel sheet coating, the first thing is to check if the chromium content of chemical conversion film is controlled at 30-34 mg/m2. In addition, due to the defects of chemical processing section is mainly appears on the phenomena of complete lack of film, local lack of film, film yellowing and hanging with ash. So the inspection should also be targeted.
After research to hundreds of color coated manufacturer, CAMELSTEEL found that the sampling method of inspection are mainly the followings:
In the production process, leave a small amount of coated plated which do not cover with organic ppgi steel sheet coating in the begin and end of the roll as a test sample. Regularly sent to the chemical masking liquid manufacturer to test the coating chromium content.
Chromium content can also be determined by x-ray fluorescence, diffraction, plasma, atomic absorption or chemical analysis method. In addition, the causes of defects can also be found on the basis of film quality of surface described below:
A. The reason of complete lack of film
1. The proportion of phosphatizing liquid composition is imbalance. The content of dihydrogen phosphate is low or free acid is too high.
2. Uncleaned decreasing.
3. The ambient temperature during chemical process is low, and the time is short.
4. There are many contained impurities, like SO.
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