③ Phosphoric acid

The corrosion resistance of enamels to phosphoric acid changes in a much different way from that to other kinds of acids. Usually corrosion resistance of enamels increases with the increase of concentration of the acids, but in case of phosphoric acid, corrosion resistance decreases with the increase of concentration. As phosphoric acid often contains fluorine as a impurity, the corrosion test for a period as long as more than one week is necessary.