When Ce4+ ions coexist in chromium etching reaction at a concentration greater than a
certain value, a stable etching rate is achieved owing to the reaction below. Conversely, if the
concentration of Ce4+ is smaller than 0.24 M , etching solution should be renewed, because
the etching rate becomes extremely low.
If the concentration of (NH4)2Ce(NO3)6 is lower than 12% in both nitric acid- and perchloric
acid-system etching solution, the etching rate shows a tendency to decrease extremely as the
fatigue is increased. Perchloric acid-system etching solution apparently changes into a
composition close to that of nitric acid etching-system solution. The reason why cerium-
system etching solution is made strong acid by adding nitric acid is to inhibit sludge produced
by the hydrolysis of (NH4)2Ce(NO3)6 and to stabilize the oxidation-reduction potential.