Muratic acid can be considerably more harmful than sandblasting. Sand cannot penetrate the pours or open spaces in the metal. Muratic acid can and does.
The primary culprit here is not the acid itself but the hygroscopic salts it leaves behind as it becomes neutralized. These salts continue to attract moisture and will continue to rust a trap.
After treating traps with muratic acid, they should be boiled
clean, preferably in a mild basic solution (baking soda). This neutralizes any remaining acid and
the boiling water removes any of the salts that may be left behind.
(I address this issue from experience.