PLEASE if you decide to use muriatic (or any strong acid), when diluting add the acid to the water rather than water to the acid. Whatever bucket you are using for mixing, put the water component in first then pour in the acid and blend smoothly (wooden paint
stir works well). DO NOT add the acid first and then pour the water on top - a splash due to a bit of overzealous water pouring will result in nearly pure acid being splashed back up onto the person doing the pouring.
As someone with professional experience with (industrial) heat exchangers, muriatic acid is excellent for the proposed task. Just don't treat it lightly, it's much stronger than your average household cleaner.