I am with CampyGuy on this one. Oxalic Acid is strong stuff. You could also damage any chrome that is on the derailure.

Better choice is to wash the derailure to remove any oil or grease and soak in white vinegar.

Be advised that rusty derailures are often teamed up with rusty cables/housings. I have found many times that what I thought was a faulty derailure was actually cause by a bad cable.