Drain cleaner is strong base (Sodium Hydroxide), not really dissolving the typical calcium-and-stuff build-up in exhaust elbow.
I tried hot oxalic acid solution, milder than brick acid (a.k.a. muriatic acid, hydrochloric acid) and worked reasonably - took some hours to dissolve the build-up and did not see any evident corrosion.
BR,
Tommi
s/y Anniina