Corrosive Mist

You create a 10-foot-radius sphere of foul smelling, copper-orange miasma centered on a point within range.  Non-magical ferrous metals such as iron and steel not being worn or carried inside the mist start to develop a layer of rust on them.  If left within the mist for the full duration, they rust away completely.  Any creature wielding metallic weapons or armor must make a Constitution save if they start their turn within the mist, or enter it for the first time on their turn.  Upon failure, the creature makes weapon attacks with disadvantage until the spell ends.  A successful save ignores disadvantage proposed by the mist.  The sphere spreads around corners, and its area is heavily obscured.  It lasts for the duration or until a wind of moderate or greater speed (at least 10 miles per hour) disperses it.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the radius of the fog increases by 10 feet for each slot level above 1st.