You are correct in stating you have a decay. This is because the base of the exponential 0.82 is in the form 1+r where 1+r = 0.82 solves to r = -0.18
The negative r value means we have decay.
The rate of decay is 18% since 0.18 = 18/100 = 18%
For example, if we started with 100 items then after 1 unit in time, you'd have 82 items since 100-18 = 82