A corrosion inhibitor is described which includes several supramolecular oxo-anion compositions useful for preventing the propagation of pit corrosion on aluminum and aluminum alloys. The apparatus comprises of a specimen holder for receiving a specimen to be tested, and an impact device. The retainer nut is attached to the shaft and includes a cylindrical ring portion and an annular flange disposed on one end of the ring portion. The detection unit is supported by a first substrate.