A rotating parallel disc device for corrosion studies

This work presents a novel Parallel Disc Device (PDD) to study flow accelerated corrosion (FAC). The device consists of two discs separated by a precision controlled gap. The bottom disc is immobile and accommodates the working electrode and the top disc rotates. The gap between the discs may be set within the range from 50 μm to 5 mm with the precision of ±5 μm. The device has the following features: • it is capable of generating wall shear rates at least as high as 200 000 s-1 at the surface of the working electrode while maintaining laminar flow conditions within the gap; • the shear of liquid at the working electrode is purely tangential; • the value of the wall shear rate may be accurately determined from the geometrical parameters and known rotation speed of the top disc. To experimentally evaluate the performance of the Parallel Disc Device the oxygen corrosion of mild steel was investigated with this device. Results were compared with data obtained with the rotating disc electrode (RDE) and with the rotating cylinder electrode (RCE). Comparisons of data demonstrate that for the same rotation speeds, the PDD can achieve substantially higher wall shear rates. At the same time, the PDD maintained well-defined laminar flow conditions over the entire range of rotation speeds tested. Flow for the RDE and RCE was distinctly turbulent and could be characterised using simple laminar boundary layer equation over a limited range of wall shear rates only (up to 380 s-1).