Flowing mixed gas corrosion test-gas flow rate effects

The effect of gas flow rate on the corrosion mechanisms in the flowing mixed gas (FMG) test developed by the Battelle Institute have been studied. Cathodic reduction was used to measure the corrosion film thickness and to analyze the composition of the corrosion films. The IEC/SC 50B/WG6, Battelle and other companies have specified gas concentrations, relative humidity, temperature and volume changes per hour in the corrosion test procedures to reach the reactivity target set by Battelle. The gas flow rate, however, is not specified and may be very high compared to the gas flow rate estimated from the volume changes per hour. High gas flow rates may arise when fans are used in the two chambers system mixing the humid air from the outer chamber with the corrosive gas mixture in the inner reaction chamber. The chamber geometry will probably also influence the gas flow rate. The study of the gas flow rate dependence shows a significant effect on the corrosion film growth rate and film composition. This will significantly effect studies of for example electrical contacts, wiping efficiency, ageing and self-healing mechanisms that are dependent on the mechanical and thermal properties, and thicknesses of the corrosion films. The conclusion is that not only the gas exchange rate but also the gas flow rate in the chamber has a significant effect on the corrosion process. The gas flow rate effects should be further studied and should also be a well specified parameter in FMG test standard procedures.