A study of the effect of concentration build-up of electrolyte on the atmospheric corrosion of carbon steel during drying

Impedance and cathodic polarization measurements for carbon steel under a thin layer of Na2SO4 solution of varying concentrations were conducted to investigate the effect of concentration build-up on the atmospheric corrosion of carbon steel. Electrolyte thickness was controlled by two methods. One is a microsyringe method which allows a predetermined amount of the electrolyte to be dropped onto the metal surface to make electrolyte layers of different thickness with the same electrolyte concentration; the other is a drying method which controls a water layer thickness through drying, causing concentration build-up of electrolyte during thinning. The concentration build-up has two opposite effects on the atmospheric corrosion rate of carbon steel depending on electrolyte layer thickness. For a comparatively thick electrolyte layer in which the atmospheric corrosion of carbon steel is controlled by a cathodic oxygen reduction reaction, the concentration build-up reduced the corrosion rate. However, it increased the corrosion rate for a very thin electrolyte layer. The critical thickness for maximum corrosion rate also changed by the concentration build-up, which indicates that the concentration build-up is the important factor responsible for determining the critical thickness.