Critical concentration of dissolved oxygen to form protective iron oxides under combined water treatment conditions

Abstract

The corrosion behaviour of carbon steel under combined water treatment (CWT) conditions was investigated for the purpose of determining the minimum adequate oxygen concentration. Corrosion tests were carried out in owing water (pH 9 . 0) under simulated CWT conditions at 250°C for 500 h, and electrochemical measurements were also carried out during these test periods. The corrosion potential of carbon steel shifted in the noble direction and the polarisation resistance increased with increasing oxygen concentration in the range of 5 to 15 l g L 1 . The corrosion rate calculated from weight loss measurements in oxygenated water containing 15 l g L 1 of dissolved oxygen was less than that in deaerated water. In the case of the water containing 25 l g L 1 of dissolved oxygen, the corrosion loss was much smaller than that for deaerated water. In addition, no signi cant change of corrosion behaviour was observed for oxygen concentrations in the range 25 to 100 l g L 1 . The present work has shown that the oxygen dosing concentration for CWT could be decreased, the minimum oxygen concentration required to maintain protective oxide lms having been estimated to be in the range 15 -25 l g L 1 .

Bathymetry changes caused by sedimentation in an unlined canal of the Chókwè irrigation scheme, Mozambique

“…The shallow water depth of 2.5 m, on average, may have had significant influence on the water velocity and, therefore, on the sedimentation process (Kawai et al, 2003). The low level of water in the canal was a result of different factors, including an increased demand for water from farmers, and reduced inflow due to a lack of rainfall during the season.…”