Oxygen is highly corrosive when present in hot water. Even small concentrations can cause serious problems. Liquids contain oxygen. In such cases, another chemical reaction takes place. On a cathode, it is not hydrogen but oxygen ions that are receiving electrons. In the basic mechanism of iron corrosion, the oxygen in the air dissolves in water and causes rust (iron oxides) to form.
The resulting potential is considerably higher that the potential resulting from a hydrogen corrosion; in this case even metals are attacked that were not attacked by an acid corrosion. As a general rules the impact of oxygen on metals corrosion is to increase the corrosion rate. The normal level of oxygen in freshwater is approx 6-8 ppm. At this concentration the corrosion of iron in freshwater is approx 0.2 - 0.5 mm/y depending on factors like the temperature and fluid velocity. High velocity increases the amount of oxygen that can reach the metal surface hence the metal corrosion rate.
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