Corrosion of Carbon Steels in Fresh Waters

Natural fresh waters are saturated with air and contain dissolved oxygen in the range of 8-10ppm at room temperatures. The dissolved oxygen in the water causes corrosion by the reactions. 

It is clear from this that the dissolved oxygen in water diffuses to the surface of the metal and causes corrosion. The diffusion of oxygen to the metal surface is accentuated by an increase in turbulence of water through increase in velocity of water. The corrosion reaction proceeds as fast as the oxygen reaches the steel surface. 

In the absence of a diffusion-barrier films on the steel surface the corrosion current density, i (A/cm2) of steel in stagnant air-saturated water is given by  

Hard waters contain high concentrations of calcium and bicarbonate and steel samples exposed to hard waters result in the deposition of calcium carbonate deposit on the steel 

Typically waters with hardness of less than 100 ppm are soft, 100-150ppm are slightly hard and greater than 150 ppm hard. The corrosivity of natural fresh waters is due to pH, dissolved oxygen, hardness, chloride and sulphate. The chloride is responsible for the localized corrosion. 

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The effect of velocity on the corrosion of carbon steels in fresh water is shown in Table 9.14 [31]. 

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