48 130150

<50 >1500

a CaCO3 as determined against methyl orange indicator. b CaCO3 given as total hardness. c Dissolved solids.

a CaCO3 as determined against methyl orange indicator. b CaCO3 given as total hardness. c Dissolved solids.

and alkaline solutions. The concentration of oxygen varies with temperature, being around 15 mg/L at 0°C and 8 mg/L at 25°C. Carbon dioxide can lead to a decrease in the pH of water, but its effect is dependent upon the bicarbonate (HCO3-) content of the water. Depending upon this relationship, temperature and other constituents within the water, precipitation of carbonate on the metal surface may occur leading to some degree of protection. A fuller explanation of this is given in reference [19].

Seawater environments are naturally more aggressive than natural waters due to the presence of a high concentration of salts, most notably chloride. Seawater is a complex electrolyte, the composition of which varies around the world. Table 11.2 presents a typical composition for seawater.

Seawater in equilibrium with atmospheric CO2 is slightly alkaline with a pH of 8.1-8.3 and contains a number of dissolved gases, the most prevalent being O2. Oxygen saturation in seawater is approximately 7-8 mg/L (ppm) although this value varies with depth being a minimum (<4 mg/L) at depths of 400-800 m.

The corrosion rates of metals in seawater are higher than those experienced in fresh (natural) water, typically being on the order of 0.65 mm/yr and 0.25 mm/yr, respectively.

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