Water Compositions

The composition of water can be quite different, dependent upon the materials and contaminants picked up from the atmosphere during rainfall, the surrounding soil, and man-made pollutants dumped or spilled into waterways. The compounds most responsible for general water corrosivity are dissolved gases and salts. There may additionally be dissolved compounds present in specific areas from pollutants.

The primary dissolved gases affecting corrosion in waters are oxygen and sulfurous gases. Oxygen is by far the biggest concern, as it directly relates to higher corrosion rates for many metals. The concentration of oxygen is greatest at water surfaces and in the presence of algae. Sulfur dioxide and hydrogen sulfide significantly increase corrosivity and are found in waters as a result of pollutants and/or microorganisms. Sulfate reducing bacteria converts sulfates to sulfides. Nitrogen, like in atmospheric environments, is less abundant, but will increase corrosion rates of metals where present.

The ions from dissolved salts mostly responsible for increased water corrosivity are chlorides and sulfates. These ions react with metal cations to produce corrosion reactions. A run down of the most common constituents and ions present in seawater are listed in Table 5. Cations present may reduce corrosion by reacting with available anions. The measure of calcium and magnesium solid precipitates determines the hardness of water. Table 6 represents the general characteristics of some natural waters. A measure of the water’s electrical resistivity gives a general indication of corrosivity.