1141 Environment

The rate of a corrosion reaction will be determined by the nature and concentration of reacting species of the environment (and the nature and form of the corrosion products). For atmospheric corrosion these include water content (humidity), the presence of chloride (Cl-), or oxides of sulfur (SOx) and nitrogen (NOx), where x can vary from 1 to 3 depending upon the source reactions. Additionally carbon in the form of CO2 influences the rate of corrosion by causing a drop in the pH of aqueous phases or in solid form by depositing soot particles on the surfaces of metals, which act as cathodic sites.

When the environment is predominantly aqueous in nature, the corrosion rate is influenced by solution conductivity, acidity (pH), dissolved gases and solids, and temperature. Natural waters vary considerably in their composition, being dependent upon the nature of the rock/soil composition through which they permeate following rainfall or being high in chloride concentration as in the case of seawater.

Typically natural waters vary from being very soft (low CaCO3 content) to very hard. Table 11.1 provides a typical water analysis (in milligrams/liter or parts per million) for the above cases.

Dissolved gases are present in all waters, oxygen being the most important, as it often takes part in the cathodic reaction (oxygen reduction) for neutral

TABLE 11.1 Typical Composition/Properties of Soft and Hard Waters
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