The concentrations of various substances in water in dissolved, colloidal, or suspended form are typically low but may vary considerably, depending on the components and usage. For example, hardness values of up to 400 parts per million (ppm) of calcium carbonate (CaCO3) is sometimes tolerated in public supplies of potable water, whereas 1 ppm of dissolved iron would be unacceptable. In treated water for high-pressure boilers or where radiation effects are important, as in some nuclear reactors, very small concentrations of impurities can be significant, and these are measured in very small units such as parts per billion. Water analysis for drinking water supplies is concerned mainly with toxins, pollutants, and bacteriological tests. For industrial supplies, a mineral analysis is of more interest. The important constituents can be classified as follows:
• Dissolved gases (oxygen, nitrogen, carbon dioxide [CO2], ammonia [NH3], and sulfurous gases)
• Mineral constituents, including hardness (principally calcium and magnesium) salts, sodium salts (chloride, sulfate, nitrate, bicarbonate, etc.), salts of heavy metals, and silica
• Organic matter, including that of both animal and vegetable origin, oil, trade waste (including agricultural) constituents, and synthetic detergents
• Microbiological forms, including various types of algae and slime-forming bacteria
The pH of natural waters is rarely outside the fairly narrow range of 4.5 to 8.5. Higher values, at which corrosion of steel may be suppressed by passivation of the surface, and lower values, at which gaseous hydrogen evolution occurs, are not often found in natural waters. Copper is affected to a marked extent by pH value. In acidic waters, slight corrosion occurs; and the small amount of copper in solution causes green staining of fabrics and sanitary ware. In addition, redeposition of copper on aluminum or galvanized surfaces sets up dissimilar metal corrosion cells, resulting in severe pitting of the more active metals.
From a corrosion standpoint, the most significant contaminant is dissolved oxygen (DO) from ambient air. Oxygen is a cathodic depolarizer that reacts with and removes the hydrogen ion film from the cathode surface during electrochemical corrosion, thereby permitting corrosion attack to continue. In a closed vessel, corrosion rates increase with temperature, hence the importance of removing DO from hot water systems and boilers. In a range of about pH 5 to 9, the corrosion rates of steel and most other metals can be expressed in terms of the amount of DO present (e.g., μm/y per mL of DO per liter of water). At about pH 4.5, acid corrosion is initiated, overwhelming the oxygen control. At about pH 9.5 and above, deposition of insoluble ferric hydroxide [Fe(OH)3] tends to stifle the corrosion attack.
Other constituents that contribute to corrosion are chlorides, CO2, calcium, and sulfides or NH3 from industrial or natural sources. Of course, many other manmade contaminants can be found in local water resources where industries are permitted to discharge their waste products. As with other chemical reactions, corrosion increases with elevated temperature, unless stifled by insoluble scales, the removal of corrosive gases, or the addition of corrosion inhibitors.
Scales precipitated uniformly onto metal surfaces can provide excellent protection of the substrate but can accentuate pitting at pores, cracks, or other voids in the film. If the film attains any significant thickness, the loss of heat transfer through the metal and deposited scale can be a problem in certain applications. Thus, the development and control of scale formation on metal surfaces is an important consideration when using metals in waters.
This article is adapted by MP Technical Editor Norm Moriber from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 514-516.