Freshwater corrosion forms

Table 1 shows the most common forms of corrosion that affect metals in the freshwater environment. These corrosion forms are described in ASTM G 15, Terminology Relating to Corrosion and Corrosion Testing. 

The minerals from which the metals are extracted, existed for millions of years in the earth s crust and are the most stable form of the metal. A considerable amount of energy is required to convert this mineral into the metal. Once this pure metal comes into contact with the natural environment such as sea-water or soils, the metal slowly converts back to its original starting material. Iron, for example, is obtained from the mineral, haematite, an oxide of iron. Once the pure iron comes into contact with water and air (oxygen), it slowly converts back to the oxide. This is called corrosion and the product is familiar to everyone as red rust. Nearly all metals will corrode in natural environments although the rates of corrosion will vary from metal to metal and alloy to alloy. In addition, the rates of corrosion will vary from one natural environment to another. Iron will corrode at approximately 50 pun per year in freshwater but at 120 pm per year in seawater. The reason for this is due to the difference in chemical composition between freshwater and seawater. The latter contains salt (sodium chloride) and this is very deleterious to the corrosion behaviour of the metal. Silver artefacts may be excavated after several hundred years buried in soils with only minimal amounts of corrosion. Those recovered from marine sites after a similar period of burial, have completely corroded and have reverted back to 100% mineral. This is entirely due to the presence of chlorides in seawater. 

The expected forms of corrosion can be anticipated once the water chemistry, operating conditions, materials of construction, and system layout are deBned. A corrosion test matrix can be designed by determining the expected forms of corrosion. Some of the common forms of corrosion found in freshwater systems are described in the chapter on Freshwater Testing in Section V of this manual. A list of test methods is presented in this chapter. The following section describes the applicability and significance of these standardized tests. 

The solubility of elements in freshwater is limited and the solubility of calcium and magnesium carbonates are of particular importance in freshwaters. The solubility of carbonates is inversely proportional to the temperature of the water. In other words, as the water temperature increases, calcium and magnesium carbonates become less soluble. If the solubility decreases sufficiently, carbonates will precipitate and form a scale on the surfaces of the system. This scale can provide a protective barrier to prevent corrosion of the metallic elements in a system. Excessive scale deposits can interfere with water flow and heat transfer. The quality of the scale is dependent on the quantity of calcium that can precipitate as well as water flow and the chloride and sulfate content of the water. The tendency of water to precipitate a carbonate scale is estimated from corrosion indices such as the Langelier Saturation Index (LSI) and Caldwell-Lawrence calculations [6-8] which use calcium, alkalinity, total dissolved solids, temperature and pH properties of the water. Other indices, such as the Ryznar Index... 

Crevice corrosion, and concentration cell corrosion, susceptibility can be measured directly, by exposing test specimens to the environment. ASTM F 746, G 48, and G 78 can be used for evaluating these forms of corrosion. These methods were designed to measure crevice corrosion in severe electrolytes such as salt solutions, but can be modified for freshwaters. 

In order to influence either the initiation or the rate of corrosion in the field, microorganisms usually must become intimately associated with the corroding surface. In most cases, they become attached to the metal surface in the form of either a thin, distributed film, or a discrete biodeposit. The thin film, or biofihn, is most prevalent in open systems exposed to flowing seawater, although it can also occur in open freshwater systems. Such thin films start to form within the first 2 to 4 hours of immersion, but often take weeks to mature. These films will usually be spotty rather than continuous but will nevertheless cover a large portion of the exposed metal surface [10]. 

Pure magnesium should have a driving potential of 850 mV to protect steel but in practice the metal corroded very rapidly with a very low efficiency. The metal suffers low polarization in the presence of chloride or sulfate ions and produce highly soluble chloride and sulfate salts. These ions are usually artificially introduced into the electrolyte as a backfill when a deficiency is expected, the hydroxide which is preferentially formed because of its low solubility becomes enriched with the backfill anions and itself functions as a backfill. Uniform general corrosion can then be obtained and well-designed inserts help to keep most or all of the anode metal available for sacrificial consumption. In freshwater or electrolytes which contain none of these ions, the hydroxide and carbonate may form, but these do not seriously polarize the anode (Morgan, 1993). 

The resistivity of freshwater is between 1000 and 3000 fi cm , depending on its salinity. The resistivity of seawater is only lOft cm . Seawater, therefore, facilitates ionic conductivity. However, for alloys commonly used for marine applications, the two most common forms of corrosion are pitting corrosion and galvanic corrosion. 

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