Laserfiche WebLink
<br />Table 2. Chemical Factors InOuencing Corrosion <br />and Corrosion Control <br /> <br />Factor <br /> <br />Effect <br /> <br />pH <br /> <br />Alkalinity <br /> <br />Dissolved <br />Oxygen <br /> <br />Chlorine <br />Residual <br /> <br />Total <br />Dissolved <br />Solids <br /> <br />Calcinm <br /> <br />Cbloride, <br />Sulfate <br /> <br />Low pH may increase corrosion <br />rates; high pH may decrease <br />corrosion rates for some materials, <br />but increase corrosion rates of others. <br /> <br />Alkalinity may reduce corrosion by <br />helping to form a protective CaCO, <br />coaling and resisting undesired pH <br />changes. <br /> <br />High levels of dissolved oxygen <br />increase the rate of many corrosion <br />reactions, but also act to form <br />protec1ive oxide films, <br /> <br />Chlorine increases the rate of most <br />corrosion reactions. <br /> <br />High IDS increases conductivity <br />and corrosion rates. <br /> <br />Calcium may precipitate as CaCO, <br />and thus provide protection and <br />reduce corrosion rates. <br /> <br />High levels of chlorides and sulfates <br />increase corrosion of iron, copper, <br />lead, and galvanized steel. <br /> <br />Temperature The rate of corrosion lends to <br />increase with increasing temperature. <br /> <br />Corrosion Control In addition to the use of non- <br />corroding pipe materials, several other corrosion <br />control methods interfere with the corrosion circuit <br />These methods include applied coatings and chemical <br />precipitates. <br /> <br />Paints, cements, or bituminous materials are <br />frequently used as pipe coatings, These coatings <br />isolate the pipe from the water, preventing corrosion. <br />Portland cement, mortar, and concrete iinings also <br />establish a high pH environment at the surface of the <br />coated metal. <br /> <br />Engineers and chemists can also adjust water <br />chemistry to cause precipitation of adherent chemical <br />scales that produce an internal coating. This process <br />is not rigorously understood and therefore cannot be <br />precisely controlled. Those who use this method must <br />consider the effect of these chemical adjustments on <br />other water quality parameters and chemical relation- <br />ships. To do the best job, they must strike a balance <br />between all competing factors. <br /> <br />One of the simplest and most common methods <br />of corrosion control is pH adjustment. Generally, <br />operators attempt to raise the pH to the point at which <br />chemical scales precipitate from the water, depositing <br />on either the anode or the cathode, and sometimes on <br />both. These deposits isolate the pipe from the water, <br />interfering with the "corrosion battery." CaCO, deposi- <br />tion is one of the most widely used techniques for gen- <br />eral corrosion control. Operators raise the pH to the <br />point at which the water is slightly oversaturated with <br />CaCO,. The CaCO, precipitates and deposits on the <br />inside of the pipe.' Calcium and alkalinity concentra- <br />tions also influence the degree of oversaturation, <br /> <br /> 100 <br /> 90 <br /> 80 <br />~ 70 <br />E <br />;., <br />0 60 <br />() <br />. <br />() <br />'" <br />" 50 <br />>- <br />!:: <br />z 40 <br />::; <br />" <br />'" <br />-' '0 <br />" <br /> 20 <br /> 10 <br /> 0 <br /> 6.5 7 7.5 8 B.5 9 9.5 10 10.5 11 <br /> pH <br /> <br /> <br />Figure 1. Relationship between carbon dioxide <br />and the three forms of alkalinity at varions pH <br />levels (values calculated for water with a total <br />alkalinity of 100 mg/l at 25'C). <br /> <br />[ill BROWN AND CALDWELL <br /> <br />3 <br />