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<br />Corrosion Any metal in contact with water is sub-
<br />ject to chemical corrosion. A complex interaction of
<br />physical and chemical factors determines the rate of
<br />this process. Corrosion is primarily an electrochemi-
<br />cal process that works somewhat like a battery.3
<br />
<br />For a "corrosion battery" to work, four elements
<br />must be present: an anode, a cathode, an external
<br />circuit, and an internal circuit Areas on a pipe having
<br />different electric potentials are the anodes and the
<br />cathodes, with the pipe itself serving as the external
<br />circuit connecting them. The water in the pipe is the
<br />electrolyte or internal circuit Within the pipe wall,
<br />electrons flow from anodic to cathodic areas causing
<br />metal ions to be released into solution, resulting in
<br />corrosion. In the solution, positively charged ions
<br />migrate towards the cathode and negatively charged
<br />ions migrate toward the anode. The goal of corrosion
<br />control is to interfere with this circuit
<br />
<br />Factors Affecting Corrosion Rate Both
<br />physical and chemical factors affect the rate at which
<br />corrosion occurs, Physically, the coupling of two
<br />dissimilar metals causes greater corrosion of the more
<br />active metal than would occur if this metal existed
<br />alone. The galvanic series, shown in Table 1, lists
<br />metals used in water systems in order of their ten-
<br />dency to corrode. Choosing metals that are close
<br />together in the galvanic series limits corrosion by mini-
<br />mizing the difference in electric potential so that less
<br />current flows from anode to cathode.
<br />
<br />Table 1. Galvanic Series--Order of Electro-
<br />chemical Activity of Melals Used in Water
<br />Systems
<br />
<br />Metal
<br />
<br />Activity
<br />
<br />Zinc
<br />Cadmium
<br />Aluminum
<br />Steel
<br />Cast iron
<br />Stainless sleel (active)
<br />Tin
<br />Lead
<br />Brass
<br />Copper
<br />Stainless steel (passive)
<br />
<br />More active
<br />
<br />,
<br />
<br />. .
<br />
<br />Less active
<br />
<br />Many water quality parameters affect the rate of
<br />corrosion, These include pH, alkalinity, dissolved
<br />oxygen, chlorine, TDS, calcium, chloride, sulfate, and
<br />
<br />temperature. Table 2 is a simplified summary of the
<br />effects these factors have on corrosion.' These
<br />parameters are also discussed below,
<br />
<br />o pH The effect of pH on corrosion is probably
<br />more significant than any other parameter. Many
<br />studies have shown the inverse relationship between
<br />pH and the corrosion rate of metals such as lead,
<br />copper, and iron, For example, low pH increases the
<br />driving force for lead dissolution. Furthermore, pro-
<br />tective lead carbonate scales cannot form at low pH.
<br />However, although increasing pH generally tends to
<br />reduce corrosion rates, exceptions may occur. High
<br />pH has been reported to encourage pitting, a particu-
<br />larly destructive form of corrosion, in iron and steel
<br />systems, Engineers and chemists should not make
<br />generalizations about the effect of pH on corrosion
<br />reactions.
<br />
<br />o Alkalinity Alkalinity, which is affected by pH,
<br />also plays an important part in corrosion reactions.
<br />The pH will determine the mix of alkalinity species
<br />present, as shown by Figure 1.5 As pH increases, the
<br />proportion of carbonate ion increases and carbonate
<br />precipitates tend to deposit as protective scales. For
<br />example, lead may precipitate as PbC03 or
<br />Pb3{C03)2(OH)2' Deposits of CaCO, may also form
<br />when the carbonate reacts with calcium,
<br />
<br />o Dissolved Oxygen Dissolved oxygen is an
<br />important factor in the corrosion process. Most
<br />metals corrode very slowly in the absence of dis-
<br />solved oxygen, and, in general, the rate of corrosion
<br />increases with an increase in dissolved oxygen.
<br />
<br />o Chlorine Residual Chlorine, like oxygen, is
<br />an oxidizing agent that tends to accelerate the
<br />corrosion reaction,
<br />
<br />o Total Dissolved Solids A water with a
<br />high TDS concentration has a high conductivity, which
<br />increases its corrosivity.
<br />
<br />o Calcium Calcium ions can combine with
<br />carbonate to form a CaC03 scale, Some evidence in-
<br />dicates that calcium alone can reduce corrosion rates,
<br />even without formation of CaCO, scales.
<br />
<br />o Chloride, Sulfate Chlorides and sulfates
<br />are aggressive anions, tending to accelerate the rate
<br />of corrosion,
<br />
<br />o Temperature As with most chemical reac-
<br />tions, the rate of a corrosion reaction increases with
<br />increasing temperature.
<br />
<br />2
<br />
<br />WATER STABILITY
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