Water systems corrosion

The three major forms of concentration cell corrosion are crevice corrosion, tuberculation, and underdeposit attack. Each form of corrosion is common in cooling systems. Many corrosion-related problems in the cooling water environment are caused by these three forms of wastage. The next three chapters—Chap. 2, Crevice Corrosion, Chap. 3, Tuberculation, and Chap. 4, Underdeposit Corrosion — will discuss cooling water system corrosion problems. 

Cooling water system corrosion causes immediate and delayed problems. Difficulties spread from a failure like ripples from a pebble thrown into a pool. A single failure may force an unscheduled outage, redirect worker efforts, contaminate product, compromise safety, increase equipment expense, violate pollution regulations, and decrease productivity. 

Many sources contain scattered information concerning cooling water system corrosion and defects, and many literature studies describe corrosion processes and mechanisms from a predominantly theoretical viewpoint. Until now, however, no source discusses cooling water system corrosion with emphasis on identification and elimination of specific problems. Much of the information in this book is unique every significant form of attack is thoroughly detailed. Color photos illustrate each failure mechanism, and case histories further describe industrial problems. 

Bahadur, A., 1993, Chromate substitutes for corrosion inhibitors in cooling water systems. Corrosion Reviews, 11, 105 - 122. 

Rice, R.G. and Wilkes, J.F., 1991, Fundamental aspects of ozone chemistry in recirculating cooling water systems. Corrosion 91 Paper 205, Nat. Assn. Cor. Engnrs. Houston. 

Components in Water System Corrosion Control Method... 

Hot Water System Corrosion A schematic diagram of the domestic hot water system is given in Figure 5.22. Visual inspection led to the following observations ... 

Selby, K. A. and Hess, R. T. 1996. The Use of Polymers for On-Line Cleaning of Building Water Systems, CORROSION 96, Paper No. 524, NACE International, Conferences Div., Houston, Texas. 

Uses Emulsifier and coemulsifier in solv./water systems corrosion inhibitor in cutting oils lubricant for metal surfaces surfactant in cosmetics... 

With hot water tanks, the bottom is more liable to attack than the sides because deposits on the bottom (such as iron filings from holes drilled elsewhere in the tanks) produce bimetallic or differential aeration cells. Such cells also can form on the sides in some waters, especially near the waterline, however. If the water is made more aggressive (e.g., by the presence of copper salts, possibly dissolved from elsewhere in the water system), corrosion cells will form more readily. The presence in water used in domestic... 

Yang, B., Real-Time Localized Corrosion Monitoring in Industrial Cooling Water Systems, Corrosion, Vol. 56, No. 7, July 2000, pp. 743-756. 

Potable Water Systems. Corrosion is experienced in potable water transportation pipes of steels and cast iron. Inhibitors, such as Ca(HC03)2 and polyphosphates are commonly used to combat corrosion. 

Lenard, D. R., and Welland, R. R., Corrosion Problems with Copper-Nickel Components in Sea Water Systems, CORROSION/98, Paper 599. 199 Houston, Tex., NACE International. 

Contact with steel can accelerate attack on Al, but in some natural waters and other special cases, Al can be protected at the expense of ferrous materials, particularly when the Al is "passive." Titanium appears to behave in a similar manner to steel. Stainless steel in contact with Al may increase attack on Al, notably in seawater or marine atmosphere, but the high electrical resistance of the two surface oxide films minimizes bimetallic effects in less aggressive environments. Where Al is coupled to copper, or exposed to metallic copper contamination (such as in water systems), corrosion of the Al is very rapid. This is because Cu is particularly efficient at supporting cathodic reactions (e.g., oxygen and water reduction). Limiting cathodic currents measured for pure copper are reported to be in the vicinity of 1.5 mA cm , whereas limiting currents on pure Al are three orders of magnitude lower (0.5-1 pA cm- ) [52]. 

Cooling water systems are dosed with corrosion inhibitors, polymers to prevent solid deposition, and biocides to prevent the growth of microorganisms. 

Many instances of intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel-based alloys have occurred in the reactor water systems of BWRs. IGSCC, first observed in the recirculation piping systems (21) and later in reactor vessel internal components, has been observed primarily in the weld heat-affected zone of Type 304 stainless steel. 

Alkalinity Reduction. Treatment by lime precipitation reduces alkalinity. However, if the raw water alkalinity exceeds the total hardness, sodium bicarbonate alkalinity is present. In such cases, it is usually necessary to reduce treated water alkalinity in order to reduce condensate system corrosion or permit increased cycles of concentration. 

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

Continuous chlorination of a cooling water system often seems most pmdent for microbial slime control. However, it is economically difficult to maintain a continuous free residual in some systems, especially those with process leaks. In some high demand systems it is often impossible to achieve a free residual, and a combined residual must be accepted. In addition, high chlorine feed rates, with or without high residuals, can increase system metal corrosion and tower wood decay. Supplementing with nonoxidizing antimicrobials is preferable to high chlorination rates. 

Chromates are used to inhibit metal corrosion in recirculating water systems. When methanol was extensively used as an antifree2e, chromates could be successfully used as a corrosion inhibitor for cooling systems in locomotive diesels and automobiles (185). 

S. Arrington and G. Bradley, "Service Water System Cleaning with Ammoniated Citric Acid," paper presented at Corrosion 87-NACE, No. 387, San Francisco, Calif., 1987. 

The thermodynamic data pertinent to the corrosion of metals in aqueous media have been systematically assembled in a form that has become known as Pourbaix diagrams (11). The data include the potential and pH dependence of metal, metal oxide, and metal hydroxide reactions and, in some cases, complex ions. The potential and pH dependence of the hydrogen and oxygen reactions are also suppHed because these are the common corrosion cathodic reactions. The Pourbaix diagram for the iron—water system is given as Figure 1. 

Eig. 2. The thermodynamic regions of corrosion, immunity, and passivation of iron in an iron—water system assuming passivation by a film of Ee202 (H)-... 

Cathodic Protection This electrochemical method of corrosion control has found wide application in the protection of carbon steel underground structures such as pipe lines and tanks from external soil corrosion. It is also widely used in water systems to protect ship hulls, offshore structures, and water-storage tanks. 

As the name implies, crevice corrosion occurs between two surfaces in close proximity, such as a crack. Table 2.1 gives a partial listing of common crevice corrosion sites in cooling water systems. 

A test water box was installed during a 2-week trial to monitor corrosion and fouling in a utility cooling water system. A baffle plate from the test box was removed after the test. Small, hollow incipient tubercles dotted surfaces (Fig. 3.28). Small amounts of carbonate were present atop and around each tubercle. Each tubercle capped a small depression no deeper than 0.005 in. (0.013 cm) (Fig. 3.29). This indicated local average corrosion rates were as high as 130 mihy (3.3 mm/y). 

Attack always occurs beneath a deposit. Cooling water system deposits are ubiquitous. Deposits can be generated internally as precipitates, laid down as transported corrosion products, or brought into the system from external sources. Hence, underdeposit corrosion can be found in virtually any cooling water system at any location. Especially troubled... 

Equipment in which water flow is slow or intermittent is subject to deposition and associated corrosion. Hence, service water-system components that operate intermittently frequently suffer attack. Deposits accumulate in narrow orifices, screens, long horizontal pipe runs, sumps, or at regions of constricted flow. 

Almost all cooling water system deposits are waterborne. It would be impossible to list each deposit specifically, but general categorization is possible. Deposits are precipitates, transported particulate, biological materials, and a variety of contaminants such as grease, oil, process chemicals, and silt. Associated corrosion is fundamentally related to whether deposits are innately aggressive or simply serve as an occluding medium beneath which concentration cells develop. An American... 

Calcium carbonate makes up the largest amount of deposit in many cooling water systems (Fig. 4.16) and can be easily detected by effervescence when exposed to acid. Deposits are usually heavily stratified, reflecting changes in water chemistry, heat transfer, and flow. Corrosion may be slight beneath heavy accumulations of fairly pure calcium carbonate, as such layers can inhibit some forms of corrosion. When nearly pure, calcium carbonate is white. However, calcium carbonates are often intermixed with silt, metal oxides, and precipitates, leading to severe underdeposit attack. 

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