Corrosion cooling system

Cooling System Corrosion Corrosion can be defined as the destmction of a metal by chemical or electrochemical reaction with its environment. In cooling systems, corrosion causes two basic problems. The first and most obvious is the failure of equipment with the resultant cost of replacement and plant downtime. The second is decreased plant efficiency to loss of heat transfer, the result of heat exchanger fouling caused by the accumulation of corrosion products. 

To increase equipment reliability and plant efficiency, corrosion inhibitors are used in boiler and cooling water programs to control fouling and deposition on critical heat-transfer surfaces. In cooling systems, corrosion inhibition is commonly achieved through the use of passivators, which encourage the formation of a protective metal oxide film on the metal surface ( 1). ... 

Desulfovibrio desulfuricans Anaerobic sulfate reducer that causes corrosion and precipitation of iron by sulfide. Corrosion can be serious, developing deep pits under slime it is especially troublesome in paper mills, steel plants, refineries, and other large cooling systems. Corrosion rates are enhanced by the removal of hydrogen from the cathode by sulfate reducers (cathodic depolarization), as follows ... 

Corroding towers are not acceptable. They present a risk of premature failure, are a source of contaminants for fouling and deposition elsewhere in the system, and may be unsafe. The use of fully softened makeup water (as required by some authorities and some cooling system operators around the world) is not recommended, as it represents a primary factor in cooling system corrosion and generally creates more problems than it solves. 

Mass flow and current density Cell voltage and cooling systems Corrosion phenomena Recycling of impurities Continuous versus batch processing Selectivity versus mass yield... 

In the early 1950 s, combinations of alkali chromate (an anodic inhibitor) and polyphosphate (generally accepted as cathodic) came into prominence for cooling system corrosion inhibition. The combination of chromate with phosphates proved highly efficient in comparison with straight phosphate or straight chromate, and could be used at substantially lower concentrations. 

A. Minhaj, PA. Saini, M.A. Quraishi, I.H. Earooqi (1999). A study of natural compounds as corrosion inhibitors for industrial cooling systems. Corrosion Prevention and Control 46(2), pp. 32-38. 

All three types may be present in heat exchangers. Any object such as loose scale or other deposits resting on a metal surface can give rise to a concentration cell and hence to crevice corrosion, which is reported to be the most potentially damaging type of corrosion in a water cooling system. Corrosion products can nucleate crystallization from supersaturated solutions or subcooled melts, anchor... 

At home, corrosion can be seen on vehicle body panels, barbecue grills, outdoor furniture, and metal tools. The main reason for replacing car radiator coolant every year is to replenish the corrosion inhibitor that limits corrosion of the cooling system. Corrosion protection is built into all major household appliances such as water heaters, furnaces, washers and dryers. 

Water-side deposits are of many types. Hardness (calcium and magnesium)-based deposits and iron oxide are the most common water-side deposits and often affect boUers and cooling systems. Process and oil leaks can foul boilers and cooling systems. BiofouUng, mud, and debris are often found in cooling systems. Treatment chemicals, if not properly controlled, can add to deposits and scales. Silica can form hard, adherent deposits in boUers, steam turbines, and cooling systems. Corrosion products can add to deposits. 

The production of hydroxide ions creates a localized high pH at the cathode, approximately 1—2 pH units above bulk water pH. Dissolved oxygen reaches the surface by diffusion, as indicated by the wavy lines in Figure 8. The oxygen reduction reaction controls the rate of corrosion in cooling systems the rate of oxygen diffusion is usually the limiting factor. 

The most serious form of galvanic corrosion occurs in cooling systems that contain both copper and steel alloys. It results when dissolved copper plates onto a steel surface and induces rapid galvanic attack of the steel. The amount of dissolved copper required to produce this effect is small and the increased corrosion is difficult to inhibit once it occurs. A copper corrosion inhibitor is needed to prevent copper dissolution. 

Theoretically, controUed deposition of calcium carbonate scale can provide a film thick enough to protect, yet thin enough to allow adequate heat transfer. However, low temperature areas do not permit the development of sufficient scale for corrosion protection, and excessive scale forms in high temperature areas and interferes with heat transfer. Therefore, this approach is not used for industrial cooling systems. ControUed calcium carbonate deposition has been used successhiUy in some waterworks distribution systems where substantial temperature increases are not encountered. 

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. 

Foulants enter a cooling system with makeup water, airborne contamination, process leaks, and corrosion. Most potential foulants enter with makeup water as particulate matter, such as clay, sdt, and iron oxides. Insoluble aluminum and iron hydroxides enter a system from makeup water pretreatment operations. Some well waters contain high levels of soluble ferrous iron that is later oxidized to ferric iron by dissolved oxygen in the recirculating cooling water. Because it is insoluble, the ferric iron precipitates. The steel corrosion process is also a source of ferrous iron and, consequendy, contributes to fouling. 

Corrosion Inhibition. Another important property of antifreeze solutions is the corrosion protection they provide. Most cooling systems contain varied materials of constmction including multiple metals, elastomeric materials, and rigid polymeric materials. The antifreeze chosen must contain corrosion inhibitors that are compatible with all the materials in a system. Additionally, the fluid and its corrosion inhibitor package must be suitable for the operating temperatures and conditions of the system. 

Service Life. The service life offered by a coolant is dependent on many factors, including the initial condition of the coolant and the cooling system, the type of water used for dilution, the metals of constmction in the system, the type of corrosion inhibitors and SCAs used, the system operating... 

When antifreeze becomes unsuitable for use, either because of depletion of inhibitors, presence of corrosion products or corrosive ions, or degradation of the fluid, recycling and reuse of the antifreeze, rather than disposal, may be considered. Although ethylene glycol is readily biodegraded in typical municipal waste treatment faciHties, antifreeze disposal becomes problematic because the coolant may contain hazardous quantities of heavy metals picked up from the cooling system. Recycling may be economically preferred over coolant disposal and reduces the concern for environmental impact. 

Sodium benzoate is also finding increasing appHcation as a corrosion inhibitor. It is incorporated into paper wrapping materials for the prevention of mst or corrosion in the production of such diverse items as razor blades, engine parts, bearings, etc. It is also used in the automotive industry as a corrosion inhibitor in engine cooling systems (at 1.5%), mainly in Europe and Japan. Unlike in its appHcation as a preservative where free benzoic acid is required to provide antimicrobial action, it appears to be the benzoate ion that provides the corrosion protection. 

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). 

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. 

Whatever the water composition, corrosivity can be increased by evaporation, which may elevate pH by increasing concentrations of ions in the remaining liquid. This is the reason that cooling systems that experience boiling and/or large evaporative losses without sufficient makeup water additions may be especially prone to attack. Cycling may also increase dissolved species concentrations. 

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