Cooling systems, corrosion inhibition

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). ... 

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. 

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. 

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. 

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. 

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). 

Open recirculating systems These are more amenable to inhibition since it is possible to maintain a closer control on water composition. Corrosion inhibition in these systems is closely allied to a number of other problems that have to be considered in the application of water treatment. Most of these arise from the use of cooling towers, ponds, etc. in which the water is subject to constant evaporation and contamination leading to accumulation of dirt, insoluble matter, aggressive ions and bacterial growths, and to variations in pH. A successful water treatment must therefore take all these factors into account and inhibition will often be accompanied by scale prevention and bactericidal treatments. 

Zinc salts rapidly generate zinc hydroxide or salt protective films on cathodic surfaces when they are added to cooling water. They are generally used in conjunction with other corrosion inhibitors, such as organophosphates. BETZ manufacturers a combination HEDP-zinc inhibitor. They have also been used with chromate systems to inhibit the chromate concentrations required. A disadvantage of zinc is its tendency to precipitate in pH environments greater than 8.0 (Roti 1985). While the toxicity of zinc to humans is far lower than that of chromium, its toxicity to marine and aquatic life is high. 

The most appropriate system for the plotform wos considered to be on indirect seawater cooling system which uses seawater to cool recirculating inhibited fresh woter in o closed loop system. The fresh water is circulated on the shell side of the process exchangers to minimise corrosion problems ond eliminate the need to use exotic materials which would be... 

Similarly, cooling water that is fully softened tends to lack sufficient buffering capacity and creates the potential for significant corrosion within the cooling system, so care and attention to design detail are needed if future corrosion problems are to be avoided. Also, most chemical treatments incorporate polymers or phosphonates of some sort to provide a measure of corrosion inhibition. These products usually require the presence of at least 25 to 35 ppm of calcium hardness to provide satisfactory inhibition. 

Both formulations are designed to provide excellent corrosion inhibition and deposit control in comfort cooling systems, with a cooling water reserve of approx. 75 to 125 ppm product. 

For control of deposits and scale, closed systems require hardness-free (or preferably mineral-free) makeup water. Microbicides are not required for closed systems, but dispersants to keep metal oxides and other insoluble contaminants in suspension are useful. They aid in preserving clean heat transfer surfaces, which provide maximum response to corrosion inhibition. Long chain ethylene oxide polymers also are being used to improve "wetting" properties of cooling water and minimize turbulence, where impingement and cavitation problems are observed. 

Influence of Hydrodynamic Conditions Inhibitors are often applied under conditions of liquid flow (e.g. in cooling systems or pipelines) and, consequently, the effect of the flow rate on their performance also has to be considered. First, the hydrodynamic conditions determine the transport of all dissolved species in the solution (inhibitor, H" ", OH , anions, corrosion products) and hence their concentration at the surface of the corroding metal. Since both inhibiting and corrosion-accelerating species are affected, the effect of transport on the inhibition efhciency is not easy to estimate and wiU depend strongly on the inhibition mechanism. Second, the liquid flow may exert mechanical forces on the metal surface, which may promote corrosion and counteract... 

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