Cooling corrosion inhibition

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. 

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. 

Other common poly glycol-based antifoams include certain derivatives of polyethylene glycol (PEG), which are condensation polymers of ethylene glycol. An example is polyethylene glycol-8 dioleate. Apart from its antifoam properties, PEG-8 dioleate is also used in cooling water inhibitor formulations as a surface cleaner, in the formation of a corrosion-inhibiting surface film. Additionally, it is employed as an oil-soluble emulsifier for other defoamer chemistries. 

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). ... 

It is rare in actual applications that only one corrosion inhibitor is used. Synergistic blends of two or more inhibitors can take advantage of the strengths of each. Table 10-1 compares the performances of several of these blends to that of a chromate-zinc blend under different operating conditions. Note that while the chromate-zinc blend offers the best corrosion inhibition with no contaminants present and at high temperatures, other blends are close, and do not present the environmental problems that chromates do. With petroleum ether, hydrogen sulfide, or hexane contaminants present in the cooling water, the Polyphosphate-HEDP-Carboxylate blend performs the best, followed by Zinc-HEDP. 

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. 

Antifreeze contains corrosion-inhibiting compounds and dye for identification purposes. Dilution is 1 1 glycol to water mixture, which gives protection down to -40°C. Newer developments have seen the advent of organic acid technology (OAT) antifreezes such as Dex-cool. OAT antifreezes have extended service lives of up to 150,000 miles. The properties of OAT coolant are shown in Table 7.7. OATs can mix with non-OATs however, the change interval will decrease to 2 years or 30,000 miles. OATs work by a mechanism different from that of traditional antifreeze mechanisms. As can be seen from Table 7.7, the additive package of the OAT contains... 

One early approach for cooling water corrosion inhibition was the use of inorganic polyphosphates. These complex molecularly dehydrated phosphates came into widespread industrial use beginning in the 1930 s. 

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. 

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. 

Chem. Analysis Sodium benzoate (90-92%), sodium nitrite (8-10%) Uses Corrosion inhibitor for antifreeze, closed cycle heating and cooling systems, domestic and industrial heating systems, summer coolants for internal combustion engines, hydraulic systems, emulsion paints, textile emulsions and adhesives corrosion-inhibiting impregnant for wrapping papers... 

---