Inhibitors cooling waters

PerforMax . [Drew Ind. Div.] Corrosion inhibitor, cooling water treatment... 

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

Organophosphonates are similar to polyphosphates in chelation properties, but they are stable to hydrolysis and replace the phosphates where persistence in aqueous solution is necessary. They are used as scale and corrosion inhibitors (52) where they function via the threshold effect, a mechanism requiring far less than the stoichiometric amounts for chelation of the detrimental ions present. Threshold inhibition in cooling water treatment is the largest market for organophosphonates, but there is a wide variety of other uses (50). 

Petroleum greases and oils can be excellent corrosion inhibitors on a variety of alloys. The hydrophobic layer produced by oil or grease can prevent water from contacting surfaces and can, therefore, almost eliminate corrosion. Unfortunately, the addition of oil and grease cannot be recommended as a corrosion-reduction measure in cooling water systems for three basic reasons. 

Environment Internal Cooling water treated with scale inhibitor External Refractory... 

Environment Internal Cooling water treated with corrosion inhibitors and sodium hypochlorite biocide, 75°F (24°C), 50 psi (345 kPa), pH 7-8... 

Corrosion inhibitor 1 Bacterial control J Cooling-water system Consumption depends on make-up Chemicals are used to provide adequate reserve... 

Dissolved solid and gaseous impurities can also affect the pH of the system and this may often lead to decreased inhibitor efficiency. In industrial plant, cooling waters can take up SOj, HjS or ammonia and pH control of inhibited waters will be necessary. The leakage of exhaust gases into engine coolants is an example in which corrosion can occur despite the presence of inhibitors. 

Since corrosion inhibitors are used in a wide range of applications, no universal test method exists. Recognised methods tend to relate to a product or process in which the inhibitor forms a part rather than to the inhibitor per se. Thus, tests exist for inhibited coolants, cooling waters, cutting oils, pickling liquids, etc. 

Industrial Cooling Water Systems Waters used for recirculating cooling systems can either be scaling or corrosive. Corrosive waters are treated with corrosion inhibitors which require monitoring for overall assessment of the treatment programme. 

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. 

Amine salts of ether carboxylates inhibit internal corrosion of oil storage tanks and pipelines [230]. Furthermore it is possible to use ether carboxylates as corrosion and scale inhibitors for industrial recirculating cooling water systems, metalworking fluids, and hydraulic fluids [28,231-233]. 

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). ... 

Organic toxic pollutants and chromium are present in the raw wastewater and normally consist of raw materials, impurities, and metals used as cooling water corrosion inhibitors. 

Therefore, the amount of blowdown required to control scaling can be reduced. Chemicals added to once-through cooling water to control corrosion or to recirculating cooling water to control corrosion and scaling is usually present in the discharges. Chromium and zinc are the active components of most of the popular corrosion inhibitors. 

Chromate compounds have been considered by many to be the best inhibitors available. They are typically composed of mixtures of sodium bichromate and chromic acid, and art through passivation of the metal surfaces. Passivation involves formation of a tough metal oxide layer or other film on the surfaces. Chromate concentrations of 200 to 1000 ppm in the cooling water are generally required, although for environments where bimetallic influences exist, chromate levels must be much higher. For instance, when steel and copper surfaces are present in the system, chromate levels often exceed 2000 ppm (BETZ 1982, pp. 207, 212). 

While minimizing blowdown volumes can be quite effective in reducing waste, removing the hazardous metals component of the waste stream can have a more dramatic effect on reducing the impact of the blowdown on the environment. Chromate based corrosion inhibitors have historically been the mainstay of cooling water treatment systems. But in recent years, some plants have elected to use nonchromate inhibitors in order to reduce the potential environmental problems associated with chromates. Types of nonchromate inhibitors that have proved useful and that have in some situations performed as well as chromates include (Roti 1985) ... 

BETZ Laboratories of Trevase, PA has developed an inorganic phosphate substitute for chromate corrosion inhibitors called Dianodic Q, which is used in 10 to 20 ppm concentrations in cooling water. 

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. 

Molybdate s low toxicity to fish and other aquatic life has helped to gain recognition in recent years as a corrosion inhibitor. Molybdate forms its protective film by adsorption on metal surfaces. When chloride and sulfate anions are present in the cooling water environment, they compete for adsorption, and high concentrations of molybdate are needed for effective passivation of the metal surfaces. In order to be able to reduce the molybdate concentration for cost-effective levels, synergistic blends are made up that include other inhibitors such as phosphorates and zinc. 

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. 

Corrosion control of metal surfaces depends on the formation and maintenance of a protective corrosion inhibitor film on the exposed metal surface. This protective film may be established during normal application of a corrosion inhibitor program however, there will be some lag time before the film is completely built up. Metal surfaces that are exposed to the cooling water before the film is completed may become candidates for accelerated corrosion during the initial system operation. Normally, localized corrosion or pitting is common during these early stages of operation. 

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