3) Chemical Treatment for Corrosion, Scale and Deposit Control

Many different kinds of chemical treatment are used to minimize problems and assure efficient and reliable operation of cooling water systems. Each open recirculating cooling system is unique. Among those characteristics that make one system different from another are:

• System design, including size, basin depth, materials of construction, flow rates, heat transfer rates, temperature drop and other factors.
• Water, including make-up water composition, pretreatment and cycles of concentration.
• Blowdown water discharge restrictions.

The selection of a treatment program for a specific system must take into account all of these system characteristics and others, and must contain materials to control, to an acceptable degree, each of the potential problems that may occur in that system. It is fortuitous that many of the chemicals used to treat cooling systems are effective in controlling more than one problem. For example, some carbon steel corrosion inhibitors also effectively control calcium carbonate scale.

(1) Corrosion Inhibition

Carbon steel is commonly used because of its favorable cost and good mechanical properties. Many other alloys are employed where their mechanical and thermal properties, and/or their corrosion resistance are required.

Corrosion inhibitors are almost universally used to control deterioration of carbon steel and other alloys in cooling systems. Corrosion inhibitors work by interfering with chemical reactions of metals with aqueous environment. They may restrict either the anodic or the cathodic at the anodes are called anodic inhibitors and inhibitors that work at the cathodes are called cathodic inhibitors. Many different types of chemicals are employed as corrosion inhibitors. The effectiveness of nay specific inhibitor composition, and the quantities required for good results, depend upon the conditions in each specific system.

Most practical corrosion inhibitor programs employ combinations of two or more compounds. Anodic and cathodic inhibitors are often used together to establish control over both sides of corrosion reaction. Such mixtures are said to be synergistic when the performance of the mixture is seen to be greater than the performance expected from the individual compounds added together. Because of this effect, some inhibitors that are known to be weak when used alone, can make important, cost-effective contributions to corrosion control when used in combinations with other inhibitors. For more details, please discuss with the manufacturers of corrosion inhibition chemicals. The corrosion control due to MIC is discussed here.

No discussion of corrosion control in open recirculating cooling systems would be complete without mention of microbiologically induced corrosion. MIC is recognized as a major source of under-deposit pitting attack in many systems that are otherwise well protected against corrosion. All of the metals and alloys are subject to MIC, and many corrosion failures in alloys previously thought to be corrosion-resistant in open cooling water environments are not recognized as MIC related.

MIC does not, in general, involve direct attack of bacteria on metal. Rather, MIC refers to corrosion that is induced or accelerated by the presence of products of microbiological metabolism. The most commonly seen cases of MIC are caused by sulfate-reducing bacteria. These are anaerobic bacteria that cannot live in the presence of dissolved oxygen in cooling water. They exist under deposits of corrosion products, suspended solids or biological slimes. They obtain their metabolic energy by reducing sulfate ions in the water and forming hydrogen sulfide(H₂S) or metal sulfide salts. The corrosion process generates more deposit and the process accelerates. Since the process is anaerobic (oxygen free), corrosion resistant films that depend on dissolved oxygen in the cooling water break down. This leads, eventually, to deep pitting attack.

Other bacteria can also cause MIC to occur. Acid-producing bacteria, also anaerobic in nature, form organic acids under-deposits. These acids can attack carbon steel and other metals ad alloys, producing the characteristic pitting attack. Iron-oxidizing bacteria react with ferrous (reduced) iron in cooling water and form voluminous deposits of mixed iron oxides and biological slimes, under which sulfate reducing and acid-producing bacteria can grow.

Detection and recognition of MIC in operating cooling water systems involves the use of several different techniques. Common microbiological assays of planktonic (free swimming) bacteria in the cooling water are not a useful way to detect the presence of MIC-causing bacteria. A cooling system can show very low levels of planktonic bacteria and still be severely contaminated with anaerobic sulfate-reducers and other bacteria living in and under deposits. It is essential to use spool pieces and other test devices that can allow sessile (attached) bacteria to grow and to inspect the cooling tower, heat exchangers and low-flow points in the system regularly for the presence of biological deposits. Field test kits are available that can detect the presence of sulfate-reducing bacteria in fresh deposits removed from operating cooling systems, and in some cases, a metallographic examination can show corrosion patterns characteristic of certain types of MIC-causing bacteria.

The first line of defense in protecting cooling systems from MIC must be to keep the system clean and free of deposits. With the system clean, a microbiocide program must be selected which is effective under the operating conditions in the system and compatible with the water chemistry and with other treatment programs in use. Finally, the system must be monitored regularly to detect any appearance of biological or other deposit that can encourage anaerobic bacteria to grow. Microbiological control methods are discussed in detail in another part of this manual.

(2) Scale Inhibitions

Five general methods are available for controlling mineral scale formation in open recirculating cooling systems. These are:

• External pretreatment of the make-up water
• External side stream or full flow treatment of the circulating water
• Blowdown control
• Chemical treatment of the circulating water to reduce the reactivity of one of more reacting species
• Chemical treatment of the circulating water to "stabilize" reactive species so that they will not precipitate from solution.

The optimum scale control program developed for any specific system must depend on the make-up water composition and its availability, operating parameters in the cooling systems, the number of concentration cycles to be carried in the circulating water, and sometimes on effluent considerations. Some systems, for which plenty of very soft make-up water is available, do not require any scale control program. For other systems, simple stabilization chemical treatment is sufficient. On the other hand, in many parts of the country the make-up water is both hard and in short supply, so that it must be conserved. The most cost-effective scale control program for recirculating cooling systems in such cases may include, for example, partial softening of the make-up water, strict control of concentration cycles, side stream filtration or softening and stabilization chemical treatment of the circulating water. For further chemical treatment, contact the makers of chemicals. The blowdown control shall be only discussed here.

Increasing the blowdown rate from a recirculating cooling system is a simple way to reduce the levels of calcium and alkalinity in the water, thus reducing the calcium carbonates scaling potential. However, this is frequently not a cost-effective option. Increased blowdown, which means operating the cooling system at lower cycles of concentration, requires increased make-up water and products more wastewater for disposal. Increased make-up leads to increased corrosion inhibitor usage and may require more frequent biocide applications.

Blowdown control is, however, a critical part of any good scale control program in open recirculating cooling systems. It is important to strike a technically practical and cost-effective balance between the hardness that can be removed from the make-up water by pretreatment, the cycles of concentration that can normally be achieved, the amount and quality of blowdown water that can be tolerated and the costs of acid and stabilizing treatment chemicals. The ability of the plant to control the system is also an important factor. Widely varying blowdown rates can make any scale control program costly and/or ineffective. Also, lapses in feed of stabilizing chemicals can lead to serious scaling problems if the system is operating under supersaturated conditions.

(3) Suspended Solid Control

Water-borne suspended solids, as defined here, includes silt and clay, corrosion  products and other metal oxides, and insoluble process contaminates, but not biologically active solids. Treatments for controlling deposition of these materials are developed based on experience and empirical data, since the materials are heterogeneous and poorly defined. Physical characteristic, such as particle size and density, and surface properties, including electrical charge, are probably more significant than chemical compositions. Progress is being made in understanding dispersion chemistry in aqueous systems and this is leading to the development of new and better dispersants. It does not seem likely, however, that dispersion chemistry will, in the near future, reach the level of technical sophistication now achieved for corrosion and scale control in cooling water systems.

(4) Microbiological Growth Control

The purpose of this technical manual is to explain how to control the microbiological growth in the cooling water systems. This will be discussed from the next chapter.

4) Performance Monitoring & Chemical Analysis

The success of the chemical treatment program is often dictated by the reliability and accuracy of the chemical feed system. Chemical feed systems can be as simple as operator sampling and testing for each control parameters, followed by manual addition of the proper chemical at the proper dosage, or it can involve full computer control for measurement and adjustment of all chemicals included in the treatment program. It is important to remember that operators are the most important element of the treatment program. They must have a complete understanding of the treatment program and how the use of each chemical affects the cooling water system performance. Operators must know how their chemical feed system operates in order to recognize whether or not it is working properly. Also, a chemical feed system needs periodic maintenance and responsibility to recognize when maintenance is needed.