Cooling water systems corrosion

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. 

Cooling water system corrosion causes immediate and delayed problems. Difficulties spread from a failure like ripples from a pebble thrown into a pool. A single failure may force an unscheduled outage, redirect worker efforts, contaminate product, compromise safety, increase equipment expense, violate pollution regulations, and decrease productivity. 

Many sources contain scattered information concerning cooling water system corrosion and defects, and many literature studies describe corrosion processes and mechanisms from a predominantly theoretical viewpoint. Until now, however, no source discusses cooling water system corrosion with emphasis on identification and elimination of specific problems. Much of the information in this book is unique every significant form of attack is thoroughly detailed. Color photos illustrate each failure mechanism, and case histories further describe industrial problems. 

Bahadur, A., 1993, Chromate substitutes for corrosion inhibitors in cooling water systems. Corrosion Reviews, 11, 105 - 122. 

Rice, R.G. and Wilkes, J.F., 1991, Fundamental aspects of ozone chemistry in recirculating cooling water systems. Corrosion 91 Paper 205, Nat. Assn. Cor. Engnrs. Houston. 

Yang, B., Real-Time Localized Corrosion Monitoring in Industrial Cooling Water Systems, Corrosion, Vol. 56, No. 7, July 2000, pp. 743-756. 

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

Continuous chlorination of a cooling water system often seems most pmdent for microbial slime control. However, it is economically difficult to maintain a continuous free residual in some systems, especially those with process leaks. In some high demand systems it is often impossible to achieve a free residual, and a combined residual must be accepted. In addition, high chlorine feed rates, with or without high residuals, can increase system metal corrosion and tower wood decay. Supplementing with nonoxidizing antimicrobials is preferable to high chlorination rates. 

As the name implies, crevice corrosion occurs between two surfaces in close proximity, such as a crack. Table 2.1 gives a partial listing of common crevice corrosion sites in cooling water systems. 

A test water box was installed during a 2-week trial to monitor corrosion and fouling in a utility cooling water system. A baffle plate from the test box was removed after the test. Small, hollow incipient tubercles dotted surfaces (Fig. 3.28). Small amounts of carbonate were present atop and around each tubercle. Each tubercle capped a small depression no deeper than 0.005 in. (0.013 cm) (Fig. 3.29). This indicated local average corrosion rates were as high as 130 mihy (3.3 mm/y). 

Attack always occurs beneath a deposit. Cooling water system deposits are ubiquitous. Deposits can be generated internally as precipitates, laid down as transported corrosion products, or brought into the system from external sources. Hence, underdeposit corrosion can be found in virtually any cooling water system at any location. Especially troubled... 

Almost all cooling water system deposits are waterborne. It would be impossible to list each deposit specifically, but general categorization is possible. Deposits are precipitates, transported particulate, biological materials, and a variety of contaminants such as grease, oil, process chemicals, and silt. Associated corrosion is fundamentally related to whether deposits are innately aggressive or simply serve as an occluding medium beneath which concentration cells develop. An American... 

Calcium carbonate makes up the largest amount of deposit in many cooling water systems (Fig. 4.16) and can be easily detected by effervescence when exposed to acid. Deposits are usually heavily stratified, reflecting changes in water chemistry, heat transfer, and flow. Corrosion may be slight beneath heavy accumulations of fairly pure calcium carbonate, as such layers can inhibit some forms of corrosion. When nearly pure, calcium carbonate is white. However, calcium carbonates are often intermixed with silt, metal oxides, and precipitates, leading to severe underdeposit attack. 

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. 

When a clean steel coupon is placed in oxygenated water, a rust layer will form quickly. Corrosion rates are initially high and decrease rapidly while the rust layer is forming. Once the oxide forms, rusting slows and the accumulated oxide retards diffusion. Thus, Reaction 5.2 slows. Eventually, nearly steady-state corrosion is achieved (Fig. 5.2). Hence, a minimum exposure period, empirically determined by the following equation, must be satisfied to obtain consistent corrosion-rate data for coupons exposed in cooling water systems (Figs. 5.2 and 5.3) ... 

Passive attack involving underdeposit corrosion tends to involve large system surface areas and, hence, accounts for the greatest amount of metal loss, by weight, in cooling water systems. Active attack tends to produce intense localized corrosion and, as such, a greater incidence of perforations. 

Four main kinds of bacteria have been linked to accelerated corrosion in cooling water systems ... 

Most cracking problems in cooling water systems result from one of two distinct cracking mechanisms stress-corrosion cracking (SCC) or corrosion fatigue. 

Metal surfaces in a well-designed, well-operated cooling water system will establish an equilibrium with the environment by forming a coating of protective corrosion product. This covering effectively isolates the metal from the environment, thereby stifling additional corrosion. Any mechanical, chemical, or chemical and mechanical condition that affects the ability of the metal to form and maintain this protective coating can lead to metal deterioration. Erosion-corrosion is a classic example of a chemical and mechanical condition of this type. A typical sequence of events is ... 

Note that low carbon or stabilized grades of stainless steel do not possess intrinsically greater corrosion resistance than their unadjusted counterparts. Their sole value in typical cooling water systems results from their resistance to sensitization and potential weld decay that can result when the metals are welded. It is therefore not economically justifiable to specify low carbon or stabilized grades of stainless steel for typical cooling water system components that are not to be welded. 

Operation of this cooling water system was intermittent, resulting in long periods (30 days) of no-flow conditions. After IVi years, leaks were found at welded pipe junctions. Radiographic examinations revealed numerous additional deep corrosion sites at welds that had not yet begun to leak. 

Galvanic corrosion in typical industrial cooling water systems is the net result of the interplay of these factors. Some factors may accelerate the corrosion process others may retard it. In their approximate order of importance, the more influential factors are discussed below. 

All areas of the cooling water system where a specific form of damage is likely to be found are described. The corrosion or failure causes and mechanisms are also described. Especially important factors influencing the corrosion process are listed. Detailed descriptions of each failure mode are given, along with many common, and some not-so-common, case histories. Descriptions of closely related and similarly appearing damage mechanisms allow discrimination between failure modes and avoidance of common mistakes and misconceptions. 

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

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. 

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]. 

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