by William (Bill) Harfst

Cooling water systems are subject to corrosion damage as a result of the reaction of the metal surface with its environment. This environment includes aerated cooling water, scale deposits, surface films, process contaminants, and microbiological growths. These and other conditions lead to rapid deterioration of the cooling tower, heat exchangers and piping system. Effective water treatment programs include provisions for corrosion inhibition to prolong the useful life of cooling water systems.

Cooling Water Corrosion

The corrosion mechanism is best depicted as an electrochemical corrosion cell. In this model, oxidation occurs at the anode of the corrosion cell where iron (Fe) is dissolved into the water. The electrons released at the anode travel through the metal to the cathode where oxygen (O2) is reduced to form hydroxide ions. The hydroxide is then available to react with the ferrous iron to form an insoluble by-product of corrosion, ferrous hydroxide. Frequently, the iron oxides deposit at the site of corrosion resulting in the formation of numerous tubercles along the metal surface. If the tubercles are scraped away with a putty knife or wire brush, the bare metal reveals a series of pits that have formed as a result of the oxidation reaction.

The electrochemical corrosion cell consists of four components: (1) an anodic site, (2) a cathodic site, (3) a current path (metal), and (4) an electrolyte (water). The rate of the corrosion reaction is dependent on several variables including the amount of dissolved oxygen available at the cathode, temperature, the pH of the water, water velocity, and total dissolved solids. In cooling water chemistry, the primary rate controlling factor is the amount of dissolved oxygen available at the metal surface. Effective corrosion control relies on the ability of chemical inhibitors to retard or inhibit the chemical reaction that occurs at either the anode or the cathode. Corrosion inhibitors that are effective in controlling the reactions that occur at the anode are called anodic inhibitors. Those that function at the cathode are called cathodic inhibitors. These inhibitors are thought to work by virtue of their ability to form a molecular film on the metal surface. The inhibitor polarizes the anode/cathode corrosion cell, thus slowing or stopping the corrosion reaction.

Corrosion Inhibitors

Various corrosion inhibitors are added to cooling water systems to control the rate of corrosion on mild steel, copper and copper alloys, stainless steel, galvanized steel, and aluminum. Since some inhibitors are more effective in controlling corrosion of a particular metal than others, the corrosion control program should be tailored to the system metallurgy. An effective cooling water treatment program always begins with an audit of the system metallurgy, equipment design and materials of construction. Once this is completed, an effective corrosion control program can be implemented. Here are some of the more popular and effective cooling water corrosion inhibitors.

Polyphosphate functions by forming an inhibitor film at the cathode of the corrosion cell. This inhibitor is most effective on mild steel, and does not protect copper or aluminum. The best protection occurs when the calcium level in the cooling water is maintained within 100 to 400 ppm. If the calcium exceeds 400 ppm, precipitation of calcium phosphate is possible especially in low-flow (less than 1 foot per second) areas of the system.

Typical dosages of polyphosphate are 10 to 30 ppm as PO4. The pH of the cooling water should be maintained within 5.5 to 7.5 to minimize calcium phosphate fouling.

Orthophosphate forms in the cooling water as a result of the hydrolysis (decomposition) of polyphosphate. Orthophosphate is an anodic inhibitor. It is also less soluble than polyphosphate and reacts with calcium to precipitate tricalcium phosphate at high calcium concentration and at elevated pH. Orthophosphate is not commonly used alone in cooling water treatment for these reasons.

Zinc is a cathodic inhibitor for steel, but does not provide effective protection for copper or aluminum. Typical dosages are 1 to 5 ppm at a controlled pH of 6.5 to 6.7. Zinc is less soluble at higher pH. At pH’s above 8.0 it is difficult to maintain zinc in solution, and it tends to precipitate in low-flow areas of the system.

Zinc is toxic to fish and microorganisms at concentrations above 3 ppm. Because of solubility and toxicity restraints, zinc is rarely used alone in cooling water treatment programs.

Molybdate is frequently used as a corrosion inhibitor in open and closed cooling water systems. Early recommendations called for 100 to 200 ppm sodium molybdate for mild steel inhibition. When compared to other inhibitors, molybdate is costly. This fact tended to restrict the use of molybdate to closed cooling water systems. When combined with zinc, phosphate or polysilicate, however, molybdate dosages can be reduced to 5 to 10 ppm, which significantly reduces the treatment costs. Often it is used less as a corrosion inhibitor and more as a chemical tracer to facilitate the testing for the product dosage. Molybdates were initially thought to be non-toxic. The EPA, however, is still investigating the environmental impact molybdate has on waste sludge and in the food chain.

Polysilicate is effective in protecting aluminum and copper. Generally, it is used at dosages of 10 to 15 ppm as SiO2 at a pH of 7.5 to 10.0. Because of reduced solubility, polysilicate is not applied at pH’s below 7.0. Polysilicate can be used with molybdate (1 to 3 ppm as MoO4) to provide enhanced protection of steel.

Orthosilicate offers less protection than Polysilicate. It is not very effective even at high dosages and can contribute to severe pitting if not carefully applied and controlled.

Chromate is one of the most effective corrosion inhibitors. It functions as an anodic inhibitor by forming a tenacious film on the metal surface. Traditional dosages are 100 to 500 ppm as CrO4 at pH 5.5 to 10. Blending chromates with other inhibitors such as zinc, polyphosphate, polysilicate and molybdate permit lower dosages of 5 to 30 ppm as CrO4.

The use of chromates in open cooling water systems was outlawed by the EPA because of toxicity and disposal problems. Chromates still find restricted use in closed cooling water loops, or in systems that have chromate removal systems prior to discharge of the water.

Organic inhibitors include azole compounds such as mercaptobenzothiazole (MBT), benzotriazole (BT), and tolytriazole (TT). These inhibitors are primarily used for copper and copper alloy inhibition. Typical dosages are 5 to 10 ppm for MBT and 1 to 3 ppm for BT and TT. Tolytriazole is the most popular of the yellow metal inhibitors in cooling water formulations because of its stability in the presence of chlorine and the low effective dosage. Organic inhibitors are classified as general inhibitors as it is not clear if they function at the anode, cathode or both.

Nitrites are used in closed loop cooling water systems. Because nitrite is a food source for bacteria, it is not acceptable for use in open cooling water systems.

Nitrite is an anodic inhibitor that provides excellent protection for mild steel. Typical dosages in closed chilled water systems are 800 to 1200 ppm as sodium nitrite. In closed hot water systems the recommended dosage is slightly higher, 1500 to 2000 ppm as sodium nitrite. Nitrites are blended with other inhibitors such as sodium tetraborate, metaborate, silica and tolytriazole to provide complete multi-metal protection. The borax component is adjusted to buffer the pH between 9.0 and 9.5.

Manganese phosphate is a new inhibitor that is very effective on copper and copper alloys.

Maintaining the Protective Inhibitor Film

Corrosion inhibitors must be applied continuously to establish and maintain the protective film on the metal surface. Initial dosages are generally higher than maintenance dosages to facilitate the establishment of the passivating film at the anode or cathode.

Monitoring Corrosion in Cooling Systems

The effectiveness of a corrosion control program is determined by the degree of protection afforded the system metal. One way of determining this is by periodic inspection of plant equipment. Waiting for the window of opportunity to make the inspection, however, can be costly because once the corrosion damage has occurred few options remain other than repair or replacement of the failure. It is better to detect corrosion problems before they reach the point of failure so that corrective action can be taken immediately. This is accomplished by several corrosion monitoring methods.

Corrosion coupons are the simplest tool for monitoring the corrosion rate in cooling water systems. Thee coupons are pieces of metal of known composition that are inserted in a by-pass flow of water. The corrosion rate is calculated by determining the weight loss of the metal coupon after a specific period of time, usually 30, 60 or 90 days.

Corrosion coupons are available in a wide range of metallurgies and sizes. Steel, copper, brass, stainless steel, and aluminum are commonly used in most water treatment applications. These specimens measure 3 inches long, ½ inch wide, and 1/16 inch thick. Other types and sizes of coupons are available for specific applications. Select a metal specimen(s) that matches the metal being studied in the system.

Corrosion coupons are inserted in the system in a by-pass rack. The coupon holders consist of a pipe plug and plastic rod to which the metal coupon is attached with a nylon bolt and nut. Metal fasteners should not be used to attach the coupon unless a plastic insulating washer is used to separate the coupon from the fastener. This prevents galvanic corrosion from occurring between the coupon and the holder. Likewise, the corrosion coupon rack should be made of PVC pipe, normally ¾” or 1″, unless the water is hot in which case black iron pipe is recommended. The pipe plug assembly is then inserted into one of the slots in the coupon rack. Position the coupon so that the thin edge is toward the water flow (vertical), the coupon is not touching the pipe wall, and is inserted into the main flow away from turbulence. The flow rate should be maintained between 3 and 5 ft per second.

After the coupon is exposed for 30, 60, or 90 days, it is removed for analysis. Initial evaluation involves inspection of the coupon for signs of pitting, tuberculation and deposits. A photograph of the coupon before and after cleaning is helpful for future reference.

It is best to leave a coupon in the system for at least 30 days. A clean coupon corrodes much faster than one that has reached equilibrium with the corrosive environment. A higher corrosion rate will be obtained on coupons exposed for intervals less than 30 days. Also, some error in the test is introduced during the cleaning of the coupons. A small amount of metal is unavoidably removed during cleaning. If the actual metal loss from corrosion is small, as is the case with a short test, the amount of metal removed during cleaning creates a significant error.

In the laboratory the coupon is cleaned and reweighed. Several cleaning methods are used including shot blasting, ultrasonic, or immersion in an inhibited solution of hydrochloric acid. Cleaning procedures differ based on the type of metal. The same procedure must be used, however, when making comparative analyses between corrosion tests.

The corrosion rate is calculated from the weight loss of the coupon and the exposure time.

Corrosion rate, mils per year = 22.3 x W

D x A x T

Where:

W = weight loss in milligrams

D = specific gravity of the metal in grams per cubic centimeter

A = area of coupon in square inches

T = time in days

Pitting of the metal is noted and the severity of this pitting corrosion is reported as maximum pit depth in thousands of an inch (mils). Pit depth is measured with a feeler gauge or microscope. Pitting rate can be determined by:

Pitting rate = Maximum pit depth, mils X 365

Time, days

EVALUATING CORROSION RATES

(Rates in mils per year on 90 day test)

Metal

Excellent

Good

Fair

Poor

Unacceptable

Mild steel piping

< 1

1 to 3

3 to 5

5 to 10

> 10

Mild steel HX tubing

< 0.2

0.2 to 0.5

0.5 to 1.0

1.0 to 1.5

> 1.5

Copper and alloys

< 0.1

0.1 to 0.2

0.2 to 0.3

0.3 to 0.5

> 0.5

Galvanized steel

< 2

2 to 4

4 to 8

8 to 10

> 10

Stainless steel

< 0.1

> 0.1

Electrical resistance instruments work by measuring the electrical resistance of a thin metal probe; as corrosion causes metal to be removed from the probe, its resistance increases.

The major advantage of the electrical resistance method versus corrosion coupons is that measurements can be obtained on a more frequent basis and require much less effort to perform. Continuous readings can be made, and with sophisticated data analysis techniques, changes in corrosion rates are available in as little as two hours instead of the 30 days or more required with coupons. Electrical resistance probes are basically automatic coupons and share many of the advantages and limitations of standard coupons. Localized deposition on the metal probe, however, can give misleading results. For this reason, ER probes are not widely used in cooling water applications.

Linear polarization resistance (LPR) is an electrochemical method that measures the dc current (imeas) through a metal/fluid interface. The dc current is generated as a result of the polarization of one or two electrodes fashioned from the metal under study by the application of a small electrical potential. Since the corrosion current is directly proportional to the corrosion rate, LPT techniques provide instantaneous corrosion rate measurements. This has certain advantages over corrosion coupons when constant corrosion monitoring is required.

LPR probes offer accurate indications of corrosion activity in the system. These devices are sensitive enough to detect differences in corrosion rate at various inhibitor levels. They can also be used to alert plant operators to a corrosive upset such as a low pH excursion.

LPR methods record pitting tendencies by the electrode potential difference that arises when the current flow is reversed. Then the electrodes reach equilibrium, they register general and pitting corrosion rates similar to those suggested by standard coupon measurements.

Overall, monitoring corrosion rates in cooling water systems is an integral part of a complete water treatment program. The use of corrosion coupons, electrical resistance and linear polarization resistance probes make this task simple and cost effective.

William Harfst is President of Harfst and Associates, Inc., an independent water management consulting firm,

PO Box 276, Crystal Lake, Illinois 60039;

Email: wfh@mc.net

Website: http://www.harfstassociates.com

Bill has over 43 years of water management experience helping industrial, institutional, commercial and government clients select, apply and control water treatment programs for boiler, cooling and wastewater applications. He graduated from the University of Illinois with a B.S. in Chemistry cum laud in 1972 and went on to hold various engineering, technical and management positions with three major water treatment companies before starting his consulting practice in 1991. His current focus is on helping clients conserve water, reduce chemical consumption, minimize waste and save energy.