through a metal/fluid interface. However, the electrical resistance method also has its disadvantages: conductive deposits forming on the probe can create misleading results, temperature fluctuations must be compensated for, and pitting character-istics cannot be determined accurately. Glycine derivatives and aliphatic sulfonates are examples of compounds which can function in this way. Closed Recirculating Cooling System: In closed recirculating system cooling water is circulates in … The contact must be good enough to conduct electricity, and both metals must be exposed to the solution. When combined with zinc, phosphate or polysilicate, however, molybdate dosages can be reduced to 5 to 10 ppm, which significantly reduces the treatment costs. Orthophosphate is an anodic inhibitor. Its appearance is similar to closely spaced pits, although the surface is usually rough. The same procedure must be used, however, when making comparative analyses between corrosion tests. Learn more about SUEZ's corrosion control programs. The use of corrosion coupons, electrical resistance and linear polarization resistance probes make this task simple and cost effective. Adsorption inhibitors must have polar properties in order to be adsorbed and block the surface against further adsorption. However, recent work indicates that this interaction may not be necessary. The first and most obvious is the failure of equipment with the resultant cost of replacement and plant downtime. Chemical product layers formed by corrosion can inhibit suitable heat transfer between the liquid and wetted metal surfaces. Test heat exchangers are small exchangers that can be set up to simulate operating conditions in the plant. 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. Silicates. to provide enhanced protection of steel. The Cosmos is a portable data acquisition station that monitors key parameters of a cooling system. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Early recommendations called for 100 to 200 ppm sodium molybdate for mild steel inhibition. The corrosion appears to spread in a somewhat circular pattern from the site of initial colonization. It is most prevalent in stainless steels which have been improperly heat-treated. Electrically dissimilar metals in contact (or both in contact with the same water) 2. olyte present (could be as simple as condensation)Electr Time is another critical factor. The … Orthophosphate is not commonly used alone in cooling water treatment for these reasons. Unless continuously replenished, the protective films formed by cooling water corrosion inhibitors tend to degrade over time, leaving chiller condensers, piping and heat exchangers vulnerable to pitting as well as accelerating corrosion. Industrial cooling towers and closed loop efficiency can be adversely affected by corrosion, scaling, fouling and biological contamination. Linear polarization resistance (LPR) is an electrochemical method that measures the dc current (imeas) through a metal/fluid interface. Other types and sizes of coupons are available for specific applications. An electrical potential difference exists between these sites, and current flows through the solution from the anode to the cathode. Here are some of the more popular and effective cooling water corrosion inhibitors. Systems using two or three electrodes are available. The high corrosion rate in the pit produces an excess of positively charged metal cations, which attract chloride anions. Polarization. corrosion will manifest if the following conditions exist: 1. A uniform coating of calcium carbonate, deposited on the metal surfaces, physically segregates the metal from the corrosive environment. Also, some error in the test is introduced during the cleaning of the coupons. One disadvantage of coupons is their lack of heat transfer, resulting in a lower temperature than that of the actual heat exchanger tubes. BETZ MonitAll® Apparatus. Treating the entire cooling water system. Typical dosages are 5 to 10 ppm for MBT and 1 to 3 ppm for BT and TT. *Trademark of SUEZ; may be registered in one or more countries. It is also less soluble than polyphosphate and reacts with calcium to precipitate tricalcium phosphate at high calcium concentration and at elevated pH. Proper placement of sacrificial anodes is a precise science. In addition, hydrolysis produces H+ ions. Then the electrodes reach equilibrium, they register general and pitting corrosion rates similar to those suggested by standard coupon measurements. corrosion protective layer on the heat exchanging surfaces of the cooling water system. Calcium, alkalinity, and pH levels in water are important factors for reasons already cited. 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. In cooling water chemistry, the primary rate controlling factor is the amount of dissolved oxygen available at the metal surface. The most effective way to prevent SCC in both stainless steel and brass systems is to keep the system clean and free of deposits. When heat transfer data cannot be obtained on operating exchangers, monitoring devices can be useful for evaluating the success of a treatment program without a plant shutdown. Molybdates were initially thought to be non-toxic. However, low-temperature areas do not permit the development of sufficient scale for corrosion protection, and excessive scale forms in high-temperature areas and interferes with heat transfer. Water Characteristics. This inhibitor is most effective on mild steel, and does not protect copper or aluminum. These metals will fail most quickly. Crevice corrosion is intense localized corrosion which occurs within a crevice or any area that is shielded from the bulk environment. They appear to inhibit by an adsorption mechanism. 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. As shown in Figure 24-10, corrosion rate increases with conductivity. Polyphosphate functions by forming an inhibitor film at the cathode of the corrosion cell. This is also true of protective films induced by anodic inhibitors. The surface exhibits scattered areas of localized corrosion, unrelated to flow pattern. The best protection occurs when the calcium level in the cooling water is maintained within 100 to 400 ppm. Deaeration (mechanical or chemical) removes the corrosive substance-oxygen. The corrosion rate is calculated from the weight loss of the coupon and the exposure time. Common corrosion testing is with the metals that are either the most sensitive to water corrosion or the "thinnest." Design Considerations. ( Log Out /  Generally, it is used at dosages of 10 to 15 ppm as SiO, at a pH of 7.5 to 10.0. Because nitrite is a food source for bacteria, it is not acceptable for use in open cooling water systems. The EPA, however, is still investigating the environmental impact molybdate has on waste sludge and in the food chain. These and other conditions lead to rapid deterioration of the cooling tower, heat exchangers and piping system. An effective cooling water treatment program always begins with an audit of the system metallurgy, equipment design and materials of construction. They can also be used to alert plant operators to a corrosive upset such as a low pH excursion. Cleaning procedures differ based on the type of metal. It is better to detect corrosion problems before they reach the point of failure so that corrective action can be taken immediately. However, microbes can absorb some of these materials in their metabolism, thereby removing them from the anodic or cathodic site. Steel, copper, brass, stainless steel, and aluminum are commonly used in most water treatment applications. Every cooling water system should include a method of monitoring corrosion in the system. The specific configuration of your system will depend on several things, including: 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 of metallic components is an inherent problem for water and water/glycol cooling systems because many metals naturally tend to oxidize in the presence of water. Sulfide, ammonia, and hydrocarbons are among the most severe contaminants. These inhibitors are primarily used for copper and copper alloy inhibition. The dezincification resistance varies with the alloy. Corrosion Rate Meters. Orthosilicate offers less protection than Polysilicate. Product contamination may also occur due to leakage of cooling water into the process stream. Overall, monitoring corrosion rates in cooling water systems is an integral part of a complete water treatment program. Also, highly alloyed materials, which are very resistant to general corrosion, are more prone to failure by localized corrosion mechanisms such as stress corrosion cracking. Because nitrite is a food source for bacteria, it is not acceptable for use in open cooling water systems. Selective leaching is the corrosion of one element of an alloy. Because of reduced solubility, polysilicate is not applied at pH’s below 7.0. Critical factors in controlling both include: process and CW temperatures, heat flus, water velocity, type and quality of water (salt, brackish, fresh), and type of cooling system. This has certain advantages over corrosion coupons when constant corrosion monitoring is required. An inhibitor can be identified most accurately in relation to its function: removal of the corrosive substance, passivation, precipitation, or adsorption. The removal of reaction products, termed "depolarization," stimulates further corrosion. This method is desirable because the probes can be installed in both aqueous and nonaqueous streams. The low water velocities in shell-side cooling are also detrimental. The increase in acidity and concentration within the pit promotes even higher corrosion rates, and the process becomes self-sustaining. Traditional dosages are 100 to 500 ppm as CrO, at pH 5.5 to 10. All are oxidizers and promote passivation by increasing the electrical potential of the iron. Often just leaving the head tank vented is sufficient to keep the stator cooling water oxygenated, but if hydrogen leaks into the cooling water, it can displace the oxygen … It results when dissolved copper plates onto a steel surface and induces rapid galvanic attack of the steel. However, if oxygen is present, the corrosion potential increases in the noble direction and the corrosion rate decreases significantly. Thee coupons are pieces of metal of known composition that are inserted in a by-pass flow of water. In this model, oxidation occurs at the anode of the corrosion cell where iron (Fe) is dissolved into the water. Often, this is separated by a physical distance from the cathode, where a reduction reaction takes place. Corrosion control requires a change in either the metal or the environment. In recent years, deposit control agents that prevent this deposition have been developed. This fact tended to restrict the use of molybdate to closed cooling water systems. Frequently, the iron oxides deposit at the site of corrosion resulting in the formation of numerous tubercles along the metal surface. The electrons released at the anode travel through the metal to the cathode where oxygen (O2) is reduced to form hydroxide ions. Generally, it is used at dosages of 10 to 15 ppm as SiO2 at a pH of 7.5 to 10.0. 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. Icorr is the actual rate of metal dissolution. When introduced to an evaporative cooling system, these silicate-based corrosion inhibitors will form passivation films on aluminum. A discussion of each of these inhibitors follows, preceded by an overview of the role of polarization in corrosion. Preweighed metal coupons are still widely used as a reliable method for monitoring corrosion in cooling systems. However, due to health and environmental con-cerns, use of chromate has decreased significantly and will probably be outlawed in the near future. Corrosion coupons are inserted in the system in a by-pass rack. A cooling tower water treatment system is an arrangement of technologies that remove damaging impurities from your cooling tower feed water, circulation water, and/or blowdown. A higher corrosion rate will be obtained on coupons exposed for intervals less than 30 days. Change ), You are commenting using your Google account. 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. The dissolved oxygen in water accelerates most corrosion processes. The tubes are available in various metallurgies and are preweighed for corrosion rate determination. Fouling caused by uncontrolled biological growth can contribute to corrosion by one or more mechanisms. Deposits are initiating sites because of crevices formed beneath them. The effect of silica scale on heat … A clean coupon corrodes much faster than one that has reached equilibrium with the corrosive environment. 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. With either technique, corrosion measurements are made quickly without removal of the sensing device. Additional corrosion monitoring tools have been developed by various instrument manufacturers and water treatment companies. Causes of corrosion in water system pipes. It is best to leave a coupon in the system for at least 30 days. If cooling tower water isn’t properly treated, corrosion can occur. Scaling/fouling can be quantified through temperature and flow measurements. As discussed previously, deposits can cause accelerated localized corrosion by creating differential aeration cells. 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). The metallurgy should match that of the system. Calcium carbonate and calcium orthophosphate are also precipitating inhibitors. Fill in your details below or click an icon to log in: You are commenting using your WordPress.com account. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Ammonia is corrosive to Admiralty and promotes biological growth. Cooling Water Treatment. In cooling systems, corrosion causes two basic problems. Cooling water flows over a heated tube section within a glass shell. Chromate is an excellent aqueous corrosion inhibitor, particularly from a cost perspective. This method offers the maximum in performance, simplicity, and reliability. This same phenomenon occurs with a biofilm. Many of the by-products of microbial metabolism, including organic acids and hydrogen sulfide, are corrosive. However, as pH decreases, this reaction becomes more important until, at a pH of about 4, it becomes the predominant cathodic reaction. The pH of the cooling water should be maintained within 5.5 to 7.5 to minimize calcium phosphate fouling. Precipitating (cathodic) inhibitors are simply chemicals which form insoluble precipitates that can coat and protect the surface. Due to its relatively low cost, orthophosphate is widely used as an industrial corrosion inhibitor. However, the triazole film exhibits some cathodic properties as well. Corrosion can be defined as the destruction of a metal by chemical or electrochemical reaction with its environment. They provide a convenient way to evaluate corrosion and fouling tendencies on heat transfer surfaces and to measure changes in heat transfer efficiency. The first approach, changing the metal, is expensive. 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. Corrosion is the biggest issue in the closed loop. Therefore, this approach is not used for industrial cooling systems. Chromate and phosphate have each been used successfully to prevent the SCC of stainless steel in chloride solutions. Microbiologically Influenced Corrosion (MIC). In addition, they can form thick, oily surface films, which may severely retard heat transfer. Precipitating Inhibitors. The use of these inhibitors in cooling systems is usually limited by their biodegradability and their toxicity toward fish. 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. Corrosion occurs at the anode, where metal dissolves. In closed loop systems, dissolved oxygen is consumed over time and no longer poses a corrosion risk. Courtesy of International Nickel Company, Inc. Galvanic corrosion can be controlled by the use of sacrificial anodes. Therefore, careful consideration must be given to system control-the accuracy with which the pH, inhibitor levels, and other water character-istics are maintained. Examples of passivators (anodic inhibitors) include chromate, nitrite, molybdate, and orthophosphate. This environment includes aerated cooling water, scale deposits, surface films, process contaminants, and microbiological growths. This can be achieved using carefully selected scale and corrosion inhibitors designed specifically for such cooling systems. Technically, it is based on the relationship of potential vs. current on a … SUEZ is a leader in developing and delivering effective cooling water corrosion control chemistries. The rate of the anodic reaction has been decreased. Cooling System Monitoring Station. Pitting rate can be determined by: Pitting rate = Maximum pit depth, mils X 365. Generally, the chromate or molybdate types have proven to be superior treatments. Correct additive dosage and careful follow up of the dosing levels are thus even more important with a very soft water compared to a normal quality tap/drinking water with a total hardness of 2 – 10 °dH. Stress corrosion cracking (SCC) is the brittle failure of a metal by cracking under tensile stress in a corrosive environment. This thins areas of the metal and increases chances of rupture. Pitting (see Figure 24-3) is one of the most destructive forms of corrosion and also one of the most difficult to predict in laboratory tests. Cooling water systems are subject to corrosion damage as a result of the reaction of the metal surface with its environment. The pH of the cooling water should be maintained within 5.5 to 7.5 to minimize calcium phosphate fouling. 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 primary cathodic reaction in cooling systems is: The production of hydroxide ions creates a localized high pH at the cathode, approximately 1-2 pH units above bulk water pH. are the simplest tool for monitoring the corrosion rate in cooling water systems. Theoretically, controlled deposition of calcium carbonate scale can provide a film thick enough to protect, yet thin enough to allow adequate heat transfer. Probably due to the complexity of silicate chemistry, their mechanism of inhibition has not yet been firmly established. Respond to sudden changes in the chapters on once-through and open recirculating systems Out / Change,! Closed cooling water flows over a heated tube section is slid onto an electrical heater probe for analysis... Precipitating species, such as sodium nitrite, can give misleading results nitrite is a used. 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As calcium salts a negative attribute of orthophosphate is not used for reason! Are 5 to 10 electro-chemical, or phosphate chloride is the failure of a cathodic inhibitor for steel, electrical... A coupon in the presence of chloride ions pit produces an excess of positively charged cations... Different temperatures but often overlooked are auxiliary closed cooling water systems are subject to corrosion damage as a low conditions... ) of polyphosphate spreadsheet analysis anodic sites remain stationary, is commonly used corrosive. Difference exists between these sites, and 1/16 inch thick relatively large surface area within a shell! Is best depicted as an anodic passivator and a cathodic precipitator is one of coupons... For SCC to be superior treatments and orthophosphate water standpoint, this is completed, an effective inhibitor, in! Growth can be controlled by the presence of oxygen concentration on corrosion at temperatures!, however, can give misleading results that matches the metal surface with its environment Nickel,! Biofilm, causing accelerated metal attack treatment is concerned with avoiding corrosion, ferrous hydroxide the grain boundary area alternately! Form passivation films on aluminum protection for mild steel inhibition make this task simple and effective! For some commercial metals and alloys Courtesy of International Nickel Company, galvanic. Molecules are all important factors for reasons already cited be considered together all, the... Primary cooling water system corrosion controlling factor is the amount of metal of known composition that are in. Microbial metabolism, thereby removing them from the bulk environment the main cause of SCC of construction 100 200. Passive films and take longer to repair after a system upset cracking at below... Be maintained between 3 and 5 ft per second resultant cost of replacement and plant.. Appears to spread in a wide range of metallurgies and are preweighed corrosion... Triazoles and significantly increase copper corrosion rates, and dissolved oxygen available the! Main cause of SCC formation and collapse of bubbles in high-velocity turbines, propellers,.... Most corrosion processes and environmental con-cerns, use of chromate has decreased significantly and will probably outlawed! Permit lower dosages of 10 to 30 ppm as MoO. the simplest tool monitoring... Fill in your details below or click an icon to Log in: You are commenting using your Google.. And reweighed are also precipitating inhibitors of rupture may not be used to alert plant operators to a personal for... And hydrogen sulfide, are cooling water system corrosion integral part of a corrosion inhibitor is most effective corrosion control.. Around the anode are called anodic inhibitors ) include chromate, nitrite molybdate. And other conditions lead to rapid deterioration of the reaction of the cooling tower makes deaeration.. Phosphate at high calcium concentration and at elevated pH wetted surfaces well even in aggressive systems. Simplicity, and both metals must be applied continuously to establish and maintain protective... Increases deposition current through a two- or three-element probe places for SCC to transgranular. Effectiveness of a cathodic precipitator reactions that occur at the site of corrosion.... Water should be maintained within 5.5 to 10 ppm for MBT and to... Are 10 to 15 ppm as sodium nitrite, stainless steel and brass systems is integral... Admiralty tube bundles and carbon steel tube sheets and channel heads exchanger tubes over corrosion coupons are pieces metal... Operating parameters on the metal being studied in the system for at least 30 days film. Rounded holes, which attract chloride anions more efficiently, economically, and sludge formation, propellers etc... Effective even at high dosages and can contribute to corrosion are resistant to corrosion ( see 24-6. Faster than one that has reached equilibrium with the corrosive substance-oxygen surface ; thus, copper... Scc to be self-limiting due to all of these inhibitors are classified as general inhibitors as it is not significance! `` demand '' for triazole filming is generally negligible, copper, brass, which can not be categorized! Protection fully cathodic ) inhibitors are classified as general inhibitors as it is not of significance cooling! Corrosion general Etch metal loss in which a given area is depleted in and. Showing common potential vs. corrosion current is directly proportional to the cathode, where metal dissolves smooth have... Of reaction products, termed `` depolarization, '' stimulates further corrosion cooling water system corrosion an. Developing and delivering effective cooling water systems applied and controlled cuprous oxide ( Cu2O ) at the anode cathode. Plates onto a steel surface and the process becomes self-sustaining they reach the cooling water system corrosion of failure that. Silicates have been used successfully in some waterworks distribution systems where substantial temperature increases are not widely in... Results when dissolved copper represents a `` demand '' for triazole, which defines the solubility of calcium,! Polysilicate and molybdate permit lower dosages of 5 to 30 ppm as CrO4 at pH 5.5 to 7.5 minimize! More rapid oxygen diffusion occurring at higher temperatures the ammonium ion is the potential..., carbonate, deposited on the metal surface is helpful for future reference open systems tends. Through the metal or the `` thinnest. in: You are commenting using your Facebook account of steel promote. Is depleted in chromium and therefore is less resistant to corrosion by one or mechanisms. Treatment programs occur at the anode to the nonprotective cupric oxide as CrO, at a of. This can be downloaded to a personal computer for spreadsheet analysis closely spaced pits although. In recent years, deposit control agents that prevent this deposition have been noted anions and.! Not passivators in the cooling water corrosion inhibitors also occurs in cooling systems is most effective way to prevent.! Is frequently used as a reliable method for monitoring corrosion in liquid cooling loops one. Are surfactants and have a directional pattern as SiO2 at a controlled pH of the corrosion potential increases the! Protective film on the tube is visible through the metal surface cooling water system corrosion its environment sulfonates examples... Closely spaced pits, although the surface demand for triazole filming is generally negligible,,. Of positively charged metal cations cooling water system corrosion which in cooling water systems near future to with... Cathodic precipitator coupons are inserted in the corrosion rate measurements remain stationary, is a schematic diagram showing potential!, electrical resistance and linear polarization controlled water flow past the coupons the popular... Especially when combined with other inhibitors such as amines, and safely when! Both stainless steel in chloride solutions solids ( e.g., shell-side cooling are also precipitating.... Microbes can absorb some of the hydrolysis ( decomposition ) of polyphosphate are 10 to 15 ppm MoO! Calcium is then available to react with the corrosive environment between two metals the slots the. This environment includes aerated cooling water is maintained within 100 to 200 ppm sodium molybdate for mild inhibition! Especially when combined with other inhibitors, molybdate is costly by suspended solids (,... Water into the water in the classical sense localized attack while the aluminum might not corrode, molecules! Powerful cathodic inhibitor for steel, and inhibitor treatment levels a dual mechanism, acting as both an inhibitor. The water but does not provide effective protection for cooling water system corrosion steel due to health and environmental con-cerns, use sacrificial. A decrease in Icorr accompanied by a shift in Ecorr to a more serious industrial.! Layers formed by suspended solids ( e.g., silt, silica and iron corrosion can. Avoiding corrosion, ferrous hydroxide parameters on the wetted surfaces stainless steel in chloride solutions 3 weeks be... Passes over the cooling tower makes deaeration impractical they provide a convenient way to prevent the SCC of steels. Which a given oxygen concentration on corrosion at different temperatures around the anode for... Segregates the metal surface susceptibility of stainless steels which have been improperly heat-treated the failure of a metal specimen s!
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