The alkaline ammonia does not attack steel. Hot condensate return as it contains less O2 then feed water and also saves fuel. If a pit contains red iron oxide, it is no longer active. Two types of corrosion can occur with oxygen present. Hydrazine. Analytical results and procedures must be evaluated to ensure that quality standards are maintained and that steps are taken for continual improvement. Where hot process softeners are followed by hot zeolite units, an additional feed is recommended at the filter effluent of the hot process units (prior to the zeolite softeners) to protect the ion exchange resin and softener shells. It is therefore desirable to provide alternate boiler water treatment chemicals which are generally free of the dangers inherent in the use of hydrazine, but which effectively scavenge oxygen and passivate steel surfaces under typical boiler conditions. Presence of corrosive gases such as Oxygen (O2), Carbon Dioxide (CO2), Hydrogen Sulphide (H2S) in the boiler water Without proper mechanical and chemical deaeration, oxygen in the feedwater will enter the boiler. It is also necessary to test for iron and copper oxides in order to assess the effectiveness of the treatment program. An example of this is the injection of hydrazine to the exhaust of a turbine to protect the condenser. Pitting of boiler tube banks has been encountered due to metallic copper deposits. Dissolved oxygen will react with carbon steel throughout the feedwater piping, economizers, and boiler, causing oxygen pitting and eventually leaks. Both copper and iron are oxidized in the presence of hydrogen ions and oxygen and can undergo oxygen pitting. Stress cracking of welded components can also be a problem. Different levels of treatment are required, depending on the particular blend in use. Both iron and the copper are removed from the boiler, and the boiler surfaces can then be passivated. Figure 11-13 shows rate of reaction as a function of temperature and hydrazine concentration. However, if enough ammonia and oxygen are present together, copper alloy corrosion increases. more economically efficient to maintain clean heat transfer surfaces and eliminate the use of cold water containing dissolved oxygen during cool-down and start-up periods. Is oxygen responsible for boiler corrosion? CORROSION CONTROL FACTORS Steel and Steel Alloys. High-pressure alkaline water is an effective means of cleaning the fireside areas. The pH or alkalinity that is maintained depends on the pressure, makeup water characteristics, chemical treatment, and other factors specific to the system. The decomposition products of hydrazine are ammonia and nitrogen. In fact, further oxidation and thermal degradation (in higher-pressure systems) leads to the final product of carbon dioxide. Water acts as a cathode of any corrosion cell to depolarize, thereby sustaining the corrosion process. Which type of corrosion occur in boiler? Generally, higher temperatures, high or low pH levels, and higher oxygen concentrations increase steel corrosion rates. What are boiler corrosion causes ? Fireside deposits, particularly in the convection, economizer, and air heater sections, are hygroscopic in nature. Both of these layers continue to grow due to water diffusion (through the porous outer layer) and lattice diffusion (through the inner layer). Operational problems such as water/steam hammer can also be a factor. 12th grade . Corrosion products generated both in the preboiler section and the boiler may deposit on critical heat transfer surfaces of the boiler during operation and increase the potential for localized corrosion or overheating. The ASME Consensus for Industrial Boilers (see Chapter 13) specifies maximum levels of contaminants for corrosion and deposition control in boiler systems. Steam blanketing is a condition that occurs when a steam layer forms between the boiler water and the tube wall. Resistance boilers generate heat by means of a coiled heating element. Advances have been made in formulating filming amine treatments. In general, whenever water and metals are in contact with each other in the presence of air and dissolved salts, corrosion will usually occur. Practically all ground surface supplies of water contain dissolved air in quantities depending on its source, time of exposure and its temperature. Pits are active only in the presence of oxygen. This is one reaction can be represented as an electro- of the reasons that alkaline conditions are chemical reaction, as shown in Figure 2.17.1. preferable for boilers. Table 19-1. Protection of steel in a boiler system depends on temperature, pH, and oxygen content. It is estimated that problems due to boiler system corrosion cost industry billions per year. Many corrosion problems occur in the hottest areas of the boiler-the water wall, screen, and superheater tubes. Under such conditions, an insulating superheated steam film develops. The degree of protection can be monitored by various means. As the oxide layer becomes thicker, the outer layers begin to slough off as particles of copper oxide. The choice of storage methods depends on the length of downtime expected and the boiler complexity. Economizers can be classified as nonsteaming or steaming. Feedwater heaters are generally classified as low-pressure (ahead of the deaerator), high-pressure (after the deaerator), or deaerating heaters. Off-line boiler corrosion is usually caused by oxygen in-leakage. If the superheater is drainable or if the boiler does not have a superheater, the boiler is allowed to cool slightly after firing. Superheater corrosion problems are caused by a number of mechanical and chemical conditions. Dissolved oxygen replaces hydrogen ions in the reduction reaction. Oxygen corrosion may be highly localized or may cover an extensive area. Active oxygen pits are distinguished by a reddish brown oxide cap (tubercle). The disposal of lay-up chemicals must be in compliance with applicable federal, state, and local regulations. The most common causes of corrosion are dissolved gases (primarily oxygen and carbon dioxide), under-deposit attack, low pH, and attack of areas weakened by mechanical stress, leading to stress and fatigue cracking. By complexing and dissolving iron and copper oxides, contaminants such as chloride, sulfide, acetate, and ammonia (for copper) can dissolve part or all of the oxide layer. Water also combines with any dissolved carbon dioxide to form carbonic acid that causes further corrosion. When dissolved oxygen enters the steam boiler corrosion manifests itself in the form of severe deep pits almost exclusively at the water level . Finally, as chromium and molybdenum increase, wear rate decreases. For effective yet simple boiler storage, clean, warm, continuous blowdown can be distributed into a convenient bottom connection on an idle boiler. Anything that results in a difference in electrical potential at discrete surface locations can cause a galvanic reaction. Distribution ratios are not true physical constants but are a function of pressure (Figure 19-6) and pH. Sample lines, continuously flowing at the proper velocity and volume, are required. 5 lb of a 35% solution of hydrazine and 0.1 lb of ammonia or 2-3 lb of a 40% solution of neutralizing amine can be added per 1000 gal (minimum 200 ppm hydrazine and 10.0 pH). Electric boilers are also used for heating. The stability of the passivating iron or copper oxide layer is critically dependent on condensate pH. Other contaminants in the condensate system can affect corrosion rates of iron and copper even when the pH is correctly maintained. All steel systems can use the same chemical concentrations recommended for wet storage. At 100 C the oxygen content is theoretically 0 ppm. Iron corrodes in water even in the absence of oxygen. The neutralizing capacity of an amine is inversely proportional to molecular weight (i.e., lower molecular weight yields higher neutralizing capacity) and directly proportional to the number of amine groups. reduces its oxygen content.6. Sodium sulfite, although an effective scavenger, is not recommended for use in systems operating above 1,000 psi because breakdown occurs to form corrosive hydrogen sulfide and sulfur dioxide. A microscopic examination of a properly prepared section of embrittled metal shows a characteristic pattern, with cracking progressing along defined paths or grain boundaries in the crystal structure of the metal (see Figure 11-6). Testing should be performed at least once per shift. In the boiler, either high or low pH increases the corrosion rates of mild steel(see Figure 11-10). Due to the addition of make-up/feed-water that is not deaerated (pre-treated mechanically to remove dissolved oxygen), boilers require a chemical additive to tie up or neutralize free oxygen molecules. Generally, these two basic heating systems are treated as closed systems, because makeup requirements are usually very low. Because some amine is returned with the condensate, the total amine in the system is greater than the amount being fed. Due to the high heat transfer rate at the resistance coil, a treatment that precipitates hardness should not be used. If the boiler is equipped with a nondrainable superheater, the superheater is filled with high-quality condensate or demineralized water and treated with a volatile oxygen scavenger and pH control agent. Even small concentrations can cause serious problems. Because chemical attack of the metal is normally undetectable, failure occurs suddenly-often with catastrophic results. Even low concentrations of DO can be highly destructive, causing localized corrosion and pitting of metal system components. Chemical oxygen scavengers should be fed to allow ample time for the scavenger/oxygen reaction to occur. Boiler corrosion is degradation of boiler and it's metal by electrochemical reaction or by pitting action of dissolved oxygen in boiler water. The resulting corrosion leads to deposits on boiler heat transfer surfaces and reduces efficiency and reliability. Boilers that use undeaerated water during start-up and during their removal from service can be severely damaged. Corrosion fatigue cracking occurs in deaerators near the welds and heat-affected zones. Whenever possible, tubes in this area should be inspected closely for evidence of corrosion. Dissolved oxygen is extremely reactive and is critical to monitor and control within steam-generating systems. CORROSION OF COPPER Economizer heat transfer surfaces are subject to corrosion product buildup and deposition of incoming metal oxides. Call Us: (217) 223-2017 530 S. 5th Street, Quincy, IL 62301 . Increasing blowdown lowers both phosphate and pH. The establishment of protective metal oxide lay-ers through the use of reducing agents (such as hydrazine, hydroquinone, and other oxygen scavengers) is known as metal passivation or metal conditioning. Effect of pH on corrosion of iron and copper. 1. Some have stainless steel superheater elements. If your boiler has a porous scale, then under deposit corrosion is also possible. Dissolved oxygen 2. . However, as long as a high caustic concentration exists, the magnetite is constantly dissolved, causing a loss of base metal and eventual failure (see Figure 11-2). Erosion usually occurs due to excessive velocities. As with any oxygen scavenging reaction, many factors affect the speed of the sulfite-oxygen reaction. All manholes, handholes, vents, and connections are blanked and tightly closed. water. In this reaction a temperature rise provides enough additional . The raw water used in the low pressure boiler feed water generally has an oxygen content exceeding the water quality standards. Application experience has shown that combination amines (filming and neutralizing amines with dispersant aids) provide a superior film bond, reduce deposit problems, and provide better system coverage and thus provide more complete and economical corrosion protection (Figure 19-8). Control of dissolved oxygen can be used to mitigate SAC in industrial boilers. Learn more about Veolia's boiler water treatment and how it can help avoid boiler system corrosion. For dry storage, the boiler is drained, cleaned, and dried completely. To elevate pH, neutralizing amines should be used instead of ammonia, which (especially in the presence of oxygen) accelerates copper alloy corrosion rates. The use of long sample lines should be avoided. Finally, gas solubility decreases as pressure decreases. Erosion at tube bends is also a problem in steaming economizers. It is also used widely in condensate systems. The behavior of amine bicarbonate in the deaerator affects amine requirements for the system. Experience has indicated that with the improved cleanliness of internal boiler surfaces, more attention must be given to protection from oxygen attack during storage. Oxygen is highly corrosive when present in hot water. In chemical terms, it is a measure of an amine's ability to hydrolyze in pure water. The normal method of filling nondrainable superheaters is by back-filling and discharging into the boiler. In any event, pitting would not occur in this type of boiler if no oxygen were present in the water. Due to the temperature increase across the heater, incoming metal oxides are deposited in the heater and then released during changes in steam load and chemical balances. The effective application of oxygen scavengers indirectly leads to passivated metal surfaces and less metal oxide transport to the boiler whether or not the scavenger reacts directly with the metal surface. Chemical de-oxygenation by use of oxygen scavengers i.e. 1-2, Degremont, 1991 Industrial water conditioning, BeltsDearborn, 1991 http://www.thermidaire.on.ca/boiler-feed.html, Distributieweg 3 2645 EG Delfgauw The Netherlands Phone: +31 152 610 900 fax: +31 152 616 289 e-mail: info@lenntech.com, 5975 Sunset Drive South Miami, FL 33143 USA Phone: +1 877 453 8095 e-mail: info@lenntech.com, Level 6 - OFFICE #101-One JLT Tower Jumeirah Lake Towers Dubai - U.A.E. Major sources of oxygen in an operating system include poor deaerator operation, in-leakage of air on the suction side of pumps, the breathing action of receiving tanks, and leakage of undeaerated water used for pump seals. 143 McDonough Parkway, McDonough, GA 30253 eatexcangerusacom 800) 80-3776 pg.3 Leaking of rivets and joint areas. Dissolved oxygen can destroy the protective hydrogen film that can form of many metals and oxidize dissolved ions into insoluble forms. Auxiliary heat, light firing of the boiler, cascade lay-up, or dry storage may be employed to prevent freezing problems. A truly passive oxide film does not form on copper or its alloys. Scale Formation Scale formation or deposits in the boilers results from hardness contamination of feed water. In most cases, proper feedwater deaeration and elimination of air infiltration into the condensate substantially reduce oxygen corrosion. Its dissolved oxygen attacks systems between the point of condensation and the deaerating heater. Second, most oxygen scavenger/passivators react more rapidly at the mildly alkaline conditions maintained by the amine than at lower pH levels. One major problem is the oxidation of superheater metal due to high gas temperatures, usually occurring during transition periods, such as start-up and shutdown. The dissociation constant Kb is a common measure of basicity. Deposits due to carryover can contribute to the problem. Sometimes, ammonia is fed to control condensate pH. The inhibition of embrittlement requires a definite ratio of nitrate to the caustic alkalinity present in the boiler water. For caustic embrittlement to occur, three conditions must exist: Fatigue cracking (due to repeated cyclic stress) can lead to metal failure. Steam side burning is a chemical reaction between steam and the tube metal. Usually, the most suitable point of application is the drop leg between the deaerator and the storage compartment. Because these oxides are formed under reducing conditions, removal of the dissolved oxygen from boiler feedwater and condensate promotes their formation. Deaerator performance has to be maintained limiting dissolved oxygen to less than 0.01 ppm level. Neutralizing amines are used to neutralize the acid (H+) generated by the dissolution of carbon dioxide or other acidic process contaminants in the condensate. After 25 seconds of contact, catalyzed sodium sulfite removed the oxygen completely. Boiler corrosion is a relevant problem that is majorly caused by the presence of oxygen and carbon dioxide dissolved in the feed water, and also due to low PH level of water. top of the tube and accelerating the corrosion, much as did the experiment of the flowing stream. The following chemical reaction results: This can take place as a separate step or during acid cleaning. Boiler feedwater hardness, iron, copper, oxygen, and pH should be measured. The major limiting factors of hydrazine use are its slow reaction time (particularly at low temperatures), ammonia formation, effects on copper-bearing alloys, and handling problems. Whenever, air enters inside the boiler (during maintenance, or when boiler is not operating, the air enters inside the boiler and the oxygen in the air reacts wit. Boiler corrosion is an electro-chemical reaction and usually causes pinpoint pitting on a sound boiler metal. When contaminants are present in the condensate, filming amines have a tendency to form deposits by reacting with multivalent ions, such as sulfate, hardness, and iron. High-purity makeup water is required. Oxygen pitting begins at weak points in the iron oxide film or at sites where the oxide film is damaged. This choice must be tailored to the condensate system and the process contaminants. Sodium nitrate is a standard treatment for inhibiting embrittlement in lower-pressure boiler systems. These are frequently termed "migratory" oxides, because they are not usually generated in the boiler. Relative neutralizing capacities. Water usually contains about 8 ppm of dissolved oxygen at room temp. This layer is very porous and easily penetrated by water and ionic species. Soluble copper ions and particulate copper oxides are also formed by the normal oxidation processes. Membrane contractors are also a possibility. The time required varies greatly with different amines. Even this small amount of oxygen is corrosive at boiler system temperatures and pressures. Results are described in the final technical report. Answer (1 of 2): Boilers are made from steel plates and they are continuously are in contact with the water which acts as electrolyte. The oxide layers formed by copper and its alloys are porous and "leaky," allowing water, oxygen, and copper ions to move to and from the metal surface (Figure 19-2). Therefore, corrosion rates are accelerated in the presence of oxygen. High-temperature hot water boilers operate at pressures of up to 500 psig, although the usual range is 35-350 psig. Hydrazine and sulfite have been widely used for this purpose, but they have some shortcomings. The electrochemical corrosion reaction for iron boiler metal surfaces is generally. Hydrazine control is usually based on a feedwater excess of 0.05-0.1 ppm. The problem usually occurs only in units operating at or above 1,500 psi. At even higher temperatures (above 300F), Fe2+ spontaneously forms magnetite without first forming Fe(OH)2. It is the primary corrosive agent of steam-generating systems. The major problem in low-pressure heating systems is corrosion caused by dissolved oxygen and low pH. In Figure 19-5, the recycle factor is the concentration of amine at point x divided by the amine feed rate at point z. Dissolved oxygen can cause serious corrosion damage to the carbon steel metallurgy used for boiler construction and is the primary cause of corrosion in out-of-service steam boilers. Because the material is a suspected carcinogen, federally published guidelines must be followed for handling and reporting. The best pH for protection of copper alloys is somewhat lower than the optimum level for carbon steel. Elimination of dissolved oxygen is usually accomplished by a combination of deaeration equipment and chemical treatment with oxygen scavengers. 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