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. If a pit contains red iron oxide, it is no longer active. The boiler metal gets eaten up quickly due to the action of dissolved oxygen leading to complete failure of the boiler system. The corrosion distilled water and after adding three types of oxygen scavengers Hydroquinone, Ascorbic acid and Monoethanolamine in different concentrations 40,60 and 80 ppm has been. During the period of initial film formation, old, loosely adherent corrosion products are lifted off the metal surface due to the surfactant properties of the amine. The water that does reach the overheated boiler wall is rapidly vaporized, leaving behind a concentrated caustic solution, which is corrosive. Low makeup or feedwater pH can cause serious acid attack on metal surfaces in the preboiler and boiler system. The most effective catalysts are the heavy metal cations with valences of two or more. Corrosion can be of widely varying origin and nature due to the action of dissolved oxygen, to corrosion currents set up as a result of heterogeneities on metal surfaces, or to the iron being directly attacked by the water. High PH value of boiler water4. Choosing The Best Oxygen Scavenger for Your Boiler System CALL (414) 425-3339 Choosing the Best Oxygen Scavenger for Your Boiler System Favorite this: Which Oxygen Scavenger should be used in your boiler system? Neutralizing capacity is the concentration of acidic contaminants that is neutralized by a given concentration of amine. Share. Sufficient treatment must be fed to water added to make up for system losses, which usually occur as a result of circulating pump leakage. Scale Formation Scale formation or deposits in the boilers results from hardness contamination of feed water. As the oxide layer becomes thicker, the outer layers begin to slough off as particles of copper oxide. Overall: Fe + O 2 + H 2 O Fe (OH) 2. Chemical treatment consists of sodium sulfite (to scavenge the oxygen), pH adjustment, and a synthetic polymer dispersant to control possible iron deposition. Hydrazine has been used for years as an oxygen scavenger in high-pressure systems and other systems in which sulfite materials cannot be used. Increasing blowdown lowers both phosphate and pH. The following chemical reaction results: This can take place as a separate step or during acid cleaning. Many have copper alloy and/or stainless steel feedwater heaters and condensers. The alkaline ammonia does not attack steel. As an alternative, metal surfaces can be sprayed or wiped with a light oil. Two types of corrosion can occur with oxygen present. In a boiler feedwater system, this could result in severe corrosive attack. Hydrogen permeates the tube metal, where it can react with iron carbides to form methane gas, or with other hydrogen atoms to form hydrogen gas. The second step is to form a protective layer on the inside surface of the tubes which protects the metal surface from any further corrosion attacks. Regardless of feedwater heater design, the major problems are similar for all types. Condensate systems can be chemically treated to reduce metal corrosion. Boiler corrosion is an electro-chemical reaction and usually causes pinpoint pitting on a sound boiler metal. Metallic or elemental copper in boiler deposits is dissolved in the hydrochloric acid solution by the following reaction: Once cuprous chloride is in solution, it is immediately redeposited as metallic copper on the steel surface according to the following reaction: Thus, hydrochloric acid cleaning can cause galvanic corrosion unless the copper is prevented from plating on the steel surface. Dissolved oxygen is extremely reactive and is critical to monitor and control within steam-generating systems. Auxiliary heat, light firing of the boiler, cascade lay-up, or dry storage may be employed to prevent freezing problems. This magnetite forms a nonporous, tightly adherent layer on the metal surface. The distribution ratio of an amine is: Amines with a DR greater than 1.0 produce a higher concentration of amine in the vapor phase than in the liquid phase. Dissolved oxygen is given, and temperature at various alloy levels for pH=9.05. Analytical results and procedures must be evaluated to ensure that quality standards are maintained and that steps are taken for continual improvement. Ammonia is the most common contaminant and is usually present in low concentrations. Without proper mechanical and chemical deaeration, oxygen in the feedwater will enter the boiler. Ammonia contamination is usually caused by the breakdown of nitrogenous organic contaminants, hydrazine, or amine treatment chemicals. Types of Boiler Corrosion. At a certain thickness of the outer layer, an equilibrium exists at which the oxide continually forms and is released into the water. High-temperature hot water boilers operate at pressures of up to 500 psig, although the usual range is 35-350 psig. As the water is heated; the dissolved oxygen is set free and the boiler starts corroding. Practically all ground surface supplies of water contain dissolved air in quantities depending on its source, time of exposure and its temperature. If levels are left unchecked, DO will destroy a boiler system. This thinning often leads to failures, especially at threaded sections (Figure 19-3). The acceptable dissolved oxygen level for any system depends on many factors, such as feedwater temperature, pH, flow rate, dissolved solids content, and the metallurgy and physical condition of the system. In general, the limit value of oxygen in make up water can be stared 0.10 mg/kg. For a complete protection from oxygen corrosion, a chemical scavenger is required following mechanical deaeration. Boiler feedwater heaters are designed to improve boiler efficiency by extracting heat from streams such as boiler water blowdown and turbine extraction or excess exhaust steam. The deaerator storage system and the feedwater storage tank are commonly used feed points. In modern boiler systems, dissolved oxygen is handled by first mechanically removing most of the dissolved oxygen and then chemically scavenging the remainder. 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. Steam heating boilers are used to generate steam at low pressures, such as 15 psig. The ultimate breakdown product is CO2. All steel systems can use the same chemical concentrations recommended for wet storage. Is oxygen responsible for boiler corrosion? Corrosion fatigue is influenced by boiler design, water chemistry, boiler water oxygen content, and boiler operation. Heat is applied to optimize drying. 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. Reduced boiler life and possible chances of failure of the entire system. All monitoring data, whether spot sampling or continuous, should be recorded. BOILER CORROSION Although oxygen corrosion is a complicated inversely related; as the pH increases, the process, the classical, lower temperature oxygen becomes less corrosive. Removal of this cap exposes black iron oxide within the pit (see Figure 11-5). In a steam boiler system, the primary routes for corrosion are due to the presence of oxygen in the feed, boiler, and condensate waters, with the risk of acidic type corrosion due to reduced pH in the condensate water. Other scavengers, such as hydroquinone, are relatively volatile and can be fed well upstream of a problem area. CALL (414) 425-3339 Boiler Oxygen Corrosion Favorite this: There are few problems associated with steam generation equipment that are seen more often than oxygen corrosion. Active oxygen pits are distinguished by a reddish brown oxide cap (tubercle). . Pipe walls are thinned, particularly in the bottom of the pipe. When chemicals are added, they should be mixed by one of the following methods: If the steaming method is used, the boiler should subsequently be filled completely, in keeping with the above recommendations. The initial surface corrosion that produces hydrogen usually occurs beneath a hard, dense scale. Even low concentrations of DO can be highly destructive, causing localized corrosion and pitting of metal system components. In general, whenever water and metals are in contact with each other in the presence of air and dissolved salts, corrosion will usually occur. Find information about the main problems occurring in boilers. Distribution ratios are not true physical constants but are a function of pressure (Figure 19-6) and pH. Deposits due to carryover can contribute to the problem. In any event, pitting would not occur in this type of boiler if no oxygen were present in the water. It is important to aim to achieve a boiler feedwater temperature in the range 85 to 95oC to reduce the available dissolved oxygen to around . Table 19-1 provides a measure of the neutralizing capacity of commonly employed amines. . The oxides form an outer layer over the metal surface. Even small concentrations can cause serious problems. Iron, for example, reverts to iron oxide as the result of corrosion. The inhibition of embrittlement requires a definite ratio of nitrate to the caustic alkalinity present in the boiler water. The first, generalized corrosion on the metal surface, causes a loss of metal from the entire surface. (0.2-0.7 mil) thick magnetite formed by the reaction of iron and water in an oxygen-free environment. As temperature increases, reaction time decreases; in general, every 18F increase in temperature doubles reaction speed. It is caused by excessive heat input or poor circulation, resulting in insufficient flow to cool the tubes. Among the most commonly used compounds are hydroquinone and ascorbate. Water acts as a cathode of any corrosion cell to depolarize, thereby sustaining the corrosion process. Dissolved oxygen in the feedwater will attack any of the pre-boiler systems - feed line, feedwater heaters, and economiser. What are boiler corrosion causes ? Actual behavior involves some loss of amine additive and some recirculation of carbon dioxide. The amounts of lost amine and retained carbon dioxide are a function of the amine bicarbonate stability in the deaerator. These factors include temperature, pH, initial concentration of oxygen scavenger, initial concentration of dissolved oxygen, and catalytic or inhibiting effects. Oxygen corrosion may be highly localized or may cover an extensive area. These gases evolve predominantly along grain boundaries of the metal. A truly passive oxide film does not form on copper or its alloys. Higher numbers on the scale indicate that you might want to obtain boiler water treatment due to high conductivity. Corrosion is a relevant problem caused by water in boilers. Such deposits may form during acid cleaning procedures if the procedures do not completely compensate for the amount of copper oxides in the deposits or if a copper removal step is not included. In boilers, oxygen is introduced through the raw water supply. Both of these gases can cause corrosion in the return condensate system and have been reported to contribute to stress corrosion cracking in turbines. In the boiler, either high or low pH increases the corrosion rates of mild steel(see Figure 11-10). Sulphite (Na2SO3) or . The condensate receiving tank can be designed with a cover to reduce air contact and a steam heating coil within the tank to elevate condensate temperature and thereby reduce oxygen solubility. The oxygen dissolved in the feed water is the main cause of boiler corrosion. Hydrazine. As in the case of neutralizing amines, various methods are used to monitor the effectiveness of the treatment, including corrosion test specimens installed in bypass coupon racks (Figure 19-9), iron analyses, corrosion rate meters, and submicron corrosion product filtration. Oxygen attack is an electrochemical process that can be described by the following reactions: Anode: The influence of temperature is particularly important in feedwater heaters and economizers. Water flows into the deposit and heat applied to the tube causes the water to evaporate, leaving a very concentrated solution. Resulting failures usually occur in the bottom loops-the hottest areas of the superheater tubes. For most low- and medium-pressure systems, sulfite residuals should be in excess of 20 ppm. Maintaining Water Quality in High-Pressure Boilers Dissolved oxygen, or DO, is one of the most important water quality parameters to control in a boiler system. This is commonly caused by boiler water pH is too high, steam blanketing (poor circulation) or local 'film boiling'. 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. water quality and process applications. For dry storage, the boiler is drained, cleaned, and dried completely. Boiler water pH should be a part of your logbook. Caustic corrosion (gouging) occurs when caustic is concentrated and dissolves the protective magnetite (Fe3O4 ) layer. As the value of Kb increases, more OH is formed (after all of the acid has been neutralized) and pH increases. Effect of pH on corrosion of iron and copper. Carbon dioxide (CO2) is the primary cause of decreased condensate pH. Maintenance of the proper pH, elimination of oxygen, and application of metal-conditioning agents can minimize the amount of copper alloy corrosion. The use of long sample lines should be avoided. This choice must be tailored to the condensate system and the process contaminants. Distribution ratios are not true physical constants but are a function of pressure (Figure 19-6) and pH. Concentration of caustic (NaOH) can occur either as a result of steam blanketing (which allows salts to concentrate on boiler metal surfaces) or by localized boiling beneath porous deposits on tube surfaces. Causes: Dissolved oxygen in boiler water. Oxygen corrosion may be highly localized or may cover an extensive area. Due to the high heat transfer rate at the resistance coil, a treatment that precipitates hardness should not be used. The amount of chemical fed should be recorded and compared with oxygen levels in the feedwater to provide a check on the control of dissolved oxygen in the system. Stress cracking of welded components can also be a problem. Control of pH is important for the following reasons: The pH or alkalinity level maintained in a boiler system depends on many factors, such as sys-tem pressure, system metals, feedwater quality, and type of chemical treatment applied. 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. For wet storage, the unit is inspected, cleaned if necessary, and filled to the normal water level with deaerated feedwater. Control of dissolved oxygen can be used to mitigate SAC in industrial boilers. In other components, cracks follow the lines of greatest stress. Proper operation, close monitoring, and detailed out-of-service inspections (in accordance with published recommendations) minimize problems in deaerators. These conditions may be controlled through the following procedures: maintenance of proper pH and alkalinity levels Corrosion in boilers is due to the following reasons (1) Dissolved oxygen : This is the most usual corrosion causing factor. Leaking of rivets and joint areas. In high-purity systems, raw water in-leakage (e.g., condenser leakage) lowers boiler water pH when magnesium hydroxide precipitates, resulting in corrosion, formation of atomic hydrogen, and initiation of hydrogen attack. Emulsifiers and, in some cases, small amounts of neutralizing amines can be added to improve film distribution by providing more uniform coverage. CORROSION OF COPPER The outer layer is thick, adherent, porous and comprised mainly of cuprous oxide (Cu2O). In boilers operating below 1,000 psig, sodium sulfite and a concentrated liquid solution of catalyzed sodium bisulfite are the most commonly used materials for chemical deaeration due to low cost and ease of handling and testing. 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