Generally, these two basic heating systems are treated as closed systems, because makeup requirements are usually very low. High-temperature hot water boilers operate at pressures of up to psig, although the usual range is psig. System pressure must be maintained above the saturation pressure of the heated water to maintain a liquid state.
The most common way to do this is to pressurize the system with nitrogen. Normally, the makeup is of good quality e. Chemical treatment consists of sodium sulfite to scavenge the oxygen , pH adjustment, and a synthetic polymer dispersant to control possible iron deposition.
The major problem in low-pressure heating systems is corrosion caused by dissolved oxygen and low pH. These systems are usually treated with an inhibitor such as molybdate or nitrite or with an oxygen scavenger such as sodium sulfite , along with a synthetic polymer for deposit control. Sufficient treatment must be fed to water added to make up for system losses, which usually occur as a result of circulating pump leakage.
Generally, ppm P-alkalinity is maintained in the water for effective control of pH. Inhibitor requirements vary depending on the system. Electric boilers are also used for heating.
There are two basic types of electric boilers: resistance and electrode. Resistance boilers generate heat by means of a coiled heating element. High-quality makeup water is necessary, and sodium sulfite is usually added to remove all traces of dissolved oxygen. Synthetic polymers have been used for deposit control. Due to the high heat transfer rate at the resistance coil, a treatment that precipitates hardness should not be used.
Electrode boilers operate at high or low voltage and may employ submerged or water-jet electrodes. High-purity makeup water is required. Depending on the type of system, sodium sulfite is normally used for oxygen control and pH adjustment. Some systems are designed with copper alloys, so chemical addition must be of the correct type, and pH control must be in the range suitable for copper protection.
Corrosion control techniques vary according to the type of corrosion encountered. Major methods of corrosion control include maintenance of the proper pH, control of oxygen, control of deposits, and reduction of stresses through design and operational practices.
Galvanic corrosion occurs when a metal or alloy is electrically coupled to a different metal or alloy. The most common type of galvanic corrosion in a boiler system is caused by the contact of dissimilar metals, such as iron and copper. These differential cells can also be formed when deposits are present. Galvanic corrosion can occur at welds due to stresses in heat-affected zones or the use of different alloys in the welds.
Anything that results in a difference in electrical potential at discrete surface locations can cause a galvanic reaction. Causes include:. A general illustration of a corrosion cell for iron in the presence of oxygen is shown in Figure Pitting of boiler tube banks has been encountered due to metallic copper deposits.
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. Dissolved copper may be plated out on freshly cleaned surfaces, establishing anodic corrosion areas and forming pits, which are very similar to oxygen pits in form and appearance.
This process is illustrated by the following reactions involving hydrochloric acid as the cleaning solvent. 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. A complexing agent is added to prevent the copper from redepositing. The following chemical reaction results:.
This can take place as a separate step or during acid cleaning. Both iron and the copper are removed from the boiler, and the boiler surfaces can then be passivated. In most cases, the copper is localized in certain tube banks and causes random pitting. When deposits contain large quantities of copper oxide or metallic copper, special precautions are required to prevent the plating out of copper during cleaning operations.
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. Caustic corrosion gouging occurs when caustic is concentrated and dissolves the protective magnetite Fe3O4 layer. Iron, in contact with the boiler water, forms magnetite and the protective layer is continuously restored.
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 Steam blanketing is a condition that occurs when a steam layer forms between the boiler water and the tube wall. Under this condition, insufficient water reaches the tube surface for efficient heat transfer. The water that does reach the overheated boiler wall is rapidly vaporized, leaving behind a concentrated caustic solution, which is corrosive.
Porous metal oxide deposits also permit the development of high boiler water concentrations. Water flows into the deposit and heat applied to the tube causes the water to evaporate, leaving a very concentrated solution. Again, corrosion may occur.
Caustic attack creates irregular patterns, often referred to as gouges. Deposition may or may not be found in the affected area. Phosphate buffers the boiler water, reducing the chance of large pH changes due to the development of high caustic concentrations. Excess caustic combines with disodium phosphate and forms trisodium phosphate. Sufficient disodium phosphate must be available to combine with all of the free caustic in order to form trisodium phosphate.
This results in the prevention of caustic buildup beneath deposits or within a crevice where leakage is occurring. Caustic corrosion and caustic embrittlement, discussed later does not occur, because high caustic concentrations do not develop see Figure Different forms of phosphate consume or add caustic as the phosphate shifts to the proper form.
For example, addition of monosodium phosphate consumes caustic as it reacts with caustic to form disodium phosphate in the boiler water according to the following reaction:. Control is achieved through feed of the proper type of phosphate to either raise or lower the pH while maintaining the proper phosphate level. Increasing blowdown lowers both phosphate and pH. Elevated temperatures at the boiler tube wall or deposits can result in some precipitation of phosphate. This effect, termed "phosphate hideout," usually occurs when loads increase.
When the load is reduced, phosphate reappears. Clean boiler water surfaces reduce potential concentration sites for caustic. Deposit control treatment programs, such as those based on chelants and synthetic polymers, can help provide clean surfaces.
In such cases, operational changes or design modifications may be necessary to eliminate the cause of the problem. Low makeup or feedwater pH can cause serious acid attack on metal surfaces in the preboiler and boiler system.
Even if the original makeup or feedwater pH is not low, feedwater can become acidic from contamination of the system. Common causes include the following:. Acid corrosion can also be caused by chemical cleaning operations.
Overheating of the cleaning solution can cause breakdown of the inhibitor used, excessive exposure of metal to cleaning agent, and high cleaning agent concentration. Failure to neutralize acid solvents completely before start-up has also caused problems.
In a boiler and feedwater system, acidic attack can take the form of general thinning, or it can be localized at areas of high stress such as drum baffles, "U" bolts, acorn nuts, and tube ends. Hydrogen embrittlement is rarely encountered in industrial plants. The problem usually occurs only in units operating at or above 1, psi. Hydrogen embrittlement of mild steel boiler tubing occurs in high-pressure boilers when atomic hydrogen forms at the boiler tube surface as a result of corrosion.
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. These gases evolve predominantly along grain boundaries of the metal. The resulting increase in pressure leads to metal failure. The initial surface corrosion that produces hydrogen usually occurs beneath a hard, dense scale.
Acidic contamination or localized low-pH excursions are normally required to generate atomic hydrogen. In high-purity systems, raw water in-leakage e. Maintenance of clean surfaces and the use of proper procedures for acid cleaning also reduce the potential for hydrogen attack. Without proper mechanical and chemical deaeration, oxygen in the feedwater will enter the boiler. Much is flashed off with the steam; the remainder can attack boiler metal.
The point of attack varies with boiler design and feedwater distribution. Pitting is frequently visible in the feedwater distribution holes, at the steam drum waterline, and in downcomer tubes. Oxygen is highly corrosive when present in hot water. Even small concentrations can cause serious problems. Because pits can penetrate deep into the metal, oxygen corrosion can result in rapid failure of feedwater lines, economizers, boiler tubes, and condensate lines.
Additionally, iron oxide generated by the corrosion can produce iron deposits in the boiler. Oxygen corrosion may be highly localized or may cover an extensive area. It is identified by well defined pits or a very pockmarked surface.
The pits vary in shape, but are characterized by sharp edges at the surface. Active oxygen pits are distinguished by a reddish brown oxide cap tubercle. Removal of this cap exposes black iron oxide within the pit see Figure 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. A temperature rise provides enough additional energy to accelerate reactions at the metal surfaces, resulting in rapid and severe corrosion.
Efficient mechanical deaeration reduces dissolved oxygen to 7 ppb or less. For complete protection from oxygen corrosion, a chemical scavenger is required following mechanical deaeration.
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.
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. Based on experience in thousands of systems, ppb of feedwater oxygen is not significantly damaging to economizers. This is reflected in industry guidelines. Many corrosion problems are the result of mechanical and operational problems. The following practices help to minimize these corrosion problems:.
Where boiler tubes fail as a result of caustic embrittlement, circumferential cracking can be seen. In other components, cracks follow the lines of greatest stress.
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 The cracks do not penetrate the crystals themselves, but travel between them; therefore, the term "intercrystalline cracking" is used. Good engineering practice dictates that the boiler water be evaluated for embrittling characteristics. An embrittlement detector described in Chapter 14 is used for this purpose.
If a boiler water possesses embrittling characteristics, steps must be taken to prevent attack of the boiler metal. Sodium nitrate is a standard treatment for inhibiting embrittlement in lower-pressure boiler systems. The inhibition of embrittlement requires a definite ratio of nitrate to the caustic alkalinity present in the boiler water.
Caustic embrittlement caustic stress corrosion cracking , or intercrystalline cracking, has long been recognized as a serious form of boiler metal failure. Because chemical attack of the metal is normally undetectable, failure occurs suddenly-often with catastrophic results. Fatigue cracking due to repeated cyclic stress can lead to metal failure. The metal failure occurs at the point of the highest concentration of cyclic stress.
Examples of this type of failure include cracks in boiler components at support brackets or rolled in tubes when a boiler undergoes thermal fatigue due to repeated start-ups and shutdowns. Thermal fatigue occurs in horizontal tube runs as a result of steam blanketing and in water wall tubes due to frequent, prolonged lower header blowdown. Corrosion fatigue failure results from cyclic stressing of a metal in a corrosive environment.
This condition causes more rapid failure than that caused by either cyclic stressing or corrosion alone. In boilers, corrosion fatigue cracking can result from continued breakdown of the protective magnetite film due to cyclic stress. Corrosion fatigue cracking occurs in deaerators near the welds and heat-affected zones. Proper operation, close monitoring, and detailed out-of-service inspections in accordance with published recommendations minimize problems in deaerators.
Steam side burning is a chemical reaction between steam and the tube metal. It is caused by excessive heat input or poor circulation, resulting in insufficient flow to cool the tubes. Under such conditions, an insulating superheated steam film develops. The problem is most frequently encountered in superheaters and in horizontal generating tubes heated from the top.
Erosion usually occurs due to excessive velocities. Where two-phase flow steam and water exists, failures due to erosion are caused by the impact of the fluid against a surface. Equipment vulnerable to erosion includes turbine blades, low-pressure steam piping, and heat exchangers that are subjected to wet steam.
Feedwater and condensate piping subjected to high-velocity water flow are also susceptible to this type of attack. Damage normally occurs where flow changes direction. Iron and copper surfaces are subject to corrosion, resulting in the formation of metal oxides. This condition can be controlled through careful selection of metals and maintenance of proper operating conditions.
Iron oxides present in operating boilers can be classified into two major types. The first and most important is the 0. This magnetite forms a protective barrier against further corrosion. The magnetite, which provides a protective barrier against further corrosion, consists of two layers. The inner layer is relatively thick, compact, and continuous. The outer layer is thinner, porous, and loose in structure.
Both of these layers continue to grow due to water diffusion through the porous outer layer and lattice diffusion through the inner layer.
As long as the magnetite layers are left undisturbed, their growth rate rapidly diminishes. The second type of iron oxide in a boiler is the corrosion products, which may enter the boiler system with the feedwater.
These are frequently termed "migratory" oxides, because they are not usually generated in the boiler. The oxides form an outer layer over the metal surface. This layer is very porous and easily penetrated by water and ionic species. When the boiler was idle, wet lay-up method was applied to prevent the corrosion.
During operation, boiler water was under phosphate treatment. For providing the proper alkaline environment, sodium hydroxide was used.
Operators carried out several blowdowns to control the water chemistry. During periodic investigations, a severe thickness reduction was detected in a horizontal boiler tube by non-destructive testing NDT. It was reported that there was a uniform oxide layer on the outer surface of the tube. Moreover, two parallel longitudinal trenches were observed on the internal surface. In order to examine the internal surface of the failed tube, a part of it was cut lengthwise and sent to the laboratory.
Characterization methods Three samples from different parts of the as-received specimen were selected for quantometric, metallographic, electron microscopy, X-ray diffractometery and hardness examinations. Furthermore, the oxide layer composition and corrosion products were analyzed.
Spectroscopic methods were utilized as follows: i Chemical composition. To specify the chemical composition of the tube, quantometric analysis was applied. For microstructural examination, specimens were cut cross-sectional from the adjacent and apart from the groove. The samples were then mounted and polished.
Vickers microhardness test was performed by Akashi M G1 Leco under indentation load of g for 15 s. Results and discussions 3. Visual inspection An image of the as-received tube is shown in Fig. Based on the appearance and the color of the scale around the groove, the tube could be separated to three distinct parts Fig. As it can be seen in Fig. The as-received failed superheater tube: a inner side view, b outer side view and c cross sectional view. On the outer surface of the tube Fig.
From Fig. Chemical composition analysis The quantometric analysis result is given in Table 1. With regard to the chemical composition, the tube was made of low alloy steel, SA Grade A Metallographical examination The microstructure and morphology of the tube at the damaged region was examined by optical microscopy and the images are shown in Fig. Metallographical images after polishing and etching procedure showed that the morphology of the tube at the center of the sample consisted of pearlite islands in ferritic matrix Fig.
Finally, Fig. By comparing Fig. Microhardness test The microhardness tests were carried out on the cross-section of the sample in different distances from the groove.
The brief results are given in Table 2. As it can be seen, the deposits on the surface of the base alloy consist of two different layers; Table 1 Chemical analysis of the failed boiler tube. Metallographic structures of the tube taken from a the center of the sample, b the area adjacent to the groove at dark gray scale, c the area adjacent to the groove at brown scale and d the groove region.
Additionally, EDS results showed that in dark gray region, weight percentage of some ele- ments such as Cu, Ca, Zn and Na is higher than brown area which can be attributed to presence of them in boiler circulating water. Surface of the tube at three regions as determined in Fig. No microcrack was observed on the surface of the specimen in different areas. As it can be observed, there were some needle shape crystals on the groove scales Fig.
EDS was utilized to identify the corrosion products on the inner side of the tube. The EDS samples were chosen from the groove, brown and dark gray scales. Three points was analyzed in each sample and the average is reported in Table 3.
Cu is sometimes added to the makeup water intentionally due to its great conductive properties. Al presence can be attributed to copper base alloys and zeloite that is used for boiler water treatment. Moreover, oxygen scavengers which are used for water deaeration may contain sulfur. Finally, as mentioned before, the source of Na and P is chemicals that are used for boiler water treatment [13]. For sample preparation, the damaged surface was scraped and the prepared oxide powder was used.
The XRD result is depicted in Fig. It is clear that the groove scale mostly consist of magnetite and hematite. Root cause and mechanism of the boiler failure Generally, it was reported that there are three under deposit corrosion namely: 1 hydrogen damage, 2 acid phosphate corrosion and 3 caustic gouging. All of them need heavy deposits and a concentration mechanism within those deposits [8,13,14]. Moreover, hydrogen atoms can easily penetrate in carbon steel Fig.
SEM image of the cross-section of the groove region, where a two layered deposit is formed on base metal: porous corrosion products part A and uniform scale part B. X-ray diffraction pattern of the corrosion products powder at groove region. Even small concentrations can cause serious problems. Because pits can penetrate deep into the metal, oxygen corrosion can result in rapid failure of feedwater lines, economizers, boiler tubes, and condensate lines.
Additionally, iron oxide generated by the corrosion can produce iron deposits in the boiler. The metal failure occurs at the point of the highest concentration of cyclic stress. Examples of this type of failure include cracks in boiler components at support brackets or rolled in tubes when a boiler undergoes thermal fatigue due to repeated start-ups and shutdowns. Thermal fatigue occurs in horizontal tube runs as a result of steam blanketing and in water wall tubes due to frequent, prolonged lower header blowdown.
Steam Side Burning: Steam side burning is a chemical reaction between steam and the tube metal. It is caused by excessive heat input or poor circulation, resulting in insufficient flow to cool the tubes. Under such conditions, an insulating superheated steam film develops. Once the tube metal temperature has reached F in boiler tubes or F in superheater tubes assuming low alloy steel construction , the rate of oxidation increases dramatically; this oxidation occurs repeatedly and consumes the base metal.
The problem is most frequently encountered in superheaters and in horizontal generating tubes heated from the top. The following practices help to minimize these corrosion problems: Selection of corrosion-resistant metals. Reduction of mechanical stress where possible e. Minimization of thermal and mechanical stresses during operation. Operating within design load specifications, without over-firing, along with proper start-up and shutdown procedures.
Maintenance of clean systems, including the use of high-purity feedwater, effective and closely controlled chemical treatment, and acid cleaning when required. Maintenance of proper pH throughout the boiler feedwater, boiler, and condensate systems is essential for corrosion control.
A well planned monitoring program should include the following: Proper sampling and monitoring at critical points in the system.
Completely representative sampling. Use of correct test procedures. Checking of test results. A plan of action to be carried out promptly when test results are not within established limits. An efficient plan for major upset conditions. A quality improvement system and assessment of results based on testing and inspections.
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