Boiler corrosion, tube perforation, red boiler water, excessive iron in steam condensate – a simple solution

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Boiler corrosion, tube perforation, red-colored boiler water, excessive iron content in steam and condensate – a simple solution. A materials company in Hebei installed two natural gas boilers of the WNS6-1.25-Y/Q model, with a capacity of 6 t/h each; one boiler was in use while the other served as a backup, and the two boilers were switched back and forth on a weekly basis. The boiler make-up water consisted of 30% resin-softened water and 70% recycled high-temperature steam condensate. There was no deaerator, and sodium carbonate (Na2CO3) was added to the feed water during operation as an anti-scaling agent. The boilers operated intermittently, for an average of 12 hours per day. No protective measures were taken during periods when the boilers were not in use. During operation, it was observed that the color of the boiler wastewater was red, and the recycled high-temperature steam condensate at 85 degrees Celsius had a yellowish color. The iron content in the condensate was excessive, at 0.77 mg/L, while the pH value of the steam condensate ranged from 5 to 6.5. After 18 months of operation, one of the boilers developed tube perforation and leakage. ▲Two newly installed boilers suffered from corroded and perforated furnace tubes; the boiler water turned red, and the iron content in the steam and condensate was above the acceptable levels. I. Inspection of the corroded and perforated furnace tubes as well as detection of corrosion substances: After shutting down the boilers, inspections were carried out on the internal furnace tubes of both boilers, and it was found that there was severe corrosion on the surfaces of the furnace tubes in the high-heat-load areas as well as on the surfaces of the boiler drums in both boilers. Brown deposits were attached to the surfaces of both the furnace tubes and the furnace chamber. Corroded and perforated furnace tubes, as well as those that were corroded but not perforated, were taken apart for inspection; the surfaces of these furnace tubes had many small bulges, with a yellow-brown outer layer on these bulges ； Beneath the outer layer lies a second layer of hard, black sediment with strong adhesiveness ； After removing all corrosion products, ulcer-like corrosion pits were found beneath them; these pits were numerous and ranged in depth from 0.1 to 1.7 mm, with some being nearly through-thickness. Many areas on the surface of the furnace chamber exhibited obvious dish-shaped corrosion grooves and signs of corrosion; the deepest parts of these corrosion pits reached 2–3.4 mm. Hard, dark brown corrosion products were present in the corroded areas, and these dark brown products turned out to be magnetic when a magnet was brought near them, suggesting that they might be iron tetraoxide, Fe3O4. Through metallographic examination and mechanical property testing of the cut samples from the corroded sections of the furnace tubes, it was found that there were virtually no changes in the metal of the furnace tubes, nor was there any significant decarburization; thus, it can be concluded that there was no overheating corrosion or stress corrosion in the boiler furnace tubes. Tests and analyses of the corrosion products in the furnace tubes and the furnace chamber revealed that iron oxides accounted for as high as 87.4% of the corrosion products in the furnace tubes, and 74.7% of those in the furnace chamber. ▲There is severe corrosion on the surface of the boiler’s furnace chamber in certain areas, as well as corrosion-induced perforations in the boiler tubes. II. Analysis of the causes of corrosion and perforation in boiler tubes: Iron oxides account for 87.4% of the corrosion products in the boiler tubes, while iron oxides make up 74.7% of the corrosion products in the furnace chamber. There are five sources of iron oxide corrosion products within the boiler: 1. Oxygen-induced corrosion products from the boiler’s furnace chamber and boiler tubes themselves. The boiler lacks a deaerator or any deoxygenation measures; as a result, the feedwater carries dissolved oxygen into the boiler, causing electrochemical corrosion. Iron and oxygen form two electrodes, with the electrode potential of iron being lower than that of oxygen. Oxygen has a dual effect on the corrosion rate of steel and iron; on one hand, it acts as an excellent depolarizer for both the cathode and the anode, accelerating the corrosion of the metal. On the other hand, an increase in the dissolved oxygen concentration in the boiler water leads to the formation of a dense protective film on the surface of the steel as a result of corrosion by dissolved oxygen, thereby reducing corrosion. After the protective film is formed, it reduces the number of corrosion cores on the surface of the steel, but increases the corrosion rate at each point where corrosion has already begun. Due to the localized nature of oxygen corrosion, we observe many **small bulges; after removing the ferric corrosion products, pitted ulcerative corrosion depressions can be seen. 2. Corrosion products from shutdown of the furnace. The boilers operate intermittently, running for 12 hours per day with a 12-hour shutdown period. The two 6t/h boilers take turns in operation on a weekly basis, with each boiler being taken out of service for 15 days per month. No protective measures are taken during these shutdown periods; as a result, oxygen from the outside air enters the boiler, where the water has not been drained and dried. The oxygen in such a humid environment can cause significant corrosion damage to the equipment. Experience shows that corrosion during shutdown periods is often more severe than that that occurs while the boilers are in operation. When a boiler is shut down frequently, the low-valent iron oxides formed during operation-induced corrosion are re-oxidized to high-valent iron oxides when the boiler is shut down again, as they absorb oxygen from the air. As operation and shutdown alternate with each other, this corrosive process repeats itself, leading to increased corrosion; therefore, boilers that are started and stopped frequently suffer from particularly severe corrosion. 3. Ferric corrosion products brought in by the feed water. The boiler feed water consists of 30% resin-softened water and 70% recycled high-temperature steam condensate. Since sodium carbonate Na2CO3 is used as a scale inhibitor in the boiler, large amounts of bicarbonate are present in the boiler water. When heated, these bicarbonates decompose into carbon dioxide CO2, which then enters the heat utilization system and the steam condensate recovery system along with the steam. The presence of carbon dioxide not only lowers the pH value of both the steam and the recycled steam condensate but also causes acidic corrosion to the equipment and pipelines in these systems. This is why the recycled high-temperature steam condensate at 85 degrees Celsius appears yellow in color, and why the iron content in the condensate is high, at 0.77 mg/L; furthermore, the pH value of the steam condensate ranges from 5 to 6.5, indicating an acidic environment. The iron content in the condensate is excessive, at 0.77 mg/L; the condensate is reused in the boiler without any treatment. The colorless ferrous ions are oxidized by heat in the boiler to form colored ferric ions. As the water in the boiler becomes more concentrated, the iron ion concentration increases, and the color of the water turns red. This is also the main reason why the wastewater discharged from the boiler appears red during operation. As the water in the boiler becomes increasingly concentrated, iron ions deposit on the heating surfaces of the boiler, forming iron oxide scale, that is, ferritic corrosion products. ▲There are numerous bulges of various sizes on the smoke pipes, boiler drum, and furnace chamber inside the boiler. The outer layer of these bulges is composed of a hard, highly adhesive yellow-brown deposit; beneath this outer layer lies rust, and at the very bottom is a black substance. 4. Corrosion products beneath iron oxide scale. Whether it is the oxygen corrosion products formed on the boiler’s furnace shell and tubes, the corrosion products that occur when the boiler is shut down, or the ferric corrosion substances brought in by the feedwater, all of these result in the formation of ferric corrosion deposits primarily composed of iron oxide within the boiler’s furnace shell and tubes. Iron oxide, specifically Fe2O3, acts as a depolarizer for corrosion processes inside the boiler. As a result, the following electrochemical corrosion reactions take place: In the cathode area: Fe2O3∙nH2O + 2e → 2Fe(OH)2 + 2OH– + (n–3)H2O. At the anode: Fe → Fe2+ + 2e. The secondary reaction is: Fe2+ + 2OH– → Fe(OH)2. Further oxidation occurs as follows: 4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3. Subsequent transformation: 2Fe(OH)3 + Fe(OH)2 → Fe2O3 + 4H2O. The overall reaction is: 2Fe + 2Fe2O3∙nH2O + O2 → Fe3O4 + nH2O. Thus, as long as Fe2O3 is present in the boiler, the corrosion reactions beneath the iron oxide deposits will continue, leading to the formation of bulges in those deposits. The corrosion pits beneath the iron oxide deposits become deeper over time, and eventually, perforations and leaks in the boiler’s furnace shell and tubes become inevitable. ▲Boiler corrosion, tube perforation; boiler water turning red, excessive iron content in steam and condensate; 5. Concentration of media in the furnace chamber along with alkali corrosion products. Based on the analysis of the corrosion condition on the surface of the furnace chamber and the corrosive substances, it can be seen that the corrosion in the furnace chamber differs from that in the furnace tubes; the corrosion in the furnace chamber is mainly caused by alkali concentration. Due to the addition of Na2CO3, a scale-inhibiting agent, to the boiler feedwater, the heat load on the boiler’s furnace is very high, and the circulation of water within the furnace is not effective. The hydrolysis reaction of sodium carbonate is: Na2CO3 + H2O → 2NaOH + CO2↑. Half of the alkalinity present in the feedwater is decomposed inside the boiler to produce sodium hydroxide (NaOH). As a result, corrosive concentrations of NaOH are formed beneath the accumulated ferric oxide deposits, due to the evaporation of water from the boiler. Although this concentrated water is diluted by the influx of new water, the process of infiltration – evaporation and concentration – followed by further infiltration and evaporation continues indefinitely. Overall, high concentrations of sodium hydroxide NaOH in the boiler water persist beneath the ferritic corrosion deposits, with the concentration being higher the closer to the boiler furnace. In the most severe cases, the concentration of sodium hydroxide NaOH beneath these deposits can reach 15%. At this point, the protective film Fe3O4 on the surface of the steel furnace chamber reacts first with concentrated NaOH; subsequently, the underlying base metal iron also reacts with sodium hydroxide, resulting in the formation of sodium ferrate and sodium ferrous oxide. The reaction is as follows: Fe3O4 + 4NaOH → 2NaFeO2 + Na2FeO2 + 2H2O. Sodium ferrate and sodium ferrous oxide then undergo hydrolysis reactions with the water that penetrates into the furnace, resulting in corrosion products such as iron oxide and ferrous oxide, as well as sodium hydroxide. Sodium hydroxide continues to corrode the metal, and throughout this process, it is not consumed or lost. Therefore, when this type of localized medium concentration-induced alkaline corrosion occurs, it is difficult to detect the high-concentration alkaline corrosion caused by localized medium concentration merely by measuring the pH and alkalinity of the boiler water. In summary, due to the absence of a deaerator or any deoxygenation measures in the feedwater, oxygen corrosion has occurred in various parts of the boiler system ； Corrosion products resulting from intermittent operation and shutdown of boilers ； The addition of sodium carbonate as a scale inhibitor to the boiler make-up water results in a low pH value of the steam condensate, making it acidic. This acidic condensate corrodes and carries excess iron ions into the boiler, where they deposit on the boiler’s heating surfaces and cause scaling ； The electrochemical corrosion induced by Fe2O3 continues to occur beneath the iron oxide scale, which is the product of corrosion ； Local concentration of media in the furnace chamber leads to alkali corrosion products; there are 5 sources of ferric oxide corrosion products within the boiler, and these are also the causes of corrosion in the boiler as well as tube perforation and leakage. ▲Reddish boiler water (left); steam condensate with a yellowish color due to excessive iron levels, resulting in acidity (right); softened water (middle). Sodium carbonate Na2CO3, when dissolved in water, creates an alkaline environment, which can increase the alkalinity and pH level of the boiler water. It also neutralizes carbon dioxide present in acidic feed water, thereby preventing corrosion of boilers and pipes caused by acidic softened water or deionized water. Sodium carbonate Na2CO3 is a weak electrolyte; when it dissolves in water, it forms a buffer solution of sodium carbonate and sodium bicarbonate. An electrolytic equilibrium exists in this solution. The hydroxide ions produced by the electrolysis of sodium carbonate Na2CO3 combine with the hardness components present in boiler water, such as calcium ions and magnesium ions, to form amorphous sludge, thereby preventing the formation of crystalline scale and achieving scale prevention. Sodium carbonate Na2CO3 helps to maintain a relatively stable pH level in the boiler water, with minimal fluctuations in pH, which is an advantage over using sodium hydroxide NaOH to regulate the pH and alkalinity of the boiler water. Although sodium carbonate Na2CO3 has certain advantages over sodium hydroxide NaOH in terms of regulating the alkalinity and pH level of boiler water as well as preventing scaling, there are 4 issues to consider when using sodium carbonate Na2CO3: its rate of pH adjustment is slower compared to that of sodium hydroxide, and its range of effectiveness is limited ； Adding sodium carbonate to the boiler feed water reduces the pH levels of steam and condensate, thereby causing corrosion of the heat exchangers and condensate pipelines ； Excessive addition of sodium carbonate to the boiler feed water increases the salt content in the boiler water, leading to an increase in conductivity ； Use the boiler sparingly and with caution when it is already fouled or corroded, as this can lead to localized concentration of the medium due to scaling or corrosion, thereby accelerating the corrosion, perforation, and leakage of the boiler equipment. Colleagues and students, aside from using sodium carbonate and sodium hydroxide to adjust the alkalinity and pH of boiler water as well as to prevent scaling, do you know of any other water treatment chemicals that can be used for these same purposes? How does your facility regulate the alkalinity, pH level, and scale prevention of the boiler water? Regarding issues such as adjusting the pH value of boiler feed water, furnace water, and steam condensate water; furnace water turning red; hardness in steam condensate water; boiler corrosion, scaling, and tube failures; corrosion in the steam system; excessive iron content in steam condensate water; yellowish color of steam condensate water; online descaling techniques that allow continued operation of the boiler without shutdown; and online cleaning and descaling techniques for condensers that enable continued operation without interruption, Yan Hui from Beijing University of Chemical Technology at I86OO475З86 is always available to discuss these issues, exchange experiences and insights, and welcome everyone to share their knowledge regarding the management and use of boiler equipment, so as to learn from each other in addressing various practical problems related to boilers. Boiler corrosion, tube perforation, red-colored boiler water, excessive iron content in steam and condensate – a simple solution. Compared to the various shortcomings of using sodium carbonate or sodium hydroxide to adjust the pH value of boiler feedwater, in recent years more and more users are turning to alkaline water treatment agents – the multi-functional BF anti-corrosion and scale-removal agent – to regulate the pH value of boiler feedwater. ▲The boiler water turns red, the steam condensate is acidic, and the boiler tubes corrode and perforate; can you see those corrosion pits, those perforations? III. Products that replace sodium carbonate (Na2CO3) for regulating the alkalinity and pH value of boiler water, as well as for scale prevention – the multi-functional BF anti-corrosion and scale-removal agent, which removes and prevents scale formation, disperses iron scale, provides anti-corrosion and corrosion-inhibition effects, and regulates the pH value; characteristics of the multi-functional BBF anti-corrosion and scale-removal agent. To address the severe corrosion problem in the heating systems of industrial boilers, with the strong support of the Ministry of Science and Technology, Beijing University of Chemical Technology has successfully developed a combined anti-corrosion and scale-inhibition agent for boilers, as well as corresponding anti-corrosion and scale-inhibition technologies. This technology does not require expensive deoxygenation equipment; by following the specified process conditions and adding the anti-corrosion and scale-inhibiting agent BF to the system, which has functions such as removing and dispersing scale, providing anti-corrosion and corrosion inhibition, regulating pH levels, defoaming, indicating dosing levels, and preventing water loss, it is possible to achieve both continuous anti-corrosion and scale-inhibition during operation, as well as anti-corrosion maintenance when the system is not in use. Performance tests and practical applications by numerous boiler users have demonstrated that BF anti-corrosion and scale-removal agent boasts 10 advantages: 1. It provides anti-corrosion protection by forming a protective film; its corrosion inhibition rate is ≥99%, its scale inhibition rate is also ≥99%, it offers a wide range of scale inhibition effects, and its phosphorus-free formula replaces trisodium phosphate ； 2. As a boiler shutdown protection agent, it provides a corrosion inhibition rate of ≥99% for protecting boilers and system equipment when they are not in use. It is sufficient to simply seal the boiler equipment when shutting it down, eliminating the need for frequent regular inspections. There is no need to drain the boiler water either when starting up or shutting down the boiler. It is suitable for use in situations involving long-term shutdowns, frequent start-ups and shutdowns, as well as prolonged intermittent operation of boilers ； 3. As a scale dispersant, for rust removal and dispersing iron scale ； Especially suitable for boilers in which high-temperature steam condensate is reused directly without iron removal treatment ； 4. Alkaline agents, used as pH regulators to raise the pH of feedwater and boiler water, to neutralize the weakly acidic corrosion caused by CO2, and as alternatives to sodium carbonate, sodium hydroxide, and ammonia ； 5. Scale dissolution and removal, chelation to eliminate old scale; replaces boiler acid cleaning for maintenance, enabling damage-free online cleaning of the boiler so that it can continue to operate without needing to be shut down ； 6. Acting as an antifoaming agent to increase steam dryness and ensure steam quality ； 7. Raw water + multi-functional BF corrosion and scale inhibitor; hot water boilers can be used instead of softened water, with oxygen removal procedures to protect the environment ； 8. Raw water + multi-functional BF corrosion and scale inhibition agent + BF condensate iron removal and corrosion inhibitor, to replace the softening device, deaeration equipment, and iron removal equipment in steam boiler systems ； 9. Simple operation: Add the chemical to the make-up water tank; a pH value of 10.5–12 for the water in the boiler is considered acceptable ； 10. The chelating, dispersing, and lattice-distorting effects on scale are all non-stoichiometric and exhibit a threshold effect; a low dosage of the additive is sufficient to reduce the boiler’s blowdown rate. Due to space constraints, the mechanism of action of the multi-functional BF anti-corrosion and descaling agent, which is effective in removing scale and rust deposits, as well as in providing anti-corrosion protection, corrosion inhibition, and pH regulation, is omitted here; Yan Hui welcomes inquiries at any time. ▲Boiler tube corrosion leads to perforations, and oxygen corrosion in the boiler drum results in ulcerative corrosion pits. IV. Solutions and effects for boiler tube corrosion and perforation: 1. After replacing the severely corroded and perforated boiler tubes, chemical pickling is carried out to remove scale and rust, as well as any weld slag or impurities inside the boiler. 2. During boiler operation, stop adding sodium triphosphate (Na2CO3) to the furnace; instead, use the multi-functional BF anti-corrosion and scale-inhibiting agent, which is capable of dispersing iron scale, providing anti-corrosion and corrosion-inhibition effects, and regulating the pH value, as a substitute for sodium triphosphate (Na2CO3) for scale inhibition. Maintain the pH value of the boiler water between 10 and 12. Thoroughly address the safety risks posed by the use of sodium carbonate Na2CO3 as a scale inhibitor in water supply, which leads to localized concentration of alkali and corrosion in the furnace chamber and tubes. 3. Appropriate amounts of BF condensate iron-removing and corrosion-inhibiting agent should be added to the boiler feed water in order to adjust the pH of the steam condensate to 7.5–8. This helps to protect the equipment and pipelines in the steam system and condensate recovery system from acidic and oxygen-induced corrosion, thereby completely resolving the problems of excessive iron ions in high-temperature steam condensate and yellowing of the condensate. 4. The two boilers operate in an alternating, intermittent manner. Not only are the shutdown periods for these boilers long, but they also shut down and restart once per day. The boilers run for 12 hours before shutting down for another 12 hours. During these shutdown periods, protective measures must be taken to prevent corrosion. Since BF corrosion and scale inhibitor is also a wet-shut-down protection agent, when there is an existing amount of this inhibitor in the boiler water, it is not necessary to drain the boiler water during either short-term or long-term shut downs; no pressure protection is required. The boiler can be directly sealed, and upon restarting after a short-term or long-term shutdown, there is no need to drain or replace the boiler water before starting it up. After implementing the above measures, the appearance of the boiler water and furnace water became clear and pure during operation, with their color returning to normal; there was no longer any red coloring in the wastewater discharged from the boiler, nor was there any yellowing in the condensed water from the 85-degree Celsius high-temperature steam that was reused. After the boiler had been in operation for 180 days, it was inspected after being turned on, and no local corrosion was found in the furnace shell and furnace tubes in the high-heat-load areas; moreover, a gray-black protective layer had formed on the surfaces of both the furnace shell and furnace tubes. Boiler corrosion, tube perforation, red boiler water, excessive iron in steam condensate – a simple solution (Yan Hui)