How Corrosion Can Lead to Failures in Steam Boilers

What is Furnace Corrosion?

Fireside corrosion in the superheaters and reheaters is termed coal ash corrosion in coal-fired units, oil ash corrosion in oil-fired units, and ash corrosion in waste-recovery boilers. In some instances, fireside corrosion is also named hot corrosion. The mechanism in each scenario is similar, but the low-melting species in each differ. In coal ash corrosion, the low-melting substances are usually sodium or potassium iron trisulfate (Na3Fe(SO4)3 or K3Fe(SO4)3). In oil ash corrosion, it is usually V2O5-Na2O or V2O5-Na2SO4, and in waste heat-fired boilers, it is generally chlorides of iron and zinc, along with other possibilities. High temperatures above 1,000F of the superheater/reheater encourage the formation of these low-melting compounds.

 The corrosion in coal ash corrosion ranging between 1,100F and 1,250F is very high. In such a temperature range, Na2SO4/K2SO4 reacts with surface oxides along with SO3 to form complex liquid sulfates (Na3Fe(SO4)3 or K3Fe(SO4)3) that cause corrosion.

The elements in oil ash corrosion include vanadium, sodium, potassium, and sulfur. The liquid fuel combustion generates low-melting species V2O5-Na2O, V2O5-K2O, V2O5-Na2SO4, and V2O5-K2SO4. Depending on the composition, the melting points of the compounds are between 1,000F and 1,550F. The oil ash corrosion is similar to coal ash corrosion with low-melting-point liquid forms and dissolves the protective iron oxides.

In waste-recovery boilers, chloride, and sulfate cause the formation of low-melting-point liquids on the tube surface that includes zinc, iron, lead, and sodium. It dissolves the protective iron oxide and brings the bare steel in contact with the corrosive environment, leading to significant wall loss. Lowering the conditions in the furnace results in the formation of iron sulfide rather than iron oxide. The carbon and iron sulfide present in the ash deposits indicate reducing conditions. Hydrogen chloride can impact iron sulfides instead of oxides to form iron chloride as a corrosion product. The iron chloride consists of a relatively low boiling point and forms vapors in the superheater/reheater temperature range. The corrosion mechanism is by the loss of iron as a vapor.

Furnace wall tubes are also impacted through fireside corrosion, but the low-melting elements differ from superheater/reheaters. Sodium and potassium pyrosulfate (Na2S2O7 or K2S2O7) causes the furnace wall corrosion. Both the elements melt below 800F, where the furnace wall tubes operate. The melting point of Na2S2O7 is 750F and of K2S2O7 is 570F. Combining the two compounds can melt at even lower temperatures.

Effects of Corrosion:

Lost Efficiency

Corrosion and scale deposits impact steam boiler efficiency. Even scale buildup as small as an eighth of an inch can reduce the efficiency.

Scale buildup contributes to more corrosion by trapping sodium under the scale, pitting the inside surface. It leads to further damage inside the boiler and its tubing. Holes in the metal result in leaks that cause severe operational problems and boiler shut down.

Shorter Lifespan

Overlooking corrosion reduces the lifespan of the steam boiler system. Corrosion increases with time if the water chemistry is neglected. The efficiency loss degrades until the system shuts down.

Higher Costs

Corrosion can lead to excessive costs for repairing the system or replacing damaged parts. In many cases, pitted tubes or parts need to be replaced rather than repaired. The system should be shut down to fix the damage that reduces productivity. Additionally, the downtime cuts into the operations and profits.

Holes

Holes occur while operating a system with severe pitting. The pits do not repair themselves or reverse their severity. It deteriorates as the chemical reaction that caused the erosion continues until the metal has a breech.

Pitting

Steam boilers having high levels of oxygen react with the metal to cause pits on the surface. If overlooked, the pits continue to deepen till they create holes in the metal. The holes can lead to the failure of the system.

Preventing Corrosion in Steam Boiler:

Monitoring the System

Maintaining the acid-base balance of the water is crucial to prevent acidity from damaging the steam boiler. Even with proper attention to the feedwater, there may be contaminants in the boiler itself. Monitoring the pH helps in determining when to blow down the system to remove contaminated water and reduce its impact.

The pH for feedwater is between 7 and 9, slightly alkaline. Sodium phosphate salts or sodium hydroxide are added to control the water within the range. However, the pH inside the heat of the boiler can be directly monitored and instead measured from a cooler, lower-pressure side stream.

Checking the water and steam for sodium also helps in preventing corrosion by determining the need to control the minerals present in the system.

Adjust Feedwater

Feedwater adjustment prevents the impact of dissolved oxygen on the metal surface of the system. Depending on the results, scavenging agents or added or a deaerator is used for getting rid of oxygen.

Three chemicals are common for scavenging oxygen

• Sodium sulfite: Usually used in medium or low-pressure systems
• Hydrazine: Ideal for high-pressure boilers
•  Sodium erythorbate: Non-toxic and can replace the other two chemicals for use in food processing.

Conclusion:

Rakhoh Boilers is one of the trusted boiler manufacturers in Pune since 1983. With 38+ years of experience and expertise in thermal solutions, we deliver highly efficient and reliable industrial steam boilers, waste heat recovery systems, thermic fluid heaters, and boiler accessories to over 20 process industries in 26 countries worldwide. We provide excellent boiler services to boost the efficiency and productivity of the boilers.

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