Steam Boiler: Corrosion in the steam boiler

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Corrosion in Boiler

Corrosion can be caused by oxygen and carbon dioxide which contained in condensed steam. Corrosion is reforming of metal from its natural original form such iron to be oxide iron, aluminum, etc. Corrosion can be caused by the following below:

- Corrosive gases such as O₂, CO₂, H₂S

- Crust and deposit

- Different metal type

- Low PH

- etc

Type of corrosion which is generally found in steam boiler is general corrosion, pitting corrosion, and embrittlement corrosion. The presence of dissolved oxygen and carbon dioxide in boiler water is the major cause of occurrence of general corrosion and pitting corrosion. The solubility of these gases in boiler water will be decrease if temperature rises. Most of oxygen will be separated in the steam room but small amount of residue is left in solution of pockets or under deposit, it can cause corrosion on metals boiler. Therefore it is important to make deoxygenating process on boiler water.

The average amount of corrosion or electrochemical attack would increase if the pH value decreases. Therefore boiler water must be maintained chemically in high pH value. Form of corrosion that is not common but dangerous is embrittlement corrosion or inter crystalline crack which can occur in steel if it is at high pressure and chemical environment that is not appropriate. Caustic embrittlement occurs in the connection plug and extends to the end tube when slit develop in concentrated caustic environment.

Hydrogen embrittlement is another form of inter crystalline crack that occur on water wall tube which is caused by high pressure and certain temperature condition.

Corrosion can be prevented by the following action:

- Eliminate corrosive gases

- Prevent crust and deposit in boiler tube

- Using a substance that can inhibit corrosive event

- Adjust pH and alkalinity

- etc

Reason of Corrosion in Steam Boiler

Corrosion can occurs in every point of location in steam boiler. Each location of occurrence, corrosion can be caused by some reasons depend on type of corrosion. The following below is the classification of corrosions in steam boiler based on reason for occurrence:

1. Inter granular Corrosion
Inter granular corrosion occurs in the metal of chromium which may be starved by formation of chromium carbide so chromium oxide as its protective layer is not produced.

2. Fatigue Corrosion
Fating corrosion is caused by stress which working cyclic and fluctuating.

3. Pitting Corrosion
Reason of occurrence this corrosion is the presence of pitting because of electrochemical reaction which can break protective oxide layer

4. Caustic Attack
Caustic attack is caused by high concentration PH which can break protective layer of iron oxide.

5. Acid Attack
Acid attack is caused by low concentration PH in the water wall tubes which can accelerate occurrence of corrosion.

6. Stress Corrosion
Stress corrosion commonly occurs because of combination of stress, environment and material.

7. Galvanic Corrosion
Electric potential will be generated on two dissimilar material such section superheater and reheater tubes and cause particle of metal move from one to other (anode to cathode).

Location of Corrosion in Steam Boiler

There are some types of corrosions which possible occur in steam boiler. Each type has reason for occurrence and location of occurrence. Base on them corrosion can be classified as pitting corrosion, fatigue corrosion, hydrogen embrittlement, caustic attack, acid attack, galvanic attack, inter granular, and stress corrosion. The following below is the description of corrosion based on location of occurrence in steam boiler area:

1. Pitting Corrosion
Pitting Corrosion usually occurs in economizer and tube which is not drained during maintenance.

2. Fatigue Corrosion
This type always causes cracking in location near welded area of water wall tubes.

3. Hydrogen Embrittlement
Hydrogen embrittlement generally occurs in water wall tubes

4. Caustic Attack
This corrosion often occurs in the grooving area under deposit on water wall tubes.

5. Acid Attack
Acid attack commonly occurs in water wall tubes area.

6. Galvanic Attack
Galvanic attack corrosion usually occurs in joint welding between two materials of tube both in superheater and reheater

7. Stress Corrosion
Stress corrosion commonly occurs in austenstic steel such as superheater tubes and reheater tubes.

Impact of Ash on Steam Boiler

Ash is product of combustion, it is unburned matter. Ash will flow with flue gas through furnace, superheater, reheater, economizer, air heater, multi cyclone / baghouse / electrostatic precipitator and then be discharged out to atmosphere through stack.

Ash can gives bad impact to steam boiler if maintenance program is not performed. Erosion, corrosion, slagging and fouling are the influence of ash. Erosion is reducing of surface metal because of impingement particle in ash with surface metal tubes, so the abrasion occurs. Erosion in steam boiler can be reduced but can not be eliminated, because particle abrasion ash include other unburned particle always presence in flue gas content, can not be eliminated 100%. Some of ways to reduce possibility of erosion are choose highest fuel efficiency and doing routine maintenance such us clean surface tubes with soot blower.

Another impact of ash is corrosion which is caused by either low temperatur corrosion or high temperature corrosion. Low temperature corrosion generally occur in economizer and air heater area because at temperature under 200 C, the water vapor will react with SOx content in flue gas to form sulfuric acid which lead to corrosion. High temperature corrosion generally occur in superheater and reheater area but the causes can not be fully known because amount of reduction regarding to corrosion is very small approximately microns per year.

Another impact of ash are slagging and fouling. Slagging and fouling can be distinguished based on heat transfer. Slagging is the deposits which attached on the surface furnace tubes by radiant heat while fouling attached on superheater, reheater, economizer and air heater by convection heat. Slagging and fouling influence on heat transfer from flue gas to water inside tube can not be optimal.

Erosion, Corrosion Water Hammer and Leaks in Economizer

Economizer is one of heat exchanger equipment in steam boiler that absorbs heat from flue gas then transfers it to feedwater. Economizer can help steam boiler to increase efficiency. Some of failure regarding to operation of economizer can be occur because of erosion, corrosion, water hammer and leaks. So, the preventive maintenance should be performed.

Possibility of corrosion can be occur if there is concentration of oxygen either high or low concentration. Corrosion can be prevented by removing oxygen content in feedwater. Deaerator has important role to make feedwater free from oxygen.

Steam boiler which uses coal as fuel combustion will has a lot of fly ash particle. It will impact to metal tube erosion even steam boiler is operated with high excess air and on maximum continuous rating; possibility of erosion is highly occur. If certain amount of fly ash is localized in certain area then forming plugging, this condition will reduce flow area and increase the velocity of flue gas, so potential erosion on economizer tubes will mostly occur and the result economizer tubes will leak and rupture. Preventive maintenance to clean plug and fly ash attached on surface tube must be executed routinely. Rotary soot blower type is suitable applied to this preventive maintenance in economizer.

Another warning regarding to economizer operation is water hammer. At certain condition, level water gauge can not read water level in steam drum correctly, the result is boiler control allow feedwater is not distributed into steam drum for a while. So steam will flow enter and become trapped in economizer. When feedwater is distributed again through economizer although with low pressure, it will impact to water hammer because of steam is encountered immediately with feedwater on certain pressure. To overcome this condition, venting should be installed to make the steam out from economizer or supply feedwater constantly.

Sulphur Corrosion in Steam Boiler

All fuels (gaseous fuel, liquid fuel, and solid fuel) has comprise some sulphur if it is burned will generate compounds which can cause corrosion. Corrosion must be prevented because it would damage the pressure part in the steam boiler. Sulphur may be separated from the fuels but it would require an expensive process. When in burning process of sulphur, two products of sulphur such as sulphur dioxide (SO₂) and sulphur dioxide (SO₃) will be generated. Water that be produced from combustion of hydrogen will combine with these compounds (SO₂ and SO₃) to generate acids.

At acid dew point, these acids will be condensed between temperature 120^o C and 175^o C depending on the number of sulphur content in the fuels. These acids will be absorbed by some soot deposits contained in stack, economizer, air heater, and other pressure part also in flue gas will be fused and formed acid smut. The water dew point will be achieved when the temperature down to reach 55^o C. and these acids will melt. It is the process which will cause corrosion.

The corrosion can be prevented if the metal temperature in the steam boiler is maintained in high temperature. The flue gas temperature exit of steam boiler is usually maintained at minimum 200^o C to avoid the formation of acid in air heater, economizer, stack and other pressure part also flue gas. For best design, the stack must be insulated to avoid cooling and corrosion. If the steam boiler is shut down, all parts should be cleaned to remove all acids in the steam boiler.