Objective 04: Corrosion Mechanisms in Power Plants Flashcards
1
Q
What 4 factors make boilers susceptible to corrosion?
A
- Water is the process fluid
- High Temperature
- High Pressure
- Materials are under stress
2
Q
Magnetite Layer
A
Very thin, hard, protective layer of iron oxide
3
Q
How is Magnetite Layer formed?
A
Iron on the water side oxidizes and reacts with water on the waterside to form magnetite (iron oxide).
4
Q
What effect does Magnetite have on the metal surface of boilers?
A
It passivates them, preventing futher oxidation
5
Q
How many layers of magnetite are usually present on boiler surfaces?
A
2
6
Q
How does the outer magnetite layer differ from the inner layer?
A
It’s more porous and easily penetrated by water and aggresive ions
7
Q
How thick does the magnetite layer become?
A
0.01 - 0.025 mm
8
Q
What is the necessary pH level needed to maintain magnetite layers?
A
8.5 - 12.7
9
Q
How is periodic weakening or damaging of magnetite layers rectified?
A
Proper internal boiler water treatment
10
Q
What are 3 mechanisms that can deplete the magnetite layer?
A
- Oxidation
- Caustic Corrosion
- Hydrogen Damage
11
Q
What is the largest source of boiler system corrosion?
A
Dissolved gases, e.g. oxygen, carbon dioxide, and ammonia
12
Q
Which dissolved gas is the most aggressive?
A
Oxygen
13
Q
What 3 factors affect the degree of oygen attack?
A
- Concentration of dissolved oygen
- pH
- Water temperature
14
Q
How does oxygen react with the magnetite layer?
A
It reacts to form hematite
15
Q
What is problematic about Hematite?
A
It often appears as a hydrate that can act as both a base and a weak acid to promote pitting
16
Q
What does Caustic Corrosion involve?
A
The direct reation of sodium with the metal in a boiler
17
Q
Where is Caustic Corrosion most often seen?
A
In furnace tubes in regions of high heat fluctuation
18
Q
What is a source of sodium that can ultimately cause Caustic Corrosion in boilers?
A
Sodium Hydroxide is added to the boiler water in non-corrosive concentrations
19
Q
What are 2 conditions that can induce Caustic Corrosion in a boiler?
A
- Steam Blanketing
2. Localized Boiling
20
Q
Describe the process of Steam Blanketing
A
- A steam layer forms between the boiler water and the tube walls
- This causes insufficient water contact with the tube surface
- The water that does reach the tube surface is rapidly boiled away, leaving behind a concentrated and corrosive caustic solution
21
Q
In which boilers does Localized Boiling occur?
A
Boilers that use phosphate-treated water
22
Q
Describe the process of Localized Boiling
A
- In areas of high heat transfer, porous deposits of phosphate may develop on the tube surface
- Water then flows into the deposit and boils beneath it, leaving a concentrated caustic solution
23
Q
Describe the process of Caustic Corrosion
A
- Caustic Soda concentrates until the pH becomes extremely
- Localized attack begins and the magnetite layer is destroyed
- The concentrated caustic sodium hydroxide then reacts with the exposed boiler metal to produce hydrogen, among other things
24
Q
What are 2 ways that Caustic Corrosion is stopped?
A
- Porous deposits are rmeoved
2. Caustic corrosion is reduced to normal
25
How does Caustic Corrosion appear in the metal?
1. Irregular patterns and gouges
2. White salts may appear in the metal sample
3. After an extended period of time, black magnetite iron oxide may appear in low flow area, e.g. mud drum, due to stripping away of the magnetite film
26
Describe the process of Hydrogen Damage
1. The boiler water is contaminated with acid
2. The acid dissolves the magnetite layer and attacks the boiler metal
3. The corrosion produces hydrogen which penetrates the steel
4. The hydrogen reacts with carbon in the steel to form methane
5. The methane is too large to escape and exerts pressure between the metal grains
27
What is the result of Hydrogen Damage on the steel?
The metal is weakened and becomes brittle, thus being suseptible to brittle failures
28
What is problematic about economizers producing steam before feedwater enters it?
When feedwater enters, the economizer may experience thermal quenching. This causes severe shocks and possible stress cracks or fatigue at connections and support brackets
29
What are 4 corrosion mechanisms that can occur in Economizers?
1. Pitting
2. Flow Accelerated Corrosion
3. Corrosion Product Buildup
4. Gas-side Corrosion
30
How can Pitting occur in economizers?
Free oxygen can come from poorly dearated feedwater or from inadequate drainage or storage during boiler shutdown.
31
How can Flow Accelerated Corrosion occur in Economizers?
A combination of excessie dearation and high interface velocity at economizer elbows can cause general thinning of the tubes as iron is dissolved into the feedwater
32
What 4 conditions accelerate Economizer tube thinning?
1. Low pH
2. Excessive oxygen scavenger chemicals
3. Chemicals (e.g. chelants) that increase iron solubility
4. Thermal decomposition of organic material
33
How can Gas-Side Corrosion occur in Economizers?
Contaminants in the flue gas may condense out of the flue gas and produce a low-pH environment
34
What makes Superheaters particularly susceptible to corrosion in general?
They operate at very high temperatures
35
What are 6 ways that Superheaters are susceptible to corrosion?
1. Oxidation
2. Magnetite Layer Sloughing
3. Carryover Depositing
4. Pitting
5. Stress Corrosion Cracking
6. Gas-Side Corrosion
36
How can oxidation occur in Superheaters?
1. At above 500ºC, the steel reacts directly with the steam, forming iron oxide and releasing hydrogen
2. This may lead to fissures and hydrogen blistering
37
How can oxidation in Superheaters be reduced?
Firing rates must be limited during startup so combustion gases at the superheater do not exceed 500ºC until the boiler reaches operating pressure, all superheater tubes are cleared of condensate that may have accumulated duing shutdown, and steam flow through the superheater is confirmed
38
How does Magnetite Layer Sloughing occur in Superheaters?
When the Superheater cools during shutdown, the magnetite layer tends to break down and upon restart, carries over and can enter the steam turbine where it can cause hard particle erosionof the turbine nozzles.
39
What are sources of Carryover Deposits in Superheaters?
1. Water droplets containing boiler chemicals and other contaminants
2. Direct contact spray attemperators
40
How can pitting occur in Superheaters?
Exposure of water to oxygen during downtime, particularly in pendant loops.
41
How can Superheaters have water in them, thus making them susceptible to pitting?
Water can be leftover due to improper drainage of drainable superheaters, low spots in sagging horizontal tube banks, or non-drainable superheater design. Some Superheaters are flooded when the boiler is shut down.
42
Explain the process of Pitting in Superheaters
Some of the soluble deposits carried into the Superheater are dissolved into the water and, when combined with oxygen, form corrosion cells, usually where there is an interface between air and water. This can eventually lead to pitting.
43
Which Superheaters are susceptible to Stress Corrosion?
Superheaters made of austenitic stainless steel
44
How does Stress Corrosion in Superheaters occur?
After long periods of exposure to high temperatures, cyclic stresses, and creep fatigue, the superheater headers may develop internal stress corrosion cracks, particularly in the ligaments between tube holes
45
Which boilers are particularly susceptible to Gas-side corrosion?
Boilers that burn solid fuels
46
Which type of fossil fuel does not promote corrosion on the flue gas side of boiler components?
Natural Gas
47
What are the main fuel impurities that cause fuel side corrosion?
1. Sulphur
2. Vanadium
3. Sodium
48
What additional fuel impurities come from Solid Fuels?
1. Chlorine Gas
| 2. Flyash
49
What are the 3 Temperature Zones in which Fuel Side Corrosion occurs?
1. Furnace (1000-1700ºC)
2. Superheater/Reheater (650-1000ºC)
3. Economizer/Air Heater (<150º)
50
Dewpoint Corrosion
Corrosion due to the cooling of flue gases which causes water vapour to condense and combine with the sulphur products to form sulphurous and sulphuric acids
51
What is the dewpoint at which corrosion occurs?
The exact dewpoint depends on the concentration of the sulphur gases, but it is around 150ºC
52
Are areas of flue gas leak susceptible to corrosion?
Yes
53
How does Flyash create a corrosive environment?
It accumulates on surfaces throughout the fuel gas path and acts as a sponge, collecting moisture, sulphuric acid, and other corrosive elements that originate in the fuel.
54
Explain the corrosion mechanism of Flyash
Flyash deposits on tube surfaces and dissolves the protective magnetite layer, exposing bare metal to the corrosive elements of the deposit
55
How does fuel ash corrosion cause fatigue cracks?
1. Some of the ash deposit melts, creating a thin liquid film between the tube surface and the solid ash deposit
2. The ash layer thickens until the film cannot support the increased mass
3. The ash layer falls off, exposing the tube surface to furnace heat
4. Local heat flux increases, causing a sudden increase in tube temperature and an area of high local stress
5. As the ash layer re-forms, it insulates the tube and reduces the metal temperature and stress
6. This cycling of temperature and stress repeats continuously, causing corrosion fatigue cracks
56
What is the corrosion pattern for Superheaters and Reheaters in Oil-fired boilers?
It depends on the mass and aerodynamics of the flue gas as it flows through the tube banks
57
How do overheated areas in coal fired boilers present?
Thickening of the oxide scale with a series of cracks and grooves in the tube surface.
58
How does fuel ash corrosion present in waterwall tubes?
Their external surfaces may show a series of circumferential grooves or cracks while a cross-section of the tube may show shallow cracks or grooves through the tube wall.
59
What kind of fuel ash corrosion occurs in the burner area?
An erosion pattern may be evident due to unburned coal particles, flame impingement, or both
60
How does fuel ash corrosion present in the furnace of a refuse burner?
Waterwall corrosion appears as smooth, uniform wastage with generalized thinning of the tubes
61
What kind of tube corrosion occurs on condensers with copper alloy tubes?
1. Ammonia Grooving
2. Ammonia Stress Corrosion Cracking
3. Erosion
4. Under-Deposit Corrosion
5. Galvanic Corrosion
62
How does Ammonia Grooving in Copper Alloy Condenser Tubing?
1. When hydrazine is used for oxygen scavenging, it breaks down into ammonia compounds
2. It enters the condenser with the steam and combines with condensate
3. The resulting ammonia solution then attacks the tubes at the center of the condenser
4. The copper removed from the tubes is then carried with the condensate ultimately back to the boiler where it may deposit on boiler tubes
63
How are Copper Alloy Condenser Tubes susceptible to Ammonia Stress Corrosion Cracking?
The ammonia combines with the copper alloy tubes under high residual stress and causes tube leaks
64
How are Copper Alloy Condenser Tubes susceptible to Galvanic Corrosion?
The cooling water acts as an electrolyte between the dissimilar metals of the waterboxes and the tubes
65
What kinds of corrosion are Stainless Steel Condenser Tubes susceptible to?
1. Chloride Stress Corrosion Cracking
2. Crevice Corrosion
3. Pitting
4. Microbiological Corrosion
66
How are Stainless Steel Condenser Tubes susceptible to Chloride Stress Corrosion Cracking?
If it contains nickel then the combination of stress, chlorides, and a temperature of 300oC+ causes Chloride Stress Corrosion Cracking
67
What are the main sources of corrosion in Feedwater Heaters?
Oxygen and improper pH
68
What are 2 kinds of corrosion that Feedwater Heaters are susceptible to?
1. Stress corrosion cracking of welded components
| 2. Erosion in the shell side
69
What are 5 types of corrosion that Deaerators are susceptible to?
1. Flow Accelerated Corrosion
2. Oxygen Pitting
3. Erosion
4. Corrosion Fatigue Cracking
5. Stress Corrosion Cracking
70
How are Deaerators susceptible to Flow Accelerated Corrosion?
In areas where water flow is high, the protective oxide layer may be removed from the surface, exposing it to continual loss of thickness
71
How are Deaerators susceptible to Oxygen Pitting?
Since free oxygen is released inside the deaerators, oxygen pitting may occur around the vent and at any location where oxygen may become trapped
72
How are Deaerators susceptible to Erosion?
High velocity water impinges on metal surfaces such as the inlet baffles at the steam inlet
73
How serious is Corrosion Fatigue Cracking and Stress Corrosion Cracking for Deaerators?
It affects a large percentage of Deaerators and their associated water storage compartments and all designs of deaerators are susceptible to it.
74
When does Cracking start to occur in Deaerators?
After 3+ years in service
75
Where does Cracking occur in Deaerators?
In the water storage compartments, at or below the water level. They begin at welds, corrosion pits, and heat-affected areas, most often at head-to-shell welds, but also on the internal surface of the shell where attachments are welded.
76
What causes Corrosion Fatigue Cracking in Deaerators?
Cyclic stresses due to startups and shutdowns, vibration, flow, and level fluctuations
77
What causes Stress Corrosion Cracking?
Areas of high local stress, if caustic is carried in with the condensate.
78
What are 3 ways that Corrosion be minimized in Deaerators?
1. Maintaining steady operation
2. Ensuring good control of oxygen scavenging chemicals
3. Maintain all internal components in good working order
79
What general examination must be carried out in Deaerators?
Regular magnetic particle testing of internal welds, along with immediate repair of any detected cracks
80
What makes Deaerators susceptible to different types of Corrosion?
1. Design
2. Flow patterns through them
3. Mixing of water, steam, and gases
81
What factors affect the corrosion susceptibility of a particular heat exchanger?
1. Fluids involved
2. Flow rates
3. Temperatures
4. Metals used for exchanger construction
82
What are 7 types of corrosion Heat Exchangers are susceptible to?
1. Crevice Corrosion
2. Galvanic Corrosion
3. Stress Corrosion Cracking
4. Pitting
5. Erosion Corrosion
6. Under-Deposit Corrosion
7. Microbiological Corrosion
83
How are Heat Exchangers susceptible to Crevice Corrosion?
If there is any gap (crevice) between the tubesheet and tube, deposits may collect and initiate corrosion in the crevice
84
How are Heat Exchangers susceptible to Galvanic Corrosion?
To promote heat transfer, often the tube of the heat exchanger are made of a different metal than the shell or the tubesheet
85
How is Galvanic Corrosion minimized in Heat Exchangers?
By using different metals that are as close as possible to each other on the galvanic series
86
How are Heat Exchangers susceptible to Stress Corrosion Cracking?
Stresses are produced due to constant startup and shutdown and the fact that the tube and shell are at different temperatures and thus expand and contract at different rates.
87
How are Heat Exchangers susceptible to Pitting?
If the fluid(s) handled contain entrained gases
88
How are Heat Exchangers susceptible to Erosion Corrosion?
High flow velocity through tubes can cause loss of protective film and of bare metal which may occur if the exchanger flow exceeds design or when some tubes are plugged thus forcing excess flow through the remaining tubes.
89
What areas in Heat Exchanger tubes are particularly susceptible to Erosion Corrosion due to turbulence?
1. Tube ends
| 2. Tube bends
90
How are Heat Exchangers susceptible to Under-deposit and Microbiological Corrosion?
If the Heat Exchanger is operated with the tube or shell flow below acceptable turndown rate, deposits and microbes (if present) may collect
