Objective 2: Causes & Effects of Boiler System Corrosion Flashcards
Corrosion
Loss of metal (specifically iron) due to chemical interaction between the metal, the water, and certain impurities in the water.
What are 2 undesirable effects of Corrosion?
- Thins and weakens the boiler metal to the point of rupture
- Redeposition of iron as scale onto metal surfaces in hotter areas of the boiler
What are the 2 main areas of concern vis-a-vis Corrosion in a Boiler System?
- Inside the boiler itself
- In the Condensate system which can weaken the condensate piping and then cause iron to be transported into the boiler where it can become scale
What is a desirable corrosion activity in boilers?
Formation of a protective magnetite layer on the boiler surfaces
How is Boiler Magnetite Layer formed?
Natural corrosion activity between water and mild steel takes iron from the metal and creates a very thin magnetite layer which becomes a protective layer against other corrosion and can reach a maximum thickness of about 2.7 mm, at which point water can no longer react with the boiler metal.
Why is a Magnetite Layer vulnerable?
It is composed of iron and therefore susceptible to corrosion from other sources.
What are the 4 Proponents of Corrosion?
- Low boiler water pH (Acidic)
- High boiler water pH (Caustic)
- Oxygen corrosion
- Carbon Dioxide corrosion
What range of pH should boiler water be kept within to avoid corrosion?
8.5 - 12.7
What pH conditions promote Acidic Corrosion?
<8.5
What occurs when a boiler is run continuously at low pH?
- Loss of the magnetite layer
- General thinning of the metal
- Localized thinning in areas of high stress
What are 2 causes of low pH in boiler water?
- The feedwater (particularly returning condensate) becomes contaminated with process substances of low pH
- Improper operation of pretreatment demineralizers, which use acid as a regenerant
What should be monitored for pH to prevent low pH from occurring in the boiler water?
- Make-up water
- Condensate returns
- Boiler water
What should be done if a solution has persistently low pH?
Injection of a caustic
What pH conditions promote Caustic Corrosion?
12.5
What form does Caustic Corrosion take?
Deep, irregular, localized gouges in the metal surface.
What are 2 causes of Caustic Corrosion in high-pressure boilers?
- Caustic, eg. NaOH, entering with the treated water from a demineralization system which can cause widespread corrosion
- Accumulation of caustic underneath scale or iron deposits which occurs when caustic-containing water flows into these voids and boil, leaving the caustic. The caustic dissolves the magnetite layer, allowing water to form another layer, consuming metal to do so
At which pressure is Caustic Corrosion a problem?
High-Pressure boilers
What should be done in low-pressure boilers to combat Caustic Corrosion?
A good scale prevention program to eliminate the possibility of caustic scale accumulating
What should be done in High-Pressure Boilers to prevent Caustic Corrosion?
Scale Prevention Program and Phosphate Program
What are 3 Phosphate Programs that can be used to combat Caustic Corrosion?
- Coordinated Phosphate
- Congruent Phosphate
- Equilibrium Phosphate
Coordinated Phosphate Program
Ratio of Na ions to PO 4 ions is kept below 3:1 so that the formation of trisodium phosphate is avoided in order to keep pH at a level below which free OH ions will exist in water that could form NaOH.
Congruent Phosphate Program
Used in boilers above 6900 kPa, a ratio of between 2.15:1 to 2.85:1 of sodium to phosphate is maintained in order primarily to mitigate under-deposit corrosion.
What is the advantage of the Congruent Phosphate Program over the Coordinated Phosphate Program?
It further reduces the risk of free caustic in the boiler water.
Hideout of Phosphate
A serious problem in Congruent Phosphate Program where larger boilers with tube areas subjected to high heat and perhaps reduced circulation can cause some phosphate to precipitate out of the water and “hide out” on the tube surfaces. During load changes, this phosphate may go back into solution and cause spikes of NaOH.
How closely is the phosphate level maintained in Coordinated and Congruent Phosphate Programs?
There is a tendency to inject more phosphate chemical then necessary to maintain a residual amount. It is difficult to maintain steady conditions when boiler load changes.
Equilibrium Phosphate Program
Predetermined amount of trisodium phosphate and small amounts of OH are added to maintain boiler water pH between 9.0-9.7 in order primarily to control phosphate hideout.
What is the primary purpose of injecting phosphate in low-pressure boilers?
Scale control
What is the primary purpose of injecting phosphate in high-pressure boilers that used demineralized water?
pH control
Caustic Embrittlement
Caustic in the water accumulates at a boiler leak, in an area of high metal stress. The leak creates a build-up of caustic, which reacts with the highly stressed metal, changing the crystalline structure of the metal and making it brittle, which can lead to sudden rupture or cracking.
What 3 conditions must exist for Caustic Embrittlement to occur?
- Mechanism to concentrate the boiler water must be present
- Boiler metal must be under high stress
- Boiler water must have an embrittling characteristic, e.g. contain high concentration of NaOH
What are some embrittlement locations?
- Rivets
- Tube Sheets
- Any boiler part subject to stress and leakage
- External leaks like gauge glasses and columns
What is the treatment to prevent Caustic Embrittlement in high-pressure boilers?
Keep the water relatively free of caustic using a Phosphate Program
What is the treatment to prevent Caustic Embrittlement in low-pressure boilers?
Inject sodium nitrate to combat embrittlement
Oxygen Corrosion
When oxygen is present in hot water, even in very small concentrations, it is extremely corrosive to any surrounding metal
What is the maximum acceptable concentration of oxygen in boiler water?
5-7 ppb
How can oxygen be introduced in a boiler system?
- Make-up water
- Leakage into condensate return pumps
- Leakage of cooling or sealing water on transfer pumps
What would happen if oxygen was allowed to get past the pretreatment deaeration equipment and reach the boiler?
Much of it will be released with the steam but some will directly attack the boiler metal. The oxygen that exits with the steam will become very corrosive to the condensate system or anywhere that the steam condenses.
What are the most vulnerable parts of the boiler system to Oxygen Corrosion?
- Economizer tubes
- Feedwater injection pipe
- Steam drum
- Downcomers
What form does Oxygen Corrosion take?
Small but potentially deep pits in the metal which can be randomly dispersed over a very localized area or a wide area. Oxygen pitting can progress very rapidly and result in rapid failure of the metal.
How is Oxygen Corrosion prevented?
Addition of chemical upstream of the boiler water, e.g. the deaerator or suction of feedwater pumps, which will react with the free oxygen and eliminate it from the water.
What are 2 Chemicals commonly used to prevent Oxygen Corrosion?
- Sodium Sulphite
2. Other Oxygen Scavengers
How does Sodium Sulphite prevent Oxygen Corrosion?
It reacts with oxygen to produce sodium sulphate, which can be removed by blowdown. Sodium sulphite is continuously fed to the feedwater in quantities to maintain a sufficient residual like 10-15 ppm.
What are the 2 disadvantagesof the Sodium Sulphite Program?
- Formation of Sulphate which necessitates additional boiler blowdown
- In boilers over 6000 kPa and a sulphite residual of greater than 15 ppm, the sulphate can further break down into sulphur dioxide and hydrogen sulphide that will carry over with the steam and contribute to condensate return line corrosion.
Oxygen Scavengers
Volatile chemicals that partially vaporize due to heat used to control oxygen
What are 2 advantages of using Oxygen Scavengers?
- Vaporized chemical carries over with the steam and provides some corrosion protection for condensate return lines
- Does not add to the dissolved solids in the boiler
Why is hydrazine no longer used as an Oxygen Scavenger?
Its toxic properties
Carbon Dioxide Corrosion
Carbon dioxide is formed inside a boiler when the carbonate and bicarbonate impurities in the water decompose and carry out with the steam as carbon dioxide. When the steam condenses in the external condensate system the CO2 combines with condensate to produce carbonic acid.
What form does Carbon Dioxide Corrosion take?
Generalized corrosion pattern, identified by grooves in the metal.
What are the 2 methods of preventing Carbon Dioxide Corrosion?
- Neutralizing Amines
2. Filming Amines
Neutralizing Amines
Amine-based volatile chemicals that have the ability to react with and neutralize acidity of carbonic acid
What are the most common amines used to neutralize the acidity of carbonic acid?
- Morpholine
- Cyclohexylamine
- Diethylaminoethanol (DEAE)
- Dimethylpropanolamine (DMPA)
How is the amount of Neutralizing Amine to add to a boiler managed?
The amount of chemical injected into the boiler water is adjusted to raise and maintain the pH of the returning condensate above 8.0 as this indicates that sufficient neutralization is occurring.
Are Neutralizing Amines effective against Oxygen Corrosion in return lines?
No
What can make the use of Neutralizing Amines expensive?
If the boiler make-up water has a high ‘M’ alkalinity (carbonates and bicarbonates)
Why is the use of Ammonia as a Neutralizing Amine restricted?
It is corrosive to copper and zinc in condensate systems.
Filming Amines
Amine-based volatile chemicals that provide corrosion protection by establishing a continuous, physical barrier that consists of a thin chemical film that bonds to the metal surface between the metal and the corrosive condensate
What are some common Filming Amines?
- Octadecylamine
- Hexydecylamine
- Dioctadecylamine
Do Filming Amines protect against Oxygen Corrosion?
Yes
How can Filming Amines be applied?
They can be injected at a fixed feed rate, independent go the amount of ‘M’ alkalinity in the feedwater.
How does the expense of Filming Amines compare to Neutralizing Amines?
Fixed feed rate makes a filming program generally less expensive than a neutralizing program
What strategy is commonly applied to combat Carbon Dioxide Corrosion?
A combination of both neutralizing and filming amines to provide dual protection against corrosion either as separate chemicals or a blend of amines into one chemical.
