Flashcards in Objective 2: Causes & Effects of Boiler System Corrosion Deck (61)
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?
1. Thins and weakens the boiler metal to the point of rupture
2. 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?
1. Inside the boiler itself
2. 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?
1. Low boiler water pH (Acidic)
2. High boiler water pH (Caustic)
3. Oxygen corrosion
4. 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?
What occurs when a boiler is run continuously at low pH?
1. Loss of the magnetite layer
2. General thinning of the metal
3. Localized thinning in areas of high stress
What are 2 causes of low pH in boiler water?
1. The feedwater (particularly returning condensate) becomes contaminated with process substances of low pH
2. 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?
1. Make-up water
2. Condensate returns
3. Boiler water
What should be done if a solution has persistently low pH?
Injection of a caustic
What pH conditions promote Caustic Corrosion?
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?
1. Caustic, eg. NaOH, entering with the treated water from a demineralization system which can cause widespread corrosion
2. 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?
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?
1. Coordinated Phosphate
2. Congruent Phosphate
3. 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?
What is the primary purpose of injecting phosphate in high-pressure boilers that used demineralized water?
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.