Steam turbin LEFT
Feed water managementCorrosionDeposits Testing Treatments Phosphate treatment Feed Heating
(A) Corrosion:-Corrosion found in the Boiler and feed systems
Corrosion and tube failure caused by water chemistry
Metals obtained from their oxide ores will tend to revert to that state. However , if on exposure to oxygen the oxide layer is stable , no further oxidation will occur. If it is porous or unstable then no protection is afforded.
Iron+O2 --- magnetite(stable and protective) + O2----ferrous oxide (porous) Two principle types of corrosion
Direct chemical-higher temperature metal comes into contact with air or other gasses (oxidation, Sulphurisation ) Electrochemical-e.g. Galvanic action , hydrogen evolution , oxygen absorption Hydrogen Evolution (low pH attack)
Valency = No of electrons required to fill outer shell
Pure water contains equal amounts of hydrogen and hydroxyl ions . Impurities change the balance . Acidic water has an excess of hydrogen ions which leads to hydrogen evolution
For hydrogen absorption to occur no oxygen needs to be present, a pH of less than 6.5 and so an excess of free hydrogen ions is required.
The Protective film of hydrogen gas on the cathodic surface breaks down as the hydrogen combines and bubbles off as diatomic hydrogen gas.
<A name=oxy>Oxygen Absorption(high O2 corrosion)
pH between 6- 10, Oxygen present. Leads to pitting. Very troublesome and can be due to ineffective feed treatment prevalent in idle boilers. Once started this type of corrosion cannot be stopped until the rust scab is removed , either by mechanical means or by acid cleaning. One special type is called deposit attack, the area under a deposit being deprived of oxygen become anodic. More common in horizontal than vertical tubing and often associated with condensers. Boiler corrosion
General Wastage Common in boilers having an open feed system. . . Pitting -Most serious form of corrosion on the waterside -Often found in boiler shell at w.l. -Usually due to poor shape -In HP blrs found also in screen and generating tubes and in suphtr tubes after priming. <A name=cfc>Corrosion fatigue cracking
Cases found in water tube blrs where due to alternating cyclic stresses set up in tube material leading to a series of fine cracks in wall. Corrosive environment aggravates. Trans crystalline
more in depth: Occurs in any location where cyclic stressing of sufficient magnitude are present
Rapid start up and shut down can greatly increase susceptibility.
Common in wall and supht tubes, end of the membrane on waterwall tubes, economisers, deaerators . Also common on areas of rigid constraint such as connections to inlet and outlet headers
Other possible locations and causes are in grooves along partially full boiler tubes (cracks normally lie at right angle to groove ), at points of intermittent stm blanketing within generating tubes, at oxygen pits in waterline or feed water lines, in welds at slag pockets or points of incomplete fusion , in sootblower lines where vibration stresses are developed , and in blowdown lines.
<A name=cc>Caustic cracking (embrittlement) or stress corrosion cracking
Pure iron grains bound by cementite ( iron carbide).
Occurs when a specific corrodent and sufficient tensile stress exists
Due to improved water treatment caustic stress- Corrosion cracking ( or caustic embrittlement ) has all but been eliminated.
It can however be found in water tubes , suphtr and reheat tubes and in stressed components of the water drum.
The required stress may be applied ( e.g. thermal, bending etc. ) or residual ( e.g. welding)
Boiler steel is sensitive to Na OH , stainless steel is sensitive to NaOH and chlorides
A large scale attack on the material is not normal and indeed uncommon. The combination of NaOH , some soluble silica and a tensile stress is all that is required to form the characteristic intergranular cracks in carbon steel.
Concentrations of the corrodent may build up in a similar way to those caustic corrosion i.e.
- Evaporation at water line[li]And also by small leakage
[/li]Caustic corrosion at temperatures less than 149oC are rare
NaOH concentration may be as low as 5% but increased susceptibility occurs in the range 20- 40 %
Failure is of the thick walled type regardless of ductility.
Whitish highly alkaline deposits or sparkling magnetite may indicate a corrosion sight.
To eliminate this problem either the stresses can be removed or the corrodent. The stresses may be hoop stress( temp', pressure) which cannot be avoided bending or residual weld stresses which must be removed in the design/ manufacturing stage.
Avoidance of the concentrations of the corrodents is generally the most successful. Avoid DNB , avoid undue deposits prevent leakage of corrodents, prevent carryover. Proper water treatment is essential.