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Caustic stress corrosion cracking
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in Failure of Boilers and Related Equipment
> Analysis and Prevention of Component and Equipment Failures
Published: 30 August 2021
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Published: 15 January 2021
Fig. 6 Photographs of boiler plates exhibiting caustic stress-corrosion cracking (arrows). Source: Ref 5
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Published: 15 January 2021
Fig. 32 Micrographs of caustic stress-corrosion cracking that developed in austenitic stainless steel piping. The micrographs in (b) correspond to the box in (a). Original magnification: 100×
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Series: ASM Failure Analysis Case Histories
Volume: 2
Publisher: ASM International
Published: 01 December 1993
DOI: 10.31399/asm.fach.v02.c9001323
EISBN: 978-1-62708-215-0
... stainless steels. Conclusion and Recommendations Most Probable Cause The cause of the U-bend failures was caustic gouging for the carbon steel U-bends and caustic-induced stress-corrosion cracking for the stainless steel U-bends. The root cause of the failures is carryover. Remedial Action...
Abstract
Original carbon steel and subsequent replacement austenitic stainless steel superheater tube U-bend failures occurred in a waste heat boiler. The carbon steel tubes had experienced metal wastage in the form of caustic corrosion gouging, while the stainless steel tubes failed by caustic-induced stress-corrosion cracking. Sodium was detected by EDS in the internal deposits and the base of a gouge in a carbon steel tube and in the internal deposits of the stainless steel tube. The sodium probably formed sodium hydroxide with carryover moisture and caused the gouging, which was further aggravated by the presence of silicon and sulfur (silicates and sulfates). It was recommended that the tubes be replaced with Inconel 600 or 601, as a practical option until the carryover problem could be solved.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.chem.c9001407
EISBN: 978-1-62708-220-4
... of the solution of caustic soda and possibly the temperature also were in the range known to produce stress-corrosion cracking of mild steels in the presence of stresses of sufficient magnitude. In this instance the location of the cracking suggested that residual stresses from welding, which approach yield point...
Abstract
The onset of leakage adjacent to two butt welds in a 2 in. bore pipe was traced to the development of fine cracks. The pipe carried 40% sodium hydroxide solution. The actual temperature was not known, but the pipeline was steam traced at a pressure of 30 psi, equivalent to a temperature of 130 deg C (266 deg F). Magnetic crack detection revealed circumferential crack-like indications situated a short distance from the butt weld. Cracking originated on the bore surfaces of the tube and was of an intergranular nature reminiscent of caustic cracking in steam boilers. The strength of the solution of caustic soda and possibly the temperature also were in the range known to produce stress-corrosion cracking of mild steels in the presence of stresses of sufficient magnitude. In this instance the location of the cracking suggested that residual stresses from welding, which approach yield point magnitude, were responsible. As all other welds were suspect, the remedy was to remove the joints and to reweld followed by local stress relief.
Series: ASM Failure Analysis Case Histories
Volume: 2
Publisher: ASM International
Published: 01 December 1993
DOI: 10.31399/asm.fach.v02.c9001324
EISBN: 978-1-62708-215-0
... Abstract A type 430Ti stainless steel flue gas expansion joint cracked because of caustic-induced stress-corrosion cracking. Energy-dispersive X-ray spectroscope analysis of the fracture surface deposits revealed the presence of sodium and potassium—caustics in hydroxide form. Primary fracture...
Abstract
A type 430Ti stainless steel flue gas expansion joint cracked because of caustic-induced stress-corrosion cracking. Energy-dispersive X-ray spectroscope analysis of the fracture surface deposits revealed the presence of sodium and potassium—caustics in hydroxide form. Primary fracture surfaces were all similar in appearance, and a primary crack origin could not be identified. A secondary crack brought to fracture in the laboratory showed brittle, cleavage features rather than classic, tensile overload features. This suggested that the material was embrittled.
Series: ASM Failure Analysis Case Histories
Volume: 2
Publisher: ASM International
Published: 01 December 1993
DOI: 10.31399/asm.fach.v02.c9001367
EISBN: 978-1-62708-215-0
... approach to failure analysis to confirm that the cracking was caused by a caustic stress-corrosion cracking mechanism is presented. Caustic cracking Chemical processing equipment Compressor blades Sodium hydroxide, environment 4320 UNS G43200 Stress-corrosion cracking Background Severe...
Abstract
Cracking was discovered in an in-service, second-stage turbine impeller during a downtime inspection. The fabricated 4300 series low-alloy steel impeller was used in a compressor in an industrial petrochemical plant. It was also reported that a process upset had allowed a 10% NaOH solution to be ingested by the unit. Routine magnetic particle inspection revealed numerous cracks in the hub area and vane tips of the second-stage impeller Additionally, the outside surface of the backing plate showed a cyclic pattern of cracks. An overview of a conventional, systematic metallurgical approach to failure analysis to confirm that the cracking was caused by a caustic stress-corrosion cracking mechanism is presented.
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Published: 15 January 2021
Fig. 3 Plot demonstrating the susceptibility of some austenitic stainless steels to caustic stress-corrosion cracking (SCC) with respect to temperature and caustic concentration. Stress-corrosion cracking has not been observed in these austenitic stainless steels exposed to conditions
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Published: 15 January 2021
Fig. 5 Photographs of a cracked rivet (top) and dislodged rivet heads (bottom) from a boiler that experienced caustic stress-corrosion cracking. Source: Ref 5
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Published: 15 January 2021
Fig. 4 Map of the United States with red circles identifying the locations of wells whose water was linked to caustic stress-corrosion cracking in boilers in the early 20th century. Source: Ref 5
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Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.usage.c9001545
EISBN: 978-1-62708-236-5
.... Caustic, not chloride, stress corrosion was the culprit. Had material substitutions been made on the original premise of countering chloride stress corrosion, most of the loop's highly stressed components would have eventually failed. Bearings Bellows Caustic stress corrosion cracking 304 UNS...
Abstract
The presence of secondary, branching intergranular stress-corrosion cracking in a type 440C stainless bearing caused the analyst to overlook the real culprit, which was a mechanically-initiated, primary transgranular crack that propagated through the steel's hard chromium carbide. Failure was actually caused by overload. Had the original conclusion been accepted, a relatively exotic alloy would have been specified. In another case, brass heat exchanger tube failure was automatically attributed to attack by an acidic cleaner, and a decision was made to stop using the solution. A more thorough analysis showed failure was caused by tube vibration. In a third case, a type 304 stainless steel bellows in a test loop was thought to have failed because of chloride stress corrosion. The report concluded with a recommendation that carbon steel be used as an alternative bellows material. Caustic, not chloride, stress corrosion was the culprit. Had material substitutions been made on the original premise of countering chloride stress corrosion, most of the loop's highly stressed components would have eventually failed.
Series: ASM Handbook
Volume: 11
Publisher: ASM International
Published: 15 January 2021
DOI: 10.31399/asm.hb.v11.a0006785
EISBN: 978-1-62708-295-2
... was encountered in copper alloys, caustic embrittlement was experienced in boilers. (Caustic embrittlement is also known as caustic cracking. Both of these terms are considered archaic; currently, the term caustic stress-corrosion cracking is used to describe the same phenomenon.) Cracking was observed around...
Abstract
Stress-corrosion cracking (SCC) is a form of corrosion and produces wastage in that the stress-corrosion cracks penetrate the cross-sectional thickness of a component over time and deteriorate its mechanical strength. Although there are factors common among the different forms of environmentally induced cracking, this article deals only with SCC of metallic components. It begins by presenting terminology and background of SCC. Then, the general characteristics of SCC and the development of conditions for SCC as well as the stages of SCC are covered. The article provides a brief overview of proposed SCC propagation mechanisms. It discusses the processes involved in diagnosing SCC and the prevention and mitigation of SCC. Several engineering alloys are discussed with respect to their susceptibility to SCC. This includes a description of some of the environmental and metallurgical conditions commonly associated with the development of SCC, although not all, and numerous case studies.
Series: ASM Failure Analysis Case Histories
Volume: 3
Publisher: ASM International
Published: 01 December 2019
DOI: 10.31399/asm.fach.v03.c9001832
EISBN: 978-1-62708-241-9
... of oxygen. Stress-Corrosion Cracking Stress-corrosion cracking (SCC) requires the combination of stresses, specific environment, and materials susceptibility. Ferritic steels are known to be susceptible to SCC in nitrate solutions, caustic solutions, and various other environments.[ 3 ] Nitrate...
Abstract
High-level radioactive wastes generated during the processing of nuclear materials are kept in large underground storage tanks made of low-carbon steel. The wastes consist primarily of concentrated solutions of sodium nitrate and sodium hydroxide. Each of the tanks is equipped with a purge ventilation system designed to continuously remove hydrogen gas and vapors without letting radionuclides escape. Several intergranular cracks were discovered in the vent pipe of one such system. The pipe, made of galvanized steel sheet, connects to an exhaust fan downstream of high-efficiency particulate air filters. The failure analysis investigation concluded that nitrate-induced stress-corrosion cracking was the cause of the failure.
Series: ASM Failure Analysis Case Histories
Volume: 2
Publisher: ASM International
Published: 01 December 1993
DOI: 10.31399/asm.fach.v02.c9001344
EISBN: 978-1-62708-215-0
.... Boiler tubes Caustic cracking Residual stress Welded joints 304H UNS S30409 Stress-corrosion cracking Background Stainless steel superheater tubes fractured in stressed areas within a few hours after caustic regenerant leaked into the boiler feedwater system. Applications In typical...
Abstract
Several 304H stainless steel superheater tubes fractured in stressed areas within hours of a severe caustic upset in the boiler feedwater system. Tests performed on a longitudinal weld joint, which connected two adjacent tubes in the tertiary superheater bank, confirmed caustic-induced stress-corrosion cracking, promoted by the presence of residual welding stresses. Improved maintenance of check valves and routine inspection of critical monitoring systems (conductivity alarms, sodium analyzers, etc.) were recommended to help avoid future occurrences of severe boiler feedwater contamination. Additional recommendations were to eliminate these short longitudinal weld joints by using a bracket assembly joint between the tubes, use a post-weld heat treatment to relieve residual welding stress or select a more stress-corrosion cracking resistant alloy for this particular application.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.power.c9001406
EISBN: 978-1-62708-229-7
... Abstract Caustic cracking is the term used to describe one of the forms in which stress-corrosion cracking manifests itself in carbon steels. In the present study, persistent leakage occurred after ten weeks of service from tube expansions in the steam and mud drum of a two-drum D type boiler...
Abstract
Caustic cracking is the term used to describe one of the forms in which stress-corrosion cracking manifests itself in carbon steels. In the present study, persistent leakage occurred after ten weeks of service from tube expansions in the steam and mud drum of a two-drum D type boiler, which failed to respond to repeated expansion. The leakage was traced to circumferential cracking in the portion of Fe-0.11C-0.46Mn-0.018S-0.011P tubes within the expanded region. Microscopic examination indicated that all cracks started from the outer surface of the tubes in the expanded portion. The form of cracking which was mostly intergranular. Examination at higher magnification disclosed that a selective attack had taken place on the carbide constituents of the pearlite grains. An alkaline deposit on the fireside surface of the tube resulted from the evaporation of boiler water which had found its way past the tube expansions. This indicated that this operation had not resulted in a satisfactorily tight joint.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.design.c0048819
EISBN: 978-1-62708-233-4
... with the nozzle weld. Caustic embrittlement Potassium hydroxide Pressure vessels ASTM A516 Grade 70 UNS K02700 Stress-corrosion cracking A large pressure vessel ( Fig. 1a ) that had been in service for about 10 years as a hydrogen sulfide (H2S) absorber developed cracks and began leaking...
Abstract
A large pressure vessel that had been in service as a hydrogen sulfide (H2S) absorber developed cracks and began leaking at a nozzle. The vessel contained a 20% aqueous solution of potassium hydroxide (KOH), potassium carbonate (K2CO3), and arsenic. The vessel wall was manufactured of ASTM A516, grade 70, low-carbon steel plate. A steel angle had been formed into a ring was continuously welded to the inside wall of the vessel. The groove formed by the junction of the lower tray-support weld and the top part of the weld around the nozzle was found to have a crack. Pits and scale near the crack origin were revealed by microscopic examination and cracking was found to be transgranular. Periods of corrosion alternated with sudden instances of cleavage, under a tensile load, along preferred slip planes were interpreted during examination with a microscope. It was concluded that the combination of the residual plus operating stresses and the amount of KOH present would have caused stress corrosion as a result of caustic embrittlement. It was recommended that the tray support should be installed higher on the vessel wall to prevent coincidence of the lower tray-support weld with the nozzle weld.
Book Chapter
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.modes.c9001405
EISBN: 978-1-62708-234-1
... Intergranular fracture Stress-corrosion cracking During the microscopical examination of a number of cases of caustic cracking, a certain feature has been recognised that appeared to be associated only with caustic cracking, this being a preferential attack on the carbide envelopes and lamellae...
Abstract
During microscopic examination of a number of cases of caustic cracking, a certain feature has been recognized that appeared to be associated only with caustic cracking. This was a preferential attack on the carbide envelopes and lamellae of the pearlite grains. Evidence suggests that the intergranular path of caustic cracks in steam boilers may be due largely to the presence, probably on a submicroscopic scale, of carbides at the grain boundaries, thus rendering these regions susceptible to preferential attack. It is known that some steels are more liable to develop caustic cracks than others, although their microstructures may not show any significant differences, and it seems probable that this behavior may be related to the amount and continuity of the grain-boundary carbides.
Series: ASM Failure Analysis Case Histories
Publisher: ASM International
Published: 01 June 2019
DOI: 10.31399/asm.fach.power.c9001398
EISBN: 978-1-62708-229-7
... was subsequently treated with phosphate. It would appear that concentration of sodium hydroxide occurred in the bellows unit and the stress corrosion cracking which develops was of the form known as caustic cracking. For many years the use of sodium sulphate — either added as such or formed in situ from sodium...
Abstract
Several articulated bellows of 10 in. ID developed leakage from the convolutions after a service life of some 18 months. One of the units received from examination showed cracking at the crown of a convolution and at the attachment weld to the pipe. Sectioning of the bellows revealed many others cracks on the internal surface which did not penetrate to the outside. Microscopical examination showed multiple intergranular, tree-like cracking typical of stress-corrosion cracking. Concentration of sodium hydroxide occurred in the bellows unit and the stress-corrosion cracking which developed was of the form known as caustic cracking. It was recommended that water for de-superheater use should be taken after the deaerator and prior to the addition of salts which may deposit or concentrate in the desuperheater.
Series: ASM Failure Analysis Case Histories
Volume: 3
Publisher: ASM International
Published: 01 December 2019
DOI: 10.31399/asm.fach.v03.c9001775
EISBN: 978-1-62708-241-9
... a caustic solution (NaOH) to remove HCl from the stream, before the latter was directed to a burner. Metallographic examination showed that the cracks were intergranular and were frequently branched. Although nominal chemical composition of the component was found within the specified range, energy...
Abstract
A bent Ni-Cu Monel 400 alloy tube, which operated as part of a pipeline in a petrochemical distillery, failed by through-thickness cracking. The pipeline was used to carry a stream of gaseous hydrocarbons containing hydrochloric acid (HCl) into a reaction tower. The tower provided a caustic solution (NaOH) to remove HCl from the stream, before the latter was directed to a burner. Metallographic examination showed that the cracks were intergranular and were frequently branched. Although nominal chemical composition of the component was found within the specified range, energy dispersive x-ray analysis (EDXA) indicated significant segregation of sulfur and chlorine along the grain boundaries. Failure was attributed to hypochlorous-acid (HClO)-induced stress-corrosion cracking (SCC). The HClO was formed by the reaction of HCl with atmospheric O 2 that entered the tube during shutdowns and startups. Residual stresses, originating from in situ bend forming of the tube during assembly of the line, provided a driving force for crack growth, and the segregation of sulfur on grain boundaries made the material more susceptible to cracking.
Series: ASM Failure Analysis Case Histories
Volume: 2
Publisher: ASM International
Published: 01 December 1993
DOI: 10.31399/asm.fach.v02.c9001354
EISBN: 978-1-62708-215-0
... of sodium on crack surfaces, which is indicative of NaOH-induced stress-corrosion cracking. Failure was attributed to superheater problems that resulted in caustic carryover from the boiler. Two options for disk repair, installing a shrink-fit disk or applying weld buildup, were recommended. Weld repair...
Abstract
An A-470 steel rotor disk was removed from the high-pressure portion of a steam turbine-powered compressor after nondestructive testing revealed cracks in the shoulder of the disk during a scheduled outage. Samples containing cracks were examined using various methods. Multiple cracks, primarily intergranular were found on the inlet and outlet faces along prior-austenite grain boundaries. The cracks initiated at the surface and propagated inward. Multiple crack branching was observed. Many of the cracks were filled with iron oxide. X-ray photoelectron spectroscopy indicated the presence of sodium on crack surfaces, which is indicative of NaOH-induced stress-corrosion cracking. Failure was attributed to superheater problems that resulted in caustic carryover from the boiler. Two options for disk repair, installing a shrink-fit disk or applying weld buildup, were recommended. Weld repair was chosen, and the rotor was returned to service; it has performed for more than 1 year without further incident.
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