Some Adverse Effects of Stress Corrosion in Steam Turbines

1977 ◽  
Vol 99 (2) ◽  
pp. 255-260
Author(s):  
R. E. Sperry ◽  
S. Toney ◽  
D. J. Shade
Keyword(s):  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
Field Experiences ◽  
Corrosion Cracking ◽  
Material Behavior ◽  
Corrosion Mechanism ◽  
Steam Turbines ◽  
Critical Flaw ◽  
Size Growth

The performance and reliability of a steam turbine can be seriously impaired by the admission of steam of unacceptable quality. Of particular concern are contaminants such as caustic, chlorides, and sulfides which can promote the occurrence of corrosion attack and stress corrosion cracking. Critical flaw size growth from cracks initiated by the stress corrosion mechanism can result in brittle-type failure. The importance of stress corrosion as one of the failure mechanisms responsible for turbine outages indicates the need for some understanding of the essential aspects of this phenomenon. The metallurgical aspects of stress corrosion cracking, field experiences with turbine components in aggressive steam environments and results of a study of turbine-material behavior in caustic and sulfide environments are discussed. Field experiences discussed are limited to fossil-fueled industrial and small utility turbines. To further emphasize the serious consequences of stress corrosion failures, typical turbine outage times based on our experience are discussed.

scholarly journals Hydrogen-Induced Stress Corrosion Cracking of Pipe Lines of Russia

1996 ◽  
Author(s):  
Tatyana K. Sergeyeva ◽  
Igor A. Tychkin ◽  
Gennady G. Vasiliev
Keyword(s):  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
Large Diameter ◽  
Corrosion Cracking ◽  
Slow Strain Rate Test ◽  
Corrosion Mechanism ◽  
Hydrogen Distribution ◽  
Induced Stress ◽  
Rate Test ◽  
Large Diameter Pipes

The results of expert studies of large diameter pipes damaged due to external stress corrosion cracking are presented in the report. These data obtained in the 1993–1995 are typical for various regions of Russia. The results of laboratory studies of the stress-corrosion mechanism for pipe steels in suspensions of soils from the places where the operating failure had occurred are given in the report also. The mechanism of hydrogen-induced stress-corrosion cracking (HISCC) realizing through local hydrogenation of steel during plastic deformation has been determined by means of the technique of slow strain rate test (SSRT) of samples in the soil under cathodic, anodic and free corrosion potentials in combination with hydrogenation and hydrogen distribution analyses along length of a sample. No hydrogenation of volumes of pipes non-subjected to cracking was observed but hydrogenation took place in the zones subjected to stress corrosion.

Stress Corrosion Cracking in Steam Turbine: Two Case Studies

2017 ◽  
Author(s):  
Vamadevan Gowreesan ◽  
Kirill Grebinnyk
Keyword(s):  
Electron Microscope ◽  
Fracture Surface ◽  
Scanning Electron Microscope ◽  
Stress Corrosion ◽  
Steam Turbine ◽  
Stress Corrosion Cracking ◽  
Corrosion Cracking ◽  
Steam Turbines ◽  
Root Section ◽  
Scanning Electron

Stress corrosion cracking in steam turbines had been an old problem though some modern steam turbines have almost eliminated this problem by several methods. The methods include design modification to reduce the stress levels below the threshold stress level for stress corrosion cracking, inducing compressive stress by different means and using pure steam [1, 2]. Some of the older steam turbine discs are prone to stress corrosion cracking. Two cases where such machines experienced stress corrosion cracking in their discs are discussed here. The row 6 disc of an integral steam turbine rotor developed cracks in the root sections. Some of the cracks were mechanically opened for the evaluation. Evaluation of the fracture surfaces with a scanning electron microscope showed evidence of intergranular mode of cracking. Optical microscopy of a cracked root confirmed intergranular mode of cracking. In addition, it showed branching of cracks. Based on these findings, it was concluded that stress corrosion cracking was the reason for the cracks. In addition, finite element analysis was used to calculate the stress distribution in the blade root of the disc. The location of the maximum equivalent stress coincided perfectly with that of the actual crack location in the disc root section. Unfortunately, redesign of the root geometry to minimize the local stress concentration is very difficult due to the size limitation of the blade roots. Small amount of chlorine was identified on the fracture surface and the chlorine could have come from the steam used. The customer was advised to analyze their steam quality and to improve the quality of the steam if needed. The cracked portion was removed from the disc and weld-build up to machine new root sections with the same type of roots. Root section of the row 6 disc of another steam turbine developed failure. This disc had radial entry type blades. Portion of the disc root and some blades were liberated from the disc due to the cracking. The fracture surface had heavy oxide layer on it. Evaluation of the fracture surface with a scanning electron microscope revealed intergranular mode of failure. Energy dispersive spectroscopy analysis of the fracture surface found oxides on the fracture surface. Optical microscopy showed secondary cracking and branched cracking. All these evidences confirmed that the failure occurred due to stress corrosion cracking. In addition, it was suspected that forging was not heat treated properly due to measured lower toughness and different microstructure. The lower toughness was believed to be a result of improper heat treatment rather than that of embrittlement. Methods to mitigate the risk of stress corrosion cracking were proposed.

Stress Corrosion Cracking and Corrosion Fatigue Analysis of Nuclear Steam Turbine Rotor

2014 ◽  
Author(s):  
Gang Chen ◽  
Puning Jiang ◽  
Xingzhu Ye ◽  
Junhui Zhang ◽  
Yifeng Hu ◽  
...  
Keyword(s):  
Stress Corrosion ◽  
Steam Turbine ◽  
Corrosion Fatigue ◽  
Stress Corrosion Cracking ◽  
Nuclear Power ◽  
Power Plants ◽  
Fatigue Cracking ◽  
Corrosion Cracking ◽  
Major Influence ◽  
Steam Turbines

Although stress corrosion cracking (SCC) and corrosion fatigue cracking can occur in many locations of nuclear steam turbines, most of them initiate at low pressure disc rim, rotor groove and keyway of the shrunk-on disc. For nuclear steam turbine components, long life endurance and high availability are very important factors in the operation. Usually nuclear power plants operating more than sixty years are susceptible to this failure mechanism. If SCC or corrosion fatigue happens, especially in rotor groove or keyway, it has a major influence on nuclear steam turbine life. In this paper, established methods for the SCC and corrosion fatigue-controlled life prediction of steam turbine components were applied to evaluating a new shrunk-on disc that had suffered local keyway surface damage during manufacture and loss of residual compressive stress.

Role of Mexican Clay Soils on Corrosiveness and Stress Corrosion Cracking of Low-Carbon Pipeline Steels: A Case Study

CORROSION ◽  
10.5006/3515 ◽  
2020 ◽  
Vol 76 (10) ◽  
pp. 967-984
Author(s):  
A. Contreras ◽  
L.M. Quej ◽  
H.B. Liu ◽  
J.L. Alamilla ◽  
E. Sosa
Keyword(s):  
Corrosion Rate ◽  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
Corrosion Cracking ◽  
The State ◽  
Clay Soils ◽  
Corrosion Mechanism ◽  
General Corrosion ◽  
Southern Mexico ◽  
Gauge Section

This work analyzed the physicochemical effect of different types of Mexican clay soils on corrosion and stress corrosion cracking (SCC) behavior in contact with X60 and X65 steels. Four soils were obtained from the right of way land in southern Mexico at 1.5 m depth close to pipelines. Two soils were from the state of Oaxaca (SO1 and SO2), and two others from the state of Veracruz (SV1 and SV2). Physicochemical and textural analysis of soils was performed and correlated to SCC susceptibility and corrosion mechanism. It was observed that soil texture might be related to corrosivity. A texture index (ratio between sand and silt + clay), which was seen to have a relationship with the corrosive tendency of soils, was estimated. It showed that soil with a higher index (SV1) has a higher corrosion rate. Electrochemical impedance spectroscopy and polarization curves were performed and correlated to the corrosion rate and the SCC susceptibility of steels. Steels exposed to SV1 soil exhibited a higher corrosion rate related to a higher content of chlorides and acid pH than those seen in other soils, which resulted in the pitting of such steels. Two types of corrosion were observed. Soils from SV1 and SV2 generated pitting, and soils from SO1 and SO2 produced general corrosion. Inclusions caused pitting in the gauge section of X60 and X65 steels exposed to SV1 soil by anodic dissolution. Galvanic coupling between inclusions and the base metal and dissolution of the inclusions might have enhanced the nucleation of pits at these sites. SCC susceptibility was evaluated using slow strain rate tests (SSRT). After SSRT, the fracture surfaces were analyzed through scanning electron microscopy. The SCC index obtained from SSRT indicates that X60 and X65 steels exhibited good resistance to SCC. A highly corrosive soil, such as SV1, causes the formation of pits instead of cracks, which is attributed to the dissolution process; however, lower SCC indexes were obtained for this system. The higher corrosion resistance of X60 steel is related to a more homogenous microstructure and a higher content of elements, such as Ni and Cr, than those of X65 steel that decrease the corrosion rate.

Nano-Scale Characterization of Stress Corrosion Cracking in a Failed Alloy C-276 Component

CORROSION ◽  
10.5006/3742 ◽  
2021 ◽  
Author(s):  
Des Williams ◽  
Jared Smith ◽  
Kevin Daub ◽  
Matthew Topping ◽  
Fei Long ◽  
...  
Keyword(s):  
Electron Microscopy ◽  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
Ion Beam ◽  
Microstructural Analysis ◽  
Chemical Characterization ◽  
Corrosion Cracking ◽  
Corrosion Mechanism ◽  
Characterization Methods ◽  
Cracking Mode

A failure analysis was performed on an alloy C-276 pull rod which underwent unexpected brittle, intergranular fracture after exposure to 280°C-300°C aqueous solutions designed to replicate secondary side environments in nuclear energy systems: Pb-containing alkaline (pH300°C 8.5-9.5), and sulfate-containing acidic solutions (pH280°C 3-5). The component was characterized using advanced electron microscopy methods to demonstrate the benefits of these techniques for determining the nanoscale chemical, mechanical, and material factors contributing to failure, and to provide insight into the mechanisms of stress corrosion cracking (SCC) responsible for failure. Site-specific transmission electron microscopy specimens containing crack tips were prepared using focused ion beam. Nanoscale chemical characterization methods revealed that Pb was present in some oxidized regions of cracks, suggesting that the element may be inhibiting or impairing the passivity of the Cr-rich oxide. Complementary nanoscale microstructural analysis was performed. At an intergranular to transgranular cracking mode transition, it was observed that the transgranular crack (and corrosion process) propagated along the (110) crystallographic plane. Also, the cracking mode was highly dependent on the tensile stress direction relative to grain boundary orientation, the crystallographic orientation of grains and geometrically necessary dislocation structures. A comparison of results with proposed mechanisms for SCC of Ni alloys in similar environments are discussed; the highly directional nature of cracking is consistent with a slot-tunnel corrosion mechanism.

Stress-Corrosion-Cracking Characterization Procedures and Interpretations to Failure—Safe Use of Titanium Alloys

1969 ◽  
Vol 91 (4) ◽  
pp. 614-617 ◽  
Author(s):  
R. W. Judy ◽  
R. J. Goode
Keyword(s):  
Titanium Alloys ◽  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
Corrosion Cracking ◽  
Flaw Size ◽  
General Level ◽  
Critical Crack ◽  
Dual Purpose ◽  
Critical Flaw ◽  
Failure Conditions

Ratio analysis diagram (RAD) interpretive procedures have been evolved recently to provide generalized engineering solutions for fracture toughness assessments of structural titanium alloys. Failure-safe design also requires consideration of possible sub-critical crack propagation (slow fracture) due to stress-corrosion cracking (SCC). Procedures for incorporation of SCC characterizations into the RAD system have now been developed. These procedures serve the dual purpose of providing simplified interpretations of critical flaw size-stress instability conditions by consideration of resistance of the material to both fast fracture and SCC. The failure conditions are expressed in terms of KI/σys ratios which provide an index of the general level of critical flaw sizes. The combination RAD also features limit lines that indicate: (a) the highest level of KIscc/σys ratios for which accurate plane strain interpretation to flaw size-stress conditions for SCC can be made for 1-in-thick plate, and (b) the highest level of SCC resistance measured in extensive surveys of plate material of this thickness.

Exfoliation Corrosion Mechanism: Interplay between Intergranular Corrosion and Stress Corrosion Cracking during Exfoliation Corrosion of AA7449

2006 ◽  
Vol 519-521 ◽  
pp. 693-698 ◽  
Author(s):  
C. Hénon ◽  
G. Pouget ◽  
Timothy Warner
Keyword(s):  
Crack Propagation ◽  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
Intergranular Corrosion ◽  
Corrosion Cracking ◽  
Corrosion Mechanism ◽  
Thermal Treatments ◽  
Quench Rate ◽  
Exfoliation Corrosion ◽  
Slow Quench

It is generally considered that exfoliation corrosion is due to the build-up of corrosion products that create a wedging stress that lifts up the surface grains. However, the exfoliation mechanism is still under discussion: possible operating mechanisms include intergranular corrosion of in plane grain boundaries accelerated by the wedging effect, or crack propagation by a “purely” stress corrosion mechanism. The sensitivity to exfoliation corrosion of AA7449 in relation to the intergranular and stress corrosion cracking sensitivity has been addressed in a program of controlled quenches followed by thermal treatments. Our observations demonstrate that the quench rate has a strong effect on intergranular corrosion and exfoliation corrosion sensitivity and in a lesser extent on stress corrosion cracking. In the first moments of the EXCO test, the initiation of corrosion follows the same trends as those revealed by the ASTM G110 test. We observe intergranular initiation for the slow quench rate (~5°C/s) and pitting initiation for samples quenched between 50 to 500°C/s. On the contrary, the final EXCO corrosion quotations do not seem to correlate with the intergranular resistance but rather with SCC resistance.

The Effect of Water Chemistry on the Reliability of Modern Large Steam Turbines

1979 ◽  
Vol 101 (3) ◽  
pp. 477-482 ◽  
Author(s):  
B. W. Bussert ◽  
R. M. Curran ◽  
G. C. Gould
Keyword(s):  
Water Chemistry ◽  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
Power Plants ◽  
Corrosion Damage ◽  
Corrosion Cracking ◽  
Steam Turbines ◽  
Steam Generators ◽  
Drum Boiler ◽  
Effect Of Water

The results of a survey of 50 once-through steam generators and 60 drum boiler steam supplies for modern large steam turbines are presented in this paper. A questionnaire was circulated and the information gathered was used to assess current industry practices related to feedwater treatment, boilerwater chemistry and steam purity measurements. Further information was gained by visiting 28 of the surveyed power plants and developing more information concerning their water chemistry practices. The results of the survey and visitation information are then related to individual cases of major corrosion damage that have occurred in the turbines of the participating utilities: The once-through steam generators provide the best measure of steam purity because, in general, the steam purity is equal to the feedwater purity. Major corrosion damage, in many instances stress corrosion cracking, appears to be related to upset conditions of steam purity and water chemistry.

Modeling Damage of the Hydrogen Enhanced Localized Plasticity in Stress Corrosion Cracking

2010 ◽  
Vol 20 (6) ◽  
pp. 831-844 ◽  
Author(s):  
S. Benbelaid ◽  
M.A. Belouchrani ◽  
Y. Assoul ◽  
B. Bezzazi
Keyword(s):  
Hydrogen Embrittlement ◽  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
High Performance ◽  
Nuclear Reactor ◽  
Corrosion Cracking ◽  
Corrosion Mechanism ◽  
Structural Metals ◽  
Material Solution ◽  
Viable Mechanism

Stress corrosion cracking is an important and complex mode of failure in high-performance structural metals operating in deleterious environments, due to metallurgical, mechanical, and electrochemical factors. Depending on the material/solution system, the stress corrosion cracking mechanism may involve a combination of hydrogen embrittlement (HE) and anodic dissolution. In this article, a numerical model for predicting the mechanical behavior of hydrogen-induced damage in stress corrosion cracking is described. The methodology of modeling used in this study is based on the thermodynamics of continuum solids and elastoplastic damage. This model is based on a stress corrosion mechanism that occurs through the simultaneous interaction of hydrogen and plasticity. This mechanism is also called hydrogen-enhanced localized plasticity, which is a viable mechanism for hydrogen embrittlement. The model is applied to the fatigue damage problems of nuclear reactor pipe, and the results are compared with published fatigue life data obtained experimentally.

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