initiation and growth of corrosion pit and its effect on

Initiation and Growth of Corrosion Pit and Its Effect on Corrosion Fatigue Strength in 12Cr Stainless Steel

by Makoto HAYASHI*, Kazuo AMANO** and Yoshiharu UEYAMA**

The structural components used in power generating plants sometimes fail due to the corrosion fatigue. The corrosion

fatigue cracks often initiate from the pits. In order to prevent the corrosion fatigue failure, 12Cr stainless steel is employed for the components used in the corrosive environments. Even if employing 12Cr stainless steel, the components failed by the fatigue and the corrosion pits were observed at the origins of fatigue cracks. In this study, the effect of surface finishing on the nucleation of corrosion pits was investigated using 12Cr stainless steel. The corrosive environment is pure water with chlorine ion concentration. The initiation and growth of the corrosion pits are remarkably affected by the chlorine ion concentration. So-called “One-third rule” was obtained and the aspect ratio, that means the ratio of pit depth to length on the surface, was estimated as about 1/4. The surface finishing affected the nucleation and growth of corrosion pits. Especially they are reduced when the surface was finished to the mirror-like surface by the emery paper polishing.

The fatigue tests were conducted in air at the ambient temperature and in de-oxygenated water at 463K. The fatigue strength decreased with the increasing the corrosion pits. The reduction of fatigue strength can be explained by the short crack theory. On the other hand, all the fatigue cracks initiated from the corrosion pits in the high temperature water. The surface observation of test specimens suggested that the corrosion pits were mainly nucleated from the manganese sulfide. This indicates that the corrosion fatigue strength could be improved by controlling the chemical compositions and the impurity atoms. Key words: Corrosion fatigue, Cr stainless steel, Corrosion pit, Inclusion

1 Introduction About 60 to 70 % of the pumps in thermal power plants in

Japan have been used more than thirty years. As a result, deterioration of these pumps has been accumulated. Most plants are now operating on a DSS (Daily Start and Stop) or WSS (Weekly Start and Stop) basis in response to electricity supply and demand. This tends to increase the pump start/stop frequency and the operating time in the low flow rate range. To maintain stable operation of these deteriorated thermal power pumps, which were designed and manufactured on the assumption of base load operation, various countermeasures must be taken against severe operating conditions(1). For this purpose more accurate inspection techniques for maintenance and control have been developed(2-4).

Pump shaft failure mechanism seems to be as follows. Corrosion pits are nucleated on the pump shaft surface because of long-term operation. Fatigue micro-cracks are initiated from the bottoms of corrosion pits due to stresses at the start and stop or the low flow rate operation. Micro-cracks initiated in close proximity coalescence each other and grow into the macro-crack. The growth of macro-crack leads to the pump shaft failure. Thus the nucleation of corrosion pits which are initial corrosion damage have to be prevented in order to maintain the stable operation of pump.

Komai proposed the corrosion fatigue crack growth mechanism based on stress assisted dissolution(5). First corrosion pits are nucleated on the surface. When the corrosion pit is grown to some depth, the extremely wide slot-like crack is initiated at the bottom of pit and grown to the depth direction. The stress assisted dissolution is limited in the vicinity of crack tip and the crack width is decreased. At that time the crack tip is blunted due to the dissolution.

The crack growth is caused by the electro-chemical reaction due to the slip deformation at the crack tip. On the other hand, Kondo proposed that the corrosion pit is nucleated on the surface and grown to the depth, finally the fatigue crack is initiated and grown when the corrosion pit size exceeds the critical value determined by the crack growth stress intensity threshold Kth(6,7). However they have not examined the mechanism of corrosion pit nucleation.

There are many factors influencing the nucleation of corrosion pits. At the present the chemical compositions, the inclusions, the surface roughness, the surface finishing and the environment are considered as the key factors. In this study, effects of surface finishing, inclusions and corrosive environment on the nucleation of corrosion pits and the configuration of corrosion pits were examined.

2 Mechanism of corrosion fatigue in pump shaft Figure 1 shows the mechanism of stress corrosion

cracking and corrosion fatigue crack initiation and growth due to long-term exposure in corrosive environment. At first, surface oxide film is ruptured even in pure water environment. This rupture is accelerated by the chlorine ion and the stress amplitude. At the bottom of corrosion pit the oxide film is newly formed and simultaneously the corrosion pit is grown. When the corrosion pit is sufficiently grown up to the size of fatigue crack growth stress intensity threshold Kth, micro-cracks are initiated from the bottoms of corrosion pits. The micro-cracks grow due to the cyclic stress and coalesce each other and become short cracks. The short cracks coalesce each other and grow into the macro-crack. When the macro-crack sufficiently grows to exceed the material’s fracture toughness, the fracture takes place. This mechanism

☨ Received Dec. 28, 2016 ©2017 The Society of Materials Science, Japan *Member:Neutron Science Center, Comprehensive Research Organization for Society and Science, Tokai-mura, Naka-gun, Ibaraki, 319-1106Japan **Tsuchiura Works, Hitachi Ltd., Kandatsu, Tsuchiura, Ibaraki, 300-0013Japan

｢材料｣ (Journal of the Society of Materials Science, Japan), Vol. 66, No. 12, pp. 957-962, Dec. 2017Original Papers

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Fig.4 Effect of chlorine ion concentration on corrosion pit growth behavior of ground surface.

Fig.5 Relationship between pit size on the surface and pit depth.

between the depth and the surface length is slightly scattered, the aspect ratio is estimated about 0.5. This means that the surface length of corrosion pits is 4 times as large as the depth of them. Kondo obtained 0.7 of a/2c for 12CrMo stainless steel in sodium chloride water at 80℃ and 0.35 for 2.5NiCrMoV and 3.5NiCrMoV low alloy steels in water with oxygen concentration of 100 ppb at 90 ℃ (6,7). These differences seem to be caused by the differences in chemical compositions of materials and water environments.

3.2 Corrosion fatigue strength Rotating fatigue tests of specimens with the corrosion pits

were conducted in air and water environments at ambient temperature. In the water environment, the specimen was surrounded by the acryl case to prevent the contaminations from the air, and the de-oxygenated pure water was dropped on the rotating specimen. The rotating frequency was 3,300rpm. The corrosion pit sizes are 25–50 m, and larger than 50 m in AISI414 steel, and smaller than 25 m, 25-50 m and 50-150 m in type 403 steel, respectively. Before the fatigue tests the specimen surfaces were cleaned in acetone solution using the ultrasonic cleaning machine to remove the chlorine ion from the pit bottoms.

S-N curves for AISI414 stainless steel are shown in Fig.6. The fatigue endurance limits without the corrosion pit both in

Fig.6 S-N curves for AISI414 stainless steel with various pit size under rotating bending.

Fig.7 S-N curves for Type 403 stainless steel with various pit

size under rotating bending. air and pure water are both 460 MPa. This means the corrosive environment of de-oxygenated water does not affect the fatigue strength of AISI414 stainless steel. On the other hand, the fatigue endurance limits for the specimens with the pits are decreased with the increase of pit size. The fatigue endurance limits for the specimens with the corrosion pit size of 25-50 m and larger than 50 m are 370 MPa and 350 MPa respectively. This means that the small size corrosion pit affects the fatigue strength even in the pure water.

S-N curves for type 403 stainless steel are shown in Fig.7. The fatigue endurance limits without the corrosion pit both in air and pure water are both 400 MPa. There is no effect of pure water environment of the corrosion fatigue even in type 403 stainless steel. The fatigue endurance limit of type 403 stainless steel is about 13% lower than that of AISI414 steel. The fatigue endurance limits for the specimens with the pits are decreased with the increase of pit size similarly with AISI414 stainless steel.

The relationship between the fatigue endurance limit and the corrosion pit size is shown in Fig.8. For the shallower pits, the fatigue endurance limit almost coincides with that of the smooth specimen. Two oblique lines show the relationships between the fatigue endurance limit and the pit depth

0 100 200

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σw=350MPa


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σw=350MPa

Fig.1 Corrosion fatigue fracture mechanism suggests that the suppression of the corrosion pit nucleation is the most important in order to prevent the corrosion fatigue fracture.

3 Rotating bending fatigue strength 3.1 Corrosion pit growth behavior

Materials used in this experiment were AISI414 and Type403 stainless steels. Chemical compositions and mechanical properties are shown in Tables 1 and 2 respectively.

The nucleation of corrosion pits is affected by the surface finishing and the corrosive environment. Thus the effects of the surface finishing and the chlorine ion concentration were examined. High-temperature and high-pressure autoclave made of stainless steel was used for corrosion tests. The pressure was 7.5 MPa, and the test temperature was 150℃. The chlorine ion concentrations were 1, 10 and 100 ppm, and the exposure time was 100, 300, 500, 800 and 1000 hours.

The shape and dimensions of corrosion and rotating bending fatigue test specimen is shown in Fig.2. After the corrosion tests the same specimens were fatigue-tested. The specimen surface of gauge length was preliminarily worked. The finishing conditions were grinding, emery paper polishing and buff polishing. The grinding condition is the same with actual pump shaft. The roughness for each finishing is 0.8S, 0.1S and 0.05S respectively. The specimens were exposed in the autoclave without loading and taken out after the given time. The corrosion pit sizes on the surface were observed by the optical microscope and the depths were measured using the laser microscope.

Figure 3 shows the effect of surface finishing on the

corrosion pit growth behaviors of AISI414 stainless steel in water with chlorine ion of 1 ppm. In the figure, the maximum pit sizes observed on the specimen surfaces are plotted. The maximum pit size depends on the surface finishing. The corrosion pit size becomes larger in the order of buff polishing, emery paper polishing and grinding. The pit size on the ground surface is about 2.4 times as large as that of the buff polished surface. This means that the grinding introduces the high dislocation density surface layer and easily nucleates the corrosion pit, but the mirror-like surface finishing can restrain the corrosion pit nucleation and growth.

The effect of chlorine ion concentration on the corrosion pit growth on the ground surfaces is shown in Fig.4. The corrosion pit size increases with the chlorine ion concentration. The pit size in 10 ppm chlorine ion environment is about 1.2 times as large as that in 1 ppm chlorine ion environment. However, the pit size in 100 ppm chlorine ion concentration is about 9.1 times as large as that in 1 ppm chlorine ion environment. The slopes of the pit size growth in Figs.3 and 4 are both about 3. This means that so-called “One-third rule” is established in the pit growth of AISI414 steel in chlorine ion environment.

In order to evaluate the fatigue strength of the specimen with the corrosion pits, the pit depths have to be estimated from the surface observation. Since the configuration of corrosion pit is affected by the corrosive environment, the ratios of corrosion pit depth to the length on the surface were measured using optical microscope and the laser microscope. The relationship between the pit depth a and the length on the surface 2c is shown in Fig.5. Although the relationship

31

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imum

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Cl:1ppm Surface finishingGrindingEmery polishingBuff polishing

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b (

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Surface finishingGrindingEmery polishingBuff polishing

Fig.3 Corrosion pit growth behaviors of AISI414 stainless

steel in 1 ppm sodium chloride ion concentration.

Bulk WaterChemistry

CrevicedWaterChemistry

CrackInitiationProcess

CrackGrowthProcess

Film Rupture

Corrosion Pit

Micro-cracks

Short Cracks

Macro-cracks

Corrosion Pit

Short Crack Growth

Macro-crack Growth

Fracture

Film Rupture

Micro-crackInitiation

Coalescence ofMicro-cracks

Coalescence ofShort Cracks

Table 1 Chemical compositions of test materials (wt%). Mat. C Si Mn P S Ni Cr MoAISI414 0.10 0.47 0.62 0.021 0.007 2.41 12.63 0.55Type403 0.13 0.34 0.54 0.029 0.012 3.7 11.64 0.55

Mat. C Si Mn P S Ni Cr MoAISI414 0.10 0.47 0.62 0.021 0.007 2.41 12.63 0.55Type403 0.13 0.34 0.54 0.029 0.012 3.7 11.64 0.55

Table 2 Mechanical properties.

Yield stress Tensile strength Elongation Reduction(MPa) (MPa) (%) of area (%)

AISI414 745 902 21.0 57.0Type403 668 700 25.0 68.0

Mat. Yield stress Tensile strength Elongation Reduction(MPa) (MPa) (%) of area (%)

AISI414 745 902 21.0 57.0Type403 668 700 25.0 68.0

Mat.

Fig.2 Shape and dimensions of corrosion and fatigue test specimen.

958 M. Hayashi，K. Amano，Y. Ueyama

13-2016-0151-(p.957-962).indd 958 2017/10/11 18:30:09

Fig.4 Effect of chlorine ion concentration on corrosion pit growth behavior of ground surface.

Fig.5 Relationship between pit size on the surface and pit depth.

between the depth and the surface length is slightly scattered, the aspect ratio is estimated about 0.5. This means that the surface length of corrosion pits is 4 times as large as the depth of them. Kondo obtained 0.7 of a/2c for 12CrMo stainless steel in sodium chloride water at 80℃ and 0.35 for 2.5NiCrMoV and 3.5NiCrMoV low alloy steels in water with oxygen concentration of 100 ppb at 90 ℃ (6,7). These differences seem to be caused by the differences in chemical compositions of materials and water environments.

3.2 Corrosion fatigue strength Rotating fatigue tests of specimens with the corrosion pits

were conducted in air and water environments at ambient temperature. In the water environment, the specimen was surrounded by the acryl case to prevent the contaminations from the air, and the de-oxygenated pure water was dropped on the rotating specimen. The rotating frequency was 3,300rpm. The corrosion pit sizes are 25–50 m, and larger than 50 m in AISI414 steel, and smaller than 25 m, 25-50 m and 50-150 m in type 403 steel, respectively. Before the fatigue tests the specimen surfaces were cleaned in acetone solution using the ultrasonic cleaning machine to remove the chlorine ion from the pit bottoms.

S-N curves for AISI414 stainless steel are shown in Fig.6. The fatigue endurance limits without the corrosion pit both in

Fig.6 S-N curves for AISI414 stainless steel with various pit size under rotating bending.

Fig.7 S-N curves for Type 403 stainless steel with various pit

size under rotating bending. air and pure water are both 460 MPa. This means the corrosive environment of de-oxygenated water does not affect the fatigue strength of AISI414 stainless steel. On the other hand, the fatigue endurance limits for the specimens with the pits are decreased with the increase of pit size. The fatigue endurance limits for the specimens with the corrosion pit size of 25-50 m and larger than 50 m are 370 MPa and 350 MPa respectively. This means that the small size corrosion pit affects the fatigue strength even in the pure water.

S-N curves for type 403 stainless steel are shown in Fig.7. The fatigue endurance limits without the corrosion pit both in air and pure water are both 400 MPa. There is no effect of pure water environment of the corrosion fatigue even in type 403 stainless steel. The fatigue endurance limit of type 403 stainless steel is about 13% lower than that of AISI414 steel. The fatigue endurance limits for the specimens with the pits are decreased with the increase of pit size similarly with AISI414 stainless steel.

The relationship between the fatigue endurance limit and the corrosion pit size is shown in Fig.8. For the shallower pits, the fatigue endurance limit almost coincides with that of the smooth specimen. Two oblique lines show the relationships between the fatigue endurance limit and the pit depth

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Env.AirWaterWaterWater

Pit size (m)00

25-50＞50

σw=460MPa

σw=370MPa

σw=350MPa

Fig.1 Corrosion fatigue fracture mechanism suggests that the suppression of the corrosion pit nucleation is the most important in order to prevent the corrosion fatigue fracture.

3 Rotating bending fatigue strength 3.1 Corrosion pit growth behavior

Materials used in this experiment were AISI414 and Type403 stainless steels. Chemical compositions and mechanical properties are shown in Tables 1 and 2 respectively.

The nucleation of corrosion pits is affected by the surface finishing and the corrosive environment. Thus the effects of the surface finishing and the chlorine ion concentration were examined. High-temperature and high-pressure autoclave made of stainless steel was used for corrosion tests. The pressure was 7.5 MPa, and the test temperature was 150℃. The chlorine ion concentrations were 1, 10 and 100 ppm, and the exposure time was 100, 300, 500, 800 and 1000 hours.

The shape and dimensions of corrosion and rotating bending fatigue test specimen is shown in Fig.2. After the corrosion tests the same specimens were fatigue-tested. The specimen surface of gauge length was preliminarily worked. The finishing conditions were grinding, emery paper polishing and buff polishing. The grinding condition is the same with actual pump shaft. The roughness for each finishing is 0.8S, 0.1S and 0.05S respectively. The specimens were exposed in the autoclave without loading and taken out after the given time. The corrosion pit sizes on the surface were observed by the optical microscope and the depths were measured using the laser microscope.

Figure 3 shows the effect of surface finishing on the

corrosion pit growth behaviors of AISI414 stainless steel in water with chlorine ion of 1 ppm. In the figure, the maximum pit sizes observed on the specimen surfaces are plotted. The maximum pit size depends on the surface finishing. The corrosion pit size becomes larger in the order of buff polishing, emery paper polishing and grinding. The pit size on the ground surface is about 2.4 times as large as that of the buff polished surface. This means that the grinding introduces the high dislocation density surface layer and easily nucleates the corrosion pit, but the mirror-like surface finishing can restrain the corrosion pit nucleation and growth.

The effect of chlorine ion concentration on the corrosion pit growth on the ground surfaces is shown in Fig.4. The corrosion pit size increases with the chlorine ion concentration. The pit size in 10 ppm chlorine ion environment is about 1.2 times as large as that in 1 ppm chlorine ion environment. However, the pit size in 100 ppm chlorine ion concentration is about 9.1 times as large as that in 1 ppm chlorine ion environment. The slopes of the pit size growth in Figs.3 and 4 are both about 3. This means that so-called “One-third rule” is established in the pit growth of AISI414 steel in chlorine ion environment.

In order to evaluate the fatigue strength of the specimen with the corrosion pits, the pit depths have to be estimated from the surface observation. Since the configuration of corrosion pit is affected by the corrosive environment, the ratios of corrosion pit depth to the length on the surface were measured using optical microscope and the laser microscope. The relationship between the pit depth a and the length on the surface 2c is shown in Fig.5. Although the relationship

31

102 103 104

102

10

1

Time t (h)

Max

imum

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size

b (

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31

102 103 104

102

10

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Time t (h)

Max

imum

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size

b (

m)


Surface finishingGrindingEmery polishingBuff polishing

Fig.3 Corrosion pit growth behaviors of AISI414 stainless

steel in 1 ppm sodium chloride ion concentration.

Bulk WaterChemistry

CrevicedWaterChemistry

CrackInitiationProcess

CrackGrowthProcess

Film Rupture

Corrosion Pit

Micro-cracks

Short Cracks

Macro-cracks

Corrosion Pit

Short Crack Growth

Macro-crack Growth

Fracture

Film Rupture

Micro-crackInitiation

Coalescence ofMicro-cracks

Coalescence ofShort Cracks

Table 1 Chemical compositions of test materials (wt%). Mat. C Si Mn P S Ni Cr MoAISI414 0.10 0.47 0.62 0.021 0.007 2.41 12.63 0.55Type403 0.13 0.34 0.54 0.029 0.012 3.7 11.64 0.55

Mat. C Si Mn P S Ni Cr MoAISI414 0.10 0.47 0.62 0.021 0.007 2.41 12.63 0.55Type403 0.13 0.34 0.54 0.029 0.012 3.7 11.64 0.55

Table 2 Mechanical properties.

Yield stress Tensile strength Elongation Reduction(MPa) (MPa) (%) of area (%)

AISI414 745 902 21.0 57.0Type403 668 700 25.0 68.0

Mat. Yield stress Tensile strength Elongation Reduction(MPa) (MPa) (%) of area (%)

AISI414 745 902 21.0 57.0Type403 668 700 25.0 68.0

Mat.

Fig.2 Shape and dimensions of corrosion and fatigue test specimen.

959Initiation and Growth of Corrosion Pit and Its Effect on Corrosion Fatigue Strengthin 12Cr Stainless Steel

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Fig.12 S-N curves for Type 403 stainless steel in air and CWT water environment.

Fig.13 Effect of amount of type A inclusion on fatigue strength in air and in CWT water environment.

MPa and 415 MPa respectively. This means that the fatigue strength in CWT water environment is about 15 % lower than that in air, and the CWT water environment is more severe compared with the de-oxygenated pure water (See Fig.6). In this figure, Type-A inclusions in AISI414 steel is 0.11 weight %.

S-N curves for type 403 stainless steel are shown in Fig.12. The fatigue endurance limits of the specimens in which the amounts of Type-A inclusions are 0.13 % and 0.04-0.07 % are 350 MPa and 400 MPa in air, respectively. The fatigue endurance limits of the specimens in which the amounts of Type-A inclusions are 0.13 % and 0.04-0.07 % are 310 MPa and 400 MPa in CWT water environment, respectively. Although the fatigue strength in CWT water environment of the specimens with the inclusions higher than 0.1 % is reduced about 12 % compared with that in air, the fatigue strengths of the specimens with the inclusions lower than 0.1 % are the same in spite of the environment.

The relationship between the amount of Type-A inclusion and the fatigue strength ratio in air and CWT water environment at 180℃ is shown in Fig.13. As can be seen in Fig.13, the fatigue strength is not affected for the amount of Type-A inclusion lower than 0.07 wt%. However, it abruptly decreases if the amount of Type-A inclusion is higher than

0.07 wt%. This suggests that the suppression of formation in Type-A inclusion can prevent the reduction in the corrosion fatigue strength of 13Cr stainless steel.

5 Discussion Kawai et al.(10) and Kondo(6,7) reported that the corrosion

fatigue crack is initiated from the corrosion pits. The appearance of corrosion fatigue crack observed in type 403 stainless steel specimen fatigued at =330MPa in CWT water environment is shown in Fig.14. The crack length on the surface is about 1.3 mm. This corrosion fatigue crack was observed about a half of the fatigue life which is about 2x106 cycles. This means that the corrosion fatigue crack is initiated at the early stage of the fatigue life even in the corrosion environment. The corrosion pit can be observed at the center of fatigue crack in Fig.14. Its size is about 250 m and the corrosive product rises on the specimen surface.

SEM observation of fracture surface is shown in Fig.15. The corrosion pit can be observed at the crack initiated region. Its size is 980 m on the surface and 640 m in the depth. As mentioned before, the corrosion fatigue crack is initiated at the early stage of the fatigue life. Thus the corrosion pit seems to be grown up even after the crack initiation and the corrosive product is removed from the surface.

It is obvious that the corrosion fatigue crack is initiated from the corrosion pit from the observations of the specimen surface and the fracture surface shown in Figs.14 and 15. Next the corrosion pit nucleation behavior is examined. The corrosion pits are not observed in the specimen exposed

Fig.14 Corrosion fatigue crack initiated from the corrosion pit in type 403 stainless steel in CWT water environment.

Fig.15 SEM observation of fracture surface in type 403 stainless steel in CWT water environment.

0.01 0.1 1

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igue

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igue

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igue

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igue

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σw=400MPa

σw=350MPa

σw=310MPa

Fig.8 Relationship between the fatigue endurance limit and the pit size on the surface. determined by the stress intensity range threshold Kth of 6.2 MPa√m in air and 2.2 MPa√m in 3 % salt water. It is said that the fatigue endurance limit agrees with the oblique line for the stress intensity range threshold for the deeper pits, and is smoothly connected between the fatigue endurance limit for the smooth specimens and Kth line. The broken line indicates the mean values of the experimental results. It tends to be connected with Kth of 6.2 MPa√m in air. This means that the corrosive environment of de-oxygenated water does not affect the fatigue strength of specimens with pits and the reduction of fatigue strength can be explained by the short crack theory(8).

4 Tension-Compression fatigue strength 4.1 Corrosion pit growth behavior

As shown in Fig.1, the corrosion fatigue crack is initiated from the corrosion pit. There are many corrosion pit nucleation site on the metal surface. They are mechanical scratches, grain boundary triple points, inclusions and so on. The inclusions are the most probable corrosion pit nucleation sites.

Surface configurations of inclusions in type 403 stainless steel are shown in Fig.9. Type-A inclusions are mainly composed by MnS, Type-B inclusions are aluminum oxide and Type-C inclusions are granular oxide. Among these inclusions, the corrosion pits are the most probably nucleated from MnS inclusuions(9).

The effect of inclusion on the corrosion pit growth is shown in Fig.10. The amounts of Type-A inclusions, dA, are 0.13, 0.07 and 0.04 in weight % and the total amounts of

(a) A-type inclusion (b) B-type and C-type inclusion Fig.9 Configurations of inclusions in type 403 stainless steel.

Fig.10 Effect of type A inclusion on corrosion pit growth behavior in type 403 stainless steel.

inclusions, dT, are 0.15, 0.09 and 0.06 in weight % respectively. The chlorine ion concentration was 1 ppm in Fig.10. In the figure, the maximum pit sizes observed on the specimen surfaces are plotted. The maximum pit size is affected by the amount of type-A inclusions. However, the effect of amounts of inclusions is divided into two schemes. Namely the corrosion pit growth rate for the specimens in which the amount of inclusions higher than 0.1 weight % is about 60 % higher than that for the specimens in which the amount of inclusions is lower than 0.1 weight %. 4.2 Corrosion fatigue strength The corrosion fatigue tests were performed under tension-compression load in air and pure water environments at 180℃. The fatigue test specimens are hour-glass type and 4 mm in diameter. The water chemistry simulates Combined Water Treatment (CWT) water environment in fossil thermal power plants, and the dissolved oxygen concentration was 0.2 ppm, the conductivity was 0.3S/cm. The frequencies were changed from 5 to 15 Hz depending on the stress amplitude. The stress ratio was -1.0.

S-N curves for AISI414 stainless steel are shown in Fig.11. The fatigue endurance limits in air and CWT water are 485

Fig.11 S-N curves for AISI414 stainless steel in air and CWT water environment.

0.001 0.01 0.1 1 10

1000

500

100

50

σw=400-460MPa

⊿Kth=6.2MPa√

m(in Air)

⊿Kth=2.2M

Pa√m(in 3%NaCl)

AISI414

SUS403

Fat

igue

end

ura

nce

lim

it （

ＭＰ

ａ）

Corrosion pit depth (mm)0.001 0.01 0.1 1 10

1000

500

100

50

σw=400-460MPa

⊿Kth=6.2MPa√

m(in Air)

⊿Kth=2.2M

Pa√m(in 3%NaCl)

AISI414

SUS403

Fat

igue

end

ura

nce

lim

it （

ＭＰ

ａ）

Corrosion pit depth (mm)

50m50m 50m50m50m

31

102 103 104

102

10

1

Time t (h)

Max

imum

pit

size

b (

m)

Cl:1ppm dA=0.13%dA=0.07%dA=0.04%

b＝1.4ｔ1/３

b＝2.3ｔ1/３

31

102 103 104

102

10

1

Time t (h)

Max

imum

pit

size

b (

m)

Cl:1ppm dA=0.13%dA=0.07%dA=0.04%

b＝1.4ｔ1/３

b＝2.3ｔ1/３

10８10３ 10５ 10６ 10７200

600

500

400

Stre

ss a

mpl

itude

(MPa

) Environment

180℃ AirCWT Water

InclusiondA (%)0.11

300

10４


σw=485MPa

σw=415MPa


13-2016-0151-(p.957-962).indd 960 2017/10/11 18:30:11

Fig.12 S-N curves for Type 403 stainless steel in air and CWT water environment.

Fig.13 Effect of amount of type A inclusion on fatigue strength in air and in CWT water environment.

MPa and 415 MPa respectively. This means that the fatigue strength in CWT water environment is about 15 % lower than that in air, and the CWT water environment is more severe compared with the de-oxygenated pure water (See Fig.6). In this figure, Type-A inclusions in AISI414 steel is 0.11 weight %.

S-N curves for type 403 stainless steel are shown in Fig.12. The fatigue endurance limits of the specimens in which the amounts of Type-A inclusions are 0.13 % and 0.04-0.07 % are 350 MPa and 400 MPa in air, respectively. The fatigue endurance limits of the specimens in which the amounts of Type-A inclusions are 0.13 % and 0.04-0.07 % are 310 MPa and 400 MPa in CWT water environment, respectively. Although the fatigue strength in CWT water environment of the specimens with the inclusions higher than 0.1 % is reduced about 12 % compared with that in air, the fatigue strengths of the specimens with the inclusions lower than 0.1 % are the same in spite of the environment.

The relationship between the amount of Type-A inclusion and the fatigue strength ratio in air and CWT water environment at 180℃ is shown in Fig.13. As can be seen in Fig.13, the fatigue strength is not affected for the amount of Type-A inclusion lower than 0.07 wt%. However, it abruptly decreases if the amount of Type-A inclusion is higher than

0.07 wt%. This suggests that the suppression of formation in Type-A inclusion can prevent the reduction in the corrosion fatigue strength of 13Cr stainless steel.

5 Discussion Kawai et al.(10) and Kondo(6,7) reported that the corrosion

fatigue crack is initiated from the corrosion pits. The appearance of corrosion fatigue crack observed in type 403 stainless steel specimen fatigued at =330MPa in CWT water environment is shown in Fig.14. The crack length on the surface is about 1.3 mm. This corrosion fatigue crack was observed about a half of the fatigue life which is about 2x106 cycles. This means that the corrosion fatigue crack is initiated at the early stage of the fatigue life even in the corrosion environment. The corrosion pit can be observed at the center of fatigue crack in Fig.14. Its size is about 250 m and the corrosive product rises on the specimen surface.

SEM observation of fracture surface is shown in Fig.15. The corrosion pit can be observed at the crack initiated region. Its size is 980 m on the surface and 640 m in the depth. As mentioned before, the corrosion fatigue crack is initiated at the early stage of the fatigue life. Thus the corrosion pit seems to be grown up even after the crack initiation and the corrosive product is removed from the surface.

It is obvious that the corrosion fatigue crack is initiated from the corrosion pit from the observations of the specimen surface and the fracture surface shown in Figs.14 and 15. Next the corrosion pit nucleation behavior is examined. The corrosion pits are not observed in the specimen exposed

Fig.14 Corrosion fatigue crack initiated from the corrosion pit in type 403 stainless steel in CWT water environment.

Fig.15 SEM observation of fracture surface in type 403 stainless steel in CWT water environment.

0.01 0.1 1

1.0

0.6


Rat

io o

f fat

igue

stre

ngth

(C

WT)

/(A

ir)

Type 403AISI414

1.2

0.8

0.01 0.1 1

1.0

0.6Rat

io o

f fat

igue

stre

ngth

(C

WT)

/(A

ir) 1.2

0.8

0.01 0.1 1

1.0

0.6


Rat

io o

f fat

igue

stre

ngth

(C

WT)

/(A

ir)

Type 403AISI414

1.2

0.8

0.01 0.1 1

1.0

0.6Rat

io o

f fat

igue

stre

ngth

(C

WT)

/(A

ir) 1.2

0.8

500mLoaddirection

500mLoaddirection

500m500m

10８10３ 10５ 10６ 10７200

600

500

400

Stre

ss a

mpl

itude

(MPa

) Environment

180℃ AirCWT Water

InclusiondA (%)

0.04-0.07 0.13

300

10４


σw=400MPa

σw=350MPa

σw=310MPa

Fig.8 Relationship between the fatigue endurance limit and the pit size on the surface. determined by the stress intensity range threshold Kth of 6.2 MPa√m in air and 2.2 MPa√m in 3 % salt water. It is said that the fatigue endurance limit agrees with the oblique line for the stress intensity range threshold for the deeper pits, and is smoothly connected between the fatigue endurance limit for the smooth specimens and Kth line. The broken line indicates the mean values of the experimental results. It tends to be connected with Kth of 6.2 MPa√m in air. This means that the corrosive environment of de-oxygenated water does not affect the fatigue strength of specimens with pits and the reduction of fatigue strength can be explained by the short crack theory(8).

4 Tension-Compression fatigue strength 4.1 Corrosion pit growth behavior

As shown in Fig.1, the corrosion fatigue crack is initiated from the corrosion pit. There are many corrosion pit nucleation site on the metal surface. They are mechanical scratches, grain boundary triple points, inclusions and so on. The inclusions are the most probable corrosion pit nucleation sites.

Surface configurations of inclusions in type 403 stainless steel are shown in Fig.9. Type-A inclusions are mainly composed by MnS, Type-B inclusions are aluminum oxide and Type-C inclusions are granular oxide. Among these inclusions, the corrosion pits are the most probably nucleated from MnS inclusuions(9).

The effect of inclusion on the corrosion pit growth is shown in Fig.10. The amounts of Type-A inclusions, dA, are 0.13, 0.07 and 0.04 in weight % and the total amounts of

(a) A-type inclusion (b) B-type and C-type inclusion Fig.9 Configurations of inclusions in type 403 stainless steel.

Fig.10 Effect of type A inclusion on corrosion pit growth behavior in type 403 stainless steel.

inclusions, dT, are 0.15, 0.09 and 0.06 in weight % respectively. The chlorine ion concentration was 1 ppm in Fig.10. In the figure, the maximum pit sizes observed on the specimen surfaces are plotted. The maximum pit size is affected by the amount of type-A inclusions. However, the effect of amounts of inclusions is divided into two schemes. Namely the corrosion pit growth rate for the specimens in which the amount of inclusions higher than 0.1 weight % is about 60 % higher than that for the specimens in which the amount of inclusions is lower than 0.1 weight %. 4.2 Corrosion fatigue strength The corrosion fatigue tests were performed under tension-compression load in air and pure water environments at 180℃. The fatigue test specimens are hour-glass type and 4 mm in diameter. The water chemistry simulates Combined Water Treatment (CWT) water environment in fossil thermal power plants, and the dissolved oxygen concentration was 0.2 ppm, the conductivity was 0.3S/cm. The frequencies were changed from 5 to 15 Hz depending on the stress amplitude. The stress ratio was -1.0.

S-N curves for AISI414 stainless steel are shown in Fig.11. The fatigue endurance limits in air and CWT water are 485

Fig.11 S-N curves for AISI414 stainless steel in air and CWT water environment.

0.001 0.01 0.1 1 10

1000

500

100

50

σw=400-460MPa

⊿Kth=6.2MPa√

m(in Air)

⊿Kth=2.2M

Pa√m(in 3%NaCl)

AISI414

SUS403

Fat

igue

end

ura

nce

lim

it （

ＭＰ

ａ）

Corrosion pit depth (mm)0.001 0.01 0.1 1 10

1000

500

100

50

σw=400-460MPa

⊿Kth=6.2MPa√

m(in Air)

⊿Kth=2.2M

Pa√m(in 3%NaCl)

AISI414

SUS403

Fat

igue

end

ura

nce

lim

it （

ＭＰ

ａ）

Corrosion pit depth (mm)

50m50m 50m50m50m

31

102 103 104

102

10

1

Time t (h)

Max

imum

pit

size

b (

m)

Cl:1ppm dA=0.13%dA=0.07%dA=0.04%

b＝1.4ｔ1/３

b＝2.3ｔ1/３

31

102 103 104

102

10

1

Time t (h)

Max

imum

pit

size

b (

m)

Cl:1ppm dA=0.13%dA=0.07%dA=0.04%

b＝1.4ｔ1/３

b＝2.3ｔ1/３

10８10３ 10５ 10６ 10７200

600

500

400

Stre

ss a

mpl

itude

(MPa

) Environment

180℃ AirCWT Water

InclusiondA (%)0.11

300

10４


σw=485MPa

σw=415MPa

961Initiation and Growth of Corrosion Pit and Its Effect on Corrosion Fatigue Strengthin 12Cr Stainless Steel

13-2016-0151-(p.957-962).indd 961 2017/10/11 18:30:13

longer than 1,000 hours in pure water without loading. On the other hand, the corrosion pits are observed in the specimen exposed only 50 hours with loading (number of cycles=106). This means that the cyclic loading accelerates the nucleation of corrosion pit. The surface observation indicates that many inclusions are observed in the specimen where the corrosion pits are initiated, and the inclusions are not observed in the specimen where the corrosion pits are not initiated. SEM observation manifests that the corrosion pits are initiated where the inclusions are observed on the specimen surface. The example of the corrosion pit observed on the specimen surface is shown in Fig.16. There are many Type-A inclusions. The corrosion pits are initiated in the inclusions.

In order to clarify the corrosion pit nucleation sites, the X-ray analysis were conducted. Fig.17 shows the X-ray analysis near the corrosion pit. Left side photograph shows SEM observation of the corrosion pit. The corrosion pits are initiated at the center of the inclusion. Right side photograph shows Mn distribution obtained by EDX analysis. Although Mn is not detected at the center of inclusion, Mn is detected both at end side of inclusion. Similar observation was obtained for S distribution analysis. This means that the inclusion consist of MnS.

The most important thing in the actual machine components subjected to the cyclic load in the corrosive environment is to prevent the corrosion fatigue crack

Fig.16 Corrosion pits initiated from MnS inclusions.

(a) SEM observation (b) EDX analysis Fig.17 X-ray analysis near the corrosion pit.

initiation. It is clarified in this experiment that the corrosion fatigue strength is affected by the amount of Type-A inclusion and the corrosion pits are initiated from the Type-A inclusion. Thus the reduction of Type-A inclusion in 13Cr stainless steel like AISI414 and type 403 stainless steels leads to the

prevention or restriction of the corrosion fatigue crack initiation.

6 Conclusions The corrosion and fatigue tests of 12Cr stainless steels

were conducted in air at the ambient temperature and in de-oxygenated water at 180℃ to clarify the effect of surface finishing and the inclusions on the initiation of corrosion pits and the fatigue strength. Obtained results are summarized as follows. 1) The growth of corrosion pits depends on the surface

finishing, and its size increases in the order of buff polishing, emery paper polishing and grinding.

2) The corrosive environment of de-oxygenated water does not affect the fatigue strength of AISI414 and type 403 stainless steels. The fatigue strength decreased with the increasing the corrosion pits. The reduction of fatigue strength can be explained by the short crack theory.

3) The fatigue cracks are initiated from the corrosion pits in water environment. The surface observation of test specimens suggests that the corrosion pits are mainly nucleated from the manganese sulfide. Thus the corrosion fatigue strength could be improved by controlling the chemical compositions and the impurity atoms.

References

(1) H. Komada, T. Imai and Y. Misumi, “Life Prediction and Up-grade for Pump”, Thermal and Nuclear Power, Vol.40, pp.123-128 (1989)

(2) M. Hayashi, Y. Ueyama and Y. Hashimoto, “Life Diagnostic System for Boiler Feed Water Pump Shaft in Fossil-Fuel Power Plant”, Materials Evaluation, Vol.50, pp.616-621 (1992)

(3) M. Hayashi, Y. Ueyama, and Y. Hashimoto, “Preventive Maintenance System for Boiler Feed Water Pump Shaft in Fossil Power Plant”, Proc. Int. Conf. on Power Engineering-93 (ICOPE-93), Vol.2, pp.307-312 (1993)

(4) M. Hayashi, Y. Ueyama and Y. Hashimoto, “Automatic Inspection System for Pump Shaft by Image Processing of Corrosion Damage”, Trans. JSME, Vol.65, pp.1179-1184 (1999).

(5) K. Komai, “Design for Corrosion Strength of Structural Materials”, YOKENDO, pp.87 (1993)

(6) Y. Kondo, “Prediction Method of Corrosion Fatigue Crack Initiation Life Based on Corrosion Pit Growth Mechanism”, Trans. JSME, Vol.53, pp.1983-1987 (1987)

(7) Y. Kondo, “An Approach to Quantitative Evaluation of Corrosion Fatigue Crack Initiation Codnition”, Trans. JSME, Vol.57, pp.1057-1062 (1991)

(8) S. Usami, and S. Shida, “Effect of Environment, Stress Ratio and Defect Size on Fatigue Crack Threshold”, J. Society of Materials Science, Japan, Vol.31, pp.493-499 (1982)

(9) K. Amano, M. Hayashi, H. Suzuki, S. Nishino and K. Katahira, “Relationship Between Corrosion Pits Initiation and Inclusions of Stainless Steels during Fatigue Process in Pure Water”, Trans. JSME, Vol.64, pp.1831-1836 (1998)

(10) S. Kawai and K. Kasai, “Consideration about Allowable Stress of Corrosion Fatigue”, Trans. JSME, Vol.51, pp.23-28 (1985)

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initiation and growth of corrosion pit and its effect on

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