07253 cavitation corrosion.pdf

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CAVITATION CORROSION BEHAVIOUR OF CARBON STEEL, AL-BRONZE AND

COBALT-BASED ALLOY IN SEAWATER

A. Al-Hashem, H.Tarish and A. Akbar

Petroleum Research & Studies Center

Kuwait Institute for Scientific Research

P.O.Box 24885,13109, Safat, Kuwait

[email protected]

ABSTRACT

An ultrasonically induced cavitation facility was used to study the cavitation corrosion behavior of

carbon steel (UNS G10200), Al-Bronze (UNS C95300), and cobalt-based alloy (UNS R30006) in

seawater. The work included measurements of free corrosion potentials, and mass loss in presence and

absence of cavitation. The cavitation tests were made at a frequency of 20 KHz and at temperatures of 250C. Cavitation conditions caused a noble shift in the free corrosion potential for carbon steel and an

active one for Al-Bronze and cobalt-based alloys. Cavitation also increased the rate of mass loss of these

alloys by several orders of magnitude with respect to stagnant conditions. Cavitation made the surface

of these alloys very rough, exhibiting large cavity pits in the middle region of the attacked area as

revealed by the scanning electron microscope (SEM). Mechanical factors were determined to be theleading cause of metal loss.

Keywords: Cavitation, corrosion, carbon steel, Al-Bronze, Stellite 6B, seawater, Free corrosion-

potential.

INTRODUCTIONIn this investigation, the cavitation corrosion behavior of carbon steel (UNS G10200), Al-Bronze

(UNS C95300), and a cobalt-based alloy, Stellite 6B(1)

(UNS R30006) in seawater was studied. This

study was conducted in our laboratory due to repeated failures of valves and pumps in seawater systems

of refineries and petrochemical plants in Kuwait1-4

. The cause of such failures was attributed tocavitation-erosion, erosion corrosion and under-deposit corrosion. However, in this investigation, the

results of laboratory cavitation studies made on the above mentioned alloys will be summarized.

Cavitation damage is a form of localized attack found on many types of materials

exposed to turbulent flows. Although mainly mechanical in nature, this type of damage(1) Stellite 6B is a trade name of Deloro Stellite Inc.

is more severe in mediums where the cavitation mechanism acts synergistically with corrosion5 -6. The

mechanical action of cavitation usually attacks protective surfaces, exposing un-protective ones to

corrosion. Cavitation is often defined as the growth and collapse of vapor bubbles because of local

1

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07253

Paper No.

Government work published by NACE International with permission of the author(s). The material presented and the views expressed in this paper aresolely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.

Copyright



pressure fluctuations in a liquid. If the pressure suddenly falls below the vapor pressure, these bubbles

then collapse violently when they are submitted to a higher pressure. This collapse is accompanied by

the sudden flow of liquid, which imposes stress pulses capable of causing plastic deformation on solid

surfaces7.

Selection of corrosion resistant materials depends on a better understanding of materials response

to corrosive liquids and cavitation stresses. Therefore, continuous reappraisal of proposed materials of

construction is necessary to confirm that the least expensive material, which will still fully satisfy the

requirements of the service, has been selected.

EXPERIMENTAL METHOD

Apparatus

The vibratory apparatus used for this test method works at a frequency of 20 kHz and peak-to-

peak amplitude of 25 m. This test method produces axial oscillations of a test specimen inserted to a

specified depth in the test liquid. The vibrations are generated by a magnetostrictive or piezoelectric

transducer, driven by a suitable electronic oscillator and power amplifier. Figure 1 shows a schematic

view of such an apparatus.

Material

Specimens were cut from 15mm rods of UNS G10200, UNS C95300, and UNS R30006. Each

specimen had a diameter of 1.59 cm and a thickness of about 0.27 cm. Before experimental testing,

specimens were mechanically polished with silicon carbide papers up to 1200 grit. For morphologicalexamination, some specimens were etched before testing to reveal their microstructure. Two specimens

of each alloy were tested. The test specimens were fixed on a special holder which was placed at a

distance of 0.125 cm from the apparatus horn. At the end of each cavitation test, detailed morphological

examinations were carried out on the specimens. The optical microscope and scanning electron

microscopy (SEM) were used to identify the initiation and mode of damage in addition to the role played by the constituent phases of the alloy.

Test Solution

The chemical composition of the seawater used in this study is shown in Table 1. The seawater was contained in an open 600 ml glass beaker surrounded by a copper coil in a water bath. Inside the

beaker, the seawater was maintained at 251C.

RESULTS

Mass Loss of UNS G10200

Figure 2 shows the cumulative mass loss and rate of mass loss of UNS G10200 versus exposure

time for this alloy in seawater at 250C. A gradual increase in mass loss with time was observed for this

specimen with a very small incubation period in seawater at the testing temperature and a sharp increase

in mass loss occurred for the same specimen after 5 h of testing as shown in Figure 2a. This behavior has also been reflected for rate of mass loss for this specimen where a sharp increase (accumulation

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period) has occurred in the seawater at 250C reaching a maximum value of 0.00325 g/h and then

decreasing (attenuation period) after 5 h of cavitation where it reached steady state value of 0.00025 g/h

at the end of testing as shown in Figure 2b.

Mass Loss of UNS C95300

Figure 3 shows the cumulative mass loss and rate of mass loss versus exposure time for this alloy in

seawater at 250C. A steady gradual increase in mass loss with time was observed for this alloy with an

incubation period of 2.5 h in seawater at the testing temperature as shown in Figure 3a. This behavior has also been reflected for rate of mass loss for this specimen where a sharp increase (accumulation

period) has occurred in the seawater at 25oC reaching a maximum value of 0.00050 g/h and then

decreasing (attenuation period) after 1h of cavitation where it reached steady state value of 0.00010 g/h

at the end of testing as shown in Figure 3b.

Mass Loss of UNS R30006

Figure 4 shows the cumulative mass loss and rate of mass loss versus exposure time for this alloy in

seawater at 250C. A very steady small gradual increase in mass loss with time was observed for this

alloy with an incubation period of about 11 h in seawater at the testing temperature as shown in Figure

4a. This behavior has also been reflected for rate of mass loss for this specimen where a very slightincrease (accumulation period) has occurred in the seawater at 25C reaching a maximum value of

0.00010 g/h and then decreasing (attenuation period) after 1h of cavitation where it reached steady state

value of 0.00001 g/h at the end of testing as shown in Figure 4b.

General Observation on the Rate of Mass Loss

In order to compare the cavitation erosion performance of the three alloys in the seawater at 250C,

many investigators6,7

recommend comparing rate of mass loss in the steady state period of the curves.

Initial rate of mass loss is affected by many factors such as the type of alloy, aging temperature, surfaceinhomogenuities, alloy microstructure and the surface of a sample. Therefore, the steady state or semi-

steady state period will be considered as a measure of the erosion performance of each type of alloy.Based on the steady state rate of mass loss values, the performance of the three alloys is as follows:

UNS R30006 › UNS C95300 › UNS G10200 in seawater at 250C. It was observed that the rate of mass

loss values tend to converge to relatively close values at the end of cavitation testing inspite of the initialdifferences in the rate of mass loss.

Electrochemical Measurements

Potential-time Measurements of the Investigated Alloys

Figure 5 shows the potential-time measurements for carbon steel in seawater under stagnant and

cavitation conditions. As shown in Figure 5, the open circuit (OC)-potential of carbon steel was morenegative (active) under stagnant conditions compared with that under cavitations conditions. A shift of

120 mV towards the noble direction was observed for carbon steel upon the introduction of cavitationsconditions.

Figure 6, b shows the OC-potential versus time for Al-bronze in distilled, sea and formation

waters under stagnant and cavitation conditions. As shown in Figure 5, the OC-potential began with a

less negative value above 200 mV under stagnant conditions. However, it immediately shifted towards a

more negative value upon the introduction of cavitation conditions to the sample. This occurs in the

3




three different solutions with slightly varying initial and final values. The OC-potential becomes less

negative if cavitation conditions are no longer imposed on the sample.

Figure 7 shows the potential-time measurements for UNS R30006 in distilled, sea and

formation water under stagnant and cavitation conditions. Figure 7 indicates that the OC-potential of UNS R30006 begins with a less negative value under stagnant conditions and then immediately shifts

towards more negative (active) direction upon the imposition of cavitation condition. The approximate

value of the negative shift for UNS R30006 potential under cavitation conditions is 70 mV. The OC-

potential of UNS R30006 returned to less negative values when cavitation conditions were lifted.

General Observations about the Electrochemical Measurements.The OC-potential versus time for all the alloys investigated started with a less negative (noble)

value under stagnant conditions compared with their potential values under cavitation conditions with

the exception of carbon steel. The OC-potential behavior of carbon steel under the cycle of stagnation-

cavitation-stagnation versus time was opposite to those observed for the rest of the alloys in the threesolutions.

Assessment of Surface Damage

Figure 8 shows SEM micrographs revealing the specimen surface of UNS G10200 after 35 h of cavitation with randomly scattered cavity pits. Figure 8 a, b and c shows an area at the edge of the

cavitation attacked region at different magnifications. Several cavity pits of different sizes are shown in

the previous micrographs as well as a plastically deformation in the attacked region. Figure 8d shows a

plastically deformed region including both large cavity pits as well as small micro-cavities.

Figure 9 shows SEM micrographs of UNS C95300 after 35 h of cavitation in seawater. Figure 9a

shows a portion of the specimen with an attacked and un-attacked surface of this alloy revealing large

and small cavities. Figures 9b, c and d show a plastically deformed region of UNS C95300 covered with

both large and small micro-cavities. In addition, ductile tearing and grain boundary attack were also

observed under cavitation condition. The presence of cavities and ductile tearing is readily explainable

in terms of the known devastating effects of cavitation. On the other hand, the presence of grain boundary attack (i.e., corrosion of the boundaries of columnar grains) indicates that electrochemical

dissolution due to structural heterogeneity is indeed contributing in the surface damage during

cavitation.

Figure 10 shows SEM micrographs of UNS R30006 after 63 h of cavitation in seawater. Figure

10a show the edge of an intact surface and that of an attacked region of plastically deformed surface as a

result of the cavitation action. Figures 10b,c and d also show the microstructure of this alloy at higher

magnifications revealing the carbides of Cr 7C3 and W6C in a cobalt-rich matrix. . Figure 10 c and d

clearly show the carbides are selectively eroded while the carbide-matrix interface acts as an initiatingsite for erosion. Also material is lost, to a lesser extent, from twin intersections in the cobalt-rich matrix

phase of this alloy.

DISCUSSION

Cavitation damage is associated with components that are being driven at high velocity through a

fluid, for example propellers, impellers, water turbines, valves and pumps. Numerous studies4-7

have

been made on cavitation. Reports8-10

have been published which show that both mechanical and

4




electrochemical factors are involved. Cavitation may be defined as the growth and collapse of vapor

bubbles due to localized pressure changes in a liquid. The bubbles are formed in the liquid in regions of

very low pressure, for example, behind the leading edge of a propeller blade. The collapse process takes

place extremely rapidly, producing a strong shock wave that damages the material. Because a fewmillions of bubbles may collapse in a second, damage quickly occurs.

Results of the corrosion potential versus time tests indicated that cavitation caused a rapid

positive shift for carbon steel and negative shift in the free-corrosion potential for Al-Bronze and cobalt-

base alloys, which increased slightly during the cavitation period. The potential shift for Al-Bronze andcobalt-base alloys quickly reversed when cavitation was stopped, suggesting reformation of the

protective oxide layer on the surface of these alloys.

In the present study, cavitation made the specimen surface very rough, containing many cavities

(Figures 8-10). The impact of loading, which is a series of discrete events occurring in a time frame of microseconds, produces relatively high stress and strains in the metal surface. The cumulative effect

will lead to the formation of fatigue cracks. These cracks intersect causing small fragments of the metal

surface to be dislodged, leaving the surface in a spongy, porous condition11-13.

The exceptional cavitation erosion resistance of the UNS R30006 alloy can be attributed to the

drastic changes in mechanical properties brought about by the formation of mechanical twins and by the presence of platelets of the epsilon (E) HCP phase.

Woodford14

in 1971 in his metallographic studies of cobalt base alloys, postulated the concept of

stacking fault energy (SFE) as a key factor in the cavitation resistance of an alloy rather than itshardness. SFE is a measure of the resistance to dislocation motion on a microscopic scale and directly

related to the ability to work harden, which is key to improved cavitation resistance. Reduced SFE is

associated with increased cycles to nucleate a crack in high cycle fatigue, which is the mechanism

producing metal loss in cavitation11

. Figure 8-10 shows that this mechanism applied to these alloys

where metal loss occurs due to the formation of micro-cracks as a result of the high cycle fatigue.

CONCLUSIONS

The ultrasonically induced cavitation conditions on carbon steel, Al-Bronze and cobalt-basedalloys in seawater at 25oC resulted in the following:

1. An increase in mass loss and in the rate of mass loss with exposure time for the three

alloys.

2. A shift in the OC-potential towards the less negative direction for UNS G10200 and

towards the more negative direction for UNS C95300 and UNS R30006.3. The formation of rough surfaces for the three alloys, containing small and large

cavities as well as plastically deformed regions.

4. The creation of cyclic stresses which are considered to be the main cause of metal lossin these three alloys.

ACKNOWLEDGEMENT

The authors would like to acknowledge the support provided by the Kuwait Foundation for the

Advancement of Sciences (KFAS) under Contract Number (2002-14-01).

5




REFERENCES

1. A. Al-Hashem and J. Carew, "Consultancy Services for Laboratory and Onsite Studies for theFinalization of the Process and Process Design Parameters of Seawater Injection Project, North

Kuwait", KISR Report No. 4808, August, (1995).

2. A. Al-Hashem,K. Al-Muhanna and M. Salman, "Consultancy Services for the Laboratory Studies

for Effluent Disposal West Kuwait", Kuwait Institute for Scientific Research (KISR) Report No.

4809, December, (1995).3. V. K. Gouda; A. H. Al-Hashem; A. M. Abdullah and W. T. Riad, "Effect of Ultrasonically

Induced Cavitation on the Behaviour of Nodular Cast Iron in Seawater" Br. Corros. J., 26, 2,

(1991): p. 109.

4. A. Al-Hashem, H. Tarish, and J. Carew, “The Effect of Ultrasonically Induced Cavitation

Conditions on the Behavior of Copper and Nickel Based Alloys in Seawater”, CORROSION/06, paper No. 299 (NACE International, Houston, Tx, 2006).

5. J. M. Hobbs in Erosion by Cavitation or Impingement, STP 408; Philadelphia PA, ASM (1967): p.

159.

6. T. McGuiness and A., Thijuvengadam in Erosion Wear and Interfaces with Corrosion, STP 567:

P. Philadelphia, PA, ASTM. (1974), 30.

7. VC. M. Preece. Cavitation Erosion, in Treatise on Material Science and Technology. Vol. 16, C.M. Preece, ed. New York, NY: Academic Press, (1979), p. 249.

8. R. L. Chance in Engine Collant Testing, State of Art, (ed. W. H. Ailor), STP 705: Philadelphia,

PA, ASTM (1980), p. 270.

9. A. H. Al-Hashem, H. M. Shalaby and V. K. Gouda, Proc. 12th

Int. Cong. On Metallic Corrosion,Sept. 19 - 24, Vol. 5B, p. 3600, Houston, Texas, (1993).

10. J. G. Auret, O.F.R. Damm, G. J. Wright and F.P.A. Robinson, Corrosion 49, 11: p. 910, (1993).

11. C. McCaul, "An Advanced Cavitation Resistant Austenitic Stainless Steel for Pumps"

CORROSION/96, paper No. 415 (NACE International, Houston, Tx, 1996).

12. A. R. Marder and G. Krauss, "Hardenability concepts with Applications to Steel," AIMEPublication, (1978), p. 238.

13. C. J. Heathcock, B. E. Protheroe, and A. Ball, Wear, 80, (1982), pp. 311 - 327.14. D. A. Woodward, "Cavitation Induced Phase Transformation in Alloys", Metall. Trans3, (1972),

p. 1137.

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TABLE 1.

CHEMICAL COMPOSITION OF ARABIAN GULF SEAWATER COMPARED WITH

SYNTHETIC SEAWATER (ppm).

Parameter Synthetic*

Seawater

Arabian Gulf Seawater at

Doha Plant

(ppm SD)

Sodium

Magnesium

Potassium

Calcium

Chloride

Sulfate

Bicarbonate

Carbonate

Total Hydrocarbon

TDS

PH

10768

1297

388

408

19360

2702

143

-

35146

-

-

12,300 20

1700 150

470 20

570 45

24,000 700

3400 300

185 18

14 8

0.204

47,000 2000

8.2 0.1

* Seawater as prepared by the hydrographic laboratories of Copenhagen, Denmark

SD = Standard Deviation

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Figure 1. Schematic of Vibratory Cavitation Test Apparatus.

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Al-Bronze ( 25 o

C)

0.00000

0.00500

0.01000

0.01500

0.02000

0.02500

0.03000

0 5 10 15 20 25

Time (Hrs)

M a s s L o s s ( g m )

(a)

Al-Bronze ( 25 o

C)

0.00000

0.00010

0.00020

0.00030

0.00040

0.00050

0.00060

0 5 10 15 20 25

Time (Hrs)

(b)

Figure 3. Typical cumulative mass loss and rates of mass loss of UNS C70600 specimen exposed to

sea water at 25oC.

10




Stellite 6B ( 25 o

C)

0.00000

0.00200

0.00400

0.00600

0.00800

0.01000

0.01200

0 10 20 30 40 50 60 70

Time (Hrs)

M a s s L o s s ( g m )

(a)

Stellite 6B ( 25 o

C)

0.00000

0.00001

0.00002

0.00003

0.00004

0.00005

0.00006

0 10 20 30 40 50 60 70

Time (Hrs)

(b)

Figure 4. Typical cumulative mass loss and rates of mass loss of UNS R30006 specimen exposed to

sea water at 25o

C.

11




Figure 5.The effect of cavitation conditions on the free-corrosion potential of UNS G10200 in

seawater at 25oC.

Figure 6.The effect of cavitation conditions on the free-corrosion potential of UNS C95300 in

seawater at 25oC.

12




Figure 7.The effect of cavitation conditions on the free-corrosion potential of UNS R30006 in

seawater at 25oC.

13




(a) (b)

(c) (d)

Figure 8. SEM micrograph of the surface of UNS G10200 after 35 h of cativation

in seawater at 25oC showing dispersed cavities.

14




(a) (b)

(c) (d)

Figure 9. SEM micrograph of the surface of UNS C95300 after 35 h of cativation

in seawater at 25oC showing dispersed cavities.

15




(a) (b)

(c) (d)

Figure 10. SEM micrograph of the surface of UNS R30006 after 63 h of cativation in seawater at

25oC showing dispersed cavities.

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