Influence of cavitation erosion on corrosion fatigue and the effect of surface coatings on resistance

Wear, 63 (1980) 51 - 70 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

51

INFLUENCE OF CAVITATION EROSION ON CORROSION FATIGUE AND THE EFFECT OF SURFACE COATINGS ON RESISTANCE TSUNENORI OKADA* University of Fukui, Fukui (Japan)

Faculty

of Engineering,

Mechanical

Department,

Bunkyo

3-9-1,

KAORU AWAZU industrial

Research Institute,

Ishikawa,

Yoneizumi

4-l 33, Kanazawa

(Japan)

HISAMITSU KAWASAKI Toyoda

Coki Co. Ltd., Asahi l-l,

(Received July 20,1979;

Kariya, Aichi Prefecture

(Japan)

in final form October 8,1979)

Summery The fracture of S35C test pieces under cavitation erosion in 3% salt water occurs in the area of corrosion as in the case of corrosion fatigue without cavitation erosion. However, erosion fatigue strength decreases more than corrosion fatigue strength owing to the formation of a macrogalvanic cell between the erosion and corrosion areas. When the surface of the test piece is coated with either a less noble or a more noble metal than that of the matrix, its fatigue strength is recovered. The effects of different materials, test liquids and distances between the disc and test piece are also considered.

1. Introduction Some components exposed to cavitation erosion such as hydraulic machinery, ship propellers and pumps often fracture by the combined action of erosion and alternating mechanical stress. For instance, fatigue cracks in a hydraulic plunger pump quickly propagate from cavitation erosion pits around the plunger hole where the alternating stress is concentrated [ 11. One cause is assumed to be that the fatigue strength of the material is lowered owing to the stress concentration at the bottom of the erosion pits formed by the loss of erosion particles. In other cases electrochemical actions in both the components and the atmosphere are increased by the *Visiting Professor, Cavitation and Multiphase Flow Laboratory, Engineering Department, University of Michigan, U.S.A., 1979 - 1980.

Mechanical

52

relative flow of corrosive liquid, the growth of cavitation bubbles, the increase of dissolved oxygen due to incorporation of air, the activation of the damaged surface and the higher temperature of the liquid used. Generally corrosion is accelerated by dissolved oxygen in the liquid. If areas of different oxygen concentrations develop in the liquid, an oxygen concentration cell is likely to be created such that the area of higher concentration is a cathode and the area of lower concentration is an anode. A temperature difference in the liquid will probably also create a thermogalvanic cell. In the case of cavitation erosion in an atmosphere of greater electrical conductivity an electrochemical cell is formed between an eroded and an uneroded area, and the corrosion is accelerated in the uneroded area. Therefore it is necessary to consider the corrosion fatigue strength of both the eroded area and the uneroded area surrounding it. Torsional fatigue tests carried out on steel bar specimens partially exposed to cavitation in salt water or in ion-exchanged water are reported and the mutual relation between erosion and fatigue is made clear mechanically and electrochemically. The corrosion fatigue strength of materials under cavitation erosion can be increased by protecting the surface of the corrosion zone surrounding the erosion zone. Fatigue tests under cavitation erosion were also carried out on steel specimens surface plated with either a less noble or a more noble metal than steel.

2. Test apparatus

and test procedures

Figure 1 shows a schematic view of the test apparatus which consists of a 4 kgf m Schenk type torsional fatigue tester, a magnetostrictive oscillator

Path

of

Fig. 1. Schematic view of the test apparatus: 1,4 kgf m Schenk type torsional fatigue tester; 2, revolution counter; 3, test piece; 4, stress measuring instrument; 5, recorder; 6, dial gauges to set up a stress; 7, ultrasonic generator; 8, magnetostrictive oscillator; 9, amplifying horn; 10, disc; 11, dial gauge to set the distance between the disc and the test piece; 12, cooling bath for the horn; 13, test liquid container; 14, filter; 15, electronic cooling instrument.

53

used for the cavitation erosion test and a cooling bath to keep the temperature constant. Carbon steel containing 0.35% carbon (S35C) was the main test material together with carbon steel containing 0.55% carbon (S55C) and 18-8 stainless steel (SUS 304). The chemical compositions of the materials are listed in Table 1. The shape and dimensions of the fatigue test piece are shown in Fig. 2. The heat treatment used before and after the manufacturing process and the mechanical properties of the test pieces are listed in Table 2. Some S35C steel test pieces were plated with molten zinc and molten tin, the latter first being plated with molten copper. The thickness of the plating was approximately 20 pm in zincification and approximately 10 pm in tinning. TABLE

1

Chemical

compositions

of fatigue

test pieces

Materials

C

Si

Mn

P

S

Cu

Ni

Cr

s35c S55C sus 304

0.37 0.54 0.08

0.24 0.27 0.60

0.74 0.82 1.58

0.011 0.016 0.039

0.026 0.017 0.020

0.08 0.02 -

0.02 8.68

-

TABLE

2

Heat treatment Mate&&

0.09 18.44

and mechanical

properties

of fatigue test pieces

Heat treatment

Tensile strength (kgf mmm2)

Elang&ion (%)

Reduction in area (5%)

Hardness Hv

Before manufacturing process

After manufacturing process

s35c

880 “C, 1 h, normalizing

600 “C, 30 min, annealing

58

33

57

172

s55c

830 ‘C, 1 h, normalizing

600 ‘C, 30 min, 66 annealing

28

46

210

lllO”C, water quenching

-

60

67

72

334

sus

304

The test pieces were set parallel in water a small distance away from the disc surface which was screwed into the free end of the ~p~fying horn of an oscillator. Alternating torsional stress was applied by the fatigue tester at a frequency of 1200 cycles min- I. The shape of the disc is shown in Fig. 3. The test surface was polished with no. 2000 emery paper. The disc material was 18-8 stainless steel with a high cavitation erosion resistance. However, as

54 Ml0

P=l .2i

Fig. 2. Shape and dimensions of fatigue test piece. Fig. 3. Shape and dimensions of disc.

the disc was eroded by cavitation in long fatigue tests a new disc was used for each test. The resonance frequency of the oscillator was 21.5 kHz. The double amplitude of the free end of the disc was measured by a microscope. The amplitudes were almost proportional to the output current of the oscillator and were kept constant during the tests by controlling the currents. The test liquids used were ion-exchanged water (specific resistance greater than 5 X lo6 52 cm) and 3% salt water (which was made by adding 3 wt.% salt to ion-exchanged water). The water temperature was kept at 25 “C and the water circulated at 2.8 1 mine1 between the container and the electronic cooling bath. The distance h between the disc and the fatigue test piece was 0.5 mm for conventional tests and 0.25,1.0, 5.0 or 10.0 mm for special tests. The erosion damage to the fatigue test piece is affected by the diL+mce h [ 2, 31, so h was measured with the dial gauge shown in Fig. 1. The test surface of the disc was immersed to a depth of 3 - 4 mm maintained by the weir in the container.

3. Experimental

results and discussion

3.1. Fatigue strength under cavitation erosion in salt water To study fatigue failure under cavitation erosion a test was carried out where cavitation and alternating torsional stress acted jointly on a 0.35% carbon steel test piece in 3% salt water. The distance h between the vibrating disc and the fatigue test piece was 0.5 mm and the double amplitude A,, of the disc end was 30 pm. The relation between the number of stress cycles N required to induce fracture and the torsional stress amplitude T, is shown in Fig. 4 and is denoted by EF. The relations between 7, and N under corrosion in 3% salt water, denoted by CF, and in air are also shown in Fig. 4. At large stress amplitudes and less than lo6 cycles the fatigue strength is unaffected by cavitation erosion. However, at smaller stress amplitudes and more than lo6 cycles the fatigue strength under cavitation erosion decreases more markedly than that under corrosion.

55

16

6 106 FRACTURE CYCLES

10’ N

Fig. 4. S-N curves of S35C fatigue test pieces in 3% salt water.

The fatigue strengthsat 10’ cycles are 17.0 kg-f mmm2 (24 200 lbf inm2) in air, 11.5 kgf mmd2 (16 400 lbf inm2) under corrosion in 3% salt water and 8.0 kgf mm-s (11400 lbf inT2) under cavitation erosion in 3% salt water. The fatigue strength under corrosion is decreased by 32% and that under cavitation erosion by 53% compared with the value in air. Under cavitation erosion, however, the fatigue fracture of test pieces occurs near the fillet in the corrosion area surrounding the erosion area. The stress concentration at the fillet is 1.06, which is much smaller than that at the bottom of either the erosion or corrosion pits. Therefore the marked decrease of fatigue strength under cavitation erosion is considered to be due to the acceleration of corrosion in the corrosion area. 3.2. Characteristics of erosion damage and fatigue crack propagation Figure 5(a) shows a test piece after erosion fatigue testing. The erosion damage due to the cavitation generated by vibrating the disc occurs on the surface of the test piece within an elliptical area with a major axis of about 18 mm. However, there is considerable resistance to erosion and corrosion in an area about 2 mm wide forming a ring around the erosion area. Thus the test piece surface can be broadly divided into an erosion area (I), an area of corrosion resistance (II) and a corrosion area (III). Figure 5(b) shows the front and back surface profiles of the test piece with readings taken down a centre line following the major axis. On the front, i.e. the side facing the disc, the surface roughness is greatest in the erosion area while the surface of the corrosion resistance area is almost the same as the virgin surface. On the back, i.e. the side facing away from the disc, the surface roughness is greater near the fillet than at the central part of the test piece.

56

(a) Erosion area

The lnagniflcatlon

Back aide

erosion

of

area

-----L 1,

20

*

a.1

10

12

0

11

0

Dis+.ance from ths center

-~-

-.

-

of A-B

~-

pOait.iOn -_ Failure 8 1 0 *. 11 10

of test

piece

L;+_ .‘I

20

mm

(b) Fig. 5. Example of a test piece surface in 3% salt water: 7, = 12.0

kgf mm-2;

h =

0.5 mm; N = 1.95 X 106.

Figure 6 shows the relation of the number N of stress cycles under torsional stress amplitudes T, = 12.0 kgf mmm2 (17 100 lbf in2) to (1) the depth of the maximum pit itself and (2) the distance from the virgin surface to the tip of the maximum depth of the erosion pit. The maximum depth of the corrosion pit remains constant at about 20 pm when N > 10’ cycles but the depth of the eroded area continues to increase with the number of stress cycles. The maximum depth of the erosion pit at N = 2.98 X lo6 when fracture occurs reaches twice the maximum depth of the corrosion pit. Moreover, the maximum distance from the surface is about six times that in the case of corrosion. The distribution of crack lengths of a cross section along the major axis of the test piece is shown in Fig. 7. The number of these cracks is greater in the erosion area than in the corrosion area but the crack lengths of the former are shorter. The maximum crack lengths in the corrosion and erosion areas are plotted against the number of stress cycles under T, = 12 kgf mm-’ (17 100 lbf inm2) in Fig. 8. The crack in the corrosion area initially deepens gradually as the number of stress cycles increases until it reaches a length of about 1 mm. It then propagates very rapidly and eventually causes a frac-

57

a) Max.

0

I

2

3 Cycle

depth

4 number

of

5x106 n

Fig. 6. Depth of the maximum pit itself and distance from the virgin surface to the tip of the maximum pit (maximum depth of erosion). , I I-?~=12.O~q/~',h=a.5~ I

.In 3% salt water

Distance from the center of test piece mm

-

Crack length 0 Corrosion area l

,5540

40

1460 rracrure

_

Erosion area

60 cycles

100 80 ratio n/N %

Fig. 7. Distribution of cracks in 3% salt water: r, = 12.0 kgf mmh2; h = 0.5 mm; IV = 2.98 x 10”. Fig. 3. Relation between the maximum crack length and the cycle number.

ture in the test piece. Cracks in the erosion area reach only half the length of the cracks in the corrosion area and barely propagate even with an increase in the number of stress cycles. Figure 9 shows examples of cracks at the tips of erosion and corrosion pits. The crack in the corrosion area propagates farther in the radial direction, but cracks in the erosion area grow only slowly even with a large number of stress cycles and as they propagate in various directions erosion particles detach from the surface.

58

(a) Fig. 9. Examples of cracks in 3% satt water: (a) erosion 12.0 kgf mmW2; h = 0.5 mm; N = 2.27 X 106.

lb) area; (b) corrosion

area. T, =

Figure 10 shows the Vickers mi~roh~dness below the tips of erosion and corrosion pits measured along the radial direction of the test piece at the ferrite matrix etched with 3% nital. The bottoms of erosion pits have a larger work-hardening layer than those of corrosion pits and a depth of 1 - 2 mm. 3.3. Galvanic cell formation It is assumed that a galvanic cell is formed between the erosion and corrosion areas as a result of differences in temperature, dissoived oxygen concentration, ion concentration, speed of liquid flow and materials. The corrosion potential of the test piece surface is measured with a calomel halfcell as a reference half-cell under test conditions of 7, = 12.0 kgf mmW2 (17 100 lbf ine2) and h = 0.5 mm; the resulting d~t~bution is shown in Fig. 11. A potentiometer with input resistance greater than 1000 Ma is

200 $

190

=

: 2

180

i

170 160 150 140 0

2 Distance

4

0

6

from the surface

mm

2 Distance

4

6

from the surface

mm

(b)

(4

Fig. 10. Distributions of the hardness of the pit bottoms in 3% salt water: (a) erosion = 0.5 mm; N = 2.27 x 106. area; (b) corrosion area. T, = 12.0 kgf‘mm-2;h

74 -520 : 2 & -530 ” : z -540 z d

0 5 10 15 DlstanCe from the center Of (4

test

piece

0 5 10 15 DlStanCe from the center

20

Of

mm

test

piece

20 nm

(b)

Fig. 11. Distributions of galvanic potential in the test piece in 3% salt water: (a) after the start of the test;(b) N = 1.4 X lo6 (15.8 h). 7, = 12.0 kgf mme2; h = 0.5 mm.

used and the tip of the Luggin capillary of the electrode is closed about 0.2 mm away from the surface to be measured. However, since the potential of the eroded surface cannot be measured directly, readings are taken down the major axis along the side and the back at intervals of 5 mm. During the initial period of erosion (Fig. 11(a)) the potential at the side is about 10 mV higher than that at the back but both potentials remain fairly constant in the direction of generation. During the development stage (Fig. 11(b)) the potential close to the eroded area is higher. It is assumed that a galvanic cell is formed such that the erosion area becomes a cathode and the corrosion area an anode. The test piece is cut at a point about 10 mm from the centre as shown in Fig. 12 and annealed in a vacuum. The shorter of the two pieces is coated with insulating paint except for a band 8 mm wide and the other larger piece is coated except for the eroded surface area. The separation between the two cut surfaces is set at about 1 mm, and the shorter piece is connected to the negative terminal of a potentiometer

60

Cutting

urfaceicoating) Insulating

ii

Test

piece

3% salt

water

coating

\

Fig. 12. Measurement of potential between the erosion area and the corrosion area.

and the larger piece to the positive terminal. Before the cavitation test is carried out the potential is zero, but under cavitation conditions of h = 0.5 mm and A0 = 30 pm it is +50 mV. When a galvanometer is used instead of a potentiometer the corrosion current is +0.45 - 0.50 mA, i.e. a current density of about 0.2 mA cmP2 which is large compared with that of 5 PA cmh2 obtained for low carbon steels in 3% salt water [4]. Therefore the decrease of fatigue strength under cavitation erosion is caused by the accelerated corrosion of the corrosion area. However, the fatigue strength of the erosion area is expected to increase so that the erosion area becomes more cathodic, a larger work-hardening layer is formed at the bottom of the erosion pits and crack propagation is slowed. 3.4. Effect of h As the cavitation intensity depends on the distance h [2,3] changing h may affect the relation between the fatigue strength and the erosion rate. Thus similar fatigue tests were carried out with h = 0.25 mm and h = 5.0 mm. Figure 13 shows the S-N curves under corrosion and under erosion with h values of 0.25,0.5 and 5.0 mm. In the low stress range with N > lo6 the fatigue strength under erosion decreases more than that under corrosion for all values of h, and this tendency is more marked as h becomes larger. However, it is expected that when h is further increased the fatigue strength under erosion will improve. The fracture of all test pieces occurs at the corrosion area (Table 3). The size of the erosion area when h = 0.5 mm is similar to that when h = 0.25 mm, but the erosion area when h = 5.0 mm is half the size of that when h = 0.5 mm and is not clearly separated from the corrosion resistance area. As h increases the macrogalvanic cell caused by cavitation erosion is hardly formed, although the corrosion caused by local action cells may be accelerated owing to liquid flow and the increase of dissolved oxygen in the testing liquid. Thus when h is small fracture occurs consistently at the fillet. However, when h is large the fracture points are

61

61

I

I

I

I

107

106

105

FRACTURE CYCLES

N

Fig. 13. Effect of the distance h on the fatigue strength. TABLE 3 Fracture positions of S35C fatigue test pieces in 3% salt water

Distance from the centre of the test piece

Flat surface

Fillet

O-5mm

5-10mm

lo-15mm

CF EF, h = 0.25 mm EF, h = 0.5 mm EF, h = 5.0 mm

0 0 0 0

0 0 0 1

3

5

0

0 2 1

3 10 2

0 0 0

15-20mm

20 mm

scattered throughout the area and fracture may occur even near the middle of the test piece. A relation between the fatigue strengths of the erosion area and of the corrosion area cannot be determined from Fig. 13. The hardness and the cracks of a cross section of the test piece were observed after fatigue testing under conditions of 7, = 12.0 kgf mmw2 (17 100 Ibf inm2), h = 0.25 mm and 7, = 12.0 kgf mmw2, h = 5.0 mm. Figure 14 shows the d~~bu~on of the hardness measured in a radial direction from the tip of the pits in the erosion area. The hardness of the corrosion surface is not changed by h and is similar to that shown in Fig. 10(b), but the hardness of the eroded surface increases markedly for smaller values of h. Figure 15 shows the crack initiation points and cracklengths. When h = 0.25 mm cracks propagate deeply in the corrosion area near the fillet. When h = 5.0 mm comp~atively large cracks were observed even in the erosion area. When h increases further the cracks propagate easily even in the erosion area because of the decrease in the rate of removal of erosion particles. Thus the fatigue strength in the erosion area must be considered to increase rather than decrease with increasing cavitation intensity.

62

0

? Distance

2 from

3 the

4

0 1 2 Distance from

5

surface

mm

(a)

3 the

4

5

surface

mm

(b)

Fig. 14. Distribution of hardness of pit bottoms (in the erosion area): (a) N = 2.85 106, h = 0.25 mm; (b) N = 2.20 x 106, h = 0.5 mm. In 3% salt water; Ta = 12.0 kgf mmv2.

x

T:.

1589

(a)

3465

lb)

Fig. 15. Distribution of cracks: (a) h = 0.25 mm, N = 2.85 2.20 X 106. In 3% salt water; 7, = 12.0 kgf mmm2.

X

106; (b) h = 5.0 mm; N =

3.5. Fatigue strength in ion-exchanged water Figure 16 shows the results of fatigue tests on S35C test pieces in ionexchanged water, in particular under test conditions where A,, = 35 pm with values of h of 0.25,0.5,1.0, 5.0 and 10.0 mm. In the high stress range with N < 10” the effect of cavitation on the fatigue strength is slight. flowever, in the lower stress range with N > lo6 the fatigue strength under erosion increases more than that under corrosion for the same value of N, in contrast to the results obtained in salt water. This tendency is more marked for smaller values of h.

63

x103 28

16

FRACTURE CYCLES

N

Fig. 16. S-N curves of S35C fatigue test pieces in ion-exchanged water.

The surface appearance of the test pieces is similar to that obtained in salt water. All the fracture points of the test pieces (Table 4) are clustered in the corrosion area, except for h = 0.5 mm. This result (Fig. 16) means that the fatigue strengths of both the erosion and the corrosion areas increase. It is thought that the surface of the test piece is quickly passivated as the dissolved oxygen in the liquid increases because of the lack of Cl- ions in ion-exchanged water. The increase of the fatigue strength of the erosion area is assumed to be caused by the removal of erosion particles and work hardening of the erosion surface. However, the fracture at the eroded area when h = 5.0 mm (Fig. 17) is due to the formation of a relatively deep pit at the centre of the eroded area. A small number of deep local cracks occur when the cavitation intensity is small [ 51. This explains why the fatigue strength is lower for h = 5.0 mm than for h = 10.0 mm (Fig. 16).

TABLE 4 Fracture positions of S35C fatigue test pieces in ion-exchanged water Distance from the centre of the test piece

Flat surface 0-5mm

5-1Omm

lo-15mm

15-20mm

20 mm

CF EF, EF, EF, EF, EF,

1 0 0 0 2 0

1 0 0 0 0 0

2 1 1 1 1 2

3 3 6 4 2 4

0 0 0 0 0 0

h h h h h

= = = = =

0.25 mm 0.5 mm 1.0 mm 5.0 mm 10.0 mm

Fillet

64

Fig. 17. Example of fracture in the erosion kgf mmv2; h = 5.0 mm; N = 4.54 X 106.

area in ion-exchanged

water:

T, = 14.0

3.6. Material effects

Figures 18 and 19 show the S-N curves of 0.55% carbon steel (S55C) and stainless steel (SUS 304) respectively, The asterisk represents the test piece fractured at the erosion area. The surface of the S55C test piece is divided into three areas similar to S35C. That of SUS 304 can be divided into the erosion area I and the uncorroded area II with no corroded area even at n = 10’. These erosion areas form an ellipse with a major axis of about 18 mm on both materials but the depth of erosion differs. When compared under the same test conditions (Table 5) the erosion resistance under ~~~at~g stress is higher on S55C and SUS 304 than on S35C. S55C has high erosion resistance but easily forms a galvanic cell so that it is easily corroded. It is also notch sensitive. Thus fracture always occurs at the corrosion area and the fatigue strength under erosion is lower than that under corrosion. SUS 304 does not form a galvanic cell due to cavitation erosion as it has a much higher erosion resistance as well as being a passivation metal. Therefore in the low stress range the fatigue strength under erosion decreases more than that under corrosion, and the fracture occurs at the erosion area because erosion damage is greater than corrosion damage. In X103 28 ._ 24 ; 20

t-"

12

135

106 FRACTURE CYCLES

Fig. 18. S-N

curves of S55C fatigue

N

test pieces in 3% salt water.

65

SUS 304 STAINLESS U,EaF.,

STEEL

h=0.5 NM

10s

107

106 FRACTURE CYCLES

N

Fig. 19. S-N curves of SUS 304 fatigue test pieces in 3% salt water (the asterisk indicates fracture in the erosion area).

TABLE 5 Maximum depth of erosion on every test piece in 3% salt water Materials

Stress amplitude (kgf mm-z)

N

S35C s55c sus 304 sus 304

12.0 12.0 12.0 16.0

2.27 2.22 10’ 2.34

Maximum depth of erosion (Ctm) x 106 x 106 (no fracture) x lo6

125 100 150 64

the high stress range where the test piece undergoes a marked increase of heat due to repeated stress the fatigue strength is higher under erosion than under corrosion owing to the greater cooling effect of the cavitation liquid flow in the erosion area. In this case fracture occurs near the fillet where the cooling effect is poor. 3.7. The fatigue strength of ph ted test pieces Figures 20 and 21 show the S-N curves of test pieces plated with molten zinc and molten tin respectively. The results of Fig. 4 are shown by the dotted and chain curves. In Fig. 20 the fatigue strength of the zincified test piece under corrosion decreases with N but is larger than the corrosion fatigue strength of a normal (unplated) test piece at N = 107. Moreover, the corrosion fatigue strength when A,, = 35 pm differs little from that when A,-, = 20 pm in the high stress range although the strength of the former decreases more markedly than that of the latter in the low stress range. The

66 22

24

ta 16

i -o- C.F.

20 k"

No PLATED S35C STEEL -=C,F. --- E.F., A,=30NM ----IN AIR 6 106

105

107 FRACTURE CYCLES

N

Fig. 20. S-N curves of zincified S35C fatigue test pieces in 3% salt water (compare with the results of Fig. 4).

c G

z

rn

$

,NNED s35C STEEL __. ,TcuC.F. IL -6- E.F., A =35 PM s3! C STEEL No PLATED .,,

r-l

I

A,=30 PM

I 105

I

1

106 FRACTURE CYCLES

10' N

Fig. 21. S-N curves of tinned S35C fatigue test pieces in 3% salt water (compare with the results of Fig. 4).

fatigue strength of the zincified test piece under cavitation erosion eventually approaches the erosion fatigue strength of the normal test piece after about 5 X lo6 cycles for A,, = 35 I.trnand after a somewhat larger number of stress cycles for A,, = 20 ,um owing to the different rates at which the plated zinc is dissolved. In Fig. 21 the corrosion fatigue strength of the tinned test piece is shown to be similar to that of a normal test piece. The fatigue strength of the tinned test under cavitation erosion is lower than either of these corro-

67

sion fatigue strengths and decreases with N but it does not reach the even lower fatigue strength of the normal test piece under cavitation erosion. Whether the plated metals are more noble or less noble than the matrix, plating is one effective method for increasing the fatigue strength of material components subjected to cavitation erosion. 3.3. Crack propagation in plated test pieces Normal test pieces fractured in a zigzag manner across the surface perpendicular to the major axis because the many cracks which occur around the piece connect at this position. However, in corrosion fatigue of the plated test pieces (Fig. 22) the cracks propagate in the direction of principal stress and the intersection of two cracks leads to fracture. Cracks in the zincified test piece propagate quickly to fracture after crack initiation. The cracks in the tinned test pieces propagate more slowly. Crack propagation of the zincified test piece in water is similar to that of a normal test piece in air as cathodic protection prevents corrosion of the matrix. In the tinned test piece a local corrosion cell with the tinning layer acting as cathode and the matrix as anode is formed at defects such as pinholes and the crack tips are dissolved. Thus the cracks propagate slowly. Examples of the surfaces of plated test pieces after fatigue testing under cavitation erosion are shown in Fig. 23. An elliptical area with a major axis of about 16 mm is quickly eroded by cavitation and the plating layer is broken down. In the zincified test piece a corrosion resistance area in the form of a ring 1 - 2 mm wide appears around the-eroded area but the surface of the ring area is composed of the base matrix as the zinc layer has

Fig. 22. Fracture in corrosion fatigue: (a) zincified S35C test piece, ra = 16.0 kgf mme2, N = 7.64 x 10”; (b) tinned S35C test piece, 7, = 16.0 kgf mmv2, N = 1.10 X 10”.

(a)

(b)

Fig. 23. Fracture in erosion fatigue: (a) zincified S35C test piece, 7, = 15.0 kgf mmU2, h = 0.5 mm, N = 1.95 X 106; (b) tinned S35C test piece, 7, = 11.0 kgf mmF2, h = 0.5 mm, N = 6.84 X 106.

68 TABLE 6 Depth of maximum pit for the fractured test pieces of Fig. 20

Stress amplitude (kgf mmw2)

r,

Fracture cycles N

Depth of the maximum pit (pm) xx

YY

Ao=35pm 16.0 15.0 14.5 14.0 13.0 12.0

1.4 1.9 4.7 5.5 9.1 11.2

x x x x x x

106 106 106 106 106 106

12 12 10 17 72 119

42 103 103 125 153 145

1.5 2.4 4.5 > 10

x x x x

106 lo6 106 106

10 12 12 11

64 93 109 104

Ao=20pm 16.0 15.0 14.5 14.0

dissolved. The zinc layer adjacent to the ring area is also gradually corroded as the test proceeds. When the zinc layer is entirely dissolved the effect of the sacrificial anode is lost and red rust appears on the surface of the matrix metal. On the tinned test piece a copper band around the elliptical area appears as the corrosion resistance area. Another band 1 - 2 mm wide around the copper layer is markedly corroded. Outside this area the intact tin layer protects the matrix material. The area of crack growth related to the fracture of the zincified test piece differs between values of N greater and less than 5 X 105. In the higher stress range one or two cracks originate at the point within the erosion area farthest from the disc and propagate quickly. In the lower stress range many cracks grow at the point of minimum distance from the disc and propagate from the corrosion area through the corrosion resistance area in a zigzag manner perpendicular to the major axis of the test piece. In the tinned test piece many cracks grow at severely eroded places at the extremes of the eroded area and at many places in the corrosion area outside the copper layer, and some of these cracks propagate in the direction of principal stress. The propagation rate is slower, particularly in the erosion area, and one of these cracks leads to fracture. Figure 24 shows (1) the mean distance from the virgin surface to the erosion surface, (2) the maximum distance from the virgin surface to the

69

0' 0

I

10

Degreesfrom

20

30

the position

40

50

60

of mlnimum

distance

Fig. 24. Maximum and mean distance from the virgin surface to the erosion surface and depth of the maximum pit on the zincified fatigue test pieces.

erosion surface and (3) the depth of the maximum pit itself (actually the mean value of the depths of the three largest pits). All of these values were measured in the direction of the major axis of the test piece by a surface profilometer starting at the position closest to the disc and rotating the line of measurement in increments of 10” around the curved surface. Table 6 shows the depth of the maximum pit (again calculated as above) for the fractured test pieces of Fig. 20 at the position of minimum distance from the disc and at a second position rotated by about 35” from the first. In general when the distance from the disc is small new erosion particles fall off continuously and erosion damage is severe. However, when the distance from the disc is large cracks caused by collapsed pressure propagate more deeply because the work-hardening layer is improperly formed. Large pits are created but erosion damage is slower. Therefore in the higher stress range the fatigue cracks originate more easily at a greater distance from the disc than at the minimum distance. However, when N >, 5 X lo6 the test has a long duration and the minimum distance becomes larger than the original value owing to the increase in erosion damage. Then large pits are formed even at the point of minimum distance and the fatigue cracks propagate easily. For even longer test durations the zinc layer dissolves completely, corrosion pits are formed and fracture occurs even in the corrosion resistance area. Consequently, the behaviour of these fatigue cracks is related to the fatigue strength of the plated specimens shown in Figs. 20 and 21. 4. Conclusion (1) In the case of S35C and S55C carbon steel in 3% salt water the fatigue strength under cavitation erosion decreases more than the corrosion

fatigue strength but fracture of the former also occurs in the corrosion area outside the erosion area. (2) The fatigue strength of the corrosion area in fatigue under cavitation erosion decreases more than that in the case of corrosion fatigue owing to the formation of a macrogalvanic cell between the corrosion and the erosion areas. The fatigue strength of the erosion area increases owing to the formation of a thick work-hardened layer and to the interference of erosion particle detachment in crack propagation at the bottom of the erosion pits. (3) In the case of ion-exchanged water fracture of the test pieces also occurs at the corrosion area except in a few cases. The fatigue strength under cavitation erosion increases more than the corrosion fatigue strength as the surface of the test piece is quickly passivated owing to the increase of dissolved oxygen in the liquid from cavitation activity and because the fatigue strength of the erosion area also increases. (4) In the case of SUS 304 stainless steel the fatigue strength under cavitation erosion increases more than the corrosion fatigue strength in the high stress range and fracture of the test pieces occurs at the periphery where the influence of cavitation flow is slight. In the low stress range, however, fracture occurs in the erosion area owing to cavitation erosion damage and the erosion fatigue strength decreases more than the corrosion fatigue strength. (5) When the surface of the test piece is coated with either zinc, which is a less noble metal than that of the matrix, or tin, which is a more noble metal, the erosion fatigue strength increases more than that of a normal test piece. Thus plating is one effective method of increasing the fatigue strength of material components on which cavitation erosion occurs.

Acknowledgments Sincere thanks are due to Professor F. G. Hammitt, University of Michigan, and Professor K. Endo, Kyoto University, for their kind suggestions and discussions. The typing and reproduction were supported by Office of Naval Research Contract N00014-76-C-0697 at the Cavitation and Multiphase Flow Laboratory, Mechanical Engineering Department, University of Michigan, Ann Arbor, Michigan.

References 1 Kikoi no Songai, 10 (4) (1967) 94 (in Japanese). 2 K. Endo, T. Okada, T. Nakano and M. Nakashima, Trans. Jpn Sot. Mech. Eng., 32 (237) (1966) 831 - 841. 3 T. Okada and Y. Nishimura, Mem. FQC. Eng. Fukui Univ., 19 (1) (1971) 19 (in Japanese). 4 H. H. Uhlig, Corrosion and Corrosion Control (transl. S. Matsuda and I. Matsushima), Sangyotosho, Tokyo, 1968, p. 85 (in Japanese). 5 K. Endo and Y. Nishimura, Tmns. Jpn Sot. Mech. Eng., 38 (309) (1972) 924.