University of Michigan Department of Mechanical Engineering Cavitation and Multiphase Flow Laboratory Report No. UMICH 014456-45-I Influence of Cavitation Erosion on Corrosion Fatigue and Effects of hic si*. tance to thllat influence by the Surface Coatings?T Okdca,!\. iwazu tT, Kawasaki Tsunenori OKADA: University of Fukui, I'aculty of Engineering, Mechanical Department, Bunrfnyo 3-9-1, Fulkui. Professor. iKaoru AHA.AZU: Industrial Research Institute, Ishikawa, Yoneizumi 4-133, Kanazawa. ELngineer. Jlisaimits u Ai'&,1~SAKI: Toyoda OQoki Co., Ltd.L, Asahi 1-1, lKariya, Aichi Prefocture. Engineer "Office of Naval Research Contract NO. NOOO14-78-C-0697 June, 1979

ABSTRACT When fatigue tests are carried out under cavitation erosion in 3% salt water, the fracture of S35C test pieces occurs in the area of corrosion, as in the case of corrosion fatigue without cavitation erosion. However, the erosion fatigue strength decreases more than the corrosion fatigue strength by means of the formation of a macro-galvanic cell between the erosion area and the corrosion area. When the surface of the test piece is coated with eithe 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.

I. Introduction Some components exposed to cavitation erosion, such as hydraulic machinery, ship propellers and pumps, have often been fractlared by the joint action of erosion and alternative 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 activity is concentrated. in this case, one of the causes is considered to be that the fatigue strength of the material was lowered Uue to the stress concentration at the bottom of the erosion pits wthich were formed by the fall-off of erosion particles. in othier cases, electrochemical actions in both components and atmosphere are increased by the relative flow of corrosive licquid, the growth of cavitation bubbles, the increase of dissolved oxygen due to the incorporation of air, activation of tlhe damag;cd surface, higher temperature of the liquid used, c o (I. In general, corrosion is accelerated due to the presence of dissolved oxygen in the liquid. Furthermore, if areas ol' different oxygen concentratiors 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 liqaid will probably also create a thormo-galvanic rell. In the case of cavitation erosion in an atmosphere of larger electrical crond:u~ct;ivity, an electrochemical cell is formed 0 etween an

2 eroded area and an uneroded area, and the corrosion is accelerated in the latter part. Therefore, we have to consider the corrosion ~Ca".:.i;ue strength of both the eroded part and the uneroded part around it. in this report, torsional fatigue tests are carried out on steel bar specimens, which are e xposed partially to cavitati.on in salt water or in ion-exchanged water, and the mutual relation between erosion and fatigue is made clear mechanically and ele ctrochemically. In addition, in order to increase the corrosion fatigue -trength of the materials under cavitation erosion, it is very AI' Cc ctiLveo to protect the surface of the corrosion zone around thle srosion zone by some suitable method. In this paper, fatigue -tests under cavitation erosion are also carried out on specimens where the steel surface is plated with either a less noble metal or a more noble metal than itself, and the results of such plating i, considered as well. II. Testing Apparatus and Test Procedures Figure 1 shows a schematic view of a testing apparatus, which consists of a 4 kilograml-meter Schenk-type torsional fatiglc teDster, a malnetostrictive oscillator used for the cavita'tion erosPion test and a cooling bath to keep the temperature constant. A piece of carbon steel of 0.35YC(S35C) is thle mlain tos(:.ait;.terl.. d.sed, alcoity with carbon steel of 0.55r.C (S55C) rw(l;l:3-, stainless stoel (LUS 304). T'l'he chemical compositions

3 of t;liese materials are listed in Table 1. The shape and dimensions of the fatigue test piece are shown in Fig. 2. The heat Lre-wilomnt used before and after the manufacturing process and thie iechanical properties of these test pieces are listed in rlyabli, 2. Encldiition, some S35C steel test pieces are plated on the surface with molten zinc and molten tin, respectively, the latter being first plated with molten copper. The thickness of the plating metals is approximately 20 microns in zincification arid appro;ximately 10 microns in tinning. I'he te:.t pieces are set parallel in water and held a small distance away froi;i the disc surface, which is s crewed into the free end of the amplifying horn of an oscillator, and alterlnative torsional stress is applied by the fatigue tester. Trhe Crequency of -this fatigue tester is 1200 cycles per minute..'Thle shape of the disc is shown in lFig. 3. The testing surface of the disc is polished with /i2000 emery paper. The material of the disc i.s 18-3 stainless steel having; a high resistance to cavitation erosion. However, as the disc is eroded by cavitation in long fati,-rue tests, the disc is exchanged with a now one after a sin!gle test. VWhlen this disc is screwed into the free end of the ampliifying horn, the resonance frequency of the oscillator is 21.5 KH-Iz.'The double amplitude of the free end of the disc is measured by a reading microscope in the air. These amplitudes are nearly proportional to the output current of the oscillator, and tl;-ey arc kept constl;ant during the tests by controlling t1hei' c( lrront.

'iTest liquids used are ion-exchanged water (specific resistance: over 5 x 10~'cm) and 37. salt water (which is made by adding ~ weight-percent of salt to ion-e,:changed water.) The water termperature is kept at 25~ C and circulates at 2.8 1/min between the container and the electronic cooling bath.'T'he distance h between the disc&ad the -fatigue test piece is h11).5ranm under, conventional tests, with special instances whlere h10.25, 1.0, 5.0, or lO.U mm. As erosion damage to the (2) (3) fatigue test piece is affected by the distance h, this h is ineas ur d with the dial gauge shown in Fig. 1. The testing s.i-,face of the disc is immersed to a depth of 31v4 mm from the water surface, which is kept level by the weir in the -ietainer. III. Experimental iesults and Discussion II1.1 The faLtigue strength under cavitation erosion in salt water''o clarify the fatigue failure under cavitation erosion, a test is carried out where cavitation and alternative torsional stres aict jointly on a test piece of 0.357, carbon steel in 37 salt water, under conditions where the distance h between the vibrating disc and the fatigue test piece is 0.5mm and the double amplitude A.of the disc end is 30 microns. The relation between the number of stress cy-cles N required lt; induce fracture aidL the torsional stress amplitude *'is showun in F'ig. 4 (notation L.F.). The relation T -N under corrosion in 3Y, salt water (notation C.F.) and that in air

is also shown in i'sig. 4. At a large stress amplitude with less than 106cycles, the fatijuea strenogth is not affected by cavitation erosion. ~1owevqr, Atd a smaller stress amnplitude with more thanlO~ cycles, the f'ati. gue s tren1gth under cavitation erosion decreases more mnarkedly than that under corrosion.'The fatigue strengths at l0 cycles are 17.0 kg/nun2 (24,200 psi) (16,400 psi) in the air, 11.5 kg/mm2 /nder corrosion in 3%. salt water, (11,400 psi) and 3.0 kg/rmn2/under cavitation erosion in 3Yo salt water. That is, the fatigue strength under corrosion: is decreased by 32%. f riom that in the air, and that under cavitation erosion by 53%. Under cavita tion erosion, however, the fatigue fracture of test pieces occurs near the fillet, in the corrosion area outside of the erosion area. The stress concentration at the fillet is 1.06, much smaller than that at the bottom of either the e rosion pit or the corrosion pit. r'herefore, the remarkable decrease of -the fatigue strength under cavitation erosion is considered to be due to the acceleration of corrosion in the corrosion area. I]I.2 The feature of erosion damage and fatigue crack propagation Ain examLrple of a test piece as it appears after the erosion fatigue test is s hown in Fig. 5(a). The erosion damage occurs on the surface of the test piece within an elliptical area w. th a major axis of about 18nun, due to the cavitation generated by vibrating the disc. However, the area of about 2mnu in width thlat forms a ring around the erosion area offers great

resistance against erosion and corrosion. Thus, the test piece surface m;-ay be broadly divisible into an erosion area (I), an area of corrosion resistance (II), and a corrosion area (III). 1iigulre 5(b) shows the front and back surface profiles of ti.c t esL; piece, witil readin,;s taken down a center line follow-:inr the major axis. O!. the front, the side facing the disk, tile surface roughntess 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, or the sidefacingaway fromn the disc, the surface roughness is greater near the fillet than at the central part of the test piece. Y'igcure 6 shows the relationship of N (the number of stress cycles under torsional stress ailplitudes (C=12.0 kg/mm2(17,100 psi 1) the depthl of the maximum pit itself, and 2) the distance from the virgin surface to the tip of the maximum depth of erosion The maximumr depth of the corrosion pit remains constant at about 2)Omln at more than N-=1C5 cycles, but the depth of the eroded area continues to increase with the number of stress cycles. The maximum erosion pit depth at N-2.93 x 106, when fracture occurs, roaches twice the depth of the maximum corrosion pit. The maximum distance from the surface, moreover, comes t o about six timres that in the case of corrosion.'Jhe distribution of crack lengths at the cross section along the m!ajor axis of the test piece is sho;'n in Fig. 7. The number of these cracks is greater at the erosion area than at the corrosion area, buti; the crack lengths of the former are shorter.'or

examnle, the maximum crack lengths in the corrosion area and in the erosion area are plotted in ii'ig. 8 against the number of (17,100 psi) stLc; cycles unielor S=12 kg/n;l2/n In the early stages, the crack in the corrosion area de-pens','radually with the increase of' Lho numb-er of s tress cycles until it reaches about lmm in length. ifter that it propagates very rapidly and eventually.ives rise to a fracture in the test piece. On the other hand, from the outset the cracks in the erosion area reach only half the length of the corrosion-area cracks, and they hardly proa.)gaate even with increase of stress cycles. Figure 9 shows an example of cracks at the tips of erosion pits and corrosion its. TiThe crack in the corrosion area propagates farther in tlic rad.ial direction, but the cracks in the erosion area grow on.ly slowly even with a large number of s tress cycles, and as they propagate in various directions erosion particles fall off f. rcjr the surface. -ig-ure 10 shows the micro-Vickers hardness below the tips of' erosion pits and corrosion pits. These hardnesses are measured along the radial direction of the test piece only at the ferrite matrix etched with 3o nital. The bottom of the e rosion pits has a larger work-hardening layer than that of corrosion pits, and its depth reaches 1-2mnm. 11i.3 Galvanic cell formation It can'be considered that a galvanic cell is formed between the erosion area and the corrosion area by differences of tempera;ture, disoolvoed oxygen concentration, ion concentration,

8 speed of liquid flow, materials, etc. The corrosion potential of t he1 t est piece surface is measured with a calomel half cell] (17,10 as a reference hal.r-ccll under test conditions of -= 12.0 kg/rnm/, h= 0. $mi,; the resllting distribution is shown in Fig. 11. A potent;iometer with in.ut resistance larger than 1000 IvTL is used, and the t ip of the Luggiin capillary of the electrode is closed aboult QC.;.. -vway from the surface to be measured. iiowever, since the )otential of the eroded surface cannot be itieasured directly, readings are taken along the side and the bac!, dovn the major axis, at intervals of 5mrtn. iJuring the initial pevriod of erosion, as shown in Fig. 11(a), the potential of the side is about 10mV higher than that of the back, but both potentials remain fairly constant in the d irec'ticn of the generation. ~ uring the development stage, as shown in i'ig. 11(b), thle potential of the place close -to tihe e roded area is higher. It is considered that'u galvanic cell is formed sauch that the erosion area becomes a cathode and the corrosion area an anode. Also, as shown in Fig. 12, the test piece is cut at a point about 10ramr from the center and is annealed in a vaccura. Mfter that, the shorter of the two piecets is coated with insulating paint except for a band 81nm wide, and the other larger piece is coated except fozi the eroded surfacc area. The separation between the two cut surfaces is closed to about 1 imiu; the shorter piece is connected with the (+)terminal of the potentiometer and the larger piece with the (-) Lerm.nal]..iefoJiO the cavitation test i~ carri.,d out the potenti.

is zero, but under the cavitation conditions of h-0.5mm and A0=C6UPm it reaast 50nNV. On the other hand, when the galvai:lomete:) is used inst;ead of the potentiometer, the corrosion curI-rent,i~ves -t- 0.45-0.50miA. Converted to current density tl-,.his becoles about 0.2mki/cm22. This value is very large in comparison with the less than 5MA/cm2 value of low carbon steels in 367 salt water. Consequently, the decrease of fatigue strength under cavitation erosion is caused by the accelerated corrosion of the corros;ion area. On the other hand, the fatigue strength of: the erosion area is expected to increase, so that the erosion ar.:ea becones more cathodic, a larger work-hardening is formed at the bottom of the erosion pits, and crack propagation is slowed (as jlmade clear in the previous chapter.) Tii.4 The effect of h As the cavitation intensity depends on the distance h, chuang;ing-s h mnay affect the relation between the fatigue strength and t iic erosion rate.'Thus sifmilar fatigue tests are carried out.;ith h=0.25 rmml and h=0.50 nmm. iigure 13 shows the S-N curves under corrosion and under erosion with H=0.25, 0.5, and 5.0 mm. In the low stress ranlje of more than Nw106, the fatigue strength under erosion uccrc:Asczs more tlhan that under corrosion for all values of h. A; i]lusbrautod in'able 3, Uhli fracture of all test pieces occurs at the corrosion area. The size of the erosion area under h"i.5i,;i al;lost aftreos with that under h0O.25mn, but the erosion area under h-O.f5mrn is halrf the size of the former

10 and it s area is not clerly separated from the corrosion resistau.nco area.'.iith increased h the macro-galvanic cell causedi b.y cavitation erosion is hardly for,,Led, although the corrosion caaUsedi oy tlle local-action cells may be accelerated due to La11 liqui.d flow and the i:ncrease of dissolved oxygen in the test in,; liquid. Thus when h is small fracture occurs consistenitly at the filleL;. ilowever, when h-is large, the fracture po:ints are scattered throughout the area and fracture may occur even near the mi.ddle of the test piece. But the relation between the fatigue strengths of the erosion area and of the corrosion area cannot be clarified from i,'ig. 13.'lhe hardness and the cracks of a cross section of the test piece are observed after the fatigue test run under the test conditions of = 12.0 i)/rnl2/, (hA25P d a=1.0 kg/mm2 h=5.0m;,. Figure 14 shows the distribution of hardness measured in a. radial direction from the tip of the p)its in the e rosion Uare.'i"'le hardness of the corrosion surface is not changed by h, similarly to what was shown in Fig 10(b), but the hardness of the e roded surf ace increases extremely in the case of s;l alLcr 1i. il'igure 15 shows the crack initiation points and -the crack lengths. Under h=0.25rmn, the cracks propagate deeply in -the corrosion area near the fillet. however, under h0O.5mm, coirnparatively large cracks are observed even at the erosion area. iWhen. h becomes larger, the cracks propagate easily even in the erosion area because of thedecrease of the Oll-off of the eroci oH pa~.nrticl.;s. That is, the fatigue strength at t;he erosion

11 area must be considered to increase rather than decrease with an increase of cavitation intensity. 111.5 1Fatigue strength in ion-exchanged water 2igure 16 shows the rsults of fatigue tests run on S35C test pieces in ion-exchanged water, in particular under the test conditions of a double amplitude Ad 35pm and the distances i:O0.25mm, 0.5J1-'m, 1.0,mm, 5.0mu, and 10.0Om. In the high stress range of less than N=106, the cffect of the cavitation on the:atigue strength is slight. hlowever, in the lower stress range of' moore than N 106, the Ei. strength increases more than the C.B. strenigth at the same stress cycle, in contrast with the result in salt vwater. This tendenc;y is more remarkable the smaller the distance h is.'he appearance of the s urface of the test pieces is similar to Lth-oat i.n salt water, discussed in the preceding sections. All the fracture points of the test pieces, as illustrated in Table 5, are clustered at the corrosion area, except in the case of h=b.Ormil. Consequently, this result, shown in Fig. 16, means thal the fatigue strengths of both the erosion area and the corrosion area increase. It is thought that the surface of the test piece is quickly passivated with the increase of dli, Asolved oxygen in liquid because of the lack of C1 ions in ion-exchanged water. On the oth1::r hand, the increase of the fatigue strength of the erosion area, as described in the previous section, is considered to be caused by the fall-o'f of erosion particles and the work-hardening of the erosion surface.

12 The fracture at theeroded area under h 5.0Unmm, however, is due to the fact trnat, as show n in Fig. 17, a relatively deep pit is f orlned at the center of the e roded area and leads to fracture. Tt -is rep)orteal that a small number of deep local cracks is caused () wriien the caU7iLtation intensity is small. This characteristic property appears to explain the phenomenon that the fatigue strengt is smaller for h=O-.5rmma than for h=lO.0mm, as shown in Fig. 16. 111.6 The effects of materials lic'ures 13 anci 19 show the S-N curves of 0.554C carbon steel (S55C) and stainless steel (SKS 304), respectively. In the figure: t.hec asteriskt represents the test piece fractured at the erosion area.'i'The surface of the b$5C test piece is clearly divided into t-lirse areas, similar to S35C. That of SUS 304 can be divided into the e rosion area(I) and the uncorrosion area (II), with no corroded area even at n=107. These erosion areas form an ellipse with a major axis of about 18mm on both materials, but the depth of e:josion differs. WVhen they are compared under the samne test conditions, as illustrated on Table 5, the erosion resistance under the alternative stress is higher on S55C and SUS than on 335C. The nature of 555C material is to have high erosion resistance but it easily formas a galvanic cell so that it is easily corroded. it also has a large notch sensitivity. Thus, the fracture always occurs at the corrosion area and, on the whole, tne fatigure strer under erosion is lower than that under corrosion. On the other he SUS 304

material does not form a galvanic cell due to cavitation erosion, for it has much higher erosion resistance as well as being a pasivitation metal. So in the low stress range the fatigue strength under erosion decreases more than that under corrosion, and the fracture occurs atL the erosion area because erosion damage is greater than corPosi:3ion. In the high stress range, where the test piece undergoes extreme increase of heat due to the repeated stress, the f'atigue strength is higher under erosion than under corrosion. This is due to the greater coolin, eEffect in the erosion area bj the cavitation liquid flow. in this case, the fracture point belongs near the fillet where the cooling effect is bad. 111.7 The fatigue strength of plated test pieces. i'igures 20 and 21 show the S-N curves of test pieces:,,)atctd with molten zinc and molten tin, respectively. In addilition, the results of Fig. 4 are also shown by a dotted line and a chained line. In Fig. 20 the fatigue strength of under corrosion the zincified test npiece/decreases with N but it is larger than the corrosion fatigue strength of a normal (unplated) test piece oreover, the /corrosion fatigue at i 10k/ Moreover, tile strength uncder the double amplitude A6=35pm differs little from that under I A.=20pm in the high stress range, tnough the strength of L.to forltiov LU Iocr'ass lioro remarkably than that of the latter in tle low stress range. In the long run, the fatigue strength under cavitation erosion of the zincified test piece/approaches the erosion fatigue strength of normal test uiece al'ter about n-5 x 106 for Ar35jm and after

14 a somewhat larger number of stress cycles for Ao=20um, due to thle different rates at which the plated zinc is dissolved. corrosion fatigue In Fig., 21, the / strength of the tinned test piece is shown to be almost the same as the corrosion fatigue s-trength of a normal test piece. The fatigue strength of the tinned test under cavitation erosion is lower than either of these. corrosion fatigue strengths and decreases with N, but it does not reach the even lower fatigue strength of the normal under cavitation erosion. test piece/. oghether the plated metals are more noble or less noble than the matri:c, then, plating is one effective method for increasing the fatigue strength of material components on whi ch cavitation erosion occurs. i[[.a Crack propagation in plated test pieces.'his section will consider the various surface phenomena Uthat oc;,I2r on the test pieces as brought forward in the previous sections. The normnal tetl; piece fractured in a zigzqg across the s urface perpendicular to the major ais because the many cracks which occur around the piece at this position connect. However, in the corrosion fatigue / of the plated test pieces, as shown in Fig. 22, the cracks propagate in the principle stress direction, and two cracks crossing in an X lead to fracture. The cracks in the zicri cied test piece propagate quickly after crack initiation, and lead to fracture. The cracks in thile tinned test pieces propagate more slowly. In other worus, the crack propagation of the zincified test,piece in water shows tendencies similar

15 to tha t of a normal test piece in the air, since the cathodic protection due to zincification keeps the matrix from corroding. In thle tinned test piece, a local corrosion cell with thhe tinning layer acting a s cathode and the matrix as anode is formed at d.ecctLs such as pinholes, so that the tips of the cracks are dissolved. Thus the cracks propagate slowly. some examples of the surfaces of plated test pieces after under cavitation erosion fatigue teAt runs/are shown in Fig. 23. An elliptical area with a major axis of about 16mm is quickly eroded by cavitation and the pla ting layer is broken down. Moreover, in the zincified tet&t eiece, a corrosion resistance area in the form of a ring ofl 1l-ian width appears around the eroded area, but the surface of the ring area is the base matrix because the zinc layer is dissolved. The zinc layer adjacent to the ring area is also gradually corroded with the increase of the test duration.'ihen the zinc layer is entirely dissolved, the effect of the 3-)acrifi.cial anode is lost and red rust'appears on the surface o. the matri:. On the tinned test piece, a copper band around the elliptical area appears as the corrosion resistance area. I'urthermore, another band of 1-2 mm width around the copper layer is remarkably corroded. Outside this area, the tin layer remains com plete a&-)d protects the matrix. The asea of the crack growth related to the fracture of the zincified test piece differs between cycles of more and less than.1=5 x 105. In the nigner stress range, one crack,.or;wo cracks in an X., originato at thle point within the erosion area tilat is fart;iest fromn thn disc, and propagate quickly.

16 in the lower stress range, many cracks grow at the point of nini. lum distance from the disc and propagate from the corrosion area through the corrosion resistance area in a zigzag perpendi'cular to the major axis of the test piece. On the other hand, in the tinned btest pie c.A, malny cracks grow at severely eroded pl.aces in the far edges of the eroded area and at many places in thne corrosion area outside of the copper layer, and s:ome of these cracks propauate in the principal stress direction. The propagation rate is slower, iarticularly in the erosion area, and one of these cracks leads to fracture. F0igure 24 shows: 1) the mean distance from the virgin surfac.e to the -rosion surface; 2) the maximum distance from the virgin sur:'face to the erosion surface; and 3) the depth of tilhe iilaxil.um pit itself (actually the mean value of the three laxrgest pits' depths). All of these values are measured in tthe uirection of the maj.or a xis of the test piece by a sur-;face rorofilometer, beginning at the position closest t o the disc and. rotating the line of measurement by units ol' 10 deg;recs around the curved surface. Table 6 also shows the depth of the maximum pit itself (again calculated as above) for the fractured test pieces of Fig 20, at the position of mini-,nui distance to the disc and at La seconc position rotated by hb.1ub t 35 degr.ees from the first. fin general, when LIne caistance Froin the di~jc is small, new ero.sion particles fall off continuously and erosion damage is severe. On the othcr hand, when the distance from the disc

17 is large, the cracks inaue O,; collapse Lressure propagate more deeply because the w ork-hardening layer is hardly formed, and lare soits are created. irut erosion damage is slower. Therefore in the higher stress range the fatigue cracks originate Inorc eas:Lly at the position of greater distance from the disc trian at the minimum.1 distance. Howe, er, at more than about N-5 x lu6, the test duration time is long and the mini.-uam distance becomes larger than it was originally, due to the increase of erosion darae.'iThen large pits are r'ormed even at the point of nmin.inlllml distance, and the fatigue cracks propagate easily. lit'L even longer test duration the zinc layer dissolves completely, corrosion pits arPe formed, and fracture occurs even in thie corrosion resistance area. Consequently, the behavior of these fatigue cracks is related to the f atigue strength of tile plated specimens shown in Figs. 20 and 21. V. Conclusion To clarify the effect of cavitation erosion on fatigue strencgth, fatigue tests have been carried out in 3Y salt water and ion-exchanged svater. The results obtained are as follows: (1) In the case of S35C and S55C carbon steel in 3 salt water, tne r'atigue strength under cavitation erosion decreased more than the corrosion fatigue strength, as explained in (2) below, but fracture of the former also occurs in the corrosion area out;i.de ot. the erosion area.

18 (2) T''he fatigue st-iength oel the corrosion area in the fatigue test under cavitation erosion decreases ilore than that in the case of corrosion fatigue, due to tle macro-galvanic cell formed between CLe corrosion area and the erosion area. On the other hand, the fatigue stren-gth ol' the erosion area increases due to the formation of a thicXi layer of work-hariening and to the interference of erosion particle fall-off in crack: orouagation at the bottom of the erosion pits. (3) In the case of ion-exchanged water, the fracture of the test pieces occurs, similarly, at the corrosion area, except in a fev~ cases, but the fatigue strength unde cavitation erosion corrosion fatigue increases more than the / strength. This is because the surface of the test piece is quickly passivated du-., (to thie increase of dissolved oxygen in the liquid from cavitation activity, and because the fatigue strength of the erosion area also increases as explained in (2). (4) in the case of 3US 304 stainless steel, the fatigue under cavitation erosion corrosion fatigue strength/increases more than the strength in the high stress range and the fracture of the test pieces occurs at the periphery where the influence of cavitation flow is sligh.t. In the low stress rang5e, however, the fra'.ture occurs in the erosion ar a due -to th( damaec of cavitation erosion fatigue under erosion and thle / stren;tl /uocreases more than the corrosion fatigue strength. ( F) )When the surCace of' he tCest piece is couted with

19 either zinc, a less noble metal, or tin, a more noble metal than that of the matrix, the E.F. strength increases more than that of a normal test piece. That is, plating is one effective method for increasing the fatigue strength of material components on which cavitation erosion occurs. Acknowledgment Sincere thanks are due to F. G. Hammitt, Professor of The University of Michigan and K. Endo, Professor of Kyoto University for their kind suggestions and discussions. Typing and reproduction supported by ONR Contract N00014-76-C-0697. References 1) Kikai no Songai, 10-4 ( 1967 ), 94. ( in Japanese ) 2) K. Endo et al: JSME, 32-237 (1966-5), 831. 3) T. Okada and Y. Nishimura: Memoirs of the Faculty of Engineering Fukui University, 19-1 (1971-3), 19. ( in Japanese ) 4) Uhlig, H.H.(Matsuda and Matsushima; Translation); Corrosion and Corrosion Control, (1968), 85, Sangyotosho. 5) K. Endo and Y. Nishimura: JSME, 38-309 (1972-5), 924.

Table 1 Chemical compositions of fatigae test pieces'laterials C Si Mn P S Cu Ni |Cr S35C 0.37 0.24 0.74 0.011 0.026 0.08 S55C 0.54 0.27 0.82 0.016 0.017 0.02 0.02 0.09 SUS304 0.08 0.60 1.58 0.039 0.020 -8.68 18.44 Table 2 Heat treatment and mechanical proderties of fatigue test pieces Heat treatment Teneile Elon i Reduct Hardness Materials srength a ess BgforB~~~~enth alon in ans __ _ g- Prcer oc ce. s kg/mm2 % |% H 880~C 600oC S3SC l h 30 min 58 33 57 172 Normalizing Annealing 830 C 6000 C S55C 1 h 30min 66 28 46 210 Nor.malizing Annealing S1 110. SUS 304 || tqSgnching | 1~3 60 67 | 72 334

Table 3 Fracture positions of S35C fatigue test pieces in 3% salt water Flat surface Fillet Di stance from the 0 —5 5-10 10-15 15~v20 20mm center of test piece mm mm mm mm CF 0 0 3 5 0 EF h =0.25 mm 0 0 0 3 0 EF h =0.5 mm 0 0 2 10 0 EF h=5.0 mm 0 1 1 2 0

Table 4 Fracture positions of S35C fatigue test pieces in ion-exchanged water Flat surface Fillet Distance from the 0-5 5 —10 10-15 15-20 center of test piece mm mm mm mm CF 1 1 2 3 0 EF h=0.25 mm O O 1 3 0 EF h =0.5 mrm O 1 6 0 EF h=1.0mm O O 1 4 0 EF h=5.0 mm 2 O 1 *2 0 EF h=10.0 mm O O 2 4 0 _~~~~~~~~~~~~~~~~~~~~~~

Table 5 Maximum depth of erosion on every test piece in 3% salt water I Fracture Maximim depth Stress amplitude et Materials kg/MM2 cycles number of erosion S35C 12.0 2.27x106 1 125 S 55C 2.22 x 106 100 SUS 27 " 107 No15 fracture 1 /, 16.0 2.34 x 106 64

,I 0 Path of cooling water 1 4 Kg-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 up the distance between the disc and the test piece 12 Cooling bath of the horn 13 Container of test liquid 14 Filter 15 Electronic cooling instrument Fig.1 Schematic view of a testing apparatus

Fig.2 Shape and dimensions of fatigue test piece M10 P=1.25 2-4 dia, depth 5 20 -- Fig.3 Shape and dimensions of disc

(Eu22 20 -.-.:~. 18 16.14 -- E12' 8 0S35C ~F 0 10 - In the air ----- In 3~/o salt water i E F h-0.5 mm 10 10 6 107 Fracture cycles N Pi eL.-N curves of S35C fatigue test pieces in 3%~ salt water

(a) Erosion area --- r... - IT i The magnifiction of A B Back side of the erosion area --- Failure position. ~ 20 10 0 10 20 Distance from the center of test piece mm (b) In 3% salt water, t= 12.0 Kg/rnrn, h=0.5 rm, N=1.95x10" Fig.5 Example of a test piece surface.

160 _________/. Ta=12.14, 1 kI/mmz "40 " iEX o... Max. depth of ere sion 120o"'. w —. "... -'-.''a m. Depth of max. pi' -100 - m:' A a Corrosion pit (front), - -;-' -.e _ i Corrosiaon pit(back) 80 60 40 D. -._. /........ 20 000 —-=..... 20 —.1. 2 3.- 4 Sx506 Cycle number n Fig.6 Depth of the maximum pit itself and distance from the virgin surface to the trip of the maximum pit (maximum depth of -erosion)

Distance from the center of test piece mm 0 5 10 15 20 b.0Z g 200 M- 400. i o ~ 0) 85.0 40 bO 200 Ile d o 6;00 0_> 5540 1460 In 3% salt water, 0a=12.Okg/mm2, h=0.5mm, AN2.98x106 Fig.7 Distribution of cracks

-. - r l — a 12. O k / O2 h=05mmrn In 3 %,.salt water 5E 5, Crack length _- 0 Corrosion area C Erosion area z 330 40 50 60 70 80 90 100 Fracture cycles ratio n/N %/o Fig. 8 Relation between the maximum crack length and the cycle number

~~ii:::::ii: ~~~ ~ liii! iii i!~a~i~;IE~-~l~:~.~;i i}I.~/':"::'~~~~~~ ~~~~~~~~"-'~'?,~: ~~;.iI:: ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ _~: ~.. ~. [111:icii~i~~i:~i:::i:::::: -- ~ i:~%,::::,':i:: i ~:: ilii:i:i'i:?~,::....:~': ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~~~~::i":: ~ ~;:/ii ~:~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.. E. -.:...:;:::i:: i::::::(:::I::::::::::::::::l:::::i:::: iiii~ ~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~:11:j::: _?:::i::i'i~:,!s:::::~i::I f:i.::::::i:i:.:::::: ~iiic:-:::ii-i3:~i-iii:::- -:::::icii:;:e:i:i::::::c:: -:i'::iii~~~i I:0.2 m m ~!i:: i~i:i:i::::ii::::::::i::i~~~~~~~::i i::j~~~~i i i.:::..::.:~~~~~~~~:::::::il:::i:: i~~~~~~~~ ~ i:::li::l:"i'~:~:: i::.':.: J' -~-,.a~ ~ ~~~~~~~~~~~~~~::':. i ~.~'::ii:ii~::~c( b):i Corrosion area::::::~~::::::::i:: i::::;Fig.9:: ~::: E xa mpl~-'_:i:-::i:::ii~~i::::::iof cracksi:i:::..: i:::-j::':::::'::::

200 T a=12. k./mm 200.. 2a= 12.0 k- /mm2. 0 I1 3 % salt water x In 3 % salt water 190 v_... h=05mm 0 190 h=0.5mm 0j I \o { N=2.27xl Oo 2 N= 2.21 x10 0 9d U310 00 40 140 160 160 150 0- - _ 0 )00 o.. 140 _______ 140 0 2 4 6 0 2 4 6 Distance from the surface mm Distance from the surface r.m (a) Erosion area (b) Corrosion area Fig.10 Distributions of hardness of pit bottoms

r-H 00 -540 0 5 10 15 20 Distance from the center of test piece m M (a) After the start of test H-6 10 ide 0 -620 6 -530 rH 0 -640 0 5 10 15 20 Distance from the center of test piece mm In 3% salt water, Va=12.Okg/mM2, h=0.5mm Fig.(11 Distributions of galvanic potential in test piece in test piece

potentiometer - f cutting surface(coating) - test piece insulating coating 3% salt water Fig.12 Measurement of potential between the erosion area and the corrosion area

E$~~~~~~~~~~~~~ 20~~~28'18 r- 16 -o 14 4 E 12 S35C E 1CF...e EF h=0.25mm 12 SO 8 EF h=0.5 mm Q EF h=5.O mm 105 106 107 Fracture cycles N Fig.13 Effect of the distance h on the fatigue strength

200 -o —-- 2 0 0 7 ~ -~ a 1 2. 0 k. /rn mm? - In 3 % salt water 0 h. a o = 0.25= 0mm O 13..0. N= 2.85x10; ) 0 1-0 150 I IDIn 3% salt water l 0 00) 00l 0 0.0 0 150 t-.....-0 -[ 0 1 2 3 4 Distance from the surface mmn'2 00............... Ta 12.0 k~/mm2 Fig In 3 of h salt water h=$.Omm!3a 0 -' O......... N=2.20x 10...... cod!~0 O O 160 150 0 1 2 3 4 5 Distance from the surface Mm (in the erosion area)

Distance from the center of test piece rr.; 0 5 10 15 20' 20 400 ________ O 200' —- 400 X _ < } 1 2 600 1.D(a) In 3% salt water. ra12.kg/mm2 Gch0 25, n, "85 x106..............2361 200. 2400 4 400 __ 600 1 7 r,_ 1342 5 3465 (ba) In 3% salt water,.a=12.Okg/mm2 h =0;25 mm, N=2.85 x 106 Distance from the center of es piece mrack 0 5. 10 15 20 so h- 6 00 O5 ~'" 400 -.. o 1342 "38 22t7 4633q 302 2 04 (b) In 3% salt water, ra=12.0kg/mm z h = A.0mm, N=2.20x106 Fig. 15 Dist ribution. of cracks

Cal x 103 E 20 2 l l l 2s E22 v16 8 ~) |24 14 S35C 2 12 - In ion exchanged water:i12 oCF dE 0 EF h-0.25mm 10 EF h=0.5 mm 4 EF h=1.0 mm -- 8 EF h=5-0 mm ~EF h=0l0 mm 105 106 107 Fract u re cycl es N Fig. 16 S-N curves of S35C fatigue test pieces

In ion-exchanged water, ta=14.0 kg/mm, h -5.0 mm 6 N-4.54xl 0 Fig. 17 Example of fracture in the erosion area

2 00 E ~ ~16 240 a) _ 14 a-12 __ rrS 10 5 5 _ _ _ _ _ _ _ (I) In 3O/o salt water f7j 8 OCF 0 E F h-0.5 mm 6 105 1 0 6o 100 Fracture cycles N Fiv. 12 q- r- -

I1C E 2 0 E 28 18 -24 t~16 214 ~20 " i2 16" E r (J)lo~SUS 304t to In 31I0 salt water (1) 12 8 OCF @ EF h=0.5mm 65 6-_ 10 16 0 Fracture cycles N Fig.19 S-N curves of SJS3C4 fatigue test pieces in 3% salt water (k: fracture in the erosion area"

22 O4 E E 20 18'8, <d~ ~~~~...... 24 16 In 3% salt water (D D ~Zincified S35C:314' ~~~~~~~~~~~~~~-C -4 C. Fe,' c. -~- E.F., Ao=35 pm __-' E 12 EF. AI2OPM rb E, F, * A,:eO0 )m i No plated S35C m 10-0 a,~~~~~~~L~ mW C.F. iy) 8 -- In the air ----- E.F., Ao=30 mn 105 10"6 107 Fracture cycles N Fig.20 S-N curves of zincified S35C fatigue test pieces

22 E 20 -- 18.. 4 16 In 3% salt water 0, 1 4 Tinned S35C... -0- CoF, - 12- 2 G E.F., =A35 m -- E No plated S35C {} 10 -- cF 8 p -8 --- In the air - ) MNA E.F., Ao=30 pm 105 106 107 Fracture cycles N Fig.21 S-N curves of tinned S35C fatigue test pieces in 73 salt water(to becomparedwith the resultsof Fig.4)

Zincified S35C test piece, 1?,L 6.0 Kg/.nmn, N=7.64x106 Tinned S35C test piece, %t=16.0 Kg/mrnm, N=1.10x106 Fig.22 Fracture in the corrosion f& e

Zincified S35C test piece, Zt=15.0 Kg/nrnIm, h=0.5 mm, N=1.95x106 Tinned S35C test piece, Ca-=l 1.O Kg/mmn, h=0.5 mm, N=6.84x10 Fig. 23 Fracture in erosion fatigue

UNIVERSITY OF MICHIGAN 3 9015 03482 8775 300 Zincified S35C T, =14.5 kg/mm2 N ^ 4 69/106 Ao- 355rnm 200 _. Depth of -~~t' \t /~I erosion pits 100 o 10 20 30 40 50 Dgree from the position of minimum distance Fig.224 Maximum and mean distance from the virgin surface to the erosion surface and depth of the maximum pit itself on the zincified fatigue test pieces. ~i\$ 4Ql V M a~d AneC 4;st-ice fvo1 i ei t J - s tfAe t Ck Te a nfre^.' fat;y'4 f4es Pt /ece&