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Effects of hard chromium plating on cavitation erosion
189
Wear, 128 (1988) 189 - 200
Y. IWAI, T. OKADA and T. FUJIEDA
Fukui University, ~e~~rnent
of Me~~n~~~i ~~i~ee~~,
Bunkyo 3-S-1, Fukui (Japan)
K. AWAZU Industrial Research Institute lshikawa, Tomizumachi,
Kanazawa (Japan)
(Received January 4, 1988; revised April 19, 1988; accepted July 5, 1988)
summary The cavitation erosion of hard chromium plating on steel was studied by vibratory erosion tests. The mass loss as a function of the exposure time curves is divided into three stages: the first stage involves erosion of the chromium itself; the base metal below the interface begins to be damaged in the second stage; the third stage occurs when large particles of the plating layer are removed. Mass loss rates in each stage are greater in a 3 wt.% NaCl solution than in ion-exchanged water because of intercrystalline fracture. The damage in each stage is also affected by the plating thickness. The plating life increases with plating thickness in ion-exchanged water. In 3 wt.% NaCl solution, however, the life is lower and does not increase with plating5 more than about 65 pm thick.
1. ~troduction
The effectiveness of hard chromium plating as a method of preventing cavitation erosion has been reported by many investigators [l - 41. Nowever, chromium plating has many surface cracks and consequently cavitation erosion may initiate from these cracks (51. We have shown that a c~orni~ or nickel plating, which is more noble than the base metal, forms a rough surface because of the large difference in hardness and electrochemical corrosion between the plating and the base metal when the erosion attack extends to the interface [6 J. Therefore further studies are necessary to improve the reliability of hard chromium plating. With regard to the effects of platii thicket, Gliiman [‘I) showed by vibratory tests that chromium plating breaks down after 1 h at a thickness of 10 - 20 pm, but exhibits a relatively high erosion resistance for 3 h for platings more than 40 ym thick. Tsuyuki et al. [8] reported that the critical plating thickness above which cavitation damage scarcely
190
occurs is more than 50 pm from measurements of the number of cavitation pits and the natural electrode potential related to the failure of the plating layer. These reports suggest that the critical thickness is in the range 40 - 50 pm. However, these results were obtained within limited test times, so that substantial effects of plating thickness may not have been fully determined. In the present paper, cavitation erosion tests of hard chromium plating with various thicknesses carried out using a vibratory test apparatus are reported. The cavitation erosion mechanism is clarified and the effective thickness for reducing cavitation erosion is discussed. 2. Test piece and experimental procedure 2.1. Plating test piece The test material is a structural carbon steel (0.18% C, 0.15% Si, 0.58% Mn, 0.035% P, 0.032% S) fully annealed at 880 “C for 1 h. The test pieces were machined to the form of a disc 36 mm in diameter and 7 mm thick. The surfaces were polished with grade 1200 emery paper and were then vacuum annealed at 600 “C for 30 min. They were then commercially plated with chromium of various thicknesses. The plating was carried out in solution (250 g 1-I CrO,, 2.3 g 1-i HzS04, 3 g 1-r Cr,O,). The temperature of the solution was 50 t 2 “C and the electrical density was 40 - 50 A dme2. The plating rate was about 0.2 pm min -I. However, it was impossible to obtain the specified plating thickness accurately, so that the edges of the test pieces were cut off and the plating thicknesses were measured in crosssection using an optical microscope. The plating thicknesses were 20, 40, 65, 110 and 170 pm. The surfaces of the test pieces were glossy but many cracks were observed, as shown in Fig. 1. The cracks were shallow and did not penetrate through the plating. The cracks on the plating surface are usually described by the number of cracks crossing a unit length [9]. Figure 2 shows the number of cracks crossed on 1 mm as a function of plating thickness. Vickers’hardness (testing loads 0.49 and 4.9 N) and residual tensile stress are also shown. The residual stress was measured by the two-axis oscillation method using an X-ray diffraction stress analyser in which the characteristic X-rays are Cr Ka. The diffraction was obtained from the (211) plane and the detecting area was 4 mm X 5 mm. Vickers’ surface hardnesses were about 1000 for various thick platings and were almost the same as those of their cross-sections. The hardness of the base metal is 225 (testing load 0.147 N) and did not vary near the interface. However, the residual tensile stress reached a maximum at 65 pm, and decreased gradually with increasing thickness, being similar to the curve of the number of cracks. 2.2. Cavitation erosion test Cavitation erosion tests were performed by vibrating a disc (18 mm in diameter) attached to the amplifying horn of a magnetostrictive oscillator in close proximity to the surface of the plating test piece [6]. The vibration
191
2
u" u >
0 Hv (0.05/30) (0Hv(0.5 130) . Residual stress
Plottng thickness
Ml!
Fig. 1. Scanning electron micrograph of the original surface of the test piece. Fig. 2. The number of cracks, Vickers’ hardness and residual stress as a function of plating thickness.
disc was made of 18-8 stainless steel with relatively high cavitation erosion resistance. The horn frequency was 14.5 kHz and the double amplitude (peak to peak) was kept at 50 pm. The distance between the disc and the test piece was 1 mm. The test liquid was ion-exchanged water and 3 wt.% NaCI solution (made by adding 3 wt.% NaCl to ion-exchanged water). The liquid temperature was maintained at 25 + 1 “C by circulating liquid between the container and cooling bath at 1.5 1 min-l. The mass loss was measured at a given interval using a precision balance (~nsitivity, 0.01 mg) after ultrasonic cleaning in ~troleum benzene and acetone. 3. Experimental results and discussion 3.1. Mass loss curue Figures 3 and 4 show the mass loss of the test piece, including the plating layer, as a function of the time exposed to cavitation in ionexchanged water and 3 wt.% NaCl solution respectively. For reference, the results of the unplated specimen (base metal) and the stainless steel AISI316 (0.06% C, 1.22% Mn, 10.38% Ni, 16.95% Cr, 2.28% MO, balance iron; tensile strength, 578 MPa; Hv = 241) are also shown. Mass losses occur from the start of the tests of the plating specimens. In ion-exchanged water, the mass loss curves can be divided into three stages. In the first stage, the mass loss increases in proportion to time for all plating thicknesses (stage 1 shown
Fig. 3. Mass loss curves in ion-exchanged
0
20
l!rn?
40 'I
water.
60
Fig. 4. Mass loss curves in 3 wt.% NaCl solution.
in Fig. 3). The mass loss rate of the plating 20 pm thick increases after 5 h (stage 2 in Fig. 3), and greatly increases after 10 h (stage 3 in Fig. 3). For the plating 40 pm thick, the mass loss rate in the first stage is less than that of the plating 20 pm thick, but the second stage appears at 20 h, followed by the third stage at 37 h. For the platings 65 pm and 170 pm thick, however, the second stage starts at about 35 h and 60 h respectively. However, the plating 170 I.tmthick did not exhibit a third stage even though the test time exceeded 60 h. In 3 wt.% NaCl solution, the mass loss curves are similar to those in ion-exchanged water. However, the second and third stages appear earlier and much larger mass losses occur at the start of these stages than in ionexchanged water. For example, at a plating thickness of 40 pm, the second stage starts at 10 h, which is half that in ion-exchanged water. The third stage starts at 25 h, when the mass loss is three times larger than that in ionexchanged water. For platings more than 65 pm thick, the second stage starts at about 20 h regardless of the thickness, but the third stage begins later in proportion to the thickness. However, the plating 170 pm thick did not exhibit a third stage after 70 h.
193
3.2. Cavitation erosion in the first stage Figure 5 shows the mass loss rate as a function of plating thickness. The mass loss rate in 3 wt.% NaCl solution is larger than that in ion-
water. Therefore it is found that cavitation erosion resistance of hard chromium plating decreases remarkably because of the corrosive action of the liquid. In both test liquids, the mass loss rate decreases with increasing thickness. However, in 3 wt.% NaCl solution, it yields a minimum value at a thickness of 65 pm and increases thereafter. To clarify the differences of mass loss rate in both liquids, the eroded surfaces were observed in a scanning electron microscope. Figures 6 and 7 show the erosion damage on the surface of a plating 65 pm thick in ionexchanged water and 3 wt.% NaCl solution respectively. In ion-exchanged water, the intercrystalline boundaries appear because of the slip of grains and then small erosion pits occur on both the boundaries and the points at which the boundaries intersect. The pits increase in size because of chipping at the edge, and increase in number along the cracks, finally combining with each other. Afterwards, the pits fracture inwardly at the bottom when they grow to a given size. Thus the surface exposed to cavitation is eroded all over, although each pit grows slowly. However, in the 3 wt.% NaCl solution, the boundaries appear similar to the ion-exchanged water tests but the erosion process is quite different. After 3 h, a grain detaches along the boundary and then some neighbouring grains fall off in the form of a massive particle. Therefore the erosion pits enlarge in the radial direction and join with their neighbours. Simultaneously, intercrystalline fracture propagates inwardly and reaches the interface between the plating layer and the substrate. However, there are grains which were scarcely changed after long exposure to cavitation, so that the plating forms a rough surface on which some large eroded craters exist locally. Therefore the incubation period is not observed at the start of the test and the first stage is the process of the chromium layer itself eroding. The difference of the detachment of erosion particles between the liquids relates to the mass loss rate shown in Fig. 5. Figure 8 shows the average grain size of the measurements of 40 grains on the virgin surface. The grain size, which is smaller than the area exchanged
0 l
0
,
I
In ion ex. water In 3% NoCl soln.
50 100 Plotlng thickness
150 urn
Fig. 5. Mass loss rates in the fist stage.
(a) I hour
(b) 10 hours
(d) 30 hours (c) 20 hours Fig.6.Erosiondamage on the surface in ion-exchanged water.
surrounded by the original cracks, becomes smaller and yields a minimum value at 65 E.crnand again increases with increasing thickness. This result is opposite to that of the number of cracks (Fig. 2). The curve in Fig. 8 agrees with the variation in mass loss rate as a function of the plating thickness in 3 wt.% NaCl solution in Fig. 5, because corrosion-aided fractures initiate near grain boundaries. To investigate the reason why intercrystalline fracture occurs in 3 wt.% NaCl solution, the virgin surface of the plating was analysed using an ion microanalyser (IMA) to depths of about 2.5 pm. It was found that a small amount of iron is contained in the plating layer. It may be considered that iron ions intrude into the chromium layer during the plating process and thus intercrystalline fracture occurs because of the dissolution of iron and chromium oxide at the intercrystalline boundaries and their points of contact and also because of the galvanic corrosion in penetrating cracks,
195
(a)
(b) 3 hours
1 hour
(d)
(c) 10 hours
15 hours
Fig. 7. Erosion damage on the surface in 3 wt.% NaCl solution.
0
50 100 Plating thickness
150 rm
Fig. 8. Relationship between the grain size and plating thickness.
196
resulting in severe cavitation erosion. However, it is impossible to explain the differences in the amount of iron and its effect on the grain size for plating thickness. 3.3, Cavitation erosion in the second and third stages Figure 9 shows the relationship between the mass loss rate and plating thickness in the second and third stages, together with the first-stage results (from Fig. 5}. During the second stage, mass loss rates an order of magnitude greater are observed but their variations with the plating thickness are similar to those in the first stage in both ion-exchanged water and 3 wt.% NaCl solution. However, the mass loss rates in the third stage, which are an order of magnitude greater than those in the second stage for the thin plating, decrease with plating thickness. Finally, the third stage disappears for the thick plating. Figure 10 shows photographs of the eroded surfaces 40, 65 and 1’70 pm thick. When the second stage occurs in the mass loss curve in ion-exchanged water, erosion pits grow to an observable size. From measurements of the erosion pit depth, it was found that the pit bottom penetrates through the plating layer and reaches the substrate. However, in 3 wt.% NaCl solution, the pits of the plating 40 pm thick, enlarge both radially and inwardly because the exposed substrate is also subject to cavitation attack. For the plating 65 pm thick, the pit bottom extends to the substrate even though the pit diameter is smaller than that of the plating 40 pm. For the plating 170 pm thick, the number of pits increases and each erosion pit grows in a similar way to the plating 65 pm thick. After test termination, the cross-section of each test piece was observed with an optical microscope.
Fig. 9. Mass loss rates in the second and third stages.
197
Second stage
Third stage
Third stage
50 hours
30 hours
Pla
30 hours Plating thickness
65 urn
60 hours Plating thickness I.70?.Un
5mm -
5 mm ~ 70 hours (a) Ion-exchanged water
60 hours (b) 3% NaCl solution
Fig. 10. Erosion damage on the surface in the second and third stages. An example is shown in Fig. 11. A cavity, which is very large compared with the pit diameter, lies under the interface. This is because the substrate is dissolved by the local cell produced between the substrate and the chromium-plated layer. Similar cavities were also observed for test pieces less than 170 pm thick. F’rom these observations, the second stage is the process in which the mass loss rate increases because of the damage of the substrate in addition to the erosion of c~o~um-pan layer. When the plating thickness increases in ion-exchanged water, the pit diameter becomes smaller as well as the number of pits, resulting in a lower mass loss rate. In 3 wt.% NaCl solution, the mass loss rate, which is larger than that in ion-exchanged water because of the superimposed action of galvanic corrosion at the interface, decreases
198
4-Cr
Fig. Il. Photomicrograph
plating
of the cross-section of the plating 170 pm thick.
because of the smaller pit diameter. Thereafter, however, the rate increases as a result of the many pits caused by ~tererystall~e fracture in the first stage and the dissolution at the pit bottoms. In the third stage for the plating 40 pm thick in ion-exchanged water, most of the erosion pits propagate inwardly but several neighbouring pits coalesce, resulting in the removal of a small region of the plating around the pits. However, in 3 wt.% NaCl solution, severe local damage of the substrate occurs even with thick platings. The eroded craters coalesce and then the plating layer surrounding the craters detaches, and consequently the exposed substrate is eroded rapidly. To clarify the behaviour at the start of the third stage for the various plating thicknesses, the damaged region of the plating was measured and then the ratio of this region to the total eroded area was calculated. Here, the ratio is defined as the total damaged area of the plating to the total eroded area corresponding to the area of the vibration disc. Figure 12 shows this ratio as a function of test time. In ion-exchanged water, the time when the plating layer begins to detach corresponds to the start of the third stage in the mass loss curve. In 3 wt.% NaCl solution, the times when the plating layer detaches a little and then detaches uniformly agrees with the begging of the second and third stage respectively for plating thicknesses of 20 pm
Fig. 12. Ratio of the damaged area of the plating to the total eroded region as a function of time.
199
and 40 pm. For the 65 and 110 pm thicknesses, the plating layer begins to detach at the start of the third stage. Afterwards, the damaged area of the plating increases in proportion to the test time for every plating thickness. Therefore the third stage is the process in which the mass loss increases remarkably because the plating layer detaches macroscopically and the exposed substrate is eroded. With large plating thicknesses in both liquids, the mass loss rate does not increase further because the thick plating layer is restrained and does not detach. 3.4. Relationship between plating life and plating thickness If we define the start of the second or third stage in the mass loss curve as the life (time exposed to cavitation) of the chromium plating, the relationship between the life and the plating thickness is shown in Fig. 13. The time to the second stage increases with increasing thickness but the degree of increase becomes gentle for platings more than 65 pm thick in ion-exchanged water. The life in 3 wt.% NaCl solution is less than that in ion-exchanged water and does not increase for platings more than 65 pm thick. In contrast to corrosion-aided fractures of the chromium plating in the first stage, the second stage occurs as a result of the damage of the exposed substrate due to the erosion and the galvanic corrosion between the substrate and chromium plating layer. Therefore the variation in life in 3 wt.% NaCl may occur because the erosion pits propagate more quickly through the plating layer more than 65 E.tmthick, which grain size again increases. However, if we consider the time to the third stage as the life of the plating, it becomes considerably longer. Although the plating 170 pm thick appears to have a much longer life in the 3 wt.% NaCl solution, it is presumed that the plating layer may eventually detach over the large area based on the fact that a large subsurface cavity was formed, as shown in Fig. 11. From these results, we conclude that in corrosive environments, the erosion life of hard chromium plating is considered to be the time when erosion pits extend to the substrate through the plating layer, i.e. the time when the second stage begins in the mass loss curve. Thus the life Ionex. 3XNoCI water SoIn.
0
20
Plmm
Fig. 13. Relationship
Co life
between
h
60
the plating life and plating thickness.
200
of hard chromium plating decreases remarkably and does not increase for platings more than about 65 pm thick. 4. Conclusions The following conclusions are drawn. (1) Mass loss as a function of exposure time is divided into three stages. The incubation period is not observed and the mass loss occurs from the beginning of the test. The second and third stages appear earlier and greater mass loss occurs at the start of these stages in 3 wt.% NaCl solution than in ion-exchanged water. (2) The first stage is the process of the erosion of the chromium plating itself. The substrate below the interface begins to be damaged in the second stage. The third stage is the process in which the mass loss increases greatly because the plating layer detaches in massive particles and the exposed substrate is eroded. The mass loss rates in each stage are greater in a 3 wt.% NaCl solution than in ion-exchanged water because of intercrystalline fracture of the chromium plating and galvanic corrosion between the substrate and chromium plating layer. Damage in each stage is also affected by the plating thickness in which cracks and grain size of the platmg vary. (3) The life of hard chromium plating becomes longer with increasing thickness in ion-exchanged water. The life in 3 wt.% NaCl solution is shorter than that in ion-exchanged water and does not increase for platings more than 65 pm thick because of the dissolution of the substrate beneath the interface. References H. Kubo, Cavitation erosion resistances of cast iron and chromium films plated on cast iron under various conditions, J. Sot. Master. Sci., 14 (145) (1965) 833 (in Japanese). T. Nakajo, H. Kubo, T. Hamada and H. Kinoshita, On the cavitation erosion resistance of coatings, 3. Sot. Mater. Sci., 14 (136) (1965) 54 (in Japanese). C. J. Heathcock, A Ball and B. E. Protheroe, Cavitation erosion of cobalt based stellite alloys cemented carbides and surface treated alloy steels, Wear, 74 (1981) 11. A. Barletta and A. Ball, The cavitation erosion of coated and surface treated steels, ICIW, 4 (1) (1984) 421. N. Frees, Cavitation erosion of titanium carbide coatings on cemented carbides and other substrates, Wear, 88 (1983) 57. T. Okada, Y. Iwai and K. Awazu, Effects of plating on cavitation erosion, Wear, 124 (1988) 21. L. A. Glikman, Corrosion-Mechanical Strength of Metal, Butterworth, London, (1962) 126. S. Tsuyuki, K. Tarao, A. Fujii and S. Yuri, Cavitation erosion resistance of metallic coatings on water side of diesel cylinder liner, COPTOS.Eng., 13 (7) (1964) 296 (in Japanese f. S. Konishi and M. Tadagoshi, Microscopic observation of chromium deposits surface from low concentration chromium plating solution, J. Met. Finish. Sot. Jpn., 25 (2) (1974) 93 (in Japanese).
Wear, 128 (1988) 189 - 200
Y. IWAI, T. OKADA and T. FUJIEDA
Fukui University, ~e~~rnent
of Me~~n~~~i ~~i~ee~~,
Bunkyo 3-S-1, Fukui (Japan)
K. AWAZU Industrial Research Institute lshikawa, Tomizumachi,
Kanazawa (Japan)
(Received January 4, 1988; revised April 19, 1988; accepted July 5, 1988)
summary The cavitation erosion of hard chromium plating on steel was studied by vibratory erosion tests. The mass loss as a function of the exposure time curves is divided into three stages: the first stage involves erosion of the chromium itself; the base metal below the interface begins to be damaged in the second stage; the third stage occurs when large particles of the plating layer are removed. Mass loss rates in each stage are greater in a 3 wt.% NaCl solution than in ion-exchanged water because of intercrystalline fracture. The damage in each stage is also affected by the plating thickness. The plating life increases with plating thickness in ion-exchanged water. In 3 wt.% NaCl solution, however, the life is lower and does not increase with plating5 more than about 65 pm thick.
1. ~troduction
The effectiveness of hard chromium plating as a method of preventing cavitation erosion has been reported by many investigators [l - 41. Nowever, chromium plating has many surface cracks and consequently cavitation erosion may initiate from these cracks (51. We have shown that a c~orni~ or nickel plating, which is more noble than the base metal, forms a rough surface because of the large difference in hardness and electrochemical corrosion between the plating and the base metal when the erosion attack extends to the interface [6 J. Therefore further studies are necessary to improve the reliability of hard chromium plating. With regard to the effects of platii thicket, Gliiman [‘I) showed by vibratory tests that chromium plating breaks down after 1 h at a thickness of 10 - 20 pm, but exhibits a relatively high erosion resistance for 3 h for platings more than 40 ym thick. Tsuyuki et al. [8] reported that the critical plating thickness above which cavitation damage scarcely
190
occurs is more than 50 pm from measurements of the number of cavitation pits and the natural electrode potential related to the failure of the plating layer. These reports suggest that the critical thickness is in the range 40 - 50 pm. However, these results were obtained within limited test times, so that substantial effects of plating thickness may not have been fully determined. In the present paper, cavitation erosion tests of hard chromium plating with various thicknesses carried out using a vibratory test apparatus are reported. The cavitation erosion mechanism is clarified and the effective thickness for reducing cavitation erosion is discussed. 2. Test piece and experimental procedure 2.1. Plating test piece The test material is a structural carbon steel (0.18% C, 0.15% Si, 0.58% Mn, 0.035% P, 0.032% S) fully annealed at 880 “C for 1 h. The test pieces were machined to the form of a disc 36 mm in diameter and 7 mm thick. The surfaces were polished with grade 1200 emery paper and were then vacuum annealed at 600 “C for 30 min. They were then commercially plated with chromium of various thicknesses. The plating was carried out in solution (250 g 1-I CrO,, 2.3 g 1-i HzS04, 3 g 1-r Cr,O,). The temperature of the solution was 50 t 2 “C and the electrical density was 40 - 50 A dme2. The plating rate was about 0.2 pm min -I. However, it was impossible to obtain the specified plating thickness accurately, so that the edges of the test pieces were cut off and the plating thicknesses were measured in crosssection using an optical microscope. The plating thicknesses were 20, 40, 65, 110 and 170 pm. The surfaces of the test pieces were glossy but many cracks were observed, as shown in Fig. 1. The cracks were shallow and did not penetrate through the plating. The cracks on the plating surface are usually described by the number of cracks crossing a unit length [9]. Figure 2 shows the number of cracks crossed on 1 mm as a function of plating thickness. Vickers’hardness (testing loads 0.49 and 4.9 N) and residual tensile stress are also shown. The residual stress was measured by the two-axis oscillation method using an X-ray diffraction stress analyser in which the characteristic X-rays are Cr Ka. The diffraction was obtained from the (211) plane and the detecting area was 4 mm X 5 mm. Vickers’ surface hardnesses were about 1000 for various thick platings and were almost the same as those of their cross-sections. The hardness of the base metal is 225 (testing load 0.147 N) and did not vary near the interface. However, the residual tensile stress reached a maximum at 65 pm, and decreased gradually with increasing thickness, being similar to the curve of the number of cracks. 2.2. Cavitation erosion test Cavitation erosion tests were performed by vibrating a disc (18 mm in diameter) attached to the amplifying horn of a magnetostrictive oscillator in close proximity to the surface of the plating test piece [6]. The vibration
191
2
u" u >
0 Hv (0.05/30) (0Hv(0.5 130) . Residual stress
Plottng thickness
Ml!
Fig. 1. Scanning electron micrograph of the original surface of the test piece. Fig. 2. The number of cracks, Vickers’ hardness and residual stress as a function of plating thickness.
disc was made of 18-8 stainless steel with relatively high cavitation erosion resistance. The horn frequency was 14.5 kHz and the double amplitude (peak to peak) was kept at 50 pm. The distance between the disc and the test piece was 1 mm. The test liquid was ion-exchanged water and 3 wt.% NaCI solution (made by adding 3 wt.% NaCl to ion-exchanged water). The liquid temperature was maintained at 25 + 1 “C by circulating liquid between the container and cooling bath at 1.5 1 min-l. The mass loss was measured at a given interval using a precision balance (~nsitivity, 0.01 mg) after ultrasonic cleaning in ~troleum benzene and acetone. 3. Experimental results and discussion 3.1. Mass loss curue Figures 3 and 4 show the mass loss of the test piece, including the plating layer, as a function of the time exposed to cavitation in ionexchanged water and 3 wt.% NaCl solution respectively. For reference, the results of the unplated specimen (base metal) and the stainless steel AISI316 (0.06% C, 1.22% Mn, 10.38% Ni, 16.95% Cr, 2.28% MO, balance iron; tensile strength, 578 MPa; Hv = 241) are also shown. Mass losses occur from the start of the tests of the plating specimens. In ion-exchanged water, the mass loss curves can be divided into three stages. In the first stage, the mass loss increases in proportion to time for all plating thicknesses (stage 1 shown
Fig. 3. Mass loss curves in ion-exchanged
0
20
l!rn?
40 'I
water.
60
Fig. 4. Mass loss curves in 3 wt.% NaCl solution.
in Fig. 3). The mass loss rate of the plating 20 pm thick increases after 5 h (stage 2 in Fig. 3), and greatly increases after 10 h (stage 3 in Fig. 3). For the plating 40 pm thick, the mass loss rate in the first stage is less than that of the plating 20 pm thick, but the second stage appears at 20 h, followed by the third stage at 37 h. For the platings 65 pm and 170 pm thick, however, the second stage starts at about 35 h and 60 h respectively. However, the plating 170 I.tmthick did not exhibit a third stage even though the test time exceeded 60 h. In 3 wt.% NaCl solution, the mass loss curves are similar to those in ion-exchanged water. However, the second and third stages appear earlier and much larger mass losses occur at the start of these stages than in ionexchanged water. For example, at a plating thickness of 40 pm, the second stage starts at 10 h, which is half that in ion-exchanged water. The third stage starts at 25 h, when the mass loss is three times larger than that in ionexchanged water. For platings more than 65 pm thick, the second stage starts at about 20 h regardless of the thickness, but the third stage begins later in proportion to the thickness. However, the plating 170 pm thick did not exhibit a third stage after 70 h.
193
3.2. Cavitation erosion in the first stage Figure 5 shows the mass loss rate as a function of plating thickness. The mass loss rate in 3 wt.% NaCl solution is larger than that in ion-
water. Therefore it is found that cavitation erosion resistance of hard chromium plating decreases remarkably because of the corrosive action of the liquid. In both test liquids, the mass loss rate decreases with increasing thickness. However, in 3 wt.% NaCl solution, it yields a minimum value at a thickness of 65 pm and increases thereafter. To clarify the differences of mass loss rate in both liquids, the eroded surfaces were observed in a scanning electron microscope. Figures 6 and 7 show the erosion damage on the surface of a plating 65 pm thick in ionexchanged water and 3 wt.% NaCl solution respectively. In ion-exchanged water, the intercrystalline boundaries appear because of the slip of grains and then small erosion pits occur on both the boundaries and the points at which the boundaries intersect. The pits increase in size because of chipping at the edge, and increase in number along the cracks, finally combining with each other. Afterwards, the pits fracture inwardly at the bottom when they grow to a given size. Thus the surface exposed to cavitation is eroded all over, although each pit grows slowly. However, in the 3 wt.% NaCl solution, the boundaries appear similar to the ion-exchanged water tests but the erosion process is quite different. After 3 h, a grain detaches along the boundary and then some neighbouring grains fall off in the form of a massive particle. Therefore the erosion pits enlarge in the radial direction and join with their neighbours. Simultaneously, intercrystalline fracture propagates inwardly and reaches the interface between the plating layer and the substrate. However, there are grains which were scarcely changed after long exposure to cavitation, so that the plating forms a rough surface on which some large eroded craters exist locally. Therefore the incubation period is not observed at the start of the test and the first stage is the process of the chromium layer itself eroding. The difference of the detachment of erosion particles between the liquids relates to the mass loss rate shown in Fig. 5. Figure 8 shows the average grain size of the measurements of 40 grains on the virgin surface. The grain size, which is smaller than the area exchanged
0 l
0
,
I
In ion ex. water In 3% NoCl soln.
50 100 Plotlng thickness
150 urn
Fig. 5. Mass loss rates in the fist stage.
(a) I hour
(b) 10 hours
(d) 30 hours (c) 20 hours Fig.6.Erosiondamage on the surface in ion-exchanged water.
surrounded by the original cracks, becomes smaller and yields a minimum value at 65 E.crnand again increases with increasing thickness. This result is opposite to that of the number of cracks (Fig. 2). The curve in Fig. 8 agrees with the variation in mass loss rate as a function of the plating thickness in 3 wt.% NaCl solution in Fig. 5, because corrosion-aided fractures initiate near grain boundaries. To investigate the reason why intercrystalline fracture occurs in 3 wt.% NaCl solution, the virgin surface of the plating was analysed using an ion microanalyser (IMA) to depths of about 2.5 pm. It was found that a small amount of iron is contained in the plating layer. It may be considered that iron ions intrude into the chromium layer during the plating process and thus intercrystalline fracture occurs because of the dissolution of iron and chromium oxide at the intercrystalline boundaries and their points of contact and also because of the galvanic corrosion in penetrating cracks,
195
(a)
(b) 3 hours
1 hour
(d)
(c) 10 hours
15 hours
Fig. 7. Erosion damage on the surface in 3 wt.% NaCl solution.
0
50 100 Plating thickness
150 rm
Fig. 8. Relationship between the grain size and plating thickness.
196
resulting in severe cavitation erosion. However, it is impossible to explain the differences in the amount of iron and its effect on the grain size for plating thickness. 3.3, Cavitation erosion in the second and third stages Figure 9 shows the relationship between the mass loss rate and plating thickness in the second and third stages, together with the first-stage results (from Fig. 5}. During the second stage, mass loss rates an order of magnitude greater are observed but their variations with the plating thickness are similar to those in the first stage in both ion-exchanged water and 3 wt.% NaCl solution. However, the mass loss rates in the third stage, which are an order of magnitude greater than those in the second stage for the thin plating, decrease with plating thickness. Finally, the third stage disappears for the thick plating. Figure 10 shows photographs of the eroded surfaces 40, 65 and 1’70 pm thick. When the second stage occurs in the mass loss curve in ion-exchanged water, erosion pits grow to an observable size. From measurements of the erosion pit depth, it was found that the pit bottom penetrates through the plating layer and reaches the substrate. However, in 3 wt.% NaCl solution, the pits of the plating 40 pm thick, enlarge both radially and inwardly because the exposed substrate is also subject to cavitation attack. For the plating 65 pm thick, the pit bottom extends to the substrate even though the pit diameter is smaller than that of the plating 40 pm. For the plating 170 pm thick, the number of pits increases and each erosion pit grows in a similar way to the plating 65 pm thick. After test termination, the cross-section of each test piece was observed with an optical microscope.
Fig. 9. Mass loss rates in the second and third stages.
197
Second stage
Third stage
Third stage
50 hours
30 hours
Pla
30 hours Plating thickness
65 urn
60 hours Plating thickness I.70?.Un
5mm -
5 mm ~ 70 hours (a) Ion-exchanged water
60 hours (b) 3% NaCl solution
Fig. 10. Erosion damage on the surface in the second and third stages. An example is shown in Fig. 11. A cavity, which is very large compared with the pit diameter, lies under the interface. This is because the substrate is dissolved by the local cell produced between the substrate and the chromium-plated layer. Similar cavities were also observed for test pieces less than 170 pm thick. F’rom these observations, the second stage is the process in which the mass loss rate increases because of the damage of the substrate in addition to the erosion of c~o~um-pan layer. When the plating thickness increases in ion-exchanged water, the pit diameter becomes smaller as well as the number of pits, resulting in a lower mass loss rate. In 3 wt.% NaCl solution, the mass loss rate, which is larger than that in ion-exchanged water because of the superimposed action of galvanic corrosion at the interface, decreases
198
4-Cr
Fig. Il. Photomicrograph
plating
of the cross-section of the plating 170 pm thick.
because of the smaller pit diameter. Thereafter, however, the rate increases as a result of the many pits caused by ~tererystall~e fracture in the first stage and the dissolution at the pit bottoms. In the third stage for the plating 40 pm thick in ion-exchanged water, most of the erosion pits propagate inwardly but several neighbouring pits coalesce, resulting in the removal of a small region of the plating around the pits. However, in 3 wt.% NaCl solution, severe local damage of the substrate occurs even with thick platings. The eroded craters coalesce and then the plating layer surrounding the craters detaches, and consequently the exposed substrate is eroded rapidly. To clarify the behaviour at the start of the third stage for the various plating thicknesses, the damaged region of the plating was measured and then the ratio of this region to the total eroded area was calculated. Here, the ratio is defined as the total damaged area of the plating to the total eroded area corresponding to the area of the vibration disc. Figure 12 shows this ratio as a function of test time. In ion-exchanged water, the time when the plating layer begins to detach corresponds to the start of the third stage in the mass loss curve. In 3 wt.% NaCl solution, the times when the plating layer detaches a little and then detaches uniformly agrees with the begging of the second and third stage respectively for plating thicknesses of 20 pm
Fig. 12. Ratio of the damaged area of the plating to the total eroded region as a function of time.
199
and 40 pm. For the 65 and 110 pm thicknesses, the plating layer begins to detach at the start of the third stage. Afterwards, the damaged area of the plating increases in proportion to the test time for every plating thickness. Therefore the third stage is the process in which the mass loss increases remarkably because the plating layer detaches macroscopically and the exposed substrate is eroded. With large plating thicknesses in both liquids, the mass loss rate does not increase further because the thick plating layer is restrained and does not detach. 3.4. Relationship between plating life and plating thickness If we define the start of the second or third stage in the mass loss curve as the life (time exposed to cavitation) of the chromium plating, the relationship between the life and the plating thickness is shown in Fig. 13. The time to the second stage increases with increasing thickness but the degree of increase becomes gentle for platings more than 65 pm thick in ion-exchanged water. The life in 3 wt.% NaCl solution is less than that in ion-exchanged water and does not increase for platings more than 65 pm thick. In contrast to corrosion-aided fractures of the chromium plating in the first stage, the second stage occurs as a result of the damage of the exposed substrate due to the erosion and the galvanic corrosion between the substrate and chromium plating layer. Therefore the variation in life in 3 wt.% NaCl may occur because the erosion pits propagate more quickly through the plating layer more than 65 E.tmthick, which grain size again increases. However, if we consider the time to the third stage as the life of the plating, it becomes considerably longer. Although the plating 170 pm thick appears to have a much longer life in the 3 wt.% NaCl solution, it is presumed that the plating layer may eventually detach over the large area based on the fact that a large subsurface cavity was formed, as shown in Fig. 11. From these results, we conclude that in corrosive environments, the erosion life of hard chromium plating is considered to be the time when erosion pits extend to the substrate through the plating layer, i.e. the time when the second stage begins in the mass loss curve. Thus the life Ionex. 3XNoCI water SoIn.
0
20
Plmm
Fig. 13. Relationship
Co life
between
h
60
the plating life and plating thickness.
200
of hard chromium plating decreases remarkably and does not increase for platings more than about 65 pm thick. 4. Conclusions The following conclusions are drawn. (1) Mass loss as a function of exposure time is divided into three stages. The incubation period is not observed and the mass loss occurs from the beginning of the test. The second and third stages appear earlier and greater mass loss occurs at the start of these stages in 3 wt.% NaCl solution than in ion-exchanged water. (2) The first stage is the process of the erosion of the chromium plating itself. The substrate below the interface begins to be damaged in the second stage. The third stage is the process in which the mass loss increases greatly because the plating layer detaches in massive particles and the exposed substrate is eroded. The mass loss rates in each stage are greater in a 3 wt.% NaCl solution than in ion-exchanged water because of intercrystalline fracture of the chromium plating and galvanic corrosion between the substrate and chromium plating layer. Damage in each stage is also affected by the plating thickness in which cracks and grain size of the platmg vary. (3) The life of hard chromium plating becomes longer with increasing thickness in ion-exchanged water. The life in 3 wt.% NaCl solution is shorter than that in ion-exchanged water and does not increase for platings more than 65 pm thick because of the dissolution of the substrate beneath the interface. References H. Kubo, Cavitation erosion resistances of cast iron and chromium films plated on cast iron under various conditions, J. Sot. Master. Sci., 14 (145) (1965) 833 (in Japanese). T. Nakajo, H. Kubo, T. Hamada and H. Kinoshita, On the cavitation erosion resistance of coatings, 3. Sot. Mater. Sci., 14 (136) (1965) 54 (in Japanese). C. J. Heathcock, A Ball and B. E. Protheroe, Cavitation erosion of cobalt based stellite alloys cemented carbides and surface treated alloy steels, Wear, 74 (1981) 11. A. Barletta and A. Ball, The cavitation erosion of coated and surface treated steels, ICIW, 4 (1) (1984) 421. N. Frees, Cavitation erosion of titanium carbide coatings on cemented carbides and other substrates, Wear, 88 (1983) 57. T. Okada, Y. Iwai and K. Awazu, Effects of plating on cavitation erosion, Wear, 124 (1988) 21. L. A. Glikman, Corrosion-Mechanical Strength of Metal, Butterworth, London, (1962) 126. S. Tsuyuki, K. Tarao, A. Fujii and S. Yuri, Cavitation erosion resistance of metallic coatings on water side of diesel cylinder liner, COPTOS.Eng., 13 (7) (1964) 296 (in Japanese f. S. Konishi and M. Tadagoshi, Microscopic observation of chromium deposits surface from low concentration chromium plating solution, J. Met. Finish. Sot. Jpn., 25 (2) (1974) 93 (in Japanese).