Effects of Velocity on Corrosion by Water H.
R. COPSON
The International Nickel Co., Inc., Bayonne,
Velocity can be a major factor in the corrosion of metals by water, but most available information on the subject i s scattered. A general review, therefore, was undertaken with emphasis on iron, zinc, copper, and their alloys. Pertinent unpublished data are included. Generally, corrosion increases with velocity, but the effect may be just the opposite. Motion may sometimes eliminate and sometimes cause local attack. It may have a marked effect on galvanic couples. At high velocities mechanical damage accelerates attack by erosion corrosion and cavitation erosion. Experience and a detailed knowledge of operating conditions are essential for the, analysis of a particular problem. It i s important to be on the lookout for the secondary effects of velocity.
ELOCITT by itself has little inherent effect on corrosion It does, however, have a decisive effect on many factors t h a t control corrosion rates. After a general review, the specific influence of velocity on corrosion by water of iron, zinc copper, and their alloys is presented. At high velocities mechanical damage may accelerate attack by erosion corrosion and cavitation erosion. Much of the information presented is taken from the literature, but some unpublished data are included.
V
,
General Effects
of Velocity
Like all factors influencing corrosion, the effects of velocity are varied and often conflicting. A little motion generally tends t o make conditions more uniform, and this will tend t o make corroqion uniform and prevent local attack. Corrosion tests are more reproducible when there is some motion. On the other hand motion may set up turbulence, and the turbulence may produce nonuniform conditions which lead to pitting. Thus motion may sometimee eliminate local attack of one kind, and at other times motion mav cause local attack of some other kind. Motion generally increases total weight loss by supplying the corrosivee a t a faster rate. Motion thins the quiescent layers a t the metal surface so t h a t it is easier for corrosives t o reach the metal, and there is less restriction of corrosion by diffusion processes. On the other hand a t high relative motion the character of t h e corrosion products may change and become more protective. With certain alloys motion may increase the oxygen supply sufficiently t o induce passivity. Motion may also decrease corrosion by replenishing inhibitors of corrosion, particularly by increaging the supply of inhibitors t o crevices or other dead spaces. Thus, while the general rule is that corrosion increases with velocity, the effect may be just the opposite in special cases. Since motion affects different alloys differently, changes in velocity can have a marked effect on galvanic behavior when metals are coupled together. Measurements a t different velocities can lead t o conflicting conclusions with regard t o galvanic corrosion. When t h e velocity becomes extremely high, mechanical effects add t o the corrosion and increase t h e damage t o metals. Erosion may remove protective films or layers of corrosion product and
August 1W2
N. 1.
keep corrosion going a t the initial high rate. Turbulence, v i h a - , tion, and other conditions may lower t h e pressure locally suficiently t o cause cavities t o form in t h e liquid, which may then lead t o mechanical damage by the hammering action resulting from the subsequent collapse of the cavities. Damage caaused by erosion corrosion and by cavitation erosion can be severe arid rapid. From these facts, it can be appreciated t h a t velocity by itself has little direct effect on corrosion. It exercises its influenw indirectly by its effect on factors t h a t are of major importance in determining the pattern and rates of corrosion. I n neutral water solutions the oxygen concentration is of ma,ior importance in determining the corrosion rate of most metal* Local variations in oxygen content may set up oxygen conrentrstion cells which cause pitting and local attack. iLIotiori genriallv would be expected t o eliminate these differential aeration cells but would tend t o increase the total corrosion by supplying more oxygen. Another type of concentration cell affected by motion is a metal ion cell. Metal ion concentration cells are most a p t to form near crevices or recessed areas where metal ions have a chance t o build up. Conversely a fresh stream of solution impinging upon a particular area may wash away the metal ions locally and cause such areas t o become relatively anodic and t o corrode. Films of corrosion products or other deposits are of major importance in determining corrosion rates. Generally s u r h films tend t o exclude oxygen and therefore tend t o be protective. The protectiveness of the film will vary with its uniformity and the conditions under which it is formed. Since velocity affects these things, it may affect the protective character of the filni Thus at high velocity owing to a greater availability of 0x5 gen, ferric hydroxide might be precipitated in close contart with the metal surface and be protective, whereas at loww velocity feirous products might he precipitated away from the metal and be nonpyotective. Corrosion rates usually decrease with time. This is due in part t o gradual thickening of protective films. If the films are thick enough, changes in velocity might be unnoticed at the metal surface. At high velocities, however, erosive action can remove the protective films, Turbulence may cause this t o occur locltlly and lead t o rapid pitting and impingement attack. The composition of t h e water has an important bearing on corrosion. This is particularly true if insoluble material such as calcium carbonate precipitates out of t h e water. Constituents present in small concentration might soon be depleted unless some flow is present t o have a replenishing action. Thus without some flow, dissolved oxygen, carbon dioxide, or other corrosive constituents might soon be exhausted. Likewise if the water contains a n inhibitor of corrosion, some flow is neressary t o distribute the inhibitor. Temperature and p H cannot be neglected in corrosion work. However, there is little connection between these and velocity except as the other factors which have been discuased come into the picture. A rise in temperature tends t o increase reaction rate, but a rise in temperature decreases t h e solubility or oxygen in water. The net result in the case of iron is that in open water corrosion rate increases u p t o 80 C. and then fallv t o a low value a t the boiling point. The pH of the solution is more important
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
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1745
at high and low pH's than in t h e neutral range. Thus in iron? from p H 4 t o 9 corrosion rate is practically independent of pH. At higher p H values more protective films of corrosion products are formed. Below p H 4 corrosion is rapid because of hydrogen evolution. Effed of Velocity on Corrosion of Iron
The initial effect of increasing velocity in neutral waters is to increase the corrosion rate of iron (9, 42, @,' 50, 66). The data of Cox and Roetheli (9) are reproduced in Figure 1. They rotated 'specimens of mild steel in Cambridge, Mass., t a p water. I n water saturated with air at room temperature (6 ml. per liter of oxygen) the corrosion rate increased from 47 mg. per square dm. per day a t zero velocity to 208 mg. per square dm. per day at 1.2 feet per second.
I
OXYGEN
CONC.,
ML./LITER
Figure 1. Effect of Velocity and Oxygen Concentration on Corrosion of Steel ( 9 )
tubes containing foil specimens observed the maximum a t about 0.15 foot per second. Russell, Chappell, and White (48) obtained the maximum at 0.2 t o 0.9 foot per second. Turbulence is one factor that explains the difference; temperature is another. The oxygen content of air-saturated water decreases from around 10 ml. per liter near the freezing_point, . t o 6 ml. per liter a t room temperature, t o zero a t the boiling point. The inflection points at low speed were obtained in cold water which ties in with the fact t h a t cold water may contain more oxygen. The condition of the metal surface is another factor affecting the inflection point. This was studied by Russell, Chapell, and White (48). They pointed out t h a t the test results showing passivity a t high velocity were obtained by starting with clean specimens. When the test specimens were rusted at the start, corrosion at high velocity was at a rapid rate. This was confirmed bvForest. Roetheli. and Brown ( l e ) ,who also showed that the passivity induced on clean specimens at high speed would not persist after the motion was stopped. It seems evident that the accumulation of rust layers, which are not removed by t h e velocity effects, may bring about a condition where the velocity at the metal surface is low and independent of the velocity over the layer of attached rust. The work of Speller and Kendall (50)showed that in practice it would be unsafe t o place much reliance on iron becoming passive a t high velocity. Some of their results are reproduced in Figure 3. They measured the loss in oxygen content of water flowing through pipe and found that corrosion increased with velocity, in some cases approahing a maximum but never decreasing, I n some of their tests the velocity vms as high as 8 feet per second. Presumably their pipe did not have a nice clean surface a t t h e start, and the temperature, air content, and composition of the water may have been factors. They made the interesting observation that a t any given velocity, corrosion rate decreased with pipe size. This probably was a matter of decreased turbulence. Therefore it is good practice to use oversize pipe, both to decrease the velocity and t o decrease the turbulence. I
Five- to seven-day tests in water in
c 5
23' C.
~1
5 0.m 0"
A number of investigators (8, 32, 47, 4 8 ) have found that a t higher velocities the corrosion rate may decrease again. The Roetheli andcylinders Brown (47) are reproduced in Figuret a2. data ofused They rotating of mild steel in Cambridge p water. They attributed the decrease t o the formation of a protective film of ferric hydroxide, which wm a consequence of the greater supply of oxygen. Figure 1shows t h a t corrosion increased with oxygen concentration up t o 18 ml. per liter. At still higher oxygen concentrations, however, the corrosion may drop t o low values with the iron tending to become passive (WI). Groesbeck and Waldron (24) found t h a t in distilled water moving a t 0.25 foot per second the corrosion rate of mild steel was 330 mg. per square dm. per day with 16 ml. of oxygen per liter but t h a t a t 24 mil. per liter the corrosion had dropped t o 30 mg. per square dm. per day in 48hour tests at 30" C. Thus the effects of velocity and oxygen are interrelated. High velocity brings sufficient oxygen t o the iron surface so t h a t passivity may set in a t oxygen concentrations of 6 ml. per liter or less. According t o Uhlig (64) true passivity is involved with t h e iron assuming a less active potential. As might be expected the velocity a t which corrosion decreases varies with t h e conditions. I n Figure 2 the maximum corrosion occurred around 1.3 feet per second and the minimum above 2.3 feet per second. Heyn and Bauer ( 2 2 ) b y running water through a beaker containing iron specimens observed the maximum at 0.004 foot per second. Undoubtedly they had lots of turbulence. Friend (22) by passing t a p water through glass 1746
:8 0.013
I:
00
$
g
0.010
0.m
Y
t
omo
U'
Figure 2.
2
I VELOCIlY,
3
Fl ISEC
Effect of Velocity on Corrodon of Rotating Steel Cylinders ( 4 7 ) One-day test in oxygenated water
That the composition of the water is important is shown by the fact t h a t it is more difficult for high velocity t o cause passivity in sea water. Presumably the chloride ion is responsible. Some data in the Corrosion Handbook ( 3 2 ) are reproduced in Figure 4. These were obtained by pumping salt water through pipe sections a t 23" C. Similar results were obtained by Wormwell ( 5 6 ) by rotating steel cylinders in half normal sodium chloride saturated with oxygen a t 25 O C., although in his tests the curve was steeper a t low velocity. Thus Wormwell reported t h a t an increase in velocity from zero t o 0.01 foot per second trebled the corrosion. This absence of passivity a t atmospheric temperatures ties in with the fact t h a t high concentrations of dissolved oxygen likewise have not caused passivity of stpel in salt solutions (21).
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Val, 44, No. 8
,
Corrosion by Water
VELOCITY,
FT./SCC.
Figure 3. Effect of Velocity on Corrosion of '/s-lnch Pipe in Oxygenated Water (50)
On the other hand LaQue (34)has apparently induced passivity in cold sea water a t 11" C. I n his tests 5-inch diameter disks were rotated horizontally a t 1140 r.p.m. in moving sea water. T h e velocity at the outside edge of the specimen calculated t o 25 feet per second. The appearance of a cast-iron disk after 30 days is shown in Figure 5 , A . The average corrosion rate was 100 mg. per square dm. per day, but all the attack was confined t o the central portion. The outer part of the disk which was moving a t high velocity showed no attack. In addition to the colder water used in these tests, there is the possibility that galvanic effects produced the result. The greater oxygen supply a t the higher velocity may have caused the outer portion of the disk to become cathodic t o the inner portion and thus to be protected by the galvanic current. This would then be a case of local attack caused by a differential aeration cell set up by differences in relative motion. It illustrates the need of keeping such differential aeration cells and galvanic contacts in mind. Changes in motion can produce other surprising galvanic effects. Thus in sea water a t low velocity it makes little difference whether steel is coupled to stainless steel, titanium, copper, or nickel; the galvanic acceleration of the corrosion of the steel is approximately the same in all cases (34). At high velocities,
A.
Figure 5.
August 1952
Cast Iron
however, the cathodic polarization of stainleas steel and titanium is much easier than of the other materials (SI),80 that a t high velocity the galvanic corrosion produced by coupling steel to stainless steel or titanium is much less than that produced by coupling steel t o copper or nickel. Some typical data are given in Table I. One saving feature in many corrosion problems is that with time corrosion products increase in thickneas, adherence, and .protective value. This causes corrosion rates to decrease with time and mitigates against the damaging effects of moderate velocity. Thus in Wmhington, D. C., city water flowing a t 0.5 foot per second, the corrosion rate of a particular steel decreased from 33 a t 6 months t o 9 mg. per square dm. per day a t 10 years (IS).
8 200I50
e
50
10
VELOCITY,
I5
20
25
FT. / S E C
Figure 4. Effect of Velocity on Corrosion of Steel by Sea Water at Atmospheric Temperature, 38 Days (32)
Zero velocity is a special case that applies t o stagnant solutions. General corrosion tends to be comparatively low in most stagnant solutions, but the attack may localize with deep pitting. Pitting of pipe buried in the ground is an example of local attack under stagnant conditions. A little motion helps keep conditions more uniform and tends t o prevent the pitting. Stainless steel provides striking examples of this behavior. Thus in quiet sea water Type 316 stainless steel showed severe pitting uith the
9.
Admiralty Brass
Appearance after Test of Specimens Rotated Horizontally in Sea Water at 1140 R.P.M., 25 Ft./Sec. Tip Speed (34)
INDUSTRIAL AND ENGINEERING CHEMISTRY
1747
oi the spccknens was 23 feet per second. The attack at t,he leading edge after 60 days is shown in Figure 6. The weight lose ivaz: equivalent t o 878 mg. per square dm. per day, .\lloy materials, which develop more adherenc protective filnls, are more Time 16 t o I R days resistant to erosion corrosion. Thus austenitic nickel alloy cast .ireits 0 , 2 sq. dni. iroiis were substant,ially unatt>ackedin the same test. In these Veiooity, Velooit,>-, 0.5 F t . / S e c . 7.8 Ft./Sec. _____-.-.. tcsts the vertical rotation brought about extreme turbulence Corrosion Galvanic Corrosion Galvanic with the water becoming frothy. This brought itn abundant rate of steel, effect, rate of steel, effect, mg./sq. mg./sq. mg./sq. mg./sq. supply of oxygen to all surfaces, so that the coridit.ioris illusCoup1 e dm./day dm./day dm./day din./day trated by Figure 5, -4,did not develop. Fontana and Luce ( 1 7 ) studied the erosion corrosion of steel in water in some laboratory tests. I s might he expected they found that the amount of damage was affected by the aeration Ax-. of both meinbers of couple. and by the pH of the water. Anyt,hing that changed the protective character of the film would be effective. In one series of t,ests the erosion corrosion \vas severe a t low pH's, decreaeed t o a minimum a t p H 6, increased again t o p H 8, and then became average pit depth 0.033 incll in 13 ~ n o t ~ t h\\-he s, nil a t pH 10 and above. Unfort,unately the actual velocitiee moving a t 4 t o 5 feet per sccond, there \\-as no nieasurd)lc w i g h t ivcrc not stated, but a t pH 3 in some overnight testa st 50" C. loss and only one pit, 0.004 inch ctecp (SSj. the rate of attack increased from 300 a t comparatively low veZero velocity is harmful also if inhibitors atre added t o \\-ittcr. locities to 8iO mg. per square dni. per day a t high velocities. For example, in quiescent watchre little protrction was givcii Severe erosion corrosion was a180 reported by Wagner, Decker, by glassy phosphates, wherea,s with agitation, which cttrrictl :tnd Marsh (65) in tests siniulating conditions in centrifugal the inhibitor to the metal surface, definite protection n'as found boiler fced pumps handling condensed steam a t 250" F. and (46). The rate of formation of the protective film \vas a fuwtion high pressure. The velocit,y in these tests waa 6.5 to 450 feet of the rate of supply of g l a s ~ yphosphate and \v:t~ depondent per second. The water had been deaerated but contained 0.01 on the type and velocity of flow ( $ 5 ) . to 0.02 ml. of oxygen per liter. They found cast steel to suffer Other inhibitors would behave similarly. Thus Cohen ( 8 ) the most attack, with about one quarter as much attack on cast proved that the amount of sodium nitrite required to inhibit iron and one hundredth as much attack on 5% chromium steel. the corrosion of steel in Ott,awa, Ont., tap watcr varied with Other corrosion resistant materials that formed good protective velocity, less nitrite being requireti a t high velocity. He found films, such as 12% chromium steel, stainless steel, bronze, and that this effect of velocity could be followed by potential inexsurcAIonel, iwre resistant t o attack. nients with the iron becoming quite noble with complete inhibiI n some related tests (10, 1 6 ) the temperature and pH of the tion, water had large effects on t,he aniount of damage. Thus increasIt is difficult for velocity to l ~ v much c effect ii) cwviccs 01' ing the temperature from 90" t o 205" F. increwetl t,he attack other protected areas. The importance of considering this about ten times (16'1, At higher also is illustrated by the behavior of temneratures around 320' to ,385 F. stainless steel in sea water. I n a heat the effect of temperature changes exchanger it would be prcferahlc t o seemed uncertain ( I O ) . In one series have the water flow through the tubee of tests increasing the p H from 7.6 rather than through the shell around t o 8.4 doubled the attack, wheieas in the tubes. Comparatively stagnant another series treated water at pH areas outside the t,ubes might result, 9.3 caused less attack than untreated in severe pitting. Inside the tuhes v itter at p H 8. the desired velocity of flow is easicr t o More work needs to be done on control (33). erosion corrosion of iron in water, but the principles involved seem clear. 911 Erosion Corrosion of Iron the exposure conditions t h a t determine corrosion rates under normal conditions b'igure 2 s h o m that the coIrosiori \vi11 havc some effect on erosion coirorate began t o increase again a t vc10c.ision because they influence the nature ties higher than 3 feet per serontl. and rate of formation of the protecThe same effect was observed in other tive films. Alloying, which increases experiments (22, 4 8 ) and has bern the corrosion rePistance and develops attributed to erosion, a t these velocimore protective film@*will be helpful ties, beginning to strip away protectivc in erosion corrosion. Erosion corrofilms. This phenomenon is called sion is likely t o occur ui pumps, valves, erosion corrosion, and it can lead t o centrifugals, orifices, agitators, elbows, severe attack a t high velocities. Hetees, heat exchanger tub=, and in fact nioval of protective films by erosion anywheye that water itt high velocity allows the metal to detcrioratc at the is encountered. original rapid rate. Related to erosion corrosion in water Some tests in sea water a t 30' C. ir the attack t h a t nlay be caused by showed t h a t cmt iron was quite suswater droplets moving a t high speed. ceptible to erosion corrosion ( 2 7 ) . Such attack might occur on aircraft Tho specimens were attached t o a disk moving a t high speed through wet and rotated a t high speed in a vertical Figure 6. Erosion Corrosion of Cast Iron in Sea Water ( 7 7) plane so that the vrlocity at thc tip c l o u d ~or rainstorms, or it might be Table I.
1748
Effect of Velocity on Galvanic Corrosion of Steel in Sea Water (37, 3 4 ) Temperaturc .ioo E'.
INDUSTRIAL AND ENGINEERING C H E M ISTRY
VOl 44 NO. 8
by Water
-Corrosion caused by water droplets in wet steitin (6). The impingement attack by wet steam moving a t high velocity has been observed on turbines, blades, condenser tubes, valve seats and disks, piston rings, or any other parts handling t h e steam. Somctimes t h e attack is quite local and takes the form of deep clean grooves; in thie case it is called wire drawing. Effect of Velocity on Corrosion
of Zinc
Zinc is not ordinarily used under high velocity conditions such as might involve erosion corrosion and impingement attark. Like many other metals the corrosion of zinc is stimulated by relative motion between the metal and the solution ( 1 ) . Under conditions of lower velocity most of the effects or velocity tie in with differential aeration cells. I n zinc, differential aeration cells generate strong currents with the aerated portion being cathodic. Stagnant crevices may prove points of intense attack (14). A common use of zinc is for galvanic protection of iron. There is evidence t h a t in hot water of appropriate composition the polarity of the zinc-iron couple may reverse with the zinc becoming cathodic (16,49). This undesirable behavior is dependent on the composition of the water and on the ions present. As discussed previously, motion has a big effect, in maintaining the supply of trace constituents a t the metal surface. Therefore motion would be expected t o have an effert on this reversal of polarity. The current required for the galvanic protertion of iron will vary with the velocity. This is true berause the corrosion rate and cathodic polarization characteristirs of iron vary with velocity conditions. This point is illustrated by Figure 7 , which shows some cathodic polarization curves for mild steel in sea water (58). At 7.8 feet per second more current is required t o produce the same polarization. This needs t o be considered in using zinc t o protect iron because the zinc will have to supply more current, and it will be consumed faster under conditions of high velocity. Effect of Velocity on Corrosion of Copper
The corrosion rates of copper and copper alloys are comparatively low in most fresh waters. However, nonscale forming waters and aggressive waters containing carbon dioxide may cause attack (6). I n such cases the principal effect of moderate velocity may be t o set up metal ion concentration rells or differential aeration cells. Bengough and May (3)exposed two copper electrodes t o 3% sodium chloride. One electrode was sealed from oxygen, and a stream of oxygen saturated solution was forced around the other. When the stream was slow the electrode in the flowing liquid functioned as cathode, owing t o the renewal of oxygen; when t h e stream was rapid the same electrode behaved as anode, owing t o the washing away of metallic ions. Evans (14) conrluded t h a t the oxygen effect was more important in anodic metals like zinc and t h e metal ion effect more important in noble metals like copper. As a n example of a metal ion cell Evans ( 1 6 ) cited the intense corrosion of some externally cooled copper condenser tubes a t the place where the cooling water entered and washed against tho surface. The total attack was not great, but as it was roncentrated on a small area, perforation was rather rapid. Newberry ( 4 1 ) reported another example in a copper alloy pump on a water well. Current flowed between the inner portions of the pump, where the liquid was in rapid motion, and the outer portions, where the liquid was comparatively stagnant. Pitting may also occur where foreign bodies (Rand, mud, slime, weeds, leaves, silt, ashes, shells, rust) lodge against the metal surface and set up differential aeration cells or metal ion cells
August 1952
( 4 ) . Dcposit, a t t w k is met mainly where water velocity is slow. Under certain conditions favored by low velocity, high zinc brasses (60 copper, 40 zinc and 67 copper, 33 zinc) may suffer a type of attack called dezincification (6, 65). The zinc is, in effect, removed and a porous layer or plug of copper is left behind. This is likely t o occur in hot polluted unaerated waters under stagnant conditions as under thick permeable layers of corrosion products or deposits or in crevices. Dezincification can be modcrated or prevented by adding arsenic, antimony, or phosphorus t o the alloy, by lowering the zinc content, or by shifting to other copper alloys such as the cupronickcls. Ileeincification seldom -1200,
I
I
I
I
I
I
Figure 7. Effect of Velocity on Cathodic Polarization Curves of Mild Steel in Sea Water (38) if ever occurs under high velocity conditions, but t h e high vclocity may cause another complication called impingement attack. Thus in tests in sea water a t high velocity, the exposed part of manganese bronze sperimens suffered impingement attack, but the part under a Bakelite insulator where the Rolution was stagnant suffered dezincification (34). Impingement Attack of Copper Alloys
Impingement attack is a form of erosion corrosion. It occurs when a turbulent aerated watcr stream impinges against a metal surface (59). With nonresistant metals it can occur a t water velocities as low as 3 feet per second, although generally it occurs where water velocities are fast-6 t o 12 feet per second or faster. It is associated with continuous local breakdown and removal of protective films from the metal surface by impingement. With removal of protectivr films, a small area is rendered anodic to adjacent areas and suffers rapid corrosion. Impinging aerated water depolarizes cathode areas adjacent to the anodes so that ideal conditions for rapid electrochemical corrosion are maintained. Impingement a t t w k is more intense in sea water than in fresh water. Accordingly sea water or salt solutions have generally been used for studying impingement attack. Sand, debris, and air bubbles may make the attack more serious, but high velocity sea water is damaging enough. Typical impingement pits have a smooth rounded contour, and they are deeper and often undercut on the downstream side. The inlet end of heat exchanger and condenser tubes are particularly susceptible, but the attack may occur anywhere impingement occurs, as at screens, at bends in tubes and pipe, downstreani of valves, at branch connections, a t changes in cross section, and below partial obstructions such as shells. Bulow ( 7 ) observed it at the leading edges of test specimen8 immersed in sea
INDUSTRIAL AND ENGINEERING CHEMISTRY
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water moving at 2 to 3 feet per second, in which case it was manifested by a rounding of the leading edges. Much can be accomplished by better design t o avoid local high velocities and turbulence of circulating waters (11, $3). Impingement attack can generally be prevented by the use of resistant alloys t h a t form more protective film. Thus in some comparative tests in a full scale experimental condenser, handling turbulent sea water a t an average velocity of 12 feet per second,
I $15
occurred a t the margin of the protective film rather than a t the periphery. The decrease in thickness a t the outer edge was 0.017 inch. Figure 8 summarizes t h e results on disk specimens of cupronickel. This demonstrates the abrupt increme in attack beyond the velocity where protective films are maintained. It is interesting that the high iron alloy is attacked t o a lesser extent before as well as after the protective film has been impaired by erosion. I n contrast with the behavior of the admiralty brass, the attack on the cupronickels was very uniform. In fact the corroded surfaces near the edges appeared smoother than the unattacked areas near the center. The average corrosion rates of the attacked portions were about 220 and 290 mg, per square din. per day.
c U
2
10
Cavitation Erosion
LL
0 8 '
a.
gQ o
+
IO IO VELOCITY,
5
15
F?.
20 20
25 25
30 30
lSEC
Figure 8.
Effect of Velocity on Depth of Attack of 70/30 Cupronickel in Sea Water (35) Sixty-day tests at 2 6 '
C.
Freeman and Tracy (19) rated high iron 70/30 cupronickel best followed by low iron 70/30, high iron 9O/lO cupronickel, 8% phosphor bronze, aluminum brass, aluminum bronze 95/5, arsenical admiralty, and uninhibited admiralty. Tracy and eo-workers (20, 63) arrived at substantially similar results in laboratory tests in which disk specimens were rotated a t peripheral speeds from 8 t o 48 feet per second. LaQue (36, 36) has made extensive tests on the effect of velocity on copper alloys in natural sea water using several kinds of apparatus and paying particular attention t o iron modified 70/30 cupronickel. Results up t o about 12 feet per second are summarized in Table 11. The addition of about 0.5% iron to 70/30 cupronickel was uniformly beneficial, particularly a t the higher velocities and under impingement conditions. Probably the beneficial effect of iron was due to the formation of a more protective film.
Table II.
Velocity Ft./Sec.' 0-2 3-4
7;s 12'
42 days
.....
Corrosion Rate Mg./sq. dm./day Pit depth, inch * Low Hi% ' LOW Hi hTemp., Fea Fe Fea F,"b O C. 70/30 70/30 70/30 70/30 ,, 1.3-1.9 0.7-1.1 0 . 0 2 0.02 . . . 10 5 1.6 Nil hTil 12.6 124 67 25 3 3 0 .'OOOS 0 .'do04 20 13 3 0 009 0.004
.
Less than 0 . 1 2 Fe. * A b o u t 0.5% Aspirator jet impingement apparatus, air bubbles excluded.
a
.
~~~~
~
For higher velocities up t o 27 feet per second LaQue used rotating disk specimens or attached tubes and bars t o the periphery of a rotating wheel. The disk specimens were particularly useful because they showed the critical velocity above which rapid corrosion occurred owing t o erosion of the protective film. An admiralty brass specimen is shown in Figure 5, B ($4). It provides quite a contrast t o the behavior of cast iron in the same apparatus. Most of t h e attack on the admiralty brass occurred outside a radius of 1.75 inch or a t speeds greater than 19 feet per second, T h e average corrosion rate of the attacked portion was 305 mg. per square dm. per day. The attack tended to be uneven and localized with typical erosion pits. The maximum thinning 1750
(la). This mechanism emphasizes the mechanical nature of the damage (5d). Nevertheless corrosion accelerates the destruction, This might simply be due t o erosive effects removing protective films or it might be due t o increased reactivity of the water associated n-ith the cavitation ( 3 7 ) . Pitting and roughening of the surface by corrosion increases the damage. Conditions might be such t h a t a corrosion resistant material would remain unaffected, whereas a material which pitted would cause turbulence t h a t would lead t o cavitation eroeion. Several paperf have discussed the mechanism (29, SO, 44).
Effect of Velocity on Corrosion of Cupronickel on Sea Water (35)
Time 10 years 0.5years 17.5daya
8
Cavitation erosion requires t h a t the water be subjected t o rapidly alternating ranges of pressure. At high relative motion between metal and solution the pressure may be reduced locally so t h a t boiling occurs. Small cavities of vapor form in the water. Return of pressure t o normal causes an implosion as the cavities collapse. If a cavity is in contact with the metal, there is a high speed of liquid impact. The surface work hardens, roughens, and cracks by fatigue. The result is rapid, deep, spongy pitting
Figure 9. Cross Section of Cylinder of Diesel Engine Showing Cavitation Erosion on the Water Side (57)
An example of cavitation erosion is shown in Figure 9 ( b l ) . This is a cross section of a cylinder from a Diesel engine. The attack occurred on the water side of t,he cylinders a t rates as high as 16,000 mg. per square dm. per day (3 inches per year), which is much too high t o be accounted for by oxygen consumption, hydrogen evolution, or stray currents, Cavitation erosion has been experienced in high speed pumps, hydraulic turbines, valves, near orifices, near obstructions, in marine propellers, and in fact it may occur anywhere t h a t high velocity flow is encountered under medium or low pressure. J e t tests, Venturi tests, and vibratory tests (a, 88,40)have been used in laboratory studies. Some results by Beeching (9)are given in Table 111, and these serve t o illustrate the general findings. Both corrosion reaistance and hardness are important. Hence for alloys of different types there is no direct relation between
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 8
Corrosion by Water Results of Some Cavitation Erosion Tests Using the Vibratory Method (2)
Table 111.
Type of Alloy Cast High Tensile Brasses
Ni Zn A1 Admiralty Mn Bronze 39 0.3 8.0 5.9 21 2.6 5.0 10 5.0 31 3.7 3.4 12.6 41 1.0
Chemical Analysis, % Fe Mn Sn 0.5 1.6 1.0 1.7 1.0
0.3 1.8 1.0 2.1 2.0 0.1 1.7 1.9
..
Cast
Monels
..
.. ..
.. .. ..
I .
3 4
..
.. .. .. . I
..
..
.. .. .. .. ..
.. .. ..
..
0:5 .,
.. .. ..
.. ..
..
.. .. I
.
10 9 5 10 5 5 10
.. ..
.. ..
P1
.. .. .. .. .. .. .. .. .. .. .. .... i.’5 5
.. .. .. .. .... .. ..
Gray cast iron Cast Cylinder iron 1 . 4 Ni 0 . 7 Cr Iron Ni-Resist, 14‘Ni, 3.4’Cr, 6 . 4 CU and 18/2 as cast Cast 18/2 heat treated Stainless 18/8 steel a Brinell hardness number.
resistance to cavitation erosion and either one of them. I n the table t h e high tensile b r w e s have mechanical properties of the same order as t h e aluminum bronzes, b u t since they are less resistant t o corrosion they suffer much greater Cavitation erosion. On t h e other hand the high tensile brasses have mechanical properties superior t o the cast gun metals and bronzes, so that they are more resistant to cavitation erosion even though their general corrosion resistance is inferior. With alloys of the same type, such rn the copper-nickel-zinc alloys, where resistance t o corrosion is about t h e same, the resistance to cavitation erosion increases regularly with hardness. The effect of t h e corrosive is shown by the results in fresh water and sea water. There is little difference between the behavior of the stainless steels or t h e bronzes in these two media. On the other hand cast irons corrode more rapidly in sea water and hence show greater cavitation erosion in this corrosive. The best remedy for cavitation erosion is proper design. Much can be accomplished, however, by selecting alloys resistant t o this type of attack either due t o greater corrosion resistance or better mechanical properties or both. B y increasing the pressure t h e formation of cavities can be eliminated. Dissolved air or other gases come out in the cavities and cushion t h e blow when the cavity collapses. Coatings of resistant metals can be used. Acknowledgment
The author wishes t o acknowledge his debt t o F. L. LaQue who reviewed the manuscript and who supplied a large portion of t h e data t h a t have been cited. literature Cited (1)Anderson, E. A., and Reinhard, C E., “Corrosion Handbook,” p. 338, New York, John Wiley & Sons, 1948. (2) Beeching, R., Prod. Eng., 19, 110 (1948). (3) Bengough, G.D., and May, R., J . Inst. Metals, 32, 139 (1924). (4)Ibid., p. 174. (5) Bigger, T.W.,Trans. Am. SOC.Mech. Eng., 72, 24 (1950). (6) Bulow. C.L.. J . New E M . WateT Worlcs Assoc.. 59, 163 (1945). . (7) Bulow, C. L., Trans. EleEtrochem. SOC.,87, 127 (1945). ( 8 ) Cohen, M., J . Electrochem. Soc., 93, 26 (1948). August 1952
Mechanical Properties Tensile strength Elonga1000 lb./s& tion, inch % 80.3 27 156 156 85.0 36 86.0 18 145 88.8 28 101 89.5 20 175 89.5 15 175
Hard-
Cu Bi. Bal. Bal. Bal. Bal. Bal. Bal. Bal. 29
.. .. *. 88 Bal. 85 Bal. Bal. Bal. Bal. 40 48 47 38
Other
.. .. .. .. .. .. .. ..
.. ..
3 6i
....
.. .... .. .. 0:i P
.. ..
.. ..
$g&a
Wt Loss Mg &t 60 Min.’ of Exposure Salt Distilled water water 18.9 16.2 ... 13.9 11.2 9.5 8.9 8.8 ...
... ... ...
169 154 128 139 226 215 253
109.5 117.8 122.1
82
34 15
8.7 8.2 5.8
84 72 62 99 93 208 72
37.2 37.2 29.5 48.4 48.8 87.0 34.0
13 25 19 18 42 3 13
18.9 22.1 25.0 23.8 27.6 20.3 19.4
255 155 187 170
114.4 73.2 103.0 101.5
2 18 16 19
3.5 9.8 7.0 8.3
214 244 170 236 276 168
29.5
...
... ...
...
14.3 22.6 24.3 25.8 24.6 16.0 20.5
... ... ...
...
46.9
..
Z.8
(9) Cox, G. L., and Roetheli, B. E., IND. ENO. CHEM.,23, 1012 (1931). (10) Decker, J. M., Trans. Am. SOC.Mech. Eng., 72, 19 (1950). (11) Dillon, R. E., Eaton, G. C., and Peters, H.,Ibid., 59, 147 (1937). (12) Donaldson, J. W.,Metal I d . (London), 60, 383,401 (1942). (13) Ellinger, Ci. A., Waldron, L. J., and Marmlf, J. B., Proc. A m . Soc. Testing Materials, 48, 618 (1948). (14) Evans, U. R., J . Inst. Metals, 30, 261 (1923). (15) Evans, U.R., J. Soo. C h m . Ind., 43, 127T (1924). (16) Fairchild, F. B., Trans. Am. SOC.Mech. Eng., 69, 398 (1947). (17) Fontana, M.G.,and Luoe, W. A., Corrosion, 5, 189 (1949). (18) Forest, H. O., Roetheli, B. E., and Brown, R. H., IND. E m . CHEM.,23, 660 (1931). (19) Freeman, J. R.,and Tracy, A. W., Corrosion, 5, 245 (1949). (20) Freeman, J. R., and Tracy, A. W., “Symposium on Corrosion Testing Materials,” p. 32, Philadelphia, Am. Soc. Testing Materials, 1937. (21) Frese, F. G., IND.ENG.CHEM.,30, 83 (1938). (22)Friend, J. N.,J . Iron Steel Znst., 11, 62,128 (1922). (23) German, A. J,, Trans. A m . SOC.Mech. Eng., 61, 125 (1939). (24) Growbeck, E, C., and Waldron, L. J., Proc. Am. Soc. Testing Mat&B, 31, 11, 279 (1931). (25) Hatch, G.B., and Rice, O., IND. ENG.CHEM..37, 752 (1945). (26) Hoxeng, R.B.,Corrosion, 6,308 (1950). (27) International Nickel Co., Inc., Znco., 22, No. 3, 7 (1948). (28) Kerr, S. L., Trans. Am. SOC.Mech. Ertgrs., 59, 373 (1937). (29) Knapp,R.T., and Hollander, A.,Ibid., 70, 419 (1948). (30) Korrifield, M., and Suvorov, L., J . Applied Phys., 15, 495 (1944). (31) LaQue, F. L.,Am. SOC.Testing Materials, Marburg Lecture (1951). (32) LaQue, F. L., “Corrosion Handbook,” p. 391, New York, John Wiley & Sons, 1948. (33) Ibid., pp. 414,415. (34) LaQue, F. L., unpublished data, International Nickel Co., Ino. (35) LaQue, F. L., and Mason, J. F., Jr., Prm. Am. Petroleum Inst., Die. of Refining, 30 M [III],103 (1950). (36) LaQue, F. L., and Stewart, W. C., Mbtaua & Corrosion, 23, 147 (1948). (37) Marboe, E. C., Chem. Eng. News, 27, 2198 (1949). (38) May, T.P.,unpublished data, International Nickel Co., Inc. (39) May, R.,and Stacpoole, R. W., Devere, J. Znst. Metab, 77, Part 4, 331 (1950). (40) Mousson, J. M., Trans. A m . SOC.Mech. Engrs., 59, 399 (1937). (41) Newberry, E., Trans. Electrochen. SOC.,67, 223 (1935). (42) Paseano, R. F., and Nagley, F. R., Proc. Am. SOC.T ~ t i n ~ Mczterials, 33, 387 (1933).
INDUSTRIAL AND ENGINEERING CHEMISTRY
1751
(43) (44) (45)
Poluskin, E. P., and Scliuldiner, H. Id.,Curnosion, 2, 1 (1946). Poulter, T. C., J . Applied Mrc/i(znics,9. A-31 (1942). Hawdon, H. S., and Waldroii, L. ,I., Proc. Am. SOC. 'f'esliny
Materials, 35, 233 (1935). (46) Rice, O., J . Am. Water U-orks dssoc.. 39, 552 (1947) (47) Hoetheli, B. E., and Brown, 13. H., INI).ENO.CHEX.,23, 1010 (1931). (48)
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