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Erosion and corrosion of pure iron under cavitating conditions
Erosion and corrosion of pure iron under cavitating conditions W . J . Tomlinson and M . G . Talks* Department of Materials, Coventry Polytechnic, Coventry CV1 5FB, UK
Received 7 March 1990; revised 15 August 1990 Pure iron has been eroded in various aqueous solutions at 50°C using a 20 kHz vibratory facility and the erosion kinetics determined, the corrosion component during erosion determined electrochemically, the size distribution of the erosion products determined and
the eroded surface examined metallographically. Salt in the water increased the erosion rate 1.4 times and gave a smaller maximum size of eroded particle, and the erosion, corrosion and corrosion-induced erosion components of damage were estimated to be the fractions 0.71, 0.10 and 0.19, respectively. The effects of an inhibitor, pH, cathodic protection and amplitude of vibration on the erosion were also briefly examined. The results are presented and discussed in terms of the erosion/corrosion processes. K e y w o r d s : c a v i t a t i o n ; e r o s i o n , c o r r o s i o n ; iron
Stresses generated by the collapse of cavitation bubbles may deform, fracture and detach particles from the surface of nearby solids1'2. Erosion may cause pitting and weakening of the material or, in the case of sonochemical reactions, the detached particles may catalyse the reaction and/or contaminate the product 3. Although cavitation erosion in a corrosive environment is considered to be predominantly mechanical in nature, a corrosion component of the damage is also present which in some cases may be substantiaP. Thus, the corrosive effect of salt in the water may increase the cavitation erosion rate by up to seven times4. The chemical nature of the action is evident since the extent of the damage may be reduced by the presence of inhibitors or by cathodic protection (see, for example, Reference 5). In general, the corrosion factor is relatively unimportant for high intensity cavitation or for poorly resistant materials, whereas it is important for low intensity cavitation or for highly resistant materials 1. The most important aspect of cavitation erosion in a corrosive environment is the synergistic nature of the action. This makes it difficult to determine the separate physical and chemical factors, but recently an electrochemical technique has been used to measure the corrosion component of steels 6 and cast irons 4. These materials have, however, relatively complex microstructures which may mask the Underlying conjoint action. The present work therefore investigated the simultaneous erosion and corrosion of pure iron in aqueous solutions under cavitation conditions generated by 20 kHz ultrasound and, more generally, the effects of inhibitors, pH, cathodic protection and the amplitude of vibration on the erosion process.
* Part-time student, presentlyemployedat BritishCoal,Ashby Road,
Burton-on-TrentDE15 0QD, UK 0041-624X/91 / 020171-05 © 1991 Butterworth-Heinemann
Ltd
Experimental
details
A high purity iron of composition (wt %) C 0.02, Si 0.02, Mn 0.2, P 0.007, S 0.006 and Ni 0.03, with a very low concentration of non-metallic inclusions, was supplied by British Steel as bars 21 mm in diameter and was heat treated to produce a large and uniform grain size. Ground specimens 2.5 mm thick were silver soldered on a mild steel stub and machined to a working face of 113 mm 2. Prior to erosion the face was polished to a 1/~m diamond finish. Full details of the vibratory erosion apparatus with a facility for potentiostatic control, the testing procedure, electrochemical linear polarization measurements and optical and scanning electron microscopy techniques have been given elsewhere 7'8. In outline, the sample was eroded in aqueous solution at 50°C with a frequency of 20 kHz at, typically, 15 #m peak to peak and the course of the erosion followed by periodic weighing ( _ 1 mg). The polarization resistance, Rp, of the specimen was obtained during erosion in salt solution by measuring the current when the potential of the specimen was changed manually by - 5, - 10, + 5 and + 10 mV from the corrosion potential. Acid (pH 2) and alkaline (pH 12) solutions were made by the addition of AnalaR grade sulphuric acid and sodium hydroxide, respectively, and the inhibited solutions contained 2.5 vol% Fleetguard DC4 inhibitor 7. Results
Erosion followed a uniform pattern of behaviour in which the erosion rate gradually increased until a final steady state erosion rate occurred. Typical kinetics are shown in Figure 1. The start of the linear period is difficult to locate and so for convenience the extrapolation of the linear region on the time axis is called the incubation period, to. Erosion parameters from all the results are
Ultrasonics 199t Vol 29 March
171
Erosion and corrosion of pure iron under cavitating conditions: W.J. Tomlinson and M.G. Talks collected in Table 1. It is seen that 0.02 % salt in the water has no effect on the erosion rate but the presence of 3 % salt increases the erosion rate at all amplitudes. An increase in amplitude always increases the erosion rate. Alkaline conditions (pH 12) increase considerably the erosion rate. The presence of an inhibitor in neutral
solutions does not decrease the erosion rate, thus indicating that the erosion damage is essentially mechanical in nature. The incubation period, to, is relatively unaffected by the various testing conditions, except in the case of the acid solution where its value is greatly decreased ( Table 1). Cathodic polarization under both static and cavitating conditions shows clearly the influence of cavitation on the limiting currents due to the reduction of oxygen and the reduction of protons (Figure 2). At E(SCE) = - 1.0 V (SCE = saturated calomel electrode) the turbulence due to cavitation has increased the limiting current of oxygen reduction by 40 times. Cathodic protection in neutral solution eliminated the corrosive influence of the salt. Corrosion potentials, E ..... and the polarization resistance, Rp, were measured in neutral 3 % salt solution under static and cavitating conditions. Due to the increased oxygen supply and hence a decreased cathodic polarization, cavitation always increased E .... (see Figure 2). The corrosion currents, I c.... were calculated using
E
I .... = (0.035 V)/(Rp V/A) which may be considered a sufficiently accurate approximation 9 for the present purposes. The results are shown
in Table 2. Although the results obtained under cavitating conditions tend to be more variable than those obtained under static conditions, it is seen that during the steady state erosion period the corrosion currents for each amplitude may be considered roughly similar. Weights of the erosion debris in various size ranges are shown in Table 1 and the size distribution for some results in Figure 3. The colour of the particles/solution (Table 1) indicated the corrosion conditions; grey particles (presumably mainly iron) occurred in distilled water and brown particles (hydrated oxides) occurred in the corrosive salt waters. Blue particles resulted from the interaction of the iron particles and the inhibitor. The
t (min)
Figure I Erosion, w, as a function of time, t, for pure iron eroded at various amplitudes in distilled water and 3% NaCI solution at 50°C. Distilled water: (3, 10/am; r-l, 15/am; A , 20/am. 3% NaCI solution: O, 10/am; I I , 15/am; A , 20/am Table I
Erosion kinetics and the erosion debris of pure iron eroded by 20 kHz ultrasound in aqueous solution at 50°C under various conditions Erosion kinetics b
Filtered debris Mass (mg)e
Test conditions a
to (min)
r (mgh
Distilled, a = 10/am Distilled Distilled, a = 20/am 3% salt, a = 10/am 3% salt 3% salt, a = 20/am Distilled, inhibitor 0.02% salt 0.02% salt, inhibitor 3% salt, r / = --0.2 V 3 % salt, t / = - 1 . 4 V Distilled, pH 2 3 % salt, pH 2 Distilled, pH 1 2
8 14 10 c 15 15 13 17 19 18 13 15 2 9 20
23 35 52 c 32 49 85 39 39 44 39 39 81 78 42
3% salt, pH 12
19
41
1)
Specimen loss (mgper90min)
>160/am
100/am
40/am
16/am
4/am
31 47 87 40 61 110 47 46 46 40 d 39 d 117 105 48
2 16 26 8 7 18 10 3 8 4 3
6 9 17 11 12 27 8 12 24 21 8 .
15 15 21 30 29 7 22 21 62 19 20
3 0 6 5 9 12 3 4 18 3 4
2 3 1 3 2 3 2 0 14 2 3 . -
48
.
.
.
.
.
% Specimen Total loss Colour 28 43 71 57 59 129 45 40 126 49 38
90 91 82 143 97 114 96 87 274 122 97
-
-
.
.
.
.
All amplitudes, a = t 5/am unless otherwise given and r / = cathodic polarisation (see text for details) bt o = incubation time and • = steady erosion rate (see text for details) cSecond r = 79 occurs with to = 22 a For total erosion time of 75 min e 100/am indicates size range 1 0 0 - 1 6 0 / a m , etc. % Specimen loss = [ (weight of debris) / (specimen loss) ] × 1 O0 f N D = no'debris; all had dissolved
172
UItPas~nics 11991 -Vol 29 March
Grey Grey Grey Brown Brown Brown Blue Brown Blue Brown Brown ND t Grey Grey/ brown Grey
Erosion and corrosion of pure iron under cavitating conditions: W.J. Tomlinson and M.G. Talks -0.5
Discussion
El
Erosion under cavitation conditions is a fatigue process resulting from the cyclic stresses induced by the collapse of the cavitation bubbles 1°. In the present case it is assumed that erosion in distilled water is essentially mechanical in nature because neither the presence of inhibitors nor the action of cathodic protection reduced the erosion rate below the basic level (Table 1). A few per cent salt in the water does not change the physical properties of the solution and hence the cavitation processes to any significant extent 6, and so the increase in the erosion rate in 3 % salt solution is considered to be due to its corrosive action. Salt in the water will increase the corrosion current, I¢o. , due to a decrease in the electrical resistance of the corrosive environment, R,ol., since in the corrosion cell 1
-I.C
t~ -I 3
-2.0 I
I
-I
0
I
I Log ( l i c l ) (mA)
I
I
2
3
Figure 2 Cathodic polarization of pure iron in 3 % NaCI solution at 50°C under static ( O ) and cavitating ( [ ] ) conditions at 15/am amplitude, Only oxygen reduction occurred down to ~ - 1 . 2 V: below - 1 . 2 V extensive hydrogen evolution also occurred, Area = 113 mm 2
E=q = r/a + I~/cI + I . . . . Rsol.
100 90
total weight of the filtered grey particles in distilled water was 80-90% of the specimen weight loss, whereas the total weight of the filtered brown particles was 90-140 % of the specimen weight loss. Thus hydration has increased the weight of the particles in an irregular manner owing to different degrees of hydration and flocculation. Some dissolution would also have occurred in the salt solution. It should also be noted that the smallest filter used was 5/~m. The inhibitor clearly interacts with the iron to form the blue colour and can also greatly increase the weight (note 274% in Table 1). It is clear that a uniform pattern of erosion occurs (Figure 3). The most important feature is that erosion in distilled water with the relatively high amplitudes (a = 15 and 20 #m) produces more of the large particles, whereas for erosion in salt solution or in distilled water with the lowest amplitude (10 /tm), relatively few large particles are formed. Erosion produced a roughened surface that was uniform over the specimen (Figure 4a). Detailed examination showed that the metal in the surface had been severely deformed and fractured in a ductile mode, and that there were no essential differences between the various erosion conditions. For example, although the erosion rates in distilled water and in salt water with an amplitude of 20/am were different ( Table 1), the eroded microstructures (Figures 4b and c) were very similar.
80 70
\
6O v
50 40 30 20 ,°
0
5
I 16
I 40
j, (p.rn)
I I00
I 160
Figure 3 Weight of filtered erosion debris, m, as a function of the maximum particle dimension greater than y, for pure iron eroded at various amplitudes in distilled water and 3% NaCI solution at 50°C. Distilled water; O, 10/am; 1-1, 15 tam; A , 20 pro. 3% NaCI solution; O, 10 #m; I I , 15/am; A , 20/am. Note that the lines are drawn for convenience and do not indicate an assumed particle size distribution function
Table 2
Corrosion currents of pure iron in 3% NaCI solution at 50°C under static and cavitating conditions with various amplitudes after 5 and 90 min exposure Corrosion current, / (mA) Static
Cavitating
Ratio [ / (cav) / / (static) ]
Amplitude (/am)
5 min
90 min
5 min
90 min
5 min
90 min
10 15 20
0.40 0.55 0.49
0.41 0.45 0.35
(4.60) a 2.17 1.30
3.96 4.73 2.84
( 11.5) a 4.0 2.7
9.7 10.5 8.1
a Uncertain value
Ultrasonics 1991 Vol 29 March
173
Erosion and corrosion of pure iron under cavitating conditions: W.J. Tomlinson and lid. G. Talks Table 3 Erosion (E), corrosion (C), erosion in salt water (ES) and the corrosion-induced erosion (CIE) components of damage of pure iron eroded in aqueous solution at 50°C with various amplitudes Damage rate (mg h -1 )a
Damage rate (fraction)
Amplitude (#m)
E
C
CIE
ES
E
C
CIE
10 15 20
23 35 52
4 5 3
5 9 30
32 49 85
0.72 0.71 0.61
0.12 0.10 0.04
0.16 0.19 0.36
a Corrosion converted from electrochemical measurements and CI E calculated (see text for details)
Figure 4 Surface of pure iron after erosion under various conditions: (a) distilled water, amplitude 10/~m (marker 0.1 mm); (b) distilled water, amplitude 20 pm (marker 2 pm); (c) 3% NaCI solution, amplitude 20/~m (marker 2 pm)
The equilibrium cell potential, Eeq , and the anodic and cathodic overpotentials, ~/~ and r/c, respectively, will be relatively unaffected by the presence of salt 12. The corrosion will have two effects during the erosion process. First there will be a corrosion component (C) of damage and second there will be a corrosion-induced erosion component (CIE) of damage, since the formation of corrosion pits will act as stress concentrators. Hence, the erosion in salt water (ES) will have the components ES = E + C + C I E
174
Ultrasonics 1991 Vol 29 March
Assuming dissolution occurs as Fe 2+, 1 mA is approximately equal to an erosion rate of 1 mg h-~. Hence, for an amplitude of 15 /~m and using the information in Tables 1 and 2 we calculate CIE = (49 - 35 - 5) = 9 mg h - 1. Similar results calculated for erosion with 10 and 20 ~m amplitudes, and the components expressed as fractions of the damage, are shown in Table 3. It can be seen that 4 - 1 2 % of the damage is due to corrosion and the rest, 88 96 % of the damage, is due to the formation of debris ( Table 3). This is consistent with the independently obtained filtered debris (Table 1) where, in distilled water (considered the most accurate results), at least 82 9 1 % of the damage was due to the formation of solid particles. In fact, the fractions were higher since the results in Table 1 do not include those particles < 5 ~m in size. It is also seen in Table 3 that as the amplitude of erosion increases from 10 to 20 ~m the CIE increases from the fraction 0.16 to 0.35. This is interpreted as reflecting a relatively constant corrosion current (Table 2) and pitting, combined with an increasing mechanical action on the stress concentrating pits. An insight into the mechanism of erosion in a corrosive environment may be obtained by comparing the present results on pure iron with results obtained on a 0.2% P flake graphite grey coat iron 7. In the present work (15 ~m amplitude) the presence of 0.02 % salt in the water did not influence the erosion rate, 3 % salt increased the rate by 1.4 times and the damage due to E, C and CIE were fractions 0.7, 0.1 and 0.19, respectively. In contrast, with the cast iron 7, the presence of 0.02 or 3.0% salt in the water increased the erosion rates by 3.6 and 7.0 times, respectively, and the damage due to E, C and CIE were fractions 0.14, 0.02 and 0.84, respectively. Thus, the effect of salt in the water and corrosion and the corrosioninduced erosion were much greater in the case of cast iron. It would appear that the major difference is due to the different microstructures. Pure iron is ductile and deforms easily and the stress concentration at the pit root is reduced as the root is blunted by plastic flow, whereas in cast iron the brittle phases do not deform and thus fracture under the cavitating stresses. Although relatively ineffective compared with cast iron, corrosion in pure iron does influence the erosion process by forming corrosion pits. This is supported by the smaller maximum size of debris formed in corrosive solutions. Here it would appear that corrosion forms many pits that are active in concentrating the applied stress, whereas in distilled water only the cracks generated mechanically occur.
Erosion and corrosion of pure iron under cavitating conditions: W.J. Tomlinson and M.G. Talks
Conclusions
Acknowledgements
From the present work on the erosion and corrosion of pure iron (surface area 113 mm z) in aqueous solutions at 50°C under cavitating conditions using a 20 kHz
The authors wish to thank: at British Coal, Burton-on-
vibratory facility with an amplitude of 15 #m, it is concluded that: 1 2
3
4
5
6
7
Erosion in distilled water is essentially mechanical in nature. The presence of 3 %0 NaCI in the water increased the steady state erosion rate (SSER) by 1.4 times and reduced the maximum size of the eroded particles. In 3 %0NaCI solution cavitation increased the limiting current for oxygen reduction by ~ 40 times and increased the electrochemical corrosion rate in the SSER region by ~ 10 times. The electrochemical corrosion rate in the SSER region was 4.7 mA. The corrosion induced erosion rate in 3 % salt solution was calculated and the erosion, corrosion and corrosion-induced erosion components of damage were fractions 0.71, 0.10 and 0.19, respectively. Independent measurements of the weight of the eroded particles confirm that at least 8 0 - 9 0 % of the weight loss is in the form of particles. The rate of electrochemical corrosion in 3 % NaC1 solution remained at a similar level as the amplitude of vibration increased from 10 to 20 #m, but the erosion rate and the corrosion-induced erosion rate increased. The relatively small effect of the corrosive salt solution on the erosion of pure iron is considered to be due to the high ductility of pure iron blunting the crack tip and so reducing the stress concentration. Results obtained using an inhibitor, acid and alkaline solutions and cathodic protection were consistent with the above conclusions.
Trent, C.T. Massey for provision of cavitation erosion facilities and E.D. Yardley for encouragement; and at Coventry Polytechnic, C. Dawson for help with the SEM.
References 1 Preece,C.M. Cavitation erosion, in: Treatise on Material Science and Technology: Vol 16 Erosion (Ed Preece, C.M.) Academic Press, New York, USA (1979) p 249 2 Karimi, A. and Martin, J.L. Cavitation erosion of materials lnst Met Rev (1986) 31 1 3 Tomlinson,W.J. Effect of ultrasonically induced cavitation on corrosion, in: Advances in Sonochemistry Vol 1 (Ed Mason, T.) Jai Press, London, UK (1990) 173-195 4 Tomlinson,W.J. and Talks, M.G. Erosion and corrosion of cast irons under cavitation attack Tribology lnt in press 5 Ran, P.V., Sestha~miah, K. and Char, T.LR. Cavitation corrosion and its prevention by inhibitors and cathodicprotection Corr Prey Control (1972) 19 8 6 Okada,T. Corrosive liquid effects on cavitation erosion J Ships Res (1981) 25 271 7 Tomlinson, W.J. and Talks, M.G. Cavitation erosion of grey cast irons containing 0,2 and 1.0 phosphorus in corrosive waters Tribology lnt (1989) 22 195 8 Talks,M.G. Erosion and corrosion of cast irons under cavitating conditions CNAA PhD Department of Applied Physical Sciences, Coventry Polytechnic, UK, 1991 9 Fontana, M.G. Corrosion Engineering, 3rd Edn, McGraw-Hill, New York, USA (1986) 502 10 Hobbs,J.M. Experience with a 20-kc cavitation erosion test, in: Erosion by Cavitation or Impingement ASTM Spec Tech Publ, USA (1967) 408 159 11 West, J.M. Basic Corrosion and Oxidation, 2nd Edn, Ellis Horwood, Chichester, UK (1986) 105 12 Ceilings,P.J. Introduction to Corrosion Prevention and Control Delft University Press, The Netherlands (1985) 40
Ultrasonics 1991 Vol 29 March
175
Received 7 March 1990; revised 15 August 1990 Pure iron has been eroded in various aqueous solutions at 50°C using a 20 kHz vibratory facility and the erosion kinetics determined, the corrosion component during erosion determined electrochemically, the size distribution of the erosion products determined and
the eroded surface examined metallographically. Salt in the water increased the erosion rate 1.4 times and gave a smaller maximum size of eroded particle, and the erosion, corrosion and corrosion-induced erosion components of damage were estimated to be the fractions 0.71, 0.10 and 0.19, respectively. The effects of an inhibitor, pH, cathodic protection and amplitude of vibration on the erosion were also briefly examined. The results are presented and discussed in terms of the erosion/corrosion processes. K e y w o r d s : c a v i t a t i o n ; e r o s i o n , c o r r o s i o n ; iron
Stresses generated by the collapse of cavitation bubbles may deform, fracture and detach particles from the surface of nearby solids1'2. Erosion may cause pitting and weakening of the material or, in the case of sonochemical reactions, the detached particles may catalyse the reaction and/or contaminate the product 3. Although cavitation erosion in a corrosive environment is considered to be predominantly mechanical in nature, a corrosion component of the damage is also present which in some cases may be substantiaP. Thus, the corrosive effect of salt in the water may increase the cavitation erosion rate by up to seven times4. The chemical nature of the action is evident since the extent of the damage may be reduced by the presence of inhibitors or by cathodic protection (see, for example, Reference 5). In general, the corrosion factor is relatively unimportant for high intensity cavitation or for poorly resistant materials, whereas it is important for low intensity cavitation or for highly resistant materials 1. The most important aspect of cavitation erosion in a corrosive environment is the synergistic nature of the action. This makes it difficult to determine the separate physical and chemical factors, but recently an electrochemical technique has been used to measure the corrosion component of steels 6 and cast irons 4. These materials have, however, relatively complex microstructures which may mask the Underlying conjoint action. The present work therefore investigated the simultaneous erosion and corrosion of pure iron in aqueous solutions under cavitation conditions generated by 20 kHz ultrasound and, more generally, the effects of inhibitors, pH, cathodic protection and the amplitude of vibration on the erosion process.
* Part-time student, presentlyemployedat BritishCoal,Ashby Road,
Burton-on-TrentDE15 0QD, UK 0041-624X/91 / 020171-05 © 1991 Butterworth-Heinemann
Ltd
Experimental
details
A high purity iron of composition (wt %) C 0.02, Si 0.02, Mn 0.2, P 0.007, S 0.006 and Ni 0.03, with a very low concentration of non-metallic inclusions, was supplied by British Steel as bars 21 mm in diameter and was heat treated to produce a large and uniform grain size. Ground specimens 2.5 mm thick were silver soldered on a mild steel stub and machined to a working face of 113 mm 2. Prior to erosion the face was polished to a 1/~m diamond finish. Full details of the vibratory erosion apparatus with a facility for potentiostatic control, the testing procedure, electrochemical linear polarization measurements and optical and scanning electron microscopy techniques have been given elsewhere 7'8. In outline, the sample was eroded in aqueous solution at 50°C with a frequency of 20 kHz at, typically, 15 #m peak to peak and the course of the erosion followed by periodic weighing ( _ 1 mg). The polarization resistance, Rp, of the specimen was obtained during erosion in salt solution by measuring the current when the potential of the specimen was changed manually by - 5, - 10, + 5 and + 10 mV from the corrosion potential. Acid (pH 2) and alkaline (pH 12) solutions were made by the addition of AnalaR grade sulphuric acid and sodium hydroxide, respectively, and the inhibited solutions contained 2.5 vol% Fleetguard DC4 inhibitor 7. Results
Erosion followed a uniform pattern of behaviour in which the erosion rate gradually increased until a final steady state erosion rate occurred. Typical kinetics are shown in Figure 1. The start of the linear period is difficult to locate and so for convenience the extrapolation of the linear region on the time axis is called the incubation period, to. Erosion parameters from all the results are
Ultrasonics 199t Vol 29 March
171
Erosion and corrosion of pure iron under cavitating conditions: W.J. Tomlinson and M.G. Talks collected in Table 1. It is seen that 0.02 % salt in the water has no effect on the erosion rate but the presence of 3 % salt increases the erosion rate at all amplitudes. An increase in amplitude always increases the erosion rate. Alkaline conditions (pH 12) increase considerably the erosion rate. The presence of an inhibitor in neutral
solutions does not decrease the erosion rate, thus indicating that the erosion damage is essentially mechanical in nature. The incubation period, to, is relatively unaffected by the various testing conditions, except in the case of the acid solution where its value is greatly decreased ( Table 1). Cathodic polarization under both static and cavitating conditions shows clearly the influence of cavitation on the limiting currents due to the reduction of oxygen and the reduction of protons (Figure 2). At E(SCE) = - 1.0 V (SCE = saturated calomel electrode) the turbulence due to cavitation has increased the limiting current of oxygen reduction by 40 times. Cathodic protection in neutral solution eliminated the corrosive influence of the salt. Corrosion potentials, E ..... and the polarization resistance, Rp, were measured in neutral 3 % salt solution under static and cavitating conditions. Due to the increased oxygen supply and hence a decreased cathodic polarization, cavitation always increased E .... (see Figure 2). The corrosion currents, I c.... were calculated using
E
I .... = (0.035 V)/(Rp V/A) which may be considered a sufficiently accurate approximation 9 for the present purposes. The results are shown
in Table 2. Although the results obtained under cavitating conditions tend to be more variable than those obtained under static conditions, it is seen that during the steady state erosion period the corrosion currents for each amplitude may be considered roughly similar. Weights of the erosion debris in various size ranges are shown in Table 1 and the size distribution for some results in Figure 3. The colour of the particles/solution (Table 1) indicated the corrosion conditions; grey particles (presumably mainly iron) occurred in distilled water and brown particles (hydrated oxides) occurred in the corrosive salt waters. Blue particles resulted from the interaction of the iron particles and the inhibitor. The
t (min)
Figure I Erosion, w, as a function of time, t, for pure iron eroded at various amplitudes in distilled water and 3% NaCI solution at 50°C. Distilled water: (3, 10/am; r-l, 15/am; A , 20/am. 3% NaCI solution: O, 10/am; I I , 15/am; A , 20/am Table I
Erosion kinetics and the erosion debris of pure iron eroded by 20 kHz ultrasound in aqueous solution at 50°C under various conditions Erosion kinetics b
Filtered debris Mass (mg)e
Test conditions a
to (min)
r (mgh
Distilled, a = 10/am Distilled Distilled, a = 20/am 3% salt, a = 10/am 3% salt 3% salt, a = 20/am Distilled, inhibitor 0.02% salt 0.02% salt, inhibitor 3% salt, r / = --0.2 V 3 % salt, t / = - 1 . 4 V Distilled, pH 2 3 % salt, pH 2 Distilled, pH 1 2
8 14 10 c 15 15 13 17 19 18 13 15 2 9 20
23 35 52 c 32 49 85 39 39 44 39 39 81 78 42
3% salt, pH 12
19
41
1)
Specimen loss (mgper90min)
>160/am
100/am
40/am
16/am
4/am
31 47 87 40 61 110 47 46 46 40 d 39 d 117 105 48
2 16 26 8 7 18 10 3 8 4 3
6 9 17 11 12 27 8 12 24 21 8 .
15 15 21 30 29 7 22 21 62 19 20
3 0 6 5 9 12 3 4 18 3 4
2 3 1 3 2 3 2 0 14 2 3 . -
48
.
.
.
.
.
% Specimen Total loss Colour 28 43 71 57 59 129 45 40 126 49 38
90 91 82 143 97 114 96 87 274 122 97
-
-
.
.
.
.
All amplitudes, a = t 5/am unless otherwise given and r / = cathodic polarisation (see text for details) bt o = incubation time and • = steady erosion rate (see text for details) cSecond r = 79 occurs with to = 22 a For total erosion time of 75 min e 100/am indicates size range 1 0 0 - 1 6 0 / a m , etc. % Specimen loss = [ (weight of debris) / (specimen loss) ] × 1 O0 f N D = no'debris; all had dissolved
172
UItPas~nics 11991 -Vol 29 March
Grey Grey Grey Brown Brown Brown Blue Brown Blue Brown Brown ND t Grey Grey/ brown Grey
Erosion and corrosion of pure iron under cavitating conditions: W.J. Tomlinson and M.G. Talks -0.5
Discussion
El
Erosion under cavitation conditions is a fatigue process resulting from the cyclic stresses induced by the collapse of the cavitation bubbles 1°. In the present case it is assumed that erosion in distilled water is essentially mechanical in nature because neither the presence of inhibitors nor the action of cathodic protection reduced the erosion rate below the basic level (Table 1). A few per cent salt in the water does not change the physical properties of the solution and hence the cavitation processes to any significant extent 6, and so the increase in the erosion rate in 3 % salt solution is considered to be due to its corrosive action. Salt in the water will increase the corrosion current, I¢o. , due to a decrease in the electrical resistance of the corrosive environment, R,ol., since in the corrosion cell 1
-I.C
t~ -I 3
-2.0 I
I
-I
0
I
I Log ( l i c l ) (mA)
I
I
2
3
Figure 2 Cathodic polarization of pure iron in 3 % NaCI solution at 50°C under static ( O ) and cavitating ( [ ] ) conditions at 15/am amplitude, Only oxygen reduction occurred down to ~ - 1 . 2 V: below - 1 . 2 V extensive hydrogen evolution also occurred, Area = 113 mm 2
E=q = r/a + I~/cI + I . . . . Rsol.
100 90
total weight of the filtered grey particles in distilled water was 80-90% of the specimen weight loss, whereas the total weight of the filtered brown particles was 90-140 % of the specimen weight loss. Thus hydration has increased the weight of the particles in an irregular manner owing to different degrees of hydration and flocculation. Some dissolution would also have occurred in the salt solution. It should also be noted that the smallest filter used was 5/~m. The inhibitor clearly interacts with the iron to form the blue colour and can also greatly increase the weight (note 274% in Table 1). It is clear that a uniform pattern of erosion occurs (Figure 3). The most important feature is that erosion in distilled water with the relatively high amplitudes (a = 15 and 20 #m) produces more of the large particles, whereas for erosion in salt solution or in distilled water with the lowest amplitude (10 /tm), relatively few large particles are formed. Erosion produced a roughened surface that was uniform over the specimen (Figure 4a). Detailed examination showed that the metal in the surface had been severely deformed and fractured in a ductile mode, and that there were no essential differences between the various erosion conditions. For example, although the erosion rates in distilled water and in salt water with an amplitude of 20/am were different ( Table 1), the eroded microstructures (Figures 4b and c) were very similar.
80 70
\
6O v
50 40 30 20 ,°
0
5
I 16
I 40
j, (p.rn)
I I00
I 160
Figure 3 Weight of filtered erosion debris, m, as a function of the maximum particle dimension greater than y, for pure iron eroded at various amplitudes in distilled water and 3% NaCI solution at 50°C. Distilled water; O, 10/am; 1-1, 15 tam; A , 20 pro. 3% NaCI solution; O, 10 #m; I I , 15/am; A , 20/am. Note that the lines are drawn for convenience and do not indicate an assumed particle size distribution function
Table 2
Corrosion currents of pure iron in 3% NaCI solution at 50°C under static and cavitating conditions with various amplitudes after 5 and 90 min exposure Corrosion current, / (mA) Static
Cavitating
Ratio [ / (cav) / / (static) ]
Amplitude (/am)
5 min
90 min
5 min
90 min
5 min
90 min
10 15 20
0.40 0.55 0.49
0.41 0.45 0.35
(4.60) a 2.17 1.30
3.96 4.73 2.84
( 11.5) a 4.0 2.7
9.7 10.5 8.1
a Uncertain value
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Erosion and corrosion of pure iron under cavitating conditions: W.J. Tomlinson and lid. G. Talks Table 3 Erosion (E), corrosion (C), erosion in salt water (ES) and the corrosion-induced erosion (CIE) components of damage of pure iron eroded in aqueous solution at 50°C with various amplitudes Damage rate (mg h -1 )a
Damage rate (fraction)
Amplitude (#m)
E
C
CIE
ES
E
C
CIE
10 15 20
23 35 52
4 5 3
5 9 30
32 49 85
0.72 0.71 0.61
0.12 0.10 0.04
0.16 0.19 0.36
a Corrosion converted from electrochemical measurements and CI E calculated (see text for details)
Figure 4 Surface of pure iron after erosion under various conditions: (a) distilled water, amplitude 10/~m (marker 0.1 mm); (b) distilled water, amplitude 20 pm (marker 2 pm); (c) 3% NaCI solution, amplitude 20/~m (marker 2 pm)
The equilibrium cell potential, Eeq , and the anodic and cathodic overpotentials, ~/~ and r/c, respectively, will be relatively unaffected by the presence of salt 12. The corrosion will have two effects during the erosion process. First there will be a corrosion component (C) of damage and second there will be a corrosion-induced erosion component (CIE) of damage, since the formation of corrosion pits will act as stress concentrators. Hence, the erosion in salt water (ES) will have the components ES = E + C + C I E
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Assuming dissolution occurs as Fe 2+, 1 mA is approximately equal to an erosion rate of 1 mg h-~. Hence, for an amplitude of 15 /~m and using the information in Tables 1 and 2 we calculate CIE = (49 - 35 - 5) = 9 mg h - 1. Similar results calculated for erosion with 10 and 20 ~m amplitudes, and the components expressed as fractions of the damage, are shown in Table 3. It can be seen that 4 - 1 2 % of the damage is due to corrosion and the rest, 88 96 % of the damage, is due to the formation of debris ( Table 3). This is consistent with the independently obtained filtered debris (Table 1) where, in distilled water (considered the most accurate results), at least 82 9 1 % of the damage was due to the formation of solid particles. In fact, the fractions were higher since the results in Table 1 do not include those particles < 5 ~m in size. It is also seen in Table 3 that as the amplitude of erosion increases from 10 to 20 ~m the CIE increases from the fraction 0.16 to 0.35. This is interpreted as reflecting a relatively constant corrosion current (Table 2) and pitting, combined with an increasing mechanical action on the stress concentrating pits. An insight into the mechanism of erosion in a corrosive environment may be obtained by comparing the present results on pure iron with results obtained on a 0.2% P flake graphite grey coat iron 7. In the present work (15 ~m amplitude) the presence of 0.02 % salt in the water did not influence the erosion rate, 3 % salt increased the rate by 1.4 times and the damage due to E, C and CIE were fractions 0.7, 0.1 and 0.19, respectively. In contrast, with the cast iron 7, the presence of 0.02 or 3.0% salt in the water increased the erosion rates by 3.6 and 7.0 times, respectively, and the damage due to E, C and CIE were fractions 0.14, 0.02 and 0.84, respectively. Thus, the effect of salt in the water and corrosion and the corrosioninduced erosion were much greater in the case of cast iron. It would appear that the major difference is due to the different microstructures. Pure iron is ductile and deforms easily and the stress concentration at the pit root is reduced as the root is blunted by plastic flow, whereas in cast iron the brittle phases do not deform and thus fracture under the cavitating stresses. Although relatively ineffective compared with cast iron, corrosion in pure iron does influence the erosion process by forming corrosion pits. This is supported by the smaller maximum size of debris formed in corrosive solutions. Here it would appear that corrosion forms many pits that are active in concentrating the applied stress, whereas in distilled water only the cracks generated mechanically occur.
Erosion and corrosion of pure iron under cavitating conditions: W.J. Tomlinson and M.G. Talks
Conclusions
Acknowledgements
From the present work on the erosion and corrosion of pure iron (surface area 113 mm z) in aqueous solutions at 50°C under cavitating conditions using a 20 kHz
The authors wish to thank: at British Coal, Burton-on-
vibratory facility with an amplitude of 15 #m, it is concluded that: 1 2
3
4
5
6
7
Erosion in distilled water is essentially mechanical in nature. The presence of 3 %0 NaCI in the water increased the steady state erosion rate (SSER) by 1.4 times and reduced the maximum size of the eroded particles. In 3 %0NaCI solution cavitation increased the limiting current for oxygen reduction by ~ 40 times and increased the electrochemical corrosion rate in the SSER region by ~ 10 times. The electrochemical corrosion rate in the SSER region was 4.7 mA. The corrosion induced erosion rate in 3 % salt solution was calculated and the erosion, corrosion and corrosion-induced erosion components of damage were fractions 0.71, 0.10 and 0.19, respectively. Independent measurements of the weight of the eroded particles confirm that at least 8 0 - 9 0 % of the weight loss is in the form of particles. The rate of electrochemical corrosion in 3 % NaC1 solution remained at a similar level as the amplitude of vibration increased from 10 to 20 #m, but the erosion rate and the corrosion-induced erosion rate increased. The relatively small effect of the corrosive salt solution on the erosion of pure iron is considered to be due to the high ductility of pure iron blunting the crack tip and so reducing the stress concentration. Results obtained using an inhibitor, acid and alkaline solutions and cathodic protection were consistent with the above conclusions.
Trent, C.T. Massey for provision of cavitation erosion facilities and E.D. Yardley for encouragement; and at Coventry Polytechnic, C. Dawson for help with the SEM.
References 1 Preece,C.M. Cavitation erosion, in: Treatise on Material Science and Technology: Vol 16 Erosion (Ed Preece, C.M.) Academic Press, New York, USA (1979) p 249 2 Karimi, A. and Martin, J.L. Cavitation erosion of materials lnst Met Rev (1986) 31 1 3 Tomlinson,W.J. Effect of ultrasonically induced cavitation on corrosion, in: Advances in Sonochemistry Vol 1 (Ed Mason, T.) Jai Press, London, UK (1990) 173-195 4 Tomlinson,W.J. and Talks, M.G. Erosion and corrosion of cast irons under cavitation attack Tribology lnt in press 5 Ran, P.V., Sestha~miah, K. and Char, T.LR. Cavitation corrosion and its prevention by inhibitors and cathodicprotection Corr Prey Control (1972) 19 8 6 Okada,T. Corrosive liquid effects on cavitation erosion J Ships Res (1981) 25 271 7 Tomlinson, W.J. and Talks, M.G. Cavitation erosion of grey cast irons containing 0,2 and 1.0 phosphorus in corrosive waters Tribology lnt (1989) 22 195 8 Talks,M.G. Erosion and corrosion of cast irons under cavitating conditions CNAA PhD Department of Applied Physical Sciences, Coventry Polytechnic, UK, 1991 9 Fontana, M.G. Corrosion Engineering, 3rd Edn, McGraw-Hill, New York, USA (1986) 502 10 Hobbs,J.M. Experience with a 20-kc cavitation erosion test, in: Erosion by Cavitation or Impingement ASTM Spec Tech Publ, USA (1967) 408 159 11 West, J.M. Basic Corrosion and Oxidation, 2nd Edn, Ellis Horwood, Chichester, UK (1986) 105 12 Ceilings,P.J. Introduction to Corrosion Prevention and Control Delft University Press, The Netherlands (1985) 40
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