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Dissolution of copper and copper-nickel alloys in aerated dilute HCl solutions
DESALINATION ELSEVIER
Desalination 130 (2000) 89-97
www.elsevier.com/locate/desal
Dissolution of copper and copper-nickel alloys in aerated dilute HC1 solutions A.M. Shams E1 Din*, M.E. E1 Dahshan, A.M. Taj E1 Din Material Testing Laboratory, Abu Dhabi Water and Electricity Authority Research Center, PO Box 45111, Abu Dhabi, UAE Fax + 508-1506
Received 23 May 2000; accepted 5 June 2000
Abstract The dissolution of pure Cu, 90/10 and 70/30 Cu-Ni alloys and ofMone1400 in strongly aerated 0.1 m HCI is studied by the weight loss technique at 25 °C and 60°C. All four materials behave in a similar manner. The weight-loss/time curves are formed by an initial, low rate, induction period representing the dissolution of Cu in the monovalent state, CuCI~. The nickel of the alloys dissolves simultaneously, but in quantities higher than present in the solid metal. Air oxidizes Cu÷to Cu2÷.When enough of Cu2÷accumulates in solution, the dissolution of Cu is auto-catalyzed through the reaction Cu+Cu2÷= 2Cu÷.This conclusion is supported by the results of experiments in which extra Cu2÷ions are added to the solution. The rate of dissolution along the induction period increases and the period itself decreases and disappears as more Cu2÷ions are added to the acid. A rise in temperature accelerates the dissolution reaction both along the induction as well as the catalyzed stages. The activation energies of dissolution are material dependent and vary between 3 and 11 K cal/mole. The corrosion rate (CR) of the four materials along the induction period varies with the [Cu2÷]of the solution according to CR = A[Cu2+]"where A and n are constants. The value ofn depends on the Cu content of the alloy. The corrosion rates along the catalyzed reaction are independent of the [Cu2÷]. Keywords:
Pure Cu, 90/10 and 70/30 Cu-Ni alloys; Monel 400; Dissolution in aerated HC1; Induction period; Auto-catalysis; Cu2÷additions; Temperature
I. Introduction Practically all the potable water of the City of Abu Dhabi (UAE) is produced by desalinating Arabian Gulf water. Desalination is carried out by thermal distillation using the multi-stage-flash *Corresponding author.
(MSF) evaporation technique. One o f the main problems encountered in the operation of MSF distillers is the formation of alkaline scales inside the condenser tubes [1]. The scales are formed mainly of CaCO3 and Mg(OH)2 and restrict water flow and impair heat-transfer. Usually anti-scale agents are added to the flowing brine to reduce
0011-9164/00/$- See front matter © 2000 Elsevier Science B.V. All rights reserved PII: S0011 -9164(00)00077-1
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A.M. Shams El Din et al. / Desalination 130 (2000) 89-97
the tendency of scale formation. Eventually, however, the scale builds up to the extent that the operation of the distiller becomes uneconomic. An acid wash of the distiller then becomes inevitable. Enough acid, usually HCI, is added to the circulating seawater to give a solution of pH 1.8-2.0. As scales dissolve, the pH of the water rises. More acid is administered to retrieve the low value. The process is repeated a number of times until the pH remains unchanged at the low value. Our interest in Cu-Ni alloys stems from the fact that they constitute the materials of the condenser tubes of most of the MSF distillers in Abu Dhabi. The tubes of the brine heater and the first few high temperature cells are commonly made of 70/30 Cu-Ni alloy. Those of subsequent cells are manufactured of the less expensive 90/10 variety. Both materials are nobler than hydrogen in the electromotive series of metals; they do not displace H + ions from acid solutions. Experience has repeatedly shown, however, that Cu-Ni tubes are not completely immune during acid wash. A different mechanism must, therefore, be responsible for the corrosion of those materials. A reasonable hypothesis assumes the involvement of oxygen of the air in the dissolution process. Oxygen can act as a cathodic depolarizer, complementing the partial anodic dissolution reaction. Another alternative is that oxygen oxidizes the alloys to copper and nickel oxides, followed by their dissolution in the acid solution by an acid-base reaction [2]. A peculiarity of the dissolution of Cu-Ni alloys in hydrochloric acid is that it occurs at two distinct rates. After an initial long period of low dissolution rate (induction period) attack increases suddenly and linearly [3]. No satisfactory explanation for this behavior has yet been presented. The above considerations prompted us to examine in some detail the phenomenon of dissolution of Cu-Ni alloys in dilute HC! solutions. To obtain a clearer picture we
extended the work to pure copper and a Monel alloy (high nickel-copper alloy) under similar conditions. No work on these materials from the present standpoint has been published before.
2. Experimental The dissolution of pure Cu (99.999%), nominal 90/10 Cu-Ni (87.2% Cu, 10.35% Ni, 0.70% Mn, 1.65%Fe, Pb, Zn traces) and nominal 70/30 Cu-Ni (68.24% Cu, 30.55% Ni, 0.67% Mn, 0.52% Fe, Pb, Zn, S, P traces) and Monel 400 (32.40% Cu, 64.40% Ni, 0.72%Mn, 1.87% Fe, Si, Co, Ti traces) in strongly aerated 0.1M HC1 (pH-1.0) was studied by the weight-loss technique at 25 °C and 60 °C. Coupons measuring l x4cm and having a hole along one of their shorter sides for hanging in the corroding solution were used. Before use the coupons were abraded with metallographic emery paper of increasing fineness. This was followed by degreasing with cotton wool soaked with acetone and washing with bi-distilled water. The complete wetting of the surface was taken as the criterion for cleanliness. The weighed coupons were suspended in 11 of the acid solution by glass hooks. The corrosion cell at 60°C was provided by a water-cooled condenser. At increasing time intervals a metal sample was withdrawn, washed with running bidistilled water, dried between filter paper and reweighed to the nearest 0.1 mg. The pH value of the solution was monitored during the course of experiments. As a result of involvement of H + ions in 02 reduction (see later), on the one hand, and the hydrolysis of Cu and Ni ions, on the other, a rise in pH was always recorded. This was corrected for by the addition of few drops of diluted HCI, when needed, to bring the pH value to its original value. Simultaneous with the determination of the loss in weight of the coupons, small portions of the corroding solution were analyzed for their Cu and Ni contents by the
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A.M. Shams El Din et al. / Desalination 130 (2000) 89-97
X-ray technique. The feasibility of this method was proven on synthetic samples containing known amounts of Cu 2÷ and Ni 2÷ in ratios very near to that present in the alloys. Good agreement (4-2%) between present and found values was established. Experiments were also conducted in which the corroding solutions contained increasing amounts of Cu 2÷ ions, added as CuC12. The results were presented as collective loss in weight (mg.cm -2) - - immersion time curves.
3. Results and discussion Curve 0 of Fig. 1 represents the variation of the weight loss (mg.cm -2) with time (h) of pure Cu in strongly aerated 0.1 m HCI at 25°C. The curve is formed of two distinct segments of different slopes. Along the lower part (I) dissolution occurs at a rate of 0.100 mg cm -2 h-L After some 90 h of immersion (induction period) the rate increases, rather linearly to acquire another constant value of 3.230mg cm-2h -~ (part II).
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A.M. Shams El Din et al. / Desalination 130 (2000) 89-97
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Time (hr) Fig. 2. Weight loss-time curves of 70/30 Cu-Ni alloy in aerated0.1M HC! in absence (O) and presence (1-6) of increasing concentration o f Cu 2+ ions. A at 25 °C; (B) at 60°C.
The dissolution of Cu-Ni alloys in the acid solution occurs much in the same manner as pure Cu. An example of the behavior of the 70/30 alloy is represented by curve of Fig. 2 .That the corrosion of the alloys simulates that of pure Cu indicates that their behavior is decided mainly by their Cu content rather than by the Ni component. In support o f this conclusion is the fact that CuNi alloys exhibit corrosion potentials very near to those of pure Cu in the galvanic series of metals [4].
When we first recorded the dissolution pattern of Cu-Ni alloys, represented by curve (0) of (Fig. 2), we were of the opinion that segment (I) represented the attack on the air formed oxide film present on the metal surface. The subsequent rising part was considered to correspond to the true corrosion rate of the metal [5]. The
information gathered since then makes us revise this idea. That stage (I) is not due to the dissolution of the metal oxides was gleaned from the observation that removal of the oxide film required only a few minutes in the acid solution, whereas the step extended over tens of hours. Similarly, experiments in which the test coupons were freed of their oxide films through cathodic reduction still exhibited part I of the dissolution curves over practically the same time [3]. It is to be noted that the double-rate corrosion curves of Cu and Cu-Ni alloys is only recorded in HCI solutions. In dilute, aerated HESO 4 and HCIO4 the metals dissolve along a single step covering the whole span of the experiment (up to 200 h) [3]. The rates of metal loss in these acids correspond to that registered along stage (I) in HCI [3]. Upon the strength of this observation
A.M. Shams El Din et al. / Desalination 130 (2000) 89-97
one cannot escape the conclusion that the low rates in all acids describe the true corrosion of the metals, and that stage (II) of HCI is due to metal dissolution at an enhanced rate. The disparity in the behavior o f the metals in HCI solutions, on the one hand, and the other mineral acids, on the other, is readily understood in the light o f the chemistry of Cu in these media. In HCI Cu dissolves in the monovalent state, as a result of stabilization as the CuC1-2 ion. Cu + 2C1- - e
= C u f l 2-
(1)
In the other non-complexing acids, dissolution occurs directly in the bivalent state. Cu - 2e = Cu 2+
(2)
Since Cu and Cu-Ni alloys are nobler than H + in the electromotive series, a cathodic reaction other than the displacement o f H + must account for metal dissolution. This is readily available in terms of O2 reduction from solution. 0 2+
4H + + 4e- = 2H20
(3)
Some authors consider that the role of O2 is to oxidize the metal to CuO (NiO) followed by the dissolution o f the oxides in the acid solution [ 1]. The present discussion does not differentiate between the two alternatives. The increase in corrosion rate recorded at advanced times of immersion is of particular interest. It is noted with pure Cu and Cu-Ni alloys and appears to be a general feature of copper alloys in aerated HCI solutions [6]. We assume that 02 of the air plays another role in the corrosion reaction. It oxidizes monovalent Cu + ions to the divalent state: 4Cu + + Oz + 4H ÷ =
4 C u 2+ + 2 H 2 0
(4)
The concentration of Cu 2+ ions is considered to build up in solution until a definite value is
93
reached which allows the auto-catalytic process to Cu + Cu 2÷ = 2Cu+
(5)
take place. Reaction (5) allows more copper to go into solution than is produced by the cathodic reduction of 02 (stage I). Accordingly, the rate of corrosion increases as noted experimentally. Reaction (5) is favored in chloride solutions by a decrease in free energy of some 6 K cal/mole [7]. A direct support to the above assumption comes from the results of experiments in which Cu 2+ ions of increasing amounts were added as CuC12 to the corroding solution. The curves (1-5A) of Figs. 1 and 2 represent the behavior noted. From these curves one recognizes the following effects as the Cu 2+ ion content is increased: • the gradual increase of the corrosion rate o f the metals along the induction period; • a corresponding shortening of the induction period. In the presence o f enough Cu 2+ the induction period disappears completely and dissolution at the high rate of stage (II) starts from the beginning of the experiment; • the rate of dissolution along stage (II) appears to be little dependent of the Cu 2+ ion content of the solution so long as part of the induction period is recorded. When the induction period is absent, an increase in the Cu 2+ content is accompanied by an elevation of the dissolution rate. In the case of the 70/30 Cu-Ni alloy, however, the rate of stage (II) slightly increases with the rise in the Cu 2+ content of the solution (Fig. 2) before the induction period disappears. The above effects will be treated in a more quantitative manner at a later part of the paper. Additions of Ni 2+ ions to the corroding solution had no effect on the dissolution behavior of Cu and the Cu-Ni alloys. This is understood in the light that Ni 2+ ions do not oxidize Cu. The
94
A.M. Shams El Din et aL / Desafination 130 (2000) 89-97
Table 1 Ratios of Ni/Cu going into solution after different times of immersion in aerated 0.1 M HCI at 25 °C Time, h
20 44 91 165 Theor~ical
Ni:Cu ratio in solution 90:10
70:30
0.1637 0.1276 0.1257 0.1286 0.1187
0.6330 0.4763 -0.4741 0.4477
standard Ni/Ni 2+ potential amounts to - 0.250 V vs. NHE, as compared to +0.337V for the Cu/Cu* couple [8]. Also, unlike Cu, Ni does not exist in solution as monovalent Ni + ions. Simultaneous with the determination of the loss in weight of the metal coupons, small portions of the corroding solutions were extracted and analyzed for their metal contents, Of importance to us were the ratios Ni/Cu going into solution at various times. These measurements were conducted at 25°C in aerated HC! solutions free from added Cu 2+ ions. The results obtained for the 90/10 and 70/30 Cu-Ni alloys are listed in Table 1, together with the same ratios in the metals themselves. The figures of the table clearly show that, during corrosion, both alloy metals are dissolved. In fact, more Ni than is present in the alloy goes into solution. This result strongly suggests that the corrosion of Cu-Ni alloys involves denickelification. Denickelification is thought to occur as result of micro-galvanic cells developing between Cu and Ni crystallites in the solid solution of the Cu-Ni system. As the time o f immersion increases, the ratio o f Ni:Cu in solution progressively approaches that of the bulk metal. This is the result of exhaustive denickelification as well as promoted Cu dissolution due to auto-catalysis. At these extended times the solution turns blue with
streaks of green which increase in intensity with the increase of the Ni content of the alloy. On the other hand, the coupon surfaces acquire a darkbrownish black deposit. X-ray analysis revealed the black film to be mainly metallic Cu. By analogy to the case of dezincifiction of brasses, denickelification of Cu-Ni alloys can result from the preferential dissolution o f nickel or the dissolution of both alloy components followed by redeposition of Cu. In the light of the results obtained with the 90/10 and 70/30 Cu-Ni alloys, it appeared of interest to establish the dissolution pattern of alloys with higher Ni-content. Work was, therefore, carried out on Monel 400 (nominal 30/70 Cu-Ni) under the same conditions o f the study. Curve (0) of Fig. 3A represents the behavior of Monal 400 in aerated 0.1 M HC1 at 25°C. By and large the general features o f the curve are similar to those o f Cu and the cupronickels. One recognizes the regions of normal and accelerated dissolution. The transition from one to the other is, however, gradual and occurs over longer times. Apparently both effects are related to the low copper content of the alloy which furnishes little Cu 2÷ ions for catalysis. Analysis of the solution revealed the presence of more Ni than is found in the solid metal. Similarly, the metal acquired a dark appearance denoting the deposition of copper on the surface. That the same mechanism governing the dissolution of Cu and the cupro-nickels operates also in case of Monel is revealed from the results obtained when extra Cu 2÷ ions were added to the acid solution. As in clear from the curves of Fig 3A, dissolution along stage I increased and the induction period decreased as the Cu 2÷ content of the solution was raised. The way by which additions o f Cu ~÷ ions affect metal dissolution is now considered in a more quantitative manner. Amongst the various effects noted, that on the initial corrosion rate, CRI, along the induction period, is of interest. The plot of CR t as function o f [Cu 2÷] on a double
A.M. Shams El Din et al. / Desalination 130 (2000) 8 9 - 9 7
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Time Oar) Fig. 3. Weight loss-time curves o f M o n e 1 4 0 0 (30/70 C u - N i ) alloy in aerated 0.1M HC! in absence (O) and presence (1-4)
of increasing concentration of Cu2÷ions. A at 25°C: (B) at 60°C.
logarithmic scale gives straight lines for all tested materials. The two parameters are accordingly related as CR, = A [CuZ+]"
(6)
where A and n are constants dependent on the nature of the metal. The constant A is the theoretical rate that should be measured in a solution containing 1 ppm Cu 2÷. As result o f the continuous bubbling of air in the corroding solution, which causes the oxidation of Cu ÷ to Cu 2+ states, this condition might not be realized experimentally. The values of A are obtained, however, by extrapolating the straight lines to log
[CuZ+]=0.On the other hand, n is the power dependence of the corrosion rate on [Cu2+]. It is simply the slope o f the log-log plot. For pure Cu, the 90/10, 70/30 and Monel alloys, the values o f the constant A at 25°C amount successively to 3.47x 10 -3, 4.27 x 10 -3, 2.63 x 10-2 and 5.37 x 10 -2. The corresponding values o f n are 1.0, 0.76, 0.45 and 0.44. At 60°C the induction periods are curved to the extent to make the calculation o f the corrosion rate somewhat difficult• Each of the two constants of Eq. (6) seems to address a certain aspect of the dissolution reaction. Parameter A, for example, is lowest in the case of pure Cu and increases with the
96
A.M. Shams El Din et al. / Desalination 130 (2000) 89-97
increases of the Ni content of the metal. From this dependence the effect o f Cu 2+ is readily understood on the basis of accelerating the displacement reaction: Cu 2+ + Ni = Ni 2+ + Cu, which promotes denickelification. On the other hand, the facts that pure Cu exhibits a first-order dependence on [Cu 2+] (n = 1) and that n decreases with the increase of the Ni content of the alloy, is taken to indicate that the constant n is more related to the dissolution of Cu by a reaction similar to (5). The dependence of the rate of the catalyzed dissolution (stage II) on the [Cu 2+] of the medium is also of interest. In the case of pure Cu (Fig. 1), the 90/10 Cu-Ni alloy and ofMone1400 (Fig. 3), practically parallel straight lines are obtained, indicating the process to be independent of [Cu2+]. In the case of the 70/30 alloy at 25°C, there is an initial increase in the rate of dissolution (Fig. 2A). As the Cu 2+content of the solution is increased, however, the rate appears to attain a constant value. At 60°C the rate of the catalyzed reaction is once again independent of [Cu2+]. It is reasonable to conclude, therefore, that constant rates are the rule and that the behavior of the 70/30 alloy at 25°C is an exception. We are unable to give a satisfactory explanation to this behavior. That the rate of the catalyzed reaction is zero order with respect to [Cu 2+] indicates that it is a surface reaction. The weight-loss technique does not allow much to be said on the nature of this reaction. It is probable, however, that the reaction involves the transport and formation of chlorocopper complexes which occur at a lower rate than the reaction with the metal itself. As is seen from the curves of Figs. 1-3, the increase in the Cu 2÷ content of the solution is accompanied by the shortening of the induction period. When enough of the Cu 2+ions are present in solution the induction period disappears completely. This state of affairs occurs in the presence of 500 ppm Cu 2+ in the case of pure Cu and about 1000ppm in the case of the three
Cu-Ni alloys. No simple relation was found to describe the reduction in length of the induction period with increase in [Cu 2+] of the solution. Each material behaved in a particular manner. Results similar to those described above were also obtained when the experiments were carried out at 60°C (actual temperature of an acid wash of MSF distillers). As is seen from curves (B) of Figs. 1-3, a rise in temperature accelerates the corrosion reaction. This is manifested in the elevation of the corrosion rates along stages I and II of the curves, as well as the shortening o f the induction period. Also at the high temperature, the induction period is completely eliminated at smaller Cu 2+ ion concentrations compared to 25 °C. The effect of temperature on the rates of corrosion of the four tested materials is of interest. Along stage I (induction period) the increase of 35°C in solution temperature the corrosion rate increased 5.43, 5.84, 6.75 and 3.98 times in the case of pure Cu, 90/10, 70/30 and Monel, successively. The corresponding figures for stage II (catalyzed dissolution) were 2.30, 3.99,3.90 and 1.70, respectively. These figures speak for very low activation energies of the order of magnitude of 7.81 to 10.81 K cal/mole for stage I, and 2.91 to 5.00 K cal/mole for stage II. The very low values of the activation energies, i.e., the ease with which the materials dissolve, can be readily understood on the basis of galvanic corrosion between the two alloy components, on the one hand, and the occurrence of the auto catalytic reaction, on the other.
4. Conclusions
On the basis of the results o f the present investigation, the following conclusions are drawn: 1. Pure Cu, 90/10 and 70/30 Cu-Ni alloys, as well as Monel 400, readily dissolve in aerated, dilute HCI. The two alloy elements simultaneously go into solution but with more Ni than
A.M. Shams E1 Din et al. / Desalination 130 (2000) 89-97
is present in the alloy (denickelification). The general behavior of the alloys simulates, however, that of pure Cu. 2. Weight-loss measurements show corrosion to occur along an initial, slow induction period, followed by a final fast reaction. Along the induction period copper dissolves as CuC1-2. Air oxidation of Cu+ to Cu 2+ and the occurrence of the auto-catalytic reaction Cu:++Cu=2Cu + account for the subsequent fast dissolution of the metals. 3. Additions of extra Cu 2÷ ions to the acid solution shortens the induction period and enhances the corrosion reaction. The induction period completely disappears in the presence of enough Cu 2÷. 4. A rise in temperature accelerates the dissolution o f the metals. Form the dependence of the rates of corrosion on the [Cu 2+] ideas about the type of surface reaction are drawn. The activation energy of metal dissolution is material dependent and varies between 3 and 11 K cal/
97
mole. These low values are attributed to galvanic corrosion, on the one hand, and dissolution under autocatalysis on the other.
References [1] A.M.Shams El Din and R. A. Mohammed, Desalination, 99 (1994) 73. [2] A.M.Shams El Din and R. A. Mohammed, Desalination, 115 (1998) 135. [3] T.M.H. Saber, A.M.K. Tag E1 Din and A.M. Shams El Din, J. Fac. Sci. UAE University, 6(1) (1994) 297. [4] M.G.Fontana and N.D. Greene, Corrosion Engineering, McGraw-Hill, New York, 1978, p. 32. [5] T.M.H. Saber, A.M.K. Tag El Din and A.M. Shams El Din, Br. Corr. J., 27 (1992) 139. [6] A.A. El Warraky and H.A. El Dahan, J. Mater. Sci., 32 (1997) 3693. [7] W.M. Latimer, Oxidation Potentials, Prentice Hall, New Jersey, 1964, p. 183. [8] W.M. Latimer, oxidation Potentials, Prentice Hall, New Jersey, 1964, p. 198.
Desalination 130 (2000) 89-97
www.elsevier.com/locate/desal
Dissolution of copper and copper-nickel alloys in aerated dilute HC1 solutions A.M. Shams E1 Din*, M.E. E1 Dahshan, A.M. Taj E1 Din Material Testing Laboratory, Abu Dhabi Water and Electricity Authority Research Center, PO Box 45111, Abu Dhabi, UAE Fax + 508-1506
Received 23 May 2000; accepted 5 June 2000
Abstract The dissolution of pure Cu, 90/10 and 70/30 Cu-Ni alloys and ofMone1400 in strongly aerated 0.1 m HCI is studied by the weight loss technique at 25 °C and 60°C. All four materials behave in a similar manner. The weight-loss/time curves are formed by an initial, low rate, induction period representing the dissolution of Cu in the monovalent state, CuCI~. The nickel of the alloys dissolves simultaneously, but in quantities higher than present in the solid metal. Air oxidizes Cu÷to Cu2÷.When enough of Cu2÷accumulates in solution, the dissolution of Cu is auto-catalyzed through the reaction Cu+Cu2÷= 2Cu÷.This conclusion is supported by the results of experiments in which extra Cu2÷ions are added to the solution. The rate of dissolution along the induction period increases and the period itself decreases and disappears as more Cu2÷ions are added to the acid. A rise in temperature accelerates the dissolution reaction both along the induction as well as the catalyzed stages. The activation energies of dissolution are material dependent and vary between 3 and 11 K cal/mole. The corrosion rate (CR) of the four materials along the induction period varies with the [Cu2÷]of the solution according to CR = A[Cu2+]"where A and n are constants. The value ofn depends on the Cu content of the alloy. The corrosion rates along the catalyzed reaction are independent of the [Cu2÷]. Keywords:
Pure Cu, 90/10 and 70/30 Cu-Ni alloys; Monel 400; Dissolution in aerated HC1; Induction period; Auto-catalysis; Cu2÷additions; Temperature
I. Introduction Practically all the potable water of the City of Abu Dhabi (UAE) is produced by desalinating Arabian Gulf water. Desalination is carried out by thermal distillation using the multi-stage-flash *Corresponding author.
(MSF) evaporation technique. One o f the main problems encountered in the operation of MSF distillers is the formation of alkaline scales inside the condenser tubes [1]. The scales are formed mainly of CaCO3 and Mg(OH)2 and restrict water flow and impair heat-transfer. Usually anti-scale agents are added to the flowing brine to reduce
0011-9164/00/$- See front matter © 2000 Elsevier Science B.V. All rights reserved PII: S0011 -9164(00)00077-1
90
A.M. Shams El Din et al. / Desalination 130 (2000) 89-97
the tendency of scale formation. Eventually, however, the scale builds up to the extent that the operation of the distiller becomes uneconomic. An acid wash of the distiller then becomes inevitable. Enough acid, usually HCI, is added to the circulating seawater to give a solution of pH 1.8-2.0. As scales dissolve, the pH of the water rises. More acid is administered to retrieve the low value. The process is repeated a number of times until the pH remains unchanged at the low value. Our interest in Cu-Ni alloys stems from the fact that they constitute the materials of the condenser tubes of most of the MSF distillers in Abu Dhabi. The tubes of the brine heater and the first few high temperature cells are commonly made of 70/30 Cu-Ni alloy. Those of subsequent cells are manufactured of the less expensive 90/10 variety. Both materials are nobler than hydrogen in the electromotive series of metals; they do not displace H + ions from acid solutions. Experience has repeatedly shown, however, that Cu-Ni tubes are not completely immune during acid wash. A different mechanism must, therefore, be responsible for the corrosion of those materials. A reasonable hypothesis assumes the involvement of oxygen of the air in the dissolution process. Oxygen can act as a cathodic depolarizer, complementing the partial anodic dissolution reaction. Another alternative is that oxygen oxidizes the alloys to copper and nickel oxides, followed by their dissolution in the acid solution by an acid-base reaction [2]. A peculiarity of the dissolution of Cu-Ni alloys in hydrochloric acid is that it occurs at two distinct rates. After an initial long period of low dissolution rate (induction period) attack increases suddenly and linearly [3]. No satisfactory explanation for this behavior has yet been presented. The above considerations prompted us to examine in some detail the phenomenon of dissolution of Cu-Ni alloys in dilute HC! solutions. To obtain a clearer picture we
extended the work to pure copper and a Monel alloy (high nickel-copper alloy) under similar conditions. No work on these materials from the present standpoint has been published before.
2. Experimental The dissolution of pure Cu (99.999%), nominal 90/10 Cu-Ni (87.2% Cu, 10.35% Ni, 0.70% Mn, 1.65%Fe, Pb, Zn traces) and nominal 70/30 Cu-Ni (68.24% Cu, 30.55% Ni, 0.67% Mn, 0.52% Fe, Pb, Zn, S, P traces) and Monel 400 (32.40% Cu, 64.40% Ni, 0.72%Mn, 1.87% Fe, Si, Co, Ti traces) in strongly aerated 0.1M HC1 (pH-1.0) was studied by the weight-loss technique at 25 °C and 60 °C. Coupons measuring l x4cm and having a hole along one of their shorter sides for hanging in the corroding solution were used. Before use the coupons were abraded with metallographic emery paper of increasing fineness. This was followed by degreasing with cotton wool soaked with acetone and washing with bi-distilled water. The complete wetting of the surface was taken as the criterion for cleanliness. The weighed coupons were suspended in 11 of the acid solution by glass hooks. The corrosion cell at 60°C was provided by a water-cooled condenser. At increasing time intervals a metal sample was withdrawn, washed with running bidistilled water, dried between filter paper and reweighed to the nearest 0.1 mg. The pH value of the solution was monitored during the course of experiments. As a result of involvement of H + ions in 02 reduction (see later), on the one hand, and the hydrolysis of Cu and Ni ions, on the other, a rise in pH was always recorded. This was corrected for by the addition of few drops of diluted HCI, when needed, to bring the pH value to its original value. Simultaneous with the determination of the loss in weight of the coupons, small portions of the corroding solution were analyzed for their Cu and Ni contents by the
91
A.M. Shams El Din et al. / Desalination 130 (2000) 89-97
X-ray technique. The feasibility of this method was proven on synthetic samples containing known amounts of Cu 2÷ and Ni 2÷ in ratios very near to that present in the alloys. Good agreement (4-2%) between present and found values was established. Experiments were also conducted in which the corroding solutions contained increasing amounts of Cu 2÷ ions, added as CuC12. The results were presented as collective loss in weight (mg.cm -2) - - immersion time curves.
3. Results and discussion Curve 0 of Fig. 1 represents the variation of the weight loss (mg.cm -2) with time (h) of pure Cu in strongly aerated 0.1 m HCI at 25°C. The curve is formed of two distinct segments of different slopes. Along the lower part (I) dissolution occurs at a rate of 0.100 mg cm -2 h-L After some 90 h of immersion (induction period) the rate increases, rather linearly to acquire another constant value of 3.230mg cm-2h -~ (part II).
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A.M. Shams El Din et al. / Desalination 130 (2000) 89-97
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Time (hr) Fig. 2. Weight loss-time curves of 70/30 Cu-Ni alloy in aerated0.1M HC! in absence (O) and presence (1-6) of increasing concentration o f Cu 2+ ions. A at 25 °C; (B) at 60°C.
The dissolution of Cu-Ni alloys in the acid solution occurs much in the same manner as pure Cu. An example of the behavior of the 70/30 alloy is represented by curve of Fig. 2 .That the corrosion of the alloys simulates that of pure Cu indicates that their behavior is decided mainly by their Cu content rather than by the Ni component. In support o f this conclusion is the fact that CuNi alloys exhibit corrosion potentials very near to those of pure Cu in the galvanic series of metals [4].
When we first recorded the dissolution pattern of Cu-Ni alloys, represented by curve (0) of (Fig. 2), we were of the opinion that segment (I) represented the attack on the air formed oxide film present on the metal surface. The subsequent rising part was considered to correspond to the true corrosion rate of the metal [5]. The
information gathered since then makes us revise this idea. That stage (I) is not due to the dissolution of the metal oxides was gleaned from the observation that removal of the oxide film required only a few minutes in the acid solution, whereas the step extended over tens of hours. Similarly, experiments in which the test coupons were freed of their oxide films through cathodic reduction still exhibited part I of the dissolution curves over practically the same time [3]. It is to be noted that the double-rate corrosion curves of Cu and Cu-Ni alloys is only recorded in HCI solutions. In dilute, aerated HESO 4 and HCIO4 the metals dissolve along a single step covering the whole span of the experiment (up to 200 h) [3]. The rates of metal loss in these acids correspond to that registered along stage (I) in HCI [3]. Upon the strength of this observation
A.M. Shams El Din et al. / Desalination 130 (2000) 89-97
one cannot escape the conclusion that the low rates in all acids describe the true corrosion of the metals, and that stage (II) of HCI is due to metal dissolution at an enhanced rate. The disparity in the behavior o f the metals in HCI solutions, on the one hand, and the other mineral acids, on the other, is readily understood in the light o f the chemistry of Cu in these media. In HCI Cu dissolves in the monovalent state, as a result of stabilization as the CuC1-2 ion. Cu + 2C1- - e
= C u f l 2-
(1)
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(2)
Since Cu and Cu-Ni alloys are nobler than H + in the electromotive series, a cathodic reaction other than the displacement o f H + must account for metal dissolution. This is readily available in terms of O2 reduction from solution. 0 2+
4H + + 4e- = 2H20
(3)
Some authors consider that the role of O2 is to oxidize the metal to CuO (NiO) followed by the dissolution o f the oxides in the acid solution [ 1]. The present discussion does not differentiate between the two alternatives. The increase in corrosion rate recorded at advanced times of immersion is of particular interest. It is noted with pure Cu and Cu-Ni alloys and appears to be a general feature of copper alloys in aerated HCI solutions [6]. We assume that 02 of the air plays another role in the corrosion reaction. It oxidizes monovalent Cu + ions to the divalent state: 4Cu + + Oz + 4H ÷ =
4 C u 2+ + 2 H 2 0
(4)
The concentration of Cu 2+ ions is considered to build up in solution until a definite value is
93
reached which allows the auto-catalytic process to Cu + Cu 2÷ = 2Cu+
(5)
take place. Reaction (5) allows more copper to go into solution than is produced by the cathodic reduction of 02 (stage I). Accordingly, the rate of corrosion increases as noted experimentally. Reaction (5) is favored in chloride solutions by a decrease in free energy of some 6 K cal/mole [7]. A direct support to the above assumption comes from the results of experiments in which Cu 2+ ions of increasing amounts were added as CuC12 to the corroding solution. The curves (1-5A) of Figs. 1 and 2 represent the behavior noted. From these curves one recognizes the following effects as the Cu 2+ ion content is increased: • the gradual increase of the corrosion rate o f the metals along the induction period; • a corresponding shortening of the induction period. In the presence o f enough Cu 2+ the induction period disappears completely and dissolution at the high rate of stage (II) starts from the beginning of the experiment; • the rate of dissolution along stage (II) appears to be little dependent of the Cu 2+ ion content of the solution so long as part of the induction period is recorded. When the induction period is absent, an increase in the Cu 2+ content is accompanied by an elevation of the dissolution rate. In the case of the 70/30 Cu-Ni alloy, however, the rate of stage (II) slightly increases with the rise in the Cu 2+ content of the solution (Fig. 2) before the induction period disappears. The above effects will be treated in a more quantitative manner at a later part of the paper. Additions of Ni 2+ ions to the corroding solution had no effect on the dissolution behavior of Cu and the Cu-Ni alloys. This is understood in the light that Ni 2+ ions do not oxidize Cu. The
94
A.M. Shams El Din et aL / Desafination 130 (2000) 89-97
Table 1 Ratios of Ni/Cu going into solution after different times of immersion in aerated 0.1 M HCI at 25 °C Time, h
20 44 91 165 Theor~ical
Ni:Cu ratio in solution 90:10
70:30
0.1637 0.1276 0.1257 0.1286 0.1187
0.6330 0.4763 -0.4741 0.4477
standard Ni/Ni 2+ potential amounts to - 0.250 V vs. NHE, as compared to +0.337V for the Cu/Cu* couple [8]. Also, unlike Cu, Ni does not exist in solution as monovalent Ni + ions. Simultaneous with the determination of the loss in weight of the metal coupons, small portions of the corroding solutions were extracted and analyzed for their metal contents, Of importance to us were the ratios Ni/Cu going into solution at various times. These measurements were conducted at 25°C in aerated HC! solutions free from added Cu 2+ ions. The results obtained for the 90/10 and 70/30 Cu-Ni alloys are listed in Table 1, together with the same ratios in the metals themselves. The figures of the table clearly show that, during corrosion, both alloy metals are dissolved. In fact, more Ni than is present in the alloy goes into solution. This result strongly suggests that the corrosion of Cu-Ni alloys involves denickelification. Denickelification is thought to occur as result of micro-galvanic cells developing between Cu and Ni crystallites in the solid solution of the Cu-Ni system. As the time o f immersion increases, the ratio o f Ni:Cu in solution progressively approaches that of the bulk metal. This is the result of exhaustive denickelification as well as promoted Cu dissolution due to auto-catalysis. At these extended times the solution turns blue with
streaks of green which increase in intensity with the increase of the Ni content of the alloy. On the other hand, the coupon surfaces acquire a darkbrownish black deposit. X-ray analysis revealed the black film to be mainly metallic Cu. By analogy to the case of dezincifiction of brasses, denickelification of Cu-Ni alloys can result from the preferential dissolution o f nickel or the dissolution of both alloy components followed by redeposition of Cu. In the light of the results obtained with the 90/10 and 70/30 Cu-Ni alloys, it appeared of interest to establish the dissolution pattern of alloys with higher Ni-content. Work was, therefore, carried out on Monel 400 (nominal 30/70 Cu-Ni) under the same conditions o f the study. Curve (0) of Fig. 3A represents the behavior of Monal 400 in aerated 0.1 M HC1 at 25°C. By and large the general features o f the curve are similar to those o f Cu and the cupronickels. One recognizes the regions of normal and accelerated dissolution. The transition from one to the other is, however, gradual and occurs over longer times. Apparently both effects are related to the low copper content of the alloy which furnishes little Cu 2÷ ions for catalysis. Analysis of the solution revealed the presence of more Ni than is found in the solid metal. Similarly, the metal acquired a dark appearance denoting the deposition of copper on the surface. That the same mechanism governing the dissolution of Cu and the cupro-nickels operates also in case of Monel is revealed from the results obtained when extra Cu 2÷ ions were added to the acid solution. As in clear from the curves of Fig 3A, dissolution along stage I increased and the induction period decreased as the Cu 2÷ content of the solution was raised. The way by which additions o f Cu ~÷ ions affect metal dissolution is now considered in a more quantitative manner. Amongst the various effects noted, that on the initial corrosion rate, CRI, along the induction period, is of interest. The plot of CR t as function o f [Cu 2÷] on a double
A.M. Shams El Din et al. / Desalination 130 (2000) 8 9 - 9 7
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Time Oar) Fig. 3. Weight loss-time curves o f M o n e 1 4 0 0 (30/70 C u - N i ) alloy in aerated 0.1M HC! in absence (O) and presence (1-4)
of increasing concentration of Cu2÷ions. A at 25°C: (B) at 60°C.
logarithmic scale gives straight lines for all tested materials. The two parameters are accordingly related as CR, = A [CuZ+]"
(6)
where A and n are constants dependent on the nature of the metal. The constant A is the theoretical rate that should be measured in a solution containing 1 ppm Cu 2÷. As result o f the continuous bubbling of air in the corroding solution, which causes the oxidation of Cu ÷ to Cu 2+ states, this condition might not be realized experimentally. The values of A are obtained, however, by extrapolating the straight lines to log
[CuZ+]=0.On the other hand, n is the power dependence of the corrosion rate on [Cu2+]. It is simply the slope o f the log-log plot. For pure Cu, the 90/10, 70/30 and Monel alloys, the values o f the constant A at 25°C amount successively to 3.47x 10 -3, 4.27 x 10 -3, 2.63 x 10-2 and 5.37 x 10 -2. The corresponding values o f n are 1.0, 0.76, 0.45 and 0.44. At 60°C the induction periods are curved to the extent to make the calculation o f the corrosion rate somewhat difficult• Each of the two constants of Eq. (6) seems to address a certain aspect of the dissolution reaction. Parameter A, for example, is lowest in the case of pure Cu and increases with the
96
A.M. Shams El Din et al. / Desalination 130 (2000) 89-97
increases of the Ni content of the metal. From this dependence the effect o f Cu 2+ is readily understood on the basis of accelerating the displacement reaction: Cu 2+ + Ni = Ni 2+ + Cu, which promotes denickelification. On the other hand, the facts that pure Cu exhibits a first-order dependence on [Cu 2+] (n = 1) and that n decreases with the increase of the Ni content of the alloy, is taken to indicate that the constant n is more related to the dissolution of Cu by a reaction similar to (5). The dependence of the rate of the catalyzed dissolution (stage II) on the [Cu 2+] of the medium is also of interest. In the case of pure Cu (Fig. 1), the 90/10 Cu-Ni alloy and ofMone1400 (Fig. 3), practically parallel straight lines are obtained, indicating the process to be independent of [Cu2+]. In the case of the 70/30 alloy at 25°C, there is an initial increase in the rate of dissolution (Fig. 2A). As the Cu 2+content of the solution is increased, however, the rate appears to attain a constant value. At 60°C the rate of the catalyzed reaction is once again independent of [Cu2+]. It is reasonable to conclude, therefore, that constant rates are the rule and that the behavior of the 70/30 alloy at 25°C is an exception. We are unable to give a satisfactory explanation to this behavior. That the rate of the catalyzed reaction is zero order with respect to [Cu 2+] indicates that it is a surface reaction. The weight-loss technique does not allow much to be said on the nature of this reaction. It is probable, however, that the reaction involves the transport and formation of chlorocopper complexes which occur at a lower rate than the reaction with the metal itself. As is seen from the curves of Figs. 1-3, the increase in the Cu 2÷ content of the solution is accompanied by the shortening of the induction period. When enough of the Cu 2+ions are present in solution the induction period disappears completely. This state of affairs occurs in the presence of 500 ppm Cu 2+ in the case of pure Cu and about 1000ppm in the case of the three
Cu-Ni alloys. No simple relation was found to describe the reduction in length of the induction period with increase in [Cu 2+] of the solution. Each material behaved in a particular manner. Results similar to those described above were also obtained when the experiments were carried out at 60°C (actual temperature of an acid wash of MSF distillers). As is seen from curves (B) of Figs. 1-3, a rise in temperature accelerates the corrosion reaction. This is manifested in the elevation of the corrosion rates along stages I and II of the curves, as well as the shortening o f the induction period. Also at the high temperature, the induction period is completely eliminated at smaller Cu 2+ ion concentrations compared to 25 °C. The effect of temperature on the rates of corrosion of the four tested materials is of interest. Along stage I (induction period) the increase of 35°C in solution temperature the corrosion rate increased 5.43, 5.84, 6.75 and 3.98 times in the case of pure Cu, 90/10, 70/30 and Monel, successively. The corresponding figures for stage II (catalyzed dissolution) were 2.30, 3.99,3.90 and 1.70, respectively. These figures speak for very low activation energies of the order of magnitude of 7.81 to 10.81 K cal/mole for stage I, and 2.91 to 5.00 K cal/mole for stage II. The very low values of the activation energies, i.e., the ease with which the materials dissolve, can be readily understood on the basis of galvanic corrosion between the two alloy components, on the one hand, and the occurrence of the auto catalytic reaction, on the other.
4. Conclusions
On the basis of the results o f the present investigation, the following conclusions are drawn: 1. Pure Cu, 90/10 and 70/30 Cu-Ni alloys, as well as Monel 400, readily dissolve in aerated, dilute HCI. The two alloy elements simultaneously go into solution but with more Ni than
A.M. Shams E1 Din et al. / Desalination 130 (2000) 89-97
is present in the alloy (denickelification). The general behavior of the alloys simulates, however, that of pure Cu. 2. Weight-loss measurements show corrosion to occur along an initial, slow induction period, followed by a final fast reaction. Along the induction period copper dissolves as CuC1-2. Air oxidation of Cu+ to Cu 2+ and the occurrence of the auto-catalytic reaction Cu:++Cu=2Cu + account for the subsequent fast dissolution of the metals. 3. Additions of extra Cu 2÷ ions to the acid solution shortens the induction period and enhances the corrosion reaction. The induction period completely disappears in the presence of enough Cu 2÷. 4. A rise in temperature accelerates the dissolution o f the metals. Form the dependence of the rates of corrosion on the [Cu 2+] ideas about the type of surface reaction are drawn. The activation energy of metal dissolution is material dependent and varies between 3 and 11 K cal/
97
mole. These low values are attributed to galvanic corrosion, on the one hand, and dissolution under autocatalysis on the other.
References [1] A.M.Shams El Din and R. A. Mohammed, Desalination, 99 (1994) 73. [2] A.M.Shams El Din and R. A. Mohammed, Desalination, 115 (1998) 135. [3] T.M.H. Saber, A.M.K. Tag E1 Din and A.M. Shams El Din, J. Fac. Sci. UAE University, 6(1) (1994) 297. [4] M.G.Fontana and N.D. Greene, Corrosion Engineering, McGraw-Hill, New York, 1978, p. 32. [5] T.M.H. Saber, A.M.K. Tag El Din and A.M. Shams El Din, Br. Corr. J., 27 (1992) 139. [6] A.A. El Warraky and H.A. El Dahan, J. Mater. Sci., 32 (1997) 3693. [7] W.M. Latimer, Oxidation Potentials, Prentice Hall, New Jersey, 1964, p. 183. [8] W.M. Latimer, oxidation Potentials, Prentice Hall, New Jersey, 1964, p. 198.