GALVANIC CORROSION OF SOME COPPER ALLOYS COUPLED WITH IN SYNTHETIC SEA WATER

Hiroshi Kunieda, Hiroshi Yamamoto and Naomichi Nishijima

Furukawa Co., Ltd., Amagasaki-shi, Japan

Introduction

Titanium tube is a promising material for heat exchangers installed in thermal power plants, nuclear power plants, desali­ nation plants and various chemical plants because it has superi­ or corrosion resistance in sea water. However, consideration is necessary when selecting tube sheet materials because titanium shows noble potential in sea water. In Japan, thin wall tita­ nium tubes made by welding are already being widely used in the air cooling zone of condensers in thermal power plants but exam­ ples of galvanic corrosion of the naval sheets of the air cooling zone have been reported [1]. Numerous studies (1,2,3,4) have been carried out on galvanic corrosion of copper alloys coupled with titanium in normal temperature sea water and also, coating and cathodic protection of the tubes and sheets of the air cooling zone in practical condensers have been investigated.

On the other hand, desalinatio11 plants are progressing rap­ idly, and being constructed in the f'llidd_le East. The corrosion environment is deaerated, concentrated, high temperature sea water when a combination of titanium tubes and copper sheets are used in sea water desalination plants and it is be­ lieved that galvanic corrosion of copper alloy sheets indicates different behavior from that of power condenser environment. Particularly in the deaerated environment, whether galvanic cor­ rosion occurs or not is an interesting problem. Fukuzuka et al. [5] studied galvanic corrosion of copper alloys coupled with titanium by immersion test in deaerated, high temperature and concentrated NaCl solution but it is inevitable that a violent flow condition is added in practical equipment. From the prac­ tical point of view, it is necessary to study under a flowing condition in order to find out whether galvanic corrosion occurs or not and if in the affirmative, the corrosion rate of copper alloys.

In this report, the effects of various factors such as dis­ solved oxygen concentration of sea water, pH, concentration fac­ tor, temperature, flow rate and the area ratio of titanium and copper alloys were investigated by measurement of galvanic cur­ rent and by loop test for the case when titanium tubes are joined with copper alloy sheets in sea water.

Experiment

1. Materials

The materials used were specimens in sheet fo~m, these were 350 H. Kunieda et al. .

cold rolled to a thickness of 1 mm and then cut into the specified dimensions. These

were polished with #400 emery Cu Mn Ni Al Fe Pb Sn Zn H N Ti paper, degreased and washed, 0027 000!2008 mflll: Bal 61 43 -- -!0021. 0025 077 Ba1. - weighed and used. The chemical 90/lO Cu-N1 88 52 0 35 9.84 - 1.28 -- Bal - compositions of the titanium Al·Bronze BB 71 0 22 - 7.25 2.72 0002 <001 ---- and copper alloys are given in Table 1. The aluminium bronze is specified in ASTM-B 171-79 and the others are specified in JIS. 2. Electrochemical Measurement 2.1 Experimental Method

The corrosion potential of the materials and the galvanic current of the copper alloys coupled with titanium were measured with the apparatus shown in Fig. 1. Polypropylene tank, pump and pipings were used to avoid the effects of other ions. The solution Fig.1 Apparatus for electrochemical used was 2 times concentration of measurement synthetic sea water which does not contain heavy metals specified in ASTM-D-1141-52. Experimental conditions were as follows: tem­ perature=500C, flow rate=2m/s, pH=6 or 8.2. The pH was adjusted everyday with dilute HCl or dilute NaOH. Deaeration was carried .out by adding sodium sulfite to remove dissolved oxygen thor­ oughly and then bubbled by N2 gas even during measurement. The dissolved oxygen concentration was measured everyday by Winkler's azide modification method and kept below 20 ppb. The corrosion potential was measured by a Luggin capillary in the corrosion potential measurement part of Fig. 1. Saturated calomel electrode was used as the reference electrode. Galvanic current was measured by a zero resistance ammeter. The area ratio of copper alloys to titanium was 1:20 (copper alloy: titanium). The test pieces were fixed at a distance of 5 mm and parallel with the flow. As a preliminary test, measurement was carried out with the distance between 3 mm and 10 mm but as almost no difference was observed in the current value, a distance of 5 mm was used throughout the experiment. Measure­ ment time was up to 100 hours.

2.2 Results Table 2 shows the corrosion potential of the specimens after 5 and 100 hours. Under the same condition, the potentials of copper alloys are approximately Table 2 Potenlial measurt'd tor 100 hours the same irrespective of the ( 50 "C,CF=2. 2 mis, synthll!l.1~ sea water, Urn! :V. ¥5. 5.C.E. ) material. A potential difference 6 2 Spe~~ Nondeat"ralion Oeaeration Nondeaerat~· De-aeration of 360 - 480 mV was measured titanium .. 006-•0.23 0 - +0.12 +O OB-•0.15 +0.01 -+0 TO between titanium and copper alloys Na~al Brass -0.37 - -0.32 -0 t.2 - -0 36 -0.34 - -0.32 -0.41 - -0.37 in the deaerated condition. A 90/!0Cu-Ni -0.35 - -0.31 -0 t.J- -o 36 -a.JO· -0.27 -0.41 - -0.38 potential difference of 380 - 550 Al-Bronze -032--028 -0.36--0.34 -0,37--030 -0.40--0.37 mV was observed even in the non- deaera ted condition. COPPER ALLOYS COUPLED WITH TITANIUM 351

The potential difference with or without deaeration was rather small.

Fig. 2 shows the galvanic current CF=-2, 50-C,2mls,~ raliol:20(Cu-Alloy:TiJ measurement results. When the current oNaval Br.ass, •90/IOCu-Ni. •Al-Bronn value with time is observed, a steady state is reached after 20~30 hours in the deaerated condition but a steady state is not obtained in the nonde­ aerated condition. Change in pH with time was observed particularly at pH~6 and the current value showed a large fluctuation with this. Also, discrepancy in the current value was observed in the nondeaerated condition at pH~8.2 depending on the kind of copper alloy and this tendency became clear with time but no difference among the alloys was observed in the nondeaerated condition at pH=6 and Timt'(lir) the deaerated condition. Fig. 2 Variation of galvanic current 3. Loop Test 3.1 Experimental Method It is necessary to know the valency of ions for calculating low meter

the corrosion rate from the measure­ lrn/s ment of galvanic current by Faraday's 2m/S law. Also, in case of an alloy, it Jm~ is difficult to make an accurate Test section calculation of the corrosion rate because it is necessary to predict Fig 3 loop test apparatus the proportion of corrosion of each metal. This is particularly difficult when a constant value cannot be obtained such as in the measurement results of Pl..i;" Cu·-Jjl2M5,lO~OI the nondeaerated condition. In view of this, the effects of several factors were studied by the gravimetric method . Plastic using the loop test apparatus shown in· boll& Fig. 3. . • nuts Specimens c2cf~~~~0'!1100.20•500l

Fig. 4 Test specimens and holder The loop test apparatus was made in test section chiefly with FRP(fiberglass reinforced plastic), and a ceramic pump was used. As shown in Fig. 4, the titanium sheet and copper alloy sheet were fastened Tableo 3 Eap.rirM"nlal condilions

to each other with plastic bolts and ~val Brass . Aluminum Bronze ( Alloy DJ . Tnl SPt'cimens nuts, and fixed to the test section 90110 Cupro , Titanium with a plastic holder. Temp.("CJ RT, 50. 90 Concen1ra11on !actor l , 2 pH 6, 8.2 Details of the experimental Veloc11y (mis) 1 . 2. 3 condition .are listed in Table 3. Sur tac::• ¥E'a ratio I , 20. 100 ( Titanium J Cu-Alloy )

CF means concentration factor. Disolveod OJygen Noncleoaeralion • O.aeration For each condition, two test Test PHIO

in distilled water, weighed. The mean value of test pieces was adopted as datum.

3.2 Results Fig. 5 shows typical experimental results indicated as weight loss of copper alloys. When the weight loss of copper alloys are compared, aluminium bronze indicated the smallest value for all conditions, followed by 90/10 cupronickel. Consequently, the difference among copper alloys is reduced considerably by lowering pH or by deaeration treatment. The results at pH~6 is about 10 times that of pH=8.2. The weight loss decreases to less than 1/10 when deaeration treatment is carried out. Fig. 6 shows the effect of temperature. The weight loss of test pieces was converted to corrosion rate on assumption that copper alloys corrode uniformly. In the nondeaerated condition, corrosion rate in CF=1 increases with increase in temperature with the exception of aluminium bronze. The highest corrosion rate in CF=2 was shown at 50°C. In a deaerated condi­ tion, naval brass indicated the highest corrosion rate at 90°c, while corrosion rate of 90/10 cupronickel and aluminium bronze decreased with increase in temperature. Fig. 7 shows the effect of flow rate. A tendency of slight increase in corrosion rate with increase in flow rate is indi­ cated but the effect of flow rate is not necessarily large in the experimental range.

Fig. 8 shows the effect of area ratio. Corrosion rate increased remarkably with increase in area ratio, and linearly particularly increased in the deaerated condition up to 50°c.

Oeaeration

1 :100

pH•8.2 pH=G

N~ 0 E

Area ratio: Cu-Alloy:Ti 1: 20 1: 100

20

1 3 1 3 1 2 J Velocity (mis) c:::::::iNaval Brass , ~At-Bronze • -90/10Cu-Ni • Corroded away Fig. 5 Weight loss alter 10 days loop test ( CF=2, SO'C ) COPPER ALLOYS COUPLED WITH TITANIUM 353

20 of------....~:::====~ o Naval Brass .. ---_;.:,-'i':------. • 90/10 Cu-Ni .1 """ .. · 4 Al-Bronze ------Nondeaeration,pH=6

ol Corroded ;rway

--- Nondeaeration. pH•8. 2 ~ 0. ~::~~«~ -·-- Deaeration. pH=6

c: .!!

g0.1 --- Oeaeration .~. 2 0 u s::::::. ~ __..,__,.~ 0.0 ,,..------..,...--

CF=2. SO'C, Area ratio 1: 20(Cu-Alloy: Ti)

T Temp. <·c > v ocity ( m/s) Fig. 6 Effect of temperature on corrosion Fig. 7 Effect of velocity on corrosion rate rate

Corrosion is not evaluated only from the loss of weight. As it is known that there are c~ses of· selective corrosion in case of copper alloys, so it is necessary to check the micro­ structures for finding out whether there is localized Velocity 2 m/s corrosion or not. For this purpose, the test pieces were --·Nondeaeration,50'C,CF=2. pH=B.2 inspected after completion of ---Nondeaeration,RT,CF,1,pH:S.2 the experiment. Although no · -··-Oeaeration,SO'C,CF,2, pH:6 abnormality in titanium was observed, particularly the vicinity which is in contact ~ 'f 0. with titanium had corroded E considerably in case of copper alloys. Dezincification was observed in naval brass and c: 0.1 dealuminization in aluminium ·;;;0 brone in case of the nondeaer­ 0 ~O. ated condition. In the deaer­ u ated condition, dezincification was observed in naval brass but 6 Al-Bronze dealuminization did not take 0.01 place in aluminium bronze. -----ae-ation, 50'C,CF•2, pH=S.2 Photo. 1 and Photo. 2 show 0.00 typical cross sectional micro­ structures of copper alloys on 10 00 which experiments in the Area ratio ( Ti I Cu-Alloy ) Fig. 8 Effect of area ratio on corrosion rate deaerated condition were carried out. 354 H. Kunieda et al.

Photo. 1 Cross sectional microstructures of copper alloys Photo. 2 Crass seoctional microstructures of copper alloys ( pH•B.2. so•c. Cf:2. 2 m/s, Oeaeration, ~ ) CPH=6. 50 'C. CF= 2 • 2m/s. Deaeration • ~ )

Atta ratic Naval Brass 90/10 Cu-Ni Al-Bronze 90/10 Cu-Ni Al-Bronze

1: 1 .~ ,J - -, -- '

1 : 20

...... --- 1: 100

Discussion

Potential difference between two metals provides a driving force whether galvanic corrosion takes place or not. As shown in Table 2, a potential difference of 360 - 480 mV will be enough to take place galvanic corrosion in the deaerated condition. As it is difficult to discuss, however, the effects of dissolved oxygen concentration, etc. from the concept of potential differ­ ence based on the equilibrium theory of thermodynamics, they are discussed from measurement of galvanic current and from experi­ mental results of loop tests.

Decrease in pH generally results in large increase in the corrosion rate because protective film is not formed and it has been confirmed by this experiment that this is the same in case of galvanic corrosion. When the cases of pH=6 and pH=8.2 are compared, the corrosion rate of pH~6 was about 10 times faster, although there is some difference due to other factors.

As dissolved oxygen acts as a cathodic depolarizer by redox reaction to increase the cathodic reaction, corrosion rate is increased in an environment in which this is rate determining. When the nondeaerated and deaerated conditions used in Fig. 5 are compared, the corrosion rate of the former was more than about 10 times that of the latter. Next, the effects of temperature and CF are considered on the basis of the results of Fig. 6. It can be considered from chemical reaction kinetics that corrosion rate increases with temperature but probably increase in temperature causes decrease in dissolved oxygen concentration and CaC03 and Mg(OH)2 scales precipitate depending on the pH due to synergism with CF and also, formation of protective film on the copper alloys is affected. As these phenomena participate mutually, it is diffi­ cult to state clearly the effect of these factors.

At pH=6 in the nondeaerated condition, corrosion rate increased with temperature up to 50°C but decreased at 90°C. This is due probably to decrease in dissolved oxygen. As it can be assumed that dissolved oxygen concentration is constant (below 20 ppb) at pH=8.2 in the deaerated condition, only the effects of precipitation of scale and formation of protective film can be considered. The corrosion behavior of 90/10 cupro- COPPER ALLOYS COUPLED WITH TITANIUM 355 nickel, aluminium bronze and naval brass at 90°C must be considered that this is due to difference in the protective films formed on copper alloys. The effects at pH=S.2 in the nondeaerated condition are very complicated. It is probably suitable to consider that the effect of scale precipitation is large in case of CF=2 and the effect of protective film is large in the aluminium bronze phenomenon of CF=1. Increase in flow rate results in increase in corrosion rate as the speed of supplying oxygen to the metal surface and the speed for removing corrosion products increase. Yamaguchi et al. (1) measured galvanic current for naval brass and aluminium bronze coupled with titanium in normal temperature sea.water and reported on the effects of flow rate. According to their results, corrosion rate increases parabolically with increase in flow rate as the corrosion rate at a flow rate of 2 m/s was more than 10 times of the stationary condition. It is indicated in the results of Fig. 7 that the effect of flow rate is not necessarily large in a range of 1 ... 3 m/s. Fukuzuka et .al. (5) studied galvanic corrosion of-copper alloys coupled with titanium by a dip test using deaerated 6% NaCl solution of 100°C at pH=6 and reported that corrosion rate was 0.03 mm/year in case of 90/10 cupronickel, 0.03 mm/year for aluminium bronze and 0.045 mm/year for naval brass when the area ratio was 1:10 (copper alloy:titanium). Fig. 8 shows the results when the temperature was 50°c and when the value at area ratio of 1:10 was read, it was about 0.3 mm/year, which is about 10 times the results obtained by Fukuzuka et al. It is probably suitable to consider· that the main cause of this difference is the flow condition.

The effect of area ratio differs with the mode of control of polarization. Corrosion rate increases almost proportionally with the cathode area in case of cathode control as shown in the results of deaerated condition of Fig. 8. In the results of nondeaerated condition, however, corrosion rate is not necessa­ rily proportional to the cathode area. It is necessary to take into consideration the effect of anodic reaction in this case. For example the experimenal results at pH::S.2 in the deaerated condition which is shown in Fig. 2 can be considered based on the insufficient protective film formation, and the results in the nondeaerated condition will be explained as the difference of protective film intrinsic of each copper alloys. When considered in this manner, the increase in corrosion rate not being proportional to the a~ea ratio at pH~S.2 in Fig. 8 can be said to be due to the inhibiting effect of protective.film.

The above is an evaluation of the various factors. The effects of the ·various factors on galvanic corrosion are very complicated and it is difficul t 1·to express this unconditionally but pH, dissolved oxygen concentration and area ratio would be important factors. Harding et al.[6] have pointed out that there is possibili­ ty of getting a mistaken conclusion from results of experiments carried out in an ideal environment when estimating the life of practical sea water desalination plants. This suggests that sometimes pH and dissolved oxygen concentration are not being steadily operated in an ideal condition from the stand point of 356 H. Kunieda et al. corrosion engineering and we think this is also important when considering galvanic corrosion of copper alloys coupled with titanium. For instance, aluminium bronze whose corrosion rate is the smallest under most experimental conditions will be considered as an example. The corrosion rate is 0.057 mm/year at pH=8.2, CF=2, 50°c, flow rate 2 m/s and area ratio 1:20 under a deaerated condition but a large increase to 0.53 mm/year is indicated when pH becomes below 6. Also, the corrosion rate increases remarkably to 0.56 mm/year at pH~8.2(CF=2, 50°c, flow rate 2 m/s, area ratio 1:20) under nondeaerated condition in which air leakage is assumed.

Furthermore, the corrosion rates mentioned here refer to the results when the area ratio of titanium to copper alloy was taken as 1:20. This figure is obtained by Morgan's equation (7] which is normally applied for the impressed current cathodic protection of power condensers. However, there are cases in which the temperature and CF of the sea water are high in a desalination environment. It can be considered that specific resistanc~ decreases and current arrival distance in the tube increases in such a case. Now, when the area ratio of titanium to copper alloys is calculated with only decrease in specific resistance in a high temperature concentrated sea water considered, this value becomes 2-3 times larger than that of normal temperature sea water. The value of area ratio of titanium to copper alloys to be taken is not clear because data on the arrival distance in tube of cathodic protection current in a practical desalination environment are not available at present but if an area ratio of 1:100 is taken, the corrosion rate of aluminium bronze becomes 0.21 mm/year at pH~8.2(CF=2, flow rate 2 m/s, 50°C) in a deaerated condition.

According to the results of the present experiment, galvanic corrosion of copper alloy sheets by titanium tubes does not necessarily_ become large if operation of a practical desalination plant is carried out under an ideal condition. The use of aluminium bronze which indicated the best corrosion resistance is desirable for minimizing corrosion troubles of practical equipment by taking into consideration the various environmental changes and even when aluminium bronze is used, it is necessary to apply a suitable corrosion protection method.

Conclusions An experiment on galvanic corrosion of copper alloys coupled with titanium in synthetic sea water was carried out and the following results were obtained:

(1) The potential difference of titanium and copper alloys in sea water was 360 - 480 mV even when dissolved oxygen concentration is low(below 20 ppb), which does not differ very much from the case of high dissolved oxygen concentration.

(2) Corrosion rate of copper alloys coupled with titanium increased about 10 times when pH is lowered to about 6 from about 8.2. COPPER ALLOYS COUPLED WITH TITANIUM 357

(3) Corrosion rate of the nondeaerated condition increased about 10 times from that of the deaerated condition.

(4) The effects of temperature and concentration factor are very complicated as the corrosion rate of copper alloys vary considerably in accordance with change in dissolved oxygen concentration and by interaction of precipitation of scale and formation of protective film.

(5) A tendency of increase in corrosion rate by increase in flow rate was indicated but almost no increase was observed in a range of 1 .... 3 m/s. (6) Corrosion rate indicated a large increase with increase in the area ratio of titanium to copper alloys. This tenden­ cy was particularly marked in a condition protective film has not formed thoroughly on copper alloys. (7) Aluminium bronze was the best corrosion resistant among naval brass, 90/10 cupronickel and aluminium bronze.

(8) It is necessary to take into consideration suitable protection method for practical equipment even for aluminium bronze sheets because the corrosion rate differs considerably with change in pH and dissolved oxygen concentration.

References 1. Y. Yamaguchi, K. Onda, H.• Hirose, S. Sato, z. Tanabe, M. Shimono and T. Nosetani: The Thermal and Nuclear Power, 25, (1974), 172. 2. T. Fukuzuka, K. Shimogori, H. Sato, H. Kusamichi, H. Itoh, T. Hiwatashi, R. Araki, Y. Mitomi and Y. Imoto: The Thermal and Nuclear Power, 30, (1979), 79. 3. Y. Hisatomi, S. Sato, N. Suzuki, K. Suzuki, K. Tanno, A. Tomita and Y. Oshima: The Thermal and Nuclear Power, 29, (1978), 771. 4. A. Kawabe, Y. Ikushima, s. Iijima, S. Sato, K. Nagata and S. Yamauchi: The Thermal and Nuclear Power, 27, (1976), 569. 5. T. Fukuzuka, K. Shimogori, H. Sato, F. Kamikubo and T. Hara: Pre-print of Japan Society of Corrosion Engineering, Spring Conference, p. 231, (1978). 6. K. Harding and D. A. Bridle: Desalination, 28, (1979), 89. 7. J. H. Morgan: Corrosion, 15, (1959), 417.