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Galvanic Corrosion of Some Copper Alloys Coupled with Titanium in Synthetic Sea Water

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Galvanic Corrosion of Some Copper Alloys Coupled with Titanium in Synthetic Sea Water GALVANIC CORROSION OF SOME COPPER ALLOYS COUPLED WITH TITANIUM IN SYNTHETIC SEA WATER Hiroshi Kunieda, Hiroshi Yamamoto and Naomichi Nishijima Furukawa Metals 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 brass 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 alloy 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 density 3. Loop Test 3.1 Experimental Method It is necessary to know the valency of metal 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 Nickel , 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<l (day l 10 pieces were removed after 10 days, descaled in the dilute HCl, washed 352 H. Kunieda et al. 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.
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