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CuNi Alloys in Chlorinated Substitute Ocean Water.
-Effect of Sulfide in Surface Preconditioning with Substitute Ocean Water-
G. Schmitt1, R. Feser2, C. Kapsalis2,3, K. Steinkamp3, B. Sagebiel3,
1
IFINKOR-Institute for Maintenance and Corrosion Protection Technology
Kalkofen 4, D-58638 Iserlohn, Germany, gue.schmitt@t-online.de
2
Laboratory for Corrosion Protection, South-Westfalia University of Applied Sciences
Frauenstuhlweg 31, D-58644 Iserlohn, Germany, feser.r@fh-swf.de
3
KME Germany AG &Co KG; Klosterstr. 29, D-49074 Osnabrück
bernd.sagebiel@kme.com Abstract
Six weeks exposure tests were run with CuNi 70 30 (C71500) and CuNi 90 10 (70600) tubes in a flow loop with artificial ASTM seawater (pH 8.2; flow rate 0.5 m/s) at room temperature and 40°C in the absence and presence of 1 ppm hydrogen sulfide. The effect of hypochlorite addition (0.5; 1.0; 3.0 ppm; sensor controlled) was investigated on tube surfaces i) in the as-received surface condition, ii) after conditioning with additive-free artificial seawater and iii) after conditioning with artificial seawater containing 1 ppm bisulfide. The conditioning was performed for 6 weeks at room temperature under access of air.
On a non-conditioned as-received tube surface the presence of hypochlorite increases the mass loss rate. The effect increases with the hypochlorite concentration and decreases with increasing temperature (40°C). Pre-conditioning of the inner tube surface with additive-free artificial seawater is beneficial to improve performance in the presence of hypochlorite.
The presence of bisulfide in the flowing seawater significantly decreases the corrosion resistance of both types of CuNi alloys in the as-received surface condition, as well after 6 weeks conditioning with additive-free and bisulfide containing seawater.
1 Introduction
It is well documented [1] that copper-nickel alloys exhibit excellent corrosion performance in clean seawater. This is due to the formation of protective layers and a high resistance to biofouling. However, when sulfide is present, e.g. due to the activity of sulfate reducing bacteria (SRB) under oxygen deficient conditions, the corrosion resistance is reduced considerably even at sulfide concentrations as low as 0.01 ppm [1,2]. This is due to a change in the nature of the corrosion product layers containing copper, nickel and iron sulfides and become inhomogeneous and poorly adherent. Therefore, under such conditions the material becomes susceptible to flow induced localized corrosion. The susceptibility is higher for CuNi 90/10 than for Cu 70/30 [2]. Furthermore, the electronic conductivity of such sulfides seems to cause bimetallic effects which stimulate the anodic dissolution of the base material [3].
Sulfide containing surface layers can convert to oxide-based layers when contacted with oxygen saturated seawater [1,2]. This process may take more than a week. Protective oxide based layers seem to endure periodic exposure to sulfide contaminated seawater without significant impairment of the protective layer. Longtime exposure, however, is not recommended.
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1
The objective of the present work was to investigate the effect of hypochlorite on the corrosion performance of CuNi 70 30 and CuNi 90 10 tubes in flowing (0.05 m/s) artificial ASTM seawater at room temperature and 40°C. Special emphasis was put on the influence of surface preconditioning in additive-free and sulfide-contaminated (1 ppm) artificial seawater [4].
2 Experimental
2.1 Material
Tubes (20 x 1 mm, length 200 mm) of CuNi 90 10 (C70600) and CuNi 70 30 (C71500) (chemical composition in Table 1) were tested in horizontal position (Fig. 1-3).
Table 1 - Chemical composition of CuNi alloys
Element [Mass-%] Alloy
Cu Ni Fe Mn Others
Rest 10-11 1.5-1.8 0.6-1.0 max.0.3 CuNi 90 10 (C70600)
Rest 30-32 0.4-0,7 0.5-1.0 max. 0.08 CuNi 70 30 (C71500) 2.2 Test Media and Conditions
The tests were performed in artificial seawater according to ASTM D 1141 with a pH of 8.2 without and with additions of hypochlorite (0.5; 1.0; 3.0 ppm as NaOCl ; sensor controlled) and bisulfide (1 ppm as NaHS), respectively. The effect of hypochlorite addition was investigated on tube surfaces in the as-received surface condition, after conditioning with additive-free artificial seawater and after conditioning with artificial seawater containing 1 ppm bisulfide. The conditioning was performed for 6 weeks at room temperature under access of air. The tests were run at room temperature and at 40°C with a flow rate of 0.05 m/s.
2.3 Experimental Setup
Prior to the testing the marked probes were pretreated with ultrasonic support subsequently for 5 min in a C6-7 hydrocarbon fraction, acetone, distilled water and methanol. A sufficient number of tube probes were exposed for 6 weeks in stagnant, air-saturated additive-free artificial ASTM seawater at room temperature at pH 8.2. Another number of tubes was treated for 6 weeks at room temperature at a low flow rate of about 0.05 m/s (Fig. 1) with artificial ASTM seawater containing 1 ppm bisulfide (dosed as a sodium hydrogen sulfide solution). The HS—-concentration was colorimetrically controlled using the MERCK Microquant® test kit.
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CuNi 7030 CuNi 9010Medium Inlet Dosage PumpsStorage Tanks
Figure 1: Six weeks preconditioning of CuNi alloy test tubes with flowing (0,05 m/s) 1 ppm
HS—-containing ASTM seawater at RT and pH 8,2. Each type of alloy was treated
in a separate flow loop.
The final 6 weeks corrosion experiments at RT and 40°C were run in test loops with test tubes having surfaces i) as-received , ii) 6 weeks preconditioned in stagnant ASTM seawater, iii) 6 weeks preconditioned at RT in slow flowing ASTM seawater containing 1 ppm bisulfide. The test solution was ASTM seawater (pH 8.2) circulated at a flow rate of 0.05 m/s. The test solution was i) additive-free, ii) conditioned with addition of hypochlorite (as NaOCl solution: 0.5, 1.0 and 3.0 ppm), or iii) conditioned with addition of 1 ppm bisulfide. The loop test rig is shown in Figs. 2-4.
The artificial ASTM seawater was produced in 30 ltr. tanks and conditioned – if necessary - with hypochlorite by dosing NaOCl-solution or with bisulfide by dosing NaHS-solution. The hypochlorite concentration was monitored amperometrically with an electrode Model CS2.3 of Sensortechnik Meinsberg GmbH (Germany) using the transmitter MU2060 (delivered from the same company) with a digital reading. The accuracy was ±0.01 ppm. The electrode was calibrated colorometrically with the \"CHECKIT Comparator\" of Tintometer GmbH (Germany). The transmitter signal was used to trigger pumps to dose NaOCl-solutions. This allowed to keep the hypochlorite concentration constant within ±0.05 ppm. In a given loop, the test medium was pumped from the storage tanks into test tubes at a flow rate of 0.05 m/s, through the hypochlorite electrode cell back to the storage tank. Each test run was started with freshly produced test solution which was renewed each week.
The experimental parameters of the loop tests are summarized in Tab. 2. The experimental matrix is shown in Tables 3 and 4.
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Current Noise Diagnosis OCl—-Dosage PumpsStorage Tanks Figure 2: Test loops with control of OCl— concentration and ECN online diagnosis
OCl—Analysis (Electrode + Transmitter 40°C- Experiments (insulation) RT-Experiments(no insulation)
Fig. 3: Test loop (close-up)
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Fig. 4: Test loops from different views.
Table 2: Parameters of loop tests Material Medium
OCl—Concentrations [ppm] HS—Concentration [ppm] Flow Rate [m/s] Temperature [°C] Exposure Time [weeks] Sampling
CuNi 9010; CuNi 7030
Artificial seawater (ASTM D1141), pH 8,2 0 ; 0,5 ; 1,0 ; 3,0 1 0,05 RT; 40 6
after 2; 4; 6 weeks
The runs no. 1 to 10 (Table 3) included only experiments which started with tubes in the as-received surface condition (indicator (a) for runs with CuNi 70 30 and (c) for runs with CuNi 90 10; Table 3) or with tubes that had been preconditioned in stagnant additive-free artificial ASTM seawater (indicator (b) for runs with CuNi 70 30 and (d) for runs with CuNi 90 10; Table 3). The runs no. 11 to 20 (Table 4) where performed with tubes that had been preconditioned with artificial ASTM seawater containing 1 ppm HS— (indicator (a) for runs with CuNi 70 30 and (b) for runs with CuNi 90 10 (Table 4).
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Table 3: Experimental Matrix (1)
7030 As-received
a a a a a a a a a a
Pre-cond. b b b b b b b b b b
9010 As-received
c c c c c c c c c c
Pre-cond. d d d d d d d d d d
HS- [ppm] 0 1 0 1 0 0 0 0 0 0
OCl- [ppm] 0 0 0 0 0,5 1 3 0,5 1 3
Run-Nr. 1 2 3 4 5 6 7 8 9 10
Temp. [°C] RT RT 40 40 RT RT RT 40 40 40
Precond.: 6 weeks preconditioning in stagnant additive-free artificial ASTM seawater Table 4: Experimental Matrix (2) Run-Nr. 11 12 13 14 15 16 17 18 19 20
CuNi 70 30 Preconditioned
a a a a a a a a a a
CuNi 90 10 Preconditioned
b b b b b b b b b b
Temp. [°C] RT 40 RT 40 RT 40 RT 40 RT 40
HS- [ppm] 0 0 0 0 1 1 0 0 0 0
—
OCl- [ppm] 0 0 0,5 0,5 0 0 3 3 1 1
Preconditioned: 6 weeks preconditioning in slow flowing artificial ASTM seawater with 1 ppm HS
The corrosion intensity was monitored online using electrochemical noise (ECN) diagnosis. Two tubes of the same type and preconditioning were connected over a zero resistance ammeter (ZRA) and the
electrochemical current noise exchanged between these two free corroding tube electrodes was measured and diagnosed using the CoulCount™-data evaluation software. This software converts the current noise into noise charge vs. time curves with always positive slope. The slope of such curves correlate with the
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corrosion intensity on the electrode surfaces, thus allowing real-time indication of relative corrosion rates [5].
2.4 Evaluation of experiments
After exposure times of 2, 4 and 6 weeks the tubes were demounted from the rig and dried at 40°C. The tubes were than cut longitudinally in the 4:30 – 10:30 h plane. This should allow to investigate special features in the 6 and 12 o'clock position of the inner tube surface. One half shell was left as it came from cutting. The other half shell was pickled with 10% citric acid for 5 minutes in the ultrasonic bath. Both unpickled and pickled half shells were fotographed together and then investigated microscopically in order to evaluate the deepest penetration at sites of localized attack. Some surfaces were additionally investigated with the scanning electron microscope. Furthermore, characteristic \"worst case\" surface areas with localized attack were scanned topographically with the topographic measuring device \"MicroProf®\" from Fries Research & Technology GmbH, Bergisch-Gladbach (Germany) allowing a resolution in the z-axis of 10 nm. This method was also used to measure maximum penetration depths.
3 Results 3.1
Effect of hypochlorite on as-received and bisulfide-free conditioned surfaces
3.1.1 Absence of Hypochlorite
CuNi alloys develop uniform, protective surface layers when exposed for six weeks in stagnant additive-free ASTM seawater. This continues when such preconditioned surfaces are subjected to additive-free ASTM seawater for another 6 weeks under slow flow conditions (0.5 m/s). Similar results are obtained when starting with an as-received surface and subsequent 6 weeks treatment with additive-free flowing ASTM seawater. Rising the temperature to 40°C yields no significant decrease of the corrosion resistance. Under none of the conditions tested, localized attack was observed at both types of CuNi alloys.
3.1.2 Presence of Hypochlorite
Exposure of CuNi 70 30 tubes in the-as received surface condition to flowing hypochlorite-containing ASTM seawater at room temperature yields an increase of corrosion attack, which increases with increasing hypochlorite concentration. However, at 40 °C under otherwise same conditions the corrosion enhancing effect of hypochlorite decreases. This positive temperature effect is also observed when starting with a CuNi 70 30 surface that had been preconditioned for 6 weeks at room temperature in stagnant additive free ASTM seawater.
At CuNi 90 10 this \"inhibiting\" effect of hypochlorite is not seen. With increasing hypochlorite concentration the corrosion attack increases significantly at room temperature and even more at 40°C, regardless of the starting surface condition. The presence of hypochlorite promotes the denickelification at sites of localized corrosion. The positive temperature effect, observed at CuNi 70 30, is not found at CuNi 90 10. Preconditioning in stagnant additive-free ASTM seawater yields a slight reduction of the stimulating effect of hypochlorite.
3.2 Effect of hypochlorite on bisulfide-influenced preconditioning
Whenever bissulfide is involved, either during preconditioning or during exposure, significant, widely uniform materials attack is encountered at both alloys. The tendency to denickelification is more pronounced at CuNi 90 10 than at CuNi 70 30. Bisulfide preconditioned CuNi 70 30 surfaces experience some \"corrosion inhibition\" when subjected to slow flowing hypochlorite containing ASTM seawater. The \"inhibiting\" effect seems to increase at CuNi 70 30 with increasing hypochlorite concentration. This is not seen at CuNi 90 10.
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Although all the above evaluations are based on visual inspection, the conclusions made are nicely backed-up by microscopic quantification of maximum materials penetration related to sites of localized attack. While it is appreciated that these microscopic wall loss measurements give only rough estimates and extrapolation from wall loss during the experimental period to a linear max. yearly corrosion rate may not be totally valid, these considerations, nevertheless, give clear trends (Table 5 and 6).
Table 5: Microscopical wall loss measurements at CuNi 70 30 tubes after 6 weeks flow testing Temp. Preconditioning Additive Concentration in Flowing Range of Maximum [°C] Wall Loss Local ASTM Seawater
during Penetration Bisulfide Hypochlorite Concentration Rate Concentration Experiment[ppm]
[µm] [mm/a] 0.5 1.0 3.0 [ppm
RT - - - - + 75 - 145 1.26 RT SW - - - + 28 - 80 0.70 40 - - - - + 70 - 85 0.74 40 SW - - - + 34 - 61 0.53 RT SW + HS— - - - - 70 - 227 1.97 —RT SW + HS + - - - 28 - 249 2.16 —RT SW + HS - + - - 0 - 45 0.40
—RT SW + HS - - + - 17 - 78 0.68 —RT SW + HS - - - + 58 - 209 1.82 —40 SW + HS - - - - 91 - 149 1.30
—40 SW + HS + - - - 28 - 266 2.31 40 SW + HS— - + - - 25 - 38 0.33 40 SW + HS— - - + - 16 - 39 0.34
—40 SW + HS - - - + 101 - 184 1.60 SW: preconditioning for 6 weeks at room temperature in additive-free stagnant ASTM seawater
SW + HS— : preconditioning for 6 weeks at room temperature in slow flowing ASTM seawater with 1 ppm HS—
Table 6: Microscopical wall loss measurements at CuNi 90 10 tubes after 6 weeks flow testing
Additive Concentration in Flowing Range of Maximum Temp. Preconditioning ASTM Seawater [°C] Wall Loss Local
during Penetration Bisulfide Hypochlorite Concentration Rate Concentration Experiment[ppm]
[µm] [mm/a] 0.5 1.0 3.0 [ppm
RT - - - - + 87 – 130 1.13 RT SW - - - + 115 – 127 1.10 40 - - - - + 111 – 176 1.53 40 SW - - - + 107 – 180 1.56 —RT SW + HS - - - - 109 – 151 1.31 RT SW + HS— + - - - 96 – 136 1.18
—RT SW + HS - + - - 78 – 158 1.37
—RT SW + HS - - + - 46 – 185 1.61 —RT SW + HS - - - + 136 – 213 1.85
—40 SW + HS - - - - 83 – 131 1.14 —40 SW + HS + - - - 114 – 180 1.56 40 SW + HS— - + - - 78 – 220 1.91 40 SW + HS— - - + - 94 – 158 1.37 —40 SW + HS - - - + 133 - 195 1.70 Legend: see Table 5
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With respect to localized attack and maximum local penetration rates it appears that localized corrosion at CuNi 70 30 tubes in bisulfide-containing slow flowing ASTM seawater is clearly reduced by preconditioning in additive-free stagnant ASTM seawater (Table 5). This holds for exposure at room temperature and at 40°C. Preconditioning of CuNi 70 30 tubes in bisulfide-containing slow flowing ASTM seawater and subsequent treatment with additive-free flowing ASTM seawater yields high maximum local penetration rates which are even increased by adding 0.5 ppm hypochlorite, However, addition of 1 and 3 ppm hypochlorite seems to inhibit the local attack at room temperature and at 40°C. Preconditioning of CuNi 70 30 tubes in bisulfide-containing slow flowing ASTM seawater and subsequent treatment with flowing bisulfide containg ASTM seawater causes expectedly high local penetration rates at room temperature and at 40°C (Table 5).
Table 6 demonstrates that overall CuNi 90 10 tubes are significantly less corrosion resistant than CuNi 70 30 tubes. Preconditioning with additive-free stagnant ASTM seawater is ineffective when the exposure occurs with bisulfide containing flowing ASTM seawater. This holds for exposure at room temperature and at 40°C. The hypochlorite effect is much less pronounced. Under all experimental conditions the maximum local penetration rates were well above 1 mm /a.
Additional information on system's corrosivity was obtained from electrochemical current noise diagnosis. Fig. 5 shows examples of typical noise charge vs. time plots measured in a time interval of two hours at a pair of CuNi 70 30 tubes with an as-received starting surface. More 2 hours time intervals have been monitored during the 6 weeks experiments. However, the data obtained generally gave the same information.
Exposure to flowing additive-free ASTM seawater at room temperature yields a very low slope of the noise charge curve which is in agreement with the low corrosivity of CuNi 70 30 under such conditions (Fig. 5). Adding 1 ppm of bisulfide to the flowing ASTM seawater at room temperature increases the slope of the curve, although a relative higher slope would have been expected from the visual inspection of corroded surfaces. Increasing the temperature under otherwise same conditions rises the slope considerably. Adding 1 ppm hypochlorite to the flowing ASTM seawater at 40°C yields a higher slope than found for the curves measured at room temperature in additive-free and 1 ppm hypochlorite containing ASTM seawater, which is within the expectations. Increasing the hypochlorite concentration to 3 ppm causes a significantly steeper noise charge curve indicating an increase of corrosivity. Thus, the noise charge curves give a good overview on parameter effects on relativ corrosivities.
Figure 5: Noise charge curves measured at CuNi 70 30 tubes exposed in the as-received
surface state to flowing ASTM seawater without and with additives
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The relative superior corrosion resistance of CuNi 70 30 as opposed to CuNi 90 10 is demonstrated in Figs. 6 and 7 comparing the level of noise charges sampled within 2 h from CuNi alloys preconditioned in 1 ppm bisulfide-containing ASTM seawater prior to treatment with flowing ASTM seawater containing given additives at room temperature and 40°C. The graphs show the higher levels of noise charges obatined from CuNi 90 10 specifically in the experiments with flowing additive-free and 1 ppm bisulfide-containing ASTM seawater.This correlates with the significant corrosion attack observed under these conditions. Also the hypochlorite effect is demonstrated although the stimulating effect of low hypochlorite concentrations on localized attack is not seen. This is due to the fact that in the graphs only an arbitrary 2 h sampling period has been selected from a total of 6 weeks experimental time. Better correlation would have been achieved in case of continuous sampling over the whole exposure time (which is possible). Nevertheless, ECN diagnosis of current noise exchanged between two tube sections as pair of flow-through electrodes allow real-time indication of relative corrosivities of corrosion systems.
Fig. 6: Noise charges after 2 h sampling time in different corrosion systems.
CuNi 70 30 preconditioned in 1 ppm bisulfide-containing ASTM seawater and subsequent treatment
with flowing ASTM seawater containing given additives.
Fig. 7: Noise charges after 2 h sampling time in different corrosion systems.
CuNi 90 10 preconditioned in 1 ppm bisulfide-containing ASTM seawater and subsequent treatment
with flowing ASTM seawater containing given additives.
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4 Conclusions
Preconditioning of CuNi 70 30 and CuNi 90 10 for 6 weeks at room temperature in stagnant additive-free ASTM seawater produces protective scales which reduce materials attack during exposure to flowing (0.05 m/s) additive-free ASTM seawater at room temperature and 40 °C. The performance of both alloys is similar under these conditions.
However, the perfomance of CuNi 70 30 is better than CuNi 90 10 when small amounts of aggressive additives like hypochlorite (OCl—) or contaminants like bisulfide (HS—) are present.
In general, 0.5 to 3.0 ppm hypochlorite in slow flowing ASTM seawater enhance the corrosion attack at room temperature. The intensity increases for both alloys with increasing hypochlorite concentration. However, at 40 °C this stimulating effect decreases for CuNi 70 30, but not for CuNi 90 10. The latter alloy experiences increased denickelification in the presence of hypochlorite in the tested concentration range. In hypochlorite –containing systems preconditioning in stagnant additive-free ASTM seawater at room temperature is beneficial for CuNi 70 30, but not for CuNi 90 10.
Whenever bisulfide is involved, either during preconditioning or during subsequent exposure under flow conditions, increased materials attack is encountered at both CuNi alloys, however, the tendency to denickelification is higher for CuNi 90 10 than for CuNi 70 30.
Bisulfide-influenced seawater preconditioning increases the corrosion attack when the materials are exposed to flowing additive-free ASTM seawater. However, in case of CuNi 70 30 addition of hypochlorite to the flowing ASTM seawater decreases the stimulating bisulfide effect. This is not seen at CuNi 90 10, regardless of the starting condition of the surface.
The better overall performance of CuNi 70 30 as opposed to CuNi 90 10 has been verified by wall loss measurements, topographic scanning and electrochemical current noise measurements using a special current noise diagnosis software which allows online monitoring of actual corrosivities in corrosion systems.
5 References 1
C. A. Powell, H. T. Michels, „Copper-Nickel Alloys for Seawater Corrosion Resistance and Antifouling - A State of the Art Review“, CORROSION 2000, NACE International, Houston, Texas, USA, 2000, Paper 627.
C. A. Powell, „Copper Nickel Alloys - Resistance to Corrosion and Biofouling, CDA Inc. Seminar Technical Report 7044 - 1919, The Application of Copper Nickel Alloys in Marine Systems“, 1992, http://www.copper.org, extracted 25.04.2009.
F. P. Ijsseling, „The Corrosion Behaviour of the System CuNi10Fe/Seawater. The Protective Layer of Corrosion Products - A Literature Review”, Reviews on Coatings and Corrosion: A Quarterly Reviews, 3 (1981) 269 – 324.
C. Kapsalis, \"Investigations on the performance of copper-nickel alloys under different conditions of seawater corrosion\" (in german); Master Thesis, South-Westfalia University of Applied Sciences, Iserlohn, Germany, June 2009.
G. Schmitt, R. Buschmann, Ch. Olry, B. Motko, P. Schrems, „Measuring Corrosivity as Simple as Temperature?\
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