Effect of cyclic hydrostatic pressure on the sacrificial anode cathodic ...

Effect of cyclic hydrostatic pressure on the sacrificial anode cathodic protection Shengnan Hu, Tao Zhang and Yawei Shao Corrosion and Protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology, Harbin Engineering University, Ministry of Education, Harbin, China, and

Guozhe Meng and Fuhui Wanga State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China Abstract Purpose – The purpose of this paper is to study the effect of cyclic hydrostatic pressure on the protective performance of cathodic protection (CP) system consisting of Zn-Bi sacrificial anode and Ni-Cr-Mo-V steel. Design/methodology/approach – The anode and cathode polarization curves of the driving potential and current for CP were investigated in case of cyclic hydrostatic pressure (0-3.5 MPa) and compared with that at atmospheric pressure. The morphologies of the anode material with and without corrosion products were observed by scanning electron microscopy. Findings – The experimental results revealed that the cyclic hydrostatic pressure had significant influence on the CP system. The anode potential instantaneously responded to the cyclic hydrostatic pressure and the discharge performance decreased due to the deposition of corrosion product. Also, the CP system exhibited higher slope parameter under cyclic hydrostatic pressure, indicating that the CP system cannot provide adequate protection for Ni-Cr-Mo-V steel. Originality/value – The results presented in this paper clearly show the effect of cyclic hydrostatic pressure on the sacrificial anode CP system, and present a foundation for further research on the practical application of sacrificial anode under cyclic hydrostatic pressure environment. Keywords Cathodic protection, Zn-Bi sacrificial anode, Cyclic hydrostatic pressure, Circuit current, Slope parameter, Hydrostatics Paper type Research paper

Fischer and Finnegan (1987) reported field test results for deepsea CP on the Norwegian continental shelf. Kennelley et al. (1987) investigated the performance of bimetallic anodes on deep water production platform. Chen et al. (2003) assessed the deep-sea CP system around the Gulf of Mexico. Although a publication by NACE International (1992) provides state-ofthe-art approach for CP design considerations for deepwater structures, actual field tests are both costly and time consuming. Accordingly, some effort has been devoted to laboratorysimulated experiments (Chen et al., 2002, 2003). Nonetheless, the available information remains insufficient for proper understanding the CP systems in deep-sea environments and some questions still remain. For instance, transfer-vessels usually alternate between deep sea and shallow-water environments (shallow-deep cycle), with different hydrostatic pressures. The effect of such hydrostatic pressure cycling on materials corrosion performance is still unclear. Recently, a new experimental approach to marine CP has evolved, which involves a slope parameter, based upon the freerunning approach. Wang et al. (1996) and Chen et al. (1998) considered the interrelationship between the polarized anodic and cathodic potentials in terms of the anodic or cathodic current (Ia or Ic, respectively) according to Ohm’s law:

1. Introduction In attempts to minimize exploitation of land resources, more and more effort is being devoted to make the most of marine resources because of the abundance of oil, gas and mineral reserves in the deep sea. Efficient exploration in the deep-sea environment poses great challenges for engineering and structural materials due to the turbulent and highly corrosive nature of the environment. Therefore, the subject corrosion and corrosion protection in the deep-sea (ocean) environment must be given special consideration. Cathodic protection (CP) by coupled galvanic anodes is a common anti-corrosion technique for metallic structures (usually iron or steel) deployed in service in marine environment. The fundamental criterion for marine CP is that the structure of interest be polarized to a potential around 2 0.80 V vs silver-silver chloride (Ag-AgCl), throughout the proposed design life. Recently, an important distinction has been established for materials corrosion in shallow and deep water marine environments, which must be taken into consideration when designing CP systems in order to minimize overdesign and, at the same time, maximize reliability. CP in deep-sea environments has received considerable attention in the last several decades. Fischer et al. (1987) and

Ia ¼ Ic ¼

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Ec 2 Ea Rx þ Ra þ Rc

ð1Þ

Where Rx, Rc, Ra are the external, cathode and anode resistances; Ec, Ea are the cathode and anode potential, respectively. If defined the total circuit resistance Rt as:

Anti-Corrosion Methods and Materials 58/5 (2011) 238– 244 q Emerald Group Publishing Limited [ISSN 0003-5599] [DOI 10.1108/00035591111167703]

Rt ¼ Rx þ Rc þ Ra 238

ð2Þ

Effect of cyclic hydrostatic pressure

Anti-Corrosion Methods and Materials

Shengnan Hu, Tao Zhang, Yawei Shao, Guozhe Meng and Fuhui Wanga

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Figure 2 Schematic diagram of the experimental setup for deep ocean corrosion research

Then, equation (1) can be rewritten as: E c ¼ ðRt £ Ac Þ £ i a þ E a

ð3Þ

1

Where ic is the cathodic current density, and Ac is the cathode area. Thus, the slope of the linear interdependence between DE (DE ¼ Ec 2 Ea) and I is projected to be the product of the total circuit resistance. This technique exists to conveniently quantify E-I interrelationships associated with sacrificial anode CP. The present study investigates the effect of cyclic hydrostatic pressure on the protective performance of CP systems using electrochemical measurements and surface morphological examinations. The CP system of interest consists of zinc-bismuth sacrificial anode and Ni-Cr-Mo-V high-strength steel.

Rx 2 3

2. Experimental detail Reference electrode

2.1 Materials The experimental procedure involved a cuboid-shaped cathode (20 £ 20 £ 50 mm) of Ni-Cr-Mo-V high-strength steel. The anode geometry is shown in Figure 1. The composition of anode material is 99.5 percent zinc and 0.5 percent bismuth. The cathode and anode ring were mounted using epoxy and the exposed surface area was 40:30 cm2.

Anode

Cathode

2.2 Experimental setup The pressure vessel is shown in Figure 2, which is pressurized with a hydro pneumatic pump using high-purity nitrogen. Notes: (1) Anode; (2) cathode; (3) reference electrode

2.3 Electrochemical measurements Appropriate fittings were used for electrical connections and mutual isolation of the anode, cathode, and an Ag/AgCl reference electrode for each cell. Polarization of the steel cathode was affected by electrically connecting this electrode to the anode through an appropriate sized external resistor. Cathodic current density was calculated from the voltage drop across this resistor and the exposed steel surface area. In addition, by sizing the resistor in the range of 75-8,000 V, values of practical interest for the initial current density and the extent of cathodic polarization were realized. In effect, this provided for simulation of anode-cathode surface area ratios, relevant to those on actual offshore structures. Water temperature for these tests ranged from 24 to 268C. The shallow-deep water alternating cycle (cyclic hydrostatic pressure) was simulated by immersing the CP system alternately for 16 h at atmospheric pressure and 8 h at a pressure of 3.5 MPa.

2.4 Microstructure observation The macroscopic morphologies of cathode and anode were observed using a digital camera, while the microscopic morphologies of the anode with and without corrosion products were observed by scanning electron microscopy (SEM S-400).

3. Results 3.1 Effect of cyclic hydrostatic pressure on the anodic process of the CP system The anodic potential measured under hydrostatic pressure presented in Figure 3 shows distinct differences from that measured at atmospheric pressure. First, the response of anode potential at high-hydrostatic pressure was almost totally instantaneous, with the potential shifting towards more positive values. Moreover, there is a distinct relaxation process with a characteristic time during the pressurization period (Figure 3(b)). Second, the amplitude of anodic shift increased from 20 to 146 mV with the number of cycles. Figure 4 shows representative E-I decay plots from experiments covering the range of external resistances investigated and for an exposure time of 768 h. The anodic polarization curves shifted in the noble direction with increasing immersion time for pressurized and unpressurized systems. However, the amplitude of positive shift of anodic polarization curve under cyclic hydrostatic pressure conditions was about 40 mV, compared to that at atmospheric pressure (, 10 mV). After immersion for 768 h, the anode surface was visually examined and images obtained using a digital camera (Figure 5). The anode immersed at atmospheric pressure was covered by a white powdery and flaky corrosion product, which

Figure 1 The geometry of zinc-bismuth sacrificial anode 13

25

Ø45

239




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Figure 3 The potential of anode at cyclic hydrostatic pressure (0-3.5 MPa) and atmospheric pressure during the immersion of 768 h

Figure 4 The polarization curve of anode at (a) atmospheric and (b) cyclic hydrostatic pressure at various immersion time –0.96

Atomspheric Cyclic

96 h 192 h 384 h 720 h

–0.97 –0.98

–0.84 E (VAg/AgCl)

Anode potential (VAg/AgCl)

–0.80

–0.88

–0.99 –1.00

–0.92 –1.01 –0.96

–1.02 0

200

400 Time (h)

600

–1.03

800

0.1

1 i (A) (a)

(a) –0.96 –0.86 –0.88

–0.97 –0.98

3.5M Pa E (VAg/AgCl)

Anode potential (VAg/AgCl)

3.5M Pa

Atomspheric Cyclic

–0.90 –0.92

0.1M Pa

0.1M Pa

0.1M Pa

–0.99 –1.00 –1.01

–0.94

96 h 192 h 720 h

–1.02 540

560

580

–1.03

600

0.1

1

Time (h)

i (A)

(b)

(b)

Notes: (a) Small magnification; (b) large magnification

seen that the slope parameter in the condition of cyclic hydrostatic pressure was higher than that at atmospheric pressure (Figure 10). Moreover, after 768 h of immersion, the morphology of cathode (Ni-Cr-Mo-V steel) was visually observed and captured by digital camera (Figure 11). Figure 11 shows that the cathode surface under cyclic hydrostatic pressure was severely corroded and covered by a thick layer of rust, whereas that at atmospheric pressure was only slightly corroded, which were all in agreement with the electrochemical results.

is evidence of active uniform dissolution of the anode surface (Figure 5(a)). However, under cyclic hydrostatic pressure, two typical morphologies were observed: a whitish region covered by flocculent corrosion product, and a grayish region covered by melanocratic flakes, which was difficult to be striped even by scratching with a knife (Figure 5(b)). In order to obtain more detailed information, the anode surface was further examined by scanning electron microscopy (Figure 6). The corrosion product formed under cyclic hydrostatic pressure was more compact than that formed at atmospheric pressure.

4. Discussion

3.2 Effect of cyclic hydrostatic pressure on the electrochemical properties of CP system The driving potential and circuit current of the CP system for the specific case of Rx ¼ 75 V is shown in Figure 7 and the statistical results shown in Figure 8. The driving potential seems to be independent of the cyclic hydrostatic pressure, while the circuit current was lower than that at atmospheric pressure. The relationship between the driving potential and circuit current is shown in Figure 9. The plots exhibit linear behavior and the slope parameters were calculated. It can be

The anodic potential results (Figure 3) which had instantaneous response with pressure indicate that the cyclic hydrostatic pressure significantly influenced the potential of anode. The anodic shift of polarization curves (Figure 4) implies that a corrosion product film was formed on the anode surface, which inhibited the anodic dissolution process. The higher amplitude of anodic shift revealed that the cyclic hydrostatic pressure diminished the discharge performance of the anode. The corrosion product which formed on the anode 240




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Figure 5 The macro-morphology of anode at (a) atmospheric and (b) cyclic hydrostatic pressure after the immersion of 768 h

Figure 6 The SEM morphology of anode at (a) atmospheric and (b) cyclic hydrostatic pressure after the immersion of 768 h 2

100 µm

5 mm (a)

(a) 1

100 µm

5 mm (b)

(b)

surface (Figures 5 and 6) might be the reason why the discharge performance decreased in case of cyclic hydrostatic pressure. As higher slope parameter in the condition of cyclic hydrostatic pressure as shown in Figure 10. A simplified schematic diagram (Figure 12) was drawn to elucidate the effect of slope parameter on the protective performance of the CP system, particularly the reason why the protective performance of CP system decreased in the condition of cyclic hydrostatic pressure. For atmospheric pressure, the slope followed the curve A-B, which implies that the cathode was protected at the potential A. For cyclic hydrostatic pressure, the slope followed curve C-D. As a result, the cathode was protected at the potential C, which is higher than that at atmospheric pressure. Usually, the fundamental criterion for marine CP is that the structure of interest be polarized to a potential more negative than 2 0.80 VAg/AgCl, for the proposed service duration. Therefore, the higher CP potential suggests that the potential was probably more positive than 20.80 VAg/AgCl, which implies that the cathode might not provide adequate protection. This point is further

confirmed by the trend of the cathode potential (Figure 13). In the condition of cyclic hydrostatic pressure, the cathode potential data fluctuate around 2 0.80 VAg/AgCl, while the data at atmospheric pressure remained at the lower level (about 2 0.90 VAg/AgCl). So, the anode was serious corroded at cyclic hydrostatic pressure.

5. Conclusions The cyclic hydrostatic pressure had significant influence on the CP system. The anode potential instantaneously responded the cyclic hydrostatic pressure, with the anodic potential shifting towards the positive direction due to the deposition of corrosion product, which indicates decreased discharge performance. The amplitude of anodic shift increased with the cycle number. Cyclic hydrostatic pressure decreased the circuit current of the CP system compared to that at atmospheric pressure, but had no obvious influence on the driving potential. At the cyclic hydrostatic pressure, the CP 241




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Figure 7 The (a) driving potential and (b) circuit potential of CP system at atmospheric and cyclic hydrostatic pressure during the immersion of 768 h 0.09 0.9

Atmospheric Cyclic

0.08

Atmospheric Cyclic

0.8

I (A)

E (VAg/AgCl)

0.07 0.06

0.7 0.6

0.05 0.5 0.04 0.4 0.03

0

200

400 Time (h) (a)

600

0

800

200

400 Time (h) (b)

600

800

Figure 8 The statistic results of Figure 7 0.30 0.30

Atmospheric Cyclic

0.25

0.25

0.20

0.20

Probability

Probabilty

Atmospheric Cyclic

0.15 0.10

0.15 0.10

0.05

0.05

0.00 0.04

0.05

0.06 E (VAg/AgCl)

0.07

0.00 0.3

0.08

0.4

0.5

(a)

0.6 I (A)

0.7

0.8

0.9

(b)

Notes: (a) Driving potential; (b) circuit potential

Figure 9 The relationship of Ec-Ic of CP system at atmospheric and cyclic hydrostatic pressure at various immersion time 0.08

0.07

96 h 192 h 288 h 384 h 528 h 624 h 720 h

0.05

0.04 0.4

0.5

0.6 I (A)

0.7

∆E (V)

∆E (V)

0.07 0.06

0.06

96 h 192 h 288 h 384 h 528 h 624 h 720 h

0.05

0.04

0.8

0.4

(a)

0.5

0.6 I (A) (b)

242

0.7

0.8




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Figure 10 The slope parameter of CP system at atmospheric and cyclic hydrostatic pressure during the immersion period 0.11 0.10

Slope

0.09 0.08 0.07 0.06

Atmospheric Cyclic

0.05 0

200

400 Time (h)

600

800

Figure 11 Schematic illustration of the relationship between slope parameter and cathodic polarization curve of cathode at various hydrostatic pressures

10 mm

10 mm

(a)

(b)

Figure 12 The potential of cathode at cyclic hydrostatic pressure and atmospheric pressure during the immersion period

Cyclic

Cathode polarization curve E (VAg/AgC l)

C

Atmospheric

D A B

I (A)

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Figure 13 The macro-morphology of cathode at (a) cyclic hydrostatic pressure and (b) atmospheric after the immersion of 768 h

Chen, S., Hartt, W.H. and Wolfson, W. (2002), “Deepwater cathodic protection: Part 1 – laboratory simulation experiments”, Corrosion, Vol. 58 No. 1, pp. 38-48. Chen, S., Hartt, W.H. and Wolfson, W. (2003), “Deepwater cathodic protection: Part 2 – field deployment results”, Corrosion, Vol. 59 No. 8, pp. 721-32. Fischer, K.P. and Finnegan, J.E. (1987), “Cathodic protection behavior of steel in seawater and the protective properties of the calcareous deposits”, Corrosion, Vol. 89, p. 582. Fischer, K.P., Sydberger, T. and Lye, R. (1987), “Field testing of deep water cathodic protection on the Norwegian continental shelf”, Corrosion, Vol. 87, p. 67. Kennelley, K.J. and Mateer, M.W. (1987), “Evaluation of the performance of bimetallic anodes on deep-water production platform”, Corrosion, Vol. 93, p. 523. NACE (1992), “Cathodic protection design considerations for deep water structures”, Publication No. 7L-192, NACE, Houston, TX. Wang, W., Hartt, W.H. and Chen, S. (1996), “Sacrificial anode cathodic polarization of steel in seawater: Part 1 – a novel experimental and analysis methodology”, Corrosion, Vol. 52 No. 6, pp. 419-27.

–0.72

Cathode potential (VAg/AgCl)

Atmospheric Cyclic –0.76

–0.80

–0.84

–0.88

–0.92 0

200

400 Time (h)

600

800

system exhibited higher slope parameter, which indicates that the cathode provides inadequate protection.

References Chen, S., Hartt, W.H. and Townley, D. (1998), “Sacrificial anode cathodic polarization of steel in seawater: Part 2 – design and data analysis”, Corrosion, Vol. 54 No. 4, pp. 317-22.

Corresponding author Tao Zhang can be contacted at: [email protected]

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