Study of sacrificial cathodic protection on marine structures in sea and fresh water in relation to flow conditions

This art icle was downloaded by: [ M. Shehadeh] On: 08 Sept em ber 2013, At : 04: 28 Publisher: Taylor & Francis I nform a Lt d Regist ered in England and Wales Regist ered Num ber: 1072954 Regist ered office: Mort im er House, 37- 41 Mort im er St reet , London W1T 3JH, UK Ships and Offshore Structures Publicat ion det ails, including inst ruct ions f or aut hors and subscript ion inf ormat ion: ht t p: / / www. t andf online. com/ loi/ t sos20 Study of sacrificial cathodic protection on marine structures in sea and fresh water in relation to flow conditions M. Shehadeh a & I. Hassan b a Marine Engineering Depart ment , Facult y of Engineering, Arab Academy f or Science and Technology, Alexandria, Egypt b Basic Science (Chemical Engineering) Depart ment , Facult y of Engineering, Arab Academy f or Science and Technology, Alexandria, Egypt Published online: 27 Jun 2011. To cite this article: M. Shehadeh & I. Hassan (2013) St udy of sacrif icial cat hodic prot ect ion on marine st ruct ures in sea and f resh wat er in relat ion t o f low condit ions, Ships and Of f shore St ruct ures, 8: 1, 102-110, DOI: 10. 1080/ 17445302. 2011. 590694 To link to this article: ht t p: / / dx. doi. org/ 10. 1080/ 17445302. 2011. 590694 PLEASE SCROLL DOWN FOR ARTI CLE Taylor & Francis m akes every effort t o ensure t he accuracy of all t he inform at ion ( t he “ Cont ent ” ) cont ained in t he publicat ions on our plat form . However, Taylor & Francis, our agent s, and our licensors m ake no represent at ions or warrant ies what soever as t o t he accuracy, com plet eness, or suit abilit y for any purpose of t he Cont ent . Any opinions and views expressed in t his publicat ion are t he opinions and views of t he aut hors, and are not t he views of or endorsed by Taylor & Francis. 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Term s & Condit ions of access and use can be found at ht t p: / / www.t andfonline.com / page/ t erm s- and- condit ions Ships and Offshore Structures, 2013 Vol. 8, No. 1, 102–110, http://dx.doi.org/17445302.2011.590694 Study of sacrificial cathodic protection on marine structures in sea and fresh water in relation to flow conditions M. Shehadeha∗ and I. Hassanb a Marine Engineering Department, Faculty of Engineering, Arab Academy for Science and Technology, Alexandria, Egypt; bBasic Science (Chemical Engineering) Department, Faculty of Engineering, Arab Academy for Science and Technology, Alexandria, Egypt Downloaded by [M. Shehadeh] at 04:28 08 September 2013 (Received 15 December 2010; final version received 12 May 2011) The extent of corrosion between a miniature steel ship model at different relative velocities of sea water fitted with commercial sacrificial anodes of aluminium (Al) in sea water and magnesium (Mg) in fresh water is studied. The experimental results show that the degree of cathodic protection represented by the cathode potential at a given distance from different types of sacrificial anodes decreases slightly with an increase in the relative velocity. The results also show that the amount of Al or Mg required to protect the ship cathodically increases as the relative velocity increases. Keywords: marine structures; cathodic protection; sacrificial anode; relative velocity 1. Introduction Corrosion is defined as the deterioration of a material, usually a metal, because of a reaction with its environment and which requires the presence of an anode, a cathode and an electrolyte solution (Rogers 1974; Laque 1975; ShiXer 2005). Cathodic protection is used widely to protect steel marine structures against sea water corrosion (e.g. ships, pipelines, offshore structures, etc.). Sea water is an aggressive environment on marine vessels and offshore steel structures because of its high electrical conductivity along with its high oxygen content. Aerated sea water attacks steel through the formation of different types of corrosion cells such as dissimilar metal corrosion and differential aeration cells. Dissimilar metal cells arise because of the contact between iron grains and the nobler cementite grains (Fe3 C) (Rogers 1974; Laque 1975). For example, dissimilar metal cells (bimetallic corrosion) arise as a result of the contact between the nobler bronze propeller and the less noble steel ship hull. Differential aeration cells arise when part of the marine structure is coated with marine fouling organisms that hinder dissolved oxygen access to the underlying steel. Moreover, the steel marine structure may suffer from differential strain cells at weld lines, stray current corrosion, during welding and bacterial corrosion in polluted harbours (Boronstein 1994). Despite the importance of vessel speed and the degree of turbulence on the rate of ship corrosion (Melchers 2003; Melchers and Jeffrey 2004; Guedes-Soares et al. 2006; Gudze and Mechlers 2008), little work has been done to quantify this effect, especially the effect of the relative velocity between the structure and sea/fresh water on ∗ Corresponding author. Email: ezzfahmy@aast.edu  C 2013 Taylor & Francis the performance of cathodic protection. Sedahmed et al. (2010) have studied experimentally the effect of relative velocity on the zinc (Zn) anode consumption using a ship model. They noted that the amount of Zn consumed in protecting the ship cathodically depends on the relative motion between the marine vessel and sea water; the higher the relative velocity, the higher is the rate of Zn consumption and the lower the cathode potential, the lower is the degree of cathodic protection. This paper is concerned with the effect of relative velocity on the rate of consumption of sacrificial anodes made of magnesium (Mg) in fresh water and aluminium (Al) in sea water. The study is also concerned with the potential distribution during sacrificial cathodic protection at different relative velocities of the ship model. The difference in behaviour between the commonly used anode materials used in sacrificial cathodic protection (Zn, Al and Mg) is also discussed. Al and Mg differ from Zn by the fact that they produce higher electromotive force with steel; this fact along with the fact that the three metals have different tendencies to passivate at higher current densities makes this work necessary to differentiate between the behaviours of the three metals. Steel corrosion takes place through the following reactions regardless of the type of the corrosion cell: Anode : Cathode : Or : Fe → Fe++ +2e− . 2H+ + 2e− → H2 (pH < 4). O2 + 2H2 O + 4e− → 4OH− (pH 4 − 10). Downloaded by [M. Shehadeh] at 04:28 08 September 2013 Ships and Offshore Structures It is well known that the rate of steel corrosion in the pH range 4–10 is affected by the rate of diffusion of dissolved oxygen to the cathodic sites of corrosion cells. Since the pH of sea water is almost natural in sea water (pH ≈ 7), O2 reduction is the prevalent cathodic reaction. The combination of Fe++ and OH− ions eventually leads to the formation of ferrous hydroxide, which undergoes further oxidation by dissolved oxygen to x-FeOOH porous iron rust where x may be alpha, beta and gamma (Melchers 2003). The formation of porous iron rust on the steel surface does not prevent corrosion but reduces its rate as a result of decreasing the diffusivity of dissolved oxygen through the rust. Accordingly, measures such as cathodic protection should be taken to protect steel from further corrosion. Sacrificial cathodic protection is carried out by connecting the marine structures to less noble metals, such as Zn, Mg or Al, while impressed current cathodic protection is carried out by connecting the structure to the negative pole of a direct current (DC) power supply; an insoluble anode such as platinised titanium, graphite- or oxide-coated titanium connected to the positive pole of the DC power supply completes the circuit (Guedes-Soares et al. 2006). However, the common method to protect the marine structures in sea/fresh water is by coupling the structure to a less noble metal (forming a galvanic cell; Table 1). In industry, the second method is most widely used with a galvanic cell of Al-, Zn- or Mg-based alloys (Sedahmed et al. 2010). It should be noted that Mg has the highest driving voltage among the materials used for sacrificial anodes. Therefore, 103 Table 1. The galvanic series in flowing sea water. Metal Mg Zn Al alloys Cadmium Cast iron Steel Voltage range of metal vs. reference electrode (mV) −1.60 to −1.63 −0.98 to −1.03 −0.70 to −0.90 −0.70 to −0.76 −0.60 to −0.72 −0.60 to −0.70 Corrosion tendency increasing  ⏐ ⏐ ⏐ ⏐ ⏐ More active or less noble Mg is most commonly used as a sacrificial anode in onshore or fresh water (i.e. higher resistivity electrolytes) where Zn or Al is used in sea water. Many researchers (e.g. Ashworth and Booker 1986; Florian 1987; Guedes-Soares et al. 2006; Radosevic et al. 2007) have studied different aspects of cathodic protection by using Zn and Al on different marine systems. These studies have discussed the performance of different anode materials, the effect of organic coatings on cathodic protection, cathodic protection problems such as overprotection and stray current and mathematical modelling of cathodic protection (Ashraf and Shibli 2007; Benedetti et al. 2009). 2. Experimental procedure Figure 1 shows the schematic apparatus used in the present work; it mainly consisted of a horizontal rectangular open Figure 1. A schematic diagram of the apparatus used in the experiments. 104 M. Shehadeh and I. Hassan Downloaded by [M. Shehadeh] at 04:28 08 September 2013 Figure 2. Potential measuring points on ship hull model in axial and vertical direction from Al/Mg coupons. plastic channel having the dimensions 60 × 30 cm for the base and 50 cm height. The plastic channel was connected to a plastic storage tank having a base of 80 × 60 cm and a height of 40 cm filled with 150 m3 of sea water (3.5% NaCl concentration). A stainless steel centrifugal pump of 1.5 hp was used to circulate sea water between the storage tank and the channel. A miniature steel ship model, shown in Figure 1, was fixed firmly in the middle of the flow channel at a distance of 32 cm between the ship bottom and the channel base. The steel ship model had a length of 34 cm, a maximum width of 13 cm and a height of 10.5 cm (Figure 2). During the experiments, a height of 8 cm of the ship was submerged in sea water. The solution flow rate from the storage tank to the flow channel was controlled by means of a ball valve and the volumetric flow rate was measured by means of a graduated cylinder and a stopwatch. Two different experiments have been run to determine, firstly, the rate of Al consumption used to protect the ship model against corrosion in sea water and, secondly, the rate of Mg consumption used to protect the ship model against corrosion in fresh water. In each experiment, a rectangular thin coupon measuring for Mg (33 × 12 × 1.5) mm and Al (36 × 12 × 1.8) mm was fixed by a steel screw thread to the middle of the ship hull in the submerged zone, as shown in Figure 2. Before each run, the sacrificial anode coupon was weighted accurately using an analytical balance while the steel ship model was treated with emery paper to remove any oxides from the ship surface. The sacrificial anode coupon was fixed to the ship model and then the solution was allowed to flow from the storage tank to the channel containing the ship at the required flow rate for a period of 1 hour. At the end of the experiment, the solution flow was stopped; the Al/Mg coupon was separated from the ship surface, dried with a soft tissue paper and the final weight was determined. To study the cathode potential distribution and the degree of cathodic protection at different distances from Al or Mg coupons in the horizontal and vertical directions, the potential difference between the ship hull at different locations and a Al/AlCl electrode was measured by means of a high impedance voltmeter, as shown in Figure 3. The reference electrode was placed in the cup of a glass lugging tube whose tip was placed at a distance of 0.5 mm from the location whose potential is to be measured. The lugging tube was filled with sea water (Jones 1992). Table 2 shows the analyses of two sacrificial anodes used in the present practical cathodic protection study (Bridges 1982; Morgan 1987). 3. Results and discussions Figures 4 and 5 show the effect of sea/fresh water velocity on the rate of Al or Mg anode consumption. The rate of Al or Mg consumption (R) during the Table 2. Sacrificial anode materials and their properties used in the present study. Property Al alloy Mg alloy Percentage composition Cu: <0.006 Fe: <0.1 Hg: 0.02–0.05 Si: 0.11–0.21 Zn: 0.3–0.5 Others: each <0.02 Al: remainder Density (kg/m3) Capacity (A·h/kg) Wastage by weight (kg·A·h/year) Wastage by volume (ml·A/year) Output (A/m2) Voltage range of metal vs. potential (relative to Ag/AgCl) (mV) 2695 2640 3.2 Al: <0.01 Cu: 0.02 Fe: <0.03 Mn: 0.5–1.3 Pb: <0.01 Ni: <0.001 Zn: <0.01 Mg: remainder 1765 1232 4.1 1180 2296 6.5 −700 to −900 10.8 −1600 to −1630 Downloaded by [M. Shehadeh] at 04:28 08 September 2013 Ships and Offshore Structures 105 Figure 3. A schematic diagram of the electrical circuit. cathodic protection of the ship was calculated from the formula: The solution velocity, V in cm/s, was obtained by dividing the volumetric flow rate by the wetted cross-sectional area of the flow channel; ρ, µ and L are the density and viscosity of sea/fresh water in gm/cm3 and the length of the model ship, respectively. Tables 3 and 4 show that the rate of Mg and Al anode consumptions in protecting the ship hull increases with increasing the relative velocity between the ship hull and the solution. From Figures 4 and 5, the results data can be distinguished into two regions: the first region is for relative velocities less than 45 cm/min in which the rate of metal consumption increases slightly with increasing the relative velocity, and the second region is for relative velocities more than 45 cm/min in which the rate of metal consumption increases considerably with increasing the relative velocity. The results data can be correlated as follows: Table 3. Effect of the solution velocity on the rate of Al consumption. Table 4. Effect of the solution velocity on the rate of Mg consumption. R= loss in weight (m) g/cm2 · min . coupon area × time (1) Localised Reynolds number (Re) will be considered to present the condition of the service for the model ship and can be calculated as follows: Re = Exp. no. 1 2 3 4 5 6 7 ρV L . µ (2) V (cm/min) Re Loss in weight (g) R (g/cm2·min) 31.5 37.5 45 65 72.5 80 85 1762 2098 2518 3637 4057 4477 4756 0.067 0.071 0.080 0.118 0.126 0.146 0.179 2.3 × 10−4 2.54 × 10−4 2.86 × 10−4 4.23 × 10−4 4.51 × 10−4 5.2 × 10−4 6.4 × 10−4 Exp. no. 1 2 3 4 5 6 7 V (cm/min) Re Loss in weight (g) R (g/cm2·min) 31.5 37.5 45 65 72.5 80 85 1783 2178 2904 3514 4153 4502 4647 0.0083 0.0099 0.0105 0.016 0.0188 0.0195 0.022 4.25 × 10−5 5.07 × 10−5 5.4 × 10−5 8.6 × 10−5 9.69 × 10−5 1 × 10−4 1.15 × 10−4 Downloaded by [M. Shehadeh] at 04:28 08 September 2013 106 M. Shehadeh and I. Hassan Figure 4. The effect of sea water velocity on the rate of Al anode consumption. (This figure is available in color online.) (1) For Al anode, Figure 4 shows that data fit the equations: (a) For V < 45 cm/min, R = 4.149 × 10−5 × V 0.5046 . (a) For V < 45 cm/min, (3) The average deviation is ±0.581%. (b) For V > 45 cm/min, R = 2.37 × 10−6 × V 1.463 . The average deviation is ±0.0068%. (2) For Mg anode, Figure 5 shows that data fit the equations: R = 8.89 × 10−6 × V 0.468 . (5) The average deviation is ±0.063%. (b) For V > 45 cm/min, (4) R = 2.448 × 10−6 × V 0.866 . The average deviation is ±0.0082%. Figure 5. The effect of sea water velocity on the rate of Mg anode consumption. (This figure is available in color online.) (6) Downloaded by [M. Shehadeh] at 04:28 08 September 2013 Ships and Offshore Structures 107 Figure 6. Al potential distribution in the horizontal direction (X) of the ship model versus Ag/AgCl reference electrode at different relative velocities. (This figure is available in color online.) The increase in the rate of Al or Mg consumption with the solution velocity may be explained as follows. Sacrificial cathodic protection of the steel ship hull with Al or Mg anode takes place through the galvanic cell of Al/sea water/steel, and Mg/fresh water/steel. The cell reactions are as follows: Anode (Al) : Anode (Mg) : Al → Al+++ + 3e . Mg → Mg++ + 2e . Cathode (steel) : 1/2 O2 + H2 O + 2e → 2OH− . Kinetic studies of the above reactions have revealed that galvanic corrosion of Al and Mg in the above cell is controlled by the cathodic reduction of dissolved oxygen at the steel surface (Jones 1992), and the slowness of the cathodic reduction of dissolved oxygen is attributed to the slowness of the diffusion of dissolved oxygen from the solution bulk to the cathode (steel) surface across the Figure 7. Mg potential distribution in the horizontal direction (X) of the ship model versus Ag/AgCl reference electrode at different relative velocities. (This figure is available in color online.) Downloaded by [M. Shehadeh] at 04:28 08 September 2013 108 M. Shehadeh and I. Hassan Figure 8. Al potential distribution in the horizontal direction (Y) of the ship model versus Ag/AgCl reference electrode at different relative velocities. (This figure is available in color online.) diffusion layer surrounding the ship hull (Incropera and DeWitt 1990). Increasing the solution velocity past the ship hull decreases the thickness of the hydrodynamic boundary layer and the diffusion layer (Kennelly et al. 1991) with a consequent increase in the mass transfer coefficient K (k = D/δ) and dissolved oxygen flux N according to the equation: N = K Co2 , (7) where N is the flux of dissolved oxygen in mol/cm2. s, K is the mass transfer coefficient (cm/s), Co2 is the concentration of dissolved oxygen in sea water in mol/cm3 and D is the diffusivity of dissolved oxygen in cm2/s. The considerable increase in the rate of Al and Mg consumptions for Re > 3500 may be attributed to the transfer of flow around the ship model from the laminar flow regime to the turbulent flow regime (Kennelly et al. 1991). It is noteworthy that the mass transfer coefficient K used in Equation (7) is an overall coefficient given by the equation (Zaharan Figure 9. Mg potential distribution in the horizontal direction (Y) of the ship model versus Ag/AgCl reference electrode at different relative velocities. (This figure is available in color online.) Ships and Offshore Structures et al. 2006): 4. Downloaded by [M. Shehadeh] at 04:28 08 September 2013 1 1 1 = + , K K1 K2 (8) where K 1 is the liquid phase mass transfer coefficient and K 2 is the mass transfer coefficient due to the porous oxide film formed during corrosion. The magnitude of K 2 depends on the film thickness and its porosity, i.e. the relative velocity affects only K 1 . For the uncorroded steel surface that is protected by cathodic protection, the term 1/K 2 can be neglected, in this case K = K 1 = D/δ, i.e. the rate of corrosion is mainly determined by the diffusion of dissolved oxygen across the liquid phase diffusion layer. The results data shown in Figures 6 and 7 indicate that for a given solution velocity, the cathode potential and the degree of cathodic protection decrease slightly with increasing the distance in both directions of horizontal and vertical distances from the sacrificial anode. At ship hull locations near the coupon, the influence of cathodic protection cell dominates that of the corrosion cells; as a result, the neighbouring steel acts as a cathode, which is fully protected against corrosion. As the distance increases from the anode, the influence of the cathodic protection cell decreases while the influence of the corrosion cells increases, i.e. the degree of cathodic protection decreases gradually with distance until it is lost at sufficiently large distances from the coupon. The decrease in the cathode potential with distance from the anode is attributed mainly to the voltage drop (IR) resulting from the solution resistance (Rs ) between the cathode and the anode. The solution resistance Rs is given by Rs = σ l , as (9) where σ is the specific resistance of sea or fresh water in (ohm·cm), l is the cathode–anode separation distance in cm and as is the cross-sectional area of the solution between the anode and the cathode in cm2. The decrease in the cathode potential in X and Y directions at any distance from the anode with increasing the solution velocity may be explained by the increase in current I between the steel and Mg or Al as a result of the increase in the rate of diffusion-controlled reduction of dissolved oxygen (I/ZF = KO2 ). The increase in the current I increases the IR drop between the anode and the cathode with a consequent decrease in the cathode potential. Figures 8 and 9 also show that for a given distance from the sacrificial anodes, the cathode potential and the degree of cathodic protection, as indicated by the cathode potential, decreases slightly with increasing the solution velocity in both directions of horizontal and vertical distances from the sacrificial anode. This result can be explained by reasoning similar to the aforementioned explanation. 109 Conclusion The increase of the relative velocity between the marine vessels and sea/fresh water increases the amount of anode required to protect the vessel against corrosion. Also, increasing the relative velocity decreases the cathode potential and the degree of cathodic protection at locations far from the sacrificial anode. It is therefore necessary to distribute the sacrificial anodes uniformly all over the ship to obtain uniform cathodic protection. The present results are consistent with the finding of Sedahmed et al. (2010) who used Zn anodes in their studies. Although these results apply qualitatively to large ships and higher relative velocities, the present data cannot be extrapolated to predict quantitatively the rate of Al and Mg consumptions in the case of large ships and high relative velocities. Large ships and high relative velocities need further experimental work to determine the rate of Al and Mg consumptions. Finally, the conclusion of the present study is that it alerts the cathodic protection designer to consider the effect of the relative velocity in his design. References Ashraf PM, Shibli SMA. 2007. Reinforcing aluminum with cerium oxide: a new and effective technique to prevent corrosion in marine environments. Electrochem Commun. 9:443–448. Ashworth V, Booker CJ. 1986. Cathodic protection theory and practice. Chichester (UK): Eills Herwood. Benedetti A, Magagnin L, Passaretti F, Chelossi E, Faimali M, Montesperell G. 2009. Cathodic protection of carbon steel in natural seawater: effect of sunlight radiation. Electrochim Acta 54:6472–6478. Boronstein SW. 1994. Microbiologically influenced corrosion handbook. New York (NY): Industrial Press. Bridges JM. 1982. Code of practice for the design, installation and operation of cathodic protection systems in ships. 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