Modelling stray current interference to shipboard cathodic protection system

Transcription

1 Paper No Modelling stray current interference to shipboard cathodic protection system Yueping Wang Defence Research and Development Canada Atlantic Research Centre CFB Halifax, Bldg D-2 P.O. Box 99 Stn Forces Halifax, Nova Scotia, B3K 5X5 Canada Ken J. KarisAllen FACTS Engineering Inc. P.O. Box 239 Halifax, Nova Scotia, B3R 2K9 Canada ABSTRACT Cathodic protection is commonly used for corrosion protection of the underwater hull structures of naval platforms. Cathodic protection works by establishing an electrostatic field that provides the desired potential and current distributions in the seawater surrounding the structures to be protected. Interference to a ship s cathodic protection system may occur when the ship is alongside a jetty or another naval ship due to the influence of the electrostatic field associated with the cathodic protection system employed on the jetty or another naval ship. The interference may alter the electrostatic field surrounding the ship and, therefore, the level of corrosion protection to the ship hull. In the current study, a physical scale modelling (PSM technique is used to evaluate how, and to what extent, the level of cathodic protection on a ship hull could be affected by an external current source. A 1/1 scale ship model with two-zone four-anode impressed current cathodic protection (ICCP system is used in the study. The modelling results indicate that the presence of stray current may affect the potential distribution along the ship hull. The extent of stray current effect depends on a number of factors including the relative position and layout of stray current source, the magnitude and direction of the stray current flow, and the operating mode of the shipboard ICCP system. Key words: corrosion, stray current, cathodic protection, impressed current, seawater, ships, physical scale modeling INTRODUCTION Cathodic protection (CP, either using sacrificial anodes or impressed current, is commonly used for corrosion protection of the underwater hull structures of naval platforms. Cathodic protection works by

2 establishing an electrostatic field that provides the desired potential and current distributions in the seawater surrounding the structures to be protected. In the case of impressed current cathodic protection (ICCP, a direct current (DC feedback system is used to supply current from the ICCP anodes to the underwater hull and other submerged appendages to maintain the electric potential of the structures at a pre-determined level. In the case of sacrificial anode cathodic protection, the current flows from sacrificial anodes to the underwater hull and other appendages as a result of electric potential differences between the anodes and the cathodes. In both cases, the current flows through the seawater surrounding the ship hull in order to close the electric circuit. Stray current interference to a ship s CP system may occur when there are external current sources and/or unprotected metal structures in the vicinity of the ship hull. One such case is when a ship is alongside a jetty that is constructed of steel piles. While ideally all the steel piles are protected by the jetty CP system, electric discontinuities may exist between adjacent piles. A discontinuous steel pile, when located near the anodes and/or the hull structures, will present a low resistance path and cause the current to enter the steel pile over an area closest to the anode and to leave the pile over an area that has the lowest resistance path to the hull structure. The area where the current enters the pile is cathodically protected while the area where the current leaves the pile is subject to stray current corrosion. Even under circumstances where all the steel piles are adequately protected by the jetty CP system, the flow of the electric current associated with the jetty CP system may alter the electrostatic field in the vicinity of the ship hull causing over-protection in some areas of the hull structure and underprotection in other areas. Similarly, when two ships are in close proximity to one another, the fields established by the respective CP systems may interact causing possible stray current issues which, in turn, alter the potential distributions over both structures. In this situation, the extent of the stray current interaction may be dependent on many factors, including the type of CP systems (sacrificial anode or impressed current, zoning and placement of anodes and reference s in the ICCP systems, current output from each CP system and relative position of the two ships with respect to each other. Numerical modeling techniques, such as the boundary element method, have been used to study stray current interference issues associated with shipboard cathodic protection systems [1, 2]. Stray current interaction of ship and dock cathodic protection systems has also been assessed based on the site survey potential data associated with the dock cathodic protection system [3]. An alternate technique used for the design and evaluation of shipboard ICCP systems is physical scale modeling (PSM. PSM is a methodology in which a large structure, subject to cathodic protection, is physically modelled by maintaining a linear relationship between the model size and the conductivity of the electrolyte. In this way, the resistance path for current flow through the electrolyte for the model is maintained similar to that for the full-scale structure. The potential difference at any two geometrically scaled points on the model hull is the same as that at the two corresponding points on the ship hull being modeled. The relationships for other parameters between the model and full size ship in an ICCP PSM study are presented in Table 1. PSM has been used in some NATO countries to evaluate and design shipboard ICCP systems [4, 5]. The technique has also been used for the verification and validation of numerical modeling results for the evaluation of a shipboard ICCP system [6]. One issue that affected the accurate modelling of shipboard ICCP systems using PSM techniques was the altered polarization behavior of propeller and hull materials in the seawater with reduced conductivity due to the change in the conditioning film on the materials. A novel approach was developed to address the issue [6]. This approach used a current control technique to simulate the polarization behavior of a propeller material (cathode that was obtained in undiluted seawater under static or dynamic conditions. Experimental results have demonstrated that the current control technique is capable of controlling the current to the cathode so that all potential-current density data points during the PSM testing conform to the polarization curve relationship assigned to the cathode [7].

3 Table 1. Relationship between a model and a full scale ship in an ICCP PSM study Parameter Relationship Length l(ship l( model k Area AC (ship AC (model 2 k Conductivity (ship (model k Current density i i Current C( model C(ship I IC(model k * k is the linear scaling factor C(ship 2 In the present study, the PSM technique was used to evaluate the interference of external current sources with respect to shipboard ICCP system performance. This paper describes the setup of the physical scale model and presents the PSM results on the performance of a shipboard ICCP system in the presence of external DC dipole sources. The factors studied include ICCP operating mode (singlezone or two-zone system, layout of external dipole source, and the magnitude of the external current. EXPERIMENTAL PROCEDURE Physical scale model A 1/1 scale model based on a naval vessel shipboard ICCP system was used for the current PSM study. The model was fabricated from PVC and incorporated most underwater appendages including nickel aluminum bronze (NAB propellers, shafts, A-brackets, rudder, and stabilizers. The two-zone ICCP system used onboard the naval vessel was modeled in the PSM study. The system was configured with two independently controlled zones. Each zone had its own anodes and reference (as shown in Figure 1. Each zone also consisted of a dedicated DC power supply and feedback control unit. Two NAB propellers were used in the model with the exposed surface area of the propellers appropriately scaled. The two propellers were the only cathodes considered in the current PSM study. Nineteen Ag/AgCl reference s (as shown in Figure 1 were used to obtain a detailed potential profile along the model hull. Figure 1: Schematic of positions of anodes, reference s and NAB propellers

4 The tank used for the PSM trial was 2.8 m long, 1.1 m wide and.9 m deep. The conductivity of naturally occurring seawater that was referenced in the PSM study was.39 S/cm. Accordingly, the seawater in the model tank was diluted so that the conductivity of the electrolyte was maintained at.39 S/cm for the 1/1 scale ICCP model. Discrete area current control technique A discrete area current control (DACC technique was used to impose a pre-determined potentialcurrent density relationship to the NAB propellers. The detailed description of the DACC techniques is presented elsewhere [6, 7]. In brief, the major component of the current controller was a variable voltage source inserted between the cathode and the earth ground reference. A reference (not shown in Figure 1 was also used to provide feedback potentials on the cathode to the current controller. It has been demonstrated that the current control system is capable of altering the current flowing into a cathode so that it can simulate the polarization curve relationship of the cathode based on a pre-determined polarization curve relationship assigned to the cathode [6, 7]. The application of the DACC technique in PSM studies requires the polarization curve data of each cathode material as one of the inputs in order to generate hull potential profiles. The two NAB propellers are the only cathodes considered in the present stray current interference modelling study. The polarization curve data of the NAB under quiescent flow conditions was based on the long-term potentiostatic experimental data compiled by Hack [8]. The original data set was smoothed to obtain a monotonic potential-current density relationship. The smoothing procedure was required in order to use the polarization curve data as input for the PSM experiment when the DACC technique is employed. The smoothed potential-current density relationship was expressed using the second order polynomial relationship where Φ is potential, mv and i is current density, ma/cm 2. The potential-current density relationship was applied in the range of potentials between -5 mv and -9 mv. Stray current source configuration The stray current was introduced using a dipole consisting of two cylindrical graphite s, which were connected to a potentiostat. The following three dipole layouts were used: 1. Transverse Dipole The transverse dipole was positioned at the model mid ship with a spatial separation of approximately 92 cm between the s. This dipole was used to simulate the possible current flow between two jetties with the current largely flowing across a ship. 2. Short Longitudinal Dipole The dipole was positioned on the starboard side of the model 19 cm from the longitudinal centreline of the model. Longitudinally the positions of the two dipole s correspond to the positions of the fore anodes and the aft anodes, respectively, with a spatial separation of 62 cm. The dipole was used to simulate a longitudinal stray current introduced by the second ship positioned alongside the first ship with a similar overall length and shipboard ICCP system. 3. Long Longitudinal Dipole The dipole was positioned on the starboard side of the model 19 cm from the longitudinal centreline. Longitudinally the two dipole s were positioned near the two ends of the tank with a spatial separation of 238 cm. This dipole was used to simulate a longitudinal stray current introduced by a much longer ship s ICCP system For all of the stray current configurations, the submerged ends of the graphite s were approximately 4.9 cm below the waterline of the model. (1

5 ICCP operating mode Three ICCP operating modes were used in the study: a single fore-zone system, a single aft-zone system, and a two-zone system. Fore reference and aft reference, as labelled in Figure 1, were used to provide closed loop feedback potentials for single fore-zone system and single aft-zone system, respectively. The power supply and control unit of each zone adjusts its current output based on the feedback potential in order to maintain the potential at the feedback reference at a pre-set value. The pre-set potential at respective reference s for both single fore-zone and single aft-zone systems was -85 mv (vs. Ag/AgCl. For the cases where the potential at the feedback reference potential becomes more negative than the pre-set value, there will be no current output from the respective ICCP zone anodes. Procedure for stray current interference physical scale modelling trial The PSM tank was filled with 1/1 scale (approximately.39 S/cm electrolyte with naturally occurring seawater as the base. Before the model control sequence was initiated, all Ag/AgCl reference s were calibrated to a saturated calomel. The quiescent NAB polarization curve relationship, as expressed in Equation 1, was imposed on the NAB propellers. A commercial potentiostat, which was electrically isolated from the model hardware, was used to generate the stray currents. For each combination of ICCP operating mode and stray current dipole configuration, a sequence of dipole currents was imposed on the s. The order and magnitude of the dipole currents were -.5 ma, +.5 ma,, and +. The positive current was defined as the current flowing from starboard side towards port side in the case of transverse current or the current flowing from the bow of the model toward the stern for the stray currents that flow longitudinally. A period of approximately 3 minutes was included between the successive dipole currents applied to facilitate stabilization of the model potentiostat and DACC current controller hardware. RESULTS For each of the test sequences conducted, the data generated by the model software/hardware algorithms was post processed to evaluate the effect of the imposed stray currents on the control characteristics of the ICCP system under each ICCP operating mode and resulting potential distributions along the hull of the ship model. Parameters used for the evaluation included the potential of the feedback reference, current output from the anodes for each ICCP zone and the hull potential profiles corresponding to the dipole currents imposed during the PSM trial. Stray current effect in single fore-zone ICCP system Figures 2 through 4 present the hull potential profiles on the single fore-zone ICCP system under three stray current layouts: Transverse Dipole, Short Longitudinal Dipole, and Long Longitudinal Dipole. The setpoint potential value at the feedback reference locations was 85 mv (vs. Ag/AgCl with a potential range within ±5 mv of the setpoint considered to be acceptable from a corrosion protection perspective. When the transverse dipole was activated, there were no appreciable differences in the hull potential profiles, as shown in Figure 2. The maximum deviation of the hull potentials observed at the dipole current of ± was less than 5 mv when compared to the hull potentials obtained in the absence of stray current interference. The effect of the transverse dipole current on the current output of the ICCP system was also minimal, with the current output increased.4% at a dipole current of. In the case of Short Longitudinal Dipole, the hull potential profiles were affected by both magnitude and direction of the imposed dipole current, Figure 3. The hull potential profiles shifted in a positive direction with positive dipole currents (i.e. dipole currents flow from the fore to the aft and in a negative direction with negative dipole currents, with exception of the potentials at two reference s closest to the fore anode and at the feedback reference. In addition, the imposed positive dipole currents caused under-protection in the hull surfaces around the propellers with the

6 potentials more positive than -75 mv on the hull right above the propellers at a dipole current of 1.5 ma. Moreover, the positive dipole currents prompted the model ICCP system to adjust its current output. For example, the ICCP current output decreased 4.5% at a dipole current of. Fore Anode ma.5 ma Figure 2: Hull potential profiles under various dipole currents in single fore-zone ICCP system (Transverse Dipole, 92 cm separation Fore Anode ma.5 ma Figure 3: Hull potential profiles under various dipole currents in single fore-zone ICCP system (Short Longitudinal Dipole, 62 cm separation The dipole currents from Long Longitudinal Dipole, as shown in Figure 4, have a similar effect on the potential distribution to the dipole currents from Short Longitudinal Dipole on the model hull aft of the fore anode. The positive dipole currents also resulted in under-protection on the hull surface around the propellers with potential of -76 mv observed on the hull surface above the propellers at a dipole current of. On the portion of the hull to the fore of the forward feedback reference, the positive dipole currents caused the hull potentials to shift toward negative direction while the negative dipole currents caused the hull potentials to shift toward positive direction, which was in contrast to the

7 hull potentials obtained with Short Longitudinal Dipole. In addition, the dipole currents caused the single-zone ICCP system to adjust its current output (up to ±3.9% in order to maintain the potential of the feedback reference at -85 mv. Fore Anode ma.5 ma Figure 4: Hull potential profiles under various dipole currents in single fore-zone ICCP system (Long Longitudinal Dipole, 238 cm separation Stray current effect in single aft-zone ICCP system Figures 5 through 7 present the hull potential profiles in the single aft-zone ICCP system under the three stray current layouts. As shown in Figure 5, the dipole currents from Transverse Dipole did not cause an appreciable effect on the potential distribution along the hull with a maximum potential deviation of less than 4 mv. In addition, the ICCP current output did not change appreciably in the presence of the transverse stray current. Aft Anode ma.5 ma Figure 5: Hull potential profiles under various dipole currents in single aft-zone ICCP system (Transverse Dipole, 92 cm separation

8 In the case of Short Longitudinal Dipole, the hull potential profiles were affected by both the magnitude and direction of the dipole current, as shown in Figure 6. The positive dipole currents caused the potential profiles to shift in a negative direction and the negative dipole current caused the hull potential to shift in a positive direction on the hull surfaces to the fore of the aft reference. In comparison, the flow direction of the dipole currents had exactly the opposite effect on the hull potentials on the portion of the hull aft of the aft reference. The maximum shift in the hull potential occurred near the longitudinal position of the fore dipole. All the potentials were still within the acceptable range of cathodic protection for the various magnitudes of dipole current tested. The stray currents also caused a slight adjustment in the ICCP current output (up to 1.2% in order to maintain the potential of the feedback reference at the setpoint value (i.e. -85 mv. Aft Anode ma.5 ma Figure 6: Hull potential profiles under various dipole currents in single aft-zone ICCP system (Short Longitudinal Dipole, 62 cm separation Aft Anode ma.5 ma Figure 7: Hull potential profiles under various dipole currents in single aft-zone ICCP system (Long Longitudinal Dipole, 238 cm separation

9 Similar to the Short Longitudinal Dipole cases, the dipole currents from Long Longitudinal Dipole also have an appreciable effect on the hull potential profiles, as presented in Figure 7. For the Long Longitudinal Dipole case, the maximum shift in the hull potential occurred near the bow of the model hull. The dipole current also caused the ICCP system to adjust the current output (up to 1.% in order to maintain the potential of the feedback reference at the setpoint value. Stray current effect in two-zone ICCP system Transverse Dipole The hull potential profiles under various dipole currents from a Transverse Dipole are presented in Figure 8. The variations of the current output from each of the ICCP zones and the potentials of the feedback reference s, as a function of dipole current, are presented in Figure 9. In general, the dipole current from Transverse Dipole did not have significant effect on the hull potential profile, as seen in Figure 8. The maximum variation in the hull potential was 17 mv and occurred on the surface of the hull closest to the aft dipole. The dipole currents did, however, have a noticeable effect on the current contribution from each of the ICCP zones, as shown in Figure 9. A positive dipole current caused an increase in the current output from the aft-zone ICCP unit and a decrease in the current output from the fore-zone ICCP unit, whereas a negative dipole current had the opposite effect. It was also noticed that for a dipole current of, the current output from the fore-zone ICCP unit reduced to zero and the potential reading of the fore feedback reference was -852 mv, which was more negative than the setpoint value (i.e. -85 mv. Fore Anode Aft Anode ma.5 ma Figure 8: Hull potential profiles under various dipole currents in two-zone ICCP system (Transverse Dipole, 92 cm separation Short Longitudinal Dipole The hull potential profiles under various dipole currents from a Short Longitudinal Dipole are presented in Figure 1. The variations of the current output from each of the ICCP zones and the potentials of the feedback reference s as a function of dipole current are presented in Figure 11. On most hull surfaces the applied dipole currents resulted in a negative shift in the hull potential profiles, as shown in Figure 1. The maximum potential shift was found to be -38 mv at a dipole current of. The exceptions to the negative shifts observed were in the hull surface areas around the propellers where the hull potentials shifted in a positive direction at the dipole currents of -.5 ma and. For all dipole currents tested, the potentials along the hull remained within the range of -85±5 mv.

10 Current (normalized Potential (mv Dipole Current -.5 ma.5 ma Current (fore Current (aft Potential (fore Potential (aft Time (min Figure 9: Variations of potentials of feedback reference s and current output from each of the ICCP zones as a function of dipole current (Transverse Dipole, 92 cm separation Fore Anode Aft Anode ma.5 ma Figure 1: Hull potential profiles under various dipole currents in two-zone ICCP system (Short Longitudinal Dipole, 62 cm separation The Short Longitudinal Dipole configuration also had a significant effect on the current contribution from each of the ICCP zones, as shown in Figure 11. The negative dipole currents resulted in an increased current output from the fore-zone ICP unit and a decreased current output from the aft-zone ICCP unit, while on the contrary the positive dipole currents resulted in the opposite effect. The current output from the aft-zone ICCP anodes reduced to zero when a dipole current was applied while the current output from the fore-zone anodes became zero when the two positive dipole currents were applied. Moreover, the potentials at the respective feedback reference s became more negative than the setpoint value (i.e. -85 mv when the current outputs were reduced to zero (Figure 11.

11 Current (normalized Potential (mv Dipole Current -.5 ma.5 ma Current (fore Current (aft Potential (fore Potential (aft Time (min Figure 11: Variations of potentials of control reference s and current output from each of the ICCP zones as a function of dipole current (Short Longitudinal Dipole, 62 cm separation Long Longitudinal Dipole The hull potential profiles under various dipole currents from a Long Longitudinal Dipole are presented in Figure 12. The variations of the current output from each of the ICCP zones and the potentials of the feedback reference s as a function of dipole current are presented in Figure 13. Similar to the case with Short Longitudinal Dipole, the applied dipole currents resulted in more negative hull potentials on most surfaces of the model hull than those obtained in the absence of stray current, as shown in Figure 12. The maximum potential shift was found to be -35 mv. The two exceptions observed were the surface areas to the fore of the fore feedback reference where there was a small positive potential shift under a negative dipole current, and the areas near the stern where a small positive potential shift was observed under a positive dipole current. For all dipole currents tested the potentials along the hull remained within the -85±5 mv range. Fore Anode Aft Anode ma.5 ma Figure 12: Hull potential profiles under various dipole currents in two-zone ICCP system (Long Longitudinal Dipole, 238 cm separation

12 The stray current from a Long Longitudinal Dipole configuration also had a significant effect on the current contribution from each of the ICCP zones, as shown in Figure 13. The negative dipole currents resulted in an increased current output from the fore-zone ICCP unit and a decreased current output from the aft-zone ICCP anodes, whereas the positive dipole currents had the opposite effect. Similar to the Short Longitudinal Dipole configuration, the current output from the aft-zone ICCP unit reduced to zero at a dipole current of and the current output from the fore-zone unit reduced to zero when either a.5 ma or a dipole current was applied. In addition, the potential readings at the feedback reference s became more negative than the setpoint value (i.e. -85 mv when the current output from the respective ICCP zones was reduced to zero (Figure 13. Current (normalized Potential (mv Dipole Current -.5 ma.5 ma Current (fore Current (aft Potential (fore Potential (aft Time (min Figure 13: Variations of potentials of control reference s and current output from each of the ICCP zones as a function of dipole current (Long Longitudinal Dipole, 238 cm separation DISCUSSION Effect of dipole layout and current direction on stray current interference As previously indicated, cathodic protection works by establishing an electrostatic field which provides the desired potential and current distributions in the seawater surrounding the structures to be protected. In a shipboard ICCP system, the potential field is established between the ICCP anodes and the cathode areas that receive current from the ICCP anodes (e.g. propellers. The potential difference across the electrolyte between the ICCP anodes and the cathodes can be expressed as: S I R S (2 where I is the current flow to a cathode and R S is the solution resistance between the ICCP anodes and the cathode and can be expressed as: R S x 1 A( x dx (3

13 where is the solution conductivity, which is a constant in a homogeneous electrolyte; x is the distance from the anode; and A(x is the cross sectional area of the current path at distance x. The relationships in Equations 2 and 3 indicate that the potential difference is affected by the distance under a constant current. As the distance between the cathode and the ICCP anodes increases, the potential difference becomes greater and, from cathodic protection point of view, the potential on the cathode becomes less negative. In the presence of an external current source in the vicinity of the ship, the external current source will contribute to the potential difference across the electrolyte between the ICCP anodes and the cathode. As a result Equation 2 is modified to include the contribution from the external current and expressed as follows: I R I R (4 S S E SE where I E is the portion of the external current flow that shares the same current path with the ICCP current and R SE is the solution resistance in the current path shared with the ICCP anodes. The sign of I E is dependent on the flow direction of the external current with reference to that of the ICCP current. I E is a positive value if the external current flows in the same direction as the ICCP current, and a negative value if the external current flows in the opposite direction to the ICCP current. Based on the definition, Equation 4 indicates that an external current source would lead to an increased potential difference between the ICCP anodes and the cathode which results in a positive potential shift at the cathode site if the external current flows in the same direction as that of the ICCP current. Conversely, an external current source would lead to reduced potential difference between the ICCP anodes and the cathode and a negative potential shift at the cathode if the external current flows in the opposite direction to that of the ICCP current. This has been confirmed in this PSM study. As shown in Figures 3, 4, 6 and 7, the stray current generated by the two longitudinal dipole configurations resulted in positive potential shifts for the portions of the hull where the ICCP current and the dipole current flowed in the same direction, and resulted in negative potential shifts on other portions of the hull where the two currents flow in the opposite directions. Based on the PSM results, the shift of the hull potentials in a positive direction is not favorable as it could result in under-protection in part of the hull, as shown in Figures 3 and 4. In the case of the stray current generated by a transverse dipole there was only a minimal portion of current that shared the same flow path with the current from the ICCP system. As a result of the reduced interaction, the effect of the stray current from the transverse dipole on the hull potential profile is marginal, as show in Figures 2, 5, and 8. Effect of shipboard ICCP operating mode on ICCP response to stray current interference The two-zone ICCP system can be operated in three modes: single fore-zone system, single aft-zone system, and two-zone system. Each zone has its own power supply and control unit, and reference for providing feedback potentials. In the presence of stray current, the potential at the feedback reference will be affected due to the change of potential difference between the ICCP anodes and the reference based on Equation 4. To offset the stray current effect each ICCP system responds by adjusting the ICCP current output in order to maintain the potential of the feedback reference at the setpoint value. Based on the testing results the ICCP current output varied within ±4.5% in the single fore-zone system and ±1.2% in the single aft-zone system, respectively, in order to compensate for the stray current effect. However, as the feedback reference was located relatively close to the respective ICCP anodes the hull areas away from the feedback reference were still subject to a stray current effect, as demonstrated in Figures 3, 4, 6 and 7. When both fore-zone and aft-zone ICCP units are activated, each ICCP zone works independently to maintain the potential of its feedback reference at the setpoint value (i.e. -85 mv by adjusting the respective current outputs. A balance between the contributions of the current supplied by the fore and aft anodes is established when a stable state is reached. As the aft-zone anodes and

14 reference are located relatively close to the propellers, which receive most of the ICCP current, the current output from the aft-zone system is significantly higher than that supplied by the forezone system. In the presence of stray current, the added potential difference along the ICCP current path affects the potentials at the two feedback reference s. The change in the potential difference along the ICCP current path results in each ICCP zone adjusting its current output in order to offset the effect of stray current on the potentials of the feedback reference s. As shown in Figures 11 and 13, the current output from the fore anodes increased while that from the aft anodes reduced in response to a negative dipole current that flowed primarily in the opposite direction to the current flow of the ICCP system. On the other hand, the current output from the aft anodes increased while the current output from the fore anodes reduced in response to a positive dipole current that flowed primarily in the same direction as the current flow of the ICCP system. Under certain stray current conditions, maintaining the potential of the feedback reference potential in one ICCP zone could make the potential of the feedback reference at another ICCP zone more negative than -85 mv, which automatically makes the current output from its ICCP zone reduce to zero. This scenario has been observed in the PSM experiments when dipole current was changed from -.5 ma to and when two positive dipole current values (i.e..5 ma and were introduced, as shown in Figures 11 and 13. As a result, the two-zone ICCP system responded to the stray current by shifting the hull potential profile to negative direction. This would effectively reduce the likelihood of under-protection. There is a possibility of over-protection in the presence of strong stray current, but the hull potential profiles were within the -85±5 mv limit in the range of stray current tested in this PSM study. CONCLUSIONS A physical scale modelling (PSM technique has been used to evaluate the interference of external current sources with respect to the performance of a shipboard impressed current cathodic protection (ICCP system. The following conclusions can be drawn from the present PSM study: 1. Stray current in the vicinity of a ship, whether it is generated by another ship s cathodic protection system or from nearby jetty s cathodic protection system, could affect the potential profiles of the ship s underwater hull and, therefore, the level of cathodic protection by changing the potential difference in the current path of the ship s cathodic protection system. 2. A stray current that flows in the axial direction of the ship has a greater effect on the performance of a shipboard ICCP system than the stray current that flows transversely to the axial direction of the ship. 3. Single-zone ICCP systems have limited capability to compensate for stray current interference and, therefore, are potentially subject to under-protection in the presence of stray current, in particular when stray current flows in the same direction as the ICCP current. 4. A two-zone ICCP system is more tolerant with respect to offsetting stray current interference than single-zone ICCP systems. For the two-zone configuration used in the study, the ICCP system was able to maintain the hull potentials within the -85±5 mv range for the stray currents applied to the model which indicated acceptable performance from corrosion protection perspective. REFERENCES 1. J. Trevelyan, H.P. Hack, Analysis of Stray Current Corrosion Problems using Boundary Method, Boundary Element Technology IX, pp (1994.

15 2. R.A, Adey, Y.H. Pei, Computer Simulation as an Aid to Corrosion Control and Reduction, CORROSION/99, (Houston, TX: NACE, H.P. Hack and D.C. Lynn, Interaction of ship and dock cathodic protection systems predicted from potential measurements of a seawall at Panama City, Florida, CARDIVNSWC-TR-61-95/12, Survivability, Structures and Materials Directorate Technical Report, Naval Surface Warfare Centre, USA, D. J. Tighe-Ford, J. N. McGrath, and L. Hodgkiss, Design improvement for a ship s impressed current cathodic protection system using Dimension And Conductivity Scaling (DACS, Corrosion Prevention and Control 32, 5 (1985: p E. D. Thomas, K. E. Lucas, R. L. Foster, A. R. Parks and A. I. Kaznoff, Physical Scale Modeling of Impressed Current Cathodic Protection Systems, CORRSION/89, paper no (Houston, TX: NACE, Y. Wang and K. KarisAllen, Comparison of Impressed Current Cathodic Protection Numerical Modeling Results with Physical Scale Modeling Data, Corrosion 66 (21, to Y. Wang and K. J. KarisAllen, Simulating polarization behavior of propeller materials in the physical scale modeling of shipboard impressed current cathodic protection, CORROSION/27, paper no, 782 (Houston, TX: NACE, H.P. Hack, Atlas of polarization diagrams for naval materials in seawater, CARDIVNSWC-TR-61-94/44, Survivability, Structures and Materials Directorate Technical Report, Naval Surface Warfare Centre, USA, 1995.