NOVEL USES OF CATHODIC PROTECTION SYSTEMS FOR STRUCTURE CONDITION ASSESSMENT James A. Ellor Elzly Technology Corporation Reston, VA jellor@elzly.com And Andrew D. Seelinger Naval Sea Systems Command Washington, D.C. ABSTRACT A shipboard impressed current cathodic protection (ICCP) system results in the creation of an electric field in the seawater electrolyte surrounding the ship hull. Measurement and interpretation of this electric field can provide valuable insight into the performance of the ICCP system. The following will discuss some basics behind the electric field, measurement techniques, and application of the data for the analysis of system performance, coating condition, and hull current distribution. KEYWORDS Electric field, cathodic protection, coating, current, vector, stray current BACKGROUND Simply, an ICCP system consists of hull-mounted electrodes supplying current to the hull from a DC power supply. These electrodes, or anodes, are typically constructed of an inert conductive material and are distributed over the underwater wetted hull. System output is controlled by the hull potential as referred to an independent, non-driven, electrode; this electrode is usually a silver-silver chloride / seawater type reference cell. The electric field in the electrolyte surrounding a cathodically protected hull can be measured by determining the voltage between any two electrodes. These measurements are typically performed with independent electrodes (i.e., reference cells) in the water adjacent to the hull. However, the same type of data may be obtained from hull mounted elements including non-driven anodes and reference cells. 1
To make the measurements discussed herein, the required instrumentation consists primarily of a high-speed, recording, high-impedance voltmeter. This is used to monitor the potential between any two reference points be they two reference cells, a reference cell to an anode, or between two anodes when current flows from a source, such as another anode, to the hull. To eliminate any inherent potential differences between electrodes (i.e., polarization or corrosion potential variations), the electric field is measured immediately before and after current from the driven electrode(s) is interrupted. This change in potential indicates the magnitude of the electric field between the observation points that results from current flowing from a particular source to the hull. The potential observed will be directly proportional to the current in the electrolyte; thus prior to the measurement, the magnitude of current must also be recorded. For convenience sake, the relationship between observed voltage changes between two electrodes and the current will be expressed as the ratio, ΔV/I, where ΔV represents the voltage chance (in volts) and I represents the current (amperes). The ratio takes on the units of volts / ampere or ohms. MEASUREMENT OF VOLTAGE GRADIENT OBSERVED BY HULL MOUNTED ELECTRODES The techniques described above have been applied to Navy ships. For the ships studied, the ICCP system consisted of six anodes and multiple reference cells. Thus the electric field around the hull can be measured from multiple points of reference. Current may flow from any single anode and the voltage may be observed at multiple reference points. Most informative are the voltage gradients observed from the reference cells or non-driven anodes with respect to the hull when current flows from a given anode. Figure 1 provides the results of measurements on a single ship. Data are shown for the potential gradient-per ampere current output for various current sources - potential measurement points. The data show the voltage between the hull and selected reference points--either non-driven anodes or reference cells. On Figure 1, each data point is referenced by the frame number of current source (one of the six hull-mounted anodes throwing current to the hull) and the frame number of the reference point (monitoring the potential of that electrode vs. the hull). It is clear from Figure 1 that there is a variation in the potential gradient depending on the point of reference. 2
Reference Points FR 83S-FR 83P FR 474S-FR 465P FR 276S-FR 276P FR 474S-354S REF FR 276P-364P REF FR 276S-364P REF FR 276S-354S REF FR 465P-364P REF FR 474S-364P REF FR 276P-354S REF FR 276P-FR 83P FR 465P-354S REF FR 276S-FR 83S FR 276P-FR 83S FR 83S-354S REF FR 83P-364P REF FR 474S-FR276S FR 276S-FR 83P FR 83P-354S REF FR 83S-364P REF FR 465P-FR 276S FR 465P-FR 276P FR 474S-FR 276P FR 474S-FR 83S FR 474S-FR 83P FR 465P-FR 83P FR 465P-FR 83S 0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 volts/ampere Figure 1 - Voltage Gradient / Unit Current for Referenced Current Source to Reference Point Combinations Figure 2 shows a plot of the potential gradient versus the estimated current source to reference point separation distance for a collection of data from three ships. The distance separating these elements was estimated to be represented by, 2 2 2 ( x y z S = + + ) (square root of the sum of all terms), where x = the difference in longitudinal displacement along the hull and y = the average width of the ship, for electrodes that were not located on the same side of the hull. The planar coordinate z represents the different electrode depths below the waterline. By inspection the gradient tends to be less as the current source and reference point are separated. 3
0.1000 Volts/Ampere 0.0100 0.0010 0.0001 (100.00) - 100.00 200.00 300.00 400.00 500.00 Separation (feet) Figure 2 - Volts/Ampere vs. Source to Reference Point Separation The data indicate an inverse relationship. Figure 3 plots the same volts / ampere relationship versus the reciprocal of the separation distance. A linear regression of this data shows an excellent correlation (correlation coefficient of 0.999) with a slope of 0.104 volt-feet/ampere (0 y-intercept). This confirms that the magnitude of the electric field falls off in accordance with the reciprocal of the distance from the current source. Volts/Ampere 0.0100 0.0090 0.0080 0.0070 0.0060 0.0050 0.0040 0.0030 0.0020 0.0010-0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 1/Separation (1/feet) Figure 3 - Volts/Ampere vs. 1/Separation Distance 4
This is not unexpected. By common cathodic protection theory derived from Coulomb's Law, an anode on the hull can be modeled as a hemi-spherical point source and the hull bare areas can be considered remote from the anodes and of significantly greater surface area. These points are further discussed within Appendix A. The voltage at any distance from the anode is given by: V = I ρ/2πr eq.(1) where: V = Voltage, volts I = Anode current, amperes r = Distance from the anode, feet ρ = Seawater resistivity, ohm-feet For any given magnitude of current, equation (1) may be simplified to: where: k = ρ/2π (V/I) = (k/r) For the nominal 20-25 Ω-cm seawater in which the reported data were generated, the valve for k would be 0.104 to 0.131 volt-feet/ampere. This is in excellent agreement with the 0.104 volt-feet/ampere slope of the data in Figure 3. In demonstrating the close agreement between shipboard measurements and fundamental principles, a number of practical applications become evident: 1. Estimation of The Seawater Potential Gradient Included in Hull Polarization Measurements. Shipboard ICCP systems rely on feedback from a hull mounted reference cell to control system output. Usually a setpoint is established for this control, e.g., 0.85 volts vs. Ag/AgCl. As Figures 1 and 2 demonstrate, output from an anode will induce an electric field in the sea-water. The potential gradient of this field is included in the reference cell to hull potential. The magnitude of this potential is 0.104 x anode current/anode-reference cell separation-feet (for 20 ohm-cm seawater). The contribution of each active anode is cumulative. Similarly, a NAVSEA representative 1 measured the hull potential using a portable cell adjacent to the edge of a dielectric shield of a 75-amp anode on a CG-47 class ship. At full current output, the voltage was measured at about 2.5 to 2.7 volts. Assuming that the edge of this primary shield is about 5 feet from the anode edge, by the above equation the seawater potential gradient to the remote hull would be 1.6 volts. Added to the remote hull polarization of ~ 0.85 volt, the potential predicted would be about 2.45 volts, which is in fair agreement with the measured valve. Similar estimates of the impacts of 5
driven anodes on potential readings could be made to correct typical hull potential surveys. 2. Monitoring The Degradation of a Hull Coating System. As discussed initially, the agreement between the shipboard measurements and the expected theoretical results required consideration of the anodes as current point sources with the majority of current flowing to areas remote from the reference points. This is not an unreasonable assumption for a hull with a good coating. Initially the areas around the reference cells are generally well coated and each anode is centered in an effective dielectric shield. When this is no longer the case, the shipboard measurements begin to deviate from theory and indicate a much lower resistance (i.e., volts/ampere) value. Consider the following gradient measurements for three ships, each with a different quality coating system. Figure 4 shows the effect of a deteriorating or poor hull coating on the gradient observed at a mid-ship mounted reference electrode vs. total current from the ICCP system. 0.01 Coating Resistance Data Volts/Ampere 0.001 0.0001 0.00001 Ship 1 - New Epoxy Hull Coating Ship 2-2-Year Old Coating Ship 3 - Pre-Delivery Holding Primer Figure 4 - Measurement of Hull Coating Resistance The data have been corrected for variations in the seawater resistivity. For the ship with the pre-delivery hull coating, typically only a single coat of anti-corrosive primer, the magnitude of the gradient per ampere current is almost 25 times less than that for a ship with a new hull coating. This type of data can be tracked as a function of time to monitor the degradation of hull coatings. 6
3. Monitoring of Stray Currents. As has been discussed, the flow of current to the hull results in a measurable voltage in the electrolyte. The magnitude of the voltage observed at any reference point may be calibrated in terms of current flow to or from the hull (i.e., volts/ampere). Figure 5 shows a time trace of a hull potential for a ship pierside with the ICCP system secured. Potential Change mv vs. Aft Reference Cell 800 600 Hull Potential Change vs. Time 0 Time (min) 15 Figure 5 - Observations of Stray Current on Ship Hull The "sudden" 50mV noble shifts in the hull potential coincided with welding operations on an adjacent tender. For this reference cell, measurements had indicated a gradient strength of about 1.5mV/ampere. Thus, one may estimate that about 33 amperes (50mV/l.5mV/ampere) were discharging from the ship hull during these spikes. Using similar techniques, stray currents of values estimated to exceed 1000 amperes have been observed. In these cases, significant hull corrosion was noted. Local corrosion rates could exceed 1 inch per year during these observations. 4. Monitoring Current Flux around a Hull. The direction and relative magnitude of the current flux around a hull can be measured through potential gradient readings. Most simply, the gradient can be measured using a dipole consisting of two reference electrodes located a constant, fixed distance apart. 7
At a given depth, the direction and magnitude of the current at this point can be determined by two potential gradients readings: one with the dipole parallel to the'-hull (north-south reading) and the other with the dipole perpendicular to the hull (east-west reading). The two voltage readings are resolved into a single vector representing the magnitude and direction of the current. The current is proportional to the voltage due to the uniform resistivity of the seawater around the ship. The magnitude of the vector indicates the value of the current at one point in the seawater relative to the current at another point. Derivation of the exact current would require a more extensive analysis. To demonstrate the utility of the technique, consider an analysis of the dielectric quality of a coating applied to the single, starboard propeller of a twin screw vessel. For testing, the ship was divided into a number of measurement points about the stern. The measurement points were approximately 10 feet apart and symmetrical on the port and starboard sides. Measurements were also made around the stern rail at 7.5 foot intervals. The dipole was placed overboard at these reference points. It was maintained at a constant depth below the waterline (i.e., the centerline of the propellers). Current flowed to the propellers from the installed ICCP system. The measurement techniques accounted for any stray voltage gradients or inherent potential difference in the reference cells by cycling the ICCP current on/off. The results of two sets of dipole voltage measurements on the port side, immediately fore and aft of the propeller center-line are shown in Figure 7. The dipole voltage changes when the ICCP current is turned off (shown between times 1 and 2). The direction of the voltage change on the north-south measurement (parallel to the hull, north towards the bow) reverses on each side of the propeller, indicating a change in the direction of this component of the current vector. The east-west measurement remains the same in each case, indicating current flow towards the propeller. 8
0-5 <-- Current On Current Off --> Dipole Voltage - mvolts -10-15 -20-25 East-West Aft of Prop North-South Aft of Prop East-West Fwd of Prop North-South Fwd of Prop -30 0 1 2 3 4 Time (sec) Figure 6 - Data From Dipole Measurements Figure 9 presents the results of the measurements made around the ship (more data points were obtained on the port side of the ship than starboard due to time constraints). The length and direction of the arrows show the relative magnitude and direction of current at each measurement point. The data show a great amount of symmetry between port and starboard and also around the stern of the ship. This would indicate that the coated propeller is acting similarly to the uncoated propeller. 9
90 Phase Angle Axis 180 270 0 Starboard - Coated Propeller Line Port - Uncoated Figure 7 - Resolution and Plotting of Current Vectors Visual inspection of a similar propeller under repair showed that the coating did not adhere in areas subjected to cavitation and/or high velocity (hub areas and blade tips). In addition, holiday inspections were made on the propeller blades. No single region of the coating appeared to isolate the substrate electrically. This confirms the results obtained from the dipole measurements, ie., that the coating does not form an effective dielectric barrier. 10
CONCLUSIONS 1. The measurement and interpretation of the electric field around a ship hull can provide valuable information concerning the operation of the ICCP system and corrosion control on the hull. With respect to the ICCP system, data can be developed concerning current distribution. For corrosion control on the hull, techniques are shown for monitoring coating degradation, the magnitude as opposed to simply the occurrence of stray current corrosion, and hull polarization as separated from the indicated hull potential of a shipboard reference cell. 2. Similar techniques have been used in the underground cathodic protection industry for assessment of coating quality, current directions, and stray current mitigation. They may be extended to elevated water storage tanks, for example, as a non-destructive method for characterizing coating quality. 11
APPENDIX A 1. Consider the resistance between two concentric spheres, sphere A of radius r 1, and sphere B of radius r 2 (assume r 1 << r 2 ). The resistance between these two spheres is: R = (ρ dr)/4πr 2 Where: R = Resistance, ohms ρ = Resistivity, ohm-cm r = Radius, cm 2. Integrating from r 1 to r 2, R = ρ/4π(1/r 1 1/r 2 ) 3. For r 2 >> r 1, R= ρ/4π(1/r 1 ) 4. If sphere A is made into a hemi-sphere, then the resistance is doubled and: R = ρ/(2πr 1 ) 5. If you assume that the current source within r1 is a point source, then the hemisphere existing at a radius of r 1 will be at an equipotential given by: V = IR = I ρ/(2πr 1 ) where V = Voltage, volts I = Current, amps R = Resistance, ohms ρ = Resistivity, ohm-cm r 1 = Distance from the point source, cm 6. Thus the electric field observed at a reference point on a hull can be expected to follow the equation derived. 12
REFERENCES 1) E. Dail Thomas Trip Report (9633, O5M1-16/129, 22 APRIL 1986) 13