AU652634B2 - Apparatus and method for minimising corrosion in steel structures - Google Patents

Description

65

AUSTRALIA

263 P/00/0 I.I Regoulation 3.2 PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name of Applicant: Address of Applicant: Actual Inventor: Address for Service: RAIDER ELECTRONICS PTY. LTD.

57 SPENCER STREET, BUNBURY W.A. 6230 ROBERTO ENZO DI BIAGGIO GRIFFITH HACK CO.

256 ADELAIDE TERRACE PERTH W.A. 6000

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Standard Complete Specification for the invention entitled: APPARATUS AND METHOD FOR MINIMISING CORROSION IN STEEL STRUCTURES r Details of Associated Provisional Applications: PK6229 LODGED MAY 20, 1991 The following is a full description of this invention, including the best method of performing it known to me:- 2 THIS INVENTION relates to an apparatus and method for minimising corrosion of inter alia steel structures, and has particular application in those steel structures which are insulated from but capacitively coupled to the ground potential of the surrounding environment.

In accordance with one aspect of the present invention there is provided a cathodic protection apparatus for reducing corrosion in metal structures having constituent elements prone to oxidation, said apparatus having inputs for connection to an electrical power source for the provision of electrical power thereto, and an output for providing an electrical signal to a capacitive coupling member connected to the output, said capacitive coupling member being adapted to be capacitively coupled to said metal structure so as to present a load to said output; whecein said apparatus when connected to said load, is adapted to provide said electrical signal having characteristics comprising a train of electrical pulses having a positive voltage relative to said metal structure, said train of pulses comprising a pulse set repeated at periodic intervals where each pulse set includes a leading pulse having a peak voltage between 500 volts and 10,000 volts and a duration of .i between 1 microsecond and 100 microseconds and there is an elapsed time between successive leading pulses of between 1 millisecond and 1 second.

Preferably, each leading pulse is immediately followed by a plurality of high frequency positive voltage pulses of lower voltage than said peak voltage.

Preferably, said plurality of high frequency positive voltage pulses have a cumulative duration greater than the duration of an immediately preceding leading pulse.

Preferably, said plurality off high frequency positive voltage pulse's have a frequency of between 10 megahertz and megahertz.

3 Preferably, said train of electrical pulses is output from a secondary side of said voltage transformer and said plurality of high frequency positive voltage pulses occur during a period in which a primary side of said voltage trans'former is not energised.

Preferably, said apparatus further comprises a high voltage direct current voltage source connected to said primary side of the voltage transformer through a switching means; and a first oscillator operating at a first frequency operatively connected to said switching means for selectively connecting and disconnecting said high voltage direct current voltage source to the primary side of the voltage transformer wherein said voltage transformer produces said train of pulses when said high voltage direct current voltage source is connected to said primary side.

Preferably, said high voltage direct current voltage source comprises a second oscillator operating at a second frequency higher than said first frequency having an output connected to a primary coil of a second transformer, and a rectifying means connected to a secondary coil of the second transformer to provide a high voltage direct current voltage.

Preferably, said elapsed time is selectably adjustable.

Preferably, a predetermined elapsed time can be selected for 4.44 metal structures having predetermined physical and/or metallurgical characteristics.

o .Preferably, said elapsed time corresponds to the period of a signal having a frequency in the order of 10Hz, 70Hz or 140Hz.

Preferably, the duration of each leading pulse is between 2 microseconds ard 10 microseconds.

Preferably, the duration of each leading pulse is between microseconds and 6 microseconds.

4 Preferably, said peak voltage is in the order of 4000 volts.

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Preferably, said apparatus further comprises a power control means for monitoring an output voltage from said electrical power source and prohibiting the generation of said electrical signal when the output voltage is below a predetermined level.

Preferably, said, power control means allows the generation of said electrical signal when said output voltage subsequently exceeds said predetermined level.

Preferably, a negative side of said electrical power source is connected to one of said inputs and to said metal structure at a first point and said capacitive coupling member is connected at a second point on said metal structure remote from said first point.

Preferably, said first and second points are located as far apart as possible on said metal structure.

Ea Preferably, said first and second points are located adjacent respective dielectric connections made from said metal structure to ground.

Preferably, said metal structure is a motor vehicle and each :dielectric connection is a tyre supporting said motor vehicle, and first and second points are adjacent diagonally opposite tyres.

S"Preferably, said output for providing the electrical signal is connected to said capacitive coupling member by means of an electrically conductive cable, and said cable is routed along said metal structure in such a manner as to not coincide with a free current path between said output and said capacitive coupling member.

5 Preferably, said capacitive coupling member comprises an electrode sandwiched between first and second non-conductive planar members wherein the electrode is disposed inboard of the periphery of said first planar member, and said first planar member is disposed inboard the periphery of the second planar member.

Preferably, said second planar member is adapted to receive an end of a high voltage conductor to facilitate connection of said high voltage conductor with said electrode.

Preferably, said second planar member is adapted for connection to said metal structure in such a manner that said electrode is disposed substantially parallel to a portion of said metal structure to which said capacitive coupling member is connected.

Preferably, said first and second planar member are bonded together in a settable resin or plastics material.

Preferably, said first and second planar members are encapsulated.

Preferably, said first and second planar members are formed of glass fibre reinforced plastics material.

In accordance with another aspect of the present invention there is provided a method of minimising corrosion in a metal structure comprising connecting an apparatus as hereinbefore S.defined to an electrical power source and to said metal structure, and connecting the output of said apparatus to a coupling member capacitively coupled to said metal structure.

In accordance with a further aspect of the present invention there is provided a method of minimising corrosion in a metal structure having an electrical power source with one side thereof connected to the metal structure, said method 6 comprising connecting an apparatus as hereinbefore defined to said electrical power source, and connecting the output of said apparatus to a coupling member capacitively coupled to said metal structure.

The invention will be better understood by reference to the following description of one specific embodiment thereof and which: Figure 1 is circuit schematic of a cathodic protection control unit according to the embodiment; Figure 2 is a graph showing the unloaded output of the cathodic protection control unit shown in Figure 1; Figure 3 is a graph showing the loaded pulse output of the cathodic protection control unit shown in Figure 1; Figure 4 is a graph showing the overall output of the ,cathodic protection control unit when installed in situ; 9*9*9* 9 99 e *o 7 Figure 5 is a graph showing the measured current at the discharge plate which is connected to, the output of the cathodic protection control unit; Figure 6 is a suggested cable routing between the cathodic protection control unit and the discharge plate for one type of vehicle chassis configuration; Figure 7 is a suggested cable routing between the cathodic protection control unit and the discharge plate for another type of vehicle chassis configuration; Figure 8 is a grapL showing the external EMF around ~the discharge plate where the cable connecting the oeee discharge plate and the cathodic protection unit is routed along a similar path as the return current flow through the chassis of the vehicle to which the system is fitted; Figure 9 is a graph of the external EMF around the discharge plate at the time of discharge, where the cable connecting the cathodic protection control unit and the discharge plate is routed on a path away from ~that through which the return current would flow through the motor vehicle chassis to which the system is fitted; 9. 9 Figure 10 is a plan view of an electrode for a discharge plate; Figure 11 is a plan view of support plate for the discharge plate; Figure 12 is a crosa sectional view of the discharge 8 plate; Figure 13 is a part cross sectional view of the discharge plate, the view being at the section where the cable is connected to the electrode thereof; Figure 14 is a part cross sectional view through the discharge plate, at one of the mounting holes thereof; Figure 15 is a view showing the cathodic protection system fitted to a Volkswagon Derby for the purposes of testing undertaken in the United Kingdom; Figure 16 is a view of cathodic protection system i fitted to a British Leyland Mini for the purposes of :testing conducted in the United Kingdom; 0 Figures 17 to 26 inclusive are comparative graphs showing percent surface corrosion at test sights on the motor vehicles shown in Figures 15 and 16; :Figure 27 is a diagram showing a test rig employed for evaluating the performance of the cathodic protection system; S* Figure 28 is a plan view of the blank from which the test rig shown in Figure 27 was constructed, showing the positioning of test evaluation areas; Figure 29 is a graph showing comparative development of rust in two test rigs according to a first test; Figure 30 is a graph showing comparative development of rust on the test rigs according to a second test; and 9 Figures 31 to 33 inclusive are schematic diagrams showing wiring configurations for different types of vehicles.

The cathodic protection apparatus comprising a cathodic protection control unit as shown in Figure 1, connected to a discharge plate as shown in Figures 10 to 14 inclusive, is intended to be used to minimise rusting in inter alia motor vehicles, when connected as shown in Figures 15, 16, and preferably conn'cted as shown in Figures 31 to 33 inclusive.

The cathodic protection control unit circuitry is shown in Figure 1. Details of the connections of the power supply to elements of the circuitry have been omitted for clarity accordingly, no connections to battery 107 positive (V are shown, and connections to ground of the functional blocks (0V) are not shown. However, connections to ground 11 of the components in the circuit diagram portion of the circuit schematic are shown, in order to aid understanding.

These connections to ground 11 are connected to the zero volt of the power supply, which ordinarily would be the potential of a motor vehicle chassis, relative to the +12 volt connection on a.motor vehicle storage battery 107.

The cathodic protection control unit' has an oscillator circuit 13 which is connected to a push pull amplifier which in turn drives a step up transformer 17. The output S: of the step up transformer is rectified by a bridge rectifier 19, the DC output of which is smoothed by a capacitor 21 to thereby form a high voltage direct current voltage source. The oscillator circuit"has been selected to run at a frequency of 12.5kHz, this frequency being chosen in order to minimise interference to nearby radio and television receivers. A DC voltage of approximately 200 volts is developed across the capacitor 21.

10 A timing oscillator circuit 23 is provided, which feeds output signals to a timing control circuit 25., The timing oscillator circuit 23 has a frequency control adjustment 27 connected thereto, which provides for adjusting the frequency of oscillation of the timing control circuit 23.

The frequency control adjustment 27 allows for. the frequency of oscillation of the timing oscillator circuit 23 to be selected between preset settings so that the timing oscillator circuit 23 has an output of either 10 Hz, Hz or 140 Hz. The output of the timing oscillator circuit 23 controls the timing control circuit 25 which has two outputs 29 and 31. The timing control circuit 25 in response to signals received from the timing oscillator circuit 23, produces a square wave logic 1 pulse train having pulses of a duration of 800 micro seconds at the output 31. The time between the leading edge of these

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logic one pulses produced at output 31 is either 100 *oo* milliseconds, 14.2 milliseconds or 7.14 milliseconds, depending upon the oscillation frequency of the timing oscillator circuit 23 being selected to 10Hz, 70Hz, or 140Hz respectively in accordance with the setting of the frequency control adjustment 27.

The output 29 of the timing control circuit comprises a logic 1 square wave pulse train, having logic 1 pulses of 1,200 microseconds duration, coincident with the pulses appearing at the output 31.

The output 29 has a logic 0 to logic 1 transition which occurs approximately 200 microseconds before the logic 0 to logic 1 transition of the output 31. At the end of the 800 microsecond pulse duration of the output 31, the output level of the output 31 goes from logic 1 to logic 0 200 microseconds before the output level of the output 29 goes from logic 1 to logic 0. It should be noted that the 11 timing figures quoted in relation to the comparative timing of the logic level changes on outputs 29 and 31 are not overly critical, however, it is important that the output 31 does not have a logic level 1 when the output 29 has a logic level 0.

The output 31 is connected via a current limiting resistor to the gate of an SCR Q3 and the output 29 is connected to switching transistor Q1 via transistor Q2 and the current limiting resister R located between output 29 and transistor Q2. Transistor Q1 is biassed on by resister RB when transistor Q2 is switched off, corresponding to the period when the output 29 is at logic level 0. In this condition, transistor Q1 will be switched on and will charge capacitor C via the primary winding 33 of the high tension step coil 35. When the output 29 goes from logic 0 to logic 1, the transistor Q1 is "switched off" although the current will have long ceased to flow before this point in time. 200 milliseconds later, the output 31 goes from logic 0 to logic 1, switching on the SCR Q3 which provides a discharge path for the charge stored in capacitor C, the discharge path being through the primary winding 33. This generates a 4000 volt positive going pulse in the secondary winding 37 of the high tension step up coil 35 which is available at the high tension output 39.

Resistors R are providing current limiting of the output signals from outputs 29 and 31 to the respective semiconductors.

Diodes 45 provide protection to the circuitry against any reverse EMF which is generated in the primary windings 33 of the high tension step-up coil 35, resulting from switching transients.

12 The cathodic protection control unit also includes power control means in the form of down circuitry 41 which senses the voltage of the power supply, and when that voltage falls to a value indicative of the charge in the storage battery 107 in the motor vehicle becoming'depleted, but not to the extend that there is not sufficient charge left to start the motor vehicle, a signal is applied to control line 43 for the duration of that low battery 107 voltage condition. The control line 43 is connected to the oscillator circuit 13, the timing oscillator circuit 23, and the push pull amplifier 15. In the presence of the signal, the circuits are disabled from operation, resulting in the power being drawn by the cathodic protection control unit being minimised.

Figure 2 shows the pulse voltage available at the 'high tension output 39, when there is no load connected thereto.

Figure 3 shows the pulse output when loaded by discharge plate having a capacitive coupling of a value of around 250pF.

The cathodic protection control unit is intended to be connected to the battery 107 positive terminal, and to the chassis of the motor vehicle. The high tension output is connected to a discharge plate of the type of which details are shown and Figures 12 to 14 inclusive, -by a high tension lead 51.

The discharge plate 53 comprises an electrode 55 bonded to a support plate 57. The electrode 55 is shown in Figure 10, and consists of a circular piece of copper clad epoxy resin fibreglass laminate, as commonly used in the manufacture of printed circuit boards in' the electronics industry. The laminate thickness is 0.5 millimetres.

Figure 10 is a view of the copper side of the electrode.

13 The epoxy resin fibreglass laminate board is exposed around the edge of the electrode 55 by etching the outer circumference 59 by a radius of 2 millimetres, leaving the copper cladding 61 in the centre of the electrode exposed.

The support plate 57 is formed of a piece of epoxy resin fibreglass laminate board which is not copper clad, having a thickness of 1.6 millimetres. The board has a pair of mounting holes 63 which serve to allow' mounting of the completed discharge plate to a motor vehicle chassis.

There is a recess 65 for accommodating the high tension lead which is connected to the high tension output 39 of the cathodic protection control unit circuitry. The electrode 55 and support plate 57 are bonded together in such a manner that the copper cladding 61 is sandwiched between the two layers of epoxy resin fibreglass laminate board, and thus insulated from the exterior of the structure.

The recess 65 accommodates a wire 67 which is soldered 69 to the copper cladding 61 of the electrode 55. The wire is of the type able to withstand high voltages (commonly referred to as HT lead or high tension lead).. The whole assembly is then sealed or encapsulated with an epoxy :esin moulding .71.' When the discharge plate 53 is mounted on, a flat surface of a motor vehicle chassis, the capacitance between the copper cladding 61 and the motor vehicle chassis is in the order of 250 pico-farads (pF).

Figures 15 and 16 show the cathodic protection control unit installed in two different types of motor vehicles. Figure shows the cathodic protection control unit 101 installed in a Volkswagon Derby 103 for testing purposes. The 14 Volkswagon 103 is a vehicle having the battery 107 mounted forward of the cabin area. Figure 16 shows.,the cathodic protection control unit 101 installed in a British Leyland Mini 105. The B.L. Mini 105 is a motor vehicle having the battery 107 mounted in the rear of the vehicle. These two vehicles provide examples of two common configurations regarding the placement of batteries and motor vehicles.

As it was considered that the protection from corrosion of the metal in the chassis and body work of the motor vehicles would likely be reduced by use of the cathodic protection system in each of these motor vehicles, it was also considered that maximum benefit would be most likely obtained by locating the discharge plate 53 it a position well away from the earthing point of the cathodic protection control unit, so as to cause impressed current flow through the chassis of the motor vehicle.

*f The tests were conducted in the South East of England and were intended only to provide the means of judging the effectiveness of the cathodic protection system. The test was conducted on vehicles, the Volkswagon Derby, and the British Leyland Mini, which would be driven normally and left uncovered when not in use. Two tests were run, in the S. first test, the British Leyland Mini had the cathodic ,410. protection system fitted, while the Volkswagon Derby acted as a control. In the second test, the roles of the vehicles were reversed. Whilst it was considered that the reliability of the results would be reduced because of the fact that each vehicle was being driven during the tests, resulting in the environmental conditions to which each was exposed not being identical, it was decided on balance that the advantage of the exposure to real road conditions outweighed this disadvantage.

15 Five test sites were chosen on each value, located as shown in Figures 15 and 16. These are labelled site.,1, site 2, site 3, site 4, and site 5 in each of Figures 15 and 16.

The paintwork at the test sites was removed, and the metal underneath was sanded to a bright finish. Approximately, 2 thousanths of an inch of metal was removed in order to eliminate the effect of any passivation which may have been applied to the surface of the metal work during the manufacture of the vehicles. The same test sites were used for the reverse test and were cleaned and prepared in the same manner It should be noted that during the period of the test, the vehicles were used within a twenty mile radius and were driven only a few hundred miles. They were not garaged at .0 night, and should have experienced very similar ood0 environmental conditions.

The tests were carried out with the frequency of the cathodic protection control unit timing control circuit running at an output of 70Hz. Accordingly, the period of the pulse train was approximately 14 milliseconds.

e00 g( The following table sets out the weather conditions and the distances which were travelled by each vehicle during the test.

#00 The table below shows the results of the test where the cathodic protection system was fitted to the British Leyland Mini whilst the Volkswagon Derby remained the control vehicle.

16 TABLE 1 40 4 4* *4 *4*s B.L. MINI VW Derby Day Aggregate Daily Aggregate Daily Temp C0 Humidty Weather Miles IMiles Miles Miles 0 0 0 0 0 5 40 Dry, Bright, Frost am.

1 14 14 0 0 6 58 Cloud, bright later, Frost am 2 36 22 24 24 9 70 Fog, mist later Dry 3 60 24 25 1 8 68 Cloud, Dry 4 64 4 30 5 9 70 Fog, Cloud later 121 57 46 16 10 72 Drizzle 6 163 42 46 0 11 63 Cloud Dry 7 163 0 60 14 11 65 Cloud Dry 8 208 45 90 30 10 65 Cloud Dry 9 252 44 90 0 10 55 Cloud Dry 10 357 j105 90 0 9 65 Cloud Dry Bright pm 11 392 t 35 110 20 10 48 Rain 12 403 i 1 1 110 0 10 75 Drizzle 13 431 28 131 21 9 56 Dry Cloud 14 431 t 0 155 24 13 52 Heavy showers 436 5 199 34 13 60 JShowers 16 436 0 199 0 13 74 Rain overnight, Showery day 17 436 j 0 199 0 13 60 Heavy rain 3* 4 4* 44 4.

4 4*e. 4 4..

4*4* 4 *4.4 44 4.

4* The following table provides the results of the reverse test, however this is the test where the cathodic protection control unit was fitted to the Volkswagon Derby, whilst the British Leyland Mini acted as a control vehicle -17 MaLL2 0 a* 0 B.L. MINI VW Derby FDay Aggregate Daiy Aggregate Daily Temp'C Humidity Weather Mites Mites Mites MilesI 0 0 I 0 0 0 12 62 Rain 1 9 I 9 19 19 11 60 Cloud Dry 2 19 10 48 29 13 80 Rain 3 32 13 72 24 14 40 Cloud, Sun Wind 4 40 8 72 0 11 45 Cloud Dry 67 27 76 4 9 62 Showers 6 il 44 76 0 13 56 Dry Wind 7 133 122 76 0 10 60 Drizzle Cloud 8 133 0 76 0 10 60 Clear Dry 9 153 20 96 20 10 50 Mist am Cloud Dry 177 24 96 0 9 39 Cloud Dry 11 185 8 96 0 10 36 Light cloud Dry 12 190 5 114 18 10 39 Light cloud Dry 13 210 20 114 0 8 44 Cloud Dry 14 258 48 114 0 9 62 Cloud Rain overnight 15 270 12 114 0 9 62 Fog am Cloud Dry 16 270 31 135 21 9 54 Cloud Dry 17 310 30 135 0 n 48 Mist Dry 18 315 5 135 0 9 70 Drizzle 19 358 33 159 24 12 75 Drizzle 380 22 159 0 10 50 Cloud Dry 21 380 0 159 0 10 61 Cloud Dry 22 443 63 180 21 10 68 Mist Cloud Dry 23 443 0 180 0 11 66 Cloud Dry 24 490 47 216 36 11 52 Cloud Dry 2S. 502 12 237 21 11 50 Cloud Dry The test results for the first test are reproduced in Figures 17 to 21 inclusive. The results for the second test are reproduced in Figures 22 to 26 inclusive, each 18 graph showing a site by site comparison for each vehicle and each test.

It is clear from the test results that the protection is more effective when the potential rusting site is located between the mounting position of the discharge plate 53 and the point of connection of the negative terminal for the battery 107 of each motor vehicle. This is not conclusive however, as the vehicles were driven, and road grime would have had an effect on those unfavourably placed sites, namely sites 1, and possibly site 2, in each case.

Referring back to the discussion relating to the possible benefit of obtaining impressed current flow throughout the chassis, by separating the discharge plate 53 and the connection point to the chassis of the negative terminal, of *5* o the battery 107, a test was conducted where the surface current at the test sites was compared using a small search coil, with the plate placed centrally, and with the plate extended to its full length along the chassis, in the Volkswagon Derby. It was found that the test site currents were reduced by more than a factor of 10 when the discharge plate 53 was placed in a central position.

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The' following is a discussion of the method- of observation of the level of corrosion at each of the test sites on each motor vehicle. Before each inspection, the site was cleaned with water to remove all surface dirt and stains which may have been caused by soluble corrosion products.

After drying, the extent of corrosion was assessed by estimating the percentage of total site area which appeared to be affected by rust. This rather subjective test was surprisingly repeatable, repeat assessments being within about 10% of each other, and assessments by a different observer being within about 20%. The results of the test 19 are shown, as aforementioned, in Figures 17 to 26 inclusive.

Following the testing carried out on motor vehicles, a pair of identical test rigs as illustrated in Figures 27 and 28, were constructed. Each test rig 109 was made of hot rolled mild steel sheet of a thickness sufficient to provide adequate rigidity. The test rigs were 9ft in length by 2 1/2ft by 2 1/2ft. The surface of each rig was painted, except for the test sites, which are indicated by a number in Figure 28, which were left unpainted. The test rigs were isolated from the ground on motor vehicle tyres 109 laid horizontally, in order to reproduce the leakage resistance to ground between a motor vehicle chassis and the ground upon which the motor vehicle rests, through the tyres of the motor vehicle. Both test rigs were supported on a sinle steel plate in order to ensure that the ground potential was made constant over the entire area on which the test rigs were positioned. A cathodic protection system was set up Jnside each test rig with the discharge plate 53 located at one end, the cathodic protection 101 located alongside the batteries 107 at the other end of each test rig. The negative terminal of the batteries was earthed in proximity to the batteries 107. Effectively, connection of the discharge plate 53 to each test rig and the connection of the negative terminal to each test rig were made at positions located at opposing ends of each test rig.

The preparation and observation of each test site during the test was managed in the same manner as those test sites on the earlier motor vehicle tests.

It was found that the test rig in which the cathodic protection system was operating exhibited lower rusting 20 than the control rig in which the cathodic protection system was not operating. There was some variation in the degree of rusting at various test sites, slight but this appeared to be more related to the positioning of the test rig relative to the direction of the prevailing wind, than related to the positioning of the discharge plate 53 and the negative connection to the batteries 107. Accordingly, the results showing rusting at each site on a daily basis are not reproduced here, however a graphical comparison is shown at figures 29 and Referring to figure 29, the results for thb first test are shown. It can clearly be seen that the test rig which had the cathodic protection system operational exhibited a lower degree of rusting than the test rig which did not have the cathodic protection system operational, which acted as a control. Figure 30 shows the results where the test was reversed, the roles of the two rigs being reversed. For the reverse test, all of the test sites were ground down to bright metal before the tests were repeated.

Both the first test and the reverse test were conducted until the percentage of surface rust averaged over all of the test sites on the control test rig reached The following tables give details of the temperature, humidity, whether conditions, and the electric potential developed between each test rig and the ground, on a day by day basis; for the first test and the reverse test.

21 RIG PTTILNDWTHER CQND.TINS.

DATE DAY PROTECTED RIG

RIG/GROUND

POTENTIAL

(my) CONTROL RIG

RIG/GROUND

POTENTIAL LEAKAGE 6

TEMP

0C HU14IDITY WEATHER FlI ESI 2/4 3 3/4 4 :494 4 :5e4 6 6Y4 7 8 91 10 11 11/4 12 12/4 13 13/4 14 15 16 "..616/4 17 17/4 18 *.18/4 19 20 020/4 21 *1/4 22 AEVERSnn. TE51 2 24a/4 3 29/4 4 30/4 5 5 7 8 9 10 <5 <5 <5 <5 -10 -15 -40 -40 -60 -60 -80 -90 -120 -100 -150 -160 -180 -170 -180 -10 -15 -20 -40 -100 -150 -150 -100 -200 0.5 0.5 0.5 0.6 0.8 0.9 1.2 0.9 0.8 0.6 0.7 1.0 1.0 0.6 1.2 1.0 0.7 0.8 1.0 0.4 1.0 0.9 1.5 1.5 2.0 2.0 4.0 2.0 4.0 Sun~/Dry Showers Dry/Sun Frost am Dry/Sun Frost am Sun/Dry Sun/Dry Sun/Dry Cloud/Dry Cloud/Dry Cloud/Sun/Dry Cloud/Dry Showers Dry Showers Showers Showers Light Rain Dry/Cloud Dry/Cloud Dry/Cloud Dry/Sun Dry/Sun Dry/Sun Dry/Sun Sun Condensation a Sun Condensation a Sun Condensation a Dry/Sun Cloud/Dry -10 -10 -10 -9 -10 -10 -9 -9 22 When the test rigs were first set up and before the cathodic protection system was activated, it was found that both rigs developed potentials relative to the ground, one of the test rigs developed a potential of +0.1 volts relative to the ground, and the other test rig developed a potential of -0.1 volts relative to ground. These potential appeared to be due to electrolytic potentials developed across the support tyres 109, because the potentials could be eliminated by replacing the tyre supports with ceramic insulators. Since the electrodes in this electrolytic system, namely the ground plate and the test rig metal work, were both made of the same quality mild steel, it was concluded that the effect must arise from slight differences in the chemical conditions at the "two electrode interfaces. It was also concluded that if this potential was short circuited for a sufficient length of time, the resulting current woald eventually exhaust the electro-chemical imbalance. After five days, the ground potential fell to a level of less than 5 milivolts for both test rigs. With the short circuit removed, the potentials remained at the same level for a period of two days, following which the cathodic protection system was activated on one of the test rigs. The test rig to ground potential for each test rig was measured daily thereafter.

On the control rig, the potential remained at less than 0. milivolts throughout the test, but on the live rig it became increasingly negative after a few days, reaching a value of about 0.2 volts near the end of the test.

Before the reverse test was carried out, the rigs were again short circuited to ground, repeating the overall prepatory procedure \as for the first test, but it was found that the test rig which previously had the operational cathodic protection system, retained a negative potential of several tens of milivolts. The test rig in 23 the reverse test which had the operational cathodic protection system built up a negative potential over a period of several days.

The results of both the first and reverse test show clearly that the test rig on which the cathodic protection system was operating developed surface rust more slowly than did the control test rig. Although the environmental conditions were very variable over the period of the tests (see tables supra) both rigs must have been exposed to nearly identical conditions. It seems to be an inevitable conclusion therefore, that the results obtained were due to the action of the cathodic pr.tection system.

It should be noted that the protection provided on the first test appears to be greater thaln that protection provided on the second reverse test. This also appears to be apparent from the results of the earlier test described on the British Leyland Mini and the Volkswagon Derby, but was considered to be as a result of poor control of environmental conditions. It appears that, due to the fact that once the cathodic protection system is switched off a slightly negative potential remains between the protected metal and the ground that a degree of protection remains which may last several days. Accordingly, this may have effected the rate of rusting on the control testing in the *reverse test, which had previously been the protected test rig in the first test.

Some investigation wa carried out into the nature of the wave form at the high tension output 39 of the cathodic protection control unit. It was found that following the initial pulse which had a voltage of 3.6kV, once that high voltage pulse decayed, there is a high frequency oscillation, oscillating in the vicinity of 10 to 24 The amplitude of this oscillation appears to contribute to the protection provided by the cathodic protection unit. Details of the observed output' are shown in figure 4, the portion of the wave form which is labelled is that during which the high frequency oscillation occurs. Figure 5 shows the current through the plate contributed by the initial high tension output pulse 111, and the current plate contributed by the high frequency oscillation 113 which follows the initial pulse.

Some investigation was carried out into the routing of the high tension lead 51, in relation to a vehicle chassis 115.

It was found through experimentation, that the high *tension lead 51 should not follow the return path for impressed current through a motor vehicle chassis.

•co *Referring to figure 6, the suggested path for the high tension lead is illustrated when the cathodic protection Ssystem is installed in a'motor vehicle with a continuous steel chassis. The current return path is shown at 117.

In the vehicle chassis 115 shown in figure 6, the discharge plate 53 is located in proximity to one of the vehicular tyres 119 to achieve maximum separation between the discharge plate 53 and the cathodic protection unit 121 which has its 0 volt connection earthed to the motor vehicle chassis in proximity to another vehicular tyre 123 which is located d4agonall opposite the first mentioned tyre 119. In this manner there is a large separation between the earth point to the chassis and the connection point of the discharge plate 53.

Figure 7 shows a vehicle chassis 115 which has a discontinuity in the form of a hole 125 in the centre thereof. With this arrangement, the return current paths 117 will deviate around the hole 125. Accordingly, placement of the high tension lead 51 should be in a direct 25 line between the discharge plate 53 and the cathodic protection unit and its 0 volt connection to ground 121.

Referring to figures 8 and 9 two graphs showing the external EMF generated around the discharge plate during and following the pulse from the high tension output of the cathodic protection control unit are shown. The graph in figure 8 is for a motor vehicle where the high tension lead 51 follows the return current path 117. The graph in figure 9 shows the external EMF generated around the discharge plate when the high tension lead 51 does not follow the return current path 117. As can clearly be seen, there is a much greater electric field in the vicinity of the discharge plate when the high tension lead o 51 does not follow the return current path 117.

Experimentation on the system has shown that the cathodic protection system should be arranged to produce pulses along the length of a vehicle rather than across it.

o. Furthermore, it has been shown in the course of experimentation that it is best to arrange the system so that the pulses are produced between two diagonally opposite wheels in order to get the greatest separation.

Figures 31, 32 and 33 show different wiring configurations, which should produce effective results at minimising the development the rust on a motor vehicle. The earth (0 volt ground) connection for the cat unit is shown at 127.

9.

It should be appreciated that the scope of the invention should not be limited to the scope of the embodiment described herein.

Claims (27)

1. A cathodic protection apparatus for reducing corrosion in metal structures having constituent elements prone to oxidation, said apparatus having inputs for connection to an electrical power source for the provision of electrical power thereto, and an output for providing an electrical signal to a capacitive coupling member connected to the output, said capacitive coupling member being adapted to be capacitively coupled to said metal structure so as to present a load to said output; wherein said apparatus -when connected to said load, is adapted to provide said electrical signal having characteristics comprising a train of electrical pulses having a positive voltage relative to said metal structure, said train of pulses comprising a pulse set repeated at periodic intervals where each pulse set includes a leading pulse having a peak voltage between 500 volts and 10,000 volts and a duration of between 1 microsecond and 100 microseconds and there is an elapsed time between successive leading pulses of between 1 millisecond ?nd 1 second.

2. A cathodic protection apparatus according to claim 1, wherein each leading pulse is immediately followed by a plurality of high frequency positive voltage pulses of lower voltage than said peak voltage. s**s

3. A cathodic protection apparatus according to claim 2, wherein said plurality of high frequency positive voltage pulses have a cumulative duration greater than the duration of an immediately preceding leading pulse.

4. A cathodic protection apparatus according to claims 2 or 3, wherein said plurality of high frequency positive voltage pulses have a frequency of between 10 megahertz and megahertz.

A cathodic protection apparatus according to any one of claims 2 to 4, further comprising a voltage transformer 27 wherein said train of electrical pulses is output from a secondary side of said voltage transformer and said plurality of high frequency positive voltage pulses occur during a period in which a primary side of said voltage transformer is not energised.

6. A cathodic protection apparatus according to claim further comprising a high voltage direct current voltage source connected to said primary side of the voltage transformer through a switching means; and a first oscillator operating at a first frequency operatively connected to said switching means for selectively connecting and disconnecting said high voltage direct current voltage source to the primary side of the voltage transformer wherein said voltage transformer produces said train of pulsos when said high voltage direct current voltage source is connected to said primary side.

7. A cathodic protection apparatus according to claim 6, wherein said high voltage direct current voltage source comprises a second oscillator operating at a second frequency higher than said first frequency having an output connected to a primary coil of a second transformer, and a rectifying means connected to a secondary coil of the second transformer to *eve 0 provide a high voltage direct current voltage. 0*60

8. A cathodic protection apparatus according to any one of claims 1 to 7, wherein elapsed time is selectably adjustable.

9. A cathodic protection apparatus according to claim 8; wherein a predetermined elapsed time can be selected for metal structures having predetermined physieaal and/or metallurgical characteristics.

A cathodic protection apparatus according to claims 8 or 9, wherein said elapsed time corresponds to the period of 28 a signal having a frequency in the order of 10Hz, 70Hz or 140Hz.

11. A cathodic protection apparatus according to any one of claims 1 to 10, wherein the duration of each leading pulse is between 2 microseconds and 10 microseconds.

12. A cathodic protection apparatus according to any one' of claims 1 to 10, wherein the duration of each leading pulse is between 5 microseconds and 6 microseconds.

13. A cathodic protecticn apparatus according to any one of claims 1 to 12, wherein said peak voltage is in the order of 4000 volts.

14. A cathodic protection apparatus according to any one of claims 1 to 13, further comprising a power control means for monitoring an output voltage from said electrical power source .and prohibiting the generation of said electrical signal when the output v o.tage is below a predetermined level.

15. A cathodic protection apparatus according to claim 14, wherein said power control means allows the generation of said electrical signal when said output voltage subsequently exceeds said predetermined level.

16. A cathodic protection apparatus according to any one v of claims 1 to 15, wherein a negative side of said electrical power source is connected to one of said inputs and to said metal structure at a first point and said capacitive coupling member is connected at a second point on said metal structure remote from said first point.

17. A cathodic protection apparatus according to claim 16, wherein said first and second points are located as far apart as possible on said metal structure. I 29

18. A cathodic protection apparatus according to claim 16, wherein said first and second points are located adjacent respective dielectric connections made from said metal structure to ground.

19. A cathodic protection apparatus according to claim 18, wherein said metal structure is a motor vehicle and each dielectric connection is a tyre supporting said motor vehicle, and first and second points are adjacent diagonally opposite tyres.

A cathodic protection apparatus according to any one of claims 1 to 19, wherein said output for providing the electrical signal is connected to said capacitive coupling member by means of an electrically conductive cable, and said cable is routed along said metal structure in such a manner as 1, to not coincide with a free current path between said output and said capacitive coupling member. *55*9*

21. A cathodic protection apparatus according to any one of claims 1 to 20, wherein said capacitive coupling member comprises an electrode sandwiched between first and second non- conductive planar members wherein the electrode is disposed inboard of the periphery of said first planar member, and said first planar member is disposed inboard the periphery of the pl .second planar member.

.22. A cathodic protection apparatus according to claim *O 21, wherein said second planar member is adapted to receive an end of a high voltage conductor to facilitate connection of said high voltage conductor with said electrode.

23. A cathodic protection apparatus according to claim 22, wherein said second planar member is adapted for connection to said metal structure in such a manner that said electrode is disposed substantially pai.llel to a portion of said metal structure to which said capacitive coupling member is connected. S% I 30

24. A cathodic protection apparatus according to any one of claims 21 to 23, wherein said first and second planar member are bonded together in a settable resin or plastics material.

A cathodic protection apparatus according to claim 24, wherein said first and second planar members are encapsulated.

26. A cathodic protection apparatus according to claims to 25., wherein said first and second planar members are formed of glass fibre reinforced plastics material.

27. A cathodic protection apparatus substantially as herein described with reference to and as illustrated in any one or more of the accompanying drawings. *ee Dated this 20th day of May, 1992. RAIDER ELECTRONICS PTY. LTD. By its Patent Attorneys: GRIFFITH HACK CO. Fellows Instutute of Patent Attorneys of Australia. *o of&* 4 C