Patent Publication Number: US-2015068919-A1

Title: Cathodic protection system

Description:
FIELD OF THE INVENTION 
     This invention relates to a cathodic protection system for steel structures (or cast iron) structures. This invention has particular application for cathodic protection of buried and submerged structures (on land and in marine off-shore applications). However, it is also be relevant for other types of structures in which steel (or iron) is a significant structural component. 
     BACKGROUND OF THE INVENTION 
     Steel structures are used widely in industrial structures and infrastructure due to its strength and tensile properties. However, corrosion is a major problem over time. The use of cathodic protection (CP) using sacrificial anode systems in order to inhibit corrosion of these structures is well known. Conventional sacrificial anode systems are fitted to industrial structures such as pipelines, above ground storage tanks, underground tanks, and many other structures sited on or buried in the ground. Typically, this is done by attaching a number of metal blocks (anodes) of a more active metal such as magnesium to the steel structure. The more active metal acts as a sacrificial anode preferentially corroding away. This generates a small amount of DC current. The current output capacity of a magnesium sacrificial anode system attached to a pipeline or similar structure is normally 50 mA or less. However, for poorly coated pipelines or structures, anode systems as high as 1500 mA or greater can be required. 
     Magnesium anodes are very inefficient. In order to generate 100 mA of current, one kilogram of magnesium alloy is consumed per year. 55% of consumption produces the DC current. The remaining 45% of the corrosion is consumed by self corrosion of the magnesium metal. Accordingly, the electrical efficiency of the anode is only around 55%. If the anode were disconnected from the steel structure, it would still naturally waste away due to self corrosion. 
     In order to achieve the DC current output required for corrosion protection, it is often necessary to fit a number of anodes at any one location. For a buried pipeline, anode beds are typically installed at two to four kilometre intervals at a distance of not more than two metres from the pipeline easement. A typical anode bed is comprised of 5 or more anodes in order to generate the requisite current at the installation location. Typically, the primary cost for five sacrificial anodes will be in the order of AUD$2,000.00. The installation and commissioning costs of the anode bed including civil works and commissioning will typically be in the order of AUD$5,000.00. 
     A typical magnesium sacrificial anode system has a life expectancy of ten years. Because many sacrificial anode systems are installed on pipelines often in congested city streets and have to be installed close to the pipelines. With urban development, the cost to excavate and install new anode beds on a ten year cycle is significantly higher. Replacement costs plus extensive excavation costs in congested urban areas can be in the order of at least AUD$10,000 per site. For installations along a pipeline route, being installed every two to four kilometers, the accumulated replacement costs over a fifty year lifecycle of a pipeline can be in the hundreds of thousands of dollars. 
     The cost to the environment in producing 1 kg of magnesium is approximately 40 kJ of power. So, apart from being a very inefficient anode material, the carbon footprint to the environment is grossly unacceptable. The environmental efficiency of a material is largely determined by the CO 2  footprint. 
     CO 2  Footprint: 
     The production of 1 kg of magnesium generates a CO 2  footprint of 42 kg of CO 2 . The annual average magnesium tonnage consumed by the cathodic protection industry per annum in Australia and New Zealand alone is in excess of 400 tonnes. This equates to an annual CO 2  footprint of 16,800 tonnes of CO 2 . Assuming a cost of AUD$23.00 per tonne for CO 2 , this equates to an annual cost to industry before materials of $386,400. Over a fifty year design life, this equates to a CO 2  cost of AUD$19,320,000.00 (being close to AUD$20,000,000. The environmental efficiency of a material substitution is largely determined by the CO 2  footprint. 
     SUMMARY OF THE INVENTION 
     There is disclosed herein an impressed current cathodic protection system for a target structure susceptible to corrosion (such as of steel or cast iron) which comprises an inert mixed metal oxide (hereinafter referred to as “MMO”) anode surrounded by a tightly packed conductive zone connected to a power supply source and having an input/output regulator to control the flow of current to the target structure. Preferably, the tightly packed conductive zone is in the form of a powder. In the preferred embodiment the tightly packed conductive zone is comprised of calcined petroleum coke (semi graphitized carbon particles). Further, in the preferred embodiment, the system is driven by a DC power supply stored in a sealed battery cell. 
     A number of conventional anode materials were considered for this application and rejected. Silicon, iron (chromium), graphite and scrap steel were all potential materials but considered unlikely to have the desired life expectancy for the application. Platinised titanium has/had historical technical limitations on voltage. Platinised niobium was not cost competitive. As the life of the anode is critical for cathodic protection, it was decided that MMO was currently the best available material for this application. An MMO electrode is one in which the surface contains two or more kinds of metal oxides. One of the metals, usually one or more earth derivatives (such as RuO 2 , IrO 2 , or PtO 0.12  conducts electricity and catalyzes the reaction. The amount of this (more expensive) metal is up to 10-12 g per square metre. The other metal is typically in the form of TiO 2  which does not conduct or catalyze the reaction, but prevents corrosion of the interior (and is cheaper). The interior of the MMO electrode is typically made of titanium. The MMO coating is applied to the surface of the titanium substrate to activate the surface. 
     According to the system of the present invention, the anode is surrounded by a zone of calcined petroleum coke, being a highly conductive material. This is in the form of a tightly packed powder with a low moisture content. This provides a tightly packed conductive zone around the anode and effectively increases the active zone of the anode. 
     According to one form of the invention, the specification for the calcined petroleum coke is as follows: 
     Fixed carbon: 99.35% 
     Ash: 0.6% 
     Moisture: 0.05% 
     Volatiles: Nil at 950° C. 
     Bulk Density: 74 lbs. per cubic foot. 
     Predominantly round particles. 
     All particles surface modified for maximum electrical conductivity. 
     Particle Sizing: Dust free with a maximum particle size of 1 mm. 
     Minimum calcination temperature of base materials in excess of 1200° C. 
     Base materials calcined under ISO 9002 quality control. 
     Surfactants added to assist pumping and settling. 
     No de-dusting oils used during the manufacture of base particles. 
     This material was selected as it exhibited the best electrical properties and reliability under the conditions tested. 
     In a preferred form of the invention, the anode and surrounding zone of calcined petroleum coke is contained within a tubular sleeve which is sealed at both ends. The sleeve may be of a permeable synthetic or linen cloth, or other suitable material, which is non-degradable in the environment in which the anode is to be used. The MMO wire is located along the centre of the sleeve which is sealed at one end. The calcined petroleum coke backfill is air blown into the sleeve through the other end and then sealed. The sleeve which by way of example may be typically (approx 50 mm Diameter×2000 mm and approximately 2 kg in weight) is of a compact design and allows for ease of installation. This is particularly important in congested urban areas where a more compact system is required. 
     The system is designed to be able to produce 50 to 150 mA of current subject to suitable conditions (such as soil resistivity). The number of anodes required to achieve the desired life expectancy will vary from one to a number of anodes depending upon the soil resistivity. Typically, a number of anodes are placed into a bed. The placement of the anode bed will be determined by a range of factors such as safe burial offset or below pipe distances and soil resistivity. 
     In certain circumstances, where a greater level of currentlife expectancy is required, it is possible to effectively increase the capacity of the system by adding more calcined petroleum coke. This may be achieved by using a larger sleeve or by adding calcined petroleum coke to the anode bed excavation (provided that the coke is sufficiently tightly compacted so that the entire bed becomes a working anode bed). 
     According to a preferred form of the invention, the system is provided with solar input power generation and battery storage. The batteries are charged by solar powered cells or panels. These may be mounted by various means. These may be mounted on the structure itself or in the case of a pipeline on a column designed for this purpose which may also conveniently house the hardware and service access. 
     It is anticipated that other alternative forms of environmentally friendly or cost effective forms of power supply may be utilized. These may include wind, thermo electric generators (TEGs) or turbine generation. 
     The use of solar or other environmentally friendly forms of power generation and storage provides a long-life system with environmentally favourable power generation thereby minimizing the environmental impact. It is anticipated that the total CO 2  footprint for the 1.5 mm diameter MMO/TlO 2  anode will be around 0.473 kgsm. 
     The output power to the structure is controlled by a DC input/output regulator. Preferably, the system will be capable of operating at a number of output capacity ranges. Typically, these would be 50, 150, 500 and 1000 mA. However, these figures are by way of example only. The range may vary depending upon the application. Preferably, this would also be provided with lightning and surge protection. Typically, the output regulator is approximately 30 mm×80 mm. The output regulator and control circuitry are designed to fit within a small control box which may be fitted on the structure or support column or, where applicable, on a structure within close proximity such as a suburban lamppost. 
     The system design can also accommodate a range of optional monitoring systems. 
     The system can accommodate the inclusion of a permanently installed reference cell to allow monitoring of the protection system by conventional manual methods or via an electronic interface or SCADA system. 
     Alternatively, the system can accommodate remote electronic surveillance and monitoring systems to provide continuous electronic monitoring and reporting via satellite or a GSM communications system. Accordingly, the system is capable of reducing the need to travel to sites for inspection and testing. 
     Alternatively, the system can also accommodate an automatically controlled output regulated system that incorporates the above reference cells. This may be set during commissioning to allow for a set protection level and also regulates output in automatic control mode. 
     The above system utilizes inert anodes that do not galvanically corrode and increases the active zone of the anode, thus greatly increasing the life expectancy of the system over those systems currently known and in use. It is anticipated that the system of the present invention will have a life of at least 50 years. To accord with the anticipated life expectancy, the anode bed, electronics and related electrical components are also all designed to a specification of at least a 50 year life expectancy. The advantage of the above invention over current systems is that it provides a compact system with a 50 year life expectancy which is easy to install even in difficult conditions. It is also able to be modified so as to provide a level of flexibility. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that this invention may be more readily understood and put into practical effect, a preferred form of the present invention will now be described by way of example with reference to the accompanying drawings wherein: 
         FIG. 1  is a cross-sectional view of a preferred embodiment of the impressed current cathodic protection system. 
         FIG. 2  is a cross-sectional elevational view of a further preferred embodiment of the system of the present invention showing a solar powered impressed current cathodic protection system and reference cell installation. 
         FIG. 3  is a cross-sectional side view of the embodiment in  FIG. 2 . 
         FIG. 4  is a detailed front elevational view of the solar panel and supporting column, foundation for the solar powered installation in  FIGS. 2 and 3  and of the housing for the regulator and control circuitry. 
         FIG. 5  is a further detailed front elevational view of the solar panel, supporting column, foundation and baseplate for the solar powered installation in  FIGS. 2 and 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , there is shown an impressed current cathodic protection system for steel (&amp; cast iron?) structures ( 1 ) which comprises an inert mixed metal oxide (MMO) anode wire ( 2 ) surrounded by a zone of calcined petroleum coke ( 3 ) contained within a tubular sleeve ( 4 ) sealed at both ends ( 5 ) and connected via a cable tail ( 6 ) to a power supply. Preferably, the system is driven by a DC power supply, which in this embodiment is a sealed battery cell (not shown). 
     The system also has an output regulator (not shown) to control the flow of current to the target structure. 
     In this preferred form of the invention, the specification for the calcined petroleum coke is as follows: 
     Fixed carbon: 99.35% 
     Ash: 0.6% 
     Moisture: 0.05% 
     Volatiles: Nil at 950° C. 
     Bulk Density: 74 lbs. per cubic foot. 
     Predominantly round particles. 
     All particles surface modified for maximum electrical conductivity. 
     Particle Sizing: Dust free with a maximum particle size of 1 mm. 
     Minimum calcination temperature of base materials in excess of 1200° C. 
     Base materials calcined under ISO 9002 quality control. 
     Surfactants added to assist pumping and settling. 
     No de-dusting oils used during the manufacture of base particles. 
     This material was selected as it exhibited the best electrical properties and reliability under the conditions tested. 
     Referring to  FIGS. 2 &amp; 3 , there is shown an elevational and side view respectively of a further preferred embodiment of the cathodic protection system (which may be comprised of one or a bed of anodes) of the present invention.  FIGS. 2 and 3  show the placement of the cathodic protection system ( 1 ) and a reference cell ( 7 ) relative to pipeline ( 8 ), being the target structure in this case. The cathodic protection system ( 1 ) and reference cell ( 7 ) are embedded in sand ( 9 ) at a safe burial distance from the pipeline ( 8 ). This is covered by a layer of rock/free backfill ( 10 ) and generally topped with a finished grade ( 11 ). In this embodiment, the system is provided with solar input power generation. The cathodic protection system is connected by cables ( 12 ) to one or more batteries. The batteries are charged by solar cells on one or more solar panels ( 13 ). These are mounted on a supporting column ( 14 ) with baseplate ( 15 ) in concrete foundation ( 15 ). The output regulator, control circuitry and service access are located in housing ( 17 ) at the base of the column ( 14 ) with access door ( 18 ) for ease of access. A detailed view of the column ( 14 ), foundation ( 16 ), housing ( 17 ) and baseplate ( 15 ) are provided in  FIG. 4 . Structural components and related equipment are manufactured to applicable local building and safety standards. 
     In other forms of the invention, it is envisaged that other alternative forms of environmentally friendly or cost effective forms of power supply may be utilized. These may include wind, TEGs or turbine generation. 
     It will of course be realized that while the foregoing has been given by way of illustrative example of this invention, all such and other modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this invention as is herein set forth.