CATHODIC PROTECTION NETWORK INTERNATIONAL
Sacrificial anodes used on buried pipelines and structures have a finite life and have to be replaced when their metal has been consumed or the surface area is reduce to a degree that it no longer releases sufficient charges to protect the subject metal.
Their output depends on the surface area of the anode metal in contact with the corrosive electrolyte and the resistance of the materials as the charges radiate out to 'remote earth'.
Replacement is achieved by excavation but is very expensive and inconvenient where there is congestion such as in refineries or in roads where the public has access.
The solution is to design a sacrificial system in which the anode can be replaced without excavation after the first installation.
The established manner in which sacrificial anodes are used is to pack a lump of the selected metal in a bag of electrolyte that will facilitate the corrosion reaction to dissolve the metal and discharge electricity. The CPN (Cathodic Protection Network) anode uses a similar core of sacrificial metal but it is in a container made of strengthened porous graphite (or similar carbon) and this is slid into an upright terracotta pipe set in the ground with a 'manhole cover' as permanent access.
Sacrificial anodes are traditionally buried at the time of the pipeline construction at convenient places where it is estimated that coating damage might result in corrosion to the pipeline metal. The 'anode tails' are the connecting wires that are built into the anode itself and are insulated (usually in red plastic) to be brought to the surface through test posts or junction boxes where they are connected, via removable links, to a cable that is connected to the pipeline metal itself.
The new replaceable anode can be located anywhere and at any depth required, connected in the same way through a bond box in any location.
The anode itself can be accessed through a manhole set in any surface.
The performance of the anode can be tested by putting an ammeter in circuit to measure the current that is relational to the amount of metal going into solution.
It is traditional to make a voltage measurement between a ground contact electrode and the pipeline metal but this measurement is not well understood by most practicing corrosion control engineers.
For this reason it is essential that any user of the new anode studies the information about cathodic protection on line in the web pages of Cathodic Protection Network.
A sacrificial anode system is in fact a dry cell battery where the anode is the zinc case. the ground is the paste and the metal of the pipeline is the carbon rod in the center of the battery. When we use a battery we connect a resistance between the brass cap on the carbon rod and the outside of the metal case.
The corrosion reaction on the inside of the zinc case releases electrical charges that pass to the carbon rod as it has a more noble reaction to the paste. From the rod, the energy passes through the resistance where some is converted to another form of energy (for example heat) before completing the circuit back to that zinc.
The system is in equilibrium due to the current flowing.
If we 'break the circuit open' then no current can flow and corrosion stops, this is the reason why we apply coating to seperate the metal from the electrolyte both chemically and electrically. If we apply electrical energy in the opposite direction to that of the corrosion current, we can reach equilibrium with the circuit closed and this is how we control corrosion with cathodic protection.
This can also be visualised as the electrical pressures that we call 'potentials'. We have first to understand that there is no such thing a zero potential but that we set this ourselves because our measuring devices require a datum from which to make a measurement. For the purposes of measuring the EMF of a corrosion reaction we use a reference potential but this is only valid if set up in the way described in DIN50918 and other specifications for laboratory work. The next picture shows the pipe metal being our zero and the red arrows the direction from the EMF of our corrosion reaction anode towards the cathode of this cell. The anode in DIN50918 is described as the working electrode and the cathode as the return electrode.
It can be seen that a 'reference electrode' placed anywhere in the potential zones resulting from the corrosion reaction will cause errors in the measurement. This is due to the IR drop in the electrolyte that is a function of the diffusion of the charges before they concentrate again when entering the cathode (return electrode) of the corrosion system.
The green arrows and concentric circles represent the path of charges from the disolving sacrificial anode (or impressed current anode) and the potential zones resulting from the dispersion of these charges to 'remote earth' before completing their circuit via the coating fault
It can be seen that we cannot measure the actual corrosion current as we cannot break the circuit of a corrosion cell on the surface of the pipeline to make a current measurement.
However, the Alexander Cell is a corrosion cell that we can attach to the pipeline, making it a coating fault with known dimensions and properties. It is provided with a sample of the electrolyte in which the pipeline is submerged and the corrosion current must pass through the ground to complete it's circuit. The potential of the ground at the base electrodes will determine the path that the charges take to the pipeline metal to complete the cathodic protection circuit that is integrated with the corrosion circuit when the cell is connected to the pipeline. If the electrical pressure (potential) of the electrolyte (ground) at the point of contact is greater than the electrical pressure (EMF) of the corrosion reaction at the anode then the reaction will stop because the energy component will be balanced. Users of the Alexander Cell soon realise that this is all they need to know about the success of their cathodic protection systems in controlling corrosion. The criterion for cathodic protection is now the electrical balance at which corrosion stops in each location tested.
The new design of the sacrificial anode system by Cathodic Protection Network uniquely turns the 'dry cell battery' inside out. A magnesium, zinc or aluminium core is placed in a carbon case filled with active electrolytic material and the charges flow from the anode through the carbon that becomes charged more positive than the pipeline metal. These excess charges pass to remote earth and complete the circuit through coating faults in the pipeline. The driving force of the anode to carbon EMF is greater than the driving force of the steel to electrolyte reaction of a natural corrosion cell on the pipeline metal.
These anodes can be made of any size and any large, straight sided pot will suffice so long at it is completely porous.
This first stage of the development of the CPN anode works in exactly the same way as a traditional sacrificial anode system works but in this case the anode can be replaced without excavation by simply withdrawing the carbon case from the terracotta pipe. A suitable manhole cover arrangement will facilitate access for routine monitoring and replacement.
The next stage of this system is to manufacture and install further anodes 'in series' in the same way that we put batteries in a torch. The sketches show how this is accomplished with an insulating sheath.
We now have an arrangement that will produce potential differences far greater than that of a single reaction EMF (electro-motive force). We can control the amount of current by a variable resistor that can itself be controlled by an electronic sensor detecting a state of equilibrium in which no current flows through a specimen corrosion cell.
An Alexander Cell will fulfil this purpose but coupon systems suggested by a NACE qualified corrosion engineer (as shown below) will not. It is essential that these facts are known by all followers of CPN technology as they are sound science that works in practice.
It can be seen that the steel plate (coupon) in these pictures, has both anodes and cathodes on the surface that is in contact with the electrolyte. These means that the measurements are not made in accord with DIN50918 as that requires a separate anode (working electrode) and cathode (return electrode) so that the IR drop between them can be eliminated with a Lugin capillary.
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The charge density at the surface of this steel plate will vary, depending on the current that is impressed into the ground. These variations will reflect the current passing through the resistance as a voltage between potential zones. This can be seen by using a roving copper/copper-sulphate ground contact electrode in conjuction with one of these coupon systems, both in the field and on the bench in a laboratory.
The Alexander Cell not only replicates the requirements of DIN50918 but makes the measurement of current density possible as it has a defined area of anode and cathode. The actual corrosion current is measured as well as the effect of the cathodic protection and other influences on that current.
Manufacture of the components of the CPN Anode.
The CPN anode metal should be shaped to offer the required surface area in contact with the aggressive electrolyte. It should be made with a central core of steel to which can be connected a well insulated conductor cable to complete the circuit to a junction box or test post.
The corrosion reaction releases energy into the electrolyte in exactly the same way as in any pre-packed sacrificial anode.
These charges then pass into the carbon shell increasing its potential in relation to the salts in the porous surrounding terracotta.
The terracotta pipe is then charged to a higher potential than the surrounding ground.
The charges will then equalise with the ground.
The connection to the pipeline allows the current to flow and the system to reach equilibrium.
The charges then reach equilibrium with the surrounding ground by following the paths of least resistance as per Kirchoffs law of resistances in parallel.
Field measurements using two ground contact electrodes can prove these drawings.
The top of the CPN anode contains an Alexander Cell as a trigger to a small circuit that controls the amount of current from the anode corrosion reaction to balance the EMF of the corrosion reaction in the Alexander Cell. This can be achieved as the charge density at the base plates of the Alexander Cell is in a potential zone caused by the sacrifical anode charge diffusion related to the shells of resistance at all the interfaces of the system. It must be understood that when the Alexander Cell is connected to the pipeline it adds a coating fault with a surface area of the two base electrodes to any coating faults on the pipeline in this area.
The carbon case is indeed a problem to manufacture as it has to fit inside the terracotta pipe/container. I made my small version of this container out of 'charcoal' drawing sticks, bound together with string and connected them together by threading wire through a small hole.
This carbon case is crucial to the next part of the design.
We now have a battery turned inside out that we can put in series with any number of such batteries. As with traditional batteries we must connect them positive to negative with no short cicuit positive to positive.
This is achieved by putting all but the top one in a plastic pipe between the carbon and the terracotta. This is the arrangement that is common in a multi cell torch. I have a similar arrangement that is used in demonstrations and shown in the picture below.
The charges that result from the sacrificial anode are detectable by the same instruments as the charges that are impressed into the ground by a transformer rectifier. If there are scientific differences I have never seen them demonstrated or observed any difference in field work or in the laboratory.
The charges disperse according to Kirchoffs law, through shells of resistance formed by the material through which the charges are conducted. If this were a homogenous material then the distribution is three dimensionally similar to the inverse square law of radiation. However, the ground is always stratified and there are always more conductive paths in the fixed electrolyte that will distort the real dispersion.
The only way to define the actual potential profile of the ground surrounding a cathodic protection anode is to map the surface potential gradient using a fixed electrode as the reference potential. You can then make a three dimensional graph at various settings of the anode output. This is achieved in an impressed current system by controlling the DC positive into the anode. In the new anode it can be achieved by adjusting the variable resistor between the pipeline and the anode
I will be uploading the next stages of development relating to remote monitoring and then automatic computer control of the equilibrium of networks of pipelines.
This is part of the Cathodic Protection Network Dynamic Project of software development.
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