(cathodic protection)

ADMA – OPCO TASK DETAILING MANUAL

MODULE No. E-14 : Cathodic Protection

MODULE NO.: E-14

Cathodic Protection

IHRDC OJT Module – (Rev. 0) August 2000 Page # 1



INTRODUCTION

Cathodic protection has always been used on Das Island as a form of corrosion control but originally it was confined to areas outside the potentially dangerous conditions of the Process and Storage areas. However, more recently, an updated Cathodic Protection System was commissioned in the Process and Storage area with permit and bonding procedures. All personnel involved in the supervision of work in these areas must be familiar with the basic theory of cathodic protection, the potential hazardous situations arising from its use and the means by which these hazards are eliminated.

BASIC THEORY OF CATHODIC PROTECTION.

Iron and steel are by far the most widely used metallic construction materials. They are both relatively cheap and flexibility of their properties can be easily obtained by alloying, heat treatment etc. to allow their use in all but the most arduous or specialised conditions. However, they are subject to corrosion in certain environments and need to be protected in some way to allow their long term use.

Corrosion can be seen as an attempt by the iron to return to its natural state, i.e. the ore from which it has been produced. To put it another way, the iron is unstable and if conditions are right it will revert to its most stable form - iron oxide. In practice we all know the conditions that give rise to corrosion. Most important is dampness or wetness but other factors are the acidity, the degree of oxygenation and presence of salts, i.e. high acidity causes rapid corrosion as does dissolved oxygen or salt in water (sea water, for example, is much more corrosive than fresh water).

The actual corrosion reaction is an electro-chemical one. This means that there is a transfer of electric charge or current on the material during corrosion. Figure 1 shows the reactions occuring at the surface of a piece of corroding steel. Basically a small quantity of iron dissolves leaving a negative charge (electrons) behind. These electrons migrate to areas which, due to metallurgical differences in the metal or to external conditions, are able to transfer them to dissoved oxygen (or hydrogen in acid conditions). Hence, the charges can be seen to be effectively neutralising themselves in the solution by forming ferrous hydroxide (a hydrated




form of rust). The reactive areas where iron dissolves most easily are termed anodic areas and the areas that the electrons are transferred to and which do not corrode are termed cathodic areas.

Other metals besides iron or steel corrode in just the same way. Some are more reactive than iron in any particular environment and some are less reactive or noble. Figure 2 shows a table of the reactivity of some common metals in sea water. This table is called the Galvanic Series. It can be seen that some metals, notably magnesium, zinc and aluminium, are more active or anodic than iron and steel. Hence, if a piece of zinc is connected to a piece of steel in seawater it will be found that the zinc acts as an anode and corrodes whilst the steel becomes a cathode and does not corrode. Figure 5 shows the surface reactions occuring in this situation. Electrons flow from the corroding zinc surface through the metal to the non-corroding iron surface. This is equivalent to a conventional direct current flow through the water (electrolyte) from the zinc to the iron. Compare Figure 5 with Figure 4A which shows the same basic components i.e. zinc connected to steel in an electrolyte (soil or water). The zinc corrodes and in doing so protects the steel by causing all the steel surface to act as a cathode. This is cathodic Protection’ by use of a 'Sacrificial Anode’.

Figure 4A shows the effective current flow which is giving the steel cathodic protection. Figure 4B shows an alternative method of producing this current flow using a d.c. source such as a battery or, more practically, a rectifier unit and an inert anode. This is termed 'Impressed Current Cathodic Protection* because an external current source is used.

In practice both types of Cathodic Protection Systems are used. The choice of system is dependent on the economics of each particular application, such as power availability and cost etc. In the process/tankage area on Das Island an impressed current system is used operating with thirty-three transformers / rectifier units each with boreholes down which the inert anodes are placed - in this case platinum coated titanium rod anodes. Figure 5 shows a typical station.

All buried or immersed steelwork can be cathodically protected but it should be noted that above ground metal is not protected since there is not a conductive path through soil or water to complete the circuit.




Whether or not cathodic protection is achieved depends on the amount of current being received by each unit area of bare steel - typical figures would be of the order of 20-150 mA/m2. Obviously, then, a bare structure takes a considerable current to protect it, whereas a coated structure which may be 90-99% coated only requires a small current. Note that simply coating a structure is not sufficient to protect completely against corrosion since not only are areas of coating damage inevitable on installation, but coatings also deteriorate with time. Cathodic protection is most successfully used to protect well coated structures against corrosion since the currents required are fairly low in comparison with bare structures and hence large areas can be protected with one rectifier.

Measuring the current flowing onto a metal surface is obviously not -a practicable way of checking whether the protection is effective. However, the effect of the current pick up is to charge the potential of the steel surface with respect to the soil. Any metal immersed in soil or water has a natural potential - just as the plates in a battery have a potential. Normal corroding steel has. a potential of between 0.4 - 0.7 Volts negative. Experience and theory have shown that cathodic protection is effective if the natural potential is shifted to greater than 0.85 Volts negative.

Coating damage may occur if potentials greater than 1,5 Volts negative are impressed. This potential is easily measurable using a standard (i.e. known potential) cell placed in the soil near the structure, and comparing the two potentials on a voltameter. The usual reference cell is a copper/copper sulphate cell and all voltages mentioned above are measured with respect to this. Figure 6 shows the measuring principle involved.

Figure 7 shows the cathodic protection of a large structure or pipeline. The point to note is that current is picked up by the structure at all points and returns along the structure to the negative connection of the rectifier. Hence the current flow is highest at this point.

SAFE WORKING PROCEDURES ON CATHODICALLY PROTECTED PLANT ON DAS-ISLAND.

The current flow in a cathodically protected structure can be quite high. If a rectifier is operating at, say, 10QA output it means that this current is also flowing




in the structure near the drain point. Should the current path be interrupted, by breaking a current carrying pipeline for example, then this current is quite capable of causing a spark at the moment of break. It is impossible to say what magnitude of current is required to give sparking since it depends on many variables and it is therefore imperative that precautions are taken when any work which is likely to cause interruption of the current path is to be carried out.

Possible situations leading to spark hazards are :-

a) Deliberate or accidental disconnection of pipelines, joints or associated equipment under protection. (See Figure 9A). Examples are spading a line, breaking a flange generally, cold cutting etc. Note the current path has to be wholly interrupted to cause, a hazard. Cutting a hole, breaking a by-pass etc. do not cause a hazard since the current path is not interrupted.

b) Accidental short circuit between protected and unprotected structres. (See Figure 9B). This should cause no problems on Das since all pipelines and vessels in the area are to be protected, hence the potential differences between all structures will be small.

c) Accidental short circuit of insulating flanges. The only insulating flanged joints used on Das are at the extremities of ship loading lines i.e. harbour service lines and tanker berth flow booms. Figure 10 shows the correct installation of an insulating flange in a loading line to prevent cathodic protection currents flowing in these lines and causing a hazard on connecting/disconnecting the loading line.

d) Breaking or disconnecting bonding cables or cables to rectifiers orground-beds. Care should be taken to ensure that no cables are broken or disconnected in the same way that care is taken with normal power cables etc.

Two points to note regarding operational safey are firstly that cathodic protection currents are not dangerous to personnel and will not cause electric shock since the potentials involved on pipelines are very small and the system is earthed. Secondly, precautions taken to prevent sparking are only necessary in cases where the plant carries flammable products and/or in zone 0, 1 or non-gas-tested zone 2 areas.




Precautions to prevent sparking involve the fitting of an electrical bond around the work area before work commences. This bond is not removed until work is completed and electrical conductivity is once again assured. In addition, when work is being carried out in the vicinity of cathodic, protection negative connections where higher currents are likely in the structure then the relevant rectifier would be isolated before commencing work. Bonds comprise clamps which fit onto flanges either side of the work area with a heavy flexible cable between them of length sufficien to carry out the work satisfactorily. A flameproof switch will be installed in this cableto allow the bond circuit to be broken in safety when necessary e.g. removing a piece of pipework and removing the bond afterwards.

The fitting and removal of a bond shall be carried out only by a authorised electricians. The procedure is as follows.

When an application to carry out hot or cold work is made to the Area Authority the Authority must decide whether the work is likely to involve a spark hazard due to operation of cathodic protection on the plant. If so, or in cases of doubt, then the Permit Requesting Authority should be referred to the Electrical Department.

An electrician will fit the bond, isolate the rectifiers where necessary, and issue a Danger Notice tag as shown in Figure 11. One section of the Notice is fitted to the bond and the other is returned to the Work Permit Issuing Authority. The Notice is then held by the Issuing Authority along with their copy of the Permit and the number of the Notice is written on the Permit. Only after this has been done, can work be commenced.

On completion of work the Area Supervisor or his representative shall sign and date the Cancellation Section of the Notice and inform the Electrical Department that work has finished. The Electrical Department will remove the bond and re-energise any rectifiers that have been isolated.

Figure 12 showy the bonding/permit procedure in the form of a flow diagram.




Active MagnesiumOr Anodic

Zinc

Galvanised Steel

Aluminum

Mild Steel

Cast Iron

18-8 Stainless Steel (Active)

Lead

Tin

Brass

Copper

18-8 Stainless Steel (Passive)

SilverNobleOr Cathodic Gold

Figure 2: Galvanic Series Of Metals In Sea Water




Conventional Current Flow Through Electrolyte

Fig. Galvanic Couple in Water or Soil


Cathodic



Fig. 4: A- Sacrificial Anode Cathodic Protection B- Equivalent Impressed Current Circuit


A

B



Fig. 5 : actual impressed current cathodic protection system as applied to Das.




Fig. 6: For cathodic protection to be effective the steel structure should have a potential of –0.85 V measured with a copper/copper sulphate reference electrode. Normal corroding steel should be between –0.4 to –0.7 volts.




Fig. 7: Current Flow Along Protected Line


Current is picked up by the structure at all points and returns along the structure to the negative connection of the rectifier



Figure : 8




Fig. : 9 Possible spark hazards related to cathodic protection


A

B



MODULE No. PR-23 : Cathodic Protection

TASK No. 23.4 : Continued

Fig. 10: Ship loading hose flange insulation




CATHOD1C PROTECTION

THE CORROSION PROBLEM

Steels used in platform and pipework construction are not pure materials. Although iron (Fe) forms the major pan, steel contains a certain quantity of carbon (C) up to about 1%. The amount of carbon determines the type of steel — for example from mild steels up to hard tool steels. The carbon takes the form of iron carbide (Fe3C), and, because it is not spread quite evenly through the steel, there are adjacent surface areas of pure iron and iron carbide potential than pure iron when in the presence of a conducting electrolyte throughout the steel Structure. These individual areas are minute in size and arc very close together - a matter of thousandths of an inch.

Iron carbide has a lower potential than pure iron when in the presence of a conducting electrolyte (such as seawatcr or damp soil). That is to say, a mass of iron/iron-carbide 'cells' are formed in the electrolyte, with the pure iron areas positive and the iron-carbide areas negative.

Each cell, which can be regarded as a primary battery, is short-circuited by the structure metal work, as shown in Figure 6.1.

Figure 6.1Corrosion Mechanism

At the iron surface some of the electrons surrounding the iron atoms break away, leaving the atoms of iron positively charged (Fe++). These arc called iron 'ions', and they are, in fact, positively charged at oms of the metal. They travel through




the sea or soil down the potential gradient from the positive iron areas to the negative iron-carbide areas, forming an electric current through the sea or soil as they do so. This current returns through the main structure metal conducting path.

The positive areas from which current flows into the sea or soil are called 'anodes' (or anodic areas), and the negative areas into which the current flows are 'cathodes' (or cathodic areas).

As the iron metal is removed, atom by atom, from the anode and deposited at the cathode, a corrosion pit is formed as shown in Figure 6.1. Hundreds of thousands of such pits are formed, which appear as general corrosion of the structure member or pipe.

This process can be slowed by painting or by other forms of encapsulation, but it reappears increasingly as the covering wears off. On the other hand marine growth may tend to inhibit the process with time, but no reliance can be placed on it. THE CURESince corrosion is due to the outflow of iron atoms carried by the sea or soil current, it can be reduced or even prevented if such currents could be stopped or reversed, so causing all the submerged or buried metal parts to receive current instead of giving it out -- that is, by making all areas cathodic.This can be done by placing other, independent electrodes in the sea or soil near the structure and causing them to force current into the structure.There are two quite distinct methods of achieving this, known as the 'sacrificial anode' and the 'impressed-current' systems. Either method may be used on an offshore installation and on either type of platform, steel-jacket or concrete; in the latter the well risers and their guide-tube structures still have to be protected. The two methods are shown in Figures 6.2(b) and (c).




FIGURE 6.2CATHODIC PROTECTION

Figure 6.2(a) shows an unprotected offshore structure as already described. Myriads of 'sea-cells' are formed on the surfaces of the structure, causing loops of current to flow into the sea from the positive, anodic areas into the negative, cathodic areas.

The following paragraphs describe the protection systems as applied to an offshore platform. The same methods, however, are used with an onshore installation where, instead of being immersed in seawater/ metal is buried in, or stands upon, soil, which always contains water.

SACRIFICIAL ANODE SYSTEM

The system is shown in Figure 6.2(b). An electrode, called an 'anode', is placed in the sea near the metal to be protected. It is made of a material which is 'base' relative to iron; this means that, in the seawater electrolyte, the anode material becomes negative relative to the iron. Zinc is usually the material chosen, and it is in the form of an array of plates placed near to, and dispersed at points all along, the metal to be protected. The zincs are each metallically connected or 'bonded* locally to the exposed steelwork at the many dispersed points. The zinc and iron




masses, with the seawater between acting as an electrolyte, each form a sea-cell, with the iron positive and the zinc negative. This cell is short-circuited by the bonding, which causes a current to flow from positive to negative through the structure and the bond, as indicated in Figure 6.2(b). It flows from the positive iron, through the low-resistance bonding path, into the negative zinc, and thence back to the iron again through the conducting seawater in between, Just like an ordinary short-circuited battery. This sea current is in the opposite direction to the natural electrolytic currents from the anodic areas shown in Figure 6.2(a), and may be considered to swamp the small current loops at the structure.

If the anodes chosen are of sufficient surface area, their current can be made to cancel the natural electrolytic currents exactly; if made larger, they will reverse it. As the current from the zinc anode is accompanied by continuous loss of metal, the anodes eventually become too small to be effective and must be replaced. For this reason they are called 'sacrificial anodes'. Their effectiveness must be periodically monitored to determine when replacement becomes necessary. A life of 1 0 years is envisaged.

It should noted that the sacrificial anode system requires no external power source. The energy required to produce the current is derived from the chemical energy of the anode material, which disintegrates when that energy is given up.

Typical figures for a steel-jacket platform are:

Cathodic area (structure steel, well risers and 84 000m2 Foundations) to be protected

Anode arrays (55 x 0.15 m2 ) 8.3m2 (0.01%)

Current per anode array 29A

Total protection current 1 595A




To illustrate the effect of anodes of this type, consider first what goes on inside an ordinary primary battery cell. Figure 6.3 shows a single two-metal primary battery cell which, in (a), is on open circuit. An emf E is developed between the plates and appears as a voltage between the cell terminals. But no current flows from the positive to the negative terminal, and there is therefore no return current inside the cell from the negative to the positive plate.

Now apply an external load of resistance R as in (b). A current / flows externally through the resistance R from the positive to the negative terminal and returns internally through the electrolyte, path-resistance r, from the negative to the positive plate. The terminal voltage V is reduced from the open-circuit emf E by the internal drop in the cell. so that

V= E - /r

If now the ohmic value of the external load is reduced to zero as in (c), R = 0 and the cell becomes short-circuited. The load current rises to the shot-circuit value /SC




limited now only by the internal resistance, so that E =/ SC and V = 0. The internal current in the cell from the negative to the positive plate is (hen also /SC

FIGURE 6.4 PRINCIPLE OF SACRIFICIAL ANODE PROTECTION

Consider now Figure 6.4 which shows a steel undersea oil riser pipe with a circular, collar-type zinc anode placed around it and bonded direct to the pipe at several points by cad-welded straps. The steel pipe and the zinc collar form an iron/zinc primary cell with the surrounding seawater acting as electrolyte. The welded straps form the 'external' short-circuit between the zinc and iron.




Normally one thinks of a 'cell' as being a container filled with electrolyte, with two plates immersed in it and brought out to external terminals in air, as inset in Figure 6.4. If these terminals are short-circuited, the situation is exactly as in Figure 6.3(c), the cell having a developed iron/zinc emf E = 0.32 volts (see next paragraph).

Different metals immersed in an electrolyte have differing electromotive force potentials, as the examples in the following table show:

These potential voltages reflect the energy used in the refining of the metals. Those needing the greatest energy have the largest negative potentials and corrode most easily, being the least stable and reverting most easily to their original state; these are the 'base' metals. Those needing least energy for refining have positive potentials, are the most stable and have least tendency to corrode; these are the 'noble' metals.

Thus a cell containing zinc and iron, with potentials of —0.76V and —0.44V respectively, has a potential difference, or cell emf, of 0.32V, as stated above, the iron being positive to the zinc.




If the main part of Figure 6.4 is studied, it can be seen that it is, in fact, similar to the inset. The surrounding seawater is the electrolyte and is the 'inside' of the cell,




and the welded straps are the 'external' short-circuit. The internal (or return) short-circuit current flow /y, due to the same iron/zinc emfof0.32V.is from zinc to iron and passes through the surrounding sea, as shown by the red lines. The current from the zinc collar enters into the adjoining iron of the pipe, being densest near the collar. With current thus flowing into the pipe, this is equivalent to electrons passing from iron to zinc, and the ions (positively charged metallic zinc particles) moving from zinc to iron.There is thus no loss of iron material — that is to say, the pipe is protected against corrosion — but the zincs are gradually wasted and must eventually be replaced; they are 'sacrificial', as explained in para. 6.3.

As previously stated, no external power source is needed to maintain these

protective currents, as in the 'impressed-current' system described below. The

energy is derived chemically from the breakdown of the zincs.

Although Figure 6.2(b), in order to simplify the explanation, shows the zincs 'stood off* from the structure but bonded to it, in practice the zincs are permanently fixed to the structure itself (like the collar anodes to the pipes) and are bonded to it by welded straps. The mechanism of current protection is then similar to that described for the collar anodes, the surrounding seawater forming the 'inside' of the cell and the return current path.

With the sacrificial anode system the protection current is determined solely by the anode area available and, once installed, cannot be controlled (although it may alter slowly with time as paint cover breaks up, or with marine growth, or with the shrinking anodes as they decay). It is therefore necessary to monitor the effectiveness of the protection to decide whether divers are required for maintenance or replacement of anodes.The normal electrolytic currents which flow out from an unprotected structure cause that structure to be at a potential a little below that of the sea. As protection lessens for any reason, but particularly with loss of anode surface area, the protective current falls off and the structure potential gradually rises from its ideal protected level of approximately —200mV. Therefore measurements of structure or well-riser potentials at various points relative to the sea give a measure of the protection being given at that instant. The results are logged and plotted; these will




clearly show any deterioration and will indicate when maintenance or replacement of anodes is necessary.

The effectiveness of the cathodic protection is measured by means of 'reference cells'. These are small silver/silver-chloride half-cells, mounted near the ironwork to be protected but away from any anodes and their current flow. Several reference cells are placed around the platform structure in the case of steel-Jacket platforms, or around the well risers and their mounting structures in the case of concrete platforms. The sea-cell formed between the silver of the reference cell and the nearby iron of the structure develops a natural and constant emf of approximately 600mV, the silver end being positive.




The reference cell is used to monitor the anode current. When the structure is receiving current from the anode, its potential is depressed below that of the sea, and full protection is assured when that depression is maintained at —200mV. Added to the 600mV of the reference cell, the voltage between the silver electrode and the iron structure will then measure approximately 800mV.The monitoring equipment measures the potential between the reference cell and the iron structure and notes any departure from the ideal 800mV. With the sacrificial anode system no action can be taken to correct the position other than to inspect and, if necessary, to replace the affected anode by use of divers.The principle of cathodic protection of platforms standing in a sea environment applies equally to shore pipelines buried in soil; the sea-path for the protective current is simply replaced by the conducting path in the damp soil.In a concrete platform, while protection current is flowing in the well risers, they are kept at a small potential below that of the sea, and therefore of the platform's deck structure. It is therefore necessary to insulate them as well as their guides from the deck structure - or actually from the platform 'rebar', which is an earth-rail running through the platform. Similarly the underwater part of a riser must be insulated from its top end which goes on into the platform. This is achieved by a special insulating 'Ziefle' coupling in each riser, as shown in Figure 6.5.Maintenance of this insulation is important, as loss of it could affect the protection given. Also accidental contact with an insulated section, or bad bonding connections, must be avoided because it could give rise to sparking with the large d.c. currents involved, with risk of fire. The latter must be guarded against by regular inspection, and the general insulation by regular routine monitoring.




Different manufacturers provide different monitoring arrangements, but that shown in Figure 6.6 is typical. Selector switches pick out the well riser or pipeline to be monitored, and further selector switches determine which points on the selected pipe are to be monitored.Potential readings between the reference cells and each test point are obtained by setting a dial to the desired potential; any departure from it will show as under-protection or over-protection on a centre-zero voltmeter scaled —50, 0, +50mV.A separate so-called 'Isolation Meter' (actually an insulation meter) is connected by a selector switch to read insulation as a millivolt drop across the Ziefle couplings or other points of insulation for each pipe. The reading should ideally be central. If a large deviation is observed, the area of insulation failure must be inspected.




6.5 1MPRESSED-CURRENT SYSTEMThe impressed-current system is shown in Figure 6.2(c). Unlike the sacrificial anode system, it uses an external power source to provide the protection current. This current is applied in a direction opposite to that of the natural outward electrolytic currents of Figure 6.2(a) and can be regulated at will. Hence the name 'impressed current'.Anodes are used as in the first system, but the material is not importan: as they hardly decompose; columbium wound with platinum wire is often used. The anodes are maintained positive relative to the structure, as shown in Figure 6.2(c), by a d.c. supply (in the figure a d.c. generator is shown). In this case the anodes are true 'anodes', being positive and not negative as in the sacrificial method. The structure itself is the 'cathode'.




6.6 CONTROL OF IMPRESSED CURRENTThe effectiveness of the cathodic protection at any instant is measured by means of reference cells, just as in the case of the sacrificial anode system (see para. 6.4). Ideally the potential between the silver and iron should be approximately 800mV.The reference cell is used to control the anode current. When the structure is receiving impressed current from the anode, its potential is depressed below that of the sea, and full protection is assured when that depression is maintained at —200mV. Added to the 600mV of the reference cell, the voltage between the silver electrode and the iron structure will then measure approximately 800mV, and the control system automatically regulates the current to keep it at that level.Figure 6.7 shows a typical impressed-current power circuit. A.C. power from one of the 440V platform switchboards is supplied to a number of thyristor-controlled full-wave rectifiers. Each rectifier feeds d.c. current to a group of anodes. The voltage between the appropriate reference cell and the iron structure is used as sensor, and the control circuit triggers the thyristors, and so the d.c. output current, so that each group maintains the sensing voltage at 800mV. There are a number of adjustments and setting controls, as well as test facilities.




Where impressed-current protection is used on a concrete platform, the well risers have to be insulated from the platform structure rebar just as with sacrificial anode protection. In this case also the insulation across the Ziefle couplings and at certain other points must be monitored.Different manufacturers provide different arrangements of control panel; that shown In Figure 6.8 is one of them. It has all the above potential and insulation monitoring controls as well as current-measuring and test facilities.

6.7 MECHANICAL ARRANGEMENTIn a sacrificial anode system each anode plate is bonded locally to the structure which it protects. It can only be replaced by divers. The reference cells however can be withdrawn as described below for the impressed-current anodes.The impressed-current anodes are long and narrow and can be withdrawn through guide tubes each of which terminateson the platform deck near itsallotted position.The connecting cables from all anodes are brought out to junction boxes on the platform, where they are connected in groups and joined to their respective power sources. The reference cells are similarly withdrawn through tubes and connected, through junction boxes, to their control panels.


(cathodic protection)

Engineering

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