CONCEPT CATHODIC PROTECTION SYSTEM



The basic concept of cathodic protection is that the electrical potential of the subject metal is reduced below its corrosion potential, and that it will then be incapable of going into solution, or corroding.

This mechanism has been defined by many scientists and has become established beyond dispute. Indeed the principles of corrosion reactions are used in the design and construction of expendable and re-chargeable batteries and accumulators which play such a major part in modern life.

A battery that is 'dead' has no energy left and does not corrode any further. Likewise a car battery on charge does not corrode, in fact in this case the reaction is reversible, and energy is 'pumped back in'.

However, a battery has a very carefully composed electrolyte which has qualities to ensure a predictable reaction with the other components of the battery. We know that the corrosion within a battery can be controlled very accurately, by external electrical input, as this technique is in common use with rechargeable batteries which are nowadays controlled by computers which balance the reaction equilibrium to suit their own power demands.

Unfortunately a cathodic protection system is not composed of simple elements in the way that batteries are, because the electrolyte is the ground itself. This electrolyte is uncontrollable and has an almost infinite variety of qualities. The chemical composition and electrical conductivity can span a vast range, as can the temperatures and pressures to which the reaction is subjected.

Cathodic protection of such subjects as ships hulls and storage tank bases is relatively simple as the electrolyte is likely to be almost homogeneous, but as the size of the structure increases, it extends through different electrolytes and the reaction at each interface varies.

Offshore oil rigs, for example have different temperatures and pressures at the sea bed to those at the surface, and a study of this situation has shown that it has a substantial influence on corrosion.

Pipelines can be regarded as many interface reactions connected together in parallel. The metal element can be well defined, as this is specified to a high degree by the designers, as is the coating material.

However it is accepted that no coating can be perfect, and the faults, or 'Holidays' introduce the first indefinable variable to the system.

During the construction of a pipeline all possible measures are taken to detect and repair coating faults, so it follows that those remaining are undefined. It is possible to calculate the theoretical resistance of a perfectly coated pipeline, given the specification of the coating and dimensions of the pipeline, but it is impossible to calculate the actual resistance of the total pipeline.

The electrical current measurements, taken during routine cathodic protection monitoring, show that there is little resistance in the total coating (with faults) of a pipeline and this can be explained by the difficulty in quality control, during the construction period.

Undetected coating faults are the path of cathodic protection current and a perfect coating would prevent any output from the CP system. We therefore, know that there are many unspecified 'metal to electrolyte' interfaces present on an average pipeline.

The electrical resistance of the pipeline metal itself can be calculated, and is found to be very low. In fact the effect that the pipeline resistance has on the complex current paths and variation in potentials, is so small that it can almost be ignored.





The complication is due to each interface being capable of a different reaction, electro- motive-force (EMF) which cannot be measured as it is in parallel with all other EMF’s on the same section of pipeline. The magnitude of the current from each of these reactions is dependent on the earth resistance immediately adjacent to the interface, and the direction of all the resulting currents is the result of the combined effects of all the resistances and electrical pressures caused by all the EMF's.



Although it is simple to understand each corrosion cell and the mechanism of corrosion itself, the reality of applying the science, to the field, becomes immensely complex. This becomes more obvious when the circuit has been subject to computer modelling as discussed later.
To be effective, cathodic protection must reduce the metal at each single interface, to below it's corrosion potential. This is not too difficult to achieve, as each interface is part of the same metal structure, which has a very low electrical resistance. The difficulty is knowing when all the interfaces have been reduced to below their corrosion potential in relation to the electrolyte in their reaction vicinity. ( Don't forget, if we knew where each interface was we would repair them all!!!!)


OVER PROTECTION

There are several other problems, however, as too much current passing onto a steel surface can cause embrittlement, which under certain circumstances can be as detrimental as corrosion itself. This is manifest in such applications as the protection of the external surfaces of drill pipe casings, where a considerable amount of cathodic protection current is used.


CATHODIC DISBONDMENT

Another fear of 'over-protection' is that of cathodic disbondment of the coating. This happens when the coating manufacturers specifications are exceeded. Cathodic protection current passing onto the metal causes the release of hydrogen which disbonds the coating. In reality this is rarely a problem, and a careful study reveals why.

The current will only pass onto the metal at a coating fault, and the density of the current will depend on the size of the coating fault and the current locally available. As the current blows the coating from the metal, the volts drop at the interface will decrease, and equilibrium will be reached with a very small increase in additional disbondment.

If there is no coating fault, then no cathodic disbondment will occur as recognised in the British Standard Code of Practice for testing the coating manufacturers specification. This requires a specific size of coating fault on a steel coupon, to be subjected to an increasing voltage over a specified period. The test cannot be carried out on a coupon with perfect coating as the disbondment is observed under the coating at the edge of the fault.

It is logical to deduce that if cathodic disbondment is caused by current and that if all current is prevented by a perfect coating, then no disbondment will take place. This is not common sense, however, as many excavations have been dug in areas where high 'pipe- to-soil potentials' have caused concern about cathodic disbondment. In the event, it has proved the logic (above) and no disbondment has been found.

In one particular example voltages of over 5 volts had been recorded when the electrode was place on the surface above the buried pipeline which was subsequently excavated, at several spots, for examination. A coating fault was found at one location but no disbondment. The current passing onto the metal at this coating fault, caused a drop in the voltage of the electrode as it got nearer to the pipe. Whereas at the surface the reading had been over 5 volts, this reduced to 0.950 volts when the electrode could be placed close to the actual interface between the metal and the earth.

This simple drawing shows that the earth at the surface has a higher potential than the earth close to the pipeline at the coating fault, due to the current passing from 'mass earth' into the pipe metal.



At such site it is easy to plot the 'potential gradient' using a static electrode as a reference and a moving electrode to trace the potential isobars. As soon as the coating fault is fully exposed to the air, the gradient disappears completely, as the current stops. The meter then reads 5 volts, even with the electrode placed in the ground a few mm from the metal.

by : Roger Alexander

Cathodic protection is important

An idiots guide to cathodic protection





What the heck IS cathodic protection in the first place???


Cathodic protection is an electrical way of stopping rust.

Rust is chemical and electrical. Metal dissolves in some solutions and gives off electricity. Metal can be 'plated' onto other metal electrically.
All 'batteries' work on this principle and everyone knows that batteries drive loads of the things we use daily.
Not many people know that our gas and oil comes to us through pipes that are inclined to rust, but are protected by 'cathodic protection'.
Some people know that metal boats are protected by cathodic protection, and have seen lumps of metal attached to hulls for this purpose. These lumps of metal dissolve in the water and give off electricity which prevents the hull from rusting.
When you put two different metals in contact and submerge them in liquid (or wetness) one of the metals dissolves and discharges an electrical current into the liquid. The liquid (or damp material) is the 'electrolyte' and gets 'charged up' with electricity. It's 'electrical potential' is increased.
Electricity works by 'pressure' and anything with a higher 'pressure' gives off electricity to anything with a lower 'pressure'.
The electrolyte is then at a higher electrical 'pressure' than the metal that is not dissolving and so the electricity passes into it.
The metal that is dissolving is the 'anode' from which the electrical current passes into the electrolyte and the other metal is the cathode into which the current passes because the electrical pressure must be balanced out. (everything tries to equalise).
The dissolving metal is sacrificed to prevent the subject metal from corrosion, and this method is known as 'sacrificial cathodic protection'.
There are limits to which sacrificial cathodic protection can be used but the same principle can be used by causing a manufactured electrical pressure which is 'impressed' into the electrolyte. The electricity is then 'drained' out of the subject metal....... boat hull or pipeline.... and this interferes with the natural tendency of the metal to dissolve....or rust!

Impressed current cathodic protection


Electricity is generated by a sort of pumping action which causes it to flow backwards and forwards in 'waves', but this is no use for our purposes so we have to get it going in one direction through a circuit known as a 'rectifier'. At the same time we can control the amount of current by transforming it, so the apparatus is know as a transformer-rectifier.
A transformer-rectifier can be regarded as an electrical pump which is sucking the electricity out of the pipeline (etc) and pumping it into the ground (or sea ... or swamp... or wherever else you want to pump it).
The effect of this is amazing. It stops rust! And it's cheap!
But there are some snags.
Because it's so good, it gets installed .... then ignored...... well most people don't even know it exists... and because it's cheap some people don't think it's important.


But it 's life and death to some.


The villagers in the picture are gathering water from outside a flowstation in Nigeria. A pipeline in Nigeria leaked petrol and local people collected the petrol in cans and washing up bowls and the site drew hundreds of women and children until the petrol was accidentally ignited.... cooking up to 1000 people.

Cathodic protection IS important.
A couple of years before this incident a pipeline in the USSR exploded and blew a train off it's tracks, killing many and causing ecological devastation. This was thought to be caused by corrosion.

by : Roger Alexander (my great Teacher)























Newsflash 5th December 2000


*** Natural gas spewing in Texas MONT BELVIEU, Texas (AP) - A pipeline ruptured and released a
potentially explosive cloud of natural gas, forcing evacuations of
about 40 homes and the rerouting of airplane flights around the
area. Several minor injuries were reported Monday night when the
pipeline, owned by Channel Industries Gas Co., blew open near
Houston Raceway Park. The blowout was felt and heard as far away as
Baytown, more than 10 miles to the south. There was no fire, said
Baytown police Sgt. Keith Dougherty. However, residents of the
immediate area were told to evacuate and flights east of Houston
were kept at least miles from the site as a precaution, said Texas
Department of Public Safety spokesman Richard Vasser.

Full article at: http://www.infobeat.com/fullArticle?article=405225991