Cathodic Protection Training Course




Module 9
The use of sacrificial anodes in pipeline corrosion prevention

Sacrificial anodes have many applications in the protection of steel pipelines, both in the case of offshore and overland pipelines. This information sheet is based entirely on my own personal field experience over many years in many countries.

1.
Traditional applications

1.1.
Overland
Sacrificial anodes have traditionally been attached to pipelines under construction as temporary protection pending the installation and commissioning of the impressed current system.

Most oil and gas carrying pipelines are constructed of welded steel coated with a material which protects the metal from environmental electrolyte and electrically resistant. These coatings are applied in a variety of ways but are never perfect. Coating faults are known as 'holidays' (from the days when the coating inspectors used to say that painters had taken a holiday when they missed a patch!)

Holidays are most common on parts of the pipeline where the field conditions were most difficult during the construction period. the coated pipe is more difficult to handle and the weld joints are coated 'in situ' in dirty situations where access is difficult for operatives and inspectors alike.

These situations occur at road crossings, rail crossings, river crossings etc. and it is good practice to connect sacrificial anodes to the pipeline at these locations.

The anodes are usually of magnesium and packed in cotton bags filled with bentonite (or similar compound which allows the magnesium to corrode at an even rate with a minimum of corrosion product which causes pacification.

The anode 'tail' is brought to the surface through a small diameter pipe, that serves as a test post, and joined to a conductor which is 'cad-welded' to the pipeline metal. the connection is made with a 'line tap' which can be broken to allow periodical 'in line' current readings to be recorded. these readings indicate the pipeline coating condition and the anode output.

These anodes should be placed near to a well coated section of the subject pipeline but within 5 metres of the suspect area. It will be found that the output of the anode will drop in the first few days after installation as the pipeline coating faults 'polarise'. it is probable that this is caused by a film forming over the cathodic interface between the steel and the electrolytic environment in which it is buried.

The pipeline metal will become more negative in relation to a copper/copper-sulphate electrode whether placed in and isopotential cell, close to a test coupon or in a variable remote position typical of most pipeline monitoring techniques.

Many corrosion engineers disconnect the temporary sacrificial anodes as there seems to be difficulty in understanding the monitoring of 'hybrid systems' once the impressed current system has been commissioned.

The author does not recommend such disconnection as hybrid systems are sometimes necessary to overcome local conditions.

For further information relating to the installation and monitoring of temporary sacrificial anodes on pipelines it is recommended that engineers consider the relevant module of the on line correspondence course which includes theoretical, practical and field exercises.

1.2

Overland Pipelines

Interference remedy.

Sacrificial anodes can be used to remedy accelerated corrosion caused by the imbalance between two impressed current cathodic protection systems. Such systems commonly cause when the two operators are reluctant to co-operate by fine balancing their own system with the help of their neighbour.

However, it is possible for each or either pipeline operator to alleviate their own problem with the use of two pre-packed sacrificial anodes.



The technique is given in detail in the relevant module of the on-line correspondence course.

1.3.

Overland pipeline 'hot-spot' protection.

Sacrificial anodes are sometimes installed at locations where traditional monitoring readings prove impossible to shift further in a negative direction. This is often due to a misunderstanding of monitoring or the misinterpretation of the requirements to actually stop corrosion.

The installation of such anodes can never do any harm and can quite often do some good. However, a properly designed 'hybrid system' with proper monitoring will definitely stop corrosion and can be established with great cost saving and effect.


2. Offshore and submerged pipelines.

Offshore and submerged pipelines are normally fitted with zinc or alloy anodes fitted around the pipe (known as collar anodes). they are connected to the pipeline metal and kept clean of the 'weight-coating' material.

These systems are designed and calculated to last the life of the pipeline. However they are difficult to monitor and inspect and are known to have caused severe problems at crucial locations.

For details of remedial measures and effective monitoring techniques, engineers should refer to the relevant module of the on-line correspondence course.


3. Sacrificial anode ground beds.

In certain field conditions it is possible to design sacrificial anode ground beds remote from the pipeline with controllable output to service spans of the pipe that pass through inaccessible area with mains power supply difficulties. Such options should be considered during the design of the pipeline but can certainly be installed with great success at any time.


4.

Monitoring Sacrificial Anodes

Monitoring of sacrificial anodes and hybrid sacrificial/impressed current cathodic protection systems requires a clear understanding of the electrical measuring techniques and the interpretation of the readings shown on the meter or recorded on the data-logger.
The relevant module of the on-line correspondence course clearly defines the procedures to be used in such monitoring. The practical and field exercises demonstrate the common mistakes and misunderstandings which have arisen over the years and are costing pipeline operators a considerable amount of wasted effort and money.
Sacrificial anodes are themselves extremely useful in monitoring impressed current systems both overland and offshore.


Advanced applications

Field experimentation has succeeded in developing techniques for controlling the output of sacrificial anodes and monitoring hybrid systems to resolve many cathodic protection problems that cannot be addressed by other methods.
It is now possible to manufacture an automatic sacrificial anode which will control it's own output to stop corrosion at a given location and provide protective current through a computer controlled resistance to optimise the equilibrium of the relevant potentials with the most efficient life of the anode.
This and other techniques are the exclusive intellectual property of the author and are available when engaged for actual work.


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