DEMONSTRATION
Client MOBIL NIGERIA
Client's personnel CLAIR MENNINGS AND VISITORS FROM THE USA
Date FIRST WEEK IN DECEMBER 1993
1. SPECIMEN 'PIPE TO SOIL POTENTIAL'
This shows the traditional measurement of voltages between an electrode and the pipeline. This is the most common record kept by cathodic protection engineers in pipeline operations. It is the measurement on which most cathodic protection design, commissioning and maintenance is based.
2. MOVE ELECTRODE
Moving the position of the electrode produces a change in the reading. the electrical environment determines the magnitude of the variation. It is rare to find that this variation is noted in the records or a recommended electrode position clearly designated.
3. TWO ELECTRODES SAME POSITION
This simply shows the relationship that is found between any two electrodes in field use. It is seen that the degree of error is small, within 10 to 20 millivolts.
4. MOVE ONE ELECTRODE.
It is seen that the voltage between the two electrodes varies as the position of the 'roving electrode is altered. The roving electrode cannot be a reference potential.
5. TWO ELECTRODES APART
Moving each of the electrodes to different positions produces a variety of voltages and neither can be regarded as a 'reference electrode' unless placed in a fixed location and all voltage readings are referred to this standard potential.
6. INTRODUCE ELECTRIC FIELD
The introduction of an electric field, using a battery, demonstrates that foreign influences can cause measurement errors.
7. BI-METALLIC COUPLING
This shows one possible cause of the presence of a foreign electrical field which will cause measurement errors.
8. IMMEDIATE OFF READINGS
Shows a small cathodic protection system being switched on and off to demonstrate the theory of the 'polorised potential'. It can also be demonstrated that this does not eliminate the errors caused by 'foreign' electrical fields.
9. ISOPOTENTIAL CELL
Short description of the theory of the Isopotential Cell and the Lugin Capillary
10. ALEXANDER CELL
Short description and demonstration of the Alexander Cell.
DISCUSSION
It can be seen that there are many errors in the traditional measurements that have been used for establishing various criteria for cathodic protection.
We have used the causes of these errors to develop formalised survey techniques that can tell us much about the state of corrosion and the condition of buried and submerged pipelines.
We have researched the work that has been done to eliminate these errors and produced the Isopotential Cell based on principles suggested in laboratory techniques. This makes it possible to measure the voltage between the metal and the electrolyte in its immediate vicinity with no current flowing through the electrolyte in which the electrode is placed. This removes the errors in the voltage readings, and the measurement can be closer related to the thermodynamic theory which governs corrosion reactions.
The Alexander Cell measures the equilibrium in a specimen corrosion cell and then can be subjected to the cathodic protection of the pipeline. Whether this is impressed current or sacrificial anode, the Alexander Cell will measure the effect on the corrosion reaction, directly and with no calculation necessary. If the specimen corrosion is halted on connection to the pipeline, then there is sufficient cathodic protection current density to stop corrosion at this location. Simple as that. For those that require a scientific explanation of the Alexander Cell, this was defined by the (then) Chairman of the Committee of Code of Practice 1021 (Cathodic Protection) of the British Standards Institute, Mr. J. Gosden. B.Sc. In field trials the Alexander Cell has shown remarkable accuracy in determining the condition of buried and submerged pipelines.