Sacrificial anodes can be used to supplement impressed current CP systems at locations where impressed current cathodic protection cannot be achieved.This can be due to high resistivity ground surrounding the locations at which power supply is available.
Pipelines frequently pass through high resistance ground for long sections but small pockets of active electrolyte in remote areas, can cause "long cell" corrosion.
Sacrificial anodes are an ideal solution to this problem, as they can reduce the potential of the pipe metal in relation to the immediate electrolyte. The area of influence of a sacrificial anode is limited to the low resistance active area and is normally very effective. However, interpreting the pipe-to-soil voltage readings can cause confusion and this procedure will help field engineers to analyse the readings to determine the effectiveness of the anodes.
INSTRUMENTS
- A recording voltmeter or data logger..
- 2 Cu/CuSO4 electrodes and trailing conductor.
- Sensitive ammeter.
- Tools for disconnecting the anodes from the pipeline.
- Notes
METHOD
9.1 Establish a remote earth reference electrode, convenient to the sacrificial anode site.
9.2 Activate the TR timer to switch the current off for 3 seconds and back on for 12 seconds.
9.3.1 Connect the negative terminal of channel 1, on the recording voltmeter, to the pipeline and the positive terminal to the remote reference electrode.
9.3.2 Set the range from -3 volts to +3 volts and the paper speed to 240mm/hr.
9.3.3 Mark the chart paper"Anode search ...(location.).."
9.3.4 Connect channel 2 negative terminal to the remote reference electrode and the positive terminal to the roving electrode, via the trailing conductor.
9.3.5. Using the roving electrode as a sensing probe, carry out a random search in the area where the anodes are believed to be located. The point at which the ground is most positive, with reference to the remote electrode, should be pegged. This is directly above the anode and will appear as a peak in the off mode, on the chart trace.
9.4.1. Peg out strings, knotted at one meter intervals, across the pipeline wayleave, at one meter spaces to form a grid covering the area of the sacrificial anode area.
9.4.2. Mark the chart paper "Grid Survey, starting upstream left."
9.4.3. Lift the roving electrode for one minute, and then place in contact with the ground for at least half a minute, at the first peg of the knotted string at the upstream left side of the grid.
9.4.4. Repeat the ground contact at each knot across the pipeline until the end of the first transverse (piece of string), then lift the electrode for one minute.
9.4.5. Move to the left end of the second piece of string, and work across the pipeline, repeating until the recorder has charted the voltages over the complete anode groundbed area.
If a data logging system is used on a lap top computer, the software can show an immediate three dimensional plot of the ground potentials as the survey progresses. If this is not available then the chart recorder will show the transverse runs of readings which can be overlaid on a plan of the site for interpretation.
9.5.1 Disconnect the anode from the pipeline and insert the ammeter to take a current reading. note the value of the reading and the direction of the current. The switching of the impressed current CP will result in two readings and maybe a change of polarity to be noted.
9.6 The chart trace should be interpreted on site so that further readings can be recorded if necessary. The exact reference of each of these should be clearly marked on the chart paper at the time. For example there could be confusion at one point in the grid where the lifting of the electrode coincided with the switching of the TR, and a repeat of this reading will clarify the record. It will be necessary to incorporate this revised reading in the grid plot.
INTERPRETATION
Channel 1 chart trace will show the effect of the TR switching with a lower voltage when in the off mode.
On channel 2, the downward spikes will show when the electrode is lifted from the ground between steps and the longer troughs will mark the 1 minute pause dividing the transverse runs.
The highest peaks on channel 2 will result from the sacrificial anode activity during the off mode of the TR. These peaks will have a three second duration in the 15 second switching cycle.The following 12 seconds will show if the anode current has been curtailed or reversed by the impressed current system. This will be supported by the ammeter reading of 9.5.1.
The lowest troughs will appear when the TR is on , the roving electrode is away from the anode area of influence and over a coating fault in the pipeline.
If the pipe coating is in perfect condition, the ground will not be effected by the switching of the TR, except in the area of influence of the anodes.
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