PROCEDURE 10

The uses of a bright steel coupon for short term CP measurements.

INTRODUCTION

The tests carried out in procedure 10 provide data relating to the effect of the CP at the location at which it is carried out . Each test is based on an accepted principle or a standard laboratory procedure, adapted for the field.
Bright steel would naturally go into solution as a reaction to being electrically connected to old steel in an active electrolyte. By using a bright sample of similar steel to that of the pipe, we are creating a corrosion cell of similar nature to that which is possible on the pipe itself.
The bright steel coupon can be regarded as an extension of the pipeline metal and should be placed as close to the pipe as possible. In any case it should be placed in the same backfill directly over the pipeline.
Most coatings are so resistant that they contain the majority of the IR drop in the CP circuit. Where there is a coating fault in a low resistance electrolyte, a great deal of CP current will initially pass onto the pipe at this point.. This will, theoretically, cause a hydrogen film to form an electrical resistance at the interface. The speed at which this forms and degrades, and the effectiveness of the film are tested by Procedure 10.2.2
Each test is in a specific order to take advantage of the effects of the previous test, except at 10.3.2 where the coupon has to be cleaned again to bright steel, The reason is explained in the narrative of the procedure. The steel coupon should not be cleaned during these procedures unless stated..
The final tests (ie 10.5.1, 10.5.2 and 10.5.3 have proved useful in estimating the size of a coating fault in the same location, which has been detected by a close interval potential survey over the pipeline. The known surface area of the coupon has proved a useful guide to the size and shape of the potential profile caused by such a coating fault.

INSTRUMENTS ETC.

High resistance voltmeter.
Sensitive current meter.
3 low resistance conductors.
A bright steel coupon of known surface area made of similar metal to the subject metal.
Abrasive material to clean the coupon.

METHOD

Carry out Procedure 1 and note the results.
Switch off the CP current and note the effect on the readings.
10.1 Drive the bright steel coupon into the ground, over the pipeline about 1m from the test facility.
10.1.1 Connect the coupon to the pipeline via the ammeter.
Note the direction of the current and the reading. (The coupon will probably be naturally anodic to the pipeline steel and the current should therefore flow from the pipeline to the coupon.
10.1.2 Switch the CP back on and note the reading and direction of the current .
If the CP is capable of protecting an additional area of uncoated pipe equal to the coupon, then the current should reverse and the coupon should polarize to below its corrosion potential.
10.2 (CP on) Connect a high resistance voltmeter between the coupon and the pipeline.
10.2.1 Note the voltage and polarity.
10.2.2 Switch the CP off and note the voltage and polarity. The potential of the pipeline will be lower than that of the coupon while the CP system is draining current from the pipeline. When the current is switched off, the newer metal of the coupon should, theoretically, become anodic to the older metal of the pipeline and tend to discharge current to the electrolyte. Its potential aught to be more negative than that of the pipe. The depolarization process should cause more current to be lost to the electrolyte and the voltage between the pipe and the coupon should increase.
10.3.1 Connect the coupon to the half-cell placed exactly 1cm distant, via the high resistance voltmeter and note the reading and polarity.
10.3.2 Clean the coupon to bright metal, drive it back in the same place and take the reading again as in step 10.3.1.
The last two readings should be the polarised and natural potentials of the steel at this location, referenced to a Cu/CuSO4 electrode.
10.4 (CP off) Testing the off potential.
Make a low resistance connection between the coupon and the pipeline.
Connect the half-cell to the pipeline through the high resistance voltmeter.
Place the half-cell in the ground exactly 1cm from the steel coupon.
10.4.1 Note the reading and polarity.
10.4.2 Switch the CP on and note the reading and polarity.
This reading should continue to increase until the coupon is polarised and then remain constant.
10.4.3 Note the polarised 'on' reading .
10.4.4 Switch the CP off and note the immediate reading before the gradual de-polarization commences.
There should be very little difference between the last two readings, as the error that is removed by switching the current off is known to be contained between the half cell and the coupon.(1cm)
10.5 Disconnect the high resistance voltmeter negative pole from the pipeline and connect it to a remote reference Cu/CuSO4 electrode.
10.5.1.1 (CP off) Note the reading and polarity.
10.5.1.2 Move the (roving) half-cell from 1cm to 2cm from the steel coupon and note the reading and polarity.
10.5.1.3 to n Move the roving half-cell in steps at 5,10,20,30,40,50 and100cm from the steel coupon, towards the remote electrode, noting each reading and polarity. If there is still a constant decrease in the voltage, continue to step at 1m intervals so that the results may be plotted to a steady line.(V/distance.)
10.5.2 (CP on) Switch the CP on and note the readings on the meter with the roving half-cell in the same locations as those noted in procedure 10.5.1.
The difference between the 'on' and the 'off' readings is due to the IR drop in the soil caused by the CP current flowing onto the coupon, and is the potential to be plotted.
10.5.3 (CP on) Disconnect the coupon from the pipeline and repeat the readings at the same locations .
This will show the IR drops in the soil caused by the CP current with the presence of the independent coupon which will be depolarizing. There may be unsuspected conductors in the backfill or small coating faults in the immediate area that will show up during this'run'.
The use of this procedure helps the engineer to visualize the electrical pressures and currents caused in the electrolyte by the CP on and off measuring techniques.
It is possible to use other medium term techniques, using bright steel coupons to produce visible corrosion in the field over the period of a week. Control coupons used in conjunction with these could form the basis of a criterion for cathodic protection.

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