PROCEDURE 11
Using a zinc coupon as a criteria for cathodic protection of steel.
The use of a zinc coupon to determin whether a steel pipe is cathodically protected, is possible because the corrosion potential of zinc is below the corrosion potential of steel. If a zinc coupon is electrically connected to an unprotected pipe, it will sacrifice, by corrosion, thus reducing the potential of the steel to below its corrosion potential. This is the action of a sacrificial anode.
However, this depends on the zinc corroding , and that cannot happen if it is reduced to below its corrosion potential in relation to the electrolyte at its interface. If it is in the same electrolyte and in electrical contact with the steel, corrosion current will pass through the connection and can be metered.
If the current ceases to flow through the connection, then the corrosion of the zinc must have stopped.
The whole of the zinc/steel coupling must have been reduced to below the corrosion potential of the zinc in this electrolyte, to have had this effect.
If the CP current is drawn onto the steel through a conductor from the zinc coupon, then the zinc must be at a higher potential than the steel in relation to their common electrolyte. The zinc is known to be below its own corrosion potential which is known to be below the corrosion potential of steel. It follows that the potential of the pipeline at this location must be below the corrosion potential of steel in this electrolyte and can safely be assumed to be cathodically protected.
This state can be readily determined by the direction of the current passing through the connection between the zinc coupon and the pipeline when the CP is switched on. If the current is passing towards the steel pipe then it is a reversal of the natural corrosion event caused by such a connection.
This procedure is helpful to the field engineer in understanding potential readings taken in association with locations where sacrificial anodes have been installed to supplement impressed current systems.
If a half-cell is placed over sacrificial anodes in high resistance areas, the pipe-to-soil voltage will increase when the impressed current is switched off.(provided that the sacrificial anodes are still connected to the pipe.) This is because the current discharging from the anodes, is increasing the potential of the ground in which the half-cell is placed. It will be found that if the half-cell is placed over the pipe, but away from the anodes, the reading will decrease in the normal way, when the impressed current is switched off.
This phenomenum can be expected during Procedure 11.
INSTRUMENTS ETC.
- High resistance voltmeter.
- Sensitive DC current meter.
- Three low resistance connectors.
- Zinc coupon suitable to be driven into the ground, fitted with a low resistance terminal for connecting lead.
- Cu/CuSO4 electrode.
METHOD
11.1.1 Carry out Procedure 1 and note the results.
11.1.2 Switch off the CP and note the effect on the Procedure 1 readings.
11.2 Drive the zinc coupon into the ground about 1m from the test facility over the pipeline.
11.2.1 Connect the zinc coupon to the pipeline via the ammeter.
Note the reading and the direction of the current. The coupon will be naturally anodic to the pipeline steel and the current will flow from the pipeline to the coupon.
11.2.2 Switch the CP back on and note the reading and direction on the ammeter. If the current reverses, then the potential of the zinc must have been reduced to below its corrosion potential for it was the corrosion reaction of the zinc which was causing the current to flow through the ammeter in the other direction.
11.3 (CP on) Connect a high resistance voltmeter between the coupon and the pipeline.
11.3.1 Note the voltage and polarity.
11.3.2 Switch the CP off and note the voltage and polarity.
11.4.1 Connect the coupon direct to the pipeline and place the half-cell within 1cm of it. Connect the half cell through the high resi as a criteria for cathodic protection of steel.
11.4.2 Switch the CP back on and repeat the reading of 11.4.1
By stepping the half-cell away from the coupon while observing the readings, it is possible to see the IR drop in the soil which is caused by the current discharged by the reaction at the zinc /soil interface.
This test is similar to placing the half-cell over a sacrificial anode.
If the cathodic protection is effective, it will lower the potential of the steel.
It will also lower the potential of the zinc and will decrease the current passing into the electrolyte. The IR drop in the soil will decrease and the reading on the meter will reduce.
INTERPRETATION
The current read in 11.2.1 will confirm that the meter is connected correctly and confirm that there is corrosive reaction at the zinc /soil interface. If the current reverses when the CP is switched back on, then the pipeline must be protected at this location.
This conclusion is confirmed by tests 11.4.1 and 11.4.2.which show the potential profile caused by the corrosion of the sacrificial zinc and then the effect on this corrosion , of the CP system current.
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