Corrosion Protection Monitoring

Question 1

Why is a A monitoring regime is necessary

Answer 1

A monitoring regime is necessary to ensure.
 The CP system is operating to its design specifications.
 There are no in-service affects interfering with it. The amount of current required for protection varies:
 From metal to metal.
 With the geometry of the structure.
 Differences in seawater environment (temp. & pH).
 Any other factors that affect the resistance of the
circuit.

Since the amount of current required cannot be accurately predicted, the method of checking is to measure, at various points around a structure.
The Potential & Current Density.

Question 2

Inspection requirements for CP systems are:

Answer 2

 Visual inspection of anodes for wear.
 Visual inspection of electrical connections.
 Potential measurements to ensure the structure is still the cathode
 Current density measurements
 Visual & ultrasonic inspection for corrosion

Question 3

Monitoring Methods include

Answer 3

CP Readings obtained by:
 A diver/ROV taking contact readings with a CP meter.
 Proximity readings with an ROV mounted probe.
 Proximity readings via remotely mounted sensors.
Current Density measurements are obtained by:
 Taking readings with an ROV mounted sensor.
 Remote electrodes incorporated into the ICCP system.

Question 4

How is cathode potential measured

Answer 4

The Cathodic Potential is measured by using a reference electrode, incorporated into an instrument that has a readout calibrated in mV (1V = 1000 mV).
These are, commonly:
 High Purity Zinc (Zn).
 Silver/Silver-Chloride (Ag/AgCl).
 Copper/Copper-Sulphate (CSE)
(more favoured for concrete structures).

Question 5

Explain High Purity Zinc Electrode (ZRE)

Answer 5

It may be installed as part of an Impressed Current System 99.9% pure is common Site for mounting the electrode is selected because:
 It is a representative site
 Area of marginal protection
 Area of high stress The electrode is connected to a voltmeter on the surface

Question 6

What is the most common reference cell

Answer 6

Silver/Silver-Chloride (Ag/AgCl).
 Most common reference electrode used offshore.
 Used for both contact & proximity applications.
 Referred to as half-cells, as they form a ‘cell’ when the meter is connected to the cathode.
 Probe tip is placed on the cathode (metal to metal) & the meter gives the readout in mV of the electrical potential between it & the half-cell.

Question 7

Explain a Monitored Anode

Answer 7

Similar to reference electrodes, but can be used to monitor the efficiency of any CP system.
 A sacrificial anode, isolated
electrically from the structure.
 Connected to an ammeter in
the control room. Current density may also be measured using a specialised ROV mounted probe.

Question 8

Why is Calibration Checking necessary

Answer 8

It’s necessary to confirm calibration of CP meters.
 To ensure that the readings are accurate & comparable with previous readings.
 A standard method has been adopted in the offshore industry for this purpose.

Question 9

What is the first part of calibrating

Answer 9

The first part of the procedure, is to prove that the reference electrodes, either:
K-series or Calomel Cells
are chemically saturated & sufficiently stable enough to be used as reference cells.

Question 10

What equipment is needed for calibration

Answer 10

Necessary Equipment.
 3 K-series or Calomel Electrodes c/w electrical connectors.
 High Impedance (10 MΩ) voltmeter.
 Zinc block (99.9% pure) c/w clamp & electric connector.
 Plastic bucket of fresh seawater.
 Log sheets.

Question 11

How do you prove the k cell

Answer 11

Visually inspect the electrodes to ensure they are undamaged & full of solution (Potassium Chloride).
 Label them 1, 2 & 3.
 Soak in the bucket for 24 hrs.
 Connect 1 to the neg. terminal of the
voltmeter & connect 2 to the pos. & record the
reading.
 Repeat the test with each permutation.

 If all readings are within this range, any cell may be used.
 If all readings are out of this range, either replace all cells.
 Or flush out the phials with pure distilled water, refill with Potassium Chloride & re-test.
 After the procedure, rinse the cells in freshwater.

Question 12

What is the reading is good for k cell

Answer 12

+/- 2mv

Question 13

What procedure happens next

Answer 13

Once satisfied that the electrical components are functioning correctly:
 Fully charge the CP meter batteries and soak in fresh seawater (not drawn
from the fire main).
 Remove the contact probe tip and white Delrin probe cone from the meter.
 Screw the K-series reference electrode onto the Bathycorrometer in place of
the Delrin probe and probe cone tip.
 Immerse the meter in the bucket at least far enough to submerge the semi-
permeable membrane. Gently shake the unit to remove any air bubbles
lodged in the holes. (The meter display may be left out of the water to
assist taking readings).
 Allow time for the meter to stabilise (approximately 10 minutes).
 The voltage potential between the reference electrode and the meter’s own
Ag/AgCl cell is read off the meter display directly.
 Record the reading on the log.
Acceptable readings are +42mV +/-5mV. It should be noted that values will vary dependent upon salinity and temperature of the seawater, see Figure 10.7
The calibration of other types of contact CP meters is by comparison.

Question 14

Procedure for use of cp meter

Answer 14

 Ensure meter is fully charged (approx. 15 hrs).
 Maintain a battery-charging log.
 Ensure the probe tip is sharp.
 Soak for 30 minutes before use.
 Confirm the calibration of the system.
 Record the results on the log sheet.
 Record meter serial number on the log sheet.

Question 15

During use what are the procedures

Answer 15

 Take a reference reading on zinc prior to the survey.
 Ensure correct metal-to-metal contact.
 Proximity probe must have a electrical connection to the structure connected to the positive terminal of the voltmeter & the standoff is a max. 100mm.
 Ensure that each reading is recorded in the log.
 On completion, take a reference reading on zinc.

Question 16

On Completion of use

Answer 16

 Wash in fresh water
 Dry & store
 Charge batteries as necessary
 Complete the battery-charging log
Note: If taking a large number of readings, it’s prudent to check readings periodically during the survey