Corrosion control measurements

Incorrect assumptions are making pipeline corrosion control

impossible.

Corrosion is electrochemical

• We can only measure the electrical component of corrosion so all our reasoning must be in electrical terms.

• We must restrict our theories and calculations to electrical measurements.

• We must be able to repeatedly observe that our activities result in the control of corrosion during experiments, demonstrations and site case studies.

3 nails corrosion demonstrationThis demonstrates how we can measure corrosion as it happens

A corrosion cell

Another corrosion cellThe zinc case corrodes, releasing energy in the form of electrical charges whichcan be measured with a voltmeter.

Another corrosion cellThe cloth under the central nail is more highly charged at the pointthan at the head. The whole nail is at an equal potential

High potential Low potential

Another corrosion cellThis corrosion cell consists of separate pieces of the same metalthat become the anode and cathode of a corrosion cell.

This allows measurement of the corrosion current and theelectromotive force of the reaction to a sample of electrolyte.

Opening a battery batteryA battery is cut in two to reveal the inside of the zinc case

The battery in half

Electrical measurements inside the battery

• Measuring the ‘electrical component’ of corrosion.

• The meter probes of the small meter are positioned in the acid paste/electrolyte.

• The big meter shows the total corrosion energy release of the whole of the zinc case.

• The small meter shows the ‘IR drop’ in the area between the anode and the cathode of this corrosion cell.

• This voltage is entirely dependent on the exact position of each probe and is infinitely variable between the voltage shown on the big meter and zero.

The IR drop

• The name ‘IR drop’ is used because it is impossible to measure a voltage in this area.

• The charges are dispersing to fill the whole of the area between the anode of this corrosion cell and the cathode.

• The anode is called the ‘working electrode’ and the cathode is called the ‘return electrode’.

Present field measurements

• We make a voltage measurement that is called a ‘pipe-to-soil potential’.

• We connect a voltmeter between a copper/copper-sulphate electrode and a pipeline test post.

Pipe to soil measuring circuit• This is a

schematic of the measuring circuit that is used to gather most data that is recorded in cathodic protection field work.

• I have used the term IR drop because that is commonly used between field operatives.

• It is a voltage that is displayed on the meter.

The measuring circuit and influences that show on the voltmeter

Buried pipeline where voltages are measured

Wrongly named ‘Half-cell’ ‘reference electrode’

• A ‘half-cell reaction’ occurs instantly that a metal is submerged in a specific concentration of a solution of it’s own salts at a specific temperature.

• This reaction potential can be compared with the half-cell reaction of another metal in a solution of it’s own salts in a laboratory.

• There can only be two potentials involved or the electrode is not a ‘half’ of a cell but a portion of many electrical potentials.

• This is why we cannot use a copper/copper-sulphate electrode as a ‘reference potential’ in cathodic protection field use.

• It gives a different voltage each time we move it because of the other potentials in the measuring circuit.

The meter uses one of the electrodes as zero of the displayed voltage

The recognised error in the voltage measurement

• Pipeline corrosion failures in the 1970’s forced specialists to investigate the criterion for cathodic protection.

• The criterion must be a measurable value at which corrosion is known to have stopped.

• It had been thought that a pipeline voltage of -0.85v with respect to a standard copper/copper-sulphate electrode would prove that corrosion had been stopped by cathodic protection.

• Scientists examining this assumption realised that the measurement was affected by the potential zones caused by the passage of the charges from the impressed current cathodic protection system.

• Field specialists called these potential zones ‘the IR drop in the soil’.• Scientists demonstrated in a laboratory that the voltage gradient in the

electrolyte could be removed by switching the impressed current off and observing the curve recorded of voltage during time.

• This required a static experiment with the positions of each element in a fixed position for the duration of the on/off voltage measurement.

Fixed positions of electrodes

Curve produced by oscilloscope

Polarised potential

• The ‘polarised potential’ required to be measured in order to show when equilibrium is reached is at the voltage where the small kick appears in the downward curve after the cathodic protection current is switched off.

• This kick can only be produced in a laboratory when the pH of the electrolyte is within a small range.

• This range of pH values is not always present at coating faults where corrosion occurs.

• The pH value of the pipeline backfill is only included when the Alexander Cell is used.

Alexander corrosion cell

Alexander Cell and pH sample

Corrosion current57.73 micro amps

Corrosion current overpoweredby cathodic protection current

Corrosion stopped at point when current stops or reverses

You can see corrosion stopping

• If connection to the pipeline does not stop corrosion current that is displayed on the micro ammeter, the cathodic protection system can be adjusted and the results seen immediately.

• Even when the current is impressed several miles away you can see corrosion stopping at the location of the test.

• This shows that the Alexander cell can be used as a ‘trigger’ to computer control corrosion remotely and automatically on networks of pipelines with multiple cathodic protection systems.

Software that calculates corrosion using electronic circuit analysis

Using a working mathematical model of a corrosion cell

Pipe-to-soil potentials cannot be computed

• They can only be displayed as point to point graphs of voltages.

• These graphs cannot be related to the corrosion status of a pipeline.

• They can be corrected to give valuable information.

• The voltages can be related to real corrosion using the Alexander Cell.

Close Interval Potential Surveys

• These result in graphs of voltages between two variable potentials.

• The on and off measurements are not as required by science.

• The traces do not show wave forms but point to point lines between separate voltages.

CIPS results

What the CIPS graph really shows

How we should look at CIPS graphs

What is the voltage we are trying to measure

The close electrolyte potential is the value required to work out the corrosion status

The arrangement needed to get the correct voltage

Each dot on the graph is a voltageThe instrument makes up to 40,000 voltage measurements per secondand samples a number specified in the software.

The blue spots are the on voltages

The read dots are the off voltages

What is displayed on the graph

The blue dots are joined to represent the on potentials

The red dots are joined to represent the off potentials

They are the voltages between two variable potentials

The millions of waveforms that we really need to analyse.

The meter records these voltages at time intervals thatare the related to GPS locations.

We then have to relate these locationsto the actual pipeline route.

Each location has a unique waveform.

• To define that corrosion status of the pipeline we must evaluate the polarised potential at each location.

• That means that we must know the voltage of each ‘kick’ in each waveform.

• This cannot be done as no kicks ever appear in the results of CIPS surveys.

• The measurements are not made between two isolated potentials in a closed measuring circuit.

• They are measured between all of the corrosion cells on the surface of the pipeline and the series of potentials in the measuring circuit.

How we do a CIPS survey

Manual noting of voltages

Hand held data logger

Pipeline location and data logger

Actual connections

Cathodic Protection Network CIPS training centre in Brazil

The first recorded CIPS survey in the world

Conclusions• The pipe-to-soil potential measurement is a voltage between two variable

potentials.• The graphs of voltages recorded during close interval potential surveys

cannot indicate the corrosion status of a pipeline.• The copper/copper-sulphate electrode is only a reference potential when it

is used as a half-cell in a laboratory.• The copper/copper-sulphate electrode can be used as a reference potential

if fixed at a location where there is no electrical flux in the ground itself.• The only way to ascertain the corrosion status of a corrosion cell is by

measuring the corrosion current that is passing from that reaction at the working electrode known as the anode.

• The scientific understanding of the corrosion reaction includes the pH of the electrolyte and that is only reflected in the use of the Alexander cell.

• We can control corrosion by applying the codified rules of electricity and electrochemistry but this is not being done or advocated by anyone else than Cathodic Protection Network.