Transcript Slide 1 - Pipeline Corrosion Control

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 demonstration
This demonstrates how we can measure corrosion as it happens
A corrosion cell
Another corrosion cell
The zinc case corrodes, releasing energy in the form of electrical charges which
can be measured with a voltmeter.
Another
corrosion
cell
The cloth under the central nail is more highly charged at the point
than at the head. The whole nail is at an equal potential
High potential
Low potential
Another corrosion cell
This corrosion cell consists of separate pieces of the same metal
that become the anode and cathode of a corrosion cell.
This allows measurement of the corrosion current and the
electromotive force of the reaction to a sample of electrolyte.
Opening a battery battery
A battery is cut in two to reveal the inside of the zinc case
The battery in half
Electrical measurements inside the
battery
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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/coppersulphate
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 halfcell 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
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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 current
57.73 micro amps
Corrosion current overpowered
by 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 voltage
The instrument makes up to 40,000
voltage measurements per second
and 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 that
are the related to GPS locations.
We then have to relate these locations
to 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
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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.