AC Corrosion

AC Corrosion Demystified

The following paper was presented at BHR Group’s 18th International Conference for Pipeline Protection – Antwerp, Belgium – 4th – 6th November 2009.

The paper was co-authored by Richard Lindley and Ken Lax, and presented by Ken at the conference.

DISCLAIMER: All information, references to Standards and / or criteria correct at time of paper.

 
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ABSTRACT

AC corrosion is a pressing issue with an increasing number of reports of serious corrosion damage.  Paradoxically the problem exists only on modern well coated pipelines.  The paper will explain why modern coatings are “responsible” for the increase in ac corrosion damage.

AC corrosion can be difficult for non-specialists to understand.  This paper presents an understandable description of the phenomenon and how the effects can be mitigated.  It provides guidelines for pipeline operators and constructors to assist in determining whether or not there is a risk of ac corrosion damage to the pipeline.

There have been examples of unacceptable levels of corrosion damage caused by induced ac with voltages as low as 4v ac whilst cathodic protection is applied.

The paper will show, strange as it may seem, that over-zealous application of cathodic protection current can actually exacerbate ac corrosion.

No complex mathematical or chemical formulae are included in the paper.

BACKGROUND

In the last 10 years or so there has been a marked increase in the number of incidents of ac corrosion reported.  There is a good chance that the risks of ac corrosion will increase even further now that so called “energy corridors” have been established in crowded countries and that means the overhead and buried ac power lines run close to buried coated high pressure gas and oil lines.  In addition to the proximity issue it also likely that increased energy demands will result in higher ac voltages on the overhead lines, which will in turn result in higher induced voltages and hence increased corrosion risk.

External corrosion protection is usually provided to high pressure gas and oil pipelines by a combination of coatings and cathodic protection.  Although it is hard to realise from coating specifications the main function of coatings is to provide a high electrical resistance between the steel and the surrounding environment.  A high resistance will impede the flow of electrons and hence reduce the risk of corrosion. (This is explained in more detail in the Basic Theory section).  Cathodic protection is provided to prevent electron flow from the pipe at locations where the coating is damaged or does not provide a high enough resistance.  Theoretically bare pipelines can be protected using cathodic protection but the current demands will be high.  Current density at a coating defect is the main driver for corrosion, the higher the current density then the higher the corrosion rate.  So a small coating defect in an otherwise well coated pipeline will mean that all the corrosion will take place at one spot.  From an ac corrosion point of view bare pipelines are less prone to corrosion than coated pipelines!

Cathodic protection is a method of stopping external corrosion by forcing electrons (a flow of electrons is an electric current) onto the pipeline.  These electrons can be provided by galvanic (sacrificial) anodes such as zinc or magnesium or from impressed current anodes such as silicon iron.  Galvanic anodes do not require an external power source to release the electrons but impressed current anodes need an external power source (usually a transformer-rectifier).

At the coating defect there is an increase in the steel alkalinity caused by the cathodic protection reaction.  This can play an important part in the ac corrosion process.

SOME BASIC THEORY

If you have a voltage and a resistance then you can have a current.  For direct current this can be expressed as ohms law.

Current (I amps)  = Voltage (V volts) divided by Resistance (R ohms)

Often written as I = V/R amps

In simple terms the voltage is the power that drives the current through the resistance.

The voltage has to be high enough to overcome the resistance in order to drive the current through the resistance.

So, for a given voltage the lower the resistance the higher the current.

This law shows us that the current that flows depends on the voltage and the resistance.

AC is a bit more complicated because we talk about impedance (another word for resistance really) and impedance is a combination of capacitive reactance, inductive reactance and resistance.  Reactance is influenced by the capacitance and inductance of the circuit and the frequency, so it is not a simple as a pure d.c. resistance.

SINE WAVE.png

 

The sine wave drawing shows one complete cycle.  There are normally 50 of these cycles per second. (60 in the USA).

 

 

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AC is converted to dc via a unidirectional device called a diode.  A diode can be considered as a one way valve.  When certain conditions exist either side of the diode the diode will have a low resistance and will allow voltage through. 

At all other times the diode exhibits a high resistance and does not allow anything through.

This means that an ac voltage will permit the diode to conduct each half cycle. This results in a pulse of direct current every half cycle.

This is what happens to an ac voltage on a pipeline at a coating defect.  The ac voltage is rectified and the resulting direct current causes corrosion as it leaves the structure and enters the soil.

The “diodes” in this case are created on the steel surface by the chemical reactions that are taking place.  The overall process is known as Faradaic Rectification.

Faradaic rectification may be defined as “A component of the current that is due to the rectifying properties of an electrode reaction and that appears if an indicator or working electrode is subjected to any periodically varying applied potential while the mean value of the applied potential is controlled.”  In other words it is what you can get at a coating defect that is subject to ac interference whilst a cathodic protection potential is also present.

The big question, however, is how does the ac voltage get onto the pipeline anyway?  To understand this we need to consider some basic electromagnetic principles.

  • Every electric current has a magnetic field associated with it.
  • The magnetic field strength is directly proportional to the magnitude of the electric current.
  • In the case of ac current this magnetic field will vary in exactly the same way as the ac current (i.e. 50 times per second in Europe and 60 times per second in the USA)
  • A metal conductor placed in a magnetic field will have a voltage induced in it that is directly proportional to the strength of the magnetic field and the speed at which the magnetic field changes.

When we consider voltage induction in buried pipelines and add the knowledge of the corrosion process we can conclude that if a voltage is induced in the pipeline and the pipeline has a resistance (which it always has) then there will be a current flow.  If this voltage is converted to direct current then there will be corrosion where the current leaves the pipe.  This will occur at the point of lowest resistance i.e.  a coating defect.  The smaller the defect the greater the corrosion current density.  The greater the corrosion current density the greater the metal loss.

Putting all these facts together we can see that high voltage and high current ac cables close to a pipeline with small coating defects has the possibility to cause rapid corrosion at the coating defects.

WHAT CAUSES CORROSION ANYWAY?

There are a wide range of corrosion mechanisms that result in loss of metal from buried pipelines.  One thing that they all have in common is that the consequence of the corrosion process is direct current leaves the pipe and that there is a relationship between the magnitude of the current and the metal loss.  1 amp dc for one year will result in 9.1 Kgs of carbon steel consumption.

HOW DO WE KNOW IF A PIPELINE IS AT RISK FROM AC CORROSION?

The scientific community have yet to agree on a single mechanism for ac corrosion.  That is probably because they are striving to find one theory whereas there may be several different mechanisms.

External corrosion on buried pipelines is caused by a current exchange between the soil and the pipe.  No current exchange, then no corrosion. 

Because there is no universal acceptance of the mechanism there is also no consensus on the criteria for ac corrosion. 

It would be nice if we could just measure the induced ac voltage and use that to determine whether or not there is a corrosion risk.  Unfortunately the voltage by itself is not usually a good indicator of the risk.  Knowledge of the soil resistivity close to the coating defect, the pH, the defect size, ac current density and dc current density are also required.  These cannot all be measured in the field.

If the ac powerline is more than 110kV and the pipeline is within 150m of the pipeline, and the pipeline is more than 2Km long then the risks of harmful levels of induced ac voltage increase.  Areas of particular concern are where there is a change of direction of either the cable or the pipeline route.

As a guideline the following criteria have been used:

  • If the ratio between ac and dc current densities is greater than 0.5 then the risk is low
  • If the ratio between ac and dc current densities is less than 0.5 the risk is greater
  • The risk increases if the ac current density is greater than 30A/m²
  • The risk increases if the pipe-to-soil off potential is more positive than -0.850
  • The risk increases if the pipe-to-soil off potential is more negative than -0.950

The measurements cannot be made directly on the pipe and they are, therefore, made on a corrosion coupon.

Practical field measurements have shown that by increasing the cathodic protection current density the pH at the surface of the defect changes, and actually accelerates the ac corrosion.  So just increasing the cathodic protection current is not always beneficial.

The recommendation of the authors is that a special ER corrosion probe is used to quantify the actual corrosion rates.  These probes are placed in areas of risk, and control areas where no risk is perceived, and provide an absolute measure of the corrosion rate regardless of current densities, pH levels etc.

The corrosion probes work by monitoring the extent of corrosion on a known surface area of metal exposed to the soil and connected to the pipe.  Using the relationship between resistance and metal surface area they can accurately calculate the metal loss based on the measured resistance.  The better quality systems will also measure the on and off potentials and ac and dc current densities to provide additional information.  The probes can also be used to validate the effectiveness of any remedial measures.

Another method is to use a mathematical model to predict the ac induction and hence the risks.  Not surprisingly this method is popular with the companies that offer the service!  Many operators find it comforting to be able to produce calculations and maps to show the risks.  It is always worthwhile to consider the true costs and benefits of these models versus the practical application of remedial measures.

CP < 0.85 VDC              CP > 0.95 VDC

HIGH DC INTERFERENCE            HIGH RISK                      LOW RISK

HIGH AC INTERFERENCE            LOW RISK                       HIGH RISK

METRICORR CHART.png

Graph courtesy of MetriCorr

The graph shows that as the cathodic protection is increasingly negative the ac corrosion rate increases. 

When the potential is set to -0.85v the ac corrosion rate reduces.

WHAT CAN WE DO IF WE HAVE AC INTERFERENCE?

Simple.  Make the pipeline earthy, so that the induced ac will return to earth via a dedicated earthing system.

Not quite so simple, however, since we spend a lot of time and money to provide the pipeline with a high integrity coating to isolate it from the earth!

Practically speaking we need to provide a low resistance (impedance) path for the ac that is a high resistance path to the cathodic protection current, which is dc.   This is best achieved with a polarisation cell that has special characteristics that provide a low resistance to earth for the ac but blocks the dc.

The earth can be provided by zinc ribbon, copper earthing, or any other low resistance earth (e.g. silicon iron groundbed).

CONCLUSIONS

  1. Well coated pipelines near to high voltage cables (>110kV) are at greatest risk.
  2. Typically the high risk areas are where the pipeline or the ac cable changes direction.
  3. Just measuring the ac voltage is not an indication as to whether or not there is a risk of ac corrosion
  4. Increasing the cathodic protection current may increase the ac corrosion.
  5. Coupons are required to determine key characteristics such as ac and dc current density.
  6. Practically speaking electrical resistance probes are the best way to monitor the effectiveness of ac mitigation.
  7. If the pipeline is going to run near ac high voltage cables then provision should be made in the cathodic protection design for ac mitigation.
  8. The location of the mitigation can be determined pragmatically or from mathematical models.