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Metal that has been extracted from its primary ore has a natural tendency to revert to that state. This process is called corrosion and the most common example is the rusting of steel. Corrosion is an electro-chemical process involving the passage of electrical currents. The change from the metallic to the combined form occurs by an “anodic” reaction: M -> M+ + e- A common example is: Fe -> Fe++ + 2e-
This reaction produces free electrons, with a path from one site (anode) on the metal surface to another site on the metal surface (the cathode), where it is consumed by the cathodic reaction. In acid solutions the cathodic reaction is: 2H+ + 2e- -> H2 (hydrogen ions (gas) in solution)
In neutral solutions the cathodic reaction involves the consumption of oxygen dissolved in the solution: O2 + 2H2O + 4e- -> 4OH- (alkali) Corrosion thus occurs at the anode, but not at the cathode (certain metals may be attacked by alkali). The principle of cathodic protection comprises connecting an external anode to the metal requiring protection and the passing of direct electrical current (dc) so that the whole metal surface becomes cathodic and corrosion ceases. In electro-chemical terms, the electrical potential between the metal and the electrolyte solution with which it is in contact is made more negative, by the supply of negative charged electrons, to a value at which anodic reactions become negligible and the reactions that take place are predominantly cathodic. Cathodic protection can be achieved in two ways: - Sacrificial Anode Cathodic Protection (Galvanic anodes used) - Impressed Current Cathodic Protection (Low dissolution anodes used) Galvanic anode systems employ more active metals that are directly connected
to the steel (more noble metal) to be protected. The difference in natural
potentials between the anode and the steel, as indicated by their relative
positions in the electro-chemical series, results in current flow in
the electrolyte from the anode to the steel. The outcome is that the
steel surface becomes more negatively charged and becomes the cathode.
Sacrificial anodes are typically aluminium, zinc and magnesium. Alloying
of these metals improves their long-term performance and dissolution
characteristics. In order to ease interpretation and to separate what is being monitored from other DC sources, the survey DC signal is switched ON and OFF by a switcher connected to the Transformer rectifier to interrupt the DC output. The signal is asymmetrically switched with a cycle of ? of a second OFF and ? ON. The “ON” time is shorter than the “OFF” time so that the polarity of the applied CP can be recognized for coating defect location. During a pipeline survey, the operator walks directly above or just to the side of the pipe, with the probe tips placed one in front of the other along the pipe route. As a defect is approached, the millivoltmeter will respond to the impressed DC signal, with the needle deflecting in the direction of current flow, which may be a defect or an anode system. When the defect is passed the needle deflection reverses and points in opposite direction. By retracing one's step a position is reached where the needle shows no deflection, i.e. a null. At this position the defect is located midway between the two probes. This procedure is repeated at right angles to the pipe and the defect is located at the point where the two midpoints intersect. The defect location is then marked with a numbered wooden peg to locate the defect at a later stage, with the chainage at that point measured using a surveyor wheel. The located defects are ranked according to their severity rating. Defect severity is measured by computing the %IR at the defect location. Further tests are carried out to determine the anodic/cathodic status of the located defect. A number of the located defects are then exposed to assess the coating damage and this is correlated to their individual severity ranking. Ground surface mapping of uni-potential lines can assist with determining defect shape. This requires substantial time, particularly when the defect frequency is as high as it is for a bitumen coated pipeline.  Defect CharacterisationIt should be noted that not every defect located requires repair when considering cost-effectiveness and technical justification. A system of characterizing the located defects has been developed in order to prioritize defects for repair. Their features, based on available information, determine defects requiring repair. These are defect size, shape, number, distribution of faults, corroding/protection status of individual defects. Defect sizeThis is related to the %IR assessment and the following guideline is used:
It should be noted that this table is a guideline only and does not address the holistic factors of pressure, soil conditions and related risk contributors. Measurements taken under stray current activity should be carefully interpreted, since erroneous deductions may be made. Furthermore, measurements taken depend on a number of factors such as depth of cover, soil resistivity, DC and AC interference. All these should be taken into account while surveying and during the selection of defects for repair.
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Corrosion HistoryAs the need for energy from gas, oil and other refined products increases around the world we are witnessing the unprecedented construction of pipelines multiple thousand kilometres long in certain parts of the world linking the new class of “energy rich” and “energy poor”.  When Luigi Galvani, at the University of Bologna, discovered the bimetallic driving force in the late part of the eighteenth century in a series of experiments conducted on the exposed muscles of a frog leg it is unlikely that he would have considered the impact this would have on pipeline related corrosion two centuries later. The positive aspects of Galvani's principle was later put into a practical application by Alessandro Volta who built, in 1800 , the first battery. This is the basis of all modern wet-cell batteries, and it was a tremendously important scientific discovery, because it was the first method found for the generation of a sustained electrical current. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone. For galvanic corrosion to occur, three conditions must be present: • Electrochemically dissimilar metals must be present; • These metals must be in electrical contact; and • The metals must be exposed to an electrolyte. The principle was also engineered into the useful protection of metallic structures by Sir Humphry Davy and Michael Faraday in the early part of the nineteenth century. The sacrificial corrosion of one metal such as zinc, magnesium or aluminum is a widespread method of cathodically protecting metallic structures.The 21 st CenturyMore than two centuries after Galvani's frog leg experiment the world of pipeline corrosion prevention and monitoring has matured somewhat. However, certain misconceptions remain and the benefits of sound corrosion science are often neglected in favour of project deadlines, ignorance and poor risk assessment.  Over the past ten years significant pipeline failures have occurred in the United States , Russia , Nigeria , South Africa (Durban South and other product pipelines between Kwazulu-Natal and Gauteng ) and other parts of the world. The United States responded with several initiatives to place the responsibility for pipeline corrosion firmly on the shoulders of pipeline owners. The Department of Transport (DOT) in the United States assisted with legislation that has changed the face of pipeline corrosion prevention and monitoring in the United States. This has precipitated what is largely known in the pipeline industry today as External Corrosion Direct Assessment ECDA and Internal Corrosion Direct Assessment (ICDA). The National Association of Corrosion Engineers (NACE www.nace.org ) initiated their own recommended practice in this regard namely RP0502-2002 Pipeline External Corrosion Direct Assessment Methodology , which serves as a strong guideline for pipeline corrosion assessment and decision making in terms of reducing the risk of pipeline failure. Advances in corrosion assessment and monitoring have opened the door to effective survey techniques such as the Direct Current Voltage Gradient (DCVG), Pipe Current Mapper (PCM) and Close Interval Potential (CIPS) Surveys. In line inspection ( ILI ) tools comprising Ultrasonic and Magnetic Flux Leakage (MFL) Pigs (derived from pipeline inspection gauges) add a new dimension to pipeline inspection with sub-millimeter corrosion profiling of both the inside and outside of product lines over several hundred kilometers.
A further benefit of modern communication has been the use of remote monitoring systems to monitor corrosion and cathodic protection systems over vast geographical areas.  Cost and time comparisons between the traditional manual data capture routes and the remote corrosion monitoring approaches have found that the capital cost payback period is approximately five years. Communication costs and the comparative costs of manual visits to remote sites are where the greatest savings will be realized as each site will no longer require physical inspection to determine the operational status of the systems being monitored. Annual cost savings of 40% may be realized on remuneration, travel, security and related expenses, when comparing communication costs to receive data from 45 remote sites in the eThekwini (Durban) Municipal area as opposed to the cost of visiting these sites in person. Potential vandalism and theft may be countered as early warning signals are triggered when certain security measures are breached on site. The integrated corrosion monitoring system enables alarm signaling and appropriate security measures. ConclusionMore and more pipeline owners are being charged with the responsibility of looking at corrosion monitoring systems for their pipeline networks, as the skills shortage becomes more apparent. Many solutions are available to those pipeline owners who invest their resources, finances and time wisely in pursuit of suitable corrosion monitoring systems. Pipeline owner's applying certain monitoring techniques would now have 20% additional capacity per year from a qualified technician to pursue other projects and work.  As an example, eThekwini Municipality protects an estimated R4.5 billion pipeline network with a corrosion protection system valued at 0.1% of the infrastructure cost, with ongoing integrated corrosion management at a cost of 0.03% per annum. The challenge facing pipeline owners in South Africa is the selection of suitably qualified consultants, contractors and total project companies capable of offering comprehensive corrosion prevention strategies. As
a guideline civil engineers may want to visit the following web sites
to verify whether or not organisations have the requisite skills to
manage corrosion projects professionally: • NACE Certification Search: (Search for CIP Level I and Cathodic Protection Specialist Certification) http://www.nace.org/nace/content/education/Certification/certsearch.asp • Engineering Council of South Africa (ECSA) Professional Engineer / Technician Search: Click on Who is Registered? www.ecsa.co.za Further information is available at http://www.corrosion.co.za or the writer may be emailed at craig@corrosion.co.za |
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All information in these pages © Paradigm
Projects 2007 - 2008 | |||
The Principles of
Cathodic Protection 
Chemical
World: Pipe integrity and Rehabilitation - C. Botha (4 pp)
