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The four basic elements of a corrosion cell are an anode, a cathode, and the metallic and electrolytic pathways between them. Corrosion control can be achieved by eliminating (or reducing) any of these elements. One such method is to modify the electrolytic pathway by introducing a barrier between the threatened metal surface and the corrosive medium (i.e., by applying some kind of coating).
Full article click Effects of Coating on Corrosion and Cathodic Protection
Most pipeline cathodic protection (CP) applications involve either galvanic anode or impressed current CP (ICCP) systems installed in earth for protection of external surfaces. Of the galvanic anode installations in neutral soils, magnesium is the most commonly used anode material. Rectifiers are the most common source of direct current power for impressed current systems.
Full article click Specific CP Requirements for Specific Pipeline Application
By Pierre R. Roberge on 2/29/2016 5:13 PM
The principle of a close-interval potential survey (CIPS or CIS) is to record the pipe-to-soil (P/S) potential profile of a pipeline over its entire length by measuring potentials at intervals that do not significantly exceed the depth of the pipe (often ~1 m).
The actual survey typically involves three distinct tasks: 1) locating and marking the pipeline with stakes or flags inserted at regular intervals, based on tape measurements or chaining; 2) data collection, including P/S potentials and notation of physical features along the right-of-way with global positioning system (GPS) coordinates collected separately for these features; and 3) clearing the right-of-way of survey wire and other materials. The field crew must be prepared to identify and repair breaks of the trailing copper wire; areas such as road crossings and stockyards may require the use of heavier, insulated wire that is resistant to breakage.
Because the potential of interest is at the structure-electrolyte boundary, it is important to consider possible voltage (IR) drop errors that result from the flow of current through the earth between the pipe surface and the reference electrode(s). One commonly used technique includes the synchronized interruption of cathodic protection (CP) current sources. The interruption plan must consider what groups of current sources influence different portions of a pipeline and should be interrupted during testing. This may include bonds and current sources belonging to other pipeline operators. Modern interrupters can be programmed to operate during a selected testing period each day and turn off in the conducting position overnight to minimize depolarization. It is a good practice to obtain potential waveforms at the start of a day’s survey and throughout the day to confirm that interrupters remain synchronized and that no uninterrupted current sources influence the pipeline.
The CIS technique provides a complete P/S potential profile, indicating the status of CP levels. The interpretation of results, including the identification of defects, is relatively straightforward. The most useful graphical presentation of CIS data is to plot the “on” and “off ” potentials together as separate profiles vs. distance. With the polarity convention described above, potentials are plotted as positive, with values nearer to the top of the page (typically the “on” potentials) actually being more negative (more protected). If a portion of the plot shows “off ” potentials that are more negative than the corresponding “on” values, these “reverse shifts” indicate that operation of the interrupted current sources reduces CP levels locally. This condition must be explained or investigated. Localized dips in the potential profiles (to less negative values) may indicate the presence of a poor-quality coating or low-resistivity soils. The difference between the “on” and “off ” potential values should also be noted. A reduction in this potential shift may also indicate a decrease in local P/S resistance or a problem with the distribution of protective current.
It may also be useful to complement the interrupted CIS with a “native” (depolarized) CIS; these data provides the opportunity to evaluate the 100 mV polarization CP criterion. The “native” survey may be conducted prior to the initial activation of the CP system for a new pipeline; however, the polarization criterion is more commonly applied for older piping with deteriorated coating. In that case, existing current sources must be turned off for a sufficient period to approximate full depolarization. When the “native” CIS data are plotted along with the interrupted data, the difference between the “off ” and “native” profiles represents the level of polarization at each location.
Other applications and variations of the CIS technique are presented in NACE SP0207-2007, “Performing Close-Interval Potential Surveys and DC Surface Potential Gradient Surveys on Buried or Submerged Metallic Pipelines.”
This article is adapted by MP Technical Editor Norm Moriber, Mears Group, from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 507-510.
By Pierre R. Roberge
Structures such as steel bulkheads, steel piles supporting piers or wharfs, offshore drilling platforms, and other similar structures may be cathodically protected with either sacrificial galvanic anode systems or impressed current systems.
Galvanic anode systems in seawater, for the most part, use much heavier anodes than those used in soil. The low-resistivity seawater environment tends to require greater protective current densities and also permits greater current outputs from the anodes. Consequently, the greater corrodible mass is needed to provide reasonably long life.
For structures that can be polarized, a low-potential galvanic anode material such as zinc or aluminum is generally preferable to a high-potential material such as magnesium. Magnesium will work perfectly well, but may discharge more current than needed. This results in reduced efficiency and shorter service life for the anodes. In some chloride environments, magnesium anodes have a greater tendency than other galvanic materials to self-corrode, which further reduces their service life.
Zinc or suitable aluminum alloy anodes can polarize a steel structure in seawater to within a few millivolts of the characteristic potential of the anode itself. Assume that the stabilized driving voltage between the anode and the polarized structure is 0.050 V, although it can be less. This ability to maintain polarization at a relatively modest current will consume the anodes at an efficient rate.
Compare the typical behavior of magnesium anodes: the structure does not tend to polarize to a potential more negative than ~–1.1 V vs. a silver/silver chloride (Ag/AgCl) electrode (SCE) because the hydrogen overvoltage potential is reached, resulting in the evolution of free hydrogen rather than additional polarization. This means that with a standard magnesium alloy working voltage of ~–1.4 V vs. an SCE electrode, there will be a driving voltage of ~0.3 V. Thus, on a comparable basis, the magnesium will discharge about six times as much current as is actually required to achieve the desired polarization.
Because less electronegative anodes can provide an efficient cathodic protection system, the surplus current from a more powerful anode is, in effect, wasted. However, there is one advantage offered by magnesium anodes in seawater: the greater driving voltage tends to force the more rapid formation of thicker protective calcareous deposits on the structure surface than would be obtained with less powerful anodes.
Special chemical backfills are not needed in the uniform seawater environment because galvanic anodes work satisfactorily without them.
Impressed current systems for fixed seawater structures may use suitable anode materials, also without backfill, suspended from the structure being protected or placed on the ocean floor. In the past, various materials such as treated graphite, high-silicon cast iron, platinized titanium, or lead/silver have been used as anodes. However, the introduction of highly efficient, dimensionally stable anodes (DSAs), which are basically mixed metal oxide coatings on titanium, has rendered these other anodes almost obsolete.
Particular attention must be given to the design of the rectifier positioning, header cable distribution system, and anode suspension or placement details. Above-water components are subject to severe marine atmospheric attack, whereas submerged portions must be protected from, or designed to withstand, the mechanical forces exerted by moving seawater as well as by water-carried debris or shipping traffic.
This article is adapted by MP Technical Editor Norm Moriber from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 514-516.
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