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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
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).
Full article click Close-Interval Potential Surveys
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 on 6/30/2016 3:34 PM
The sources of dynamic stray currents include direct current (DC) railway systems.
Before preparing a cathodic protection (CP) design, the possible presence of stray currents must be considered. Stray currents are defined as those that follow a path other than the one intended. Where stray currents discharge from a structure into the electrolyte environment in order to return to the source, corrosion will occur. If the corrosion is concentrated over a limited surface area, the integrity of the structure may be threatened in a relatively short period. The most severe stray current influences are often those that continually vary in quantity. The sources of dynamic stray currents include direct current (DC) railway systems, DC-powered mining operations, and DC welding operations.
When a train passes a specific location, particularly during acceleration, the rails of the transit system tend to discharge a portion of the load current into the earth as stray current. When no train is nearby, the rails tend to behave as extensions of the traction power substation’s negative bus and draw current from the adjacent earth and buried metallic structures. Typically, these driving voltages are smaller than those associated with discharges from the rails; however, they represent a majority of the transit system’s operating schedule.
The magnitudes of anodic potentials at areas of stray current discharge may be so great that they cannot be counteracted readily with the usual CP system. In some cases, the large operating voltages of the stray current sources can make an affected structure several volts positive with respect to its environment. Mitigation of such conditions may require extraordinary approaches such as limited recoating in the pickup or discharge area or relocation of the structure. Metallic bonds from the affected structure to the stray current source, to limit the amount of current discharging through the environment, are generally undesirable because they establish interdependent operation with the source system; however, there may be no practical alternative.
Another type of varying stray current is “magnetic storm” or sunspot activity. Long structures such as pipelines can be affected when the intensity of the earth’s magnetic field varies. Fluctuating potentials can be introduced in a pipeline in much the same manner as potentials are induced in an electric generator.
Manmade variable stray currents can often be identified by characteristic patterns, such as those associated with acceleration/deceleration or rush-hour peaks of transit systems. While they may be correlated with solar activity, magnetic disturbances rarely, if ever, exhibit any pattern. Because stray telluric currents are usually of relatively short duration and are seldom concentrated in any specific area, they are not expected to cause as much corrosion as uncontrolled manmade stray currents.
In addition to dynamic influences, steady-state stray currents also may be encountered. These may be caused by an impressed current CP system on an adjacent but electrically separate structure if the groundbed is too close to the foreign structure, especially where the foreign structure is positioned between the groundbed and the structure to be protected. This is an example of a critical design flaw that should be avoided. If a foreign pipeline passes through the potential gradient field around the groundbed, the soil is typically positive with respect to the pipeline and causes stray current to accumulate on the pipeline. Because there is no metallic path by which this current can flow back to its source, the stray current must complete its circuit by discharging into the earth. This current discharge may be concentrated at a crossing with the protected pipeline. Possible solutions include limited recoating, properly designed metallic bonds, or relocation of the influencing groundbed.
CP system design must consider possible stray current effects on nearby structures. This includes careful placement of groundbeds, which can be especially important for impressed current systems because of the greater driving voltages involved.
This article was 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. 501-502.
Article From: Products Finishing, Adam Blakeley, from MacDermid
Salt-spray fog disperses when the chamber is opened. Photo courtesy of H.E. Orr Co.
“Why did these parts fail at 96 hours? The last ones went for over 120 hours without any issues.”
I am sure these words have escaped your lips at least once in your career as an electroplater in some form or fashion after reading the most recent report from your neutral salt spray (NSS) chamber operator.
To be less confused and frustrated by the results the plater receives from any accelerated corrosion testing facility, you must possess a basic understanding of corrosion. What it is; how it is produced; what NSS testing is; how you can produce, handle and package a product to give you the best results possible; and how you can engage your NSS operator when the root cause for any part failure seems to be generating out of the test itself, and not from the parts or the plating process itself.
To prevent corrosion, it’s imperative to know that corrosion is the process by which a metal in a solid state—such as zinc metal (Zn⁰)—is chemically changed due to a loss of electrons, turning solid metal into something different, often the cation Zn⁺².
There are a number of different corrosion reactions and types, but the one we focus on here is called oxygen-concentration cell corrosion, because that is the one employed when NSS tests are conducted. For instance, if we were to measure the concentration of oxygen directly in the middle of a drop of water on a piece of steel, we would find that it is a lot less than the concentration of oxygen at the very edge of the drop.
The different concentrations of oxygen in this droplet set up a corrosion cell wherein the oxygen-deficient area in the middle of the drop becomes the anode (or the corroding area just like you might witness in a plating tank) corroding away the iron in the steel and also becoming more acidic. The exterior of the drop (the cathode, or the oxygen-rich area) becomes more alkaline, thus precipitating out iron hydroxide in the form of red rust. This is because the cations (or positively charged particles of iron) are reduced or brought to a neutral state on the surface of the steel with the transfer of electrons. Anything that restricts the access of oxygen to a metal surface can develop what are termed differential aeration cells. Examples of restrictions would include anything from dust particles to a simple plastic washer.
Test plates are hung in the chamber for testing. Photo courtesy of Q-Lab.
Neutral Salt Spray Testing
NSS testing uses the oxygen-concentration cell corrosion mechanism to accelerate corrosion for use in performance analysis on a variety of substrates and coatings. Adding sodium chloride (NaCl) at a concentration of 5 percent keeps the corroded metal ions in solution so that they can act as conductors to enhance the corrosive effect. Salt helps to extend the life of each corrosion cell because it allows more metal to be in solution. The salt is actually increasing the solubility point of certain elements, like metal ions. Temperature is elevated during the NSS test as well in order to increase the speed of the electrochemical reactions attempting to take place. Parts are inclined to prevent the droplets of water being formed from becoming overly saturated. If the droplets fill up with metal ions and reach their saturation point, they will cease corroding the metal, which would defeat the whole purpose of the experiment.
A typical salt-spray chamber has some basic components: an air saturation tower that stabilizes the salt concentration, a reservoir for the solution itself, an atomization nozzle for the creation of the fog, supporting mechanisms to hold the parts, a method for distributing heat inside the chamber and a temperature controller.
If you have control over the way your parts are manufactured, you can do a lot to ensure the plating process runs smoothly and the coatings will be very corrosion resistant. For instance, an area of a part that has been stressed due to a crimping, machining or stamping operation is more likely to become corroded when compared with an area that has maintained its composition without significant stress.
Once the microscopic grain or micro-crystalline structure has been tampered with due to any type of force, a corrosive environment is poised to preferentially attack areas that have been modified, over regions that have sustained less deformation. Welds are a good example of this problem. Welders often use fluxes, or intermediate glues, which unfortunately can act as poultice—a moist conglomeration of conductive material contained in a soft, but self-contained slurry. The poultice is often composed of sodium, calcium, chloride, sulfate and other corrosive ionic agents that add to the conductivity of the electrolytic cell being formed.
Welds also tend to be porous, which means they can trap chemicals from the processing solutions causing bleed out and other phenomena, and they splatter metal fragments that can also negatively impact salt-spray results. Stress can also be created in the deposit itself by the inclusion of carbon, sulfur and other elements that are usually provided by proprietary additives like brighteners and carriers. The more stress your part or the deposit has, the more likely it is that those parts will become more anodic (more prone to corrosion) to their less-stressed counterparts.
A typical salt-spray chamber has an air saturation tower that stabilizes the salt concentration, a reservoir for the solution itself, an atomization nozzle for the creation of the fog, supporting mechanisms to hold the parts, a method for distributing heat inside the chamber and a temperature controller. Photo courtesy of Q-Lab.
Designing Parts
Another factor to keep in mind is that part design has a lot to do with the ability of a part to be corrosion resistant, as do the substrate itself, the type of lubricants used, the finishing and manufacturing methods, and the overall geometric complexity of the piece. Simpler parts have a smaller or tighter range of current densities (few extreme high and low regions) and thus are able to be plated with more thickness uniformity and are thus more corrosion resistant. Parts that have deep recesses or more areas of low current density will tend to perform worse than parts that do not have recesses or crevices. Plating in these areas will be thinner, so the ability of a corrosive particle to penetrate to the base metal is greater.
If you cannot adjust the way the part is manufactured entirely, you may be able to convince the fabricators to fix a few elements that would positively affect deposit uniformity. Gently curving, convex surfaces are preferred over ones that have grooves, serrations, holes, concavities, fins, ribs, edges, valleys and recesses. Sharp angles and edges should be rounded off, softened, chamfered or beveled. Slightly convex shapes are actually preferred to flat areas. Grooves cut into a metal part should be rounded into a shallow, U-shape versus a sharp, deep, V-shape.
Try to avoid processing parts separately that will eventually need to be fitted together because the two parts together tend to form grooves that can produce a capillary that will soak up liquids during the plating process. If the manufacturer refuses to concede to making some alterations, you may have to chemically process the parts in a solution that simply has higher covering and throwing power to ensure higher thicknesses.
Packaging, Transporting
When packaging parts to be chamber tested, it is important to make sure the parts and packaging elements are free of contaminants, that they carry a preservative like a desiccant, are wrapped and cushioned to prevent mechanical damage, are compartmentalized to mitigate and moderate shock and vibration, and are not bulk packaged.
Other items to avoid when packaging parts are: cardboard, paper, and rubber (because of their sulfur components), flexible PVC (chlorine leaching is a potential hazard), any metal (because of the potential for galvanic cell formation), wood (due to the potential for resin leaks), and Ziploc bags. Permitted materials include: corrosion-safe paper products, polyethylene, polypropylene, cellophane, Formica, Styrofoam, fiberglass, and hard PVC.
The following information is not an exhaustive representation of everything that is required of an operator to perform his or her duties in running a proper salt-spray chamber, but rather it is a collection of information often missed when operators or customers analyze a chamber and its operation. Results from a salt-spray chamber can be affected by many things. Before submitting parts to a salt-spray chamber facility, ensure that the standards it employs fulfill the basics of what is expected from ASTM B117.
Sometimes the required number of collection vessels/funnels in the chamber is not followed. One funnel is to be positioned as close to the atomization nozzle as the nearest sample and as far away as the furthest sample. Verify the records your operator keeps on the salt solution’s specific gravity with their collection rate. The collection rate itself has a high amount of variability, requiring between 1-2 milliliters per hour.
Specimen are not allowed to touch the wall of the chamber as condensed salt solution will run down and flush the test specimen. Parts should not be shading one another from the spray or fog, nor should they be dripping onto each other. It would be prudent to ask that your chamber operator take a picture of the chamber with your part in it so that you can take note of your part’s orientation, its proximity to other parts in the chamber, and any other details that seem abnormal or disconcerting to you.
Keep in mind that corrosion areas should only be evaluated on the areas of the part that are at the correct angle, which is 15-30 degrees. A lot of customers will assume that every aspect of the part is under diagnosis, but this is not the case. For instance, threads are typically disregarded in salt-spray testing as they tend to accumulate and hold salt water. Corrosion runs, areas where a single area of corrosion seems to spread to the rest of the part, are to be ignored as well.
Monthly evaluations of the chamber’s corrosivity gives marked assurance that the chamber is running accurately and consistently. Steel panels are placed in the chamber, one nearest the collector and one farthest away. Panels are exposed for a determined number of hours that correspond with a range of weight loss. For instance, panels exposed for 96 hours should average a weight loss of 1.5347 grams with a variance of +/- 21 percent. What this means is that the same part, with the same thickness, the same type of deposit, and the same coating, is allowed to vary by 21 percent in either direction for each trial run in the same chamber and still be considered a well-run chamber.
In other words, if you wanted some chromated zinc-plated parts to pass 96 hours in salt spray to first white rust (FWW), the exact same part, with the exact same chromate thickness, is expected to reach anywhere between 75 and 116 hours in a good salt-spray chamber. Therefore, unless your part averages 122 hours in salt spray, on average, half your parts will fail to reach 96 hours and the other half will reach 96 hours and above. The authorized or permitted variability when testing parts in different chambers for the chamber to be considered good or accurate is plus or minus 36 percent. If you wanted some chromated zinc-plated parts to pass 96 hours in salt spray to white rust, the exact same part is expected to reach anywhere between 61 and 131 hours if placed in two different chambers. So, unless your part averages 150 hours in salt spray, on average, half your parts will fail and half your parts will get to 96 hours and above.
Corrosion Resistance
There are a number of variables involved not only in conditioning a surface to be corrosion resistant to a particular standard, but also in making sure that the test is done correctly and no other elements outside the finishing purview are the cause of failures.
The key is in learning more about how to design, plate, coat, package and transport parts and how to engage your NSS operator to ensure he is operating his chamber with every degree of accuracy and repeatability.
Adam Blakeley is a CEF 1 and a technical service representative for MacDermid. For information, visitmacdermid.com. For information on H.E. Orr Co., visit heorr.com. For info on G2MT Labs, visitg2mtlabs.com. Some information for this article came from Frank Altmayer’s NASF corrosion course.
The prediction of coating life depends on many factors, including the coating system used, preparation and application methods, and the severity of the environment. This article describes the knowledge needed to estimate coating life and the conditions that can compromise coating systems.
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Researchers with Hempel A/S (Lyngby, Denmark) have developed a novel, organic zinc-rich primer coating technology that relies on a combination of zinc dust, hollow glass spheres, and a proprietary activator to provide excellent cathodic protection (CP) with greatly improved mechanical properties (crack resistance) and adhesion. This technology activates the zinc so that the zinc in the coating can be fully utilized to act as an anode. The use of glass spheres enables the coating to have a lower pigment volume concentration and more epoxy binder, which enhances coating adhesion. The unique corrosion byproduct of the activated zinc has a self-healing effect on the coating. The glass spheres block microcracks and also create an inhibitor effect by collecting insoluble complexes of zinc, oxygen, and chlorides on their surfaces and trapping these species in the coating so they don’t reach the surface of the steel.
Organic zinc-rich primers have been used since the 1960s and are an established method of corrosion protection for steel. These systems use high levels of zinc dust as a pigment in an organic binder (epoxy) to create a galvanic effect that protects the underlying steel substrate from corrosion. The zinc particles are more active than steel and act as anodes in the coating that corrode sacrificially instead of the steel when they are exposed to water, oxygen, and/or chlorides. Zinc-rich coatings are primarily used for corrosion protection of steel structures; and organic zinc-rich primers are often preferred over inorganic zinc-rich primers because they are less sensitive to surface preparation, over application, and humidity, which makes application easier. Experience in the field, however, has indicated that organic zinc-rich primers may not provide the level of CP that can be obtained from hot-dip zinc galvanizing or inorganic zinc silicate coatings, and can experience premature breakdown due to blistering and red rust formation. Additionally, organic zinc-rich primers may crack if over-applied, which leads to coating failures often seen on welds and corners.
Electrical conductivity and effective CP depend on the amount of the zinc dust used in a zinc-rich primer, says David Morton, director of Hempel’s Protective R & D Group. If the zinc content is too low, the organic binder will completely encapsulate the zinc dust. This results in limited or no contact between the zinc particles and, subsequently, electrical conductivity is limited, which reduces or eliminates CP of the steel. The amount of zinc dust also impacts the physical properties of the coating film such as adhesion, cohesive strength, and mechanical integrity—all important factors in determining the lifetime of the coating system. Attempts to improve the coating’s CP properties by using higher levels of zinc can have a negative impact on the mechanical and adhesion properties of the coating and result in premature coating failure. Although excessive zinc in the coating provides excellent CP because there is good contact between zinc particles and good conductivity, the adhesion to steel, cohesive strength, and mechanical properties usually will be deficient because the organic binder content is too low in relation to the zinc content. This is why the formulation of an organic zinc-rich primer is so important if the overall coating system is to be successful in providing corrosion protection through CP. From a coatings manufacturer’s point of view, Morton says, it’s very important to balance the coating formulation so it has reasonable adhesion and the best mechanical properties possible while retaining galvanic corrosion protection from the zinc.
Hempel’s research and development team studied ways to improve the effectiveness of conventional zinc-rich epoxy coatings and, in 2007, made an important discovery—while a typical standard organic zinc-rich epoxy coating contains ~80 wt% zinc, just one-third of the zinc actually contributes to corrosion protection. The research showed that only the zinc located in the 20 to 30 µm closest to the steel was consumed by the galvanic reaction in a zinc-rich coating with a dry film thickness (DFT) of 60 to 80 µm. About 60% of the zinc added to the primer was not used in a galvanic reaction.
To formulate a more efficient zinc-rich epoxy primer, the team spent more than 8,000 h in the lab, evaluated more than 800 prototypes, and applied coating to more than 3,000 test panels. Their efforts led to the development of an activated zinc-rich epoxy primer coating technology that incorporates tiny hollow glass spheres (~40 µm in size) and a special proprietary additive called an “activator.” With the addition of the glass spheres and an activator that improves the conductivity of the entire coating matrix, the volume of zinc dust required (relative to the epoxy resin) for effective CP is reduced while maintaining a dry film weight percent of zinc dust that complies with current standards. With the lower zinc volume in the coating, more of the epoxy binder is available, Morton explains, which promotes better adhesion properties and mechanical performance. Because of the synergy of these components, the coating delivers three methods of corrosion protection: a galvanic effect, a barrier effect, and an inhibitor effect.
For galvanic protection, the activator increases the zinc’s ability to carry the corrosion current throughout the coating although the zinc particles are not in direct contact with each other, which greatly improves CP of the steel. The coating’s barrier properties and self-healing characteristics are delivered by the corrosion product of the zinc. Typically, Morton says, the corrosion product of a zinc-rich primer is zinc oxide (ZnO). In the activated zinc-rich primer, he notes, the corrosion product created is an insoluble salt—zinc chloride hydroxide hydrate. This insoluble salt forms a uniform protective layer on the surface of the primer, which acts as a barrier that blocks water, oxygen, and chlorides from reaching the steel surface. Additionally, byproduct from the rapidly corroding activated zinc fills any microcracks in the coating, essentially enabling the coating to heal itself.
The addition of the hollow glass spheres enhances the coating’s physical properties. The spheres improve the film’s crack resistance by blocking the propagation of microcracks, and they contribute to the coating’s low permeability as well.
Additionally, the glass spheres are important contributors to the coating’s inhibitor effect. “We can observe an accumulation of insoluble complexes of zinc, oxygen, and chlorides on the surfaces of the glass spheres that become part of the coating instead of reaching the steel substrate,” Morton explains. The zinc corrosion product created during galvanic corrosion also acts as environmental scavenger by capturing chloride ions as they diffuse into the coating from the environment.
Morton comments that the activated zinc-rich epoxy primers are normally applied as part of a three-coat protective coating system that also includes an epoxy intermediate coat and a polyurethane (PUR) topcoat, which is standard for coating protection of outdoor steel structures such as bridges, offshore oil platforms, coastal power plants, and refineries in industrial and marine environments with high relative humidity and aggressive atmospheres, including coastal and offshore areas with high salinity (described in ISO 129441 as a C5 environment). Samples coated with the activated zinc-rich epoxy primer with a DFT of ~100 µm were exposed in an accelerated salt spray exposure test (per ISO 92272) for 1,259 h and passed without signs of rust. Samples coated with a three-coat system with the activated zinc-rich primer, epoxy intermediate, and PUR topcoat passed a neutral salt spray test (per ISO 12944 for the C5 category) after being exposed for 1,440 h
The activated zinc-rich primer also showed good results when tested for adhesion (ISO 46243) and crack resistance (NACE TM03044). Scanning electron microscope (SEM) images were taken to evaluate the degree of cracking and self-healing in the activated zinc-rich epoxy primer. It was possible to observe the hollow glass spheres stopping the development of cracks, as well as the zinc corrosion product filling the empty space of the microcracks. To qualitatively evaluate the chemical composition of the deposit formed around the glass spheres in the primer, energy dispersive x-ray (EDS) elemental mapping studies were done. Chloride atoms were detected in the zinc salts that surrounded the hollow glass spheres, suggesting that the spheres work as a surface for the deposition of salts, which provides an inhibitor effect.
The new activated zinc-rich primer technology was launched in 2014 as part of the AvantGuard† product series.
Contact David Morton, Hempel—e-mail: dmor@hempel.com.
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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