INTRODUCTIONCorrosion is a naturally occurring phenomenon commonly defined as thedeterioration of a substance (usually a metal) or its properties because ofa reaction with its environment. [1] Like other natural hazards such asearthquakes or severe weather disturbances, corrosion can cause danger-ous and expensive damage to everything from vehicles, home appliances,and water and wastewater systems, to pipelines, bridges, and public build-ings. Unlike many natural disasters however, there are time-proven meth-ods to prevent and control corrosion that can reduce or eliminate itsimpact on public safety, the economy, and the environment.

With its many forms, causes, and associated prevention methods, cor-rosion obviously is highly complex and requires extensive expertise andsignificant resources to control. The 2001 US Federal HighwayAdministration-funded cost of corrosion study, “Corrosion Costs andPreventive Strategies in the United States,”[2] which was initiated byNACE International and conducted by CC Technologies, Inc., determinedthe annual direct cost of corrosion to be a staggering $276 billion – or3.1% of the gross domestic product. This finding was based on an analysisof direct costs by industry sector. When based on the cost of corrosion prevention and control, the study found that $121 billion is attributed tocorrosion control methods, services, research and development, and education and training. Corrosion is so prevalent and takes so many forms that its occurrence and associated costs never will be completelyeliminated; however, the study estimates that 25 to 30% of annual corrosion costs could be saved if optimum corrosion management practices were employed.

The science of corrosion prevention and control is highly complex,exacerbated by the fact that corrosion takes many different forms and isaffected by numerous outside factors. Corrosion professionals must under-stand the effects of environmental conditions such as soil resistivity,humidity, and exposure to salt water on various types of materials; the typeof product to be processed, handled, or transported; required lifetime of thestructure or component; proximity to corrosion-causing phenomena suchas stray current from rail systems; appropriate mitigation methods; andother considerations before determining the specific corrosion problemand specifying an effective solution.

The first step in effective corrosion control, however, is to have a thor-ough knowledge of the various forms of corrosion, the mechanismsinvolved, how to detect them, and how and why they occur. [3]

BASIC FORMS OF CORROSIONThere are 10 primary forms of corrosion, but it is rare that a corrodingstructure or component will suffer from only one. The combination ofmetals used in a system and the wide range of environments encounteredoften cause more than one type of attack. Even a single alloy can suffercorrosion from more than one form, depending on its exposure to differ-ent environments at different points within a system.

All forms of corrosion, with the exception of some types of high-temperature corrosion, occur through the action of the electrochemical cell(see Figure 1). The elements that are common to all corrosion cells are ananode where oxidation and metal loss occur, a cathode where reduction

and protective effects occur, metallic and electrolytic paths between theanode and cathode through which electronic and ionic current flows, andan electrical potential difference that drives the cell. The driving potentialmay be the result of differences between the characteristics of dissimilarmetals or surface conditions, and the environment, including chemicalconcentrations. There are specific mechanisms that cause each type ofattack, different ways of measuring and predicting them, and variousmethods that can be used to control corrosion in each of its forms.

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CORROSION—A NATURAL BUT CONTROLLABLE PROCESS

Gretchen A. Jacobson, Managing EditorMaterials Performance

NACE InternationalHouston, TX

24AMPTIAC

Figure 1. The Electrochemical Cell.

Electron Flow

Electrolyte

Voltage Difference

Corrosion OccursHere

No CorrosionOccurs Here

Ion Flow

+

-

Current [+] fromAnode Metal[Corrosion]

Current [-]from Cathodic

Reduction

ANODE

CATHODE

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General Attack CorrosionAlso referred to as “general corrosion” or “uniform corrosion,” generalattack corrosion proceeds more or less uniformly over an exposed surfacewithout appreciable localization (see Figure 2). This leads to relatively

uniform thinning onsheet and plate mate-rials and general thinning on one sideor the other (or both)for pipe and tubing. Itis recognized by aroughening of the sur-face and usually by thepresence of corrosionproducts. The mecha-nism of the attack typically is an electro-

chemical process that takes place at the surface of the material.Differences in composition or orientation between small areas on themetal surface creates localized anodes and cathodes that facilitate the corrosion process.

Most often caused by misapplying materials in corrosive environ-ments, general corrosion often can be tolerated because the effect ofmetal loss is relatively easy to assess and allowances can be made in theinitial design. Protective coatings are particularly effective in control-ling uniform corrosion. Cathodic protection (CP) – an electrochemicaltechnique used for corrosion control (see “Methods of CorrosionControl” later in this article) – can be used in underground or immer-sion situations.

Localized CorrosionUnlike general attack corrosion, localized corrosion occurs at discretesites on a metal surface. Types of localized corrosion include pitting,crevice, and filiform corrosion.

Pitting Pitting is a deep,narrow attack that cancause rapid penetration ofthe substrate (metal) wallthickness. It is character-ized by corrosive attack ina localized region sur-rounded by noncorrodedsurfaces or surfaces thatare attacked to a muchlesser extent (see Figure3). Pits initiate at defectsin a protective or passivefilm. The corrosion is

caused by the potential difference between the anodic area inside the pit– which often contains acidic, hydrolyzed salts – and the surroundingcathodic area. Pitting corrosion is controlled by using more pit-resistant materials, protective coatings, and/or CP, or by modifying theenvironment (i.e., by introducing deaeration, chemical corrosioninhibitors, etc.).

Crevice Corrosion Crevice corrosion occurs at localized sites where freeaccess to the surrounding environment is restricted, such as creviceswhere materials meet – either metal-to-metal or metal-to-nonmetal.Crevices also can form under deposits of debris or corrosion products.Also called “concentration cell corrosion,” crevice corrosion is caused bytwo basic mechanisms: oxygen concentration cell corrosion and metalion concentration cell corrosion. In the first mechanism, the differencein oxygen concentration between the areas inside and outside the crevicecauses a potential difference between these areas. The area within thecrevice becomes anodic with respect to the outside area, where the highoxygen content drives the cathodic reaction and causes corrosion tooccur deep within the crevice. In metal ion concentration cell corrosion,the difference in potential between the inside and outside of the crevice iscaused by a difference in metal ion concentration. When this mechanismoccurs, corrosion usually is concentrated at the entrance of the crevice.

Crevice corrosion control is complicated by the difficulty of reachingthe environment within the crevice. The principal options for corrosioncontrol in this instance are appropriate materials selection, improveddesign (by eliminating as many crevices as possible), the use of coatingsto seal crevices, and CP.

Filiform Corrosion Filiform corrosion is a special form of oxygen cellcorrosion occurring beneath organic or metallic coatings on materials.This form is recognizable by the appearance of a fine network of random “threads” of corrosion product beneath the coating material(see Figure 4). Filiform corrosion is associated with mild surface contamination of solid particles deposited from the atmosphere orresidue on the metal surface after processing. When exposed to humidconditions (relative humidity greater than 60%), these surfaces oftenwill suffer filiform corrosion. Corrosion proceeds because of the poten-tial difference between the head of the advancing filament, whichbecomes anodic, with a low pH and lack of oxygen compared to

the cathodic areaimmediately behindthe head. This type of corrosion, particu-larly on painted sur-faces, can be prevent-ed by proper cleaningand preparation ofthe metallic surfaceand then applying aprotective coating toa clean, dry surface.

Figure 2. General Corrosion of a SteelStorage Tank.

Figure 4. Example of Filiform Corrosion.Figure 3. Pitting Corrosion Inside of a Pipe.

Galvanic CorrosionGalvanic corrosion occurs because of the potential differences betweendifferent metals when they are in electrical contact and exposed to anelectrolyte. It also can occur between a metal and an electrically conduc-tive nonmetal such as graphite. The rate of attack of one metal or alloyusually is accelerated; in contrast, the corrosion rate of the other usuallyis decreased. Galvanic corrosion often is pronounced where the differentmaterials are immediately adjacent to one another. The mechanism is aclassic electrochemical cell – electrons flow through a metallic pathfrom sites where anodic reactions are occurring to sites where the elec-trons cause cathodic reactions to occur. At the same time, there is amigration of ions (charged particles) in the electrolyte. The positive ionsare charged metal particles dissolved at the anode; loss of these ionscauses the corrosion of the anode. The negative ions come from thecathodic reactions. Thus, the anode corrodes and the cathode does not –in fact, the cathode is protected against corrosion. There also is a voltage,or potential, difference between the anode and cathode (Figure 1). Thecell is driven by the potential differences between metals or electricallyconductive nonmetals when exposed to an electrolyte.

A galvanic series can be used to determine the likely interactionsbetween adjoining metals. Table 1shows an example of a galvanic seriesfor metals in seawater. When twometals are connected together, themore active one becomes the anode(corroding) and the less active onethe cathode.

Galvanic corrosion can be con-trolled with materials selection, barri-er coatings, CP, modification of theenvironment, electrical isolation (theconnection between dissimilar metalsis insulated to break the electrical

continuity), and design. When designing the system, unfavorable arearatios should be avoided by using metal combinations in which the moreactive metal or alloy surface is relatively large. Rivets, bolts, and other fas-teners should be of a more noble metal than the material to be fastened.

Environmental CrackingUnlike many other forms of corrosion where corrosion occurs over longperiods of time, environmental cracking can occur very rapidly. Becauseit is unanticipated, it can be catastrophic. Environmental cracking is thebrittle failure of an otherwise ductile material caused by the combinedaction of corrosion and tensile stress. It can be identified by tight cracksthat are at right angles to the direction of maximum tensile stress. Typesof environmental cracking include stress corrosion cracking (SCC),hydrogen-induced cracking (HIC), liquid metal embrittlement (LME),and corrosion fatigue (CF). There are many ways to control the variousforms of environmental cracking, including materials selection, modifi-cation of the environment, protective coatings, CP, reduction in residualsurface stress, and by changing the design to lower tensile stress levels.

Stress Corrosion Cracking An anodic process, SCC occurs in metalsexposed in an environment where no damage would result if tensilestresses were reduced or absent. Examples of media that promote SCCof specific alloys include strongly alkaline solutions with carbon steel(CS), chlorides with stainless steel (SS), and ammonia (NH3) with cop-per alloys. Usually there is incubation period during which crackinginitiates on a microscopic level, followed by propagation (see Figures 5and 6). Typically there is little metal loss or general corrosion associat-ed with SCC.

Hydrogen-Induced Cracking HIC results in the brittle failure of otherwise ductile materials when exposed to an environment wherehydrogen can enter the metal. It is caused by the combined action of tensile stress and hydrogen. A cathodic phenomenon, HIC occurs whenthe normal evolution of hydrogen at cathodic sites is inhibited and theatomic hydrogen in the cathodic reaction enters the metal. Higher-strength alloys (those with a tensile strength of 1,034 MPa or greater)are more susceptible to HIC than lower-strength alloys. Sulfide stress

cracking (SSC) is a specific form of HIC wherein the presence of sulfidessuppresses the evolution of hydrogen. This is a common problem insour service – processes or conditions involving wet hydrogen sulfide(H2S) such as in oil fields.

Table 1. Galvanic Series for Metals in Seawater.Active (More Negative) End Magnesium

ZincAluminum AlloysCarbon SteelCast Iron13% Cr (Type 410 SS (Active)18-8 (Type 304) SS (Active)Naval BrassYellow BrassCopper70-30 Copper-Nickel Alloy13% Cr (Type 410) SS (Passive)Titanium18-8 (Type 304) SS (Passive)GraphiteGold

Noble (More Positive) End PlatinumSource: NACE International Basic Corrosion Course Handbook, p. 2:17.

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Figure 5. Micrograph of SCC Fracture Surface. Figure 6. SCC Induced Failure of a Tube.

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Liquid Metal EmbrittlementLME is defined as the decrease in strength or ductility of a metal or alloyas a result of contact with a liquid metal. A normally ductile materialunder tensile stress while in contact with a liquid metal may exhibit brit-tle fracture at low stress levels. Unlike fracture by SCC, LME is not timedependent – cracking can begin immediately upon application of stress.Embrittling agents cause different reactions depending on the alloy; forexample, SS are quite resistant to degradation when contacted by liquidmetal whereas mild CS and copper-based alloys can become severelyembrittled.

Corrosion FatigueCorrosion fatigue (CF) is caused by the combined action of a cyclic ten-sile stress and a corrosive environment (see Figure 7). It is characterizedby a premature failure of a cyclically loaded part. The petroleum indus-try encounters CF problems in the production of oil – the exposure ofdrill pipes and sucker rods to brines and sour crudes causes costly failuresand loss of production.

Flow-Assisted CorrosionFlow-assisted corrosion is caused by the combined action of corrosionand fluid flow. This includes erosion-corrosion, impingement, andcavitation.

Erosion-Corrosion This form of corrosion can occur in flowing liquidsor gases with or without abrasive particles (see Figure 8). The velocity ofthe flow is sufficient to remove weakly adhering corrosion products fromthe surface or by damaging the protective oxide film, reducing the protective effect of each, and also pit or otherwise remove substrate.Mechanical erosion is caused by hard particles impacting the surface,causing craters in the metal. If erosion-corrosion can be identified and

there is no evidence of particle impingement, a possible solution is toreduce the flow rate or remove flow-disturbing surface discontinuities.

Impingement Impingement is caused by turbulence or impinging flow(directed at roughly right angles to the materials). Entrained air bubblestend to accelerate this action, as do suspended solids. This type of corro-sion occurs in pumps, valves, and orifices; on heat exchanger tubes; andat elbows and tees in pipelines. It usually produces a pattern of localizedattack with directional features. When a liquid is flowing over a surface,there usually is a critical velocity below which impingement does notoccur and above which it rapidly increases. Impingement first receivedattention because of the poor behavior of some copper alloys in seawater.

Cavitation This mechanical damage process is caused by collapsingbubbles in a flowing liquid, usually forming deep aligned pits in areas ofturbulent flow. Cavitation occurs when protective films are removed froma metal surface by high pressures generated by the collapse of gas orvapor bubbles. In general, higher-strength alloys are more resistant tothis type of corrosion than lower-strength alloys. When cavitation dam-age is caused primarily by corrosion following the removal of protectivefilms, the corrosion portion of the damage may predominate. Underextreme cavitation conditions, the cavitation itself is capable of removingthe metal directly and corrosion effects are insignificant.

With all forms of flow-assisted corrosion, proper materials selectionfor a particular environment is crucial to preventing damage. Othermethods include modification of the environment, protective coatings,CP, and controlling flow velocity and patterns through design.

Intergranular CorrosionIntergranular corrosion is the preferential attack at, or adjacent to, thegrain boundaries of a metal. Almost all engineering metals are com-posed of individual crystals, or grains, that meet at areas of relative

Figure 8. Failure of an Impeller Due to Erosion Corrosion.

Figure 7. Example of a Fracture Surface Resulting FromCorrosion Fatigue.

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impurity and misalignment. Intergranular corrosion occurs when thegrain boundries or areas directly adjacent to them are anodic to the sur-rounding grain materials. This can happen because of differences inimpurity levels or strain energy of the misalignment of atoms in thegrain boundaries. In some cases, individual grains are loosened and lostfrom the material. In other cases, the localized loss of grain boundarymaterial causes localized attack similar in appearance to cracking.Intergranular corrosion can be controlled using proper material selection, design, fabrication, and weld procedures; modification of theenvironment; and heat treatment to dissolve the undesirable con-stituents at the grain boundaries.

DealloyingDealloying is a corrosion process in which one constituent of an alloy isremoved preferentially, leaving an altered residual structure. Many alloysconsist of mixtures of elements (i.e., zinc and copper are alloyed to pro-duce brass), where one element can be anodic with respect to the otherelement(s) and can selectively corrode by galvanic action. This phenom-enon is commonly detectable as a color change or drastic change inmechanical strength. For example, brasses will turn from yellow to redand cast irons will turn from silvery gray to dark gray. Various laborato-ry techniques, such as cross-sectioning the part in question, can provideevidence of color changes. Metallographic examination at high magni-fication and x-ray spectroscopy also can provide positive identification. It can be controlled through materials selection, modification of theenvironment, protective coatings, CP, and design (e.g., controlling temperature to minimize hot-wall effects in heat exchangers).

Fretting CorrosionFretting corrosion is defined as metal deterioration caused by repetitiveslip at the interface between two surfaces in contact that were not intend-ed to move in that fashion. The motion between surfaces either removesprotective films or, combined with the abrasive action of corrosive products, mechanically removes material from surfaces in relativemotion. For fretting to occur, the interface must be under load and themotion must be sufficient for the surfaces to strike or rub together.Results of fretting include metal loss in the area of contact; production of oxide and metal debris; galling, seizing, fatiguing, or cracking of the metal; loss of dimensional tolerances; loosening of bolted or rivetedparts; and destruction of bearing surfaces. Techniques for controlling this type of corrosion include materials selection, designing to avoid motion between surfaces, and the use of lubricants such as molybdenum disulfide.

High-Temperature CorrosionDirect chemical reactions, rather than the reactions of the electro-chemical cell, are responsible for the deterioration of metals by high-temperature corrosion. The actual temperature at which corrosionoccurs depends upon the material and the environment, but corrosionusually starts when the temperature is within approximately 30 to 40%of the alloy’s melting point. High-temperature corrosion usually is asso-

ciated with the formation of thick oxide or sulfide scales, with reactionsthat cause internal swelling of the metal. It is dependent on reactionsthat include oxygen effects, sulfidation, carburization, decarburization(hydrogen effects), halide effects, and molten-phase formation. Methodsto control this type of corrosion are largely confined to materials selec-tion and design, although limited modification of the environment canbe achieved and protective coatings can be effective.

METHODS OF CORROSION CONTROLAs is true of the various forms of corrosion, there are many differentmethods of corrosion prevention and control. Each offers its own com-plexities and purposes. In general, the approach to control most corrosion is to understand the corrosion mechanism involved andremove one or more of the elements of the corrosion cell; for example, byelectrically separating the anode and cathode from each other or fromthe electrolytic environment by reducing the driving potential. The mostcommonly used corrosion control methods include materials selectionand design using corrosion-resistant alloys, plastics, and polymers;organic and metallic protective coatings; CP; and corrosion inhibitors.All of these methods are appropriate for controlling corrosion in certainsituations and not for others. They often are used together to solve a particular corrosion problem (for example, protective coatings and CP are a common and effective combination).

A complete description of all the variations of corrosion controlmethods and systems is beyond the scope of this article. Brief descriptionsof the most common methods follow.

Materials Selection & DesignThere is no one material resistant to all corrosive situations but materials selection is critical to preventing many types of failures. Whenselecting a material, the required characteristics need to be defined inadvance. If no material has every characteristic that a specific projectrequires, a corrosion control system will be needed or the service conditions must be adjusted to meet the characteristics of the candidatematerial. Factors that influence materials selection are listed in Table 2.

Appropriate system design also is highly important for effective corrosion control. Design includes the consideration of many factors,such as materials selection, process and construction parameters, geometry for drainage, avoidance, or electrical separation of dissimilarmetals, avoiding or sealing of crevices, corrosion allowance, operatinglifetime, and maintenance and inspection requirements.

Protective CoatingsPutting a barrier between a corrosive environment and the material tobe protected is a fundamental method of corrosion control. There aremany organic and metallic coating systems to choose from, and theyare available in various combinations. Coating system selection is sim-ilar to materials selection in that many factors need to be considered asseen in Table 3.

Common coating application methods include brush or roller, spray,and dipping. In addition to proper coating selection and application

methods, substrate preparation is critical to the success of the coating.The majority of coating failures are caused either completely or partiallyby faulty surface preparation, [3] such as leaving contaminants on thesurface or having an inappropriate surface morphology.

Cathodic and Anodic ProtectionCP is an electrochemical technique used on facilities like pipelines,underground storage tanks, and offshore structures that makes thestructure to be protected a cathode relative to an external anode that

discharges protective current to all exposed surfaces. The source of theprotective current may be an active (impressed current) or passive (sacrificial) system of galvanic anodes (usually magnesium, aluminum, or zinc) (see Figure 9). CP is widely used in several environments, including water and soil. It often is used in combinationwith coatings that reduce the exposed surface area to receive protectivecurrent.

Anodic protection has more limited, but important, applications in chemical environments. It is achieved by maintaining an active-passive metal or alloy in the passive region by an externally appliedanodic current.

Corrosion InhibitorsCorrosion inhibitors are substances that, when added to an environment,decrease the rate of attack. Inhibitors are commonly added in smallamounts to acids, cooling waters, steam, and other fluids, either contin-uously or intermittently. They generally control corrosion by formingthin films that modify the environment at the metal surface. Some retard corrosion by adsorption to form a thin, invisible film only a fewmolecules thick. Others form bulky precipitates that coat the metal and

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Table 2. Factors Affecting Materials Selection.• Corrosion resistance in the environment• Availability of design and test data• Mechanical properties• Cost• Availability• Maintainability• Compatibility with other system components• Life expectancy• Reliability• Appearance

Founded in Houston, Texas, in 1943 by 11 pipeline corrosion engineers, NACEInternational now has approximately 15,000 members worldwide and is involved in every industry and area of corrosion prevention and control. With the mission of providingeducation and communicating information to protect people, assets, and the environmentfrom the effects of corrosion, NACE offers a wide variety of activities, services, and benefitsto the corrosion control community:

• More than 1,500 NACE members participate in the activities of more than 250 technical committees to write and publish reports and standards, and exchange technical information.

• The NACE Annual Conference and Exhibition, held each spring, attracts approximately6,000 attendees from around the world for a week of technical symposia, meetings,training opportunities, and exhibits of the latest products and services for corrosion control. Other conferences and symposia, including the popular Pipeline IntegrityManagement Symposium series, are held on a recurring basis.

• NACE education and training courses, including the renowned Coating InspectorProgram, cover a variety of corrosion control technologies and levels of certification and are held all over the world. To date, more than 10,000 people have been certifiedor professionally recognized by NACE.

• NACE members receive the monthly journal Materials Performance and many subscribeto CORROSION, the journal of corrosion science and technology. NACE offers theworld’s largest selection of books on corrosion control, along with software and otherproducts.

• NACE Public Affairs is working with industry, academia, and government to increase thevisibility of the Society and the critical importance of corrosion prevention and control.

For more information about NACE, visit www.nace.org.

NACE International –The Corrosion Society

protect if from attack. A third mechanism consists of causing the metalto corrode in such a way that a combination of adsorption and corrosionproduct forms a passive layer.

CORROSION CONTROL OF THE FUTURESociety will continue to face critical challenges in corrosion preventionand control, where aging equipment and infrastructure, new product formulations, environmental requirements, and strict budgets requirecorrosion control programs that are designed by highly skilled profes-sionals for specific situations. The fields of corrosion science and engineering are of utmost importance to develop the experience and toolsnecessary to successfully reduce the incidence, problems, and expensecaused by corrosion. By following appropriate strategies and obtainingsufficient resources for corrosion programs, the best engineering practices can be achieved. The payoff includes increased public safety,reliable performance, maximized asset life, environmental protection,and more cost-effective operations in the long run.

REFERENCES[1] Corrosion Basics – An Introduction, L.S. Van Delinder, ed. (Houston,TX: NACE, 1984)[2] G.H. Koch, M.P.H. Brongers, N.G. Thompson, Y.P. Virmani, J.H. Payer,“Corrosion Costs and Preventive Strategies in the United States”(Washington D.C.: FHWA, 2001)[3] NACE International Basic Corrosion Course Handbook (Houston, TX:NACE, 2000)

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Table 3. Coating System Selection Factors.• Types of exposure• Operating conditions• Substrate• Ambient conditions during application• Environmental regulations• Cost• Application during operation or shutdown• Time constraints• New construction or maintenance• Shop or field application• Design/fabrication considerations

Ms. Gretchen A. Jacobson has been in the business of magazine writing, editing, and production for more than 20 years. She began her career at Petersen Publishing Company in Los Angeles, California, where she worked for several specialty automotive and outdoor enthusiast magazines. She was production manager of a group of lifestyle, business, and arts magazines in California before moving to Houston, Texas, in 1994. She was the science writer and editor for a National Science Foundation-funded Science and Technology Center on parallelcomputation research that was based at Rice University with affiliated universities and government laboratoriesaround the country. She has been the managing editor of NACE International’s membership magazine, MaterialsPerformance, since 1997.

Figure 9. Cathodic Protection Methods.

Ion Flow

Electron Flow

A. Impressed Current CP

Power Source Offset CurrentProvides Electrons

GalvanicAnode

+

-Cations [+] from

Anode

ANODE

B. Galvanic Anode CP

Cation [+] fromAnode Metal

Cations [–] fromCathodic Electrons

ProtectedStructure

[Cathode]

Ion Flow

Electron Flow

Cations [–] fromCathodic Electrons

ProtectedStructure

[Cathode]

+

-