Cathodic Protection Cuts Corrosion Costs - Print This Page

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Cathodic Protection Cuts Corrosion Costs

By Ted Huck, MATCOR, Inc.

A 1998 EPRI study, released in October of 2001, estimated the cost of corrosion in 1998 to steam generatingfacilities alone at $5.37 billion. The study, EPRI Report 1004662, also concluded that 22 percent of these

costs are avoidable. Corrosion remains an ongoing problem for power generators and the industry has spent,and will continue to spend, millions of dollars on a wide range of corrosion protection measures.

One of the proven technologies for controlling corrosion is cathodic protection. Although used in numerousapplications in power plants, there are still many untapped opportunities to apply this technology. Powerplants using cathodic protection are able to reduce the economic impact of power plant corrosionconsiderably.

What is Corrosion?

Corrosion can be defined as the

degradation and destruction of a metal byits chemical reaction with theenvironment. Corrosion reactions, withthe exception of some forms of hightemperature corrosion, areelectrochemical oxidation/reductionreactions. Figure 1 shows a typicalcorrosion cell that is formed when smallvariations in potential occur over themetal's surface. The corrosion is causedby differences in the metal or its

surrounding electrolyte. The difference inpotential results in a current flow from theanode to the cathode.

Cathodic Protection

Cathodic protection is one option forcontrolling corrosion. In the typical corrosion cell, the metal structure has both anodic (area where metal islost) and cathodic (area with no metal loss) regions resulting from electrical potential differences. Even small

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Fig. 2


differences in potential can result in significant metal loss over time. Cathodic protection is accomplished byintentionally substituting the slightly anodic region of the structure to be protected with an even more anodiccomponent called an anode.

The anode is intentionally coupled with the protected structure. In a galvanic system, current results from theinherent potential differences between the anode and the cathode. An impressed system, on the other hand,uses a DC power supply. As long as sufficient protective current is maintained, an impressed system caneliminate further corrosion.

Galvanic Systems

The simplest system is the galvanic anode system,Figure 2. With a galvanic system, the potentialdifference results entirely from the electricalcharacteristics of the anode versus the structure tobe protected. Although this type of system willprotect the entire surface, it is important that thecorrect anode be selected. Some common galvanic

anode materials are magnesium, zinc and aluminum.

Anodes are available in standard sizes weighingfrom one to 48 pounds. For example, the 17-poundmagnesium pre-packaged anode, approximately 30inches long and six inches in diameter, is the mostcommonly specified anode for many pipeline systemapplications. The package consists of a solid bar ofmagnesium alloy connected to an electrical lead thatprotrudes from a bag filled with a mixture ofbentonite and gypsum.

The bentonite and gypsum provide an environmentthat absorbs and retains moisture while assuring auniform low resistance around the anode. Prior toinstallation, the bag is thoroughly wetted and the lead is either welded, or mechanically connected, to thestructure to be protected. Typically, multiple galvanic anodes are strategically located across the entirestructure.

Although galvanic systems can be inexpensive, there is little margin for design error. The natural voltagedifferences are relatively small and fixed. Galvanic systems also do not allow for increased current outputwhen operating conditions or the environment change.

For buried magnesium anodes, the electrical potential between the anode and carbon steel is a nominal-0.85V. The current output is a function of Ohms Law (E=IR) and is thus dependent on the resistance of thesystem. Spacing, quantity and location of the anodes must be determined during the design process. The rateof consumption of the anode can be calculated and its design life reasonably predicted.

Sacrificial anodes must be replaced on a regular basis to maintain system performance. Generally, buried orsubmerged anodes are not removed at the end of their useful life; new ones are simply added to the system tomaintain the required protection.

Impressed Current Systems


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Fig. 3


The impressed current system, in contrast to the galvanic system, does notrely on the potential differences between the anode and the structure to becathodically protected. Instead, an external power source, typically arectifier converting electric power from AC to DC, is used. This creates apotential difference and helps to transmit the current from the anodethrough the electrolyte and onto the surface being protected, Figure 3.

Since the system uses a rectifier to provide the necessary current, the

current can be regulated to provide optimum protection. It can also beadapted to changes in the structure, operating conditions, or theenvironment. When this type of system is used, the number of anodesrequired can be significantly reduced. A single anode is able to transfermore current over greater distances.

Anode materials are selected based on how fast they are consumed and noton their inherent potential difference with the structure to be protected.

Many anode materials can be used in impressed current systems, including graphite, high silicon cast iron,platinum alloys and mixed metal oxides. The latter two have very low consumption rates and for this reasonthey are coated, in thin layers, onto a substrate of titanium or niobium giving the active element an inert but

conducting base. With consumption rates of milligrams per amp year, impressed current anodes can bedesigned for long life.

Reference cells tied back to a control loop on the rectifier can be used to regulate the current output of thesystem. This is required when conditions change or where excessive current must be limited to preventdamage to the system. When designing impressed current systems the design engineer must take intoconsideration control points, test stations, instrumentation, and other operational and maintenance tools.

Power Plant Applications

There are several applications within a power plant where cathodic protection can be utilized:

Underground pipingWaterfront structuresCooling water process equipmentAbove ground tanks

Federal regulations have long mandated that cathodic protection be installed on underground hydrocarbonpipelines. In power plants there are several underground piping systems, including fuel oil, natural gas, firewater, cooling water, condensate and drains that need to be protected. Although underground steel pipingsystems typically are coated, damage to the coating can result in concentrated galvanic corrosion.

Depending upon the size and layout of underground piping systems, and the soil's characteristics, thesesystems can be protected using either galvanic or impressed current designs. However, the design of thecathodic protection system should take into consideration other underground piping systems and structures:conduits, copper ground grids, electrical cables and foundation steel.

In a galvanic system it is essential that the lines be electrically isolated from the plant's grounding grid system.Otherwise, the anodes will try to protect everything attached to the grid and would be unable to generatesufficient current.

In most power plant applications, the high concentration of underground structures and piping, along with an


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extensive grounding circuit, preclude the use of galvanic systems. It is simply too difficult to assure properisolation of the piping. In these cases, an impressed current system must be used.

Waterfront Structures

Many power plants have waterfront structures such as docks, steel piers and pilings, cooling water intake andoutlet structures and inlet screens. Whether carbon steel or steel reinforced concrete, these structures are

subject to extremely corrosive environments, particularly when they are located in saltwater or brackish waterareas. Even river water can be corrosive. However, cathodic protection, typically impressed current, isextremely effective for protecting these types of structures.

Corrosion of steel in concrete (rebar) is often overlooked. Although steel should be protected by the alkalinenature of the cement used, this does not always happen. Corrosion of steel in concrete can occur fromabsorption of ions from seawater or ocean spray, industrial atmospheres (caustic, acids and other damagingchemicals), de-icing salts and chloride buildup in the re-circulating water.

When the steel in the concrete corrodes, ferric oxide particles are formed. Over a period of time the rustparticles take up significantly more volume than the steel and eventually it expands within the concrete. The

end result is cracking, spalling and ultimately separation of the corrosion from the reinforcing steel. However,cathodic protection can prevent corrosion of the steel and premature failure of the concrete.

Cooling Water Systems

Surface condensers, shell and tube heat exchangers, valves, strainers, screens and pumps can all becathodically protected from corrosion. Frequently the main surface condensers in a power plant have carbonsteel water boxes with an internal epoxy coating or rubber lining applied. However, without cathodicprotection, any defect in the coating or liner can cause corrosion to develop more quickly than on surfacesthat have no coating or liners. Still, the use of coatings and liners does significantly reduce the amperagerequired by cathodic protection systems.

Because galvanic systems require a system shutdown for inspection and replacement of the sacrificial anodes,impressed current systems are the best solution for large condenser water boxes. When an impressed currentsystem is used frequent anode replacement is not required.

Special care must be taken when providingcathodic protection on condensers using titaniumtubes and tube sheets. When these materials aresubjected to excessive current they becomesusceptible to hydrogen embrittlement. A typicalimpressed current water box system is shown on

Figure 4. To limit the current output belowthresholds that might cause hydrogenembrittlement of the tubes and tube sheet, areference cell must be connected to a rectifier.This allows automatic adjustment of the currantoutput if needed.

Storage Tanks


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The bottoms of carbon steel tanks used for fuelstorage are susceptible to corrosion. Today,cathodic protection for new fuel tanks isbecoming increasingly common. However, oldertanks can be retrofitted with cathodic protection.Tank size, geometry, and the physicalcharacteristics of the underlying surface willdetermine the type of system that is most

economical.

Operation and Maintenance

Cathodic protection systems prevent corrosion of the protected structure only when they function properly.As a result, their design should incorporate sufficient test stations and reference cells to allow plant staff toconfirm that the system is indeed operating as intended.

Systems should be thoroughly tested on an annual basis by NACE certified specialists. For galvanic systems,testing is done to confirm that the protected structure is properly polarized. In addition, taking potential

readings, and measuring instant on/off voltages in accordance to NACE standards, can confirm that properprotective current is being supplied. Impressed current systems also require similar performance testing. Therectifiers supplying the power should be checked on a regular basis and the output current and reference cellvoltage readings should be taken and compared to the initial baseline readings.

Economic Considerations

Before installing a cathodic protection system, an economic evaluation should be conducted weighing theupfront and maintenance costs of cathodic protection versus the costs associated with the failure of powerplant equipment due to corrosion. Even though the cost of providing cathodic protection is relatively low, andeasily defined, the cost of power plant equipment failures resulting from corrosion are more difficult toquantify.

Besides the cost of repairing equipment damaged by corrosion, there are often consequential costs fromequipment failing. These can include environmental costs in the event of an underground pipeline or aboveground storage tank failure, lost energy production resulting from a condenser failure, or safety related issuesstemming from firewater piping failures.

Corrosion will continue to be a problem in power plants. Still, electric utilities can save thousands of dollarsfrom reduced downtime and repair by installing cathodic protection systems.

To access this Article, go to:http://www.power-eng.com/content/pe/en/articles/print/volume-106/issue-6/features/cathodic-

protection-cuts-corrosion-costs.html


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