13104 AP Operation & Apps.pdf

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MATERIALS PERFORMANCE May 2001

Most metals exposed to

a liquid-phase corro-sive environment can

be protected from ag-gressive corrosion at-tack using an electro-

chemical preventiontechnique—either cathodic protection(CP) or anodic protection (AP). Select-

ing the appropriate protection methoddepends primarily on the metal/envi-ronment characteristics.

Whether a metal can corrode in aparticular environment can be deter-

mined thermodynamically. Figure 11 is

a diagram for iron in an aqueous envi-

ronment. It shows domains of immu-

nity, corrosion, and passivity with re-

lation to the iron electrical solution po-tential (EH ) and solution pH.

Although the thermodynamic calcu-

lations leading to the construction of

this diagram can determine whether

steel can corrode, they do not predict

the rate of corrosion. In contrast, cor-

rosion is thermodynamically impos-

sible at a given pH if the steel/environ-

ment potential is maintained in the

immunity zone (e.g., –0.80 V, pH 3 to

9). Accordingly, if a steel surface cor-

rodes actively and its solution poten-

tial shifts electronegatively—as is thecase with CP—then corrosion is pre-

ventable. Conversely, the corrosion

potential of metals exhibiting active-passive behavior in some situations(e.g., steel, nickel, titanium, and stain-

less steels [SS]) can be shifted in theanodic (electropositive) direction to

produce a passive surface and hence alarge reduction in corrosion rate.

Theory of AP A number of metals exhibit what is

known as “active-passive” transition

when polarized in the electropositivedirection (Figure 2). In this state, the

potential of a metal moves into a pas-sive region when it shifts to the elec-tropositive direction after direct cur-

rent (DC) is applied. A passive surfacefilm forms, resulting in a decrease in

corrosion current density (CD) andhence a lower corrosion rate. Increas-

ing anodic polarization into thetranspassive range may cause a break-down in the passive film and lead to

pitting-type corrosion. At greater than a crit ical current

density, formation of the passive filmis essentially a coulombic (total charge)

effect; therefore, the amount of currentrequired to pass the primary passivepotential is a function of time (e.g.,

higher currents are required when thetime period is short and vice versa).

The current required to passivate themetal surface is usually an order of

AnodicProtection—ItsOperation and

Applications J AMES I AN M UNRO AND W INSTON W. SHIM, Corrosion Service Co. Ltd.

Most metals will corrode within a certain range ofelectrical solution potential and solution pH. At

potentials more negative than this range, corrosion ceases(cathodic protection). At potentials more positive than thisrange, several metals become passive. For these metals

the potential can be shifted electropositively into the

passive range through anodic protection (AP). AP isespecially applicable to sulfuric acid (H2SO

4) and caustic

liquor environments. It often permits a designer to use alow alloy rather than a corrosion-resistant metal, at anoverall cost saving.

3104



May 2001 MATERIALS PERFORMANCE

magnitude greater than the current re-

quired to maintain passivation.

The passive potential range and theCD required to achieve passivation arefunctions of the type of metal and elec-trolyte and of factors such as pH, tem-

perature, degree of oxidation, etc. Mildsteel, SS, titanium, and many other

metals in the transition group exhibitactive-passive behavior.

Figure 3 shows a general AP sche-matic for a tank or vessel.

The AP current is supplied by a po-

tential-controlled power supply that isnecessary to maintain the structure at

a fixed potential in the passive range.It is common to predetermine the ideal

set potential for the operation either by laboratory testing or, more prefer-ably, by running potentiostatic anodic

polarization curves on coupons in theactual vessel.

The reference electrode is necessary to provide a feedback potential mea-

surement for comparison to the setpotential. The type of reference elec-

trode used depends on its compatibil-ity with the liquid medium. Typical ref-

b

a

–1.6

–1.2

–0.8

–0.4

0

0.4

0.8

1.2

1.6

–1.6

–1.2

–0.8

–0.4

0

0.4

0.8

1.2

1.6

221614121086420–2

1614121086420–2

pH

E H

Passivation

AP

CP

Corrosion

Immunity

b

a

–1.6

–1.2

–0.8

–0.4

0

0.4

0.8

1.2

1.6

–1.6

–1.2

–0.8

–0.4

0

0.4

0.8

1.2

1.6

221614121086420–2

1614121086420–2

pH

E H

Passivation

Corrosion

Immunity

Corrosion

Transpassive

Ecorr

Passive(AP)

Immunity(CP)

Icorr

Log Corrosion Current

FIGURE 1

EH-pH diagram for iron in water.

EH-pH diagram in conjunction with an AP polarization curve.

FIGURE 2



MATERIALS PERFORMANCE May 2001

erence electrodes are platinum,calomel, silver/silver chloride (Ag/

AgCl), mercury/mercury sulfate (Hg/

HgSO4 ), and SS.Cathode materials are also chosen

with respect to their durability in the

liquid medium, particularly becausethe cathode often passes through the

vapor zone and leaves it susceptible toaggressive atmospheric corrosion.Common cathode materials are steel,

chromium-nickel steel, Hastelloy C†,and platinum clad brass.

Potential

controlled

rectifier

(–)

(+)

IAP

Reference

electrode

Cathode

†Trade name.

TABLE 1

CORROSION RATE OF 316 SS (mpy) IN H2SO

4

Temperature Unprotected Protected Unprotected Protected

70°C 29 <2 — —

100°C 495 25 41 <2

93% H2SO4 98% H2SO4

Schematic of an AP system.

AP is most often used to protect

mild steel and SS sulfuric acid (H2SO4 )storage tanks and, to a lesser extent,

coolers and piping. It has also been ap-plied to mild steel in certain fertilizer solutions. Any situation in which a

metal/solution combination producesactive-passive behavior is a potential

candidate for AP if a corrosion prob-lem arises.

H2SO4 EquipmentThe corrosion rate of either steel or

SS in H2SO4 service is a function of both temperature and acid concentration.

Table 12 indicates the typical corrosion

rate reduction that can occur on 316

SS in two concentrations of H2SO4 by the application of AP.Figure 42 shows an anodic polariza-

tion curve for 316 SS (UNS S31600) in

93% H2SO4 at 70°C. Note that the CDrequired to achieve passivation is al-

most 104 µ A/cm 2 (10 A/ft2 ). Thispotentiodynamically derived polariza-

tion curve would indicate that an imp-ractically large current capacity power supply would be required to exceed

the active-passive transition point. For-tunately, smaller-capacity units are fea-

sible when operated galvanostatically to take advantage of the coulombic ef-

fect. Current densities to maintain pas-sivation are normally an order of mag-nitude lower than the primary

transition current. An interesting application of elec-

trochemical protection involves thecombined use of AP and CP for cor-

rosion mitigation in acid coolers (Fig-ure 5). The shell and tubes, made of

316L SS and exposed to the acid, areanodically protected where two

Hastelloy rods, inserted in the placeof two tubes, act as the cathodes. Inaddition, the carbon steel (CS) head

boxes exposed to cooling water areprotected with galvanic anodes. Pip-

ing carrying H2SO4 can also be pro-tected anodically.

Alkaline Process EquipmentThe use of AP in alkaline environ-

ments is almost entirely limited to thepulp and paper industry for the pro-

tection of pulp digesters and storagetanks handling caustic kraft liquors,

which are a mixture of sodium hydrox-

ide (NaOH) and sodium sulfide (Na2S).CS can also be anodically protected

in NaOH solutions at elevated tem-peratures.

Economic Considerations AP can be installed in conjunction

with new and existing structures. Theeconomic benefits are attractive in

both cases.

FIGURE 3



May 2001 MATERIALS PERFORMANCE

Active-passivetransition

+2.0

+1.6

+1.2

+0.8

+0.4

0

–0.4

P o t e n t i a l ( V v

s S C E )

10410310210

CD (µA/cm2)

Polarization curve 316 SS in 93% H2SO

4 at 70°C.

Retractable

referenceelectrode

Galvanicanodes

CS headboxSS tubes

Cathodes

AP of acid coolers.

AP may allow the vessel designer to

use a less expensive material or alter-

native corrosion mitigation technique. As an example, a tank used to storeconcentrated H2SO4 can be fabricated

from CS with AP as opposed to moreexpensive SS or CS with an organiclining.

The cost effectiveness of AP is afunction of vessel surface area. In gen-

eral, protecting a larger vessel trans-lates into a significantly reduced price

per unit surface area.

Remote Monitoring As a result of the large differences

in corrosion rates between vessels in

the active and passive state, it is im-perative that AP systems operate cor-

rectly for as much time as possible. Assuch, virtually all systems are supplied

with a remote monitoring unit (RMU).3

This computer-based unit allows offsiteexperts to review, and analyze opera-

tional data and recommend changes.

Conclusions

AP electrochemical systems supply a process or maintenance engineer

with powerful corrosion control tools

that can be applied to equipment ex-posed to various liquid-phase environ-ments, especially H2SO4 and caustic li-

quors. AP can be installed duringconstruction or at a later date by retro-

fitting. These systems require externalelectrical power and must be operated

continuously to be effective—this cansometimes be an added maintenance

requirement. New remote monitoring

technology, however, facilitates sys-tem supervision and allows for auto-

matic status checks either by the pro-cess operator or the system supplier.

References1. M. Pourbaix, Atlas of Electrochemical Equilib-

ria in Aqueous Solutions (Houston, TX: NACE, 1974).

2. R.M. Kain, P. Morris, Anodic Protection of Fe-

Cr-Ni-Mo Alloys in Concentrated Sulfuric Acid, COR-

ROSION/76 (Houston, TX: NACE, 1976).

3. W.W. Shim, J.I. Munro, “Anodic Protection in

the Sulfuric Acid Industry,” MP 39, 12 (2000).

FIGURE 4

FIGURE 5

JAMES IAN MUNRO is Vice President of Engineering at Corrosion Service Co. Ltd., 369Rimrock Road, Downsview, Ontario M3J 3G2,Canada. He has more than 26 years of experiencein electrochemical protection system design,including AP and CP, and remote monitoringsystems. He has a B.S. degree in electricalengineering from the University of Toronto. He is aNACE-Certified Corrosion Specialist and CPSpecialist, a professional engineer, and a 26-yearmember of NACE. He currently serves on the MP

Editorial Advisory Board.

WINSTON W. SHIM is Manager of the ProcessEngineering Group at Corrosion Service Co. Ltd.He has 19 years of experience in electrochemicalcorrosion protection of process piping and vessels.His clientele consists mainly of pulp and papermills, mining companies, and electric utilities. Hehas a M.A.Sc and M.B.A. and is a NACE-CertifiedCorrosion Specialist-P and CP Specialist. He hasbeen a NACE member since 1993.

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