c & Anodic Protection

Anodic Protection ofWhite and Green

Kraft Liquor Tankage, Part 1:

Electrochemistry of Kraft Liquors

J. IAN MUNRo, Engineering Corrosion Service Co., Ltd.

Anodic protection is a powerful technique used to mitigate liquor tankage corrosion. The large

currents required to protect a typical vesselled to problems in the first generation of commercial systems. This article describes what caused the problems. Part 11 (March 2002 MP) will discuss the solutions developed for successful protection systems.

22 MATERIALS PERFORMANCE February 2002

he kraft pulping process utilizes hot caustic sulfide liquors to remove the lig­nin that binds the cellu­lose (C

6H

100

5) fibers. The

resulting cellulose is used to manufacture many

products such as printing paper. The liquors are named white, green, and black, with white being the most con­centrated (100 g/L sodium hydroxide [Na OH], 30 gl/L sodium sulfide [Na

2S]).

The first successful use of anodic exl protection of a kraft liquor tank dates tha back to December 1984. 1 Since that tiot time, 34 systems have been installed duc in white and green liquor tanks and stw clarifiers. During the first several years of commercial application, unexpect- of 1

edly high corrosion rates were re- ex f ported at localized areas of several H e tanks even though the remainder o f the nu c surfaces corroded at rates of <5 mpy is ir (127!lm). act

This article discusses the most likely pro causes o f high localized corrosion rates the and the techniques necessary to pre- pre vent such occurrences. Part II, to be doe published in the March 2002 issue of the MP, will cover the parameters of an- reg odic protection design and operation. cee

The electrochemistry of kraft li- the quors is complex because it involves tial multiple oxidation states of sulfur pot compounds (Table 1), numerous pos- oxi sible Fe-S-H20 reactions , and the exist- suU ence of active-passive behavior. 1 Crowe2 published a list of 31 possible catl Fe-S-H20 reactions. Crowe and Peter- in c man34 suggest that additional reactions poi have been reported. int<

The electrochemistry may be fur- bel ther complicated because some Fe-S tive compounds are semiconductors. 3 Ac- (pa tive-passive behavior requires a nega-tive resistance portion of the anodic qm curve. Voltage-controlled negative re- cat sistance devices are uncommon in dis- res1 ciplines other than electrochemistry. Thi lt is interesting to note that a semicon- sul ductor device called a tunnel diode tw<

V ar

TABLE sul1

MAJOR SULFUR SPECIES IN bel

KRAFT LIQUORS mo rat<

Sulfur ste1 Na me Symbol Valence els

Sulfate so4·2 6 in v Sulfite so3·2 4 Thiosulfate sp3·2 2 Sulfur s o an 1 Polysulfide Sx(~= l) -1 at d

s,·tx=l) -2 dm Sulfide s·2 -2 Si VI

exhibits behavior nearly identical to that of active-passive interfaces. Addi­tional work will determine if semicon­ductor fllms are significant to corrosion studies in kraft liquors.

Tromans5 deduced a simple model of passivation in caustic sulfide that explains sulfide's role in the process. He concluded that, after the initial nucleation of magnetite (Fe30 ~. sulfide is incorporated into the lattice. The re­action creates Fe30 4-xSx, a non­protective compound. At the peak of the active-passive transition, Tromans predicted x to be -0.19. Passivation does not occur until oxidation removes the sulfide from the fllm. This reaction requires high current densities to pro­ceed, however, once devoid o f sulfide, the fllm will remain stable if the poten­tial is kept more positive than the Flade potential by reducing the reaction of oxidized sulfur species such as thio­sulfate cs2o3-2) and sz-2.

Figure 1 depicts one anodic and four cathodic idealized polarization curves, including the possible intersection points. The number and location of the intersection points create four types o f behavior, namely monostable (ac­tive), bistable, astable, and monostable (passive).

Astable behavior occurs infre­quently as it requires a single anodic/ cathodic intersection on the negative resistance portion ofthe anodic curve. This unstable operating condition re­sults in continuous oscillations be­tween active and passive potentials. Various alloys in elevated-temperature sulfuric acid (H2SO 4) exhibit astable behavior.6

These four types o f mixed-potential models are simplistic and do not accu­rately reflect the behavior of carbon steel (CS) in liquors because the mod­els assume that the reactions are time­invariant.

Figure 2 depicts typical curves from an in situ test in a white liquor clarifier at different scan rates. The passive state does not exist until after the active-pas­sive transition is traversed. Therefore,

FIGURE 1

300

200

100 .. ··. o ...

-100

-200

-300 0.001 0.01 0.1

-- Polarization curve

· · · - · Monostable (active)

-- Bistable

-- Monostable (passive)

-- Astable

... 1

·­... 10

CO (mA/cm2J -----Possible combinations of anodic/cathodic intersections.

FIGURE 2

300

--- 1 mV/s 200 -- 0.02mV/s

100

o

-100

-200

0.01 0.1 1

f /

I )

I I I \ .. ..

... , I , __

.... \

10

CO (mA/cm2J

Typical in situ polarization curve of CS in white liquor at two scan rates.

unless CS discharges sufficient anodic current density (CD) via a naturally oc­curring cathodic reaction o r an applied anodic protection current, the CS/ liquor interface remains monostable (active) as the passive fllm and its low CD properties do not exist.

As Tromans predicts and typical curves verify, bistable behavior can occur once the active-passive barrier is traversed. Reversing the direction o f the curve establishes a second stable equilibrium potential. Morris6 used this

knowledge to determine protection potentials for stainless steel (SS) and high-nickel alloys in elevated-tempera­ture H2S04 .

Anodic Reactions Prior to Traversing the Aclive­Passiva Boundary

Anodic protection and forced pas­sivity require some finite period of time of operation in the active zone. Thomp­son7 performed tests to establish the

February 2002 MATERIALS PERFORMANCE 23

& Anodic Protection FIGURE 3

300 Weight loss

200 --Curve . ... .. •• J . .

100 .. . "'

o

-100 .... . . . . .. . I

-200

.... . -300 . ·--400

0.0001 0.001 0.01 0.1 1 10 CO (mA/cm2)

Coupon weight loss data and polarization curve for CS in white liquor.

3(10

2(10

100

o

-100

-200

Scan rate 0.02 mV/s

-300+-~~~~~~~~~~~~~~~~--~----~~

-10,000 -5,000 o 5,000 10,000 15,000 20,000 Resistance <n-cm2)

Film resistance vs potential scan rate 0.02 mV /s.

corrosion rate during these periods. A CS specimen was potential-controlled at the peak o f the active zone for 21 h , during which time the current was carefully monitored. The CD averaged 12.5 mA/cm2

. Assuming all ofthe cur­rent caused iron to corrode to the Fe+3

oxidation state, the Faradaic loss would have approached 3,800 mpy (96,520 !lm/y). The actual measured corrosion rate , however, was only 362 mpy (9,195 !lm/y).

24 MATERIALS PERFORMANCE February 2002

Integrating 20 cycles of the anodic curve predicted a penetration of 0.17 mils (4.3 !lm) for a single traverse of the active zone. However, the experi­mental results indicated a value of 0.0135 mils (0.34 !lm) per cycle. Hence, iron oxidation accounts for only -6 to 10% o f the discharged cur­rent. The remaining current, there­fore , must be consumed by oxidation of sulfide in the Fe

30 4-xSx film or di­

rectly from the solution. This produces

large quantities of oxidized sulfur spe­cies (S20 3

-2 o r S2 -

2) in the diffusion

layer next to the passivated surface . The presence of these reducible ions at the metal/solution interface should significantly increase the cathodic re­action rate .

Due to the above situation, the bistable condition that occurs after passivation is not permanently stable. The oxidation of the Fe

30 4-xSx and

enhanced leveis of cathodic reactants cause the anodic and cathodic curves to intersect. This lack of stability was verified by passivating a coupon in stagnant white liquor for 3 days, after which time the current was shut off. The coupon's potential remained in the passive zone for 2 days until it re­activated. The cathodic and anodic reactions became unequal so as to cre­ate only a monostable (active) opera­tion. Spontaneous bistable conditions do not exist for CS in white liquor. Bistable conditions can occur only after passivation and will not remain if the naturally occurring cathodic reaction rate is less than the anodic reaction rate. Therefore, this bistable condition is better termed "forced bistable. "

Cathodic Reactions In order to control corrosion, an

anodic protection system must create and maintain the passive condition under all possible liquor chemistries. Under monostable (active) conditions, the system must provide the required CD to passiva te the steel as well as pro­vide the steady-state current necessary to equal the difference in the cathodic and anodic reactions at the optimum potential. If "forced bistable" condi­tions occur, the system must simply prevent reactivation. The control dy­namics for these two cases can differ considerably.

Research has concentrated on the anodic reactions with less emphasis on the cathodic curves. Most work as­sumes a single cathodic reaction of polysulfide to sulfide in even low-

CO!

Thi era evt ion clu dU< dat cer cor

ten we ass oxi ma ex] pot tiO I apf mo km SiOJ

liq1 di f pre afft

ta ir sul C r<

fatt not

ma Bie effc

ES

Th1

is ­tro' fi de ti vi anc tia! tiO I C ré

Ass thü ti v< si v;

concentration polysulfide liquors. Thiosulfate is a known corrosion accel­erator4·8.17 in white and green liquors, even though direct reduction of this ion is difficult to verify. Peterman4 con­cluded that thiosulfate was directly re­duced in the corrosion reaction. His data showed that the thiosulfate con­centration significantly affected the corrosion potential and corrosion rate.

Figure 3 is a plot o f ali available po­tential vs electrochemical equivalent weight loss data.4·15·17 The conversion assumes that iron corrodes to a Fe+3

oxidation state. These data carne from many different liquor chemistries and experimental techniques, including potentiostatic control, chemical addi­tions, and open-circuit conditions. It appears that the corrosion rate is much more sensitive to potential than other known variables. Therefore, the corro­sion rates in high-content thiosulfate liquor may not be an anomaly because different corrosion products are p resent, etc. Thiosulfate may somehow affect the cathodic reactions.

WensleyB reported that liquors con­taining high leveis of sulfide and thio­sulfate passivated CS coupled to SS. Crowe4 claimed that, although thiosul­fate appeared to be an oxidant, it could not passivate CS at any concentration.

Calculations performed by Tro­mans,5 from equations provided by Biernat and Robins, may explain the effects o f thiosulfate:

ES,0 3-' I s-' = 0.034 - 0.056 pH + 0.0093 log

([5, 0 3-'l I [S- ' J' ) (1)

The equilibrium potential (ES20 3 -

2 ;S-2)

is -921 mV vs a saturated calomel elec­trode (SCE) or - 21 mV vs a silver sul­fide electrode (SSE), assuming an ac­tivity o f 1 o-6 moles/L for the thiosulfate and 0.423 for the sulfide. This poten­tial shifts - 1 O m V in the positive direc­tion for each arder of magnitude in­crease in the thiosulfate activity. Assuming a reasonable Tafel slope for this reaction, an intercept more posi­tive than the criticai potential for pas­sivation of CS is not possible . Hence,

FIGURE 5

300

200

100

o

-100

-200

Scan rate 1.0 mV/s

~oo~~~~~-r~~~~~~~~~~~~~~~~

-2,000 -1,500 -1,000 -500 o 500 1,000 1,500 2,000

Resistance m-cm2)

Film resistance vs potential scan rate 1.0 mV /s.

increases in the concentration o f thio- a sample o f 1 cm2 as a function o f po­sulfate can increase the corrosion rate tential. The resistance is calculated but cannot passivate CS. At passive from the slope of the polarization potentials, thiosulfate may be oxidized curves. The resistance is low in the ac­and hence play no major part in the tive zone and negative from the cur­reduction reaction. rent peak to the Flade potential. It in-

Estimating18 the equilibrium poten- creases to large values in the passive tial ofS2-

2/S-2 puts it at -100 to 150 mV zone. The film resistance in the passive vs SSE. Polysulfide reduction, there- zone is time-dependent. fore , can provide sufficient current to The potential measured by a refer­passivate CS if enough polysulfide is ence electrode is the weighted average present in the solution. of ali potentials of a structure. The

Separate Active and Passive Areas in Anodically Protected Tanks

As a result of the high conductivity of white liquor, severa! "myths" sur­round the use of anodic protection.

weighting factor is the resistance (so­lution and film) from the location of the various surfaces to the reference. The situation is complicated by an IR voltage drop created by protection current flowing through the electro­lyte. As a result o f the large differences

One myth is that active areas will be in surface resistance between active galvanically polarized passive if the and passive areas, an active area is de­majority of the surface is passive. The tectable by a reference at any location forced bistable scenario, can facilitate in the structure. the coexistence of stable passive and If an active area develops, the po-active intercepts; the intercepts. tential controlled current source

The simultaneous existence o f sepa- (potentiostat) provides more current in rate active and passive areas generates response to the lower reference poten­low and marginaliy higher-than-open- tial. Unfortunately, the IR at the refer­circuit corrosion rates, respectively. ence location increases the measured This type o f operation is far from opti- potential. It masks the evidence of the mum. It is not, however, the worst active area, and the source produces possible scenario . Figures 4 and 5 in- insufficient current to passivate the dicate the effective film resistance for zone. Without sufficient CD to be pas-

February 2002 MATERIALS PERFORMANCE 25

' \.

& Anodic Protection

sivated, small active areas increase in size and produce a larger current out­put from the power supply on the ac­tive site. An equilibrium eventually forms between the size of the active area and the current output. This causes a location (usually an ellipse) to opera te at the peak anodic CD of CS in liquor, hence causing very high corro­sion rates.

System operation in this mode is easily corrected once the possibility o f coexistence of active and passive zones is verified. The potential of an active area is so dominant because of the low resistance to the reference that it can be easily detected. Passi­vating an active area is simple if de­tected and repassivated when the area is small.

Potential Measurements in Anodically Protected Tanks

Extensive potential data have been collected from - 40 white and green liquor tanks. Unprotected potentials of all tanks except two were -125 ± 10 m V vs SSE regardless o f liquor type (white and green). One of the anoma­lous tanks indicated potentials of -11 O and -80 mV at opposite points on the shell circumference. The potential of the other tank, which was fabricated using large amounts of SS, was in the passive zone (75 mV vs SSE).

The most frequent potential range o f -12 5 m V corresponds within the ac­curacy of a SSE to the minor peak at -770 mV (vs standard hydrogen elec­trode [SHE]) reported byTromans. 5 He concluded that this was the potential at which s-z ions incorporate into the lattice.

Potentials in the immersed liquid/ air interface are normally 5 to 10 mV more positive than at the bottom o f the tanks. The distribution o f potentials is linear with liquor depth. No abrupt change occurs at the interface between the wet/dry and constant immersed zones.

If a passivation attempt is aborted, the potentials move 1 O to 20 m V more

26 MATERIALS PERFORMANCE February 2002

negative than the "static" values. This change has remained for as long as 2 months.

If the anodic protection system is turned off, the potentials decay in the negative direction in two discrete steps. The initial step is rapid, decay­ing to a value of- -75 mV vs SSE. This potential remains for days and, in one case, lasted 3 weeks. The next step oc­curs over hours and is linear with time to a potential o f - -125 m V vs SSE. These decay times correspond to the effective film resistance vs potential. Unlike initial passivation, the time re­quired to reestablish the set potential is relatively short (30 min), suggesting that the passive film was still intact. Perhaps reincorporating s-z back into the film is irreversible or, more likely, very time-dependent.

When anodically protected tanks are cleaned of lime mud and iron sul­fide (FeS) deposits, the underlying sur­face exhibits a bronze-colored hue. This thin film is thought to be either pyrite (FeS2) or NaFeS2 • 2H20.

Summary Most of the problems encountered

with anodic protection of liquor ves­sels so far have resulted from an incom­plete understanding of the electro­chemistry of CS in liquors and the assumption that the current distribu­tion was secondary and not primary. This caused the creation o f active and passive coexisting areas. Earlier control philosophy did not account for the possibility of active and passive coex­isting sites. Present systems recognize and avoid these problems.

References 1. R. Yeske, E. Hill, S. Mindt, "Anodie Proteetion

of a White Liquor Clarifier," Pulp and Paper Industry

Corrosion Problems 5 , 219 ( 1986).

2. O. Crowe, O. Tromans, "The High Temperature

Polarization Behaviour of Carbon Steel in Alkaline Sul­

phide Solution, " CORROSION/87, pape r no. 203

(Houston , TX: NACE, 1987).

3. K. Mishra, K. Osseo-Asare, "Aspeets of the Jnterfaeial Eleetroehemistry of Semieonduetor

Pyrite (FeS2

) ," J. Eleetroehem. Soe. 135 , 10 (1988), p.

2 ,502.

4. L. Peterman, R. Yeske, "Thiosulfate Effeets on Corrosion in Kraft White Liquor," CORROS!ON/87, paper no. 201 (Houston , TX: NACE, 1987).

5. O. Tromans, "Anodie Polarization Behaviour of Mild Steel in Hot Alka line Sulfide Solutions ," ). Eleetroehm. Soe. 127, 6 (1980), p. 1,253.

6. P. Morris, "Anodie Proteetion of Fe-Cr-Ni-Mo, Alloys in Coneentrated Sulfurie Aeid," CORROSION/ 76 (Houston , TX: NACE, 1976).

7. C. Thompson, "Studies to Determine the Cause o f Corrosion of the Anodically Proteeted White Liquor Storage Tank ," Unpublished Report (1988).

8. O. Wensley, "Corrosion Studies in Kraft Liquor Tankage," Pulp & Pape r Industry Corrosion Problems 5, 15 (1986).

9. W. Mueller, "Reduetion and Oxidation Reae­tions in Kraft Liquor Reeovery: Sourees, Effeets and Prevention," Pulp & Paper Magazine of Canada 74 , 4

(1973), p. 69. 10. N. Tonsi-Eldaker, "Corrosivity of Kraft Li­

quors," CORROSION/80, paper no. 258 (Houston, TX: NACE, 1980).

11. R. Yeske, "Measuremems of Corrosion Rates of Carbon Steel Exposed to Alkaline Sulfide Environ­ments," CORROSION/84 , paper no. 245 (Houston, TX:

ACE, 1984). 12. O. Singbeil , D. Tromans, "Stress Corrosion

Craeking o f Mild Steel in Alkaline Sulfide Solutions," Pulp & Paper Industry Corrosion Problems 3 (1980).

13. W. Mueller, "Corrosion Studies of Carbon Steel in Alkaline Pulping Liquors by the Potential Time and Polarization Curve Methods," TAPPI Magazine 40, 3 (1957).

14. W. Mueller, "Meehanism and Prevention of Corrosion o f Steels Exposed to Kraft Liquors," Pulp & Paper Industry Corrosion Problems I , 109 (1974).

15. R. Yeske, "Anodie Proteetion for Controlling Corrosion of Reeaustieizing Components, " FKBG Projeet Report 2926-1 (1984).

16. B. Haegland , B. Roald, "The Corrosion of Steel in White Liquor," Norsk Skogindustri 9 , lO (1955) .

17. W. Mueller, "Corrosion Rates of Carbon Steel

Tubes in Kraft Liquors With and Without Anodie or Cathodie Proteetion," Pulp & Pape r Industry Corro­sion Problems 2, 140 (1977).

18. N. Moscardo-Levelut , V. Pliehon , "Sulfur Chemistry in Equimolar NaOH -H

20 Melt, " ). Eleetro­

ehem. Soe. 131 , 7 (1984).

This article is based on a paper pre­sented by the author at a joint TAPPI/ NACE International conference held Sep­tember 1999 in Anaheim, California.

JAMES IAN MUNRO is Vice President of Engineering at Corrosion Service Co., Ltd., 369 Rimrock Road, Downsview, Ontario M3J 3G2, Canada. He has more than 27 years of experience in electrochemical protection system design, including anodic and cathodic protection <CPl, and remote monitoring systems. He has a B.S. degree in electrical engineering from the University of Toronto. He is a NACE-certified Corrosion Specialist and CP Specialist, a professional engineer, anda 27-year member of NACE. He currently serves on the MP Editorial Advisory Board. /VI'

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Researchers developing new water pipeline models

Predicting the rates and locations of water and wastewater pipeline failures, as well as moni­toring the general condi­tion of these pipelines, can be a difficult task. Help may be on the way, however, as a team of Photo courtesy of lhe Massachusetts Water Resources Authority.

scientists with the Com­monwealth Scientific Industrial Re- become an indispensable planning in­search Organisation (CSIRO) Built Envi- strument as the pipeline infrastructure ronment Sector (Melbourne, Australia) continues to age and the cost of mainte­work to simplify the process while nancecontinuestoincrease. ThePARMS im.proving the quality of information it model is geared to consider operational produces. methodologies, customers' needs and

Currently, water authorities calculate preferences, and renewal and repair pipeline failure rates by extrapolating expenditures.

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into the future the number of failures In a related effort, the researchers are Here's the recorded from previous years. Using sta­tistical models that cannot predict where fractures might occur, the cur­rent method proves very limited in cases where little failure data exist. CSIRO re­searchers are developing new analytical solutions- including physical-based models to complement statistical mod­els- that they contend will become in­dispensable to authorities in the care and rehabilitation of water pipelines and in pipeline asset planning and manage­ment activities.

"Water authorities are looking be­yond the traditional reactive strategies to proactive maintenance of pipeline infrastructure to deliver the primary goals of a water service, which are reli­able delivery of clean safe water, supply above a minimum pressure levei, and reliable sewer service," says Stewart Burn, Project Manager with CSIRO Building, Construction & Engineering.

Bum and his colleagues have devel­oped the Pipeline Asset and Risk Man­agement System (P ARMS), a planning model designed to allow water authori­ties to implement long-term pipeline replacement and maintenance strategies and to assess long-term costs. The de­velopers contend that their system will

investigating a number of techniques to help authorities obtain a better grasp of pipe wall condition. "Whilst the stresses on pipelines- external from soil load­ing, internai from water pressure, and axial from thermal movement - can be quantified or estimated from operational and engineering data, the properties of the pipe wall are not as readily avail­able," says Burn, adding that pipe walls need to be measured by direct or indi­rect means.

"It needs to be kept in perspective that conditioning monitoring is an emerging technology that does not al­ways provide totally reliable informa­tion," explains Bum. "As water authori­ties become proactive, shifting their emphasis to assessing the effect of dif­ferent operational strategies on long­term costs and balancing the risk/con­sequence of failure , the role of condition monitoring will become more important. "

Contact Stewart Burn, CSIRO Sus­tainable Materiais Engineering­phone: +61 3 9252 6000, e-mail: Stewart.Burn@csiro.au. IVP

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c?.'í'?-OSIOtv '\;<'-" 0 .z. u o <{ :JJ

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Continued from MP Forum, page 13. The following NCN items relate to cathodic & anodic protection.

Please be advised that the items are not peer-reviewed, and opinions and suggestions or recommendations are entirely those o f the inquirers and re­spondents. NACE does not guarantee the accuracy ofthe technical solutions discussed. MP welcomes additional re­sponses to these items. They may be edited for clarity.

J! Low-voltage DC earth-return system

QAn electrical utility is proposing to construct a monopolar 480-MW high­

voltage-direct -<:urrent (HVDC) electricity transmission system composed of a single high-voltage cable with low-voltage earth­retum electrodes in the ocean.

Although we are ali aware of the pos­sible detrirnental effects to nearby metallic structures, I would like to hear of other people's experiences with existing projects. Some o f the existing DC systems that have subsea links include the follow­ing locations:

• Gotland 2 & 3- Sweden • Baltic Cable- Sweden, Germany • Sacoi- Italy, France • Vancouver- Vancouver Island,

Canada • Hokkaido- Honshu, Japan • Cook Strait- New Zealand

AI've worked with a -700-mile (1, 100-km) bipolar system that was

supposed to have no impact on surface structures such as pipelines. It did affect one pipeline when a test running monopolar caused shifts up to 150 mV

ously damages structures along and crossing the route. Monopolar operation should be used only in emergency situa­tions that do not tolerate switchgear fails and total shutdown.

A Monopolar lines with sea returns .fihave worked successfully in areas with no submerged or underground structures that can be affected. There are some in Denmark, I believe. I have tested bipolar lines running in monopolar mode and found effects of concern generally limited to a distance o f -60 miles (96 km) from the earth electrode.

Automatic potential control rectifiers have successfully overcome potential swings caused by monopolar operation. The problem was more severe on lines par­alleling the HVDC line than on crossing lines. In these tests, monopolar operation also disrupted cathodic protection on gas distribution systems- again, within -60 miles of the earth electrodes. I agree that monopolar construction should not be used in areas where there are underground or submerged structures.

& Mg anode testing

gis there any standard procedure for testing the high-potential Mg alloy

a e for seawater applications? W e use Mg anodes for initial polarization of struc­tures in seawater for short periods.

A Mg anodes normally are not used for .l"l.seawater application on a perma­nent basis. I have, however, designed and used Mg strip anodes along the legs of an offshore platform to protect it from corrosion at the installation and commis­sioning stage. The platform was designed with an impressed current cathodic pro­tection (ICCP) system. The time lag be­tween installing the platform and com­missioning ICCP (provision of power supply, etc.) was -4 months. The Mg rib­bon anodes were thus used to provide stopgap protection and rapidly polarize the new platform.

over - 300 km. We also learned that it A Mg could still be a questionable created serious noise on the telephone .l"l.selection for initial polarization. The systems in a city 60 to 70 km away. "on" potential for this metal coupled to

In my opinion, monopolar design is carbon steel (CS) in seawater is > - 1.6 troublesome because it inevitably seri- V. Most coating systems are not recom-

men disb< than (HIC

> - 1 \1

pinb tials peril note is p1 be f but resu

an a stru< foq thec The seav almc Also ture

c bea tion "on''

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F hav( rang Ofte sea\1 the1 app1 nom 1/2 1

A bare Witb large tuw

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mended for resistance to cathodic disbondment at potentials more negative than - 1.2 V. Hydrogen-induced cracking (HIC) can occur if "off' potentials are > - 1.415 v.

With a "brand new" and relatively pinhole-free coating system, "off' poten­tials could be quite high for a 4-month period and even longer if the ribbons are not disconnected when the ICCP system is put in service. Exposed steel would be protected from corrosion with Mg, but other damage mechanisms could result.

I have found that Mg works well as an anode in seawater for coated steel structures. Indeed, it was used widely for piers and offshore structures before the development ofln and Hg/ Al alloys. The reason it is seldom used anymore in seawater is cost. The Al alloys provide almost twice the A/h per dollar spent. Also, the Al needed to protect a struc­ture weighs significantly less than Mg.

Overprotection with Mg should not be a serious concem. The - 1.2-V limita­tion is a "polarized" potential and not an "on" potential. It is unlikely that a struc­ture could be polarized to the open­circuit potential with Mg anodes, espe­cially when only a small amount of Mg is used until ICCP can be installed.

Finally, not al1 seawater is the same. I have measured seawater resistivities ranging from 18 Q-cm to >200 Q-cm. Often, at the mouth of a big river, the seawater is affected by freshwater from the river severa! miles out to sea. If you approach 100-Q-cm seawater, the eco­nomics can change because of the extra 1/2 V you gain in driving potential.

A Most offshore structures- and I am .1"\.sure this is such a structure- are bare. There is no coating to be damaged. With moving seawater and the typically large amount of steel involved, the struc­ture likely will not reach HIC potentials. Using Mg to polarize a marine structure is not uncommon, but this does not an­swer the original question. Y ou could ask the anode manufacturer to run tests simi­lar to those run for Al anodes to deter­mine potential and consumption rate in seawater. However, the results for these tests are more pertinent to the long-

term effectiveness of anodes being the sole CP system. For your short-term pur­poses, I doubt the testing would be as cri ti cal.

A Normal seawater's specific resistiv­.1'\..ity is -30 Q-cm, depending upon a number of factors such as pollutants, temperature, and pH. Under such con­ditions, the measured "off' or "polarized" potential is considered the "on" poten­tial. Experiments have shown this.

The technical data sheets on Mg an­odes are taken from normal seawater ex­periments similar to those of the Al an­odes. I have performed experiments to check the given data by manufacturers.

In our area, Al-Zn-Hg anodes are for­bidden because of concems over Hg pollution.

A In ltaly, detailed studies have been rtconducted regarding this matter. The researchers discovered that protect­ing a structure at start-up with Mg an­odes permits fast growth of a thick and compact calcium carbonate (CaC0

3) de­

posit around the legs- so compact, in fact, that the anode consumption dur­ing the life o f the platform increases dra­matically.

Bi-alloy anodes, slender Al anodes with an ou ter layer o f Mg, are produced for this purpose. Consequently, it is not illogical to protect marine structures dur­ing commissioning using Mg alloys.

• Does grounding to copper water services cause corrosion?

Qis there any research available on the grounding of electrical services

to copper water services, primarily in residential homes?

W e have experienced accelerated corrosion on some of our copper water services. Most new services are cathodi­cally protected with a 5.4-kg zinc anode, but many services are not protected.

Might grounding electrical services to the copper water pipe ( on the street si de

Continued on page 30

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111 111

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1NKER& ASOR CORROSION MITIGATION

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INKER& ASOR CORROSION MITIGATION

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30 MATERIALS PERFORMANCE February 2002

NCN-Contínued from page 29

o f the meter) contribute to the corrosion problem? Is this shortening the life span of the anode? If there is proof that elec­trical current from this source signifi­cantly contributes to corrosion, then I will explore the necessary steps to stop this procedure and start a program to re­move existing connections and install grounding rods or plates to connect the ground wire.

A U.S. National Electrical Code Section Í\..250.52 B states that electrodes used for a metal underground gas piping sys­tem cannot be grounded. However, Sec­tion 250.52 (Grounding Electrodes) Sec­tion A, Paragraph 1, allows the use of metal underground water pipe in direct contact with the earth for ~3 .0 m for a ground electrode. The National Electri­cal Safety Code also allows the use of metal water pipe as a ground electrode (Section 94A1).

There is a difference between a grounding electrode and grounding of equipment. Gas lines cannot be used as a ground electrode, but piping within the house must be bonded to the ground­ing electrode. Metal water lines can be used as grounding electrodes, and metal water lines in a house with plastic water service must be bonded to the grmmd­ing electrode.

A Bonding copper to copper likelywill fluot accelerate corrosion. My guess is the problem is more an altemating cur­rent (AC) issue. For some reason, the copper electrical ground is introducing AC to the water line. The AC is then dis­charging to ground from the water lines, resulting in water line corrosion.

A The American Waterworks Associa­.1"\.tion (A WW A) (Washington, D .C.) re­search foundation (AwwaRF) (Denver, Colorado) conducted a research project on this subject severa! years ago. I sug­gest looking at AwwaRF's Web site: www.awwarf.com or AWWA's Web site: www.awwa.org.

A The water line and gas lines within .l"l.the house must be connected to the h ouse ground. Even if these lines are not

directly bonded to the house ground, they are often connected to it through equipment such as heaters, furnaces, ranges, etc.

Years ago, copper water piping was protected by both the main to which it was connected and the steel gas service line, which was not isolated at the meter. However, with the present use of plas­tic for services and mains, the copper is no longer protected and thus can cor­rode when placed in the ground. I do not believe that AC is causing the corro­sion; rather, the corrosion may simply be soil corrosion-possibly accelerated by the temperature difference between the hot and cold water lines.

Under normal conditions, there should be little (if any) AC going to ground at the house ground. Altemating ground currents can be measured using an AC clamp on an amp meter. For a resi­dence, the only current flowing on the neutra! line (and possibly going to ground) will result from the difference between the currents on the two hot legs. Any significant AC on the ground should be reported to the local utility for repair.

A I assume there is not electrical isola­.1"\.tion between the main and the house. Isolation accelerates corrosion from any AC or direct current (DC) dis­charging from the house. It also pro­duces galvanic corrosion of the water main, assuming it is metallic. There is a lot of information on this topic. I sug­gest Paper 519 from CORROSION/91 ("Corrosion Control Distribution ofWa­ter Mains in Stray Current Areas," Boris M. Danilyak and Emer C. Flounders) and Paper 588 from CORROSION/94 ("Cor­rosion Effects ofElectrical Grounding on Water Pipe," A.M. Horton) as a starting point .

A DC may be the problem. There are rta number of appliances with DC motors-hair dryers, personal computer fan drives, and the like. I have heard re­ports that some models of hair dryers, for instance, can produce DC feedback to the ground. Ali said, grounding to a water pipe is poor practice because of safety risks.

A Pol} se ar ham has: ing COll'

ffiÍS(

h ou: per • rosic met< not thinJ ance spot DIPI poli c tiesr elect syste tant (

sives

thel rath nize it is have

the 21). foll

A In a brochure entitled "Direct Tap­~ing o f Ductile Iron Pipe Encased in Polyethylene," the Ductile Iron Pipe Re­search Association (DIPRA) (Birming­ham, Alabama) notes that the A WW A has a policy statement opposing ground­ing electrical systems to pipe systems conveying water to a customer's pre­mises. It also indicates that grounding household electrical services to the cop­per service pipe can result in stray cor­rosion of the copper service and/or a metallic water main. lt, however, does not explain the mechanics (which I think could very well involve DC appli­ances or other effects, as another re­spondent has indicated). I believe DIPRA further endorses the AWWA policy and recommends that water utili­ties require that metallic service lines be electrically insulated from the piping system, which I think might be impor­tant (at least in some electrolyte/corro­sive soil environments).

CP for ship hulls

gFor a 300-m long ship, is it neces­sary or preferable to have two ca­

t ic protection (CP) systems- one at the fore and one at the aft of the hull­rather than only one? Is there any recog­nized independent source claiming that it is necessary (not just preferable) to have two systems for long ships?

A See "Cathodic Protection Require­.fim.ents of Ship Hull Materiais" (Feb­ruary 1995 MP, p. 29) and "Protecting the Admirai" (December 1995 MP, p . 21). Both articles help to expand on the following:

• The current requirement for a bronze propeller is much greater than for the rest of the (painted or coated steel) ship. • For a very large ship, two separate rectifiers (AC power is run through­out the ship anyway) may be more economical than pulling wire and run­ning conduit from one rectifier to provide current to anodes located at both ends ofthe ship. • For an underway (moving) ship, it is difficult to get an anode physically

remote from the protected structure. Current from any particular hull­mounted anode tends to attenuate along the length of the hull, requir­ing more anodes. • Those involved with floating struc­tures develop an aversion to putting holes through the hull. 1t is very diffi­cult to mount anodes on the hull of a ship and keep them from being dam­aged. Mmmting through the hull re­quires making a hole-some consider this cure worse than the problem. If mounted from the deck, the anodes tend to be swept away. The WWII solution was generally to use many small through-hull impressed current anodes, located in frequently traveled or inhabited spaces. Otherwise, use galvanic anodes mounted on the out­side of the hull with no through-the­hull holes. tvP

- Next Month in MP-

Focus on CORROS/ON/2002

CORROSION/2002

Advance Program

Cost of Corrosion in the

Power lndustry

Ranking of Anodized/Oxygen

Stabilízed Aluminum for

Various Environments

Carbon Dioxide and Hydrogen

Sulfide Corrosion of Steels

How to Manage Corrosion

Contrai: A Guide for

lndustry Managers

Anodic Protection of White and

Green Kraft Liquor Tankage,

Part 11: Design and Operation

of Anodic Protection

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INKER& ASOR CORROSION MITIGATION

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Febr uary 2002 MATERIALS PERFORMANCE 31

oPus & Linings

PHORGOTTEN PHENOMENA

Inorganic Zincs-How Do They Cure?

L.D. "Lou" VINCENT, Corrpro Companies, Inc.

32 MATERIALS PERFORMANCE February 2002

norganic zinc (IOZ) coatings, predominantly primers, have been universally recognized as very corrosion-resistant coatings for use on carbon steel (CS) structures as well as other met­ais and concrete. This well-

deserved recognition dates back as far as the early 1940s in Australia. As these products have become more accepted into the mainstream coating industry, however, there have been far too many problems with coating systems applied over these primers. These problems predominantly take the form ofblisters and delaminations of topcoats applied over the IOZ primers. Cohesive split­ting o f the IOZ primers can also occur after applying organic topcoats.

IOZ primer formulations have not changed appreciably since their intro­duction. Some additives now offer im­proved application and cure character­istics, but the basic curing mechanisms have not changed. What has changed is the industry's concentration on faster production techniques when IOZ/organic topcoat systems are ap­plied on CS. This article examines the basic curing mechanisms and presents severa! practical suggestions for en­hancing the cure of three different types o f IOZ primers.

Although the numerous modifica­tions of IOZ primers feature additives such as polyvinyl butyral and ceramic

pigments, the basic curing mecha- mo·

nisms ali involve the reaction of poly- cu r

silicic acid to form a silicate matrix that filrr

binds the zinc particles to each other by•

and to the CS substrate. What is differ-ent is the manner in which this silicate tio r

matrix forms. According to Munger, 1 sili<

the stages of cure of the three most Thi

common types of IOZ primers are as sili<

follows. pm

Postcured IDZ alo filrr

The first step involves the evapora- anc tion of water from the applied film, fon leaving the zinc pigments and an alka- ide: line film. Under ambient conditions of fllrr ~50°F (10°C), this evaporation stage takes only a few minutes and leaves a Sol fairly hard coating on the substrate. It is not, however, cured to an insoluble state. on

As soon as the applied film is dry wh

(turns from a glossy look to a matte fin- sili<

ish), an acid postcuring solution must ces

be applied to thoroughly wet out the rati

surface with the curing compound. plie

There is an immediate reaction be- rate

tween the acid-curing solution and both the zinc particles and the steel atic

substrate. The reaction creates a silica fro1

matrix insoluble to water and unaf- Th<

fected by exposure to weather. wit

The coating gradually cures over ani

many months as carbon dioxide (C02) filrr

and water react on the coating. This but

produces a reaction between the sílica wa1

matrix and the zinc ions, which are formed by carbonic acid (H2C0

3).

ove

It is important to remember that aci<

these curing reactions leave soluble f rol

salts on the surface, which must be re- me·

moved with clean fresh water prior to cre

applying any topcoat. c ar ing

Waterborne Self-Cured IDZ Pr; The first stage of self-curing in-volves evaporating water from the ap-plied film so that the alkali silicate and top

the polysilicate acid can react fairly tim

quickly with the zinc. During this dry- soh

ing phase , air temperature (min . cur

50°F), relative humidity (RH), and air eve