Laboratory study on sacrificial anodes for reinforced concrete

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Laboratory study on sacrificial anodes for reinforced concrete

Brousseau, R. J.; Baldock, B.

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La bora t ory st udy on sa c rific ia l a node s for re inforc e d c onc re t e

N R C C - 4 3 6 0 6

B r o u s s e a u , R . J . ; B a l d o c k , B .

M a r c h 1 9 9 8

A version of this document is published in / Une version de ce document se trouve dans:

Corrosion, 54, (3), March, pp. 241-245, March 01, 1998

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Ｏｶｾ｣｣Ｍ

.y0630

CORROSiON ENGINEERING SECTION

Laboratory Study

of Sacrificial Anodes for Reinforced Concrete

R, Brousseau and B. Baldock"

ABSTRACT

Theoutput of sacrificial current by two metallized coatings

wasmonitored under well-defined conditions. Experimen.ts

wer-e conducted at two relative humidities (RH). The peifor·

rnance ojazinc-alumin.umaHoy (78:22)was equalorsllght!y

supenorto that oj pure zln.c·coatedan.odes.However, there wasa.steady decline in.currentdensityOlltputwith ttme.

which was attributedto oxidationproducts formingatthe

sacrificial anode/concrete trtteifa.ce.

KEY WORDS: aluminum.anodes. coatfngs,current denSity, relative humidity. reiriforced concrete, zinc

INTRODUCTION

Steel reinforcement corrosion is a costly problem that has led to premature deterioration of a large number of concrete bridges and parking stxuctures, Cathodic protection {CPt is be1n.g used lncreasingly to reduce corrosion of steel reinforcement. Galvanic CP has stimulated considerable interest in the civil engineer-ing community because of its simplicity, The

protective current flow in a galvanic system is in-ducedbythe electromotive force (EMF) that arises between the steel and the sacrificial anode metal. No speclal wiring. rectifiers. eleCtrical short clearing between the anode and steel, rectifiers, and mainte-nance or corrosion monitoring are reqUired.

Thermally sprayed zinc is being used increas-ingly as an anode for CP of steel-reinforcedconcrete

Submitted Cor publicationｆｾ｢ｮＮｊ｡ｲｹ 1997: InTevtsed Conn. June

1991.

• National Research CouncU of Canada. Bldg. M·20. Montreal Road, Ottawa, Ontario. KIA ORB. Canada.

stnIctures. Zinc coatings have been implemented successfully as an impressed current anode in California. Oregon,I and Ontario. The Florida

DepartmentofTransportation is startingto use metallized zincasa sacrificial anodeon itsmaline structures.2•4Recently, a more reactive metallized coating made of an｡ｬｵｭｬｮｵｭＭｺＱｮ｣ｾｩｮ､ｩｵｭ alloy has been developed.IS-6Although this newalloyshows superior electrochemical performance, its stiffness and brittle mechanical properties make poor·quality wiresthat canbe difficulttometallize.

The scope of the present work was to examine how metallized zinc coatings perform as sacrificial anodes in well·ctefined environments (e.g., at50°C and 50%or90%relative humidity(RI-Ij). The

perfor-mance of a zinc-aluminum alloy (78:221 was investigated for comparison tothat of purezinc.

EXPERIMENTAL

Effectiveness of the galvanic protection was tested on reinforced concrete samples made With type

10normalPortlandcement. admiXedwith 1%

sodium chlOride{NaCl)bymass ofcementand made withawatertocement ratio of 0.5. Additional samples were madewith a 0.43%ｷ｡ｴ･ｲｾｴｯﾷ｣･ｭ･ｮｴ

ratio for some of the bond-strength tests of the met-alliZed coatings. Most samples were fabricatedwitha single level of two steel reinforcing bars {Figure ll. The different configurations represented concrete covers of 20 mm, 50 mm, and 125 mm (0.79 in.,

1.97in., and 4.92in.)between thesteel reinforce-ment and the arc-sprayed coatings. All samples were manufactured in triplicates for a total of 36 samples.

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CORROSION ENGINEERING SECTION

FIGURE 1.Schematic of the metalliz&d samples used to monitor rhEJ galvanic current flow.

Agraphite pseudo-reference electrode was pouredin all samples. This reference electrode was used tomeasure electrochemical potential shifts that occurred durtng the Ｔｾｨ depolarization surveys fol-lowingintenuptlon of thegalvanic current flow.

Thetopsurface ofeach concrete samplewas

grit·blastedWithno. 40 s1lica sand and air driedfora

fewdaysafter being curedfor 28 days at 100% RH.

The grit-blasting was sufficient to roughen the

sur-,face without exposing the aggregateinthe concrete.

Anydust or loose particles on the surface wereｲ･ｾ

moved by bloWing With compressed air after

ｧｲｩｴｾ｢ｬ｡ｳｴｴｮｧＮ The surface of the concrete wasｰｲ･ｾ

heated to 50"'C immediately prior to arc spraying to

ensure an

adequate bond was obtained between the metallized anode and the concrete substrate. Preheating the surface of the concrete was performed sinceitwas found to maximize the adhesion of arc-sprayed zinc on concreteinprevious studies.7_{The temperature of}

50"Cwas chosen because it would increase bond strength while remaining field practical. The samples then were arc-sprayed with zinc. or with a zlnc-aluminum alloy (78:22) using an automated applica-tlon system. All arc sprayings were performed with

620 kPa air pressure, 26 V. 300 A. and at a spray

distance of 15 em (5.90 In.), All thermal spraying was performed with wires 3 mm (0.118 in.) in diam-eter. Thicknessof the applied coatings was 0.4mm

(0.016 in.). Initialbond-strength measurements wereperfonned on a few samples using a pneumatic adhesion tester.

Oncemetallized, thereinforced concrete samples wereplacedinone of two environments: a 500/0 or 90%±50/0 RH at atemperature of 50"'C ± 2C_{C. The}

samples were connected to a data acquisition system

sothat automatedcurrentand depolarization measurementscould be performed. Currentｭ･｡ｾ

surements were obtainedbymeasuring thevoltage dropacross a ＲｾｮＺ precision shunt resistorthat was installed in series between the coating and the steel rebars. Surface area of the metallized anode on each sampleswas236cm2_(36.59_In.2

).

RESULTS

Initialbond-strength data of the metallized coat· ings are presented in Tables 1 and 2. Measurements were performed on concrete samples With two pre· heating temperatures andtwowater"to-cement ratios. lnitlal adhesion of the zinc-aluminum alloy

7

Rebar

o

Graphne re1erence cod

o

I £ I Metalllted surface (23,625mm') TABLE 1

Bond Strengths of Arc-Sprayed Zinc on Concrete

Concrete Water-ta-Cement RatIo 0.43 0,50

Preheating surface temperature Bond-strength averages (kPa) Standard deviations (kPa) Number of pull tests

50"C 1,313 212 31 12QOC 2,636 274 28

socc

1,515 400 32 120C_C 2,359 465 29 TABLE 2

Bond Strengths of Arc-Sprayed Zlnc·Aluminium (78:22) on Concrete Concrete Water-to·Cement Ratio 0.43 0.50

Preheatingsurface temperature Bond·strength averages (kPa) Standard deviations (kPa) Number of pUll tests

120°C 3,084 327 12 50C_C 1,120 286 15 120°C 2,640 494 13 242

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CORROSION ENGINEERING SECTION

FIGURE _2. _{Galvanic current density time profile for metallized}

samples stored Ina50% RHchamber.

Time (weeks)

FIGURE 3. GalvanIc current density time profile lor metaJJfzed samples stared Ina90% RH chamber.

'"

,

..

'"

..

,

1

_ Zino (avg. of all blocks) I -Zn:AI (ava. of all blocks)

60 80 100 Time (weeks)

..

2

"

"'E

12

1"

ｾ

c

• "

0

ij

•

::: ｾ 2 <.l 0

,

•

average depolarization recorded after4 h. 24 h, and 72 h of interntptionof thegalvaniC current generated bythetwometallized coatings.Thisdataindicated

4 h was insufficient to measureallthe cathodic

polarization provided tothe rebars by themetallized coatinganodes. In practice, thesurface area of the

reJ.nforcingsteel andof themetallizedconcrete surface has a ratio of ..1: 1. except in beams and columns. In this experiment. the surface area ratio of steelto anode was 1.4: L

EMFbetween the metallized coatingsandrebars was measured onallsamples before the wiring to the dataacquisition system was completed.AverageEMF values were highestfor thezinc·alumlnum alloy (78:22), as listed inTable 4.Afterthe metallized an-odesandsteel reInforcing bars were wired together electrically. theｯｰ･ｮｾ｣ｬｲ｣ｵｩｴ potentialvalueswere recordedbetweenthe anode and steel after 4 h of depolarization. Althoughthiswas not a tnle EMF, it suggested that theｺＱｮ｣ｾ｡ｬｵｭｩｮｵｭ (78:22) alloy could

produce a higher drivingvoltage.

In an earller study. it was found that wetting the surface of the metallized coatingswouldenhance the

galvaniccurrent output.2_{Sagues and Powers briefly}

applied a damppaper towel to the metallized coating

"m---;:::::;::=::::;:::::;::::::=:-i

ｾ 11 __ Zinc (avg.Ol aU blocks)

... __ Zn:Ar (avg. 01 aU blocks)

1"

f

o

III NACE Standard RP0290-90, ·StandardRecommendedPracticefor Cathodtc Protection of Relnforclng Sleelln Atmospherically

ｾｸｰｯｳ･､ Concrete Structures.· NACE Book of Standards. voL 1

{Houston. TIC: NACE, 1990J.

(78:22)wasgenerally equal. if not slightly superior.

to that of pure｡ｲ｣ｾｳｰｲ｡ｹ･､ zinc. No final

bond-strength measurements were taken since the

samples remain under investigation. However. no

apparent deterioration was observed.

The current density values of the tripl1cate samples was averaged and analyzed. Unfortunately, the magnitude of the observed current densities did

not correlate with the three different concrete covers

used to manufacture the samples. The significant

scatter in the data(± 50%) coupled with a limited

number of samples (trtplfcatesl madeitimpOSSible to observe the expected reductioningalvanic current

denSitywith thicker concrete covers. Therefore, the

current density values of all samples sprayedwith

the same metal coatings were averaged. Results for

the 50% and 90%samples are presented in Figures2

and 3, respectively, Initially, therewas an･ｸｰｯｮ･ｮｾ tial declinein the galvanic current densityat both RH, whichappearedtostabilizetoward the end of the

experimental program.Thesamples stored at90% RHgenerally had current densities twoto three times higherthan thoseat50% RH. The,current delivered

bythe zinc·alumlnum alloywas simHar tothat deliV-eredbythe pure zinc at both RH.

Depolarization shifts were measured on the samples to detennineifthe criteria developed by

NACElntemattonal would be met.rLl_This_cIiteria

stipulatesthat adequate CP is achieved ifthe

poten-tial of thesteel. after current interruption, shiftsbya

minimum of 100 mV within 4 h ofcurrent

interrup-tion. Longer depolarization periods canbe used for this criterja.

Thepotential values of thesteelreinforcement

and themetallized anode wererecorded versus a

graphite pseudo-reference electrodethat was embed-ded pemlanentlyineachconcrete sample. The averageddepolarization shifts that wererecorded

severaltimesdUringtheexperimental programare

presented in Figures4 and 5forthe two RH. Asnoted. the potential shiftofthesteel

rein-forcement shouldhaveexceeded 100 mV after the

currentwas interrupted for4 h. The ＱＰＰｾｭｖ criteria

was achievedafter 134weeksfor the zinc-aluminum

(78:22) alloysamples at90% RH, Forpurezinc, the

criteria wasmetonlyfor theinitial 80 weeks, al-though both the zinc and thezinc-aluminum(78:22)

alloy provided sirollar galvaniccurrentdensities. All

metallizedsamples maintained at a 50%RH pro-duced shifts in the40-mVto50-mVrange after 134 weeks of monitoring.

Depolarization tests usually were measured over

a 4-hperiod. However. occasionally longer､･ｰｯｬ｡ｲｾ ization periods were measured. Table 3Hatsthe

CORROSION-Vol. 54, No.3

243

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-CORROSION ENGINEERING SECTION

'"

,---,

Time (weeks)

FIGURE 5. Average depolarization shifts measured on samples stored in 90% RH.

increase being highest for theｺｪｮ｣ｾ｡Ｑｵｭｬｮｵｭ alloy (78:22), as shown in Figure 6.

Significantdrying ofthe concrete samplesｯ｣ｾ

curred. at50% RH. Since theEMFremained relativeh constant,a decrease in flow of galvaniccurrentwas' notsurpriSingas concrete drying increased. The system. however. was not passivated.Testshave shown that if the metallized samples were wetted with water, cUlTent flow would be increased.ThiS.

and the observation that higher galvaniC currents were recorded in the samples maintained at 90% RH implied that metallized coating anodes would be mosl active where concrete resistivity was lower or where there was weathering such as rain, snow. and seawa-ter splashing. Fortunately,this also occurs in most areas susceptible to greater reinforcing steel corro-sion. Galvanic CPwithmetallized coatings. therefore,

canbe descrtbed as self regulating.

Concrete drying alone could not explain all of the decreaseinthe flow ofgalvaniccurrent. This was obvious from the｣ｵｲｲ･ｮｴｾｶｳＭｴｩｭ･ curves recorded at

90% RH.where continuous concrete drying was ｵｮｾ

likely to explaln the slow but consistent decrease of

galvaniccurrent.Anotherposs1ble cause for such a decrease was the formation of zinc oXidation prod-ucts attheanode/concrete interface. These products hindered further electrolytic current flow through the pores of the concrete normally fLlledwithsolution,

Contrary to what was expected. the experimental data did not show that a lower cover thickness of t.he concrete resulted in a higher galvanlc current.

Alloy-ingzincwith22% AI increased the EMF that drove the flow of galVaniccurrentfrom the anode to the rebars. However, the increase of galvaniC current with this alloy was limited. and a more electronegaｾ

Uve coating should be developed for drier concrete or for structures With higher steel density, Another｡ｬｾ

ternative would be to develop a solution that could be sprayed on the metallized zinc to enhanceits ｰ･ｲｩｯｲｾ

mance as a sacrifiCial anode for reinforced concrete.

DiSCUSSION

'"

120

'0'

'"

"

'0 '0

..

1 __ Zn (avg. 01 aU blocks) . 1

- Zn:AJ f8Vg. of all blO¢l<sI

.0

"

20

,

,.,

_ 200

>

g

ｾ

,.,

<J) c 0

'"

ｾＮｾｾ "0

_"

0-,

_,

Time (weeks)

FIGURE 4. Average depolarization shifts measured on samples stof8d in 50% RH.

and observed a strong current Increase.!O After the paper towels were removed,ittook"'" 48 h for the

galvanic current generated by metallized zinc to de-crease back to base levels. To verify the ments of surface wettJng in activating metallized zinc coatings. the blocks usually maintained in a 50% RH were immersed periodically for 7 days in deionized water.

Aspredicted, wetting the samples increased the cur-rent output of the sacrificial coatings. W1th the

TABLE 3

(Extended Depolarization Oats Obtained

After41Weeks and 80 Weeks on the Samples Maintained at 50% RH

Sample

4·h 24.h

Depolarization (mV) Depolarization (mV)

41 WeekS 80 Weeks 41 Weeks 80 Weeks

72·h Depolarization (mV) 41 Weeks 80 Weeks

244

Zinc 20 mm Zn:A120 rnm Zinc 50 mm Zn:A150 mm Zinc 125 mm Zn:A1125 mm 60,3 41 66,3 68 93.3 78 50,0 43 75.3 72 84,0 77 54.0 30 72,7 46 79,3 64.7 46 77.0 73 83.7 71 44.3 27 62.7 44 69.3 76 53.0 50 69.7 72 73.3 73 CORROSION-MARCH 1998

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CORROSiON ENGINEERING SECTION

CONCLUSIONS

(AlMeasurements Iaken at 32 weeks and 79 weeks were taken after

only 4 h of depolarization.

TABLE 4

Open-Circuit Potential Difference

I

--- Zinc (avg, of all blocks) __ Zn:A1 (avg, 01 all blOCk:!;)

I

\

\.

ｾＬ

_l\..

,

_1'0..

"

ｾＭＮＮＮＮＮＮ［

....;::

...

ｾ

o o 500 1,000 1,500 2,000 2,500 3,000 ｾＬＵＰＰ Time (h)

FIGURE 6.Current density change during rapetitlve wetting of the samplesstored at 50% RH. 168 387 209 370 213

4n

303 384 353 451 352 397 50% 50% 90% 90% Open-Circuit Potential RH Difference (mV)lAl

Condition Initial 32 Weeks 79 Weeks Metals Coupled 2n-Fe Zn:AI (7B:22) -Fe Zn-Fe Zn:AI-Fe {7a:22)

.) Suffident galvanic CP to prevent corrosion of the reinforcing steel should be provided by the metallized anodes for at least a 2-y period in humid envirOn-ments or where there Is occasional surlace wetting of the metallized coatings.

<.

Potential depolarization shifts are higheriflonger depolarization periods are used. Compared to a metallized coating anode made of pure zinc, the zinc- aluminum alloy (78:22) had similar bond strengths, a higherEMF,andsimilar galvanic cur-rent output and cathodic polarization levels to the reinforcing steel.

ACKNOWLEDGMENTS

The authors acknowledge financial support of J. SpriestersbachｯｦｇｲｴｬＱｯｾｗ･ｲｫ･ and assistance in

providing the zinc-aJuminuffi wires. The authors also the assistance of B. Arsenault and S. Dallaire of the National Research Council of Canada with arc-spraying of the samples.

REFERENCES

1. B.S, Covino, S.J. Bullard, G.R. Holcomb, S.D. Cramer, G.E. Mean!. C,B. Cryer. 'Factors Affecting the Bonding of Arc, Sprayed Zinc toｃｯｮ｣ｲ･ｴ･Ｌｾ SSpe'95, Balancing Economlc!ii and Compliance for MaintaIning Protective Coatlnga (Dallae, TX: SSPC. 1995). p, 115·128.

2. A.A. Saguee. RoO. powers. 'U>w-C05l Sprayed Zinc Galvanic AnodeforControlof Corrosl<Jn ofReinforcing SteelIn Marine

Bridge Substructu:res,'FInalReport, Str'doteglcHighway Re-."Iearch Program.ContractNo. SHRP-68-ID024.

3. R.J.Kessler.RG.Powers. I.R. Usa,ｾｕｰ､｡ｴ･ on Sacrlflcial Anode Cathodic Protection on Steel-Rt:1nforced ConcreteStruc·

tures in Seawater: CORROSlONj95, paper no. 516 (Houston,

IX: NACE, 1995).

4, A.A, Sagues. R.O. powers,ｾｓｰｲ｡ｹ･､Ｎｚｉｉｬ｣ Sacrificial Anode for Reinforced Concrete In Marine SeIVice,' CORROSION/95, paper no. 515 (Houston. TX: NACE, 1995).

5. M.Funahashl.S.F.VaUy,W.T. Young,ｾｐ･ｲｦｯｲｭ｡ｮ｣･ ofNewly Developed Sprayed Anode CathodIc P'rotectlonｓｹｳｴ･ｭＮｾ COR· ROSIQN/97, paper no. 254 (llouston.1"X: NACE. 1997). 6. M. Funahashl. 5.1". Daily.ｾｎ･ｷ Sacrifictal Anode for Cathodic

Protection of Reinforced Concrete SlrL\dures,·· Amertcan Society ofelvl} Englneen Annual Conf.. Washington, DC, November 1996 (New York. NY: ASeE. 1996).

1. R. Brousseau. M. Amott, B. Baldock, MP 34. } (l994): p, 40.

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