Hideyoshi Tsubono

Effect of Galvanizing onHydrogen Embrittlement of

Prestressing Wire

Yukio Yamaoka,Dr. Eng.Senior ResearcherR&D DepartmentShinko Wire Company, Ltd.Amagasaki, Japan

r

=^

G alvanizing is sometimes applied toprestressing wire and strand for

corrosion protection. 1,2 However, zincon the galvanized wire reacts with thealkali ingredients of portland cementand evolves hydrogen while the con-crete is still fresh and wet as expressedby Eq.(1):

Ca(OH) 2 + Zn + 2H 20 =Ca{Zn(OH) 4} + H2(1)

There is a widespread concern thatthe hydrogen evolved in this chemicalreaction may enter into the steel latticeand embrittle the wire, but this has not

been confirmed with clear evidence.There is little data concerning the re-lationship between the volume of ab-sorbed hydrogen and the brittleness ofthe steel.3,4,5

FIP (Federation Internationale de laPrecontrainte) recommends the use ofammonium thiocyanate (NH 4 SCN) todetermine the susceptibility of pre-stressing wires to stress corrosion. It iswell known, however, that the failure ofsteel wire in this solution is due to hy-drogen embrittlement rather than normalstress corrosions''

This paper describes a basic investi-gation concerning changes in the physi-

146

cal properties of a bare wire and gal-vanized prestressing wires which wereimmersed in an aqueous solution ofammonium thiocyanate. The inhibitingmechanism of the zinc coating in hydro-gen embrittlement of steel wire is alsoinvestigated by using the hydrogenpenetration test.

TEST SPECIMENSThe chemical composition and me-

chanical properties of the specimens aregiven in Tables 1 and 2, respectively. Inorder to determine the susceptibility tohydrogen embrittlement of a drawn gal-vanized wire which is presumed to havecracks in the coating layer, a 5.0 mm di-ameter galvanized wire was cold drawnby 26 percent reduction of area.

Fig. 1 shows the microstructures ofthe specimens. The coating thickness isapproximately 40 micrometers. Mi-crocracks are visible in the zinc alloylayer of the drawn galvanized wire.

TESTING METHOD

Mechanical PropertiesThe specimens were immersed in a 35

percent aqueous solution of ammoniumthiocyanate at an ambient temperaturefor a maximum of 65 hours. After rinsingin water, the mechanical propertieswere determined by testing within 5minutes.

Measurement ofAbsorbed Hydrogen

The volume of hydrogen absorbed inthe wire while immersed in 35 percentaqueous solution of ammonium thiocya-nate for a varied period of time was de-termined by keeping the specimen for 3days at 60°C in a standard test apparatusspecified in JIS Z3113. This is shown inFig. 2. The absorbed hydrogen volumewas expressed in cubic centimeters (cc)per 100 grams of steel.

SynopsisThis paper describes comparative

tests for hydrogen susceptibility onbare wire, galvanized wire and re-drawn galvanized wire used for pre-stressed concrete.

Specimens were immersed inNH 4 SCN solution, and then the rela-tive susceptibility for hydrogen embrit-tlement was evaluated by changingthe mechanical properties. Mi-crostructures are presented as a fur-ther aid in evaluating embrittlement.Hydrogen absorption was measuredand hydrogen migration was studiedwith an optical microscope. Crystallattices were applied as basis oftheory of zinc's inhibiting nature tohydrogen embrittlement.

Observation ofHydrogen Penetration

The penetration of hydrogen intosteel can be observed visually by placinga steel specimen in an aqueous solutionof 1 percent acetic acid +20 percent hy-drochloric acid + saturated hydrogensulfide. The hydrogen penetrates intoone side of the specimen and leaves theother side. Thus, the effect of galvaniz-ing and the relation between the ab-sorbed hydrogen volume and the degreeof brittleness may be clarified.

Fig. 3 shows schematically an ap-paratus for direct observation of hydro-gen migration through metals. The ap-paratus is placed on the observationstage of an optical microscope. Theabove mentioned solution is poured onSide A of each specimen, which mea-sures 15 mm long, 4 mm wide and 2 mmthick. Side B of the specimen is coatedwith a thin layer of oil. If hydrogen en-ters from Side A and is discharged out ofSide B, then hydrogen bubbles will betrapped under the oil layer and may be ob-

PCI JOURNAUJuly-August 1988 147

Table 1. Chemical composition of specimens (weight, percent).

Element C Si Mn P S Al

5 mm diameter bare wire 0.81 0.28 0.76 0.015 0.016 0.040

5 mm diameter galvanizedand 0.77 0.30 0.78 0.015 0.014 0.035

26 percent drawn galvanized wire

Table 2. Mechanical properties of specimens.

PropertiesUTS*

kN/mm2Reduction ofarea, percent

Torsion (turns)GL = 8d, 15 rpm

5 mm diameter bare wire 1,870 50.0 35

5 mm diameter galvanized wire 1,700 44.0 28

26 percent drawn galvanized wire 1,800 45.0 24

*Ultimate tensile strength.Note: 1 kN/mm t = 144 ksi.

served as black circular patterns due tothe change of reflection of the light.

The bare wire specimen wasmachined to the above dimensions. Thegalvanized wire specimen was cut to thesame dimensions and then one side ofthe specimen was hot galvanized. Forcomparison, a 99.9 percent pure zincspecimen was similarly observed. Thesolution was replaced every 30 minutes.

TEST RESULTS

Volume of Absorbed HydrogenThe volume of absorbed hydrogen as

determined in the experiment is givenin Fig. 4. A galvanized wire and a 26percent drawn galvanized wire absorbsless than 1 cc of hydrogen per 100 gramsof steel, whereas a bare wire absorbs 4cc after 50 hours of immersion in 35 per-cent NH 4 SCN aqueous solution. How-ever, the 26 percent drawn galvanizedwire appeared to increase steeply hy-drogen absorption tendency after 50hours in the solution.

Mechanical Properties

Fig. 5 shows the relation between thevolume of absorbed hydrogen and themechanical properties of the specimenwire. The tensile strength increases asthe absorbed hydrogen volume in-creases, but ductility represented byelongation and torsion decreases as thesteel absorbs more hydrogen. It is clearthat the bare wire is more sensitive tohydrogen embrittlement due to the factthat hydrogen absorption of only 0.2 ccper 100 grams of steel decreases the tor-sion value radically and torsion fractureis brittle.

On the other hand, galvanized wiredid not exhibit significant changes inelongation, reduction of area and tor-sion, even when it absorbed 1.5 cc ofhydrogen per 100 grams of steel. Inother words, galvanized wire was lesssusceptible to hydrogen embrittlement.The 26 percent drawn galvanized wireexhibited only a slight brittleness as itabsorbed hydrogen.

These tendencies can be seen more

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H2 Bubble

Glycerine

o'y Specimen

(a) Bare wire 25 '- Heater Water Bath

(b) Galvanized wire 25 AJ-

(c) 26 percent cold drawn 10 -galvanized wire

Fig. 1. Microstructure of specimen wires.

Fig. 2. Apparatus for measuring the volumeof absorbed hydrogen (JIS Z 3113).

clearly in Fig. 6, where the hydrogenembrittlement index is expressed by:

I= A B x 100 (percent) (2)A

where A is the characteristic value ofuntreated wire and B is the characteris-tic value of treated wire.

Observation of the GalvanizedLayer of Embrittled Wire andTensile Fracture

Fig. 7 is a microstructure of a 5 mmdiameter galvanized wire which hasbeen immersed in a 35 percentNH 4 SCN solution for 48 hours. Thiswire has absorbed 0.8 cc of hydrogenper 100 grams of steel, but there is still28 micrometers of zinc layer left on thewire surface. This indicates that the cor-rosion reaction did not take place di-rectly between the base metal and thesolution. It is believed that 0.8 cc of hy-drogen is absorbed uniformly in the zinclayer because there is only little brittle-ness.

Fig. 8 shows the microstructure of the26 percent drawn galvanized wire after56 hours in the solution. This wire has

PCI JOURNAL/July-August 1988 149

15X4

Plastic plate

1°/° HAc + 20°/°HCl+H2Sa Saturated Sd.

A Adhensive agent

Specimen t=2mm

Stage B Cedar oil film

aMicroscope lens

Fig. 3. Apparatus for direct observation of hydrogenmigration.

0

5mn Bare Wire

0

260/° Cold DrawnGalva. Wire A

5mmGalva. Wire

o

0 10 20 30 40 50 60 70Dipping Time in 35°/° NH4SCN Aq. Sol. (hr)

Fig. 4. Volume of hydrogen absorbed in wires whileimmersed in 35 percent NH,SCN solution.

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166

5mm BareWire•

,•----•-----• 5mm Galva. WireA ^ ■

G5 26°/° Cold Drawn Galva. Wire

o lio Normal Fracture•■ : Brittle Fracture

•

t•

0 1.0 2.0 3.0 4.0Volume of Absorbed Hydrogen

Fig. 5. Relation between volume of absorbed hydrogen andmechanical properties of wires. Note: 1 kN/mm 2 = 144 ksi.

15 micrometers of zinc layer, but it ap-pears that the cracks created during colddrawing were widened by the corrosion.The corrosion pitting indicated by anarrow in Fig. 8 is interpreted as due tohydrogen entering into the base metal asa result of its direct reaction with thesolution. Therefore, the 26 percentdrawn galvanized wire became brittle asthe absorbed hydrogen volume in-creased, despite the remaining zinclayer.

Fig. 9 shows fracture surfaces of un-treated bare wire (a), treated bare wire

(b), untreated galvanized wire (c) andtreated galvanized wire (d) as observedby a scanning electron microscope.These fractographies confirm that barewire suffers brittleness and galvanizedwire is not affected by immersion in a 35percent NH 4 SCN solution.

Penetration of HydrogenThrough Steel

Fig. 10 shows sequential photographsof hydrogen bubbles as observed onSide B of Fig. 3. Hydrogen migrated

PCI JOURNAL/July-August 1988 151

60 •

40a 5mm

•–` Bare

20 5

Wire— 26°/o Cold Drawn Galva.E o/,:A----4 • Wirew C 0 •ti 5mm Galva. Wire0d u

2.0C -20T TI ^

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0 1.0 2.0 ao L..0Volume of Absorbed Hydrogen ( C9 0 Fe)

Fig. 6. Comparison of hydrogen embrittlement indexof wires.

through a 2 mm thick bare steel speci-men and formed bubbles in 10 minutes.The bubbles increase in number and di-ameter as time passed.

Fig. 11 shows similar photographs ofhydrogen migration through a gal-vanized steel specimen. The first bub-ble was observed after 115 minutes onSide B and the number of bubbles wassmaller than that of bare steel. Fig. 12shows the galvanizing layer remainingon the wire surface after 130 minutes oftest. The fact that hydrogen bubbles

appeared on Side B after 115 minuteswas believed to have been caused byhydrogen that evolved as a result of adirect reaction of the base metal afterthe zinc was lost, and hydrogen pene-trated into the base metal and was re-leased from Side B.

In order to assess this assumption, thepenetration of hydrogen was observedusing a 2 mm thick pure zinc specimenexposed to a solution of 1 percent aceticacid + 20 percent HCl + saturated H2S.As shown in Fig. 13, no hydrogen bub-

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Fig. 7. Galvanized layer of 5 25 AJmm diameter wire after im-mersing in 35 percentNH 4SCN solution for 48 hours.

Fig. 8. Galvanized layer of 26 10 AJ-percent cold drawn galvanizedwire after immersing in 35 per-cent NH 4SCN solution.

(a) Fracture surface of a bare wire without (b) Fracture surface of a bare wire dippeddipping in any solution. in 35% NH 4SCN solution for 50 hours.

(c) Fracture surface of a galvanized wirewithout dipping in any solution.

(d) Fracture surface of a galvanized wiredipped in 35% NH 4SCN solution for 65 hrs.

Fig. 9. Scanning electron microscope observation of tensile fracture surface 55, ,k.}-of wires.

PCI JOURNAL/July-August 1988 153

C]

(a) 10 minutes (b) 15 minutes

Fig. 10. Hydrogen bubbles that migrated through a bare steel wire specimen 100 ^.and appeared on Side B.

bles were observed on Side B after 100minutes.

The volume of absorbed hydrogenwas determined using the specimenswhich were subjected to hydrogen pen-etration tests. Although no migratedhydrogen bubbles were observed dur-ing the hydrogen penetration test, it isapparent from Fig. 14 that the pure zincspecimen absorbed hydrogen.

DISCUSSION OF RESULTSThe above experiments indicate that a

bare wire is embrittled by the hydrogengenerated as a result of corrosion reac-tion of the wire with a solution becausesuch hydrogen penetrates into the steel.Galvanized wire is not embrittled by thehydrogen evolved in a reaction of thezinc on galvanized wire with solution

154

0

Fig. 12. Remaining zinc on 25-ifSide A of the galvanized wireafter 130 minutes of hydrogen

115 minutes penetration testing.

11

0

124 minutes

Fig. 13. Observation of hydro- X00 /Jgen migration on Side B of apure zinc specimen at 100minutes.

because the hydrogen is absorbed in the• zinc and does not reach the base metal

as long as there is a zinc layer.The question may be asked why hy-

drogen can stay in a stable state in zinc.130 minutes Fig. 15 is a schematic presentation of

Fig. 11. Hydrogen bubbles 100 ^- crystal structures of iron, titanium and

that migrated through a gal- zinc. Ti and Zn have hexagonal lattice

vanized wire specimen and structures, in contrast with Fe which has

appeared on Side B. a cubic lattice structure. There are largevolumes of interstitial space betweenthe crystal lattices of Ti and Zn because

PCI JOURNAL/July-August 1988 155

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1 0/0 HAc * 20°/0HCI.HaS Saturated Sol

a 5mm Bare Wire (reacted for 25m1 •

0 oToS

lure Zinc(reacted for 30min.)

I.0

cQ8

0.6 0a

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0,2

3,̂^,L0 >;'

40 50 60 70 80 90Glycerine Bath Temp (°C)

Fig. 14. Volume of hydrogen absorbed in the specimenssubjected to the hydrogen penetration test.

eQ

°< aiI' H cD ^r

2.8664 A 2.9503A 2.6648A

Fe Ti Zn

Fig. 15. Schematic presentation of crystal structure of iron,titanium and zinc.

the C-axes for Ti and Zn are 1.6 and 1.7times greater than those for Fe, respec-tively. The properties of titanium and itsalloys are closely studied as hydrogenabsorbing metals because titanium has alarge volume of interstitial space in itscrystal structure. Therefore, it cansteadily absorb a substantial quantity ofhydrogen.9•'o

Because of the crystal structure ofzinc, there is a possibility that hydrogenmakes a stable solid solution with zinc.The hydrogen evolved in the reaction of

zinc in galvanized wire with a solutionis absorbed in the zinc layer and isstabilized in the interstitial spaces be-tween crystal lattices. Hydrogen cannotreach the steel lattice through the zinclayer; thus, hydrogen embrittlement ofgalvanized wire is prevented. It is be-lieved that hydrogen evolved in thereaction of zinc with portland cementdoes not cause hydrogen embrittlementof a galvanized wire because it is ab-sorbed in the zinc layer but does notpenetrate into the steel.

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If, however, the zinc layer is cracked,there is a chance for the base metal(steel) to react with the solution andproduce hydrogen which can easilypenetrate through the steel. Therefore,the 26 percent cold drawn galvanizedwire, despite the zinc layer, has a ten-dency to be susceptible to hydrogenembrittlement.

In addition, it will be necessary to in-vestigate carefully the possibility ofhydrogen penetration for galvanizedprestressing wire under normallystressed conditions.

As mentioned above, although zincgalvanizing is an adequate prevention ofhydrogen embrittlement, it has the ad-verse effect that a reaction with zinc andalkali solution of cement formsblowholes in the concrete. Therefore, itmust be emphasized that the most suita-ble selection should be made based onzinc galvanizing characteristics met inpractice.

CONCLUSIONIn order to clarify the hydrogen em-

brittlement of galvanized wire, me-chanical tests and hydrogen absorptiontests were made on a 5 mm diameterbare wire, a 5 mm diameter galvanizedwire, and a 26 percent drawn wire. All ofthe wires were dipped in a 35 percentNH4SCN solution for various periods oftime. Direct observation of hydrogenbubbles penetrating through metals wasalso performed. Based on the tests, thefollowing conclusions can be drawn:

1. By dipping in a 35 percentNH 4 SCN solution, a 5 mm diameterbare wire loses its ductility but gal-vanized wire maintains its ductility. A

26 percent cold drawn galvanized wiresuffers only slight loss of ductility in thesame solution.

2. In the hydrogen penetration tests, itwas demonstrated that hydrogen couldeasily penetrate into the steel, as shownby the fact that hydrogen migratedthrough a 2 mm thick steel wire in 10minutes. On the other hand, with a gal-vanized wire, hydrogen bubbles couldnot be observed for 115 minutes be-cause hydrogen did not enter into thesteel lattice until the steel directlyreacted with the solution after the zinccoating had been lost. This suggests thatgalvanized steel wire helps inhibit hy-drogen embrittlement of prestressingsteel.

3. Even if a wire is galvanized, coldworking causes cracks in the zinc layerand leads to the direct reaction of thebase metal with a corrosive solution.Thus, hydrogen can enter into the steellattice, causing hydrogen embrittle-ment. A 26 percent cold drawn gal-vanized wire suffered slight hydrogenembrittlement when dipped in a 35 per-cent NH 4SCN solution.

4. The reason the zinc coating inhibitshydrogen embrittlement of galvanizedwire is that zinc has a hexagonal latticestructure like titanium, which is knownas a hydrogen absorbing metal. Zinc hasa large volume of interstitial space andcan steadily absorb a large quantity ofhydrogen, thus inhibiting hydrogenmigration through the steel.

5. Part of the hydrogen generated as aresult of the reaction of zinc on gal-vanized wire with portland cement isabsorbed by zinc and does not penetrateinto the steel. Thus, hydrogen embrit-tlement is prevented.

PCI JOURNALJJuIy-August 1988 157

REFERENCES

1. Everrett, L. H., and Treadaway, K. W. J.,"The Use of Galvanized Steel Rein-forcement in Buildings," Proceedings ofthe 8th International Association forBridge & Structural Engineering," Riode Janairo, 1964.

2. Cornet, I., "Corrosion of PrestressedConcrete Tank," Material Protection, V.3, No. 1, 1964, pp. 90-100.

3. Strecker, E., Ryder, D., and Davies, T. J.,"Evolution of Hydrogen from Hydroge-nated Specimens of a Commercial 0.8%CPatented and Cold Worked Steel," Met-als Technology, May 1977, pp. 236-237.

4. Ishikawa, T., Cornet, I., and Bressler, B.,"Mechanism of Steel Corrosion in Con-crete Structure," Material Protection, V.7, No. 3, 1968, pp. 45-47.

5. Klodt, D. T., "Study of Corrosion of Pre-stressing Steel — Effect of Stress,Metallurgical Structure and Environ-

anent," Material Protection, V. 6, 1967, p.12.

6. Bond, M. D., "Result of Experiments ona New Stress Corrosion Testing Ap-paratus for Wire," Corrosion-NACE, V.30, No. 8, August 1974, pp. 267-273.

7. Tanaka, Y., Yamaoka, Y., and Kurauchi,M., "Effects of Tensile Strength on theStress Corrosion Behavior of SteelWires," Proceedings of FIP Third Sym-posium in Madrid, September 1981.

8. Metals Handbook, 8th Edition, V. 1,1961, pp. 1169, 1210, 1225. Published byASM.

9. Kitada, M., "Metal Hydrides and its Ap-plication," Bulletin of the Japan Insti-tute of Metals, V. 17, No. 1, 1978, pp.345-349.

10. Sakai, Y., Nakamura, K., "Metal Hy-drides," Bulletin of the Japan Instituteof Metals, V. 19, No. 7, 1980, pp. 494-502.

Note: Discussion of this paper is invited. Please submityour comments to PCI Headquarters by April 1, 1989.

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