Effect of Microstructure

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Effect of Microstructure on the StressCorrosion Cracking of X-80 Pipeline Steel

in Diluted Sodium Bicarbonate Solutions

J. G. Gonzalez-Rodriguez, * M. Casales, ** VM. Salinas-Bravo, *** J. L. Albarran, **** and L. Martinez’““’

ABSTRACT

TranSgmUlar stress corrosion cracking (TGSCC) can

develop in pipelines under normal ope rating condi-

t ions when a coating breaks down and groundwater

come s into contact wi th the outside surface. Since the

fi rst TGSC C observed in the pipelines ofTransCanada

Pipe Lines, Ltd. in 1985, many other cm mtr ies such

as Italy. Mexico, and Russia have exper ienced such

failures. Intergranular stres s corrosion crac king

(ICSCC) of pipel ines can also occur in highly concen-

trated bicarbonate-carbonate [HCO ;/COzl solution

with high pH I-9 to 11). This type of stress corrosion

cracking (SCCl occurs in a very restr icted range of

electrochemical potentials I-550 mV vs a saturated

calomel electrode [SCE] to -650 mV ,,J. Dominated by

film rupture and dissolution , this is so-called classica l

SCC.‘ .3 For instance. Might and Duquette, using a

high-purity carbon steel (0.019 wt%) in 170 g/L

amm onium carbonate I[NH,],C OJ solutions at 70°C

found that, in this case. SCC occurs by dissolution

processes (not necessar i ly stress related] and not by

hydrogen embri tt lement since the pH inside the

cracks w as 8.7, at which hydrogen evolution is ther-

modynam ical ly impossible.’ However, several in-

stances of TGSC C were detected recently at coating

disbandment areas of Canadian pipel ines.” Manyresearchers are focusing more attention on this new

SCC phenomenon because i t is associated with much

lower HCO ;/CO;- concentrations and pH values

(5.5 to 7.5) than those at which classical SCC occurs.

Many effor ts have been made to understand the

mechan ism of this SCC in this nonclassical envimn-

ment.6.” The studies have focused mainly on the

effect of electrochemical potential and solution com-

posi t ion. Howeve r. since di fferent manufacturers sup-

ply pipeline steels with di fferent thermoche mical heat

5840010-9312/021000131/$5.00+$0.50/0

f3 2002. NACE International CORROSION-IULY 2002



TABLE 1

Chemical Composit ion ofXt30 Pipel ine Steel (wt%)

c h ln MO P Nb s Si ” Al Ti Ni cr

treatmen ts, very l i t tle information is avai lable on this

matter . Lopez. et al ., carr ied out SCC tests on API” ’

X-80 steel in a NACE solution “sing modff ied wedge-

opening-loaded (MWOL ) specimens in a water-sprayed

heat treatment.” They explained that the relative S CC

susceptihl l l ty of the steel in this heat treatment was

attr ibutable to i ts microstructure, which consisted of

a” extremely f ine pear l ite in a ferr ite matr ix. Under

these condit ions. numerous hydrogen- induced

microcracks apparently formed at the crack t ip plastic

region, some of which were able to join the main crack

to promote crack gwvth. These cracks developed

preferential ly at inclusions and hard spots induced by

the heat treatment. such as fme carbides in grain

boundar ies and f ine pear l ite regions. It was suggested

that the improved SCC resistance was related to the

energy needed for microcrack propagation and br ldg-

ing through the ferr i tic matr ix. Howeve r, this condi-

t ion was sti l l considered more susceptible. as

indicated by the elongations to fai lure, than that for

the as-received condit ion.

TABLE 2

Heat Treafmenfs Given to X-80 Pipeline Sfeel

Heat Treatment

Water-quenched

Water-sprayed

Quenched and

tempered

Conditions

850°C. 30 min. quenched in static water

85O”C, 30 min. cooled with sprayed water

Water-quenched + 350°C fw 30 min.

air-cooled

grade chemicals. The fracture surfaces were then ex-

amined using scanning electron microscopy @EM ).

Potentiodynamic polar ization curves were measured

in 0.1, 0.05. 0.01. and 0.005 M NaHC O, at a sweeprate of 1.0 mV /s (slow scan1 using a ful ly au tomated

potentiostat controlled with a desktop computer.

Some addit ional polar ization cwve s were obtained in

0.01 M NaHCO, at 10 mV /s ( fast scan) to evaluate

the SCC susceptibi l i ty of the steel by f i lm rupture.

RESULTS

The objective of this work was to study the effect

of di fferent heat treatments and, therefore, micro-

structure on the SCC resistance of a pipeline steel in

di luted sodium bicarbonate INaHCO $ solutions using

the slow strain rate testing (SSR’IJ technique.

Microstructures

EXPERIMENTAL PROCEDURES

The mater ial used was a modern X-80 pipel ine

steel , wi th niobium and high manganese contents to

gain high strength, wi th cornposluon and heat treat-

men ts as speci f ied in Tables 1 and 2. respectively.

The water-sprayed heat treatment simulated a” im-

proper welding procedure. Cylindrical tensile speci-

mens with a 25.00.mm gauge length and 2.50-n,“ ,

gauge diameter were machined from an unused plpe-

line perpendicular to the roll ing direction. Before

testing, the specimens were abraded longi tudinal ly

with 600.grade emery paper, degreased. and masked

(except for the gauge length). Specimens were sub-

jected to conventional , monotonic SSRT in air as an

inert environment and in 0.05 M and 0.01 M NaHC O,

soluUon at a strain rate of 1.36 x lob /s at room

temperature. Mos t of the tests were performed at the

open-circuit potential , but a few were performed un-

der potentiostatic control . Al l potentials are vs SCE.

The test solutions were prepared from analyt ical-

The microstructures produced by the di fferent

heat treatments, including those of specimens in the

as-received condit ion, are shown in Figure 1. The mi-

crostructure of the as-received sample shows bands

of ferrite alternated with bands of pearlite [Figure

l[al), and a fine dispersion of precipitate s was also

found. The quenched and tempered sample show s

partially recrystall ized grains with the presenc e of

i”cipient acicular ferrite and isolated pea rlite grains,

as well as a fine dispersionof precipitates Figurel Ibl1. Figure l (c) presents the microstructure of the

samples cooled in water spray, and i t shows a” in-

complete transfmma tion of pear l ite in a recrystall ized

ferr ite matr ix wi th fewer precipi tates. The samples

quenche d in wate r (Figure l[dll present a typical

structure of martensi te with massive segregation at

the martensi t ic lath boundar ies: a f ine dispersion of

precipi tates is also apparent.

Effect ofNaHC0, Concentration

on the Polarization Curves

Figure 2 il lustrates the polarization curve s of the

steel in the as-received condit ion in NaHC O, at di f-

ferent concentrations. The shape of the polar ization

c”rves changed with HCO ; concentration in terms of

the presence of a passive region. The mos t d i luted

solution showed no evidence of a passive region.

probably attr ibutable to a pH effect, whereas al l the

CORROSION-VOI. 58, NO. 7 58 5



(b)

Cd)

other solutions did. A white-gray ft im formed on the

surface in the 0.01-M solution, whereas a yel low-

brown surface f i lm formed in the 0.01-M and 0.05-M

solutions. The potential at which the passive region

(Ed started increased with the solution concen tra-

t ion, but the passive current densi ty I i& increased

as the solution concentration was lowered. Al l the

curves had, however, a ccmnnon featurethe appear-

ance of a second anodic peak just before establ ishing

the passive region. This second anodic peak appeared

at potentials of - -700 mV for the 0.01-M solution,- -500 mV for the 0.05-M solution, and - -650 mV

for the 0.01-M solution. The lower solution concen-

trations resulted in a narrower passive region.

The effect of the heat treatment ( including the

as-received condition) on the polarization curves in

0.05 M and 0.01 M NaHC O, is shown in Figures 3

and 4. respectively. In al l cases, the presence of the

second anodic peak is evident al though, in the 0.01-M

solution at which it appeared, it is virtually the sam e

(- -700 mV l. and in all case s the steel presented a

566

passive region. In both solutions, the lowest i , ,

value was exhibi ted by the steel in the as-received

condit ion fol lowed by the quenched steel . In the

0.05-M solution, the water-spraye d pipeline steel had

the highest $- , value. but in 0.01 M this and the

tempered steel had vir tual ly the same a.. value. In

al l the heat treatments, al l the solutions produced

pitt ing corrosion, except the most concentrated one.

The pi t densi ty and size increased with decreasing

solution concentration despi te the microstructure.

The tendency of the steel to fai l by SCC causedby f%m rupture was evaluated on the basis that there

was at least one order of magnitude di fference in i , ,

between the slow and fast sweep~‘~~‘~ and that the

iP.. in the fast sweep was >l mA/cn? The results

of the $=a values in the active-pa ssive region for the

di fferent heat treatments using slow and fast sweeps

are given in Table 3 . Using these criteria. Table 3

shows that the heat treatments of the quenched and

the quenched and tempered samples made the steel

more susceptible to SCC , whereas the water-sprayed

CORROSION-JULY 2002



and the as-received samples were the least suscep-

tible.

SSRT

The stress-elong ation curve s in air for the differ-

ent heat treatments are given in Figure 5. I t can be

seen that the highest ducti l i ty was exhibi ted by the

water-sprayed steel ( -4 mm ) and the lowest by the

quenched and tempered steel , al though the quenched

steel had a total elongation very close to i t (1.8 mm

and 2.0 mm , respectively) . The steel in the as-

received condition had an elongation-to-failure of

2.8 mm . As expected, the quenched steel had the

highest ul t imate tensi le strength (UTS) value

(-1,400 MPa). fol lowed by the quenched and tem-

pered steel ( -1.200 MPa), whereas the sprayed steel

and the steel in the as-received condition had the

lowest UTS values (-650 MPa to 700 MPa).

When tested in 0.05 M and 0.01 M NaHC O, solu-

tion, the pipeline steel in the as-received condition

was barely affected since the total elongation-to-

fai lure in air and in the two solutions was between

2.5 mm and 3.2 mm , a loss of 22% ducti l i ty. whereas

the LIT3 lay between 620 MPa and 680 MPa [Figure

6). Howeve r, the si tuation was di fferent for the

quenched steel (Figure 7). In this case, the minimum

elongation-to-fai lure obtained in 0.01 M NaHC O,

solution Isiml lar to the as-received condit ion) was

1.25 mm . compared with that in air (2.2 mm ), a lossof 45% ducti l i ty. The UTS values in the three envi-

ronments ranged be tween 1.200 MPa and 1,400 MPa,

the lowest being recorded in 0.01 M NaH CO,.

For the quenched and tempered pipeline steel .

again. the lowe st elongation-to-failure was obtained

in 0.01 M NaHC O, solution (1.1 mm ) compared with

the value obtained in air (1.9 mm , a loss of 42% duc-

t i l ity) , but the UTS values were very close in the three

environments [Figure 8). Final ly. for the water-

sprayed steel . once again, the lowest elongation-

CORROSION-VOI. 8,NO.7

1E-3 0.01 0.1

Current Density (mAfcm2)

FIGURE 3. Effect of heat treatment on the polarization cuwe for

X-80 steel in 0.05 M NaHCO,.

sr 5ouw

0

-500

-I,0001 ““’0.01 0.1 1

Current Density (mAJcm2)

FIGURE 4. Effect of heat treatment on the polarization curye forX-80 steel in 0.01 M NaHCO,.

TABLE 3

Effect o f Heat Treatment on the tw,. Values Obtained

Using Fast and Slow Sweeps in 0.01 M NaHCO,

Heat ivy (mAtem’) iv (mAtem’)

TW*tme”t at stow Scan at Fast Scan

As-received 0.008 0.04

Waterquenched 0.05 2.00Water-sprayed 0.09 0.4

Quenched and tempered 0.09 1.05

to- failure value was obtained in 0.01 M NaHC O,

(-3.00 mm ) whereas the highest value was obtained

in air Is l ightly longer than 4.00 m m, a loss of 25%

ducti l i ty) (Figure 9). In this case. the lowest UTS

value was obtained in 0.01 M NaHC O, also (-600 MP a)

whereas the highest value was obtained in air

I-700 MPal. These resul ts are summ arized inTable 4.

To see the effect of the electrochemical potential

on the SC C behavior of pipeline stee ls in diluted

NaHC O, solutions, some tests were performed on the



1.4wJ ,_. .... . . .._. Air data 1

Water-quenched

mos t susceptible mater ial ( i .e.. the water-quenched

steel) . and the resul ts are shown in Figure IO.

Marke d reductions in the total elongation-to-failure

were found for samples at anodic potentials wi th

respect to E, whereas. at cathodic potentials, the

ducti l i ty increased.

Apical fracture surfaces in 0.01 M NaHC O, are

presented in Figure 11. The quenched steel showed

transgranular cracking whereas the water-sprayed

steel exhibi ted quasicleavage features. Cross sections

of the cracks in the quenched and sprayed steel areshown in Figure 12. which reveals evidence of corro-

sion products inside the cracks.

DISCUSSION

In highly concentrated bicarbonate-carbonate

solutions with high pH l-9 to 11). the SCC of plpe-

lines is intergranular and is domina ted by tl lm rup-

ture and anodic dissolution (AD). ‘The tendency for

SCC to occur at any given specimen potential by f i lm

1,400 -

Pzoo.

~i,OOO-

As-received

0 ’

0 1 2 3 4

Elongat ion (mm)

0

0 1 2 3 4

Elongat ion (mm)

rupture was evaluated on the basis that there was at

least one order of magn itude difference in i-. be-

tween the slow and fast s~eeps,‘~~” and that the &

in the fast sweep was >l mA/cn? . The di fference of

1 mA/cm = between the slow and fast sweeps (Table 3)

means that an intermediate passivation rate was

ideal for a dissolution mecha nism. A high passivation

rate caused the crack t ip to be protected continual ly

whi le a low rate resul ted in excessive dissolution at

the crac k tip. leading to blunting. Also, data in fig-

ure IO-where anodic potentials, which enhance thesteel dissolution, also increased the SCC tendency-

suggest that the cracking mechan ism of pipel ines In

0.01 M NaHC O, solutions is AD.

It has been shown that, even under condit ions

that favor AD. the electrochemical condit ions inside

the pi ts are such that hydrogen discharge is possible.

Parkins . working with X-65 pipeline steel in NS-4

solutions containing diluted carbon dioxide (COJ

l=O.O05 M ) at pH 6.5, considered that the resul ts

suggested dissolution and hydrogen ingress into the



steel .” Simi lar resul ts were found by Gu, et al ., for

X-80.type steel in the same solution.” However, in

the NS-4 solutions, the X-80 steel did not show any

passive region” whereas, in the present study, i t did

Figure 4). regardless of the microstructure. demon-

strating the importance of the f i lm rupture. Another

di fference is that. in the NS-4 solution. the SCC sus-ceptibil ity increased as the applied poten tial was

more cathodic, unl ike the present case. in which

cathodic potentials decreased the SCC susceptibi l i ty.

These resul ts confhm those of Parki”s.15 that see of

carbon steels in carbonate solutions occurred by the

dissolution process of metal at the crack t ips. The

presence of corrosion products inside the cracks sup-

ports this idea. The dissolution rates at the crack

tips were high enough to cause the crack wal ls to

passivate. providing a large cathode inside the crack,

coupled to a smal l anode at the crack t ip where f i lm

rupture took place. The role of the applied potential

was to maintain the al loy surface in the potential

range where rapid rupture of the pass ive fi lm led to a

mzdm um dissolution rate at the crack t ip, whi le at

the same time fudng the potential of the crack wal ls

in the passive regime.

The quenched steel was mos t susceptible to SCC

because the martensi te was a highly stressed micro-

structure as a resul t of excess carbon trapped inter-

~ti tial1y.l~ Hence, i t is assumed that grain boundruy

carbon segregation coupled with severe internal

stresses renders grain boundar ies susceptible to

stress corrosion crack propagation. So i t is expected

that, under these conditions. the crack propagation

is mainly intergranular. In this work, the authors

were not able to detect intergranular cracking. as evi-

dented by Figures 11 and 12. Howeve r, as mentioned

in the Introduction, in the nonclassical SCC found in

lower concentrations of carbonate/bicarbonate solu-

t ions. the SCC propagation was transgranular, as

opposed to the IGSCC found in highly concentrated

carbonate/bicarbonate solutions. In the quenched

condit ion, i t is expected that carbon segregation at

grain and interlath boundaries comb ined with an in-

ternally stressed marte nsite will give rise to an in-

creasingly susceptible steel condit ion.

Quenching followed by temp ering, or normalizing

and temper ing, general lyjmproves the SCC resis-

tance of steels.” When crack growth occurs mainly

by hydrogen embri tt lement (HE). i t is apparent thatthe distribution and type of hydrogen traps in the

form o f inclusions and second phases play a key role

on the overal l steel susceptibi l i ty. Among the micro-

structural features that can act as noxious traps of

hydrogen are i”clusions and segregation bands con-

taining lower-tempe rature produ cts (bainite and

martensi te) . Therefore, the inherent SCC resistance

is strongly affected by metal lurgical var iables respon-

sible for the presence of inclusions, carbides, and

segregation bands.”

CORROSION-W. 58, NO. 7

1,400t

TABLE 4

Effect of Heat Treatment on the Elongation-to-Failure

in Air and in NaHCO. Solut ionsElongation-to-Failure (mm)

Heat 0.01 M 0.05 M

Treatment Air NaHCO, NaHCO,

I0

-200 -?@I 0 100 200 300

Overpotential (mV)

CONCLUSIONS

9 SSRT resul ts showed that X-80 steel is highly sus-

ceptible to SCC in 0.01 M NaHC O, solutions at room

temperature. depending upon the heat treatment.

and the SCC increased as the solution concentration

was lowered.

0 Quenched steel and steel that was quenched fol-

lowed by temper ing were the mos t susceptible to SCC

58 9



CORROSION SCIENCE SECTION

(a) (b)

FIGURE 11. Micrographs ofX-80 specimens fractured in 0.01 M NaHC O, shown in: (a) Ihe quenched sfeel and (b) the

water-sprayed steel .

in 0.01 M NaHC O,, whereas the as-received and wa-

ter-sprayed steel samples were the least susceptible.

0 Tbe mechan ism of SCC in X-80 pipel ine steel in

0.01 M NaHC O, solutions was dominated by f i lm

rupture and AD.

REFERENCES

(b)

quenched steel and (b, the waler-sprayed steel

CORROSION-JULY 2002

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Effect of Microstructure