Stress corrosion cracking of carbon steel in ethanol-gasoline blends

Materials and Corrosion 2013, 64, No. 9999 DOI: 10.1002/maco.201206899 1

Stress corrosion cracking of carbon steelin ethanol–gasoline blends

J. Torkkeli*, T. Saukkonen and H. Hanninen

Stress corrosion cracking of carbon steel in aerated ethanol–gasoline blend was

studied using notched slow strain rate testing. Characterization of the fracture

surface was made using SEM and SEM–EDS. Intergranular stress corrosion

cracking (SCC) was produced in ethanol–gasoline blend with 15.5wt% ethanol

that was produced by evaporation of light C4 and C5 fractions from the ethanol–

gasoline blend with 10.4wt% of ethanol. Chloride concentration of 2mg/L was

found to cause transition from intergranular SCC to fully transgranular SCC in

ethanol-gasoline blend with 85wt% of ethanol. Transgranular SCC was found to

initiate mainly at the pearlite phase and intergranular SCC initiated equally on

the pearlite and ferrite phases. Chloride caused localized crystallographic

pitting on the transgranular SCC fracture surfaces near the lamellar cementite

left on the steel surface due to selective dissolution of ferrite from pearlite.

1 Introduction

Recently many countries have started to promote the use of

biofuels or other renewable fuels to reduce greenhouse gas

emissions and to diversify fuel sources. Due to this reason the

ethanol content in ethanol–gasoline blends has increased lately.

The material used for handling gasoline is mainly carbon steel,

which is susceptible to stress corrosion cracking (SCC) in

ethanol–gasoline blends with sufficient amount of ethanol. There

have already been some failures in the industry due to this

phenomenon mainly in storage tanks or piping handling fuel-

grade ethanol (FGE) [1]. Due to this reason it is important to find

out how much ethanol can be added to gasoline until there is a

risk for SCC in the existing carbon steel piping and equipment.

Recently there is only one publication where SCC in ethanol–

gasoline blends has been considered [2].

Vast majority of the information on SCC of carbon steels in

ethanol available is from laboratory studies made using commer-

cial or simulated FGE as the environment. Most common method

to study SCC of carbon steel in FGE has been slow strain rate

testing (SSRT). Fracture mode of the SCC in the majority of the

laboratory studies has been transgranular, while in the industrial

cases the fracture mode has been mainly intergranular [1]. The

reason why mainly transgranular SCC has been found in the

laboratory studies may be because small amount of chlorides has

J. Torkkeli

Neste Jacobs Oy, P.O.Box 310, FI-06101 Porvoo (Finland)

E-mail: [email protected]

T. Saukkonen, H. Hanninen

Aalto University, P.O.Box 14200, FI-00076 Aalto (Finland)

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been found to cause transition in the fracture mode from

intergranular to cleavage-like transgranular SCC [3]. The SSRT

method also promotes transgranular SCC [4]. In many studies

simulated FGE with some added chlorides has been used or

chlorides have leaked from the reference electrode used.

American Petroleum Institute was the first to publish an

extensive study on the subject [1]. It was shown that SCC of

carbon steel can occur in FGE meeting the specification ASTM D

4806. Oxygen, chloride, water, and galvanic contact between new

carbon steel and old rusted carbon steel were shown to have the

strongest influence and methanol a minor influence on the SCC

susceptibility [1]. SCC of carbon steel occurs most likely when the

corrosion potential is above 25mVand below 600mV versus SCE

[3, 5].

The more exact role of the environmental parameters on the

ethanol SCC (eSCC) was later confirmed by several studies [2, 3,

6]. It was found that addition of 4.5wt% or higher water to ethanol

prevents SCC of carbon steel [6], although significant pitting

attack was observed in this environment [6, 7]. High pHe was also

found to inhibit the SCC of carbon steel in ethanol [6].

Dissolved oxygen from ambient aeration has been proven to

be the most significant impurity contributing to the SCC

susceptibility. Oxygen concentration in ethanol can be as high as

80 ppm which is a much higher value than the oxygen

concentration in air-saturated water [8]. Without oxygen SCC

of carbon steel in ethanol can bemitigated [1, 2, 3, 9], although de-

aerated conditions can lead to a similar amount of metal

dissolution as observed in aerated ethanol [2, 6, 10]. Oxygen can

be removed from ethanol by a chemical agent, nitrogen purging

or steel wool [2, 11].

Beavers et al. [2] proposed a qualitative model for SCC

of carbon steel in ethanol. This model is based on anodic

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 Torkkeli, Saukkonen and Hanninen Materials and Corrosion 2013, 64, No. 9999

Table 1. Chemical composition of the FGE used to make the ethanol-

gasoline blends

Component Method FGE Unit

Ethanol NM40 95.10 vol%

Methanol NM40 0.02 vol%

ETBE NM40 3.40 vol%

MTBE NM40 0.11 vol%

Phosphorus ASTM D3231 <0.2 mg/L

Copper NM122 <0.1 mg/kg

Sulfur EN15486 <5 mg/kg

Sulfate EN15492 <1 mg/L

Water EN15489 0.184 wt%

Chloride UOP779 <1 mg/L

Acetic acid ASTM D1613 0.0025 wt%

Table 2. The ethanol-gasoline blends used

Component E85 E30 E25 E20 E15 E10 Unit

Ethanola) 82.2 32.3 27.0 20.9 17.9 10.4 wt%

Waterb) 0.190 0.091 0.110 0.064 0.074 0.044 wt%

Conductivityc) 0.74 0.17 0.10 0.02 0.01 <0.01 mS/cm

Measured using methods,a)NM40,b)EN15489, andc)EN15938.

Table 3. Chemical composition (wt%) of the tested carbon steel

ASTM A106 Grade B

Fe C Si Mn P S Cr Mo

98.140 0.197 0.211 0.985 0.048 0.013 0.063 0.027

ASTM A106 Grade B

Ni Al Co Cu Nb Ti V W

0.092 0.029 0.012 0.152 0.003 0.002 0.010 0.013

polarization measurements with microelectrodes in ethanol

purged with nitrogen–oxygen mixtures. The measurements

showed that oxygen increases the passivity of the carbon steel

in ethanol additionally increasing its corrosion potential. Anodic

dissolution rate of the carbon steel increased, when the oxygen

concentration was decreased. Other observations were that an

addition of chloride increased the anodic dissolution, while an

addition of gasoline decreased the dissolution rate. According to

the proposedmodel, the enhanced anodic metal dissolution at the

de-aerated crack tip can lead to SCC of carbon steel in ethanol [2].

According to the proposed model [2] outside the crack and

along the crack walls occurs the cathodic reduction of oxygen and

passivation of steel due to the dissolved oxygen. At the crack tip

fresh metal is exposed by plastic deformation. Oxidation of the

exposed steel consumes the dissolved oxygen rapidly and creates a

fully deoxygenated environment near the crack tip. The coupling

of the passivated steel in oxygenated environment outside the

crack to the exposed steel surface in the de-aerated environment

at the crack tip leads to crack tip dissolution and subsequent crack

advance. In this situation, there should be a zone of active steel

surface near the crack tip and passivated oxide covered steel

surface on the crack walls and outside the crack [2]. Results from a

recent study by Lou et al. [12] indicated that the initiation of the

SCC cracks is associated with plastic deformation in the steel,

which leads to a surface film breakdown and to the propagation of

the cracks controlled by the competition between active anodic

dissolution and repassivation at the crack tip as proposed by

Beavers et al. [2]. Results from a recent study showed that crack

initiation may also favor pearlite phase of the steel due to strain

accumulation or localized corrosion [13].

We studied SCC of carbon steel in aerated ethanol–gasoline

blends using SSRT without any electorochemical measurements.

The role of chloride leaking from the commonly used reference

electrode on the SCC mechanism was also studied. Commonly

available FGE and gasoline was used to make the ethanol–gasoline

blends for the SSR tests, which were used to study the SCC

susceptibility of carbon steel in the ethanol–gasoline blends with

different amounts of ethanol. All samples were characterized

using SEM–EDS. Average crack growth rate was measured, when

SCC was found and the results were used to compare the SCC

susceptibility between the N-SSR test specimens. The purpose was

to find out how much ethanol can be added to gasoline until there

is a risk for SCC as well as to see if the commonly used reference

electrode can affect the SCC mechanism.

2 Experimental procedures

2.1 Environmental conditions

All tests were made in ethanol–gasoline blends made from FGE

conformable to specification EN 15376 and gasoline conformable

to specification SFS-EN 228. Chemical composition of the FGE is

shown in Table 1. Ethanol and water concentration as well as

conductivity of the ethanol–gasoline blends were measured and

results are shown in Table 2.

All tests were performed in an environmental cell. Body of

the cell was made from glass. Bottom and top of the cell were


made from Teflon. The ethanol–gasoline blends were circulated

between the cell and a beaker, where the ethanol–gasoline

solution was actively purged with dry air and stirred slowly.

Composition of the air was 20 vol% of oxygen and 80 vol% of

nitrogen. Air purging was started 30min before the tests and

continued throughout the test.

2.2 Test materials

Test specimens for slow strain rate tests (SSRT) were machined

from seamless carbon steel pipes conformable to specification

ASTM A106 Grade B. The designation and composition of the

test material used is shown in Table 3. The test specimens had

gauge length of 32mm, width of 5mm, and thickness of 1.5mm.

The notch was made on the thin side of the test specimen with

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Materials and Corrosion 2013, 64, No. 9999 SCC of carbon steel in ethanol–gasoline blends 3

Table 4. Corrosion potential (mV) versus Ag/AgCl/EtOH reference

electrode in FGE for all the samples tested

1E10 1E15 1E20 1E25 1E30 1E85

208 237 164 199 191 201

2E10 2E15 2E20 2E25 2E30 2E85

223 164 242 284 209 212

Figure 1. Stress-elongation curves of all the N-SSRTs and the reference

sample tested in air

Figure 2. Cross-section image of nickel-plated N-SSRT specimen 1E85

(above) and fracture surface of the same sample (below) showing

mainly intergranular SCC. No oxide film is present on the cross-section

of the fracture surface. Pearlite bands (A) are visible on the fracture

surface, causing transition to transgranular SCC

depth of 0.57mm and radius 0.3� 0.02mm, respectively (the

stress concentration factor, kt, was 2.3). Schematic of the notched

test specimen including optical micrographs of the microstruc-

ture from both sides of the test specimen was shown in the

previous study [13]. All the surfaces were first ground up to 1200

grit longitudinally, then degreased with ethanol and acetone and

finally dried in blowing air.

Before the SSRTs a corrosion film was formed on the surface

of the test specimens by a potentiostatic method at 800mV versus

SHE in neutral water environment for 12 h. Black corrosion film

was formed on the sample surface. After oxidation the samples

were rinsed with ethanol and exposed to air for 24 h. During

exposure to air the color of the corrosion film turned reddish and

it was expected to simulate a rusted pipe wall from service with

low oxygen content as closely as possible. Corrosion potential of

the pre-oxidized samples was measured in the same FGE, which

was used to make the ethanol–gasoline blends, to confirm that

the properties of the corrosion films formed were similar for

all of the samples. Corrosion potential was measured using a

silver/silver chloride/ethanol (Ag/AgCl/EtOH)þ 0.1M lithium

chloride (LiCl) reference electrode (Metrohm 6.0726.108) via a salt

bridge containing the test solution [2]. Potential of the reference

electrode used versus SCE reference electrode was 50mV.

Corrosion potentials for all the samples are shown in Table 4.

2.3 Slow strain rate tests

All the N-SSRTs were performed according to NACE standard

TM0111-2011 with the exceptions that pre-oxidized test speci-

mens were used and the dimensions were different from the

standard values due to practical reasons. All tests were performed

in the environmental cell described above using the ethanol–

gasoline blends from Table 2. Two SA106 steel samples were

tested with each blend. The purpose was to investigate howmuch

ethanol is needed to induce SCC of carbon steel in the ethanol–

gasoline blends. A displacement rate of 2.2� 10�5mm/s was

used for all the N-SSRTs. The fracture surfaces of the specimens

were characterized using SEM and SEM–EDS. One sample was

also tested in air as a reference sample. This sample was prepared

identically with the other samples.

3 Results

Stress corrosion cracking was observed with almost all the

samples tested. Stress-elongation curves from all tests are shown

in Fig. 1. Only two samples 2E10 and 1E15 did not show any SCC.

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Fracture surface of sample 2E10, 1E15, and the reference sample

tested in air showed only ductile fracture. When characterized

from the notch bottom, all other samples did show mainly

intergranular SCC as seen, e.g., from fracture surface of sample

1E85 (Fig. 2). Selective dissolution of ferrite from pearlite phase



Figure 3. Notch bottom surface of N-SSRT specimen 2E20 (above)

showing how pearlite bands have become visible at notch bottom and

fracture surface of N-SSRT specimen 2E30 (below) showing the lamellar

cementite left on the steel surface due to selective dissolution of ferrite

from the pearlite phase. The arrow indicates the crack growth direction

Figure 4. Fracture surface of N-SSRT specimen 1E30 showing SCC

crack, which has started as intergranular SCC until transition to

transgranular SCC after the first (A) or second (B) pearlite band of the

steel

Figure 5. SEM images from side surface of N-SSRT specimen 1E20.

Grain boundaries are attacked over the whole surface and initiation

sites of intergranular SCC as shown in detail A were found on the whole

surface

was observed as seen from Fig. 3 showing the notch bottom

surface of sample 2E20 and fracture surface of sample 2E30.

Pearlite bands became visible on the notch bottom as the lamellar

cementite is left on the steel surface due to selective dissolution.

Distance between the pearlite bands varied approx. from 8 to

30mm.Withmost samples the SCC cracks on the side surfaces of

the sample started as intergranular SCC until transition to

transgranular SCC usually after reaching the first or second

pearlite band in the steel as can be seen from Fig. 4, which shows

SCC fracture surface of sample 1E30.

Grain boundaries were found to be visible over the whole

surface of the N-SSRT specimens as seen in Fig. 5 showing the

side surface of sample 1E20. There were many intergranular SCC

initiation sites even 5mm away from the main fracture at the

notch bottom as shown in detail A in Fig. 5. The light areas in

Fig. 6 are lamellar cementite left on the steel surface due to

selective dissolution of ferrite from pearlite phase. As can be seen

some areas of the steel surface on the sample sides are covered

with cementite in locations, where the pearlite band is on the steel

surface. In these locations, there are cementite around the SCC

cracks. There is a difference in the appearance of SCC initiated at

the pearlite and ferrite phases of the steel as seen in Fig. 6


showing the side of sample 1E30. Appearance of SCC initiated on

the pearlite phase was wider and promoted transgranular fracture

mode while SCC initiated on the ferrite phase was more narrow

and promoted intergranular fracture mode.

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Figure 6. SEM images from side of N-SSRT specimen 1E30 showing the

difference between the SCC initiation at the pearlite and ferrite phase

of the steel. The steel surface is covered with lamellar cementite in

locations, where the pearlite band is on the steel surface

Figure 7. Cross-section image of nickel-plated N-SSRT specimen 2E85

(above) and fracture surface of the same sample (below) showing

transgranular SCC. Black dots on the cross-section of the fracture

surface are the residues of the extremely thin oxide film inside the

crack

Figure 8. Fracture surface of N-SSR test specimen 2E85. SCC initiated

at the notch bottom of the sample. Due to banded pearlite structure of

the steel, only narrow area of the steel surface is covered by cementite

(A). Corroded zone appears as a circle around the cementite

One experiment with sample 2E85 was prepared in a slightly

different way compared to the other experiments. Before starting

the experiment the reference electrode was set up for measuring

the corrosion potential in the same environmental cell, which was

used in the other experiments. The reference electrode was left in

the environmental cell for 2 h before starting the experiment.

Corrosion potential could not be measured due to low

conductivity of the test solution. After this procedure the

reference electrode setup was removed and the experiment

was made in a similar way as with the other samples. After

the experiment the amount of chloride leaked from the reference

electrode was measured. Chloride concentration after the

experiment was measured using method UOP779 to be 2mg/L.

The fracture mode of this sample was fully transgranular as seen

from Fig. 7 showing the fracture surface of sample 2E85.

Corroded steel surface zones consisting of crystallographic pits

were found on the fracture surfaces near the cementite left on the

steel surface due to selective dissolution. The corroded zones

were either in circles around the cementite on the SCC fracture

surface at the notch bottom as shown in Fig. 8 or very near

the cementite on the SCC fracture surface initiated at the pearlite

band on the side surface of the sample as shown in Fig. 9. At some

locations thin oxide film appeared to cover the corroded zone.

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SEM–EDS analyses were made from the thin film and the steel

surface next to it. More oxygen was found by EDS from the thin

film covered fracture surface area.



Figure 9. Fracture surface of N-SSR test specimen 2E85. SCC initiated

at pearlite band located on the side surface of the sample. Corroded

zone consisting of crystallographic pits is present on the fracture

surface in locations, where cementite is covering the steel outer

surface

Table 5. Chemical composition of the ethanol-gasoline blends before

and after every experiment as well as the average crack growth rate

(ACGR) for each test specimen in N-SSR test

Ethanol(wt%)

Water(wt-%)

Conductivity(mS/cm)

ACGR (mm/s)

E85 82.2 0.190 0.74 –

1E85 83.0 0.232 1.42 1.17E�06� 0.18E�06

2E85a) 83.0 0.237 3.56 4.88E�07� 1.27E�07

E30 32.3 0.091 0.17 –

1E30 56.0 0.460 2.27 2.92E�07� 0.47E�07

2E30 42.3 0.200 0.30 3.67E�07� 1.81E�07

E25 27.0 0.110 0.10 –

1E25 41.1 0.253 0.57 3.39E�07� 0.45E�07

2E25 39.9 0.215 0.41 1.69E�07� 0.11E�07

E20 20.9 0.064 0.02 –

1E20 31.0 0.175 0.21 2.72E�07� 0.69E�07

2E20 31.2 0.161 0.20 1.43E�07� 0.31E�07

E15 17.9 0.074 0.01 –

1E15 23.5 0.196 0.09 No SCC

2E15 18.7 0.168 0.02 1.16E�07� 0.60E�07

E10 10.4 0.044 <0.01 –

1E1O 15.5 0.150 0.01 8.60E�08� 2.28E�08

2E10 19.8 0.185 0.03 No SCC

a)Contained 2 mg/L of chlorides.

The crack depth was measured for all samples as seen from

Fig. 10 and average crack depth was calculated from the results.

The average crack growth rate (ACGR) was calculated by using the

average crack depth and the duration of the SSRT from the yield

Figure 10. Fracture surface of N-SSRT specimen 2E25 showing

measured SCC crack depths in four locations, which were (a) 23.13 mm,

(b) 21.66 mm, (c) 25.34 mm, and (d) 23.50 mm


point of the steel until the final fracture. During the N-SSR testing

the ethanol concentration increased because the light C4 and C5

fractions from the ethanol–gasoline blends were evaporated due

to dry air purging. The ethanol and water concentration as well as

conductivity was analyzed after every N-SSRT. All results are

shown in Table 5.

4 Discussion

Steel SA106 Grade B was found to be susceptible for SCC in the

ethanol–gasoline blends in aerated conditions. When SCC was

observed, there was significant reduction in elongation of the

steel as seen from Fig. 1. There was reduction in elongation of

sample 2E15 even though no SCC was found on the fracture

surface. This reduction in elongation can be caused by the

corrosion attack at grain boundaries. Selective dissolution of

ferrite from pearlite phase was observed in ethanol–gasoline

blends in a similar way as in earlier studies with FGE [13–15] as

seen from Figs. 3 and 9.

Susceptibility to SCC in the ethanol–gasoline blends was

evaluated using ACGR obtained in N-SSR tests. As seen from

Table 5 the ACGR increases as the ethanol concentration

increases. Crack growth rate even with sample 1E10 was still

noticeable. It should be noted that the crack growth rate depends

highly on the applied stress and on dynamic strain rate. In the

previous study [15], the crack growth rate in FGE was found to be

5.71E�08mm/s for steel SA106 in constant load test, when stress

level was at the yield strength of the steel. Comparison of this

value to the crack growth rate of 4.88E�07mm/s for the sample

2E85 shows, that dynamic strain rate in N-SSR test strongly

affects the crack growth rate as FGE is expected to be more

aggressive than ethanol–gasoline blend with 85 vol% of ethanol.

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Due to this reason the ACGR should only be used to compare the

ethanol–gasoline blends tested in N-SSRT.

During the N-SSR testing the light C4 and C5 fractions were

evaporated from the ethanol–gasoline blends due to dry air

purging. The light fractions are not considered to affect the SCC

mechanism but because of the evaporation, ethanol concentra-

tion increased during the N-SSR testing. Due to this reason it is

difficult to evaluate the exact amount of ethanol required to

induce SCC of the carbon steels. Because SCC was found in

sample 1E10, which had 15wt% of ethanol after the experiment,

it is clear that SCC can occur with only 15wt% of ethanol in the

ethanol–gasoline blend. Nevertheless, because in the beginning

of the experiment there was only 10.4wt% of ethanol, there is a

risk for SCC of carbon steels in ethanol–gasoline blends with over

10wt% of ethanol. The range of ACGRs from our N-SSR testing

was from 0.86E�07 to 11.7E�07mm/s, which is similar to the

range of ACGRs from approx. 0.6E�07 to 3.9E�07mm/s

published by Beavers et al. [2].In most cases, the fracture mode was found to be mixed

starting as intergranular and ending as transgranular SCC. As

observed in the previous study [13] the pearlite bands affect the

fracture mode and mechanism. As seen in Figs. 3 and 6, the

pearlite bands extend through the steel in a way that they are

parallel to the pipe surface and in this case perpendicular to the

notch bottom surface. The distance between the pearlite bands

varies from 8 to 28mm. Due to this reason the pearlite bands are

visible on the notch bottom as stringers of lamellar cementite,

which is left on the steel surface due to the selective dissolution.

On both sides of the samples, pearlite bands are visible as wide

zones covered with the lamellar cementite at the locations, where

the pearlite band is on the steel surface. When a specimen was

characterized from the notch bottom, fracture was mainly

intergranular but the pearlite bands were visible on the fracture

surface (Fig. 2). When a specimen was characterized from the

side surface, the fracture started as intergranular SCC, which

switched to transgranular SCC after the first or second pearlite

band in the steel (Fig. 4). At some locations where the SCC had

initiated at the pearlite band, cracking started as transgranular

SCC. The results indicate that cementite can affect the SCC

fracture mechanism even when there are no chlorides in the

ethanol–gasoline blend. It can be that due to active dissolution of

ferrite the conductivity inside the SCC crack increases, whichmay

cause more localized corrosion near the cementite leading

eventually to transition in the fracture mode. The transition in the

fracture mode may also be caused because the stress distribution

inside the crack is different, when the pearlite band is

perpendicular to the SCC crack.

When there was 2mg/L of chloride in the ethanol–gasoline

blend with 85 vol% of ethanol, the fracture mode changed to

transgranular cracking (Fig. 7). The chloride also caused localized

corrosion on the fracture surfaces near the cementite (Figs. 8

and 9). Corroded zones consisting of crystallographic pits were

formed on the fracture surfaces near the cementite left on the

steel surface due to selective dissolution. When SCC initiated on

the pearlite band located on the side surface of the sample,

cementite was covering the steel surface around the SCC crack. In

these locations there were corroded zones covering the whole

fracture surface near the cementite left on the steel surface

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(Fig. 9). When SCC initiated on the notch bottom of the sample,

only narrow regions on the steel surface were covered with

cementite due to the banded pearlite structure of the steel. In

these locations corroded zones formed circles around the

cementite (Fig. 8). Similar results were also found previously,

when SCC of carbon steel was studied in FGE [13]. In the earlier

study more chloride was leaked into the FGE due to the

electrochemical measurements during the testing. In the earlier

tests, the corroded zones were found for the first time in the

middle of the fracture surface far away from the cementite or

from the crack tip. According to the previous and new results

together, it appears as field is formed around the cementite,

which causes localized corrosion inside the SCC crack. It is

obvious that conductivity of the test solution affects this

phenomenon. It was observed (Table 5) that chloride does not

affect the SCC crack growth rate. The crystallographic pitting is,

however caused by the chloride.

It is important to understand that both transgranular and

intergranular SCC can occur in FGE and ethanol–gasoline

blends. The intergranular SCC mechanism is more severe from

the two different types of SCC mechanisms because of high

ACGR and easy SCC crack initiation. In FGE with sufficient

amounts of chloride inducing only transgranular SCC, the SCC

crack initiation takes place mainly in pearlite phase of the steel

and the SCC cracks initiated at the pearlite phase grow faster than

the SCC cracks initiated in the ferrite phase [13]. In ethanol–

gasoline blends without any chloride, the grain boundaries were

attacked over the whole steel surface and SCC crack initiation

takes place equally on the pearlite and ferrite phases of the steel.

Only 2mg/L of chloride in the ethanol–gasoline blend with

85 vol% of ethanol caused transition from intergranular to

transgranular SCC. Due to this reason it is very difficult to make

electrochemical measurements to study the intergranular SCC

mechanism. In most of the previous studies, the fracture

mechanism has been mainly transgranular, because of the

chloride leakage from the reference electrode or because

simulated FGE has been used with more than 2mg/L of added

chloride. Due to this reason there is not much information

available about the intergranular SCC mechanism of carbon

steels in FGE. Most failures in the industry have been due to

intergranular SCC, which also indicates that the intergranular

SCC mechanism is more important and severe than the

transgranular SCC mechanism.

Because the grain boundaries are attacked over the whole

steel surface as seen from Fig. 5, it is apparent that the

intergranular SCC has not initiated due to film breakdown as

proposed earlier [2, 12]. If sufficient amount of stress is applied,

the whole steel surface appears to be susceptible for intergranular

SCC, in the ethanol–gasoline blend without any chloride. There

are intergranular SCC crack initiation sites 5mm away from the

main fracture surface at the notch bottom, which indicates that

the threshold stress for the intergranular SCC can be quite low. In

a previous study transgranular SCC initiated but did not

propagate at 80% of yield strength stress level [15]. It is possible

that the threshold stress for the intergranular SCC is lower and

the cracks may propagate at a low stress level. This is why similar

constant tensile load testing in FGE without any chloride is

important for evaluation of the threshold stress for intergranular



SCC of carbon steel in FGE and ethanol–gasoline blends. New

approach is required to study electrochemically the intergranular

SCC mechanism of carbon steel in FGE and ethanol–gasoline

blends to fully understand the SCC mechanism.

The oxide layer on the steel surface may also affect the SCC

mechanism in FGEs and ethanol–gasoline blends. SCC failure

from old pipeline handling FGE was investigated in a previous

study [14]. The pipeline surface was covered with thick multi-

layered oxide film consisting of magnetite and hematite. In the

previous study pre-oxidation of the SSR test specimens was made

in a way that hematite layer was formed on the test specimen

surface [13]. When the test specimens were characterized after

the SSRTs, only a very thin oxide film if any was observed on the

test specimen surfaces. It appeared as the oxide film formed

during pre-oxidation had dissolved during the SSRTs. To see the

differences between magnetite and hematite the pre-oxidation

parameters were changed for this study in a way that a magnetite

layer was formed on the test specimen surfaces. No obvious

differences were observed as only very thin oxide film if any was

observed on the test specimen surfaces after the SSRTs. It may be

that due to aeration the properties of the oxide layer in both cases

were changed during the SSRT in a way that any oxide on the test

specimen surface was removed while the test specimens were

cleaned for SEM characterization.

5 Conclusions

� I

�

ntergranular SCC of carbon steel was produced in SSRT in

ethanol–gasoline blend with 15.5wt% ethanol that was

produced by evaporation of light fractions from a solution

with 10wt% of ethanol.

� T
he average SCC crack growth rate decreased as the ethanol
content decreased in the blend yielding a range of ACGR from

1.17E�06 to 8.60E�08mm/s.

� O
nly 2mg/L of chloride was found to cause transition from
intergranular SCC to fully transgranular SCC in ethanol–

gasoline blend with 85wt% of ethanol. Small chloride leakage

did not affect the SCC crack growth rate.

� T
ransgranular SCC was found to initiate mainly at the pearlite
phase and intergranular SCC was found to initiate equally on

the pearlite and ferrite phases.

� C
hloride was found to cause localized crystallographic pitting
on the transgranular SCC fracture surfaces near the lamellar

2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

cementite left on the steel surface due to selective dissolution of

ferrite from pearlite.

Acknowledgements: The authors wish to acknowledge Neste Oil

Oyj for financing the study as well as for providing the ethanol–

gasoline blends tested. Special thanks are also due to the experts

of Neste Oil Oyj for providing information related to the ethanol–

gasoline blend chemistry.

6 References

[1] R. D. Kane, D. Eden, N. Sridhar, J. G. Malonado, M. P. H.Brongers, J. A. Beavers, API Technical Report 939-D, 2nd ed.,Stress Corrosion Cracking of Carbon Steel in Fuel Grade Ethanol:Review, Experience Survey, Field Monitoring, and LaboratoryTesting, American Petroleum Institute, Washington, DC,2007.

[2] J. A. Beavers, F. Gui, N. Sridhar, Corrosion 2011, 67, 025005.[3] N. Sridhar, K. Price, J. Buckingham, J. Dante,Corrosion 2006,

62, 687.[4] R. C. Newman, Corrosion 2008, 64, 819.[5] J. G. Maldonado, N. Sridhar, CORROSION/2007, paper no.

07574, NACE International, Houston, TX, 2007.[6] X. Lou, D. Yang, P. M. Singh, Corrosion 2009, 65, 785.[7] X. Lou, P. M. Singh, Corros. Sci. 2010, 52, 2303.[8] F. Gui, N. Sridhar, J. Beavers, CORROSION/2009, paper no.

09531, NACE International, Houston, TX, 2009.[9] R. D. Kane, N. Sridhar, M. P. Brongers, J. A. Beavers, A. K.

Agrawal, L. J. Klein, Mater. Perform. 2005, 44, 50.[10] F. Gui, N. Sridhar, J. A. Beavers, Corrosion 2010, 66, 125001.[11] J. A. Beavers, M. P. Brongers, A. K. Agrawal, F. A. Tallarida,

CORROSION/2008, paper no. 08153, NACE International,Houston, TX, 2008.

[12] X. Lou, D. Yang, P. M. Singh, J. Electrochem. Soc. 2010, 157,C86.

[13] J. Torkkeli, T. Saukkonen, H. Hanninen,Mater. Corros. 2012,DOI: 10.1002/maco.201206844.

[14] V. Hirsi, J. Torkkeli, H. Hanninen, Weld. Cutting 2011, 10,188.

[15] J. Torkkeli, V. Hirsi, T. Saukkonen, H. Hanninen, presentedin EUROCORR 2011, Stockholm, Sweden, September 4–8,2011, Paper No. 1036.

(Received: October 5, 2012)

(Accepted: December 19, 2012)

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Stress corrosion cracking of carbon steel in ethanol-gasoline blends

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