Materials Sciences and Applicatio ns, 2010, 1, 191-198
doi:10.4236/msa.2010.14030 Published Online October 2010 (http://www.SciRP.org/journal/msa)
191
The Effect of the pH of Ammonum Nitrate
Solution on the Susceptability of Mild Steel
to Stress Corrosion Cracking (SCC) and
General Corrosion
Fathia S. Mohammed1, Alyaa G. Elramady2, Salheddin E. Abu Yahya2
1Department of Chemical Engineering, American University of Sharjah, Sharjah, United Arab Emirates; 2The Petroleum Institute,
Abu Dhabi, United Arab Emirates.
Received April 18th, 2010; revised August 6th, 2010; accepted August 16th, 2010.
ABSTRACT
This work investigates the rela tive agg ressiveness of n itrate so lu tions at different pH va lues on mild steel towa rds stress
corrosion cracking (SCC) and general corrosion. Electrochemical behavior and stress corrosion cracking susceptibility
measurements were carried out in 52 Wt% ammonium nitrate solutions at 368° K and various pH values ranging from
0.77 to 9.64. Constant load stress corrosion test at 90% yield stress was conducted. Tested specimens were prepared
and examined using the scanning electron microscope (SEM). The potentiod ynamic polarizatio n curves for different pH
values again emphasized the validity of the gravimetric measurements and hence the mechanism of cracking was at-
tributed to the stress that assisted the dissolution process.
Keywords: Stress Corrosion Cracking, Ammonium Nitrate Solution, Mild Steel, Constant Load Test, Effect of PH
1. Introduction
Stress corrosion cracking (SCC) can lead to rapid and
catastrophic failure in many different metals and alloys.
This phenomenon occurs under conditions where a com-
ponent is exposed to a mildly corrosive environment
while under applied and/or residual tensile stress. Hence,
metal parts with severe SCC can appear bright and shiny
whilst being filled with microscopic cracks [1]. These
factors along with the rapid progress of SCC make it
common for SCC to go undetected prior to failure. Steel/
nitrate interaction is an issue in nitrogenous fertilizer
plants, waste heat recovery boilers (WHRBs) in power
generating plant and nuclear wastes [2].
The effect of pH and types of nitrate solution has been
investigated [3], which concluded that the order of de-
creasing aggressiveness of nitrate solutions corresponded
to the order of increasing (initial) pH for a given chemi-
cal strength, i.e., NH4+, Ca++, K+, Na+.
The aggressiveness of ammonium nitrate when com-
pared to other nitrates was attributed to its lower pH.
Parkins [4] reported that the marked decrease in potency
at initial pH values in the region number 4 compared to
either slightly higher or lower values depends probably
upon pH changes in the solution during the test. The
above work was concerned with the changes in pH of the
bulk of solution and not with the pH at the crack tip re-
gion, which undoubtedly more acidic. Steel has been
characterized as being very susceptible to SCC at near-
neutral pH [5].
In this work a comprehensive study on th e effect of the
pH of ammonium nitrate solution on the susceptibility of
mild steel to stress corrosion cracking and general corro-
sion was carried out. The results indicate that in some pH
values the general and localized corrosion were the cause
of failure. In other pH values the stress corrosion crack-
ing were the cause of failures. The severity was con-
firmed by the calculation of crack growth rate, morphol-
ogy of the fracture surface by SEM and by polarization
work.
2. Experimental Approach
2.1. Material
The work was carried out on mild steel of the following

The Effect of the pH of Ammonum Nitrate Solution on the Susceptability of Mild Steel to
Stress Corrosion Cracking (SCC) and General Corrosion
192
composition (wt%) :
C 0.070, Mn 0.300, Si 0.093, S 0.044, P 0.019. The
Material was supplied in the form of 19 mm diameter
rods. The corroding solution was prepared by using am-
monium nitrate.
2.2. Specimen Preparation
2.2.1. Electrochemical Measurement
The steel rods were hot-rolled at 1200° K to 4 mm thick
strips. This was reheated to 1200° K in the furnace for
900 s, and then allowed to cool to room temperature.
Most of the oxide film was removed by pickling in 30%
HCl s olution, and the surface was finally c leaned for cold-
rolling by mechanical abrasion. The strips were reduced
to 0.5 mm thick by cold-rolling.
Samples of 20 mm by 13 mm were prepared. A 3 mm
holes were drilled at one end to suspend the samples, then
the specimens were degreased with ether, annealed at
1200° K for 3.6 ks. The specimens were attached to the
hold er and the who le assemb ly coated ap art from an ar ea
of 100 mm2 on one face.
2.2.2. Stress Corrosion Measurement
The steel rod was hot-rolled at 1200° k and swaged cold
to approximately 10 mm diameter. It was then annealed
at 1200° k for 900 s, furnace cooled to 850° k, followed
by air cooling to room temperature. The specimens were
machined from the rod as shown in Figure 1. They have
a gauge length of 15.8 mm and a gauge diameter of 3.2
mm.
2.2.3. General Corrosion Testing
Samples of 40 mm by 15 mm were prepared. A 3 mm
holes were drilled at one end to suspend the samples,
then the specimen were degreased with ether, annealed at
1200 k for 3. 6 ks.
2.3. Apparatus
2.3.1. Electrochemical Measurements
For electrochemical measurements on unstressed speci-
mens, a glass cell comprising two compartments was
designed. The main compartment contained the working
electrode and the platinum counter electrode. The refer-
ence compartment contained a saturated calomel elec-
trode. The complete cell is shown in Figure 2. The two
compartments were connected by a salt bridge with a
Luggin capillary. The glass joints that carried the work-
ing and the counter electrodes also had a screw cap joint
for the thermometer. There was another two openings in
the main compartment, one for water condenser, and the
other for gas and solution inlet, when working with
de-aerated system. The reference compartment has a
thermometer gas inlet and liquid inlet together with the
Figure 1. Stress corrosion test specimen (dimensions in
mm).
Figure 2. Electrochemical polarization cell (schematic). 1)
Main compartment; 2) Specimen/working electrode; 3)
Platinum electrode; 4) Luggin capillary; 5) Thermometer; 6)
Condenser and gas outlet; 7) & 13) Solution inlet; 8) & 14)
Gas inlet; 9) Salt bridge; 10) Gas Outlet; 11) Reference
compartment; 12) Saturated calomel reference el ectrode.
saturated calomel electrode in one joint. The cell capacity
is 0.4 dm3 of test solution. Only the main compartment of
the cell was immersed in an oil bath controlling the re-
quired temperature, the reference compartment was held
at room temperature.
2.3.2. Stress Corrosion Measurement
The majority of the work was conducted using the con-
stant load method. The tensile properties of the material
were measured in triplicate using Instron Tensile Testing
Machine. In all the constant load tests the load applied
was 90% of the predetermined yield stress.
For electrochemical measurements on stressed speci-
mens, a glass cell consisting of two compartments was
used; the main compartment contained the stress corro-
sion specimen and the platinum counter electrode. The
reference compartment contained the saturated calomel
electrode, similar to the reference compartment described
before. The two compartments were connected by a salt

The Effect of the pH of Ammonum Nitrate Solution on the Susceptability of Mild Steel to
Stress Corrosion Cracking (SCC) and General Corrosion
193
bridge with a Luggin capillary. The capacity of the cell is
0.25 dm3, and the details are shown in Figure 3.
2.3.3. General Corrosion Measurement
A flat-bottomed one-liter glass vessel with two necks
was used. One neck held the water condenser; the speci-
mens were suspended by a glass hook from the lower end
of the condenser. A thermometer, inserted through the
second neck.
3. Results and Discussion
To validate the results, all measur ements were conducted
at least three times under each specific environment.
3.1. Stress Corrosion Life and Corrosion
Potential
The entire stress corrosion test carried out under a con-
stant load of 90% of the yield stress (206.5 MN m-2). A
series of stress corrosion tests were carried out to deter-
mine the stress corrosion life in 52 Wt% NH4NO3 solu-
tion at 368° k and various pH values ranging from 0.77 to
9.64. The corrosion potential was also recorded during
the tests.
Figures 4, 5 and 6 show the changes in the corrosion
potential during the stress corrosion test at different pH
values. In solution of pH 2.78 and above jumps in the
corrosion potential to more negative values were ob-
served before failure occurs. At pH 0.77, no oscillations
were observed.
Figure 7 shows the whole range of potential change,
the initial and the final pH of the solution, and the stress
corrosion life for some of the above tests. For lower pH
values, the pH was more basic at the end of the test,
while at high pH values (5.7 and above), the pH became
more acidic at the end of the test. While results indicate
that the critical range for cracking (i.e., where the stress
corrosion life is minimum) is between pH 3.0 and pH 7.5,
cracking occurs even at pH 9.6 but after very long period
of time.
Staehle [6] reported that with respect to pH, a change
of one unit of pH chang es the solubility of ox ide by three
orders of magnitude for three valent ions such as Fe3+ and
by two or ders of magnitude for two valent i on such as Fe2+.
It was reported that nuclear waste are all alkaline, with pH
in the range 11-14. Even under these highly alkaline con-
ditions, the presence of certain constituents, such as ni-
trates can make the carbon steel susceptible to SCC [7].
Other researchers indicated that elevated pH are long
considered corrosion inhibitors and did not have a domi-
nant effect on the susceptibility to SCC in the range 10-
13.5 [8,9].
Figure 8 summarizes the relation between the pH of
Figure 3. Stress corrosion test cell (schematic). 1) Main
compartment; 2) Specimen/working electrode; 3) Platinum
electrode; 4) Luggin capillary; 5) Thermometer; 6) P.T.F.E
nut; 7) Rubber washers; 8) Stainless steel nut; 9) Top
shackle; 10) To condenser and gas outlet; 11, 18) Solution
inlets; 12, 17) Gas inlets; 13) Salt bridge outlet; 14) Gas
outlet; 15) Reference compartment; 16) Saturated calomel
referenc e e lectrode.
Figure 4. Effect of pH on the corrosion potential/time beha-
vior of mild steel during stress corrosion testing in 52 Wt%
NH4NO3 at 368° K.
the solution and the corrosion potential at different time
during the test. The grap h shows no straight forward sys-
tematic correlation between the pH of the solution and
the potential.
3.2. Metallographic Examination
Selected specimens were prepared for examination using
the scanning electron microscope and the ordinary met-
allographic microscope. The results are in Table 1.
Figures 9(a) and 9(b) show a fracture surface of a
specimen which was stress corroded in a solution of pH

The Effect of the pH of Ammonum Nitrate Solution on the Susceptability of Mild Steel to
Stress Corrosion Cracking (SCC) and General Corrosion
194
Figure 5. Effect of pH on the corrosion potential/time beha-
vior of mild steel during stress corrosion testing in 52 Wt%
NH4NO3 solutions at 368° K.
Figure 6. Effect of pH on the corrosion potential/time beha-
vior of mild steel during stress corrosion testing in 52 Wt%
NH4NO3 at 368° K. (pH 9.64).
0.77. The first shows an area where localized attack has
occurred, whilst the second shows the ordinary structure.
The crack growth rate was very small (3 nms-1) with
big reduction of the specimen diameter (660 μm) which
indicates the high general and intergranular corrosion
occurs during the test.
Very heavy attack was observed when specimens were
broken in a solution of pH 4.2.
Figure 10 Illustrates part of the fracture surface. It is
clear that that cracking occurs and the cracking rate was
very high (71 nms-1), with very small reduction in the
diameter (8 μm) which indicates the severity of such en-
vironment.
Figure 11 shows a specime n which was stre ss corroded
in a solution of pH 9.64.
Figure 7. The relationship between stress corrosion life, cor-
rosion potential variations and solution pH changes for
mild steel in 52 Wt% NH4NO3 solutions at 368° K.
Figure 8. Effect of pH on the corrosion potential of mild
steel under applied stress in 52 Wt% NH4NO3 solutions at
368° K.
(a) (b)
Figure 9. Fracture surface morphologies of a stress corro-
sion specimen after failure in NH4NO3 solution of pH 0.77
at 368°K (X 950). (a) localized intergranular attack and
cracking; (b) Ductile failure.
In summary the results of the present work indicate the
following facts: 1) a presence of stress corrosion cracking
at high pH values (pH 9. 64, Figures 6, 11 and Ta ble 1). 2)

The Effect of the pH of Ammonum Nitrate Solution on the Susceptability of Mild Steel to
Stress Corrosion Cracking (SCC) and General Corrosion
195
Table 1. The Effect of pH on the stress corrosion life and morphology of attack in 52 Wt% NH4NO3 solution at 368° K.
Type of Attack and Number
of Locations
pH Average Stress Corrosion Life
(Ks) A B C
Maximum Depth of
Penetration in
Grains
Reduction in
Diameter (μm) Crack Growth Rate (nm/sec)
0.77 62.64 3 5 660 3.2
2.78 56.5 7 25 - 17.7
4.2 27.3 6 1 44 8 71
8.8 108 1 7 - 2.6
9.64 970.2 2 - - -
A: Fine cracks; B: Wide cracks; C: Cracks visible to the naked eye
Figure 10. Fracture surface morphology of a specimen after
stress corrosion failure in 52 Wt% NH4NO3 solution of pH
4.2 at 368° K showing the high degree of intergranular at-
tack (X 250).
The high potency of the solution at pH 2.78 compared to
lower or higher pH values (Figure 4). 3) In a solution of
pH 0.77, the S CC wa s associ ated with high rate of gen eral
attack (Figure 4, Figure 9 and Table 1).
Naris Sridhar et al. [10] reported that intergranular
stress corr osi on crac king ( IG S CC) has gene rall y not b een
observed when the pH greater than 11.0. According to
potential pH or pourbaix diagram [11] dangerous zones
Figure 11. An illustration of catastrophic crack formation
in a stress corrosion specimen after failure in 52 Wt%
NH4NO3 solutions of pH 9.64 at 368° K.
where SCC caused by nitrate solution is between pH 2.2
and 5.2, with corresponding potential between 0-I000
mVSCE.
Parkins et al. [12] reported that changes in the pH
during the test show more relation to the results than the
initial pH values. They showed that the time to failure
correlated more significantly with the final pH of the
solution than with initial values.
Relating the results sho wn in Figures 4, 5 and 6 to the
stages of SCC indicate the followings:
 There are no sudden jumps to more active potentials
before failure at a very low pH value (0.77), and
hence no indi cations of fast propagati on period [13].
This suggests that at this pH value, general corro-
sion and not stress corrosion is the predominant
factor.

The Effect of the pH of Ammonum Nitrate Solution on the Susceptability of Mild Steel to
Stress Corrosion Cracking (SCC) and General Corrosion
196
 In the pH range 2.0 – 8.0, oscillations in potential
are evident, occurring over a period representing 20
– 25% of the total life. This suggests th at variations
in pH above and below th e natura l pH of 4.2 do not
affect the percentage of the total time taken up by
the fast propagat i on period [13].
 At relatively high pH (9.64), the duration of the fast
propagation period is relatively unchanged, but it
occupies only about 1% of the total life. This can be
attributed to the very low rate of attack at such high
pH value.
3.3. Corrosion Rate and Corrosion Potential
Measurement on Unstressed Specimens
Figure 12 shows the effect of pH of the so lution on both
the general corrosion rate and the stress corrosion life.
This figure indicates that the increase in the stress corro-
sion life at lower pH values was associated with high
general corrosion, while at higher pH values (greater than
7.5) it was associated with low general corrosion.
The variation of the corrosion potential with time for
unstressed specimens was measured for period of ~70 ks
at different pH values, ranging from 1.05 to 9.13 (Figure
13 and 14). The potential changed to the more noble di-
rection as the test proceeded over almost the whole range
of the investigated pH except at pH 9.13. At this pH the
potential at the beginning was less noble, and after ap-
proximately 30 ks more noble values were observed.
From Figures 12, 13 and 14, the following are observed:
 High general corrosion at lower pH values is ac-
companied by a more active potential.
 The non dependence of pH on the corrosion rate
between pH values of 4.2 and 6.9 is associated with
unchanged corrosion potential.
 The decrease in the corrosion rate with increasing
pH in the range 6.0 – 8.5 is characterized by erratic
changes in the corrosion potential, e.g. it is com-
paratively noble at pH 7 and yet more active at
higher values.
The abov e behav ior prob ably refl ected the differen ce of
the solubility of the corrosion product in solutions of
different pH values.
Figure 15 shows the effect of stress on the maximum
corrosion potential during testing in 52 Wt% NH4NO3 at
368° K at different pH values. The stress appears to cause
a shift in the corrosion potential to more active values in
the range of the critical pH.
3.4. Crack Growth Rate
From the microscopic examination and the results of
stress corrosion life in different pH values reported in
Table 1, the crack growth rate was calculated by dividing
Figure 12. Effect of pH on the corrosion rate and stress
corrosion life of mild steel in 52 Wt% NH4NO3 solutions at
368° K.
Figure 13. Effect of pH on the corrosion potential/time be-
havior of mild steel in 52 Wt% NH4NO3 solutions at 368° K.
Figure 14. Effect of pH on the corrosion potential/time be-
havior of mild steel in 52 Wt% NH4NO3 solutions at 368° K.

The Effect of the pH of Ammonum Nitrate Solution on the Susceptability of Mild Steel to
Stress Corrosion Cracking (SCC) and General Corrosion
197
Figure 15. Effect of pH on the maximum corrosion potential
value attained for mild steel in 52 Wt% NH4NO3 solutions
at 368° K.
the maximum measured crack depth by the total time to
failure. The obtained value gives an estimate of the rate
since it does not take into account the time to initiate
cracks. The growth rate of any observed cracking is as-
sumed to be constant throughout the exposure period.
These estimates provide a simple semi quantitative di-
agnostic to classify the SCC propensity. The following
points regar ding the crack growt h rate a re concl uded from
Table 1.
 In the solut ion of pH 0.77 the crack growth was only
3.2 nms-1 bu t there was a big reduction (1 8%) in the
diameter which clearly indicates that general cor-
rosion was dominating.
 The crack g rowth rate at pH 2.78 is 17.7 nms-1 while
it is 71 nms-1 at pH 4.2; thus it is obvious that the
latter environment is more prone to SCC.
 In the solution o f pH 8.8 the crack growth rate was
2.6 nms-1 and this is probably because of the longer
initiation period.
3.5. Potentiodynamic Polarization
Figure 16 shows the p otenti ody nam ic pol arizat ion c urves
for different pH values of 52 Wt% NH4NO3 solution at
368 K ranging from 2.04 to 8.35.
The scanning started approximately 200 mv more ne-
gative than the open circuit potential (OPC) in the noble
direction t o m ore tha n +1 500 mv usi ng s weep rate of 0.33
mVs -1.
Several distinct characteristics for solution of different
pH are as below:
 For pH 2.04 1) An active dissolution regime be-
tween −500 mVSCE and −150 mVSCE, 2) First pas-
sive plateau at a current density of 2 × 103 Am-2
between potential −150mVSCE and zero mVSCE, 3)
Figure 16. Effect of pH on the potentiodynamic anodic po-
larization behavior of mild steel in 52 Wt% NH4NO3 solu-
tion at 368 K (sweep rate 0.33 mVs-1).
Broad active to passive transition peak starting at
zero mV with corresponding current density of 0.85
× 103 Am-2, 4) Second dissolution regime between
+ 500 mV and 600 mV, 5) Second passive p lateau at
current density of 1.3 × 103 Am-2 between 600 mV
and 1300 mVSCE, 6) A transpassive regime starting
at 1300 mVSCE.
 For pH 4. 2 1) An acti ve diss olu ti on r e gime between
−500 mVSCE and −350 mVSCE, 2) First active-pas-
sive transition starting at −350 mV with a corre-
sponding current density of 1 × 103 Am-2, 3) An
active dissolution regime between −300 mVSCE to
−150 mVSCE, 4) A second active-passive transition
starting at −150 m VSCE with a corresponding current
density of 3 × 103 Am-2, 5) Transition peak at po-
tential 0 mV wi th a corres ponding c urrent de nsity of
0.6 × 103 Am-2, 6) A passive plateau at a current
density of 0.4 × 103 Am-2 between 500 mVSCE and
1400 mVSCE, 7) The initiation of the transpassive
regime at 1450 mVSCE.
 For pH 6.97 1) An active dissolution regime be-
tween −500 mVSCE and −150 mVSCE, 2) A passive
plateau at a current density of 3.5 × 103 Am-2 be-
tween −100 mVSCE and +500 mVSCE, 3) Sudden
decrease in the current density to 0.2 × 103 Am-2 at
−520 mVSCE, 4) passi ve plateau at cu rrent de nsity of
0.2 Am-2 between 550 mVSCE and 1200 mVSCE, 5)
The initiation of transpassive regime at 1200 mVSCE.
 For pH 8. 35 1) A slow diss olution fr om −250 mVSCE
to 800 mVSCE to a maxim um current de nsity of 0.1 ×
103 Am-2, 2) A nother disso lut ion beh avi or from 800
mVSCE to 1200 mVSCE to a maximum current den-
sity of 0.9 × 103 Am-2, 3) Passive plateau at a cur-
rent density of 1 × 103 Am-2 between 1200 mVSCE

The Effect of the pH of Ammonum Nitrate Solution on the Susceptability of Mild Steel to
Stress Corrosion Cracking (SCC) and General Corrosion
198
and 1600 mVSCE, 4) The initiation of transpassive
regime at 1600 mVSCE.
From above it is clear that the potentiodynamic po-
larization curves again emphasize the validity of the gra-
vimetric measurement and show the influence of pH on
the anodic dissolution characteristics of mild steel. These
results were found to be in acco rd with previou s research
[14].
4. Conclusions
At very low pH values, stress corrosion cracking is asso-
ciated with very high rate of general corrosion. In the
region of p H 2.0 to 4.2 the stre ss corrosion life is rel atively
unchanged and general corrosion rate decreases with
increasing of the pH level. Between pH 4.2 and 6.0, the
corrosion rate and stress corrosion life are alm ost constant.
In the region of pH 6.0 to 7.5, the stress corrosion life
increases slightly and the corrosion rate decreases. Above
pH 7.5, there is a noticeable increase in the stress corro-
sion life while the general corrosion rate shows a marked
decrease. Experim ental observation suggests that an oxide
film of critical physical properties is fo rmed at immersion.
This film suffers localized breakdown at the grain boun-
dary. These limited grain boundary micro fissures will
only propagate if aided by stress which facilitates the
continuing action of the corrosion process. Subsequently,
the precipitation of a layer of stifling corrosion product
re-occurs and the cyclic process is repeated until failure.
The local dissolution rate at the crack tip is accelerated
with test time, which would be attributed to the continu-
ous increase in the stress concentration. This reflects the
interaction of stress and anodic dissolution during the
SCC process.
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