Effects of Temperature on Microstructure and Corrosion
behavior of API N80 Carbon Steel
Fatiha Chelgham1,2*, Noureddine Bouzid3, Mokhtar
Saidi1, Amira Ouakkaf4, Adel Taabouche2, Souhila Boudjema2, Hanane Largot5, Abdellatif
Mamanou2, Noura
Meddoura1
1Laboratoire
De Valorisation et Promotion des Ressources Sahariennes, Université Kasdi
Merbah,
Ouargla- 30000, Algerie.
2Faculté Des Hydrocarbures, Energies Renouvelables, Science De La Terre et
De l’univers,
Université Kasdi Merbah,
Ouargla - 30000, Algerie.
3Structures, Properties and
Inter Atomic Interactions Laboratory (LASPI2A), Faculty of Science and
Technology, Unversity of Abbes Laghrour, Khenchela- 40000, Algeria.
4Faculté
Des Sciences Exactes, Université Mohamed Khider, Biskra-07000, Algerie.
5Faculté
Des Sciences Exactes, Université Echahid Hamma Lakhdar, Eloued- 39000, Algerie.
ABSTRACT:
Recently,
Pipeline corrosion is a major problem facing many oil and gas industries today
because of the enormous downtime associated with corrosion related failures. In
this study, the effect of tempering temperature (250, 300 and 550°C) on the
corrosion behavior of American Petroleum Institute (API) N80 steel in albian
water at different gradient temperatures were investigated using X-ray
diffraction (XRD), the electrochemical measurements combined with hardness
test. XRD patterns have shown that the API N80 steel samples crystallize in
ferrite type structure with a strong (110) orientation. We remarked that all
samples the N80 are a nanometric grain size, the values of grain sizes given in
the range from 211 to 485 Ĺ. Corrosion rates of samples are correlated with
structural changes (grain size, strain) in samples with increasing tempering
temperature. Steel N80 with higher tempering temperature exhibited excellent
corrosion resistance with lower corrosion current density. The distinction of
corrosion resistance can be attributed to increased grain sizes and decreased
residual stress and hardness.
KEYWORDS: API N80,
Residual stresses, tempering temperature, Corrosion, hardness.
1.
INTRODUCTION:
Carbon steel is one of the most
widely used in petroleum and gas industry. The materials K55, J55, L80, P110
and N80 are
used as tubing material or geothermal well casings1,2,3,4,5.
The materials used for the transportation pipelines,
the injection well casings and other equipment must be resistant to corrosion
by injected gas/fluids1,2,3,4,5. The N80 American Petroleum Institute
(API) is of the material used as tubing in oil well, due to their relatively
low cost, good properties and easy fabrication4. However, carbon
steels are vulnerable to corrosion in environment containing oil production.
Corrosion in gas and oil pipelines is one of the most
interesting operation problems to predict and control. The dangers of
corrosion; loss of production as a result crude oil or water may leak as a
result of corrosion which may cause environmental and water pollution and the
decrease in pipeline lifetime6,7. Corrosion rate of carbon steel
will depend on properties of the environment like pressure, pH, flow,
temperature, etc. and the base alloy such as composition and microstructure.
Recent studies have shown that the microstructure of steel pipeline and
chemical composition significantly influence its resistance to corrosion in
(hydrogen sulfide) H2S solution8,9.
Many studies on that The API N80 like Finšgar et al
shows that used in well construction (our study we used this steel), and it has
generally been used as the main construction material for down hole tubular and
transmission pipelines in the petroleum industry10,11,12,13. Also
the lifetime of concentric oil tube steel which are international specification
steels API N80 was 3 years old and over when water albian (aquifer water) was
used for pressure maintenance system in the Hassi-Messaoud (Haoud Berkaui
region), It is located about 100 km in south Algeria.
The aim of this study was to investigate the influence
of tempering temperature on the API N80 steel pipeline on the microstructures,
hardness and corrosion resistance were investigated by the experimental
observations.
MATERIAL
AND METHODS:
Instrumentation:
Chemical compositions of API 5CT N80 is given in
(Table 1). Then, the specimens were tempering at different temperature, heating
rates are 50°C/min for 2 h and subsequently were cooling in oven.
After these treatments, all specimens were polished
with wet abrasive paper with the grit size of 400, 600, 800, 1000 and 2000, respectively.
The working electrode was prepared from a
representative sample of API 5CT N80, it was 14 cm2 areas and then
specimens were put inside the support made by plastic.
X-ray diffraction (XRD) allows qualitative and
quantitative analysis of the material in the condensed state. The shape of the
diffraction peaks can be related to the crystalline microstructure. The
knowledge of the positions of the diffraction peaks and the intensities of the
diffracted beams allows the identification of the phases present, the
measurement of the residual stresses, the mesh parameter, the grain size and
the texture study.
The x-ray diffraction analyzes were carried out using the
PANalytical X’PERT Pro Philips diffractometer (λ = 1.54059 Ĺ). The
hardness profile was determined by using Vickers microhardness tester with a
load of 200 g.
The
experimental measurements used for electrochemical studies are Potentiostat-
galvanostat Type PGZ 301, connected to PC computer utilizing Volta Master-4
software.
The
electrochemical cell, we use a new Test bench consists of two coaxial
cylinders. The first cylinder is composed of a glass tank of 30 x 20 x 20 cm
dimension, provided with two holes for the introduction of the reference electrode
in saturated KCl calomel and platinum auxiliary electrode section 1 cm2.
The
second cylinder consists of a cylindrical tank 13 x11cm2 dimension
provided with an opening at the bottom which is placed in steel study
representing the working electrode.
Filled cylindrical vessel was heated using a heating
resistor controlled by a thermostat. The temperature reading is also done using
a digital thermometer.
Results
processing is done using a software voltamaster 4 allowing direct reading of
the corrosion current and the corrosion rate by the method of extrapolation of
Tafel straight, the scan rate used was 30 mV / min. Electrolyte used is water
untreated Albian recovered on site Haoud Berkaoui. (This system similar by well
production water) The chemical composition of water albian used in this study
is shown in (Table 2).
Table
1: The Chemical Composition of N80 Steel.
|
Elements %
|
Fe
|
C
|
Mn
|
Si
|
P
|
S
|
Ni
|
Cr
|
Mo
|
Cu
|
V
|
|
Composition
%
|
98.48
|
0.21
|
0.75
|
0.1
|
0.007
|
0.007
|
0.05
|
0.12
|
0.05
|
0.16
|
0.01
|
Table
2: Chemical composition of albien water
|
Elements %
|
Ca+2
|
Mg+2
|
Na+
|
K+
|
Cl-
|
SO4-2
|
HCO3-
|
NO3-
|
PH
|
|
Composition
(mg/l)
|
197
|
142.1
|
365
|
55
|
500
|
1107
|
00
|
1.3
|
7.79
|
RESULTS AND DISCUSSION:
Structural Properties:
X-ray diffraction is the important method for study of
the structure of N80 samples, before and after tempering temperature is
presented in (Fig. 1) As can be seen from this figure, main peaks were located
at 44, 64, 82, 99 and 116° correspond respectively to (110), (200), (211),
(220) and (310) peaks of ferrite phase (JCPDS card No. 00-006-0696), growth is
better after the first three peaks of tempering temperature for N80 steel.
Fig. 1: Spectra of X-ray
diffraction of tempering temperature for N80 steel.
However, we note that there are no metallic Fe, Mn,
Si, Ni, Mo, Cr or Cu peaks in N80 samples. No new phases have been detected in
the XRD pattern, are clearly revealed and indicate the formation of ferrite
phase.
In (Table 3) we can see 2θ values of the peaks
and structural parameters of N80 samples. To calculate the average grain sizes
of the samples, the Debye-Scherrer equation is used:
Where;
D is the crystallite diameter,
l is the X-ray wavelength used,
θ is the Bragg diffraction angle,
β is the full width at half-maximum (FWHM).
Table 3: Variation of structural parameters of
tempering temperature for N80 steel
|
Tempering temperature (°C)
|
2θ (°)
|
hkl
|
Interplanar spacing dhkl (Ĺ)
|
Grain sizes: D (Ĺ)
|
Strain ε (%)
|
|
untreated
|
44.759
|
(110)
|
2.02317
|
415
|
0.246
|
|
64.975
|
(200)
|
1.43413
|
211
|
0.324
|
|
82.327
|
(211)
|
1.17029
|
318
|
0.179
|
|
250
|
44.938
|
(110)
|
2.01553
|
296
|
0.333
|
|
65.149
|
(200)
|
1.43074
|
216
|
0.316
|
|
82.480
|
(211)
|
1.16851
|
287
|
0.196
|
|
300
|
44.844
|
(110)
|
2.01952
|
452
|
0.228
|
|
65.060
|
(200)
|
1.43248
|
279
|
0.249
|
|
82.399
|
(211)
|
1.16945
|
310
|
0.183
|
|
550
|
44.798
|
(110)
|
2.02151
|
485
|
0.214
|
|
65.016
|
(200)
|
1.43334
|
284
|
0.245
|
|
82.357
|
(211)
|
1.16994
|
348
|
0.164
|
We remarked that all samples the N80 are a nanometric
grain size, the values of grain sizes given in table 3 are found to be in the
range from 211 to 485 Ĺ. It is observed that the small one corresponds to the
N80 with (200) orientation (Fig. 2).
Fig. 2: The values of grain sizes with (110), (200) and
(211) orientation.
(Table 4) shows the different values of a lattice
parameters of tempering temperature for N80 steel compared with theoretical
values (2.8664 Ĺ). The samples shows greater a value compared to the bulk value
[JCPDS 00-006-0696], this mean that these samples have a strain.
Table 4: Evolution of the lattices parameters a and c,
Hardness and strain of tempering temperature for N80
|
Tempering temperature(°C)
|
Dmoy (Ĺ)
|
a=b=c (Ĺ)
|
Hardness (HV)
|
Strain εmoy (%)
|
|
untreated
|
293
|
2.86849
|
289.13
|
0
.250
|
|
250
|
307
|
2.86620
|
274.40
|
0,243
|
|
300
|
305
|
2.86710
|
275.41
|
0,206
|
|
550
|
334
|
2.86779
|
277.93
|
0,190
|
We measured the microhardness on the different tempering temperatures.
The results obtained are shown in (Table 4) and (Fig. 3).
From this figure, the hardness of API N80 steel
decreases from 289.13 to 274.40 HV with increasing the tempering temperatures
in the range of 250- 550°C. The hardness of the steel without tempering is the
highest and reaches 289.13 HV. And also the Strain decreased at high tempering temperature from 0 .250 % to 0,190
(%).
Fig.
3: The variation of the hardness and grain sizes a function of tempering
temperature for N80
It is observed that the grain size increase and the hardness
decrease when the tempering temperature increases (Fig. 3).
Differential Scanning Calorimetric Analysis (DSC):
There are several methods that make it possible to
follow the structural evolution of steel during a heat treatment, the
differential scanning calorimetry (DSC) is the most widely used thermal
analysis technique, it allows the measurement of enthalpy variations in a
material due to the evolution of its chemical and physical properties as a
function of temperature or time. On DSC curves, the formation of a phase is
represented by an exothermic peak, while its dissolution is represented by an
endothermic peak.
The results of the differential scanning calorimetric
analyses carried out on API N80 pipes with tempering temperature 250, 300 and
550°C are shown in (Fig. 4).
The thermal
cycle applied consists of heating from room temperature to 600°C, with a
holding time of 3 min at this temperature, followed by cooling to room
temperature; the heating and cooling rate used is 10 °C/min. The DSC curves
obtained are interpreted in the light of the Fe-C equilibrium diagram and show
the following effects:
· An endothermic peak between 200 to 400°C with which
can be attributed to the dissolution of cementite (Fe3C) or (Fe,Mn,Si.)3C
type substitutional carbides14,15.
· An exothermic peak observed at a temperature of 570°C
with a minimum not very clear (difficult to specify) which is due to the
pearlitic transformation, at the expense of the ferritic phase α (this
point corresponds on the iron-carbon equilibrium diagram to AC1)
Fig. 4: DSC curve of API N80 untreated and tempered at different
temperatures at V=10 °C/min: a) Untreated, b) 250 °C, c) 300 °C, d) 550 °C
Electrochemical Experiments:
The results of the electrochemical kinetic parameters
(i corr, E corr, ba and bc) obtained from Tafel plots for the N80 electrode in
Albian water for untraeted steel for different tempering temperature and
different gradient tempereature are shown in (Table 5).
Table
5: Corrosion paraméters at different Gradient temperature for different
tempering temperature of steel N80
|
Gradient Tempéreature (°C)
|
Tempering temperature (°C)
|
Ecorr
(mv)
|
ba
(mv/dec)
|
- bC
(mv/dec)
|
Icorr
(μAcm-2)
|
Corrosion rate
V (μm/y)
|
|
ΔT=
0
|
untreated
|
-676,3
|
74,6
|
104,6
|
5,6451
|
66,02
|
|
250
|
-544.3
|
39.9
|
52.3
|
2.8539
|
33.37
|
|
550
|
-582.6
|
43.1
|
103.2
|
3.1926
|
37.34
|
|
ΔT=
3
|
untreated
|
-691.8
|
57.9
|
87.1
|
9.1990
|
107.5
|
|
250
|
-699.1
|
39.6
|
43.6
|
6.0495
|
70.75
|
|
550
|
-707.4
|
54,0
|
76,4
|
9,3853
|
109,7
|
|
ΔT=
6
|
untreated
|
-674.7
|
74,7
|
111,6
|
11,2376
|
131,4
|
|
250
|
-679,2
|
42,6
|
59,3
|
7,3086
|
85,48
|
|
550
|
-688,5
|
74,3
|
118,2
|
12,9262
|
151,1
|
|
ΔT=
12
|
untreated
|
-684,2
|
80,5
|
112,2
|
12,7339
|
148,9
|
|
250
|
-682,0
|
50,1
|
66,4
|
10,0209
|
117,2
|
|
550
|
-680,7
|
76,1
|
120,1
|
16,1352
|
188,7
|
The (Table 5) shows that the corrosion rates for the
untreated steel were significantly higher than tempering temperature from
albian water. This is due to the aggressive nature of its ionic species on the
untreated steel and breakdown of the steel.
The effect of the Cl- ions seems more
deleterious than SO2-4 due to its relatively small size
and strong electronegativity thus accelerating the dissolution rate of the
steel. It must be noted that the presence of Cl- and SO2-4
ions accelerated the redox electrochemical mechanisms responsible for corrosion16.
Polarizations results for API N 80 specimens tested at
∆T= 0, 3, 6 and 12°C in (Fig. 5) shows a general decline in corrosion
rate values with increase in tempering temperature. But the steel tends to be
low resistant to corrosion with increase in gradient temperature, we can say,
the corrosion potential was not affected by variations in the gradient
temperature.
In the electrochemical tests performed at different
tempering temperature, all the studied specimens showed poor resistance and
pitting corrosion .The N80 steel tempered at 250 °C showed the best corrosion
resistance from all the gradient temperature, while the N80 steel tempered at
550°C presented the worst response in electrochemical tests.
Fig.
5: Polarization results for untreated and tempering temperature of different
gradient Temperature for N80 steel in albian water
At
Gradient temperature (ΔT=0°C), the potentiodynamic polarization curves in
(Fig.5). illustrate that the corrosion resistance of API N80 steel has been
significantly improved the corrosion potential increased from -676, 3mv to
-544, 3mv also the corrosion current density reduced from 5,6451μA/cm2
to 2,8539μA/cm2 but in Tempering temperature 550°C the
corrosion potential decreased to -582, 6mv and the corrosion current density
increased to 3,1926μA/cm2.
This
explains that ferrite grain size are increased from 293 Ĺ to 334 Ĺ at 250, 300,
550°C and slow down the corrosion process17,18.
CONCLUSION:
The structure (XRD), mechanical (hardness) and
chemical (corrosion) properties were investigated after tempering at 250, 300
and 550°C temperatures in mooring chain steel. The major results of this study
can be summarized as follows:
· All samples the API N80 are a nanometric grain size
(293 to 334 Ĺ) and crystallize in ferrite type structure with a strong (110)
orientation;
· It is observed that the grain size increase and the
hardness decrease when the tempering temperature increases;
· The density of residual stresses and hardness were
decreased greatly with increasing tempering temperature, which is the most
important factors to strengthen steel;
· The tempering temperature had a strong effect on the
microstructure and corrosion resistance of the mooring chain steel. With
elevating tempering temperature, the grain size was increased, also residual
stresses decreasing which indicated the active sites for corrosion attack
reduced and then the corrosion resistance of the tempered steels became better;
· The tempering temperature act as efficient corrosion
inhibitor for N80 steel in albian water solution, EIS measurements show that
decreases in the corrosion rate;
· Gradient Temperature played a significant role in the
corrosion rate of N80 steel in albian water environment. The corrosion rate
reached maximum value at 12 °C.
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