Sulfidation and Naphthenic Acid Corrosion of UNS

Sulfidation and Naphthenic Acid Corrosion of UNS S31603, UNS S31703 and UNS S44400 Series
Stainless Steels in Crude Processing
Gloria Isabel Duarte Poveda
ICP-ECOPETROL S.A.
Piedecuesta / Santander
Colombia
Edwin Alberto Morantes
ICP-ECOPETROL S.A.
Piedecuesta / Santander
Colombia
Nubia Esperanza Mejía Cajicá
Ambiocoop Ltda.
Bucaramanga / Santander
Colombia
Darío Yesid Peña Ballesteros
Universidad Industrial de Santander
Bucaramanga / Santander
Colombia
ABSTRACT
Materials selection has become one of the most important factors in the design and repair of refinery
processing equipment. Processing crude oil with high naphthenic acid (TAN) and high sulfur content
can lead to high corrosion rates in stainless steels at temperature approximately of 350 °C. This paper
presents the results of laboratory and field tests conducted to assess the corrosion resistance of
different materials used in refinery construction, especially stainless steels type 316L (UNS S31603),
type 317L (UNS S31703) and a new generation of 400 series stainless type 444 (UNS S44400). The
laboratory tests were carried in a dynamic autoclave using a rotation speed of 600 rpm. Field test
consisted of exposing corrosion coupons in the transfer line between furnace and atmospheric tower.
Results have shown that stainless type 444 is an alternative material at processing crudes with high
sulfur and naphthenic acid content.
INTRODUCTION
Due to the declining reserves of light crude and the increase of heavy oil reserves, the materials used
in crude processing are more susceptible to corrosion mechanisms such as naphthenic acid corrosion
and sulfidation, which has increased the need to explore new options in corrosion resistant materials. It
is known that heavy oils with a TAN greater than 0.3 to 0.5 mgKOH/g1 and refined crude oil fractions
with a TAN higher than 1.5 mgKOH/g2 have been considered to be potentially corrosive due to the
presence of naphthenic acids. Stainless steels containing molybdenum (Mo) are considered an
alternative in the materials selection resistant to these corrosive environments. Mo acts as stabilizer of
passive protective films formed on surface of stainless steel3. On the other hand, the presence of
chromium (Cr) in austenitic stainless steel such as UNS S31603 and UNS S31703 forms a passive film
of non-porous oxide (Cr2O3), protecting against naphthenic acid and sulfur compounds present in the
heavy crude. Another interesting alternative in the materials selection resistant to naphthenic corrosion
and sulfidation is the ferritic stainless steel UNS S44400, whose chemical composition includes Cr
(~18%) and Mo (< 2%), elements that promote and stabilizes the formation of passive film.
This work presents the evaluation of 300 and 400 series stainless steel, specifically of type 316L (UNS
S31603), 317L (UNS S31703) and 444 (UNS S44400), in gravimetric laboratory tests simulating
conditions closer to those found in crude processing units of Colombian refineries. The characterization
of the deposit layer formed on surface of stainless steels after exposure in the oil were performed by
scanning electron microscopy (SEM) and analysis by X-ray diffraction (XRD). To validate the results in
laboratory, coupons of these metallurgies were installed in the refinery facilities localized in the transfer
line between the furnace exit and entry of atmospheric tower. The results obtained show that although
stainless UNS S44400 presents corrosion rates higher than presented by stainless steels UNS S31603
and UNS S31703 in laboratory tests, the corrosion rates obtained in field tests are within required limits
in the materials resistant to naphthenic corrosion and sulfidation, being considered as an alternative
material at crudes processing units of refineries.
EXPERIMENTAL PROCEDURE
Laboratory Gravimetric Tests
The gravimetric laboratory tests were performed in a dynamic autoclave model Cortest made of
Hastelloy C276 (UNS N10276) with an inner volume of 5.7 L. The UNS S31603, UNS S31703 and
UNS S44400 stainless steel coupons installed in the autoclave according to Figure 1, were exposed to
heavy crude processed in Colombian refinery (GRB) at temperature of 350 °C, 200 psi (1.378 MPa) of
pressure and flow rate of 600 rpm (5 ft/s). The stainless steel coupons were cleaned, measured and
weighed according to standard ASTM G1-03 “Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens.”4
Figure 1: Assembling of coupons in the dynamic autoclave
Field Gravimetric Tests
The gravimetric tests were performed out in the facilities installed in the transfer line between the
furnace exit and entry of atmospheric tower of crude processing unit, Figure 2, this location is
considered highly susceptible to damage mechanisms by sulfidation and naphthenic corrosion.5,6 The
processing unit is located in Barrancabermeja Refinery (GRB) in Colombia.
Figure 2: Scheme of coupon location in transfer line of refinery
The coupons were exposed during a period time of 672 h (28 days). The temperature at the furnace
exit is in the range between 350 °C and 370 °C.
Characterization of Crude
The heavy crude sample processed in refinery used in the tests was characterized in terms of Total
Acid Number (TAN) determined by the ASTM D 664-11a7 in the equipment Mettler Toledo DL50
titrator; sulfur content determined by ASTM Method D 4294-108 in Horiba equipment SLFA-1800 and
2800 and API gravity analyzed by ASTM D 5002-(99) 20109 in density meter Anton Paar DMA at 48
and DMA 4500. Table 1 shows the results of different analysis performed on the sample.
Table 1
Analysis of heavy crude sample
SULFUR
g/100g
0.99
SAMPLE
Heavy oil
TOTAL ACID NUMBER
mg KOH/g
1.87
API GRAVITY
22.1
Chemical Composition of Materials
The determination of the chemical composition of materials to be evaluated, UNS S31603, UNS
S31703 and UNS S44400 stainless steel, was determined using technique of optical emission
spectrometry – OES in the equipment Shimadzu model OEF-5500II/6000, in agreement to technical
specifications of ASTM E 415-08 "Standard Test Method for Optical Emission Vacuum Spectrometric
Analysis of Carbon and Low-Alloy Steel."10 Table 2, presents the results of chemical composition of
different materials evaluated.
Table 2
Chemical composition the coupons of different materials
SPECIMENS
%C
%Cr
%Ni %Mn %Mo
%P
% Si
%S
%N
%Cu
%Nb %Fe
UNS S31603
0.03
<18
<14
2
<3
0.04
1
0.03
-
-
-
B
UNS S31703
0.08
<20
<14
2
<4
0.045 0.75 0.03
0.1
-
-
B
UNS S44400
0.003 18.19 0.24 0.19 1.83
0.05 0.53 0.008
0.25
0.023 0.197
B
The main difference between the composition of austenitic UNS S31603 and UNS S31703 and ferritic
UNS S44400 stainless steel is the content of Nickel (Ni) and Molybdenum (Mo). The content Chromium
(Cr) is relatively similar. On the other hand, type 444 (UNS S44400) stainless presents Niobium (Nb),
element not found in these austenitic steels.
Corrosion Rate
The corrosion rate in mpy was calculated using the method of mass loss expressed by Equation (1):
CR = (K * W) / (A * t * ρ)
(1)
Where:
K : 3.45x106
W : mass loss in g
A : area of the coupon in cm2
t : time of test in h
ρ : steel density in g/cm3
After of the exposure time, the coupons are submitted to a cleaning process, etching and weighed. The
corrosion rate obtained is directly related to mass loss by exposure of the material in the aggressive
environment. The cleaning procedure applied to different metallurgies was according to ASTM G1-03
“Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens” - Table A1.1 item C 7.5.1, where for stainless steels is used a fresh solution of nitric acid (HNO3), hydrofluoric acid
(HF) and water type 4, for 4 cycles of 5 min at room temperature.
Characterization of Surface After Gravimetric Tests
The stainless steel coupons after exposure in heavy crude were characterized by scanning electron
microscopy (SEM) model Leo 1450VP equipped with energy dispersive X-ray system (EDX), to identify
the elements present on the corrosion layer and infer what compounds formed and their relationship
with the corrosion rate. The crystalline phases were identified with a X-ray diffractometer model D8
Advance Bruker with CuKα1 radiation and nickel filter.
RESULTS AND DISCUSSION
Laboratory results
To determine the required exposition time of UNS S31603, UNS S31703 and UNS S44400 coupons in
the heavy oil sample for performing the autoclave tests, time periods of 12, 24, 48, 72 and 96 h were
tested. Corrosion rate varied with exposure time used in these tests but tended to stabilize after certain
exposure time. According to the corrosion rates obtained the metallurgies of 316L and 317L reached
stability between 48 and 72 h. In the case of stainless steel UNS S44400, this stability was found
between 72 and 96 h. The results of corrosion rates calculated for each one of the tests at different
times of evaluation are presented in Table 3.
As observed in the behavior of UNS S31603 and UNS S31703 there are minimal differences in the
corrosion rates at 48 and 72 h, possibly attributed to depleted naphthenic acids during this period time.
Similar analysis was determined in the performance of UNS S44400 steel between 72 and 96 h. Thus,
the exposure times used in gravimetric laboratory tests were 48 h and 72 h for metallurgies of 300 and
400 series, respectively.
Table 3
Corrosion rates of metallurgies evaluated at different times of exposure
Exposure time
(h)
12
24
UNS S31603
48
72
12
24
UNS S31703
48
72
48
UNS S44400
72
96
Metallurgy
Corrosion rate
mpy(mm/y)
6.56 (0.17)
1.35 (0.034)
1.14 (0.029)
1.02 (0.026)
5.49 (0.139)
1.19 (0.030)
1.02 (0.026)
0.83 (0.021)
21.79 (0.55)
9.58 (0.24)
8.66 (0.22)
Figure 3, shows the comparison of corrosion rates in function of exposure times of metallurgies
evaluated.
Corrosion Rate (mpy)
25
20
15
10
5
0
12
24
48
72
96
Time of Exposure (h)
UNS S44400
UNS S31603
UNS S31703
Figure 3: Comparison of corrosion rates of UNS S31603, UNS S31703 and UNS S44400 metallurgies
in different times of exposure
UNS S31603 Coupon (Exposure: 48 h)
The results of SEM-EDX and DRX performed on the deposit layer in surface of UNS S31603 coupon
after exposition in crude with TAN of 1.87 mg KOH/g and 0.99 % S are presented in Figure 4. As
observed in Figure 4(a), the layer formed on surface by the reaction between the crude components
and alloying elements of UNS S31603 steel during exposure time presents a dense morphology and
homogenous layer with some particles adhered on the deposit. Per Figure 4(b), this deposit layer is
composed mainly of iron, sulfur and chromium, with minor proportion of carbon, silicon, manganese
and nickel. The carbon and silicon peaks detected are probably due to traces of hydrocarbon adhered
as particles in the surface of deposit (dark areas). The light areas corresponding to compounds formed
by sulfur, iron, chromium and nickel, as can be seen in Figure 5.
(a)
(b)
Figure 4: SEM image of UNS S31603 coupon (a) micrograph of surface (b) EDX of deposit layer
(a)
(b)
Figure 5: SEM image of UNS S31603 coupon 2000X (a) micrograph of surface (b) EDX of detailed
area
Crystalline phases indexed by DRX in this deposit are presented in Figure 6.
Figure 6: Diffractogram of surface layer in UNS S31603 coupon
As expected and in accordance with elements identified by SEM-EDX analyses, during the exposure
time of UNS S31603 coupon in heavy oil at 350 ºC are formed films on steel surface composed mainly
by iron sulfides and nickel sulfate (FeS, NiSO4), which probably leads to corrosion rates close to 1 mpy
(0.025 mm/y) acting as protective films against naphthenic corrosion and sulfidation.
UNS S31703 Coupon (Exposure: 48 h)
The morphology of the layer formed on surface of UNS S31703 coupon exposed in the heavy oil at
temperature of 350 ºC is showed in Figure 7(a). This is a dense and homogeneous layer with some
dark areas. The EDX observed in Figure 7(b) reveals the presence of elements such as iron, sulfur,
chromium and carbon. The carbon peak can be derived of traces of hydrocarbon in the layer formed.
(a)
(b)
Figure 7. SEM image of UNS S31703 coupon (a) micrograph of surface (b) EDX of deposit layer
To identify the elements in the dark areas was realized an EDX on image of SEM at magnification of
2000X, the result is shown in Figure 8.
(a)
(b)
Figure 8: SEM image of UNS S31703 coupon 2000X (a) micrograph of surface (b) EDX of detailed
area
As shown, the dark areas are indexed primarily as silicon peak possibly derived of traces of
hydrocarbon on surface of layer formed. The crystalline phases identified in this layer are iron sulfide
and nickel sulfate (FeS, NiSO4), Figure 9. According to Figure 7(b), the film formed on surface of UNS
S31703 steel could have chromium oxide, which is not identified in the diffractogram possibly because
of minor proportion of this oxide in the film.
Figure 9: Diffractogram of surface layer in UNS S31703 coupon
The corrosion rate in the range of 0.8 to 1 mpy (0.02-0.025 mm/y), can be attributed to protective film
formed on surface of stainless steel UNS S31703 as consequence of reaction between the elements of
steel and hydrocarbon at 350 ºC identified as iron sulfide and nickel sulfate in the similar way of
behavior presented in the stainless steel UNS S31603.
UNS S44400 Coupon (Exposure: 72 h)
The surface on the UNS S44400 coupon after gravimetric laboratory test presents a morphology
heterogeneously distributed. The elemental analysis by EDX identified the presence of carbon, silicon,
sulfur, iron and chromium, Figure 10.
(a)
(b)
Figure 10: SEM image of UNS S44400 coupon (a) micrograph of surface (b) EDX of deposit layer
An EDX more detailed was realized to identify the elemental composition of dark and light areas in the
surface of stainless steel UNS S44400. As observed in Figure 11, the light zone is composed mainly by
iron and chromium, and the dark zone by carbon and silicon.
(a)
(b)
Figure 11: SEM image of UNS S44400 coupon 2000X (a) micrograph of surface (b) EDX of detailed
areas
The diffraction pattern of Figure 12 reveals the presence marked of iron sulfides. The chromium initially
observed in EDX of Figure 10, is not identified as chromium oxide in the DRX possibly due to minor
proportion in the film formed on surface of UNS S44400 coupon.
Figure 12: Diffractogram of surface layer in UNS S44400 coupon
In this study, the corrosion rates in the UNS S44400 stainless steel evaluated at 350 ºC in heavy oil
with TAN of 1.83 mgKOH/g and 0.99 % S were in the range between 8 and 9 mpy (0.20 and 0.23
mm/y). This behavior could be attributed to less content in Ni (0.24%) and Mo (1.83%) in comparison
with higher content of these elements in UNS S31603 and UNS S31703 stainless steels (<14% Ni and
<2-4% Mo). As known, Mo stabilizes the passive protective film against organic acids like naphthenic,
and probably in this case due to minor concentration of Mo, the protective film of iron sulfide is not
stable enough to resist naphthenic corrosion and sulfidation leading to higher corrosion rates.
Field results
The corrosion rates obtained in field gravimetric test of UNS S31603, UNS S31703 and UNS S44400
coupons exposed during 672 h in heavy crude processed at temperature between 350 ºC and 370 ºC
with TAN between 1.8 and 1.9 mgKOH/g and 0.99 %S, are presented in Table 4. As observed, the
lower corrosion rates were calculated to UNS S31703 coupons with values between 0.28 and 0.37 mpy
(0.007 and 0.009 mm/y), followed by corrosion rates in the UNS S31603 coupons between 0.33 and
0.74 mpy (0.008 and 0.018 mm/y). In the case of UNS S44400 coupons the corrosion rates were
between 1.12 and 2 mpy (0.028 and 0.05 mm/y). The higher corrosion rates in UNS S44400 as
mentioned above are possible consequence of minor content of Mo which acts as stabilizer of
protective film.
Table 4
Corrosion rates of metallurgies evaluated at field tests during 672 h
Metallurgy
UNS S31603
UNS S31703
UNS S44400
Corrosion rate
mpy (mm/y)
0.56 (0.014)
0.74 (0.018)
0.33 (0.008)
0.28 (0.007)
0.30 (0.007)
0.37 (0.009)
1.12 (0.028)
1.40 (0.035)
2.02 (0.051)
For UNS S31603 and UNS S31703, although the corrosion rates obtained in field tests were lower than
those obtained in laboratory tests, they were close 0.83-1.35 mpy (0.021-0.034 mm/y) as compared
with 0.28-0.74 mpy (0.007-0.018 mm/y). In contrast, corrosion rates observed in UNS S44400 coupons
in field tests were significant lower than those obtained in laboratory test, this probably due to longer
exposure time which could influence the formation of a more dense and compact protective film of iron
sulfide, that would improve its stability and resistance to naphthenic acids.
Although the corrosion rates in UNS S44400 obtained in this study are higher compared to metallurgies
of UNS S31603 and UNS S31703, the values, 1.12 to 2 mpy (0.028 and 0.05 mm/y) are within
acceptable limits to control of naphthenic corrosion and sulfidation and could be considered as an
alternative material in crude units processing this heavy crude oil TAN between 1.8 and 1.9 mgKOH/g
and 0.99% S.
CONCLUSIONS
Exposure time in these weight loss corrosion coupons significantly affects the results and this was
confirmed with the tests conducted in these different types of stainless steels. Corrosivity studies based
on these laboratory tests to evaluate corrosion by naphthenic acids, sulfur, or both should therefore
ensure that the time used corresponds to stable corrosion rate conditions.
The films formed on the surfaces of UNS S31603 and UNS S31703 stainless steels in 48 h of exposure
in the heavy oil with TAN of 1.83 and 0.99 % S at 350 ºC are mainly iron sulfides and nickel sulfate,
which can be characterized as stable protective layer, leading to lower corrosion rates (< 1 mpy (0.025
mm/y)).
According to observed laboratory tests, although the ferritic stainless steel UNS S44400 in presence of
heavy oil at 350 ºC also produces protective film of iron sulfide, it is possible that this film is
continuously removed in the presence of naphthenic acids, leading to higher corrosion rates in
comparison with the stainless steel UNS S31603 and UNS S31703. On the other hand the field tests
demonstrated that corrosion rates in metallurgy UNS S44400 with longer exposure times are in the
range of 1 to 2 mpy (0.0254-0.05 mm/y) and can be considered as alternative material resistant to
naphthenic corrosion and sulfidation in crude processing units.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the support of Laboratory Electron Microscopy, Laboratory of Xray Diffraction and Laboratory of Materials and Corrosion of Colombian Petroleum Institute, and also to
Ecopetrol refinery (GRB) for providing the facilities to evaluate different metallurgies in crude
processing unit.
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