Applied Mechanics and Materials
ISSN: 1662-7482, Vol. 252, pp 271-275
doi:10.4028/www.scientific.net/AMM.252.271
© 2013 Trans Tech Publications, Switzerland
Online: 2012-12-13
Study on corrosion behavior of the 304 stainless steel in the heavy oil
with high salt, high sulfur and high acid value1
Xiaohong WANG1,a, Yu WEI2,b, Chunyun SHAO2,c, Yijun SHI1,d, Wei Xue2,e,
Jianfeng ZHU2,f
1
College of Material Science and Engineering, Southwest petroleum University, Chengdu,
610500,China
2
Xinjiang design branch, China petroleum engineering & construction corp, Ürümqi,830019,China
a
[email protected], b [email protected], [email protected],
d
[email protected], [email protected], [email protected]
1
Correspondence author: Yijun SHI,E-mail: [email protected]
Keywords: 304; high acid value; high sulfu; high salt; heavy oil; corrosion
Abstract:The uniform corrosion and localized corrosion of 304 stainless steel in heavy oil with
high salt, high sulfur and high acid value were researched by weight loss method and
electrochemical method. The corrosion morphology of samples with and without corrosion product
films and the compositions of the corrosion product films were observed using SEM and EDS. The
results show that the 304 stainless steel is inapplicable to the refinery equipment of this heavy oil
because of the severe local corrosion even if the uniform corrosion rate is as low as 0.0107mm/a.
The salt corrosion and sulfur corrosion occur on the local surface of 304 stainless steel because of
the chloride ion formed by hydrolysis of salt in the small emulsifying water. The water soluble iron
naphthenate produced by chemical reaction between naphthenic and ferrous sulfides impel the local
corrosion of the 304 stainless steel.
Introduction
High-sulfur, high acid heavy oil has paid more and more attention for its price advantage and
more than 55% of the proportion of the world crude oil production[1,2]. By the end of 2010, China
Petroleum & Chemical Corporation processing high-sulfur, high-acid crude oil has reached 50% of
the proportion[3]. High-sulfur, high-acid heavy crude oil often contains more salt which were ape to
have severe corrosion of sulfur, naphthenic acid corrosion, salt corrosion. The serious corrosion
problems about equipment and pipelines brought by the metal sulfide corrosion because it’s rate
was 1 to 2 order of magnitude higher than that of oxidation corrosion at high temperature[4].
Reasonable material using scientific methods of corrosion resistance evaluation for the original and
newly built refining equipment was a decisive and was also an important factor for processing of
high-sulfur, high-acid crude oil[3]. At 2010, the standard of “sour crude processing unit and piping
design selection guidelines” and "high-acid crude oil processing unit equipment and piping design
selection guidelines” were revised based on the material application experience of oil refining
enterprise at home and abroad and latest achievements of API and NACE on processing of
high-sulfur, high-acid crude oil, but until now there was no uniform standard to follow about
reasonable material of refining equipment which used to process the crude oil of higher sulfur
content and higher acid value.
The 304 stainless steel had a wide range of applications in the low-sulfur, high-sulfur crude oil
refining equipment and piping for it’s good mechanical properties, processing properties and
corrosion resistance, and was considered a suitable material to prevent high-temperature sulfur
corrosion[5,6,7]. In this paper, in order to provide experimental and theoretical basis for reasonable
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Advanced Research on Applied Mechanics and Manufacturing System
material of refining equipment, the corrosion of 304 stainless steel was evaluated by the weight loss
method on the higher sulfur content and higher acid value heavy oil and electrochemical methods
on the aqueous solution which content of corrosive component agree with the given oil.
Experimental Method
The corrosion behavior of 304 stainless in the heavy oil which main corrosive ingredients as shown
in Table 1 was studied by autoclave. The chemical composition (wt%) of 304 stainless steel was: Cr
17.02, Ni 8.00, Si 1.14, Mn 0.96, Al 0.40, C 0.07, S≤0.03, P≤0.045, allowance of Fe which were
provided by an oilfield. The 304 stainless steel were machined metallic sheets of size 30 mm × 10
mm × 3 mm which were polished with 400, 600 and 1000 grit SiC paper to ensure the same surface
roughness, ultrasonically cleaned in acetone, rinsed by clean water, removed water by anhydrous
ethanol and then kept in desiccator for use. However totally 6 sheet specimens were putted in the
autoclave, 3 of them for the weight loss experiment to calculate the average corrosion rate, 1 of
them for the SEM, 1 of them for electrochemical experiment and 1 of them for reserve. Prior to
testing, the reactor was sealed, purged with nitrogen, and pressurized to 1.5 MPa. During testing,
the hesvy oil in the autoclave was maintained at a constant temperature of 150±5℃, using a
thermocuple in a thermowell and the heavy oil was stirred using a stirrer rotating at 100rpm to
ensure the uniformity of corrosion medium. The corrosion cycle was 96h in our experiments.
water content
/(%)
0.69
Table 1 The main corrosive elements in crude oil
NaCl/(mg/L)
NaS /(ppm)
CH3OOH (mgKOH/g
Crude Oil)
69.139
19900
1.62
The polariztiong curves of the film sample and original sample were measured by PGSTAT302
electrochemical workstation. The film sample and original sample which were sealed with epoxy
(only stay out of 1cm2 work area) were used as the working electrode, platinum electrode and
calomel electrode were used as auxiliary electrode and reference electrode respectively. The
working medium was the aqueous solution which was configured according to the corrosive
component content proportion showed as Table 1.
Experimental results and analysis
Uniform corrosion rate of 304 stainless steel.The uniform corrosion rate of 304 stainless steel in
this given crude oil was 0.0107 mm/a in accordance with the formula (1). The 304 stainless steel
was a Class III (4) based on the alloy corrosion resistance 10 standard[8], which was considered to a
corrosion resistant alloy without considering the localized corrosion in the given environment.
V = g ∙ 8.76/(ρ ∙ t ∙ S)
（1）
Where g was sample weightlessness; ρ was the density of the material, value was 7.93g/cm3 ; t was
the test time, unit h; S was the surface aresa of sample, units m2; V is average corrosion rate, unit
mm / a.
Localized corrosion of the 304 stainless steel. Figure 1 was the surface morphology of the 304
stainless steel after removing the corrosion product film. From figure 1, there were many "pitting"
localized corrosion on the surface of the sample (Figure 1a) and some irregular corrosion crack
which extended from the "pitting" to the matrix (Figure 1b). The potential difference between the
stainless steel to the solution was constituted by the potential difference between the lateral to the
medial of the product scale and the electric double layer potential difference between the lateral of
the product scale to the solution according to passivated metal corrosion theory by Chunan
Applied Mechanics and Materials Vol. 252
273
Cao[9,10]. Trace amounts of water was existed as oil-in-water emulsion form in the high viscosity
crude oil. The chloride ion in the crude oil was dissolved in the emulsified water. Some of the
chloride ion, which was adsorded to the surface of passive film, promoted the local dissolution of
the passive film until the solution direct contacted the local metal that was active, and happened
anodic dissolution reaction. Figure 2 was a polarization curve of the original sample and the etched
sample with corrosion product film in the aqueous simulation. Table 2 showed the analytical results
of the polarization curve. From the figure 1, the figure 2 and the table 2, the corrosion potential of
the area covered by the corrosion product film was about 0.17 V higher than that of active matrix
metal and the corrosion current was about 3 order of magnitude lower than that of the matrix metal
in the activated state. Moreover, the area of the active matrix metal was very small because the
emulsion water content of heavy oil was less and less, so there were typical large cathode and small
anode effect that the local corrosion behavior of the active matrix metal was accelerated in the
aqueous simulation.
Fig.1 SEM images for corrosion morphology
Whitout corrosion product scale on
304 stainless steel’s surface
Fig.2 Tafel polarization curves of original of
and corroded 304 stainless steel with
corrosionproduct scale in simulated medium
Table.2 Eigenvalue of the polarization curve for
304 stainless steel before and afer covering
Icorr/[A•cm-2]
Ecorr/[V]
304 before covering
1.7×10-3
-0.82
304 after covering
6.6×10-6
-0.65
Fig.3 SEM images for corrosion morphology
of 304 and the distribution of the points of EDS
Corrosion morphology and corrosion mechanism of the 304 stainless steel.Figure 3 was the
morphology of the 304 stainless steel which was corroded in the given heavy oil. The corrosion
product film was evenly covered on the specimen surface (Figure 3b A), but tthere were some pits
(Figure 3b C) and some ulcer spots (Figure 3b, B). The micro-areas composition of A, B and C was
analyzed by EDS, and the results was shown in Table 3.
Table 3 The EDS results of 304 stainless steel / [%]
Fe
C
O
Si
Cr
Mn
Ni
Ca
S
A
68.55 4.90
0.54 17.51 0.97 7.53
B
60.74 8.81 3.65 7.34 9.67
2.62 1.13 3.94
C
12.82 13.35 36.50 19.69 2.87
0.76
Al
Mg
Na
2.10
1.49 6.46 0.34 5.71
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Advanced Research on Applied Mechanics and Manufacturing System
From figure 3 and table 3 shown that there wasn’t elemental sulfur, oxygen, magnesium, sodium at
the A area, but there were elemental carbon, sulfur, oxygen, sodium at the B area and the elemental
at the C area increased magnesium on base of the B area. So, from the global aspect, the 304
stainless steel had better salt resistance, sulfur resistance and acid resistance in the high-salt,
high-sulfur, high acid value of heavy oil when temperature was 150 ° C and pressure was 1.5MPa,
but, at the same time, there were seviour local anodic dissolution. The chloride ions produced by
salt hydrolysis (reaction equation 2-4) in the emulsified micro-droplets would first destroied the
local passivation layer, which made the reaction happened between the active metallic to the
hydrochloride (reaction equation 5) and the sulfur (reaction equation 6-7) in the emulsified
micro-droplets. The water soluble iron naphthenate produced by chemical reaction between
naphthenic and ferrous sulfides with the flow of the heavy oil which further impeled the local
corrosion of the 304 stainless steel and produced the pit and ulcer spots, finally (reaction equation
8).
CaCl + H O → Ca(OH) + 2HCl
（2）
MgCl + H O → Mg(OH) + 2HCl
（3）
2NaCl + H O → NaOH + 2HCl
（4）
2HCl + Fe → FeCl + H
（5）
FeCl + H S → FeS + 2HCl
（6）
Fe + H S → FeS + H
（7）
（8）
FeS + 2RCOOH → Fe(RCOO) + H S
The contents of element Cr and Ni were significantly reduced.at the area B and C of Figure 3b
which shown the reaction happened between the element chromium to the gaseous water which
changed from the emulsion micro water droplets when temperature reached 150 ° C and the Cr2O3
which had dense microstructure had generated (reaction equation 9) [12]. The boiling point of H2S
was subzero 60.2℃, so, the H2S would evaporated too when the heavy oil temperature reached 150
° C. The gaseous H2S would reacted with element Cr and Ni and generated the CrS and NiS
(reaction equation 10-11), so the contents of element Cr and Ni at the area B and C of Figure 3b
significantly reduced.
2Cr( ) + 2H O(
)
↔ Cr O + 3H
2Cr( ) + H S( ) ↔ CrS( ) + H
Ni( ) + H S( ) ↔ NiS( ) + H
( ) ∆G
( ) ∆G
( ) ∆G
°
°
°
= −298500(J/mol)
notavailable
= −18600(J/mol)
（9）
（10）
（11）
In summary, the corrosion mechanism of 304 stainless steel in the high-salt, high-sulfur and high
acid value heavy oil was that the emulsified micro-droplets included large soluble salt and the
chloride ions was formed by hydrolysis of soluble salt which would destroied the local passivation
film. The corrosion potential of the small metal without passivation film decreased significantly.
The salt corrosion and ulfur Corrosion increased because the effect of large cathode and small
anode. The naphthenic acid reacted with ferrous sulfide and the water-soluble iron naphthenate was
formed which would decreased the protective effect of ferrous sulfide on the matrix and promoted
the localized corrosion.
Applied Mechanics and Materials Vol. 252
275
Conclusion
(1) Despite the uniform corrosion rate of 304 stainless steel is low in a given high-salt, high-sulfur,
high-acid value heavy oil, but there were significant localized corrosion phenomenon. It is a
inappropriate material for this crude oil refining equipment.
(2) The localized corrosion mechanism of 304 stainless steel in the given crude is that: the chloride
ions produced by the salt in the emulsified micro-droplets destroy the passivation layer firstly, the
water soluble iron naphthenate produced by chemical reaction between naphthenic and ferrous
sulfides further impel the local corrosion of the 304 stainless steel. This cycle reaction exacerbated
the localized corrosion.
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