metals

Article
Investigation on Multiphase Erosion–Corrosion of Elbow in
LPG Desulfurization Unit
Yan Li 1 , Jianwen Zhang 1, *, Guoqing Su 1 , Abdul Sandy 2, * and Yanan Xin 3

1 College of Mechanical and Electrical Engineering, Beijing University of Chemical Technology,

Beijing 100029, China
2 College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China
3 Chengdu Advanced Metal Materials Industry Technology Research Institute Co., Ltd.,
Chengdu 610000, China
* Correspondence: zhangjw@mail.buct.edu.cn (J.Z.); rhazac@yahoo.com (A.S.)

Abstract: Severe leakages of the elbow occur in the regeneration tower return pipeline of the LPG
desulfurization unit, leading to the unplanned shutdown of the unit frequently over the period of
four months. It is forced to apply additional steel plates to prevent the leakage. Elusively, it is found
that the first wall contact with the fluid is fully eroded away in the vicinity of the bend, however, the
walls of additional steel plates are intact. The clarification of this problem is required to ensure safe
production. This strange phenomenon can be investigated by failure analysis and computational
fluid dynamics (CFD) simulation. The failure analysis showed that the gas–liquid two-phase erosion–
corrosion was the main cause of elbow leakage. The simulation shows that droplet erosion plays a
dominant role in the erosion–corrosion process, and the elbow will leak in 4.3 months, which matches
the actual situation very well. Furthermore, multiphase erosion–corrosion behavior was thoroughly
investigated to expose the feature of the mentioned strange phenomenon. It was shown that when
the corrosion holes are formed, the gas forms a fluid vortex in the holes. The vortex acts as flexible
substrates, which plays a buffer layer to the droplet erosion, thus protecting the additional steel plate.
The formation of the holes provides an effective way for elbow failure prevention.

Keywords: LPG desulfurization unit; erosion–corrosion; CFD

Citation: Li, Y.; Zhang, J.; Su, G.;
Sandy, A.; Xin, Y. Investigation on
Multiphase Erosion–Corrosion of
Elbow in LPG Desulfurization Unit. 1. Introduction
Metals 2023, 13, 256. https://doi.org/ Liquefied petroleum gas (LPG) is mainly composed of hydrocarbons and has attracted
10.3390/met13020256 increasing attention as a primary chemical raw material and a fuel. Sulfur compounds,
Academic Editor: Sundeep
especially hydrogen sulfide (H2 S), are highly toxic and corrosive substances and are widely
Mukherjee present in LPG [1]. An LPG desulfurization unit is mainly used to remove the sulfur
compounds from LPG by methyl diethanolamine (MDEA) [2,3]. The LPG desulfurization
Received: 29 December 2022 unit consists of an absorption part and a regeneration part [4]. The poor amine solution
Revised: 22 January 2023
absorbs hydrogen sulfide first, and then turns into the rich amine solution, and finally
Accepted: 24 January 2023
desorbs hydrogen sulfide and is regenerated to the poor amine solution. In the process,
Published: 28 January 2023
the reboiler heats the poor amine solution discharged from the bottom of the regeneration
tower to resolve the H2 S, which makes the internal components of the reboiler change
from the liquid phase flow to gas–liquid two-phase flow, thus making the reboiler and the
Copyright: © 2023 by the authors.
attached pipelines suffer serious erosion–corrosion [4–6]. The elbow at the reboiler outlet is
Licensee MDPI, Basel, Switzerland. one of the parts most prone to erosion–corrosion damage in the LPG desulfurization unit.
This article is an open access article Erosion–corrosion is a phenomenon of metal damage caused by mechanical and elec-
distributed under the terms and trochemical action [7]. The erosion–corrosion phenomenon widely exists in the oil and gas
conditions of the Creative Commons chemical industry and results in substantial economic losses and potential safety hazards
Attribution (CC BY) license (https:// because of the harsh work environments [8–10]. In a wet hydrogen sulfide environment,
creativecommons.org/licenses/by/ hydrogen sulfide reacts with carbon steel to form a passivation film, which can protect
4.0/).

Metals 2023, 13, 256. https://doi.org/10.3390/met13020256 https://www.mdpi.com/journal/metals

Metals 2023, 13, x FOR PEER REVIEW 2 of 21

Metals 2023, 13, 256 2 of 19

hydrogen sulfide reacts with carbon steel to form a passivation film, which can protect
metal materials
metal fromfrom
materials corrosion. The The
corrosion. mechanism
mechanism is similar
is similarto that of aofstainless-steel
to that a stainless-steel pas-passi-
sivation
vationfilm [10].
film However,
[10]. However, the the
droplets erode
droplets erodethe the
passivation
passivation film,film,
exposing
exposingthe material
the material
beneath the passivation film to the corrosive medium.
beneath the passivation film to the corrosive medium. The combined effectThe combined effect of erosion and and
of erosion
corrosion eventually leads to material
corrosion eventually leads to material failure. failure.
At present,
At present,the research
the researchon multiphase
on multiphase flowflow erosion–corrosion
erosion–corrosion of theof elbow focuses
the elbow focuses
on the erosion–corrosion containing solid particles, mainly including
on the erosion–corrosion containing solid particles, mainly including gas–solid two-phase gas–solid two-phase
flowflow
erosion–corrosion
erosion–corrosion [11–15], liquid–solid
[11–15], liquid–solid two-phase
two-phaseflow flowerosion–corrosion
erosion–corrosion [16–19],
[16–19], and
and gas–liquid–solid
gas–liquid–solid multiphase
multiphaseflow flowerosion–corrosion
erosion–corrosion [20,21].
[20,21]. The The erosion–corrosion
erosion–corrosion mecha-
mechanism
nism of of different
different materials
materials under
under different
different working
working conditions
conditions has has
beenbeen explored
explored through
through experiments
experiments andcomputational
and the the computational fluid fluid
dynamics dynamics(CFD)(CFD)
method.method.
CurrentCurrent
dropletdrop-
erosion–
let erosion–corrosion
corrosion research research concentrates
concentrates on droplet
on liquid liquid droplet impingement
impingement (LDI) [22–25].
(LDI) [22–25]. When the
When the droplets
droplets hit the hit thewall,
solid solid wall, momentum
momentum is transferred
is transferred to the material’s
to the material’s surface and surface
the stress
and the
duestress due toexceeds
to collision collisionthe exceeds the material’s
material’s yield stress, yield stress, resulting
resulting in material indeformation
material de- [26].
formation [26]. LDI
LDI focuses focuses
more on the more on the mechanics
mechanics of droplet
of droplet erosion on erosion
materials onwithout
materials with-
considering
out considering the corrosion reaction between the droplets
the corrosion reaction between the droplets and materials. There have been few and materials. There havereports
beenonfew reports
droplet on droplet erosion–corrosion.
erosion–corrosion. Compared toCompared to the LDI,
the LDI, droplet erosiondroplet erosion
can cause can to
damage
causematerials
damageat torelatively
materialslow at relatively low speeds
speeds because because
the yield stressthe
onyield stress onsurface
the material the material
is reduced
surface
when is reduced
corrosion when corrosion occurs.
occurs.
The elbow
The elbowin theinregeneration
the regeneration tower return
tower line of
return linetheofLPG desulfurization
the LPG desulfurization unit unit
leaksleaks
frequently
frequentlyoverover
the period
the periodof four months,
of four which
months, leadsleads
which to unplanned
to unplanned shutdown,
shutdown,as shown
as shown
in Figure 1. To guarantee production, the leakage is repaired
in Figure 1. To guarantee production, the leakage is repaired by an additional steel plate, by an additional steel plate, as
shown
as shown inin Figure
Figure 2. 2.
AA strange
strange phenomenon
phenomenon is is found
found where
where thethe outer
outer wallwall
of of
thethe elbow is
elbow
entirely
is entirely eroded,
eroded, however,
however, thethe additional
additional plates
plates are
are intact,
intact, asas shown
shown ininFigure
Figure3.3.

(a) (b)
Figure 1. Leakage
Figure occurring
1. Leakage in theinelbow:
occurring (a) outside
the elbow: view;view;
(a) outside (b) inside view.view.
(b) inside

To investigate the strange phenomenon of the elbow, failure analysis was performed
first to investigate the main reason for the leakage based on the process data, the morphol-
ogy of the elbow, and the corrosion product composition analysis. CFD simulation was also
performed to characterize the flow pattern within the elbow. The erosion–corrosion rate
and the ratio of the erosion rate and corrosion rate can be obtained by the mathematical
model. Based on the erosion–corrosion rate, the erosion–corrosion map was established
to explore the mechanism of erosion–corrosion on the elbow. This paper focused on the
following two issues:
(1) The main reason for the leakage in the elbow;
(2) The reason why there is no obvious corrosion–erosion on the additional steel plate.
MetalsMetals 2023,
2023, 13, 13, 256
x FOR PEER REVIEW 3 of 213 of 19
Metals 2023, 13, x FOR PEER REVIEW 3 of 21

(a) (b)
(a) (b)
Figure 2. The2.elbow
Figure The beforebefore
elbow and after
and repair:
after (a) elbow;
repair: (a) (b) elbow
elbow; (b) with additional
elbow with plates.
additional plates.
Figure 2. The elbow before and after repair: (a) elbow; (b) elbow with additional plates.

(a) (b)
(a) (b)
Figure 3. The corrosion profile of the elbow: (a) inside wall of the elbow; (b) outside wall of the
Figure
Figure 3.
3. The corrosion
corrosionprofile
profileofofthe
the elbow:
elbow: (a)(a) inside
inside wall
wall of elbow;
of the the elbow; (b) outside
(b) outside wall
wall of the of the
elbow.
elbow.
elbow.
2. Method
To
2.1.investigate
Failure the strange
Analysis
To investigate phenomenon
the strange phenomenon of theofelbow, failure
the elbow, analysis
failure was was
analysis performed
performed
first first
to investigate
toProcess
2.1.1. the
investigate main reason
the main
Flowsheet for the
reason forleakage based
the leakage on the
based onprocess data,data,
the process the morphol-
the morphol-
ogy ogy
of the elbow,
ofThe and and the corrosion
thedesulfurization
elbow, the process
corrosionproduct composition
product composition analysis. CFDCFD
analysis. simulation was
simulation
uses the desulfurizing agent MDEA, the purifiedwasgas
also also
performed
performedto characterize
to the
characterize the flow pattern
the flow within
pattern the
within elbow.
thefuel The
elbow. erosion–corrosion
discharged from gas absorption tower is sent to the gasThe erosion–corrosion
pipeline network, the
rate rate
and and
the ratioratio
of the oferosion rate rate
and and
corrosion rate rate
can be obtained by the
bymathemat-
purified the
gas discharged the from
erosionthe liquid corrosion
absorption towercan be obtained
is sent to the liquefied thegas
mathemat-
recovery
ical model.
ical Based on the erosion–corrosion rate, the erosion–corrosion map was estab-
unit,model.
the richBased
amineonliquidthe erosion–corrosion
discharged from the rate,
twothe erosion–corrosion
absorption map was
towers is gathered estab-
and sent
lished to explore
lished the mechanism
to explore thetower,
mechanism of erosion–corrosion
of erosion–corrosion on theon elbow. ThisThis
theiselbow. paper focused
to the regeneration the regenerated poor amine liquid sent back topaper focused
the absorption
on theon following two two
the following issues:issues:
tower, and the desorbed H2 S is sent to the sulfur recovery unit. The desulfurization process
(1) The main
flowchart reason
and for
failure the leakage
position
(1) The main reason for the leakage in shown
are theinelbow;
theinelbow;
Figure 4.
(2) The reason
(2) The why
Thereason
material there
why is
of there no
the elbowobvious
is no was corrosion–erosion
20# steel,
obvious the diameter
corrosion–erosion on theonadditional
was steel
600additional
the mm, the plate.
wall
steelthickness
plate.
was 10 mm, and the length-to-diameter ratio of the elbow was 1.5. The material of the addi-
2. Method
tional
2. Method steel plates was 20# steel, and the thickness was 10 mm. The chemical composition
steel is listed in Table 1. The working temperature at the elbow is 120 ◦ C, and the
of 20# Analysis
2.1. Failure
2.1. Failure Analysis
pressure
2.1.1.2.1.1.
Process is 0.05 MPa. The composition of the fluid in the elbow is shown in Table 2.
Flowsheet
Process Flowsheet
The desulfurization
The desulfurization process usesuses
process the desulfurizing
the desulfurizing agent MDEA,
agent MDEA,the purified
the purifiedgas dis-
gas dis-
charged
charged from the gas absorption tower is sent to the fuel gas pipeline network, puri-
from the gas absorption tower is sent to the fuel gas pipeline network, the the puri-
fied fied
gas discharged
gas discharged fromfromthe liquid absorption
the liquid tower
absorption is sent
tower to the
is sent toliquefied gas recovery
the liquefied gas recovery
unit,unit,
the rich amine
the rich liquid
amine discharged
liquid discharged fromfromthe two absorption
the two absorptiontowers is gathered
towers is gatheredand and
Metals 2023, 13, x FOR PEER REVIEW 4 of 21

sent to the regeneration tower, the regenerated poor amine liquid is sent back to the ab-
Metals 2023, 13, 256 4 of 19
sorption tower, and the desorbed H2S is sent to the sulfur recovery unit. The desulfuriza-
tion process flowchart and failure position are shown in Figure 4.

Figure 4. Desulfurization process and failure location.

Figure 4. Desulfurization process and failure location.

TableThe material
1. The chemicalof components
the elbow was 20#
of the steel,
20# steel.the diameter was 600 mm, the wall thickness
was 10 mm, and the length-to-diameter ratio of the elbow was 1.5. The material of the
Components C Mn
additional Si plates was
steel S 20# steel,
P and the Cr thicknessNiwas 10 Mo Cu
mm. The chemical Fe
compo-
Contents (wt%) 0.21 sition
0.66 of 20# 0.28
steel is listed
0.011in Table 1. The working
0.014 0.072 temperature
0.045 at the elbow
0.005 0.009is 120 °C, and
balanced
the pressure is 0.05 MPa. The composition of the fluid in the elbow is shown in Table 2.
Table 2. The properties and composition of the fluid.
Table 1. The chemical components of the 20# steel.
Properties Composition (wt%)
Components C Mn Si S P Cr Ni Mo Cu Fe
◦
Temp/(wt%)
C Pressure/KPa Flow/Kg/h Water 0.072 0.045
MDEA
Contents 0.21 0.66 0.28 0.011 0.014 0.005 0.009Hbalanced
2S

120 50 12,334 99.45 0.12 0.43

Table 2. The properties and composition of the fluid.
2.1.2. Morphology and Corrosion Product Composition Analysis
Properties Composition (wt%)
In order to study
Temp/°C the corrosion failure
Pressure/KPa mechanism ofWater
Flow/Kg/h the elbow, MDEA
the number ofHholes
2S as
well120
as the diameter size50 of the holes were counted. The measuring
12,334 99.45 instrument
0.12 was0.43
a SATA
electronic digital display vernier caliper with a division value of 0.01 mm and an accuracy
of ±0.03
2.1.2. mm.
Morphology and Corrosion Product Composition Analysis
After counting
In order to studythethe
number of holes
corrosion andmechanism
failure their diameters,
of thesamples were
elbow, the taken from
number the
of holes
severely corroded areas for microscopic morphological analysis and compositional
as well as the diameter size of the holes were counted. The measuring instrument was a analysis.
X-ray diffraction
SATA electronic (XRD,
digitalRigaku
displayUltima
vernierIV,caliper
Tokyo,with
Japan) was usedvalue
a division to determine
of 0.01 mmthe and
surface
an
phase compositions of the samples. The working voltage was 40 kV, the working current
accuracy of ±0.03 mm.
was 40 mA, the radiation target was Cu Ka, and the scanning speed was 6◦ /min. The
After counting the number of holes and their diameters, samples were taken from
scanning ranged from 10◦ to 90◦ (2θ). The surface morphology of the samples was observed
the severely corroded areas for microscopic morphological analysis and compositional
by scanning electron microscope (SEM) and the distribution of elements was detected by
analysis. X-ray diffraction (XRD, Rigaku Ultima IV, Tokyo, Japan) was used to determine
energy dispersive X-ray spectrometer (EDS). In this study, SEM and EDS observations were
the surface phase compositions of the samples. The working voltage was 40 kV, the work-
performed with a Zeiss Sigma 300 field emission scanning electron microscope (ZEISS,
ing current was 40 mA, the radiation target was Cu Ka, and the scanning speed was
Oberkochen, Germany). The working voltage was 20 KV, and the current was 40 mA.
6°/min. The scanning ranged from 10° to 90° (2θ). The surface morphology of the samples
A secondary electron (SE) was used for imaging. The time of the spectra acquisition
was observed by scanning electron microscope (SEM) and the distribution of elements
was 5 min.
was detected by energy dispersive X-ray spectrometer (EDS). In this study, SEM and EDS
observations were performed with a Zeiss Sigma 300 field emission scanning electron mi-
2.2. CFD Simulation
croscope (ZEISS, Oberkochen, Germany). The working voltage was 20 KV, and the current
ANSYS Fluent (19.0, ANSYS, USA) was applied to investigate the erosion–corrosion
was 4090
of the mA. A secondary
◦ steel elbow under electron (SE) was
gas–liquid used for flow
multiphase imaging. The time
conditions withof high
the spectra ac-
order dis-
quisition was 5 min.
cretization of the equations. The Eulerian model is used to calculate the flow field dis-
tribution of the gas–liquid two-phase flow. The dispersed phase model (DPM) and the
2.2. CFD Simulationmathematical model were used to track the droplet trajectory and calcu-
erosion–corrosion
latedANSYS Fluent (19.0, ANSYS,
the erosion–corrosion USA)
rate. The RNGwas applied
k–ε to investigate
turbulence model was theapplied
erosion–corrosion
to calculate
of
thethe 90° steelinside
turbulence elbowthe
under
elbow.gas–liquid multiphasesolver
The pressure-based flow was
conditions
used towith
solvehigh order
the above
models. PRESTO was used for pressure, second-order upwind schemes for continuity,
momentum, k and ε equations. SIMPLE was used for the pressure–velocity coupling.
When the residuals are less than 1e-5, the calculation is considered to be convergent. The
 culated the erosion–corrosion rate. The RNG k–ε turbulence model was applied to calcu-
late the turbulence inside the elbow. The pressure-based solver was used to solve the
above models. PRESTO was used for pressure, second-order upwind schemes for conti-
nuity, momentum, k and ε equations. SIMPLE was used for the pressure–velocity cou-
pling. When the residuals are less than 1e-5, the calculation is considered to be convergent.
Metals 2023, 13, 256 5 of 19
The specific settings will be explained in detail below. Some assumptions should be
adopted before the calculation:
(1) The formation of droplets is mainly due to the entrainment of the gas phase. The
specific settings will be explained in detail below. Some assumptions should be adopted
main component of the droplets is the MDEA solution, composed of MDEA and non-
before the calculation:
evaporated water, and the MDEA solution is saturated by H2S.
(1) The formation of droplets is mainly due to the entrainment of the gas phase. The
(2) The shape of the particles is spherical, and the force between the droplets is ignored
main component of the droplets is the MDEA solution, composed of MDEA and
because of the low liquid content.
non-evaporated water, and the MDEA solution is saturated by H2 S.
(3) (2)
The The
corrosion
shapeoccurs only at the
of the particles is places where
spherical, andthere is MEDA
the force solution.
between the droplets is ignored
because of the low liquid content.
(4) The diameter of the droplets can be calculated according to [27]. The monodisperse
(3) Theiscorrosion
droplet adopted.occurs only at the places where there is MEDA solution.
(4) The diameter of the droplets can be calculated according to [27]. The monodisperse
(5) The temperature of the inlet and the outlet of the elbow is 120 °C, and the heat trans-
droplet is adopted.
fer is ignored.
(5) The temperature of the inlet and the outlet of the elbow is 120 ◦ C, and the heat transfer
is ignored.
2.2.1. Geometric Model and Grid Independence Analysis
The geometric
2.2.1. Geometric model
Model ofandthe elbow
Grid is shown in
Independence Figure 5. The size of the elbow is
Analysis
shown inThe Figure 5a. The geometric model of the elbow
geometric model of the elbow is shown in Figure with 5.
corrosion
The size holes
of the is shown
elbow in
is shown
Figure 5b, where
in Figure thegeometric
5a. The area of severe
modelerosion–corrosion is coveredholes
of the elbow with corrosion withiscorrosion
shown in holes.
Figure 5b,
After
where the area of severe erosion–corrosion is covered with corrosion holes.the
the field measurement, the number of holes with a 19 mm diameter was highest.
After the field
Therefore, the diameter
measurement, of the of
the number holes was
holes withseta to
1919
mm mm and thewas
diameter depth
the of the hole
highest. was 10 the
Therefore,
mm.diameter of the holes was set to 19 mm and the depth of the hole was 10 mm.

(a) (b)
Figure 5. Geometric
Figure models:
5. Geometric (a) elbow;
models: (b) elbow
(a) elbow; withwith
(b) elbow holes.
holes.

The The
erosion raterate
erosion is anisimportant
an importantparameter to analyze
parameter the erosion–corrosion
to analyze behavior
the erosion–corrosion behavior
of the
of elbow. TheThe
the elbow. maxmax erosion raterate
erosion in the elbow
in the elbowis used to verify
is used grid
to verify independence
grid independence with
with a
different
a different number
number of of grids,
grids, and and
thethe results
results areare shown
shown in Tables
in Tables 3 and
3 and 4. The
4. The relative
relative error
error
ER |ER f ER
ine −ERmax |
is defined as as
is defined ER . ER. ER fis
inethe erosion
is the erosionraterate
from 2 million
from for the
2 million elbow,
for the andand
elbow,
ER f ine

6.1 million for the

6.1 million for elbow withwith
the elbow corrosion holes.
corrosion holes.

Table 3. Validation of the grid independence of the elbow.

Number of Grids Max Erosion Rate

Error (%)
(Million) (mm/y)
0.7 35.1 31.0
1.0 31.0 15.5
1.2 29.3 9.3
1.4 28.2 5.2
1.6 27.6 2.2
2.0 26.8 -
Metals 2023, 13, 256 6 of 19

Table 4. Validation of the grid independence of the elbow with corrosion holes.

Number of Grids Max Erosion Rate

Error (%)
(Million) (mm/y)
1.0 35.3 35.6
2.1 30.7 17.8
3.1 28.6 9.8
4.2 26.8 3.1
5.5 26.4 1.5
6.1 26.0 -

2.2.2. Boundary Conditions

The properties of the fluid are shown in Table 5. The gas phase comprises hydrogen
sulfide and water, and the liquid phase is MDEA. The inlet is mass-flow-inlet and the
outlet is the pressure outlet [20]. The mass flow rate of gas and liquid are 3.42 kg/s and
0.004 kg/s, as shown in Table 6. The wall was set with non-slip boundary conditions. The
boundary conditions for the Eulerian model and the DPM model are almost the same, but
the droplets will be trapped by the wall when the droplets collide with the wall for the
DPM model, as shown in Table 7 [28].
Table 5. The properties of the gas and liquid.

Phase Density (kg/m3 ) Viscosity (kg/m·s)

Gas 0.84 1.3 × 10−5
Liquid 922 0.00389

Table 6. Boundary condition for the Eulerian model.

Phase Inlet Outlet Wall

Mass-flow-inlet
Gas Non-slip boundary
3.42 kg/s Pressure outlet
Standard wall function
Mass-flow-inlet
Liquid
0.004 kg/s

Table 7. Model setting and boundary condition for the discrete phase in the DPM model.

Phase Inlet Outlet Wall Total Flow Rate Diameter Turbulent Dispersion
Liquid Escape Escape Trap 0.004 kg/s 0.75 mm Discrete random walk model

2.2.3. Mathematics Model

(1) Chemical corrosion model
Reaction process [10]:
Fe + H2 S → FeS + H2 (1)
Anode reactions:
Fe → Fe2+ + 2e− (2)
Cathode reaction:
2H2 S + 2e− → 2H+ + 2HS− (3)
2HS− + 2e− → 2H+ + 2S2− (4)
2H+ + 2e− → H2 (5)
The cathode reactions consist of a series of depolarization processes involving H2 S,
HS− , and H+ . The H+ ions produced by the ionization of H2 S are finally reduced to H2 .
It is reasonably assumed that the whole reaction only occurs near the pipe wall
and the ferrous ion would not be transferred through the boundary layer before the
Metals 2023, 13, 256 7 of 19

sequential chemical reaction. The formula for calculating electrochemical corrosion is

shown below [29]:

EC = 0.0791(fd × fe )(ZH2 S /ZW )(MW /MH2 S )(DH2 S × Cb,H2 S × Ug 0.7 )/(dW 0.3 × vg 0.344 ) (6)

where fd is 2/3; fe is 1; Z is the number of electrons; M is molar mass; g/mol, DH2 S is the
diffusion coefficient, m2 /s; Cb,H2 S is the hydrogen sulfide concentration, kg/m3 ; Ug is the
gas velocity, m/s; dW is the diameter of the pipe; vg is the kinematic viscosity of gas, m2 /s;
EC is the chemical corrosion rate, kg/(m2 s).
(2) Erosion model
The erosion rate caused by the impingement between the liquid droplet group and
the pipe wall is as follows [30–32]:

ER = C (K × (UP )n × mdrop × F(α))/(ρw × A) (7)

where ER is the erosion rate in mm/year; K is the material constant, which is 2.0 × 10−9 for
carbon steel [33]; C is a converting factor from m/s to mm/year, which is 3.15 × 1010 ; UP is
the hit velocity of droplets; n is taken as 5 for the liquid droplet [34]; mdrop is the mass flow
of droplet that hit the area, kg/s; F(α) is a number between 0 and 1 given by the function
of impingement angles. ρw is the wall material density, kg/m3 ; A is the size of the area
exposed to erosion, m2 .

F(α) = ∑ (−1) (i+1) × Ai ((α × π)/180)i (8)

where the Ai is given in Table 8 [34].

Table 8. Constants of F(α).

A1 A2 A3 A4 A5 A6 A7 A8
9.370 42.295 110.864 175.804 170.137 98.398 31.211 4.17

3. Results and Discussion

3.1. Failure Analysis
3.1.1. Macro Corrosion Morphology Analysis of Elbow
The morphology of the inner and outer wall surfaces of the elbow is shown in Figure 3.
The inner and outer wall of the elbow is relatively rough, and a variety of corrosion
holes with different sizes could be observed. The inner wall of the elbow did not show
significant wall-thinning, while the mass loss on the outer wall was severe. The area of
severe wall-thinning was generally honeycombed and covered with corrosion holes. The
corrosion holes expanded and joined together to form a completely broken area. A total
of 103 complete holes with a maximum hole diameter of 21.1 mm and a minimum hole
diameter of 7.5 mm were found in the area of severe wall thinning on the outer wall of
the elbow. The diameter of the holes near the center line of the outer wall was larger and
decreased along the radial direction. In addition, there was a phenomenon worthy of
attention. The wall thinning on the outer wall was serious while the corresponding position
of the additional steel plate was intact.

3.1.2. Microscopic Morphology and Corrosion Product Composition Analysis

Samples were taken in the area of serious wall thinning for SEM, and the results are
shown in Figure 6, with magnifications of 250 and 2000 times in Figure 6a,b. It can be
seen that the surface of the sample was uneven and rough. When the magnification was
2000 times, thin flakes and lumps of corrosion products remaining on the sample surface
and in the crevices could be observed, which indicates that the bond between the corrosion
products and the substrate was not enough to produce a protective effect on the substrate.
 3.1.2. Microscopic
3.1.2. Microscopic Morphology
Morphology and Corrosion
and Corrosion Product
ProductComposition
Composition Analysis
Analysis
Samples were taken in the area of serious wall thinning for
Samples were taken in the area of serious wall thinning for SEM, and SEM, and thetheresults areare
results
shown
shown in in
Figure
Figure6, 6,
with magnifications
with magnifications of of
250 and
250 2000
and times
2000 in in
times Figure
Figure6a,6a,
b. b.
It can bebe
It can seen
seen
that thethe
that surface
surfaceof of
thethe
sample
samplewaswasuneven
uneven and
and rough.
rough.When
When thethe
magnification
magnification was 2000
was 2000
times,
times,thin flakes
thin and
flakes lumps
and lumps of of
corrosion
corrosionproducts
products remaining
remaining ononthethe
sample
sample surface
surfaceand
Metals 2023, 13, 256 8 and
of 19
in in
thethe
crevices could be observed, which indicates that the bond between
crevices could be observed, which indicates that the bond between the corrosion the corrosion
products
products and thethe
and substrate
substrate was
wasnotnot
enough
enough to to
produce
produce a protective
a protectiveeffect onon
effect thethe
substrate.
substrate.

(a)(a) (b)(b)
Figure 6. SEM
Figure images
6. SEM of of
images thethe
samples: (a)(a)
samples: ×250; (b)(b)
×250;
×250; ×2000.
(b) ×2000.
×2000.

EDS
EDSwaswas
wasperformed
performed
performed on
onfour
onfourlocations
fourlocations
locationsof the
ofofthe samples,
the shown
as as
samples, shown inFigure
in
shown Figure
in 7.7.The
Figure The
7. re-re-
results
The
show
sults
sults thatthat
show
show the four
the
that the main
four
four elements
main
main of the
elements
elements corrosion
of of
the products
corrosion
the were
products
corrosion products Fe,
were
wereC,Fe,
Fe,S,C,and O.
S, S,
C, andandO.O.

Metals 2023, 13, x FOR PEER REVIEW 9 of 21

(a)(a) (b)(b)

(c) (d)
Figure 7. EDS images of the samples: (a) location 1; (b) location 2; (c) location 3; (d) location 4.
Figure 7. EDS images of the samples: (a) location 1; (b) location 2; (c) location 3; (d) location 4.

The
The corrosion
corrosion product
product composition
composition can can be
be obtained
obtained by
by XRD,
XRD, which
which helps
helps to
to further
further
understand
understand thethe failure
failure mechanism
mechanism of of the
the elbow.
elbow. The
The XRD
XRD results
results of
of the
the four
four positions,
positions,
which
which is
is aa cumulative
cumulative chart
chart for
for cases,
cases, are
are shown
shown inin Figure
Figure 8.
8. The
The results
results show
show that
that the
the
physical phase of the samples was mainly FeS2 and Fe, and individual samples contained
a small amount of FeS.
 (c) (d)
Figure 7. EDS images of the samples: (a) location 1; (b) location 2; (c) location 3; (d) location 4.

Metals 2023, 13, 256 The corrosion product composition can be obtained by XRD, which helps to further
9 of 19
understand the failure mechanism of the elbow. The XRD results of the four positions,
which is a cumulative chart for cases, are shown in Figure 8. The results show that the
physical phase
physical phase of
ofthe
thesamples
sampleswas
wasmainly
mainlyFeS
FeS2and
andFe,
Fe,and
andindividual
individualsamples
samplescontained
contained a
2
a small amount of FeS.
small amount of FeS.

Figure 8. XRD images of the samples.

3.1.3. Failure Analysis

3.1.3. Failure Analysis
According
According to to the
thesurface
surfacemorphology
morphologyofof thethe elbow
elbow andand
thethe elemental
elemental analysis
analysis of
of cor-
corrosion products, the micro holes could be observed on both the inner
rosion products, the micro holes could be observed on both the inner and outer wall of and outer wall
of
thethe first
first plate
plate of the
of the elbow,
elbow, andand
onlyonly
the the
massmass
lossloss on outer
on the the outer
wallwall
was was serious.
serious. The
The EDS
EDS and XRD tests indicate that H 2
and XRD tests indicate that H2S corrosion S corrosion occurs in the elbow and the major corrosion
occurs in the elbow and the major corrosion
products were FeS 2 and FeS.
products were FeS2 and FeS. From the From the SEM
SEM test,
test, the
the corrosion
corrosion products
products were
were poorly
poorly
bonded to the surface, and the droplets collided with the wall and carried the corrosion
bonded to the surface, and the droplets collided with the wall and carried the corrosion
products away from the wall, causing damage to the material. In all, the gas–liquid two-
products away from the wall, causing damage to the material. In all, the gas–liquid two-
phase erosion–corrosion is the main cause of elbow leakage, and the mechanism of the
phase erosion–corrosion is the main cause of elbow leakage, and the mechanism of the
erosion–corrosion will be discussed by the CFD simulation in Section 3.2.
erosion–corrosion will be discussed by the CFD simulation in Section 3.2.
3.2. Simulation for Elucidation of Erosion–Corrosion Mechanism
3.2.1. Simulation of the Erosion–Corrosion Behavior on the Elbow
The velocity distribution of the gas–liquid two-phase in the elbow is shown in Figures 9 and 10.
The velocity distribution of the gas phase in the elbow is uniform. The gas phase had the
maximum velocity on the inside of the elbow and the minimum velocity on the outside, and
the velocity decreased from the inside of the elbow to the outside. The velocity distribution
of the liquid phase in the elbow was more complex. A small part of the droplets flowed
along the inner wall and separated near the inner wall and flow out of the elbow. Most of
the droplets collided with the outer wall of the elbow at a certain velocity and angle and
flow out of the elbow at a lower velocity after the collision. In addition, the droplet and the
wall collision area were mainly concentrated in the axial angle of 0–75◦ . When the droplet
collides with the wall, the droplet will gather in the collision area, so the liquid phase
fraction in the collision area will be larger. The volume fraction of droplets at different
axial angles is shown in Figure 11. The liquid phase was mainly concentrated in the axial
angle between 30 and 75◦ , indicating that the collision between droplets and the wall was
focused on this area. It can be assumed that the erosion–corrosion of the wall in this area
was more serious.
 addition, the droplet and the wall collision area were mainly concentrated in the axial
angleangle of 0–75°.
of 0–75°. WhenWhen the droplet
the droplet collides
collides with with the wall,
the wall, the droplet
the droplet will gather
will gather in theincol-
the col-
lision area, so the liquid phase fraction in the collision area will be larger.
lision area, so the liquid phase fraction in the collision area will be larger. The volume The volume
fraction
fraction of droplets
of droplets at different
at different axial axial
anglesangles is shown
is shown in Figure
in Figure 11. liquid
11. The The liquid
phasephase
was was
mainly concentrated in the axial angle between 30 and 75°, indicating
mainly concentrated in the axial angle between 30 and 75°, indicating that the collision that the collision
Metals 2023, 13, 256 between droplets and the wall was focused on this area. It can be assumed
between droplets and the wall was focused on this area. It can be assumed that the ero- that the10ero-
of 19
sion–corrosion of the wall in this area was
sion–corrosion of the wall in this area was more serious.more serious.

(a) (a) (b) (b)

Figure
Figure 9. Velocity
9. Velocity distribution
distribution and streamline
and streamline of theof thephase:
gas gas phase: (a) contour;
(a) contour; (b) vector.
(b) vector.
Figure 9. Velocity distribution and streamline of the gas phase: (a) contour; (b) vector.

Metals 2023, 13, x FOR PEER REVIEW (a) (a) (b) (b) 11 of 21

Figure
FigureFigure10.
10.Velocity
Velocity
10. Velocity distribution
distribution
distribution and
and streamline
of theof
streamline
and streamline of the
the liquid
liquid phase:phase:
liquid phase: (a)
(a) contour;
contour;
(a) contour; (b)
(b) vector.
(b) vector.vector.

Figure
Figure11.
11.Distribution
Distributionofofthe
thevolume
volumefraction
fractionofofthe
theliquid
liquidphase.
phase.

Theerosion–corrosion
The erosion–corrosion raterate calculated
calculated bymathematical
by the the mathematical
model ismodel
shownisinshown
Figuresin
Figures 12 and 13. The inner wall of the elbow was subjected to the
12 and 13. The inner wall of the elbow was subjected to the maximum electrochemical maximum elec-
trochemical
corrosion ratecorrosion rate of and
of 1.107 mm/y, 1.107the
mm/y, and the electrochemical
electrochemical corrosion
corrosion rate rate
gradually gradually
decreased
decreased
from from
the inner thetoinner
wall wall wall
the outer to the
ofouter wall of
the elbow. thedroplet
The elbow.erosion
The droplet erosioncon-
was mainly was
centrated in the elbow outer wall surface, and the maximum erosion rate was 26.766
mm/y.
 Figure 11. Distribution of the volume fraction of the liquid phase.

The erosion–corrosion rate calculated by the mathematical model is shown in Figures

Metals 2023, 13, 256 12 and 13. The inner wall of the elbow was subjected to the maximum electrochemical 11 of 19
corrosion rate of 1.107 mm/y, and the electrochemical corrosion rate gradually decreased
from the inner wall to the outer wall of the elbow. The droplet erosion was mainly con-
centrated in concentrated
mainly the elbow outer
in thewall surface,
elbow outer and
wall the maximum
surface, and theerosion rate erosion
maximum was 26.766
rate was
mm/y.26.766 mm/y.

(a) (b) (c)

Metals 2023, 13, x FOR PEER REVIEW 12 of 21
Figure 12. Distribution
Figure of theofcorrosion
12. Distribution rate:rate:
the corrosion (a) view from
(a) view +Y; +Y;
from (b) (b)
view from
view +X;+X;
from (c) (c)
view from
view −Y−Y of
from
of thethe
elbow.
elbow.

(a) (b) (c)

Figure 13. Distribution
Figure of the
13. Distribution oferosion rate:rate:
the erosion (a) view fromfrom
(a) view +Y; (b)
+Y; view fromfrom
(b) view +X; +X;
(c) view from
(c) view −Y. −Y (1).
from

A comparison
A comparison between the severe
between areaarea
the severe of erosion–corrosion
of erosion–corrosion obtained
obtained by numerical
by numerical
simulation
simulationandandthe the
serious areaarea
serious of wall
of wallthinning
thinning in in
actual
actualconditions
conditions is isshown
shownininFigure
Figure 14.
14. The
Thearea
areawhere
wherethe theerosion–corrosion
erosion–corrosion rate
ratereached
reached or or
exceeded
exceeded 90%90% of the maximum
of the maximum
erosion–corrosion
erosion–corrosion raterate
is defined as aasserious
is defined a seriousarea of of
area erosion–corrosion.
erosion–corrosion. The
Thesevere
severe elec-
electro-
trochemical
chemicalcorrosion
corrosionareaarea was
was located at the the elbow
elbowinside
insidethethewall
wallaxial
axialangle
angleofof ◦ while
0–30°
0–30
whilethethe
serious
serious erosion
erosion area was
area located
was located at at
thethe
elbow
elbowoutside
outside thethe
wall axial
wall axialangle
angleof of 45–◦ . In
45–75
75°. practice,
In practice,thethe
elbow
elbow serious wall
serious wallthinning
thinning region
regionis is
located
locatedininthe
theelbow
elbowoutside
outsidethethewall
◦ . The numerical simulation of the severe erosion area and the actual corrosion serious
wall50–80
50–80°. The numerical simulation of the severe erosion area and the actual corrosion
areaarea
serious matched,
matched, indicating
indicatingthatthat
thethe
droplet
droplet erosion
erosionwas wasthe main
the maincause
causeofofthetheelbow
elbowwall
thinning, also illustrating the accuracy of the numerical simulation
wall thinning, also illustrating the accuracy of the numerical simulation results. The results. The numerical
nu-
merical simulation of the severe erosion area and the actual severe wall thinning area had
a slight shift, probably as a result of neglecting the droplet particle size distribution.
 erosion–corrosion rate is defined as a serious area of erosion–corrosion. The severe elec-
trochemical corrosion area was located at the elbow inside the wall axial angle of 0–30°
while the serious erosion area was located at the elbow outside the wall axial angle of 45–
75°. In practice, the elbow serious wall thinning region is located in the elbow outside the
Metals 2023, 13, 256 wall 50–80°. The numerical simulation of the severe erosion area and the actual corrosion 12 of 19
serious area matched, indicating that the droplet erosion was the main cause of the elbow
wall thinning, also illustrating the accuracy of the numerical simulation results. The nu-
merical simulation
simulation of the erosion
of the severe severe erosion area
area and theand thesevere
actual actual wall
severe wall thinning
thinning area hadarea had
a slight
ashift,
slightprobably
shift, probably as a result of neglecting the droplet particle size distribution.
as a result of neglecting the droplet particle size distribution.

Metals 2023, 13, x FOR PEER REVIEW 13 of 21

Figure14.
Figure Comparisonbetween
14.Comparison betweenthe
thesimulation
simulationresults
resultsand
andthe
theactual
actualsituation.
situation.
3.2.2. Simulation of the Erosion–Corrosion Behavior on the Elbow with Corrosion Holes
3.2.2. Simulation of the Erosion–Corrosion Behavior on the Elbow with Corrosion Holes
When the corrosion holes formed on the outer wall of the elbow, the gas velocity
When the
distribution wascorrosion
as shownholes formed
in Figure 15. on the be
It can outer
seenwall
thatofthe
theflow
elbow, theingas
of gas thevelocity
elbow
distribution was as shown in Figure 15. It can be seen that the flow of gas in the elbow
showed almost no change compared to the elbow. When the gas flowed into the holes, the
showed almost no change compared to the elbow. When the gas flowed into the holes,
velocity decreased significantly at the entrance of the hole and formed a vortex inside the
the velocity decreased significantly at the entrance of the hole and formed a vortex inside
hole, as shown in Figure 16. To describe the flow of gas inside the hole more clearly, three
the hole, as shown in Figure 16. To describe the flow of gas inside the hole more clearly,
lines were drawn on the cross-section of the hole and the velocities of the three lines were
three lines were drawn on the cross-section of the hole and the velocities of the three lines
extracted, and the results are shown in Figure 17. There was a large velocity gradient at
were extracted, and the results are shown in Figure 17. There was a large velocity gradient
the entrance of the holes, and the velocity from 5 m/s rapidly decreased to 0–1 m/s. Sub-
at the entrance of the holes, and the velocity from 5 m/s rapidly decreased to 0–1 m/s.
sequently, a slight increase in the gas-phase velocity occurs due to a brief acceleration
Subsequently, a slight increase in the gas-phase velocity occurs due to a brief acceleration
caused by the gas caught in the vortex. Then, it will flow out of the hole with the vortex.
caused by the gas caught in the vortex. Then, it will flow out of the hole with the vortex. In
In the process of exiting the holes, the gas velocity increases first and then decreases be-
the process of exiting the holes, the gas velocity increases first and then decreases because
cause of the interaction with the incoming flow. Finally, the gas flows out of the holes.
of the interaction with the incoming flow. Finally, the gas flows out of the holes.

Figure15.
Figure Velocity distribution
15.Velocity distribution and
and streamline
streamline of
of the gas phase.
Metals 2023, 13, 256 13 of 19

Figure 15. Velocity distribution and streamline of the gas phase.

Metals 2023, 13, x FOR PEER REVIEW 14 of 21

Figure 16. 16.

Figure GasGas
vortex in the
vortex holes.
in the holes.

Figure 17.
Figure 17. Velocity
Velocityof
ofgas
gasof
ofthe
thethree
threelines.
lines.

The
The flow
flowofofdroplets
dropletsininthe theelbow
elbow with holes
with holeswaswas mainly
mainlysimilar to the
similar to elbow, but was
the elbow, but
different in theinholes,
was different as shown
the holes, as shownin Figures 18 and
in Figures 18 19.
andWhen the droplets
19. When the dropletsentered into into
entered the
holes, the velocity
the holes, the velocityat different locations
at different appeared
locations at different
appeared degrees
at different of decline.
degrees To better
of decline. To
describe the movement
better describe the movementof droplets in the holes,
of droplets in thefour cross-sections
holes, of holes of
four cross-sections were chosen:
holes were
location (0◦ , 30◦ ),
chosen: 1location location
1 (0°, (0◦ , 45◦ ),2 location
30°),2location (0◦ , 60◦ ),3and
(0°, 45°),3 location (0°,location
60°), and 4 (0 ◦ , 60◦ ) according
location 4 (0°, 60°)
to Figure 18, and the holes were divided into two by the diagonal
according to Figure 18, and the holes were divided into two by the diagonal of the hole of the hole cross-section.
Itcross-section.
can be seen that
It canthe bevelocity
seen that wasthelarger
velocityin the
wasarealargerbelow the
in the line,
area and the
below the closer to the
line, and the
diagonal, the smaller the velocity, while the velocity was relatively small
closer to the diagonal, the smaller the velocity, while the velocity was relatively small in in the area above
the
theline,
areawhere
above thethe closer to thethe
line, where diagonal,
closer to thethe
larger the velocity.
diagonal, Combined
the larger with the
the velocity. flow of
Combined
droplets in the hole, droplets flowed into the hole from the area below
with the flow of droplets in the hole, droplets flowed into the hole from the area below the diagonal, and the
velocity direction of the gas and droplet was the same, so the gas on
the diagonal, and the velocity direction of the gas and droplet was the same, so the gas on the droplet obstruction
was relativelyobstruction
the droplet small, and was the overall velocity
relatively small,wasandlarger
the in the area.
overall The closer
velocity the region
was larger is
in the
to the diagonal, the longer the droplet trajectory in this region and the greater
area. The closer the region is to the diagonal, the longer the droplet trajectory in this region the buffering
effect
and theof the gas on
greater thethe droplet, effect
buffering resulting
of thein agas
decrease
on theindroplet,
velocityresulting
near the diagonal
in a decreaseregion.
in
When the droplet collides with the wall, the kinetic energy loss is larger after bouncing
velocity near the diagonal region. When the droplet collides with the wall, the kinetic en-
into the area above the line. At the same time, the gas in this area moves in the opposite
ergy loss is larger after bouncing into the area above the line. At the same time, the gas in
direction to the droplet. These two reasons together led to a smaller droplet velocity in
this area moves in the opposite direction to the droplet. These two reasons together led to
the area above the diagonal line. It can be presumed that the collision between the droplet
a smaller droplet velocity in the area above the diagonal line. It can be presumed that the
and the hole wall is mainly concentrated in the area below the line. The presumption was
collision between the droplet and the hole wall is mainly concentrated in the area below
proven by the volume fraction of the liquid shown in Figure 20. Therefore, it was presumed
the line. The presumption was proven by the volume fraction of the liquid shown in Fig-
that the erosion–corrosion of the hole was concentrated in the hole wall in the area below
ure 20. Therefore, it was presumed that the erosion–corrosion of the hole was concentrated
the line. The erosion–corrosion in the area above the line was small.
in the hole wall in the area below the line. The erosion–corrosion in the area above the line
was small.
REVIEW

Metals 2023, 13, 256 14 of 19

Metals 2023, 13, x FOR PEER REVIEW 15 of 21

Figure 18.
Figure 18. Velocity
Velocity distribution
distribution and
and streamline
streamlineof
ofthe
theliquid
liquidphase.
phase.

Figure 18. Velocity distribution and streamline of the liquid phase.

(a) (b)

(a) (c) (d)

(b)
Figure 19. Velocity distribution and streamline of the liquid phase in the holes: (a) location 1; (b)
Figure 19. Velocity distribution and streamline of the liquid phase in the holes: (a) location 1;
location 2; (c) location 3; (d) location 4.
(b) location 2; (c) location 3; (d) location 4.
Metals 2023, 13, x FOR PEER REVIEW 16 of 21
Metals 2023, 13, x FOR PEER REVIEW 16 of 21
Metals 2023, 13, 256 15 of 19

Figure 20. Distribution of the liquid volume fraction.

Figure 20. Distribution of the liquid volume fraction.
Figure 20. Distribution of the liquid volume fraction.
TheThedistribution
distribution ofofthe
theerosion–corrosion rateon
erosion–corrosion rate onthe
theelbow
elbow wall
wall with with
the the corrosion
corrosion
holes is The
holes shown in in
Figures
distribution
is shown Figuresof21–23. The
The distribution
the erosion–corrosion
21–23. distribution andand value
ratevalue ofof
on the the
elbow
the erosion–corrosion
wall with theare
erosion–corrosion are
corrosion
similar
holes toisthose
similar toshown
thosebefore. When
Whenthe
in Figures
before. thecorrosion
21–23. holesformed,
The distribution
corrosion holes formed,
and the
the max
value
max oferosionraterate on wall
the wall are
the erosion–corrosion
erosion on the
of holes
similar was
of holes towas 15.832
15.832
those mm/y,
mm/y,
before. which
When thedecreased
which decreased
corrosionby by 39.2%
39.2%
holes compared
compared
formed, the to
maxto the
theerosion
outerouter wall.
wall.
rate The Thewall
on the
reduction
reduction
of holesinwas in the
the erosion
erosion
15.832 ratemay
rate
mm/y, may be related
be
which related to
tothe
decreased formation
theby 39.2% of
formation vortices
of intothe
vortices
compared in holes.
theouter
the holes.wall. The
reduction in the erosion rate may be related to the formation of vortices in the holes.

(a) (b) (c)

(a) (b) (c)
Figure 21. Distribution
Figure ofof
21. Distribution thethecorrosion
corrosionrate:
rate: (a) viewfrom
(a) view from+Y;
+Y;(b)(b) view
view from
from +X;+X; (c) view
(c) view fromfrom
−Y. −Y.
Figure 21. Distribution of the corrosion rate: (a) view from +Y; (b) view from +X; (c) view from −Y.
Metals 2023,
Metals 13, x13,
2023, FOR PEER
x FOR REVIEW
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17 21
of 21
Metals 2023, 13, 256 16 of 19

(a) (a) (b) (b) (c) (c)

Figure 22. 22.
Figure
Figure Distribution of the
22.Distribution
Distribution of erosion
ofthe rate:rate:
theerosion
erosion (a) view
rate: (a) from
(a)view
view +Y; +Y;
from
from (b) (b)
+Y; view from
(b) view
view +X; +X;
from
from (c) view
+X; (c) from
(c) view
view −Y. −Y.
from
from −Y (2).

Figure 23. 23.

Figure
Figure Distribution of the
23.Distribution
Distribution erosion
ofofthe raterate
theerosion
erosion on the
rateon holes.
onthe
theholes.
holes.

3.2.3.
3.2.3.
3.2.3. Comparison
Comparison
Comparison ofofthe
of the theErosion–Corrosion
Erosion–Corrosion
Erosion–Corrosion Behavior
Behavior
Behavior between
between
between the
thethe
ElbowElbow
Elbow and
andand the
thethe
ElbowElbow
Elbow
with with
with Corrosion
Corrosion
Corrosion Holes
Holes
Holes
Inaccordance
In accordance
In accordance with with
withthethethe erosion–corrosion
erosion–corrosion
erosion–corrosion behavior
behavior
behavior with with
with the
thethe formation
formation
formation ofofcorrosion
corrosion
of corrosion
holes,
holes,
holes, the
thethe elbow
elbow
elbow can
cancan be
be be divided
divided
divided into
intointothe
thetheerosion-dominated
erosion-dominated
erosion-dominated region,
region, erosion–corrosion
region,erosion–corrosion
erosion–corrosion dom-
inated
dominated
dominated region,
region, corrosion–erosion
corrosion–erosion
region, corrosion–erosion dominated
dominated
dominated region,
region, and
andand
region, corrosion-dominated
corrosion-dominated
corrosion-dominated region
region
regionby
by bycalculating
calculating
calculating the
the ratio
ratio
the ofoferosion
ratio oferosion rate
rate
erosion totocorrosion
rate tocorrosion
corrosion rate
rate [35],
[35],
rate as as
[35], as shown
shown
shown in
in Figure
in Figure
Figure 24.
24. 24. The
TheThe
erosion
erosion
erosion dominant
dominant
dominant region region
region of the of the
elbow
of the elbow
elbow is elliptical
is elliptical in shape,
is elliptical in
in shape,shape,
mainly
mainlymainly
located located
locatedbetween
between between
30–30–
30–75 ◦ axially and −25–25◦ circumferentially. With the erosion-dominated area as the
75°75°
axially andand
axially −25–25°
−25–25° circumferentially.
circumferentially. With thethe
With erosion-dominated
erosion-dominated area as the
area center,
as the center,
center, the the
areainside
thethe
area from
area from thefrom
insidethethe
to toinside
the to the
outside
outsideareoutside are erosion–corrosion-dominated
erosion–corrosion-dominated
are erosion–corrosion-dominated regions,
regions, regions,
corro-
corro-
corrosion–erosion-dominated
sion–erosion-dominated regions, regions,
andand and corrosion-dominated
corrosion-dominated regions. regions.
When When
thethe the
corrosion corro-
sion–erosion-dominated regions, corrosion-dominated regions. When corrosion
sion holes formed, the erosion-dominated region was mainly concentrated in the wall of
holes formed,
holes formed, thethe
erosion-dominated
erosion-dominated region
regionwaswasmainly
mainly concentrated
concentrated in the wall
in the of the
wall of the
the corrosion holes. Compared to the elbow, the reduction in the area of erosion dominated
corrosion
corrosion holes. Compared
holes. Compared to the elbow,
to the elbow, thethe
reduction
reduction in the area
in the of erosion
area of erosiondominated
dominated
area can be observed, which indicates a substantial mitigation of the erosion effect of the
area cancan
area be observed,
be observed, whichwhichindicates
indicatesa substantial
a substantial mitigation
mitigation of the erosion
of the erosioneffect of the
effect of the
droplets on the wall. The changes in the other regions after the formation of corrosion holes
droplets
droplets on on
thethewall. TheThe
wall. changes
changesin the
in theother regions
other regionsafter thethe
after formation
formation of corrosion
of corrosion
were not noticeable.
holes were
holes werenotnot
noticeable.
noticeable.
Metals 2023,
Metals 2023, 13,
13, xx FOR
FOR PEER
PEER REVIEW
REVIEW 18 of
18 of 21
21
Metals 2023, 13, 256 17 of 19

Figure
Figure 24.
Figure24. Erosion–corrosion
24.Erosion–corrosion map
Erosion–corrosionmap for
mapfor the
forthe outer
theouter wall.
outerwall.
wall.

Toinvestigate
To investigate the
the phenomenon of of the
the buffer
buffereffect
effecton
onthe
thedroplet
dropleterosion
erosion ononthethe
elbow
el-
afterafter
bow the the
formation
formation of of
corrosion
corrosion holes,
holes,threethreelocations,
locations,which
whichwere were location (0◦ ,70°),
location 11 (0°, 70◦ ),
location (−5◦50°),
location22(−5°, , 50◦location 3 (5°,
), location ◦ , 60according
3 (560°), ◦ ), accordingto the
to Figure
the Figure24, were selected
24, were fromfrom
selected the
severe erosion–corrosion
the severe erosion–corrosion areaarea
andandthe thedroplet
dropletvelocity
velocitynear
near thethewall
wallwas
wasextracted
extractedfor for
comparativeanalysis,
comparative analysis,asasshown
shownininFigure
Figure25. 25.The
Theresults
resultsshow
showthat thatwhen
whenthe thedroplets
droplets
collidedwith
collided withthethewall,
wall,the
thevelocity
velocityrapidly
rapidlydecreased
decreasedtoto00totothethewall
wallofofthe
theelbow.
elbow.WhenWhen
thecorrosion
the corrosion holes
holes formed
formed on onthe
theelbow,
elbow,the thedroplet
dropletvelocity
velocityfirstfirst
showed
showeda slight decrease
a slight de-
compared
crease to the to
compared elbow and then
the elbow andrapidly
then rapidly decreased to 0. Combined
decreased to 0. Combined with with
the distribution
the distri-
of the of
bution gasthe
in gas
the in
hole,
the the droplets
hole, first make
the droplets contact
first make with with
contact the gas thevortex inside
gas vortex the hole,
inside the
where the gas vortex is equivalent to a dynamic gas film, so the droplet
hole, where the gas vortex is equivalent to a dynamic gas film, so the droplet velocity velocity hitting the
wall decreases, thus having a buffer effect on the droplet erosion.
hitting the wall decreases, thus having a buffer effect on the droplet erosion.

(a)
(a) (b)
(b)
Figure 25. Cont.
OR PEER REVIEW
Metals 2023, 13, 256 19 of 21 18 of 19

(c)
Figure 25. The velocity near the wall:
Figure (a) location
25. The velocity1;near
(b) the
location
wall:2;(a)(c)location
location3.
1; (b) location 2; (c) location3.

4. Conclusions 4. Conclusions
The corrosion
The corrosion failure analysis failurewas
of the elbow analysis ofout
carried the according
elbow wastocarried out according to the actual
the actual
situation on site, and thesituation on site, andbehavior
erosion–corrosion the erosion–corrosion
of the elbow ofbehavior of the elbow
the gas–liquid two- of the gas–liquid two-
phase flow was exploredphase flowThe
by CFD. wasconclusions
explored byareCFD. The below:
shown conclusions are shown below:
(1) Based
(1) Based on the macroscopic on the macroscopic
and microscopic and microscopic
morphology morphology
as well as the elemental as and well as the elemental and
compositional analysis of the elbow wall area, it was
compositional analysis of the elbow wall area, it was concluded that the gas–liquid concluded that the gas–liquid
two-phase erosion–corrosion was the main
two-phase erosion–corrosion was the main cause of the elbow failure. cause of the elbow failure.
(2) The severe erosion (2) The severe
area obtained byerosion areasimulation
numerical obtained by numerical
and the actualsimulation
severe walland the actual severe wall
thinning area matched. The max erosion rate was 26.766 mm/y and the max corrosion and the max corrosion
thinning area matched. The max erosion rate was 26.766 mm/y
rate was 1.107 mm/y, whichrate indicates
was 1.107that
mm/y, which
droplet indicates
erosion thatmain
was the dropletcauseerosion
of el- was the main cause of
bow wall thinning. elbow wall thinning.
(3) Compared to the erosion–corrosion behaviors on the elbow, the erosion rate on the
(3) Compared to the erosion–corrosion behaviors on the elbow, the erosion rate on the
outer wall of the elbow was the same while the max erosion rate on the wall of holes
outer wall of the elbow was the same while the max erosion rate on the wall of holes
was 15.832 mm/y, which decreased by 39.2% compared to the outer wall when the
was 15.832 mm/y, which decreased by 39.2% compared to the outer wall when the
corrosion holes formed in the outer wall of the elbow. The formation of the gas film
corrosion holes formed in the outer wall of the elbow. The formation of the gas film
played a buffering role in the collision between the droplet and the wall, changing the
played a buffering role in the collision between the droplet and the wall, changing
mechanism of action between the droplet and the wall, thus creating the protection of
the mechanism of action between the droplet and the wall, thus creating the protec-
the additional plate.
tion of the additional plate.
Author Contributions:
Author Contributions: Conceptualization, A.S. andConceptualization,
Y.L.; Methodology, A.S.A.S.;
and Software,
Y.L.; Methodology,
Y.L.; Vali-A.S.; Software, Y.L.; Valida-
tion, Y.L., G.S. and Y.X.; Formal analysis, Y.L.; Investigation, Y.L.;
dation, Y.L., G.S., and Y.X.; Formal analysis, Y.L.; Investigation, Y.L.; Resources, Y.X.; Data curation,Resources, Y.X.; Data curation, G.S.;
G.S.; Writing—original draft preparation, Y.L.; Writing—review and editing, Y.L.; Visualization,Y.L.; Visualization, G.S. and
Writing—original draft preparation, Y.L.; Writing—review and editing,
G.S. and Y.X.; Supervision,Y.X.;
J.Z.;Supervision, J.Z.; Project J.Z.;
Project administration, administration, J.Z.; Funding
Funding acquisition, J.Z. acquisition,
All authors J.Z. All authors have read
and agreed to the published version
have read and agreed to the published version of the manuscript. of the manuscript.

Funding: This research wasFunding: This

funded by theresearch
Nationalwas
Keyfunded by the National
R&D Program of ChinaKey R&D Program of China (2021YFB3301100).
(2021YFB3301100).
Institutional
Institutional Review Board Statement: Review Board Statement: Not applicable.
Not applicable.
Informed
Informed Consent Statement: Consent Statement: Not applicable.
Not applicable.
Data
Data Availability Statement: applicable.Statement: Not applicable.
Availability
Not
Conflicts
Conflicts of Interest: The authors of Interest:
declare The authors
no conflicts declare no conflict of interest.
of interest.

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