| |
In natural metal corrosion, the substances are
in a stable condition, and the metals mostly exist
in the form of a compound as they are combined with
oxygen, sulfur, etc. Therefore, metal materials
are capable of returning to their most stable condition
in the environment, although they can be used in
industries.
For example, the iron rust that we normally observe
results from the act of maintaining the iron at
its most stable condition, where both water and
air exist. Likewise, the corrosion reaction of metal
can be set through the combination of the metal
and the environment. Iron in a vacuum condition
does not rust.
|
| |
| Principle behind Metal Corrosion |
| |
Metal corrosion is determined by the release of
metal ions, and the level of ease in the release
of the metal ions depends on the ease by which the
metal can be made into an ion in the solution. This
ionization tendency can be indicated as a measure
of the standard electrode potential. Like aluminum,
a metal that has a high ionization tendency radiates
so many electrodes that the standard electrode potential
is largely emitted because it becomes a minus value
based on the hydrogen ion (H+). It is thus classified
into a low-grade metal. In addition, metal-like
platinum, which has a low ionization tendency, has
a low electrode emission rate. As such, its standard
electrode potential has a plus sign. It is thus
classified into a noble metal.
Placing a metal in a solution will reveal its unique
electric potential. If metals with different standard
electrode potentials were placed in an electrolyte
solution, where they would have electric contact,
a flow of electrons (electric current) would emerge,
according to the different electric positions. The
metal ion (M M→M+ ＋e-) with a lower electric position
would flow out, and the other metal ion would receive
the electrons, so that only the metal ion with a
lower electric position would be dissolved.
This phenomenon occurs on the surface of the same
metal. Since differences in electric position could
be partially produced according to the arrangement
of the atom, the size of the particle, and the presence
of impurities or defects, corrosion action would
occur if a local cell would be formed.
|
| |
| Impact of the Environment on Corrosion |
| |
The corrosion incidents we commonly
encounter include red rust forming on our pressing
iron, blue rust forming on copper alloy, white rust
forming on zinc, etc. In environments where various
chemicals are used, such as factories or chemical
plants, various metal materials come into contact
with chemicals, and corrosion action drastically
occurs.
Corrosion action is accelerated according to various
conditions, such as those related to temperature
and density. In an extreme solution, some other
corrosion phenomena occur, unlike in a stable condition,
which requires care and concern.
|
| |
| The pH of the Solution |
| |
How does the ph of a solution affect corrosion?
The electric position of the metal in a solution,
and its pH, shows us if the metal has a stable condition.
Generally, iron undergoes corrosion within all pH
ranges, but chrome tends to corrode only within
a strong acid range.
The following figure shows a diagram of the corrosion
level of 18-8 stainless steel based on its pH, which
was tested in 4% and 90℃ solutions. A severe level
of corrosion is shown within the strong acid range,
which indicates that corrosion can also occur within
the alkali range, even at a relatively low level. |
| |
 |
| |
| Impact of Dissolved Oxygen |
| |
Oxygen dissolved in water creates ion hydroxide
(OH-) according to the chemical action that occurs,
and as it reacts with the iron atom, it creates
ferrous hydroxide, which is the cause of rust. Corrosion
thus progresses.
|
| |
 |
| |
The following figure
shows the impact of the density of the dissolved
oxygen on the corrosion of general steel types.
The corrosion speed increases in direct proportion
to the density of the oxygen. Corrosion no longer
progresses, though, once it exceeds a certain level
of density. This phenomenon occurs when a layer
is formed on the surface of the metal, which covers
the metal, thereby removing the gap between the
metal and the solution. This phenomenon is called
passivity. Stainless steel is a type of steel that
reflects this phenomenon well.
As the amount of oxygen that reaches the surface
of the metal increases in an environment where water
flows, the corrosion speed also increases in direct
proportion to the speed of the water flow. However,
when the flow of water is sufficiently large, the
oxygen that reaches the surface of the metal becomes
too much for passivity to occur. As such, the progress
of corrosion is reduced. |
| |
| Passivity of Stainless Steel |
| |
| Role of Cr as an Alloy Element |
| |
Submerging a general
type of steel in nitric acid and increasing the
density of the nitric acid will make corrosion progress.
At about 65% nitric-acid density, however, the corrosion
speed suddenly drops, and it is not dissolved. This
is because the nitric acid forms an inert layer
on the surface of the iron. This inert layer is
referred to as the passivity layer.
By observing the same phenomenon while adding chrome
incrementally to general iron, it was found that
the corrosion speed drops when the amount of chrome
becomes about or over 12%. This can be regarded
as the effect of the passivity layer formed by chrome,
and the product that is formed when this phenomenon
occurs is stainless steel.
|
| |
 |
| |
| The Passivity Layer of Stainless Steel |
| |
| Why is the surface of stainless
steel fine, and how does it maintain its corrosion
resistance? The surface of stainless steel is covered
by a dense protective film that we cannot see, called
passivity layer. This layer is very thin and consists
of chrome oxide. It is also very dense, like glass,
and has a very flexible structure and good clinging
ability. As such, it adheres well to a parent material
and maintains a stable layer. In addition, this
layer reacts to the metal material but is promptly
restored though partially destroyed by scratching,
etc. |
| |
| Destruction of the Passivity Layer by
Chloride ion |
| |
Stainless steel generally does not corrode in
neutralized water, but if there is CI- in the solution,
the passivity layer will be destroyed and a pitting
or stress corrosion crack will occur.
CI- makes metal chlorides adhere to it as it is
metathesized with oxygen or hydroxyl in the parts
of the structure where the thickness of the layer
is slightly unstable. Thus, corrosion progresses,
starting from those parts where the layer is locally
dissolved.
|
| |
| Types of Corrosion of Stainless Steel |
| |
Stainless steel shows
good corrosion resistance under many different circumstances,
thanks to the passivity layer on its surface. However,
in some environments, the protective feature of
the passive layer drops, and various kinds of corrosion
could occur. Therefore, stainless steel must be
taken care of.
The corrosion of stainless steel can be largely
classified into the dry type, which occurs in high
temperatures (e.g., sulfuration, oxidization, nitrification),
and the wet type, which occurs in general environments.
The wet type can be further classified into general
corrosion and local corrosion (e.g., integranular
corrosion, pitting corrosion, crevice corrosion).
|
| |
| Types of Corrosion of Stainless Steel |
| |
Stainless steel shows good
corrosion resistance under many different circumstances,
thanks to the passivity layer on its surface. However,
in some environments, the protective feature of
the passive layer drops, and various kinds of corrosion
could occur. Therefore, stainless steel must be
taken care of.
The corrosion of stainless steel can be largely
classified into the dry type, which occurs in high
temperatures (e.g., sulfuration, oxidization, nitrification),
and the wet type, which occurs in general environments.
The wet type can be further classified into general
corrosion and local corrosion (e.g., integranular
corrosion, pitting corrosion, crevice corrosion).
Another type of stainless-steel corrosion is discoloration.
Analysis of the Types of Corrosion-related Accidents
That Have Occurred (Examples from Japan)
|
| |
| General Corrosion |
| |
| General corrosion occurs in abnormal environments
where the surface of the stainless steel cannot
be passivated, and in hydrochloric-acid and sulfuric-acid
solutions. In these cases, corrosion or erosion
occurs evenly on the surface of the stainless steel,
and the extent of the corrosion can be measured
by reducing the weight of the steel based on time.
Generally, general corrosion is easier to anticipate
and treat compared to local corrosion, and it can
be prevented by selecting appropriate materials
or the appropriate thickness of the stainless-steel
sheet in advance after determining the kind of environment
where it will be used. |
| |
| Galvanic Corrosion |
| |
| Galvanic corrosion is the phenomenon in which
metal corrodes as it is oxidized, and in which a
reduction reaction system is created through the
movement of electrons between two metals or even
in the same metal, if there is a difference between
the electric positions of the two spots (the corrosion
environment conditions are locally different). It
is more the principle behind the occurrence of corrosion
in stainless steel rather than a type of corrosion
therein. Therefore, seeing the corrosion of all
types of stainless steel in micro terms will lead
to the conclusion that the basic principle behind
the corrosion of stainless steel coincides with
the theory of galvanic corrosion. It is thus possible
to predict the occurrence of corrosion if we know
the standard electrode potential of each metal,
which is called galvanic series. |
| |
| ⊙ Galvanic Series in a Seawater Environment |
| |
 |
| |
Metals that have a high standard electrode position
are regarded as noble metals, as shown in the above
table, and these are activated towards those with
lower standard electrode positions.
Chemically, when an active metal is combined with
a different type of metal, corrosion becomes more
serious than when the metal is made to stand alone.
This is because when a different type of metal comes
into contact with an active metal, the relatively
noble metal sacrifices the active metal by causing
it to corrode.
Therefore, when different types of metals are made
to come into contact with one another, it is important
to find their standard electrode positions in advance,
and to change their parts that come into contact
with each other so as to make them suitable. For
example, if the contacting part of a noble metal
is larger than that of the active metal, it would
accelerate corrosion. As such, it would be good
to make the contacting part of the active metal
large and that of the noble metal small, or to put
a conducting object between the two metals so that
they would not come into direct contact with each
other.
|
| |
| The following pictures show different
cases of galvanic corrosion that can be easily found
in our surroundings: |
| |
 |
| |
| Pitting occurs when the density of chloride ion,
which can destroy the passivity layer of stainless
steel, is high. In this case, the passivity layer
is locally destroyed and is the first part of the
stainless-steel sheet that is dissolved. |
| |
| The special feature of this corrosion is that
it takes quite a long time for it to start, but
once a pit is produced, the pit material parts develop
a small-anode (active) condition, and the entire
exterior develops a large-cathode (noble) condition.
The corrosion then accelerates, forming a large
pit in a few days. The entrance of the pit would
be very small, making one think that the pit is
small, but the inside is largely extended. Thus,
even if only a very small defect appears on the
surface of the stainless-steel sheet, a breakage
could occur within a few days. As such, it is better
to repair the damage immediately (refer to the picture). |
| |
| * Typical Sectional Phenomena of the Pitting
Part |
| |
 |
| |
| * Pitting Corrosion Occurrence
Mechanism |
| |
destruction of the passivity
layer → formation of a corrosion pit → staying of
the solution within the pit → exhaustion of the
dissolved oxygen → excessive cation → attraction
of choleric ion (to balance the electric charge)
→ formation of HCI(M+CI- +H₂O → MOH + H+CI-) → acceleration
of corrosion
|
| |
| * Impact and Measures against
the Pit |
| |
① The place where the CI density is advantageous
② The lower the temperature, the more advantageous
the CI density is.
③ Disadvantageous if there is dissolved oxygen or
oxidization (Fe³+, Cu²+)
④ More disadvantageous the closer the pH is to the
acidity
⑤ The use of stainless-steel types containing pitting
improvement elements (Mo, N, Cr, Ni, etc.) is advantageous.
⑥ The lower the pitting-causing factor, the more
advantageous it is: sulfide (Mns), ð-phase, o-phase.
⑦ Materials with pitting resistance are good, for
a more smoothly treated surface.
⑧ Disadvantageous if the solution that remains has
a small crevice
|
| |
* Impact
and Measurement of the Conditions Affecting the
Occurrence of Pitting
Electric Position of Pitting
|
| |
 |
| |
| * Impact
of Temperature on Pitting |
| |
 |
| |
| Crevice Corrosion |
| |
| The mechanism that corrosion occurred is the same
as that of pitting, and it is mainly generated when
foreign substance is attached on the stainless steel,
or a crevice which is structurally produced is placed
under the corrosive environment. |
| |
| * Corrosion Occurrence Mechanism |
| |
| Creation of a crevice a accumulation of solution
in the crevicea exhaustion of dissolved oxygen in
the crevice a excess of cation a attraction of chloride
ion (for the balance of the electric charge) a formation
of HCI a acceleration of the corrosion (same principle
as pitting) |
| |
 |
| |
| * Corrosion occurrence mechanism |
| |
- Corrosion is commonly generated if there is
a crevice or in the environment where deposits are
: Rivets, Bolts, Gaskets, Seaweeds, Deposits
- Generated when it is exposed to the chloride
- It takes quite a time for the corrosion to be
generated first. However once it is produced, the
progress speed of corrosion is highly accelerated
_ Since it is difficult to be observed with bare
eyes, it is found after it is considerably progressed
|
| |
| * Method to prevent crevice
corrosion |
| |
- Improvement of environment: Removal of chloride
environment
- Use of pitting resistant alloy: High Mo, N, Cr,
Ni alloy
- Design not to have a crevice: Weld rather than
connecting facilities by rivets or bolt
- Design as a structure for solution not to be pooled
but to be completely drained
- If a crevice is produced, it has to be filled
it with filler
|
| |
| Intergranular Corrosion |
| |
Intergranular corrosion is a local corrosion that
progresses according to the depth of progress of
the grain boundary inside the stainless-steel sheet
before the latter falls. This often occurs during
welding processing, particularly in the heat-affected
parts, due to the inappropriate heat-processing
processes used and to exposure to high temperatures.
Chrome easily combines with carbon, particularly
when it is highly heated, thus creating carbonated
chrome (Cr23 C60). This material is fully precipitated
to the grain boundary. The carbonated chrome loses
its chrome content to its surroundings. A chrome-exhausted
layer then emerges, whose corrosion resistance falls,
thereby corroding.
Likewise, the precipitation of carbonated chrome
is sensitive, and it is maintained in the 550~800℃
section or at a higher temperature and occurs when
it passes through this section. However, for ferrite
stainless steel, carbonated chrome is produced when
the steel sheet is suddenly cooled from a temperature
of over 900℃, unlike austenite stainless steel.
|
| An Example of the Development of Corrosion according
to the Grain Boundary |
 |
| |
| * Method to prevent crevice
corrosion |
| |
The best method to prevent corrosion, especially
for austenite steel, is to perform solution heat
treatment in about 1050~1150℃ heat. Actually, when
POSCO manufactures and delivers stainless steel
products, all the products are in the condition
that such solution heat treatment has been performed.
However, when a client company welds them, although
the corrosion resistance of the products is improved
as executing solution treatment, it is very difficult
to perform such heat treatment after welding at
site, it is better to choose the type of steels
having low concentration level of the carbon (L
grade- ex: 304: or 316 L) or stabilized carbon by
adding Ti or Nb (STS 321 or 345 etc), and it is
better to cool it as quickly as possible after welding.
Also, it is recommended to smoothly polish the welding
part and perform nitrate treatment on it.
|
| |
| * Extraction
of chrome carbonated from STS 304 steel
according to time and temperature |
* Impact
of C, N and Nb on the intergranular corrosion
of 19Cr2Mo steel |
|
| |
 |
| |
| stress corrosion cracking |
| |
When tensile stress is given to the exposed metal
that is sensitive to corrosion to the corrosive
environment, brittle crack is generated by cooperative
action of stress and corrosion, and this corrosion
is a unique phenomenon only for austenite steel.
It mainly occurs in 90° direction of the tensile
stress, and the spread of the wave of the crack
is spread regardless intergranularity.
For the corrosive environment, most of them is chloride
ion, but sometimes the stress corrosion is generated
in the high-temperature and high-concentration alkali,
high-temperature and high-pressure water or polytion
acid etc. For the sources of the stress, stress
by the materials on working, heat stress by welding
and the stress by powerful surface polish by grinder
etc. This corrosion is very fast transferred that
the part could be broken in 2-3 days or just in
a few hours. Therefore, if the concentration of
chorine is increased as supporting heavy structures
by austenite wire etc (roof structure of the swimming
pool etc), it is very dangerous and needs special
care.
|
| |
 →
Sectional organization a part which typical
SCC is generated |
| |
| Measures against SCC |
| |
As three actions of Susceptible
alloy, Corrosive, Environment, Tensile stress are
essential for SCC, it is possible to prevent it
by removing one of those three factors.
① Reduce the concentration level of chloride ion
and the temperature to use
② Removal of dissolved oxygen and oxidized substances
③ Removal of attachments on the surface (regular
cleaning)
④ Try not to make a shape or crevice that stress
is structurally concentrated
⑤ Performance of stress removal heat treatment after
welding or processing (mainly produced near welding
part)
⑥ Endowment of compressive stress by short peening
⑦ Selection of appropriate materials (SCC is not
generated in the ferrite steel but since its strength
is not strong, prudent consideration is necessary.
The type of steel that the pitting problem is improved
by adding Mo or high Ni line austenite steel is
advantageous. Recently, duplex steel that the strength,
SCC and corrosion resistance are improved at the
same time has been developed and used)
|
| * SCC occurrence
tendency by types of steel |
* Comparison
of SCC resistance by sizes of austenite
line |
|
| |
 |
| |
| Concentration of Cl ion |
| |
If a material receives weight that is periodically
changed, it is broken even under the very
low stress than its tensile strength. If a
material receives periodical weight under
corrosive circumstance, it could be broken
even under a lower weight and in shorter period
than average and, this is called as corrosion
crack.
As characteristics of the corrosion crack,
the created crack is hardly spread but makes
line pattern or sand pattern in the seaside.
Also the tensile strength is produced in 90°,
and although it can be generated at any circumstances,
but there is difference in the life of fatigue
according to the corrosiveness of the exposed
environment. Also, it has higher possibility
to be generated if there is notch on the surface
|
 |
|
| |
| * Measures to prevent fatigue
corrosion |
| |
| Give compressed stress by shot peening processing
on the surface of the materials or perform heat
processing to remove residual stress after welding.
Also, as the type of steel with high wield strength
has superior fatigue resistant corrosiveness, it
is better to choose type of steel like duplex with
high yield strength. |
| |
| Seawater Corrosion |
| |
| It is found that if stainless steel is used in
the seawater it shows corrosive feature much faster
than general environment. It is because there is
about 3.4 % of salt as a factor that generates corrosion,
so that it is easy to produce local corrosions like
pitting or crevice corrosion. |
| |
| * Composition of seawater |
| |
| Classification |
CaCl₂·2H₂O |
MgCi₂·6H₂O |
NaCi |
Na₂SO₄ |
KBr |
SrCi₂·6H₂O |
| Content
(g/ℓ) |
1.54 |
11.8 |
24.53 |
4.09 |
0.1 |
0.017 |
|
| |
| Among the corrosive features in the seawater,
crevice corrosion by attachment of seaweeds and
deposits and the pitting corrosion by high concentration
of CI ion in the solution are the largest problems.
However, the amount of general corrosion is comparatively
smaller than the general steel, it does not have
large problem in the respect of general corrosion.
Nevertheless, abrasive corrosion problem is some
times caused due to the floating matters in the
seawater. |
| |
| * Impact of sea waste environment
on pitting |
| |
- Concentration of CI ion: As the higher the CI
concentration is, the amount of pitting is increased.
- Dissolved oxygen: Pitting is difficult to be produced
under 5ppb of the dissolved oxygen, but it is largely
expanded in 40~60ppb of the dissolved oxygen.
- Temperature: The position of the pitting is moved
to the active position the temperature is higher.
Rapid occurrence of pitting is difficult under 20℃.
- Flow velocity: A the flow velocity if faster,
pitting is not caused (accumulation of salt is difficult)
When the flow velocity is slow as 1.5~1.8ㅡmm/sec,
pitting is easy to be cuased
|
| |
| * Impact of sea waste environment
on pitting |
| |
- Selection of seawater resistant steel materials:
Duplex, Super austenite
- Design for the seawater not to be accumulated,
and make the flow velocity as fast as possible
- Remove attachments often
- If facilities are stopped to be operated, wash
them in fresh water
|
| |
| Atmospheric Corrosion |
| |
| The inducing factors of the atmospheric corrosion
is generated by corrosive corpuscle in the air such
as sulfur, nitrogen, chlorines and carbon etc and
it is easy to be found from the industrial areas
where pollution is severe. |
| |
 |
→ Polluting substances are piled up on the
signboards near the street.The feature it
is dissolved in the rainwater |
|
| * Type of atmospheric corrosion
|
| |
- Pitting by direct falling under deposit
- Pitting in the places where water is pooled or
where washing is difficult
- Crevice corrosion in the crevices
|
| |
| * Type of atmospheric corrosion |
| |
- Regular cleaning
- Selection of appropriate material for the environment
- Surface treatment of the material: Less corrosion
is generated for the smoother surface.
|
| |
| * Cases of the atmospheric
corrosion under various circumstances |
| |
|
Exposed time (year) |
Finish of the surface |
Place |
최대
pit깊이,㎛ |
|
17Cr |
18Cr-10N |
17Cr-12Ni-2.5Mo |
| Exposed
washing |
Water
washing of the shield |
Exposed
washing |
Water
washing of the shield |
Exposed
washing |
Water
washing of the shield |
| 23 |
Cold
rolling,
Removal of oxidized layer after the annealing
treatment |
Ocean
Rural areas |
- |
- |
100
|
160
200
30 |
50
|
100
60
15 |
| 21 |
Rough polish |
Oceanic
heavy industrial area
Heavy industrial area in rural community |
50
30 |
250
85 |
45
30
20
40 |
150
155
35
410 |
50
10
10
20 |
80
40
15
530 |
|
| |
| Microbiologically Influenced Corrosion |
| |
| Microbiologically Influenced Corrosion (MIC) is
the occurrence of metal corrosion by bacteria etc
living on the surface of the metal as their group
provides crevice on the surface of the metal or
transforms the characteristic of the surface. |
| |
| * Cases of the atmospheric
corrosion under various circumstances |
| |
- Sulfate Reduction Bacteria
- SBR is an anaerobe that it affects on both positive
reaction and negative reaction generated on the
surface of iron as it converts sulfate to sulfide.
|
| |
| Positive
reaction: |
8H₂O=8H+
+ 8(OH)- |
| |
4Fe
+ 8H+=4Fe²+ + 8H |
| Negative
reaction: |
H₂SO₄
+ 8H=4H₂S+4H₂O |
| Corrosive
products: |
Fe2+
+ h2s=FeS + 2H² |
| |
3Fe₂+
+ 6(OH)- = 3Fe(OH)₂ |
|
| |
| As their activities are wide under the lower pH
environment and as it increases the concentration
of the sulfuric acid up to 5wt% in local parts of
the metal, it indirectly participates in corrosion |
| |
| * Impacting factors of MIC |
| |
- Temperature: Most active between 10℃ and 50℃
- Flow velocity: MIC is generated as microorganism
forms layer on the surface of the metal and as the
faster flow velocity is, the layer of the microorganism
becomes more gelatinized.
-pH: The range of the action is different according
to the type of microorganism
- Purity of water: The more floating matters are,
the more MIC is generated.
|
| |
| * Way to judge MIC |
| |
- Analysis of the corrosive products: Existence
of black sulfide corrosive products, mainly Fes
- Corrosive surface analysis: Fe-, Mn- compound,
existence of large amount of Cl-, S, P
- Existence of Bacterial slime (Exopolymer)
|
| |
| * Measures against MIC |
| |
Physical measure: Physically remove the layer
of the microorganism, deposit and sale etc on the
surface
- Chemical measure: Remove the corrosive products
and the layer of the microorganism in chemical method
- Biocide treatment: Sterilizer treatment
|
| |
| Soil corrosion |
| |
| Inducing factor for soil
corrosion |
| |
- Corrosion factor in the soil: Physical, chemical
and biological factors
- Moist in the soil
|
| |
| * Induction factor of the
corrosion of the soil |
| |
- Level of the electric conduction: The higher
the level of the electric conduction is, the corrosiveness
is larger
- Impact of chlorine: CI-, pH
- Oxygen: the cathode is the place where the oxygen
is contained more, the anode is the place where
the oxygen is contained less
Ex) The upper part of the laid pipe becomes the
anode because the amount of oxygen contained is
larger, and the lower part becomes the cathode because
there is less cathode that the lower part of the
pipe is first corroded in many cases.
|
| |
| * Measures against corrosion
of the soil |
| |
- It is required to select appropriate material
after thorough analysis about the nature of the
soil before lying
-Coating method: metallic coating, cement mortar
-Addition of chemicals: neutralization of the soil
-Replacement of the soil
|
| |
| High-Temperature Corrosion of Stainless
Steel |
| |
The
high-temperature corrosion of stainless steel mainly
occurs in boilers, heat exchangers, chimneys, etc.,
and the representative cases of high-temperature
corrosion can be classified into corrosion due to
contact with a high-temperature or high-pressure
solution, and corrosion due to the reaction between
a high-temperature gas and the metal. The corrosion
that occurs due to contact with a high-temperature
or high-pressure solution is ruled by an electronic
chemical reaction, and by a principle that is similar
to the various corrosion theories that were stated
and discussed in the previous chapter. On the other
hand, the corrosion that is due to the reaction
between a high-temperature gas and metal occurs
when a high-temperature gas, secondary reactants,
ashes, etc. are created when fuel is burned.
Depending on the fuel type and combustion method
used, various by-products are created, such as condensed
gas, liquid, and solid particles. Some impurities
from these by-products accumulate on the surface
of the metal, which reduces the efficiency of the
heat exchanger, increases the pressure of the gas,
and sometimes causes severe corrosion.
The following picture shows the corrosion of a small
incinerator. A large amount of dust and particles
accumulated on the part of the high-temperature
incinerator where waste gas is discharged, and severe
corrosion occurred within a few months after the
operation of the incinerator. Such corrosion was
highly sensitive (see the picture on the right)
as it had been exposed to a high temperature for
a long time. Moreover, as the dust and other particles
that accumulated on the part of the incinerator
where waste gas is discharged contained a large
amount of sulfur, they caused serious corrosion.
|
| |
| *
Acute Corrosion Due to Exposure to High Temperatures |
| |
 |
| |
| Results of the Analysis
of the Water-Soluble Deposits of Used Fuels Using
a Spectrophotometer |
| |
| Fuel |
H₂O |
SO₂²- |
CI- |
Fe³+ |
NH⁴ |
Other |
| Heav
Oil |
27.66 |
63.70 |
0.012 |
0.221 |
6.78 |
1.54 |
| Coal
and LNG |
43.35 |
29.08 |
0.157 |
0.379 |
2.33 |
24.70 |
| Coal |
29.24 |
53.08 |
0.213 |
0.033 |
1.29 |
16.10 |
|
| |
| * Effective Factors of Major
Fuel Components |
| |
| -
|
Sulfur:
Oxidized gas (SO₂, SO₃) forms H₂SO₄ (sulfur)
and causes general corrosion in low-temperature
areas (under 206℃) as well as local corrosion
in high-temperature areas. |
| - |
Chlorine: Cl₂ gas reacts
with H₂O and forms HCI, which causes severe
local corrosion. |
|
| |
| * Selection of Steel Materials |
| |
| Although the materials that should be used must
depend on the fuel type and temperature conditions
that will be used, under a high-temperature condition
where sulfur and CI are present, 304 steel is not
appropriate, and 309S, 310S, and 317 steel are appropriate
alternatives. However, if the design considerations
could not be sufficiently reflected with the use
of such steel types, 316L steel could be used. Since
super-heat-resistant steel or Ni alloy steel, however,
is needed in very high temperature conditions of
over 900℃, the following reference data must be
carefully considered. |
| |
| Comparison
of the Characteristics of Continuous High-Temperature
Oxidization |
- When ferrite
steel is continuously heated, the mass becomes
bigger. |
|
| |
 |
| - 304 Steel:
The mass becomes bigger with continued heating. |
- 316 Steel:
The mass becomes bigger with continued heating |
|
 |
| Limit the temperature
to be used, taking into consideration the
repeated and continuous heating that will
be done. |
|
 |
| |
| Super-Corrosion-Resistant Stainless Steel |
| |
For use in severe
environments, or for maximum use without need for
maintenance, ultra-stainless steel, which does not
corrode even when used in a highly corrosive environment,
has been developed. This ultra-stainless steel is
mainly made up of high alloys and has large amounts
of Mo and N, and it can be used not only in corrosive
environments but also in nuclear plants, 타황 facilities,
seawater facilities, and chemical plants.
Among the various indices assessing the corrosion
resistance of stainless steel, the case in which
the PRE (pitting resistance equivalent) value is
over 30 is considered the best case. If the stainless
steel’s PRE index is over 40, it can be called super-stainless
steel.
|
| |
 |
| |
| Maximum Equation
Depth, Index
Comparison of the Characteristics of the
Super-Corrosion-Resistant Stainless Steel
by Type |
|
| |
|
|
Representative Sizes |
Representative Components |
Mechanical Features |
Note |
| Duplex |
SAF
2205
STS 329J3L |
22CR5Ni3Mo |
YS
500Mp
TS 700mPA |
Comparatively inexpensive and can be used
for general purposes |
| Super
Austenitic |
254SMO |
20Cri18Ni6Mo |
YS280Mpa
TS 600Mpa |
Expensive
and can be used in environments with temperatures
of under 550℃, and where the PRE value is
highest. |
| Super
Ferrtic |
MONIT |
29Cr4Mo |
YW
500Mpa
TS 630Mpa |
The
precipitation of its intermetallics is easy,
and it is used only as 박물. |
| Super
Duplex |
SAF
2507
UR47N |
25Cr7Ni4Mo |
YS
650Mpa
TS 840Mpa |
It
can be used only in temperatures under 300℃,
and it has high SCC resistance. |
|
| |
| *
Appropriate-Materials Guide by Corrosive
Environment |
|
| |
 |
