NUPT-215Nuclear Plant Chemistry unit 2Bismarck State college
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CorrosionDesired Outcome for Unit two:Describe the causes and
effects of corrosion on metals and the type of chemistry used in
the plant to minimize corrosion. `
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Unit 2.1 and 2.2: objectives: Define the following Ionization
Conductivity Corrosion Electrolysis General corrosion Describe an
electrochemical cell with respect to the corrosion of metals. State
what happens to a metal during the oxidation step of the
oxidation-reduction process. State what happens to a metal during
the reduction step of the oxidation-reduction process. Define:
Passivity Polarization Describe the effects of passivity and
polarization on the corrosion process.
Corrosion taking place in a heat exchanger. Scale build up and
tube surface fowling causes reduced heat transfer and a reduction
in in design flow rates.
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Ionization: occurs when electrons are removed from atoms or
molecules orbital or an electron is gained in an orbital. It can be
caused by high temperatures, electrical discharges, and radiation.
You may ask what does this have to do with corrosion? To answer
this question we need to review some terms. Corrosion: is the
destructive chemical or electro-chemical reaction of a material in
its environment. Can occur wet or dry, and can take many forms.
But, at Nuclear Power Plants, we are concerned with water and steam
based systems. Corrosive attack: occurs when a metal comes in
contact with a liquid or moisture. The rate of the attack depends
on the type of metal and type of liquid. Ionization energies: this
is the amount of energy required to remove electrons from the
orbitals (usually the outer most) of atoms or molecules.
Electrochemical attack: Currents that set up deposits of corrosion
products in one area of the metal while a corrosive attack is
occurring in another area. Electrochemical theory: relates chemical
action (corrosion) to the electron flow. It sates three conditions
must be met for material corrosion. At least two places on the
metal must act as electrodes to allow the flow of electrons from
the metal to the solution and back again. The solution and the
metal must be capable of conducting some electrons (water and ions)
A driving force or electric potential must set up the current flow.
This is usually caused by IONIZATION. Electrochemical attack
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Electrolyte: when a metallic atom dissolves in a solution, it
gives up one or more electrons to form an ion with a positive
charge. The solution that the ion now exists is call an electrolyte
or an electrolytic solution. It will now conduct electricity.
Corrosion: is all about electron removal, gains, and the formation
of ions both positive and negative. Lets do a quick review of the
periodic table and see how ionization energy levels (amount needed
to remove an electron) change in the table. As stated in unit one,
the group one elements have a valence of +1 and like to get rid of
one of their electrons. The energy to remove these electrons is low
due to this fact. And as you move down the group, the number of
shells increase, moving that one outer electron even further away
from the nucleus. It is therefore even easer to remove. (lower
energy required) Looking over to the right side of the chart we
have the noble gases which have eight electrons in their outer
orbitals. They are happy with this configuration and do not want to
give up any electrons and it takes much more energy to remove an
electron from them. As we move down this group it becomes easier to
remove an outer electron because once again the outer orbital is
moving further away from the nucleus. The chart on the next page is
marked up with arrows indicating from low to high ionization energy
levels.
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low
High
Lower Lowest
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Helium does not want to give up an electron and as a result,
requires greater energy to remove It. Lithium on the other hand
likes to give them up and look at the low energy required to remove
an electron
The peaks get further apart As we go because of the increase of
the number of elements per period.
Notice the general trend down as orbital shells get larger and
more of them, its easier to remove an electron.
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We have a few more terms to discuss and work through an
electrochemical cell before we can go to specific examples of how
corrosion works on metal surfaces. Conductivity: Is a measure of
the ability of a solution to conduct electrical current. These
solutions are sometimes called electrolytic solutions. See the
image below. Note: the drawing is in the electron flow convention,
salt is dissolved in solution.
-
_
Salt dissolves giving these ions in solution: NaCl + + +
+
When the switch is thrown the ions move to the anode or + charge
in this case. Cl ion pairs are then oxidized by giving up two
electrons and coveting from two ions to chlorine gas. Lower energy
is required to produce Cl gas instead of 02 gas.
Reaction taking place. The + charged Na ions go to the cathode
(or 2Cl- => Cl2 + 2enegative charge), but the reaction that
occurs is seen below. This produces 2 gas. The reason for this, H
atoms have a higher affinity for electrons. Conductivity meters in
the plant work by this principle except an AC current is used to
eliminate gas and ion than does Na, and requires less energy.
depletion to some extent. 2H2O + 2e- => H2 + 2OHThe N+ then
reacts with the Ion current flow is slower and orders of magnitude
less than OHin metals. The current flow is dependent on the ions
used (some are slower or faster than NaCl ions the temperature, the
higher the temperature the more flow the concentration in solution
the amount of voltage applied
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The units of measure are : Micromhos/cm: ( mho/cm) this is the
older of the measurement methods but is still used today. You may
also see siemens/cm (S/cm) which is the accepted SI unit. Because
this is a large number you will normally see readings in S/cm May
be read as ppm if the substance is known in the solution and that
reading is desired pH if the substance is known and that reading is
desired total dissolved solids (TDS) can also be read using this
method Electrochemical cell: An electrochemical cell generally
consists of two half-cells, each containing an electrode in contact
with an electrolyte. The electrode is an electronic conductor (such
as a metal or carbon) or a semiconductor. Current flows through the
electrodes via the movement of electrons. Electrochemical cells are
usually classified as either galvanic or electrolytic.
In galvanic cells, reactions occur spontaneously at the
electrode electrolyte interfaces when the two electrodes are
connected by a conductor such as a metal wire. Galvanic cells
convert chemical energy to electric energy and are the components
of batteries, which usually contain several cells connected in
series.
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Electrolysis :
is the decomposition of water (H2O) into oxygen (O2) and
hydrogen gas (H2) due to an electric current being passed through
the water. and example drawing is provided below. Note: the current
flow is in conventional form.
Electrodes are normally inert metals such as platinum or
stainless Hydrogen is produced by reduction (It receives electrons)
at the cathode. the formula for this reaction is given above by the
right test tube. Oxygen is produced at the anode by oxidation
(Loses electrons) with the formula listed by the left test
tube.
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In electrolytic cells, reactions are forced to occur at the
electrodeelectrolyte interfaces by way of an external source of
power connected to both electrodes. Electric energy from the
external source is converted to chemical energy in the form of the
products of the electrode reactions.
Please click on the video below for information on galvanic
cells
Note: When the movies lecturer started to talk about the salt
bridge, the problem he was talking about is called polarization
(Equilibrium in the system is reached).
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Passivity: in the presentation above, a polarization occurred
when the ionic charges obtained an equilibrium stage. Corrosion
stopped at this point and no more redox reactions occurred. The
metals at this point are showing Passivity. Some corrosion occurred
but then stopped. Chrome for example, achieves passivity very
rapidly by forming a very then oxide layer which then resists any
further corrosion.
Unit 2.3 Corrosion of a one metal surface: objectives.
List two conditions that contribute to general corrosion.
Describe how the rate of corrosion occurring in the plant is
affected by the following. Temperature Water velocity Oxygen pH
Condition and composition of the metal surface Dissolved solids
List the three products that are formed from the general corrosion
of Iron. Identify the action taken for initial fill of a reactor
system to limit general corrosion. State the four methods used to
chemically control general plant corrosion. List the six water
chemistry conditions that limit corrosion of aluminum.
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Lets put what we have learned in the above discussion together
and look at how a concentration cell sets up the condition for
corrosion. In the drawing below, a drop of water is placed on a
steel surface. In the drop of water, indicators have been added to
show the presence of ions in solution. This type of corrosion is
called pitting corrosion on a one metal surface.
2 O drop, in the water drop is some salt to speed up the
process, and pH indicators (phenolphthalein, turns pink with OH-.
potassium ferricyanide, turns blue with ++ ions.)
Indicator turns pinkIndicator turns blue 2 absorption
2
CathodeFe(OH)3
Steel plate
Anode at pit
Fe(OH)2
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Corrosion or concentration cell: When the drop of water hits the
steel, the 2 will initially be reduced in the center of the drop by
the formation of Fe(OH)2 . This reduces the concentration of oxygen
in the middle of the drop. fresh 2 inters on the outside of the
drop but the oxygen concentration is uneven setting up a cathode on
the outer boundary and an anode at the metal surface near the
center of the drop. 2Fe2+ + 4OH- 2Fe(OH)2 The center of the drop
turns blue and the outside turns pink. The Fe(OH)2 will then be
oxidized further to Fe(OH)3 and precipitated around the anode.
Fe(OH)2 FeO + 2 O and at less than 10000 F, 2 FeO + 2 0 F2 3 + 2 H
Another reaction that can occur is3 Fe + 4 H2O Fe3O4 + 4 H2 . This
is a consideration in the steam generator secondary side and
cooling towers at a PWR. This substance is Magnetite. This will
further reduce the 2 at the center of the drop and add to the
potential difference across the cell. As the corrosion continues, a
pit will form at the center of the drop. The ferric hydroxide
excludes 2 from the metal below it, setting up another corrosion
cell. The surface rust is porous enough to let ions flow and
another layer of Fe(OH)2 forms under it. I am sure you have seen a
rusted object that looked like it was layered with rust, this is
the mechanism that causes that look. This is why 2 control of
primary and secondary plant water is so important. Indicator turns
blue 02 depletion zone Indicator turns pink 2 Fe(OH)2
CathodeFe(OH)3
2 absorption
Steel plate
Anode at pit
Fe(OH)2
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General corrosion: is the process whereby the surface of a metal
undergoes a slow, relatively uniform; removal of material. It is a
combination of micro concentration cells, as described above,
across the entire surface of a metal where differences in potential
exists at any given time. If the water is stagnant pitting will
occur at those sites.
In order for general corrosion to occur two items are required
Microcells or electrodes must be on the surface of the metal A
conducting path must be available for current flow (ions in water)
Below is a drawing from your text showing the layers forming
general corrosion.
Below is a photograph of steel general corrosion in two
different stages
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Things that effect general corrosion rates: Temperature:
increasing temperature increases the corrosion rate. This is due to
fact that ion mobility is increased with increasing temperature
thus carrying out the formation of corrosion cells faster. Later in
plant life however, this the corrosion rates are reduced at higher
temperature due to formation of tightly adhered oxides
Velocity: At high water velocities the erosion of the oxide
layer causes more metal to to be exposed and thus speeds up the
corrosion rate. O2 : as we have seen by our water drop example; O2
is our culprit in corrosion. As the concentration of O2 goes up, so
does the corrosion rate. This is why we control the concentration
of O2 in solution at our nuclear plants. < 0.005 ppm at
Westinghouse, < 0.01ppm at others.
pH: The section on pH in the text book pages 14, 15, and 16 has
good coverage on the effects of pH. It can be seen that the
corrosion rates are stable in the 4 to 10 pH levels with lower
corrosion rates at the upper end of the caustic side of the graph.
However, at nuclear power plants, a neutron absorber is added to
the reactor coolant (boric acid) which controls the reactivity in
the core over plant life. Because this is an acid, it must
counteracted with the addition of LiOH. This sets up a balancing
act that must be followed over the core life. The pH is attempted
to be maintained at a pH level around 6 to 7 at most plants but it
will vary depending on the plant. Most plants have an upper limit
on LiOH addition of 2.2 mg/kg (ppm) Metal composition will effect
the corrosion rates as well as the uniformity of alloys The more
homogeneous the mixture, the less chance for setting up corrosion
cells. Deposits: Can be good if you are doing it to form the
initial film layer of oxidation but a bad thing if the deposits are
where you dont want them Conductivity: increased conductivity
increases the corrosion rates.
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Dissolved solids: in solution add to the conductivity and
therefor increase corrosion. At nuclear plants, dissolved solids
are of particular concern in the steam generator and feed water
that is reused. You will hear the term Total Dissolved Solids (TDS)
There are methods to reduce TDS. Condensate polishing typically
involves ion exchange technology for the removal of trace dissolved
minerals and suspended matter. Commonly used as part of a power
plant's condensate system, it prevents premature chemical failure
and deposition within the power cycle which would have resulted in
loss of unit efficiency and possible mechanical damage to key
generating equipment. Below is a typical set of condensate
polishers.
Plant inlet water mixed bed demineralizers are used to clean and
demineralize fresh water interring the plant. This water is then
used for process systems, and for further treatments as required
for the plant systems.
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Unit 2.4 Prevention chemistry control
Passivators and inhibiters: are used to form preferred oxidation
films thereby protecting the metal underneath. This also occurs
during the reactor coolant system pickling process during initial
system start up. Cathodic protection: is an electrochemical means
of corrosion control in which the oxidation reaction in a galvanic
cell is concentrated at the anode and suppresses corrosion of the
cathode in the same cell. two types External voltage source: this
method involves impressing a direct current between an inert anode
and the structure to be protected. Since electrons flow to the
structure, it is protected from becoming the source of electrons
(anode). In impressed current systems, the anode is buried and a
low voltage DC current is impressed between the anode and the
cathode.
Cathodic protection rectifiers for piping runs.
Cathodic protection system for sand filter tank. 18
Sacrificial anode: By coupling a given structure (say Fe) with a
more active metal such as zinc or magnesium. It produces a galvanic
cell in which the active metal works as an anode and provides a
flux of electrons to the structure, which then becomes the cathode.
The cathode is protected and the anode progressively gets
destroyed, and is hence, called a sacrificial anode.
Here are some different types you may see on tanks and I-beams
around the plant
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Removal of corrosive agents: Demineralization or softening of
water reduces the conductivity De-aeration is use to control
dissolved oxygen Chemical addition is used in many places through
out the plant to control chemistry specifications. Various systems
require different qualities of water and chemicals are used for
pretreating of systems. Phosphates and sodium hydroxide are used to
control system pH. etc. Aluminum corrosion: see DOE handbook pages
17, 18 and 19 for this subject since most power plants use
zirconium alloys for fuel rod construction.
Unit 2.5 Crud and Galvanic corrosion: Objectives Define the
following: Crud Scale Galvanic corrosion
Identify the five problems associated with the presence of crud
in reactor coolant State the four causes of crud bursts. State the
two conditions that can cause galvanic corrosion. Explain the
mechanism for galvanic corrosion. Identify the two locations that
are susceptible to galvanic corrosion. Sate the five control
measures used to minimize galvanic corrosion.
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Crud: suspended or lightly adhered particles of different
corrosion products, substances from chemical or nuclear reactions,
or any other particles in places where you do not want them. Crud
has some unwanted characteristics, some of which are listed below:
It becomes activated by the affects of high radiation and relocates
to a spot causing local high radiation areas (hot spots). Can
increase the core flux imbalance between top and bottom flux rates.
Fouls heat-transfer surfaces. Components of crud found on fuel rod
surfaces. 3 4 84 91 % 2 3 25% NiO 39% MnO 15% CuO 0.2 1 % CoO <
0.05 % -This one emits two high energy gamma rays when it decays
after activation. (Co-60) Crud does not have to come from just the
reactor coolant system corrosion but can also come from corrosion
products in the CVCS system which is then injected into the RCS.
Because it is contaminated, crud makes disposal of primary waste
harder because of the long half-life's associated with the crud.
Crud burst: is the rapid release of radioactive crud components
into the reactor coolant system. The reason for this release can be
caused by several things, these include: An increase in the oxygen
concentration in the RCS A reduced or large change in the pH of the
RCS Large temperature swings such a heatups and cool downs Physical
shocks such as scrams, pumps starting or changing speed, or for
those equipped check valve slams or even a relief valve
lifting.
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Scale: is a corrosion byproduct that deposits on surfaces from
the formation of insoluble salts usually carbonates. It also cuts
down on heat transfer and reduces design flow. Galvanic corrosion:
is the corrosion that results when two dissimilar metals with
different potentials are placed in electrical contact in an
electrolyte. Corresponds most closely to a electrochemical cell.
But, galvanic corrosion is self generated and has no external
voltage source. A difference in electrical potential exists between
the different metals and acts as the driving force for electrical
current flow through the corrodant or electrolyte. The larger the
potential difference, the greater the probability of galvanic
corrosion. Below left is a simple electrochemical diagram of
galvanic corrosion. Right, real world example. Electrolyte
solution
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Below is a chart of the electrical potentials of various metals.
You can see by this chart which substances would not be a good
match up. The larger the difference in voltage of the two
substances the more likely galvanic corrosion would occur.
Electrode Potential at 77 F (25 C) Anodic end (this is where the
corrosion occurs) Standard Electrode Potential Element (Volts)
Lithium -3.045 Potassium -2.920 Sodium -2.712 Magnesium -2.340
Beryllium -1.700 Aluminum -1.670 Manganese -1.050 Zinc -0.762
Chromium -0.744 Iron; Mild Steel -0.440 Cadmium -0.402 Yellow Brass
-0.350 50-50 Tin-Lead Solder -0.325 Cobalt -0.277 Nickel -0.250 Tin
-0.136 Lead -0.126 Hydrogen reference electrode 0.000 Titanium
+0.055 Copper +0.340 Mercury +0.789 Silver +0.799 Carbon +0.810
Platinum +1.200 Gold +1.420 Graphite +2.250
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Galvanic corrosion prevention:
The use of cathodic protection is often used to reduce or
eliminate galvanic corrosion. A sacrificial anode is added to the
system which has an even higher oxidation potential than the metal
to be protected. Zinc is commonly used but it will ultimately be
the metal couple used that will decide the anode. By using only
metals that are close on the activity series (potentials) will
reduce the galvanic cell potential.
Electrical insulation of dissimilar metals. Using poorly
conducting electrolytes i.e. pure water.
Unit 2.6 Specialized corrosion: Objectives Define the following
terms: Pitting corrosion Crevice corrosion Stress corrosion
cracking State the two conditions necessary for pitting corrosion
to occur. State the particular hazard associated with pitting
corrosion. State the four controls used to minimize pitting
corrosion. Identify the three conditions necessary for stress
corrosion cracking to occur. Define the term chemisorption. State
the hazard of stress corrosion cracking. State the three controls
used to prevent stress corrosion cracking Describe the two types of
stress corrosion cracking that are od major concern to nuclear
facilities including: Conditions for occurrence Methods used to
minimize the probability of occurrence.
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Pitting corrosion on surface metal review When the drop of water
hits the steel, the 2 will initially be reduced in the center of
the drop by the formation of Fe(OH)2 . This reduces the
concentration of oxygen in the middle of the drop. fresh 2 inters
on the outside of the drop but the oxygen concentration is uneven
setting up a cathode on the outer boundary and an anode at the
metal surface near the center of the drop. 2Fe2+ + 4OH- 2Fe(OH)2
The center of the drop turns blue and the outside turns pink. The
Fe(OH)2 will then be oxidized further to Fe(OH)3 and precipitated
around the anode. Fe(OH)2 FeO + 2 O and at less than 10000 F, 2 FeO
+ 2 0 F2 3 + 2 H Another reaction that can occur is3 Fe + 4 H2O
Fe3O4 + 4 H2 . This is a consideration in the steam generator
secondary side and cooling towers at a PWR. This substance is
Magnetite. This will further reduce the 2 at the center of the drop
and add to the potential difference across the cell. As the
corrosion continues, a pit will form at the center of the drop. The
ferric hydroxide excludes 2 from the metal below it, setting up
another corrosion cell. The surface rust is porous enough to let
ions flow and another layer of Fe(OH)2 forms under it. I am sure
you have seen a rusted object that looked like it was layered with
rust, this is the mechanism that causes that look. This is why 2
control of primary and secondary plant water is so important. An
example of pitting corrosion in a crevice is given on page 29 and
30 of your text. Indicator turns blue 02 depletion zone Indicator
turns pink 2 Fe(OH)2 CathodeFe(OH)3
2 absorption
Steel plate
Anode at pit
Fe(OH)2
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As we can see by the above diagram and the crevice pitting
example in the text; pitting corrosion requires two conditions to
proceed: Low flow Areas of differing oxygen concentration. This is
called a concentration cell or differential aeration cell. Hazards
associated with pitting corrosion: Pitting corrosion causes rapid
penetration of materials with little loss of overall mass. This
could result in the compromise of a pressure boundary resulting in
a leak (see the example at right) Is difficult to detect without
visual examinations. Can take different shapes.
Prevention/minimizing of pitting corrosion: Avoid stagnant or low
flow conditions: example would be to have a recirculation flow in a
system. Using metals and alloys that are less susceptible to the
corrosion. Avoiding agents in the medium that cause pitting
(oxygen) Designing the system and components such that no crevices
are present.
Stress cracking corrosion: (SCC) is caused by the simultaneous
effects of tensile stress and a specific corrosive environment.
Stresses may be due to applied loads, residual stresses from the
manufacturing process, or a combination of both. The attack of this
corrosion is intergranular in nature in the molecular make up of
the metals.
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Chemisorption: the binding of a liquid or gas on the surface or
in the interior of a solid by chemical bonds or forces. Believed to
be the cause of stress corrosion cracking Formation of compounds
between the metals crystal structure. Stainless steels are
susceptible Below is a microscopic view of stress corrosion
cracking with the river like fracture effect
At right condensate piping with stress corrosion cracking. A
small leak initiated this by wetting insulation, followed by Cl
leaching.
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Chloride stress cracking corrosion: (CSCC) (stainless steels)
One of the most important forms of stress corrosion that concerns
the nuclear industry is chloride stress corrosion. Chloride stress
corrosion is a type of intergranular corrosion and occurs in
austenitic stainless steel under tensile stress in the presence of
oxygen, chloride ions, and high temperature. It is thought to start
with chromium carbide deposits along grain boundaries that leave
the metal open to corrosion. The three conditions that must be
present for chloride stress corrosion to occur are as follows:
Chloride ions are present in the environment Dissolve oxygen is
present in the environment Metal is under tensile stress Note: the
rate at which the attack occurs is affected by temperature, the
higher the temperature the faster the rate of attack At the right
is a photo of transgranular, chloride stress corrosion cracking
(SCC) on a preheater tube sheet. Circumferential cracking was
localized at the roll transition indicating that residual tensile
stress from the roll expansion process contributed to cracking
Prevention measures for CSCC include: Maintaining low oxygen levels
Maintaining low Chloride levels Use of low carbon steels
Note: A new technique has been found to limit all types of
stress corrosion cracking by use of hydraulic compression of piping
prior to installation that appears to greatly reduce this
corrosion.
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The micrograph on the right (X500) illustrates intergranular SCC
of an Inconel heat exchanger tube with the crack following the
grain boundaries. Caustic stress corrosion cracking or caustic
embrittlement: Caustic stress corrosion, or caustic embrittlement,
is another form of intergranular corrosion cracking. The mechanism
is similar to that of chloride stress. Mild steels (steels with low
carbon and low alloy content) and stainless steels will crack if
they are exposed to concentrated caustic (high pH) environments
with the metal under a tensile stress. In stress cracking that is
induced by a caustic environment, the presence of dissolved oxygen
is not necessary for the cracking to occur.
Three factors are required for caustic embrittlement: Leakage of
steam generator feed water must occur so as to permit escape of
steam and subsequent concentration of feed water at point of
leakage. Or internal crevices that allow the concentration of
caustic. Attack of the steam/feed system metal by concentrated
caustic soda (NaOH), originating from the concentrated steam
generator feed water.
High metal stress ( such as weld or applied stress) in the area
of concentration and attack.Prevention: Do not allow leakage to
continue for any length of time (prevents caustic build up)
Construction such that crevices are not present
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Boric acid corrosion *: is a general type pitting corrosion that
exists where reactor coolant is leaking and then vaporizes thus
leaving a build up of boric acid. Continual wetting by the leak
with 2 absorption sets up concentration cells and thus intense
corrosive effect around the area of leakage. Can increase corrosion
rates from 0.001 inch/year to 10 inches/year. Leakage rates under
technical specification limits of rate or air activity makes it
hard to detect at power. Review NRC order EA-03-009 in docsharing
concerning this problem To the right is a photo of the hole in the
head area at the Davis-Besse nuclear plant caused by boric acid
corrosion. Prevention or mitigation of boric acid corrosion:
Prevent RCS leakage by using good maintenance procedures Inspection
programs for critical systems if boric acid is found.
* Not in Text book
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Microbiological influenced corrosion (MIC) *: the term used for
corrosion influenced by microbes in the water. The primary concern
is that the influence of these microbes is often an extremely
accelerated rate of corrosion. MIC is not caused by a single
microbe, but is attributed to many different microbes.
Example of MIC in fire system piping
Methods for preventing or mitigating the effects of MIC:
Avoiding stagnant conditions Eliminating nutrient sources
Periodically cleaning or flushing systems to remove organic
deposits By chemical or mechanical means. Using biocides to kill
bacteria (when discharge limits allow)
* Not in text book
NOTE: Make sure that you have also read the lecture notes and
text prior to taking the exam. This concludes this
presentation.
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