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.

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Salt dissolves giving these ions in solution: NaCl + + +

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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

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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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