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44 Cards in this Set
- Front
- Back
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What is a uniform corrosion
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Corrosion that is uniform dictated by nerst equation and thermodynamics
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What are some examples of uniform corrosion
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Iron
Fe-> Fe2+ +2e- O2 + 2H2O + 4e- -> 4(OH-) Zinc: Zn + 2HCl = ZnCl2 + H2 |
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What controls a uniform corrosion reaction rate?
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The slowest cathodic or anodic process
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What materials are most likely to experience uniform corrosion?
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those that have potentials below redox potential for reduction of oxygen in water. They are not passive
Steel, zinc and magnesium are examples |
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What are some methods for controlling uniform corrosion
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1. Exclude the environment
-ex. Use paint 2. Sacrificial Anodes. -applying a more active metal (ex: zinc) 3. Impressed Current techniques -apply electrons from an external source 4. Material selection -use materials that won't corrode 5. Inhibitors -waxy brown paper surrounding tools has a vapor phase inhibitor displacing moisture needed for corrosion |
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What are some testing technology used for uniform corrosion?
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Salt spray testing
Tafel extrapolation |
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Salt spray testing
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A 5% by weight salt solution at 100% humidiy and 95F is passed into a chamber in which the samples are contained.
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What is Galvanic Corrosion?
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Occurs from two metals in contact with same environment
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How does galvanic protection work?
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introducing a more anodic metal will decrease the corrosion rate for a more noble metal.
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What are 4 factors that effect galvanization corrosion
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1. Area Effect
2. Physical Distance Effect 3. Distance apart from galvanic series 4. Temperature Effect |
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Describe the area effect
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Current, not current density is the same between anode and cathode. Current density amps/area, controls corrosion. Low area, higher corrosion.
To minimize galvanic corrosion, the anode area should be large and the cathode area should be small. |
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Describe the physical distance effect
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As physical distance increases, the less corrosion occurs due to less conductance, and inability for ions to exchange
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What is the galvanic series and how can it be used to control corrosion ?
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corrosion resistance in seawater for many metallic systems based on empirical data. Choose metals close together to reduce driving force for corrosion.
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What is the temperature effect?
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At temperatures above 180 F, zinc become cathodic to iron.
It is best to conduct a test program to determine the relative corrosion rates for the metals under the conditions they will be exposed to |
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Name some ways to minimize galvanic corrosion?
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1) Electrically Insulate the Metals.For example use non metallic spacers.
2) Material selection- avoid selecting dissimilar metals 3) Design a) create cheap replacements b) Use the area effect for a closed system 4) coating paint cathode 5) sacrifical protection 6) impressed current protection 7) inhibitiors |
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Name two types of localized corrosion
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1) Crevice corrosion
2) Pitting corrosion |
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What is the reaction of crevice corrosion and where does it occur?
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Gap between metal and other surface magnitude of 0.002".
M -> Mz+ + ze- O2 + 2H2O + 4e- -> 4(OH-) Crevice region can't replace dissolved oxygen and anode takes place in crevice, while cathodic takes place on surface |
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How does the chloride concentration and hydrogen ions increase in the crevice corrosion?
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There is a positive charge in the crevice causing it to be attracted to chloride. Hydrogen ion is associated with chloride.
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How does the corrosion rate increase in crevice corrosion?
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The process is Autocatalytic through the reaction below. The pH is also lowered and acidification occurs:
M+ + Cl- > M+ Cl- Mz+Cl-Z + zH2O -> M(OH)z + z(H+Cl-) H+Cl- > H+ + Cl- |
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What properties do metals have that make them resistant to crevice corrosion?
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1) Low critical current density
2) Low passivation potential. 3) Wide passive region. |
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How does geometry effect crevice corrosion?
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The wider the crevice the less likelihood of crevice corrosion as the stagnant solution conditions break down and the oxygen in the crevice can be replenished.
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What are some measures to avoid crevice corrosion?
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1. Do not form crevices. Use welding or brazing
2.Use sealant. Ex: chaulk 3. Design to remove stagnant areas where moisture and deposits can collect. 4. No materials which will hold moisture near the metal surface. 5. Use high pressure washing to remove deposits. |
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How can one test crevice corrosion?
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place thin Teflon sheets between pieces of metal to simulate a crevice.
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What factors favor pitting corrosion?
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Presence of halide ions. Stagnant solution
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What metals are most likley to exhibit pitting corrosions?
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Those that are the most passive. Ex. Stainless steel
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How does the reaction of pitting corrosion work?
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Works the same as crevice corrosion.
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What are the theories to explain pit initiation?
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1. Defect theory
2. Chloride Ion Dissolution. 3. Peptisation. 4. Local Cells. 5. Stress Theory. 6. Vacancy Theory. 7 Breakdown and Repair. |
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What is defect theory?
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Film defects like grain boundaries or slip steps due to dislocations emerging form the surfaces. Act as local anodes and initiate breakdown
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How does chloride ion dissolution explain pitting?
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the chloride ion replaced the hydroxyl ion to form a metal chloride that dissolved. Another chloride ion at the same location then dissolved some more of the passive layer until the bare metal was exposed.
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What is Peptisation?
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chloride ion diffused thorough the passive layer to the metal surface and then caused dissolution of the metal. Further chloride ions would then diffuse and keep the pit growing.
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How do local cells explain pit initiation?
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in alloys, secondary phases behave differently electrochemically. Some will be anodic or cathodic to the matrix.
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How does stress theory cause pit initiation?
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passive films grow stresses are induced as they do not have the same crystal dimensions as the metal on which they are growing. Locally the films rupture and chloride ions can get to the surface and promote dissolution of the metal.
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How does Vacancy Theory cause pit initiation?
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Metal ions diffuse from the metal/passivity interface to the metal/solution interface. This creates a void and a stress concentration. This ruptures the film and initiates pitting.
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How can pitting corrosion be prevented?
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1. Material selection
2. No stagnant solutions 3. Impressed Current Protection and Sacrificial Anodes. 4. inhibitors 5. Environment exclusion. a) Use grease. Paints do not work well for pitting corrosion |
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What testing technology is used for pitting corrosion?
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“Cylic Polarization Scan”
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Describe “Cylic Polarization Scan”
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Scans for electric density between 100 and 1000 A/cm2 in anodic region and cathodic region. Cathodic region is reverse direction and should have a higher current density for corrision to occur.
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Describe the "Cylcic Polarization Scan" procedure
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1. All solutions should be of high purity components.
2. Sample finish should be 600 grit. 3. Purge solution for one hour with nitrogen. 4. Immerse test sample one hour prior to start of data collection. 5. Start scan at least 50mV more cathodic than Eoc 6. Scan rate should not exceed 0.6v/hr of 1.6mv/sec. 7. Increase potential to a specified value or a specified current value. 8. To investigate pitting, reverse the scan direction at the potential or current value specified. 9. Upon reverse scan, stop test at original Eoc. |
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What are the advantages and disadvantages of the cyclic polarization scan?
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Advantages.
1. Short term test. 2. Can use actual metals and solutions. 3. Easy interpretation of data. 4. Standardized test so reproducible. Disadvantages. 1. Destructive test. 2. Data can be dependent on scan rate set for passive metals. |
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What is INTERGRANULAR CORROSION?
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Occurs when corrosion only occurs on or adjacent to grain boundary areas.
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What is exfoliation?
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top layer of aluminium delaminates from surface to to intergranual corrosion.
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What causes intergranular corrosion?
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In Ag-Mg alloy magnesium tends to collect in grain boundary. Magnesium dissolves the grain boundary, grain boundary not attack acts as cathode, creating large cathode to small anode.
Grain boundary phases act as cathodic or anodic. |
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What is weld decay?
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Passive layer dissolves (ex: chromium in stainless steel) due to carbon binding at high temperatures removing the chromium.
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What are some preventive measures of weld decay?
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1. Quench material welded
2. Don't use brazing. Use low carbon welding and electric arc welding. 3. limit amount of carbon in stainless steel.(L after name) 4. Use stabilizes steel (ex:niobium w/ tantulum, NbC or TaC) or titanium (TiN). |
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What is Knife Line Attack?
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Occurs in stabilized steel in which annealing between Between 1450 and 950 causes chromium carbide. To avoid this problem, keep annealing above 1450 to form Niobium carbide or desired carbide.
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