Corrosion Flashcards
What conditions must exist to cause fluoride stress corrosion
- High temperature water
- Presence of Oxygen and fluorides
- Tensile stress
Define GENERAL CORROSION
A uniform dissolution or attack of a metal from all surfaces in contact with water
State whether a solution is acidic, neutral or basic, given the solution pH value at 25 degrees Celsius
Basic: pH > 7
Neutral: pH = 7
Acidic: pH < 7
What controls can be used to prevent or minimize chloride stress corrosion
- Lower chloride content
- Lower disolved oxygen
- Use low carbon steels
What are the TRO limits for steady state RCS chemistry
TRM 8.4.1
Dissolved O2 <= 0.1 ppm
Chloride <= 0.15 ppm
Fluoride <= 0.15 ppm
What conditions are necessary for formation of the magnetite film on stainless steel
- High temperatures (> 400 degrees Fahrenheit)
- No dissolved oxygen
What conditions much exist to cause crevice corrosion
The crevice becomes a concentration cell
What controls can be used to prevent or minimize pitting corrosion
Eliminate/minimize dissolved oxygen and chlorides
What benefit is gained from general corrosion
If general corrosion results in a film of magnetite being created, this will slow down future corrosion as ferrous ions must pass through the film to react with water
What controls are used to prevent or minimize crevice corrosion
- Eliminate crevices
- Perform crevice cleaning
- Reduce contaminants so they cannot concentrate
What conditions must exist to caustic stress corrosion
High pH and tensile stress
What conditions must be present to cause chloride stress corrosion
- High temperature water
- Presence of Oxygen and Chlorides
- Tensile stress
What is the relationship between dissolved oxygen and general corrosion
As dissolved oxygen increases general corrosion increases
What chemical is used to intentionally initiate a CRUD burst
Hydrogen peroxide added to the RCS
What controls can be used to prevent or minimize caustic stress corrosion
Maintain system pH to prevent caustic levels from rising to a corrosive point
Why is a CRUD burst intentionally initiated
CRUD burst is initiated to dissolve the CRUD and transport it to the CVCS demineralizers to lower dose rates during an outage.
Define FLUORIDE STRESS CORROSION
Intergranular corrosion which occurs in high temperature water, stainless steel, and the presence of oxygen and fluorides
What is the relationship between temperature and general corrosion
As temperature increases general corrosion increases
What is hydrogen embrittlement
Monatomic hydrogen diffuses into the Zircaloy resulting in a loss of ductility and tensile strength at pH values greater than 11.3
What controls can be used to prevent or minimize fluoride stress corrosion
Minimize or eliminate fluorides
What conditions must exist to cause pitting corrosion
Chemical activity that begins at the metal at the bottom of the pit acts as an anode and loses electrons forming corrosion products and deepens the pit
What is the relationship between pH and general corrosion
pH at extremes will increase general corrosion (pH > 12 or pH <4)
Define GALVANIC CORROSION
Dissolution or attack of a metal which results when two dissimilar metals are in contact in an electrolyte
What are the TRO limits for transient RCS chemistry
TRM 8.4.1
Dissolved O2 <= 1 ppm
Chloride <= 1.5 ppm
Fluoride <= 1.5 ppm
What is hydrogen blistering
Monatomic hydrogen combines to form H2 which will not diffuse and causes bulges in the cladding at pH values greater than 11
What controls are used to prevent galvanic corrosion
- Use metals that are generally corrosion resistant
- Select metals that are close to each other in electronegativity
- Maintain high water purity as pure water is a poor electrolyte
Define CHLORIDE STRESS CORROSION
Intergranular corrosion which occurs in high temperature water, stainless steel, and the presence of oxygen and chlorides
What conditions must exist to boric acid corrosion
Boric acid in contact with carbon steel
What controls can be used to prevent or minimize boric acid corrosion wastage
- Minimize RCS leaks
- Cladding carbon steel
- Substitute Stainless steel for carbon steel
What conditions must be present for galvanic corrosion to occur
A difference in electrical potential between two metals which creates an electrical current flow
State the definition of pH
The negative of the base ten logarithm of the hydrogen ion concentration of a solution given in moles per liter.
pH = -log (+H)
Define BORIC ACID CORROSION WASTAGE
Localized attack of ferritic steel (carbon steel) due to high concentrations of boric acid resulting in metal wastage
By what means can CRUD bursts can be initiated
- Changing solubility by either pH, temperature, or dissolved oxygen changes
- Mechanical shock (e.g., water hammer, velocity changes, etc.)
Define CREVICE CORROSION
Localized attack at/in a mechanical crevice
Define PITTING CORROSION
A type of corrosion attack that involved a deep attack in/on a small area of the metal
Define CAUSTIC STRESS CORROSION
Corrosion that occurs in the presence of high pH levels
What are three undesirable characteristics of CRUD
- Fouls heat transfer surfaces
- Collects and then provide higher radiation source term
- Blocks flow passages or foul demineralization surfaces
