Corrosion Flashcards
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
State whether a solution is acidic, neutral or basic, given the solution pH value at 25C
Pure water = pH of 7.0; pH values less than 7.0 are acidic (Hydrochloric acid 1.0; Acetic acid 2.9; orange juice 3.5) and those with a pH greater than 7.0 are basic (sea water 8.5; sodium hydroxide 13.0)
State the definition of GENERAL CORROSION.
General corrosion is a uniform dissolution or attack of a metal from all surfaces in contact with water
List the conditions necessary for formation of the magnetite film on stainless steel
A magnetite film is formed on iron compounds at high temperatures (>400º F) and in the absence of dissolved oxygen (Fe304)
State the benefit 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 thru the film to react with water)
State the direction of change of general corrosion rate as the following factors change.
o Temperature. High temperatures increase the general corrosion rate
o pH. pH at extremes will increase the corrosion rate. A pH of > 12 causes caustic embrittlement
o Dissolved oxygen. Generally the more dissolved oxygen the more general corrosion
List and briefly describe three undesirable characteristics of CRUD.
Crud will foul heat transfer surfaces; It will tend to collect and then provide a higher radiation source term; Crud may block flow passages or foul demineralizer surfaces
State the means by which CRUD bursts are initiated.
A crud burst can be caused by changing the solubility of CRUD either by pH, temperature or dissolved oxygen changes; it can also be caused by mechanical shock (water hammer, fluid velocity changes)
State the chemical used and the reason for intentionally initiating a CRUD burst prior to shutdown.
A crud burst is initiated by addition of H202, hydrogen peroxide, to the RCS. We attempt to dissolve the crud and transport it to the CVCS demineralizers to “decon” (reduce the source term) of the RCS
GALVANIC
State the definition of the corrosion.
List the conditions which must exist to cause corrosion.
List the controls used to prevent or minimize the corrosion
Dissolution or attack of a metal which results when two dissimilar metals are in contact in an electrolyte; A difference in electrical potential between the two metals creates an electrical current flow; to prevent or minimize use metals that are generally corrosion resistant and/or select metals that are close to each other in electronegativity; maintain high water purity as pure water is a poor electrolyte
CREVICE
State the definition of the corrosion.
List the conditions which must exist to cause corrosion.
List the controls used to prevent or minimize the corrosion
Localized attack at/in a mechanical crevice; The crevice becomes a concentration cell; eliminate crevice, perform crevice cleaning, reduce contaminants so they cannot concentrate
PITTING
State the definition of the corrosion.
List the conditions which must exist to cause corrosion.
List the controls used to prevent or minimize the corrosion
A type of corrosion attack that involves a deep attack in/on a small area of the metal; Chemical activity begin and the metal at the bottom of the pit acts as an anode and loses electrons forming corrosion products and deepens the pit; dissolved oxygen and chlorides tend to promote the formation of pits and should be eliminated/minimized
CHLORIDE STRESS
State the definition of the corrosion.
List the conditions which must exist to cause corrosion.
List the controls used to prevent or minimize the corrosion
Intergranular corrosion which occurs in high temperature water, stainless steel and the presence of oxygen and chlorides; it involves selective attack along grain boundaries; minimizing or eliminating chlorides can prevent
FLUORIDE STRESS
State the definition of the corrosion.
List the conditions which must exist to cause corrosion.
List the controls used to prevent or minimize the corrosion
Similar to Chloride stress corrosion but caused by another halide, fluorine contamination
CAUSTIC STRESS
State the definition of the corrosion.
List the conditions which must exist to cause corrosion.
List the controls used to prevent or minimize the corrosion
Caustic stress implies high caustic levels (read high pH levels) and the subsequent metal attack; maintaining system pH to prevent caustic levels from rising to a corrosive point
