Corrosion

Question 1

State the definition of pH

Answer 1

The negative of the base ten logarithm of the hydrogen ion concentration of a solution given in moles per liter
pH= the negative log of the hydrogen ion concentration
ph=-log [H+]

Question 2

State whether a solution is acidic, neutral or basic, given the solution pH value at 25C

Answer 2

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)

Question 3

State the definition of GENERAL CORROSION

Answer 3

General corrosion is a uniform dissolution or attack of a metal from all surfaces in contact with water

or

The process whereby the surface of a metal undergoes a slow, relatively uniform, removal of material.

Question 4

List the conditions necessary for formation of the magnetite film on stainless steel.

Answer 4

A magnetite film is formed on iron compounds at high temperatures (>400º F)

and in the absence of dissolved oxygen (Fe304)

Question 5

Sate the benefit gained from general corrosion

Answer 5

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)

Question 6

State the direction of change of general corrosion rate as the following factors change.

Answer 6

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

Question 7

List and briefly describe three undesirable characteristics of CRUD.

Answer 7

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

Question 8

State the means by which CRUD bursts are initiated

Answer 8

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)

Question 9

State the chemical used and the reason for intentionally initiating a CRUD burst prior to shutdown

Answer 9

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

Question 10

GALVANIC Corrosion

1. State the definition of the corrosion.
2. List the conditions which must exist to cause corrosion.
3. List the controls used to prevent or minimize the corrosion

Answer 10

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

Question 11

CREVICE Corrosion

1. State the definition of the corrosion.
2. List the conditions which must exist to cause corrosion.
3. List the controls used to prevent or minimize the corrosion

Answer 11

Localized attack at/in a mechanical crevice; The crevice becomes a concentration cell;

eliminate crevice, perform crevice cleaning, reduce contaminants so they cannot concentrate

Question 12

PITTING Corrosion

1. State the definition of the corrosion.
2. List the conditions which must exist to cause corrosion.
3. List the controls used to prevent or minimize the corrosion

Answer 12

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

Question 13

CHLORIDE STRESS Corrosion

1. State the definition of the corrosion.
2. List the conditions which must exist to cause corrosion.
3. List the controls used to prevent or minimize the corrosion

Answer 13

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

Question 14

FLUORIDE STRESS Corrosion

1. State the definition of the corrosion.
2. List the conditions which must exist to cause corrosion.
3. List the controls used to prevent or minimize the corrosion

Answer 14

Intergranular corrosion which occurs in high temperature water, stainless steel and the presence of oxygen and FLUORIDES; it involves selective attack along grain boundaries;

minimizing or eliminating FLUORIDES can prevent

Question 15

CAUSTIC STRESS Corrosion

1. State the definition of the corrosion.
2. List the conditions which must exist to cause corrosion.
3. List the controls used to prevent or minimize the corrosion

Answer 15

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

Question 16

BORIC ACID CORROSION WASTAGE

1. State the definition of the corrosion.
   
   2. List the conditions which must exist to cause corrosion.
   3. List the controls used to prevent or minimize the corrosion

Answer 16

Localized attack of ferritic steel ( carbon steel) due to high concentrations of boric acid resulting in metal wastage;

minimizing RCS (boric acid) leaks or cladding carbon steel or substituting stainless steel; Electrolysis occurs under boric acid where the surface is wetted

Question 17

State the parameters listed and their steady state and transient limits in the RCS chemistry TRm 8.4.1. RCS Chemistry Limits.

Answer 17

TRM 8.4.1 Steady State Chemistry Limits
    Dissolved O2	≤  .1ppm 
	 Chloride	≤ .15 ppm
	 Fluoride	≤ .15 ppm
TRM 8.4.1—Transient Chemistry Limits
    Dissolved O2	≤1 ppm 
	 Chloride	≤1.5 ppm
	 Fluoride	≤1.5 ppm

Question 18

Describe the process of HYDROGEN BLISTERING and HYDROGEN EMBRITTLEMENT in Caustic Stress corrosion of zircaloy

Answer 18

Hydrogen Blistering
Monatomic Hydrogen is formed by Zr + H20 4H at pH > 11

Monatomic Hydrogen combines to form H2 which will not diffuse and causes bulges in the cladding.

Hydrogen Embrittlement
Monatomic Hydrogen is formed by Zr + H20 4H at pH > 11.3

Monatomic Hydrogen diffuses into the Zircaloy resulting in a loss of ductility and tensile strength.