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Alawi_CHEMISTRY > Module 2: 7-11 > Flashcards

Module 2: 7-11 Flashcards

1
Q

FIVE Prevention Chemistry Control

A
  1. Passivation (Pretreatment/Pickling)
  2. Passivation (Passivators and Inhibitors)
  3. Cathodic Protection
  4. Removing Corrosive Agents (Corrodants)
  5. pH Correction
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2
Q

process involving careful control of coolant water chemistry and temperature during the pretreatment period

A

pretreatment or pickling

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3
Q

most desired oxide layer - a tightly adhering corrosion film

A

Fe3O4

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4
Q

Acidic solution is flushed through

A
  1. Condensate System
  2. Feedwater System
  3. Boiler System
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5
Q

Acidic solution is flushed to remove

A
  1. Scale
  2. Dirt
  3. Rust (FE2O3)
  4. Any oils and greases
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6
Q

It is the condition where a naturally active metal corrodes at a very low rate due to an oxide coating

A

Passivation

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7
Q

If added to water, this chemical substances can provide passivation by undergoing reduction at the metal surface

A

passivators or inhibitors

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8
Q

common passivator

A

potassium chromate (K2CrO4)

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9
Q

another effective passivation technique

A

painting the surface of the metal

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10
Q

This cathodic protection method is the external application of an external electric current to the iron so that it acts as a cathode and has no acidic areas

A

Impressed Current Method

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11
Q

This cathodic protection method is accomplished by the use of a sacrificial anode (such as zinc) which will corrode to provide the electrical current

A

Sacrificial anode method

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12
Q

The purpose of removing corrodants in water (by mechanical means) is to _____ the dissolved oxygen, carbon dioxide, dissolved solids (ions), and insoluble solids.

A

remove/eliminate

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13
Q

one method of removing corrosive agents by mechanical process

A

using deaerators

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14
Q

We may reduce the dissolved oxygen and carbon dioxide in water by chemical means through the use of ______ such as hydrazine (N2H4) and ______ such as morpholine (C4H9NO)

A

oxygen scavengers
carbon dioxide scavengers

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15
Q

When two dissimilar metals are in close vicinity and are exposed to an aqueous solution, ion transfer is ____. The result is a perfect environment for _______.

A

high, galvanic corrosion

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16
Q

5 Preventions of Galvanic Corrosion

A
  1. Cathodic protection by introducing a third metal (zinc) to the metals being protected
  2. Choosing relative surface areas - SA of anodic metal > cathodic metal
  3. Separating dissimilar metals w/ a non-conducting material
  4. Separating metals from a conductive environment
  5. Use of poorly conducting electrolytes
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17
Q

This occurs where the anodic site becomes fixed in a small area

A

pitting

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18
Q

presence of ____ promotes pitting at areas on the metal surface that are initially anodic w.r.t adjacent area

A

oxygen

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19
Q

the localized attack on a metal surface at the gap or crevice between two joining surfaces

A

Crevice Corrosion

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20
Q

confined space where access of working fluid from the environment is limited (low flow)

A

crevice

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21
Q

ions that cause pitting of iron and steel

A

chloride ions

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22
Q

a hazard due to the rapid penetration of the metal with little overall loss of mass

A

pitting corrosion

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23
Q

Ways to minimize Pitting Corrosion

A
  1. Avoid stagnant conditions
  2. Use correct metals and alloys less susceptible to corrosion
  3. Avoid agents in the medium
  4. Design the system with no crevices present
  5. Remove dissolved gases and chlorides
24
Q

Iron is ____ of all metal that is refined today

A

90%

25
Q

alloy of iron and carbon

A

steel

26
Q

4 types of steel

A

Carbon Steel
Alloy Steel
Stainless Steel
Tool Steel

27
Q

properties are dependent on the % of carbon present

A

Carbon Steel

28
Q

low carbon steel (mild steel)

A

0.04 - 0.30% Carbon

29
Q

medium carbon steel

A

0.31 - 0.60% carbon

30
Q

high carbon steel

A

0.61 - 1.5% carbon

31
Q

a carbon steel that is alloyed w/ various elements to improve its properties

A

Alloy Steel

32
Q

low alloy steel

A

other additional elements < 4%

33
Q

high alloy steel

A

other additional elements > 4%

34
Q

Cr is the main alloying element

A

Stainless steel

35
Q

stainless steel contains ____ of Cr

A

10 - 20%

36
Q

although chromium is corrosive, it exhibits ______

A

passivity

37
Q

well-suited to be made into tools

A

tool steel

38
Q

the growth of crack formation in a corrosive environment subjected to tensile stress

A

Stress Corrosion Cracking

39
Q

immune to SCC

A

pure metals (pure iron)

40
Q

usual victims of SCC

A

alloyed metals (steel)

41
Q

different metals are physically different in terms of their ____

A

crystalline structures

42
Q

an intergranular attack corrosion at the grain boundaries under tensile stress

A

SCC

43
Q

The interface between two grains in a polycrystalline material. These are defects in the crystal structure

A

grain boundary

44
Q

SCC is caused by

A

chemisorption

45
Q

a type of adsorption involving chemical reactions between the surface and the adsorbate

A

Chemisorption

46
Q

the adhesion of atoms from a gas, liquid, or dissolved solid to a surface

A

adsorption

47
Q

occurs when ions or atoms pass into a bulky material

A

absorption

48
Q

two types of SCC

A
  1. Chloride SCC
  2. Caustic SCC
49
Q

3 conditions for chloride SCC to occur

A
  1. Presence of chloride ions in the environment
  2. Presence of dissolved oxygen in the environment
  3. Metal is under tensile stress
50
Q

______ steels are extensively used in power plant facilities

A

Stainless

51
Q

_____ ions can enter auxiliary water systems via leaks in condenser

A

Cl⁻

52
Q

_________ can enter auxiliary water systems with feed and makeup water

A

Dissolved O2

53
Q

Chloride SCC is controlled by:

A
  1. Maintaining low chloride ion and oxygen content in the environment
  2. Use of low carbon steels
54
Q

3 conditions for caustic SCC to occur

A
  1. Presence of chloride ions
  2. Exposure of metals to concentrated caustic
  3. Metal is under tensile stress
55
Q

Oxygen concentration of greater than ________ ppm is required for caustic SCC to initiate.

A

10,000

56
Q

SCC Prevention

A
  1. Proper design
  2. Reduce stress
  3. Remove critical environmental contributors
  4. Avoid stagnant areas and crevices in heat exchangers
  5. Use low alloy steels (less susceptible than high alloy steels)
  6. Use Nickel-based alloy (e.g. Monel)
  7. pH control (not >12)

Alawi_CHEMISTRY (5 decks)

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