Module 2: 7-11 Flashcards
FIVE Prevention Chemistry Control
- Passivation (Pretreatment/Pickling)
- Passivation (Passivators and Inhibitors)
- Cathodic Protection
- Removing Corrosive Agents (Corrodants)
- pH Correction
process involving careful control of coolant water chemistry and temperature during the pretreatment period
pretreatment or pickling
most desired oxide layer - a tightly adhering corrosion film
Fe3O4
Acidic solution is flushed through
- Condensate System
- Feedwater System
- Boiler System
Acidic solution is flushed to remove
- Scale
- Dirt
- Rust (FE2O3)
- Any oils and greases
It is the condition where a naturally active metal corrodes at a very low rate due to an oxide coating
Passivation
If added to water, this chemical substances can provide passivation by undergoing reduction at the metal surface
passivators or inhibitors
common passivator
potassium chromate (K2CrO4)
another effective passivation technique
painting the surface of the metal
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
Impressed Current Method
This cathodic protection method is accomplished by the use of a sacrificial anode (such as zinc) which will corrode to provide the electrical current
Sacrificial anode method
The purpose of removing corrodants in water (by mechanical means) is to _____ the dissolved oxygen, carbon dioxide, dissolved solids (ions), and insoluble solids.
remove/eliminate
one method of removing corrosive agents by mechanical process
using deaerators
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)
oxygen scavengers
carbon dioxide scavengers
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 _______.
high, galvanic corrosion
5 Preventions of Galvanic Corrosion
- Cathodic protection by introducing a third metal (zinc) to the metals being protected
- Choosing relative surface areas - SA of anodic metal > cathodic metal
- Separating dissimilar metals w/ a non-conducting material
- Separating metals from a conductive environment
- Use of poorly conducting electrolytes
This occurs where the anodic site becomes fixed in a small area
pitting
presence of ____ promotes pitting at areas on the metal surface that are initially anodic w.r.t adjacent area
oxygen
the localized attack on a metal surface at the gap or crevice between two joining surfaces
Crevice Corrosion
confined space where access of working fluid from the environment is limited (low flow)
crevice
ions that cause pitting of iron and steel
chloride ions
a hazard due to the rapid penetration of the metal with little overall loss of mass
pitting corrosion
Ways to minimize Pitting Corrosion
- Avoid stagnant conditions
- Use correct metals and alloys less susceptible to corrosion
- Avoid agents in the medium
- Design the system with no crevices present
- Remove dissolved gases and chlorides
Iron is ____ of all metal that is refined today
90%
alloy of iron and carbon
steel
4 types of steel
Carbon Steel
Alloy Steel
Stainless Steel
Tool Steel
properties are dependent on the % of carbon present
Carbon Steel
low carbon steel (mild steel)
0.04 - 0.30% Carbon
medium carbon steel
0.31 - 0.60% carbon
high carbon steel
0.61 - 1.5% carbon
a carbon steel that is alloyed w/ various elements to improve its properties
Alloy Steel
low alloy steel
other additional elements < 4%
high alloy steel
other additional elements > 4%
Cr is the main alloying element
Stainless steel
stainless steel contains ____ of Cr
10 - 20%
although chromium is corrosive, it exhibits ______
passivity
well-suited to be made into tools
tool steel
the growth of crack formation in a corrosive environment subjected to tensile stress
Stress Corrosion Cracking
immune to SCC
pure metals (pure iron)
usual victims of SCC
alloyed metals (steel)
different metals are physically different in terms of their ____
crystalline structures
an intergranular attack corrosion at the grain boundaries under tensile stress
SCC
The interface between two grains in a polycrystalline material. These are defects in the crystal structure
grain boundary
SCC is caused by
chemisorption
a type of adsorption involving chemical reactions between the surface and the adsorbate
Chemisorption
the adhesion of atoms from a gas, liquid, or dissolved solid to a surface
adsorption
occurs when ions or atoms pass into a bulky material
absorption
two types of SCC
- Chloride SCC
- Caustic SCC
3 conditions for chloride SCC to occur
- Presence of chloride ions in the environment
- Presence of dissolved oxygen in the environment
- Metal is under tensile stress
______ steels are extensively used in power plant facilities
Stainless
_____ ions can enter auxiliary water systems via leaks in condenser
Cl⁻
_________ can enter auxiliary water systems with feed and makeup water
Dissolved O2
Chloride SCC is controlled by:
- Maintaining low chloride ion and oxygen content in the environment
- Use of low carbon steels
3 conditions for caustic SCC to occur
- Presence of chloride ions
- Exposure of metals to concentrated caustic
- Metal is under tensile stress
Oxygen concentration of greater than ________ ppm is required for caustic SCC to initiate.
10,000
SCC Prevention
- Proper design
- Reduce stress
- Remove critical environmental contributors
- Avoid stagnant areas and crevices in heat exchangers
- Use low alloy steels (less susceptible than high alloy steels)
- Use Nickel-based alloy (e.g. Monel)
- pH control (not >12)
