Summary NACE CP2 Study Guide Complete
Solutions Verified
oxidation
loss of electrons, positive charge atoms (anode)
reduction
gain of electrons, negative charged atoms (cathode)
forms of corrosion
uniform, crevice, galvanic (dissimilar metals) if two metals have to be couple then best to couple
metals close together in the galvanic series, pitting, intergranular, selective leaching (dezincification,
graphitization), velocity phenomena, environmental cracking (corrosion fatigue, hydrogen
embrittlement, SCC (Carbonates/bicarbonates)
High PH
Low hydrogen, alkaline, easy polarization
Low PH
high hydrogen, acidic, harder to polarize
High oxygen/High Hydrogen concentration
harder to polarize, work at the cathode only, called cathodic depolarizers.
factors affecting corrosion
A. anything that affects polarization. (temp. etc...)
B. concentration cells (oxygen, temp, etc.) In an oxygen concentration cell, what is the anode? (i.e. the
area with more or less oxygen?) In a metal ion cell, which area is the anode (i.e. the area with the
greater of lower concentration)?
C. Anaerobic bacteria (MIC) Absence of oxygen, depolarizer
polarization
a. corrosion current reduces with polarization
b. polarization increases with CP current applied
c. Occurs at surface of both anode and cathode.
shielding
a. shorted casings
b. proximity of other structures
c. reinforced concrete
amphoteric materials
corrode at low and high pH
on potential
native + polarization + IR drop
,polarized potential (off)
native + polarization
kirchoff's laws
a. voltage (series circuits)
b. current (parallel circuits)
faraday's law
W=KIT
relates weight loss of metal in a corrosion cell with time and current flow.
rate of corrosion
directly proportional to current flow
cathodic protection limitations
-shielding issues
-attenuation
-interference
-excessive current density
-contact with other metals (shorts)
measurement methods
true voltage reading
voltmeter reading
% error calculations
effect of voltmeter resistance
IR correction methods
test stations
allow electrical connection to structure for potential measurements and bonds.
Spans allow current and direction (IR drop).
inistall during construction
pH
measure of acidity(hydrogen ion concentration)
high pH is alkaline
low pH is acid
pH is a log function therefore each increase in pH is a factor of 10.
RECTIFIER
- positive to anode
-negative to structure
- AC suppy
-AC circuit breaker
-Transformer (increase or decrease voltage)
-AC fuse
-rectifier bridge - diodes change AC to DC
-Voltmeter and Ammeter
, -Surge Protection
-Filters
Esecondary = (Eprimary x Secondary turns) / primary turns
Reference electrode location
the reference electrode should be placed as close to the structure as practical. (underground piping or
tanks- over the center of the structure, multiple readings along the structure as required)
internal surfaces of storage tanks, waterfront and offshore structures (as close to the wall as possible)
impressed current vs sacrificial
impressed current- higher current, better control
sacrificial- no power required, less maintenance, easy to install, less land
AC considerations
capacitance, inductance, and resistance
hazardous AC potential - 15 Vac
Safe method of connecting ground0 cable to ground then to pipe
arcing hazards
corrosion rates from AC (current density)
switching or pulse rectifiers
Reference cells
CuCuSO4- most common, (soil, fresh water)
AgAgCL - seawater
Calomel- lab
Hydrogen - standard (lab)
zinc- stationary
Cathodic Protection
protect the cathode
attempts to bring the cathode potential as negative as the anode potential
NACE criteria- what are the exceptions?
-high temperature
-bacteria
-sulfides
-acid environments
-dissimilar metals
Solutions Verified
oxidation
loss of electrons, positive charge atoms (anode)
reduction
gain of electrons, negative charged atoms (cathode)
forms of corrosion
uniform, crevice, galvanic (dissimilar metals) if two metals have to be couple then best to couple
metals close together in the galvanic series, pitting, intergranular, selective leaching (dezincification,
graphitization), velocity phenomena, environmental cracking (corrosion fatigue, hydrogen
embrittlement, SCC (Carbonates/bicarbonates)
High PH
Low hydrogen, alkaline, easy polarization
Low PH
high hydrogen, acidic, harder to polarize
High oxygen/High Hydrogen concentration
harder to polarize, work at the cathode only, called cathodic depolarizers.
factors affecting corrosion
A. anything that affects polarization. (temp. etc...)
B. concentration cells (oxygen, temp, etc.) In an oxygen concentration cell, what is the anode? (i.e. the
area with more or less oxygen?) In a metal ion cell, which area is the anode (i.e. the area with the
greater of lower concentration)?
C. Anaerobic bacteria (MIC) Absence of oxygen, depolarizer
polarization
a. corrosion current reduces with polarization
b. polarization increases with CP current applied
c. Occurs at surface of both anode and cathode.
shielding
a. shorted casings
b. proximity of other structures
c. reinforced concrete
amphoteric materials
corrode at low and high pH
on potential
native + polarization + IR drop
,polarized potential (off)
native + polarization
kirchoff's laws
a. voltage (series circuits)
b. current (parallel circuits)
faraday's law
W=KIT
relates weight loss of metal in a corrosion cell with time and current flow.
rate of corrosion
directly proportional to current flow
cathodic protection limitations
-shielding issues
-attenuation
-interference
-excessive current density
-contact with other metals (shorts)
measurement methods
true voltage reading
voltmeter reading
% error calculations
effect of voltmeter resistance
IR correction methods
test stations
allow electrical connection to structure for potential measurements and bonds.
Spans allow current and direction (IR drop).
inistall during construction
pH
measure of acidity(hydrogen ion concentration)
high pH is alkaline
low pH is acid
pH is a log function therefore each increase in pH is a factor of 10.
RECTIFIER
- positive to anode
-negative to structure
- AC suppy
-AC circuit breaker
-Transformer (increase or decrease voltage)
-AC fuse
-rectifier bridge - diodes change AC to DC
-Voltmeter and Ammeter
, -Surge Protection
-Filters
Esecondary = (Eprimary x Secondary turns) / primary turns
Reference electrode location
the reference electrode should be placed as close to the structure as practical. (underground piping or
tanks- over the center of the structure, multiple readings along the structure as required)
internal surfaces of storage tanks, waterfront and offshore structures (as close to the wall as possible)
impressed current vs sacrificial
impressed current- higher current, better control
sacrificial- no power required, less maintenance, easy to install, less land
AC considerations
capacitance, inductance, and resistance
hazardous AC potential - 15 Vac
Safe method of connecting ground0 cable to ground then to pipe
arcing hazards
corrosion rates from AC (current density)
switching or pulse rectifiers
Reference cells
CuCuSO4- most common, (soil, fresh water)
AgAgCL - seawater
Calomel- lab
Hydrogen - standard (lab)
zinc- stationary
Cathodic Protection
protect the cathode
attempts to bring the cathode potential as negative as the anode potential
NACE criteria- what are the exceptions?
-high temperature
-bacteria
-sulfides
-acid environments
-dissimilar metals
