Transcript ANODIC PROTECTION - Universiti Sains Malaysia

ANODIC
PROTECTION
Feasibility of anodic protection
is firstly demonstrated and
tested by Edeleanu in 1954
Corrosion control of metal structure by impressed
anodic current.
Interface potential of the structure is increased into
passive corrosion domain.
Protective film is formed on the surface of metal
structure which decrease the corrosion rate down
to its passive current.
Can be applied for active-passive metals/alloys
only.
Anodic protection can decrease corrosion rate
substantially.
Anodic protection of 304SS exposed to an aerated
H2SO4 at 300C at 0.500 vs. SCE
Acid
concentration, M
NaCl, M
Cor. Rate μm/y
(Unprotected)
Cor. Rate μm/y
(Protected)
0.5
10-5
360
0.64
0.5
10-3
74
1.1
0.5
10-1
81
5.1
5
10-5
49000
0.41
5
5
10-3
10-1
29000
2000
1.0
5.3
Metals which can be passivated and deactivated
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The metals which can be passivated by oxidation
and activated by reduction are those which have a
higher oxide less soluble than a lower oxide and
will thus each corrosion domain forms an angle.
The lower the apex of this angle in the diagram
(such as titanium, chromium and tin etc.), the
easier it will be to passivate the metal by oxidation
and it will be difficult to reactivate the passivated
metals by reduction.
Titanium and
chromium can be
passivated very
easily and their
passivation
process will occur
more often than
not,
spontaneously,
even in the
absence of
oxidizing agent.
Experimental potential - pH diagram for chromium
Anodic polarization curve of AISI
304 SS in 0.5 M H2SO4
Anodic protection parameters :
(can be obtained from anodic polarization
measurement)
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Range of potential in which metal is in
passivation state (protection range)
Critical current density
Flade potential
Optimum potential for anodic protection is
midway in the passive region
Flade potential (EF)
E F  E  n0,059pH
O
F
In which EFO : Flade potential at pH = 0
n : a constant (between 1 and 2) depends of metal
composition and environment conditions
 Metals having EF < equilibrium potential of
hydrogen evolution reaction (HER) can be passivated
by non oxidizing acid (i.e. titanium)
 Increasing temperature will reduce the protection
potential range and increase the critical current
density and therefore anodic protection will be more
difficult to be applied.
Parameters that should be
considered for anodic protection
design (Flade potential is not
included in the figure)
10
Influences of temperature and chloride concentration on
anodic polarization curve of stainless steels
(schematic figure)
Anodic polarization curves of a mild steel in 10% sulfuric acid at
22 and 600C

For metals exposed in aggressive ions
containing - environment

Interface potential of metal should be :
Eprot>Elogam>Eflade

Basically : Eflade is equal or slightly lower than
Epp.
Schematic figure of potential range for anodic protection of a
stainless steel which is susceptible to pitting corrosion in an
environment containing aggressive ions
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Increasing of chloride ions concentration
results in a significant decrease of protection
potential range.
Consequently, in aggressive ions containingenvironment anodic protection is applied only
for metals which have relatively high
protection potential and high pitting
potential.
Increasing temperature leading to a decrease
of Eprot
Schematic figure of anodic protection system for
protecting inner surface of storage tank
CATHODES FOR ANODIC PROTECTION
Should be permanent and can be used as current
collector without any significant degradation.
 Having large surface area in order to suppress
cathodic overpotential.
 Low cost.
Platinum clad brass can be used for anodic protection
cathodes because this cathode has low overpotential
and its degradation rate is very low, however it is
very expensive.

Cathodes used in recent anodic protection
systems
Comparison of anodic and cathodic protection :
Applicability
Corrosives
Anodic
protection
Active-passive
metals only
Weak to
aggressive
High
Relative
investment cost
Relative
Very low
operation cost
Equipment
Potentiostat +
cathode/s
Cathodic
protection
All metals
Weak to
moderate
Low
Mediums to
high
Sacrificial anodes or
DC power supply +
ICCP anode/s
Throwing
power
Very high
Low to high
Significant of
applied current
Often a direct
measure of
protected
corrosion rate
Operating
conditions
Can be
accurately and
rapidly
determined by
electrochemical
measurement
Complex
Does not
indicate
corrosion rate
Must usually be
determined by
empirical
testing
Typical applications of anodic protection

Anodic protection has been applied to protect storage
tanks, reactors, heat exchangers and transportation vessels
for corrosive solutions.

Heat exchangers (tubes, spirals and plates types) including
their anodic protection systems can be easily to purchase in
the market.

i.e. AISI 316 SS HE is used to handle 96-98% sulfuric acid
solution at 1100C. Anodic protection decreases corrosion
rate of the stainless steel, initially from 5mm/year down to
0.025mm/year and therefore less contaminated sulfuric
acid can be obtained.
DATA
Effect of chromium content on critical current density
and Flade potential of iron exposed in 10% sulfuric acid.
Effects of nickel and chromium contents on critical current
density passivation potential in 1N and 10 N H2SO4 containing
0.5 N K2SO4
Requirement of critical protection current densities
for several austenitic stainless steels (18-20 Cr , 8-12
Ni) exposed in different electrolytes
Protection current density : current density required
to maintain passivity
Effect of sulfuric acid concentration at 240C on the corrosion
rate and critical current density of stainless steel
Effect of stirring of electrolyte on the corrosion rate and
requirement of current density to maintain passivity on a
stainless steel at 270C
Current density requirements for anodic
protection
Anodic Protection Using a Galvanic Cathode
A cylindrical tank of 304 stainless steel for
storing deaerated sulfuric acid (pH=0) is
found to corrode rapidly. To provide anodic
protection, a galvanic cathode of platinum
will be installed. The tank has a diameter of
5 m and the depth of acid is 5 m.
a. Draw a labeled sketch of the polarization
diagram for the tank and calculate the
passivation potential versus SHE.
b. What is the area of platinum required to
ensure stable passivity?
c. What will the corrosion potential be when
the tank achieves passivity?
Data:
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304 stainless steel:
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Ecor = -0.44 V vs SCE
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icor = 10-3 A/cm2
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Tafel slope anodic = 0.07 V/decade
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icrit = 1.4 x 10-2 A/cm2
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ipas = 4 x 10-7 A/cm2
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H+ reduction on platinum
i0 = 10-3 A/cm2
Tafel slope cathodic = 0.03 V/decade
SCE = +0.2416 V vs.SHE