Materials Degradation and Prevention (MME 480)
(2-0-0-0 (2))
II Semester, 2012-13 II sem
Protection methods
CATHODIC AND ANODIC PROTECTION
OR
ELECTROCHEMICAL METHODS:
Cathode Protection:
The metal to be protected is made the cathode.
Metal dissolution: M →M +n
+ ne
+ 1
H d
Hydrogen evolution:
l ti H + e → H2
2
Therefore, Cathodic protection is achieved by supplying
electrodes to the metal structure to be protected. Above equation
indicate that the addition of electrons to the structure will tend
to suppress metal dissolution and increase the rate of hydrogen
evolution.
Cathodic protection
● This method is widely used to protect steel structures or any structure
such as pipeline buried in soil or immersed in water, water tanks, ship
hulls, chemical equipment, reinforcing rod in concrete and others.
● It can be better than coatings also (galvanizing), but in the presence of
coating,
i the
h protection
i isi muchh superior.
i
● Cathodic protection prevents both general corrosion and localized
corrosion, such as pitting, dezincification, intergranular corrosion
and SCC (stress Corrosion Cracking).
● The necessary condition is that the surface of the protected structure
be in intimate contact with the corrosive medium. That is why, it
cannot be applied to the portions above the water line in an
immersed structure.
● With cathodic protection, there is higher amount of H2 liberation on
the surface which may lead to hydrogen embrittlement.
● If a structure is electrically shielded, such as inner members in a
bundle of pipes or rods.
● Cathodic protection was introduced as early as in 1924 cby Sir
Humphery Devi to protect the Cu sheathing of the ship hulls of
that time, only to be discontinued as the stoppage of corrosion
prevented the release of toxic Cu ions and growth of marine
foulings adversely affecting the speed of the ship.
1. Locations that above the waterline and in
conditions where the environment is in vapor.
p
2. Non-conducting liquid like oil.
3. in electrically screened areas
4. in extremely corrosive environment because
then there would be huge cost due to power
requirements.
requirements
This applies to prevention of protective current flow through the electrolyte
because better conducting metallic path can be found.
Tank pprotected byy cathodic pprotection usingg sacrificial anode to p
protect the
tank below water line and the screen below.
However, it cannot protect the pipe that comes underneath the screen because it
is shielded.
Several other examples can be cited.
For example: if there is a bundle of pipes, only the external ones are protected. In
case of pipe buried in cinders, all the current taken up by the conducting cinders
and does not protect the pipes.
With regard to the magnitude of the current required for,
protection, the effect of external cathodic current on the E vs log
i (Evans Diagram) diagram of the corroding system may be
considered.
Impressing a cathodic current is essentially the polarization of the cathodic line
(dc) furthers towards the active (anodic direction) (cf) (bold line).
When the polarization curve reaches the level of EA0, the open-circuit-potential
(OCP) of anodic reaction, the corrosion stops completely as the driving force (EC
– EA) becomes zero and icorr = i0.
i applied = ic − i a ⇒ ic − i0 (at f position)
Then it means complete protection.
However, a current of lower magnitude will provide some degree of protection.
For example: the applied current corresponding to the point b and c (iapplied = bc).
If polarization is continued beyond the point f, i.e.
if the current applied is more than required for complete protection, the
situation is described as overprotection.
● An overprotected steel structure will still be protected, but this will
mean an unnecessary wastage of current.
● The risk of stray current corrosion of the neighbouring metallic
structure also enhances with the excess current.
● For amphoteric metals like Zn and Al, over protection increases
corrosion
i because
b off excess alkalis
lk li generated
t d att the
th metal
t l surface.
f S h
Such
a situation is sometimes referred to as cathodic corrosion.
ZnO + H 2 O + 2OH − → [ Zn(OH ) 4 ] 2−
ZnO + 2 H + → Zn + 2 + H 2 O
Al 2 O3 ( S ) + 2OH − ( Aq ) ⇔ 2[ Al (OH ) 4 ] − (basic)
Al 2 O3 ( S ) + 6 H + ( Aq) ⇔ 2[ Al (OH ) 6 ]3+
However, current requirement will be less if the cathodic process is concentration
However
polarization controlled.
1. Use of sacrificial anode
2. Use of impressed current
Sacrificial Anode:
A less noble metal galvanic contact with another metal
hi h up in
higher i the
h emff series
i behaves
b h as an anode
d member b in
i
the couple and sacrifices its life to protect the noble metal,
q
which acts as a cathode. All that is required is to p
provide
galvanic coupling between the two metals. This may be
achieved by direct contact, as in a coating, i.e. galvanizing,
or by electrical contact through a conducting wire.
wire Mg,
Mg Zn
and Al are used to protect steel.
A structure is protected if
current (I) enters the
structure from the electrolyte.
However, if current (I) leaves
the
h metall to theh electrolyte,
l l
corrosion is enhanced.
Without supplying
external current
Requirements to be fulfilled
1. The potential difference between the anode and the corroding structure must
be large enough to overcome the local cells on the corroding metal.
2. The anode material must have sufficient electrical energy contact to last for a
reasonably long period replacement, the electrical energy contact is expressed
in terms
e s oof “amperes
pe es hour
ou pe
per pou
pound
d oor kg”
g means
e s a pou
pound
d oof material
e will
w
last for so many hours if 1 ampere current is continuously discharged by it.
3. The self corrosion of the anode material should be minimized so that it is used
up efficiently for the current output only.
1. Mg corrodes too fast in aqueous solution and therefore can be used only in low
conducting medium soil environment. However, Mg has very low protection efficiency
(ampere-hour per pound). Sometimes, Mg is often alloyed with Zn and Al to give better
efficiency. However, main reason for the choice of Mg as sacrificial anode is its very
active potential.
2. Zn anode is used in its commercially pure form and has an efficiency of 95% of the
theoretical. Zn is generally used in sea water application.
3. The potential difference between steel and pure aluminium is not large and under
several condition Al tends to polarize and becomes passive. That time, passive Al acts
as cathode with respect to steel. To avoid such situation and also to increase efficiency,
the Al is often alloyed with tin,Zn,Hg or Fe.
4. In soil, the anodes are usually surrounded by a backfill comprising coke, gypsum and
bentonite. The purpose of the backfill is to provide conducting surroundings that help
i the
in th uniform
if di h
discharge off current,
t when
h Al alloy
ll isi used
d in
i anode,
d NaCl
N Cl is
i added
dd d to
t
the backfill, which counters the passivation.
5. Cathodic pprotection should be invariablyy employed
p y with an insulatingg coatingg on the
surface to be protected. A bare surface needs a large amount of current for protection,
which means a shorter life for the sacrificial anode. Coated surfaces need to be
protected only at pores or leaks and the current requirement decreases drastically.
Sacrificial anodes are used when:
1 The
1. Th currentt requirement
i t are low
l
2. The soil possesses low resistivity.
3 For
3. F shorter
h term protection.
i
4. When electrical power supply is unavailable.
5. When there are interferences due to stray current.
In aan impressed
p essed cu
current
e t syste
system cat
cathodic
od c pprotection
otect o syste
system tthee p protective
otect ve
current is supplied by an external dc supply, which is usually a rectifier but may
be a generator or a battery as well. Its negative terminal is connected to the
structure to be protected and the positive terminal to an auxiliary anode that
discharges the current.
Supplying
external current
Importance of Backfill?
In soil, the anode or more often, a “ground bed” comprising
a number of anodes is surrounded by backfill for better conductivity
● Common materials used for the auxiliary anode are steel scrap, graphite,
aluminium lead and high silicon iron (durion).
aluminium, (durion)
● Platinized Ti has indefinite life as an auxiliary anode and is being increasingly
used in the protection of marine structures.
● P
Proper i
insulation
l ti off cables
bl mustt beb warranted.
t d
● Protective current are determined empirically. The current requirement is low in
static environments but increases under flowing conditions of the environment.
● P t ti currentt becomes
Protective b enormously
l high
hi h in
i aggressive
i medium.
di E Fe
E.g. F in
i
concentrated H2SO4. Cathodic protection become economically nonviable
process in that case.
● Th effectiveness
The ff ti off protection
t ti is i monitored
it d through
th h potential
t ti l measurement. t
If a metallic object is placed in a strong current field, a potential difference
develops across it and accelerated corrosion occurs at points where current
leaves the object and enters the soil.
That means stray current are those that travels away from the intended circuit.
A part of the current discharged by the ground bed in an impressed current
cathodic protection system may enter a metallic structure in the vicinity of the
protected structure. To complete the circuit, it leaves the structure, enters the
electrolyte (soil) and then goes to the cathode (i.e. protected structure).
Fig below shows such situation.
The steel pipe is the neighboring structure to the cathodically protected
steel tank.
tank The portion of the steel pipe where the current is entering
from the soil gets the benefit of cathodic protection, whereas, the portion
where the current leaves the pipeline to enter the soil acts as anode and
undergoes corrosion. This type of corrosion is called stray current
corrosion. Eventually, a leak may appear in the pipeline.
Application of a paint coating to the pipeline simply aggravates the
situation as the current now gets discharged from a few defect sites in the
paint coat where anodic current density increases enormously.
Preventive Measure:
The structure can be saved from stray current corrosion by simply short
circuiting it to the protected tank.
tank In that case the current will follow the
metallic path to complete the circuit and the pipeline will also be a part of
the protected structure.
Other Cases:
Case I: In a metropolis stray currents from tram car lines often being hazards to the
metallic structures underneath, particularly the municipal water supply lines and
sewage lines
li are affected.
ff t d
Case II:
Th hull
The h ll off a ship
hi becomes
b prone to
t stray
t currentt corrosion
i if a welding
ldi operation
ti isi
carried out on it with the help of a welding motor generator located on shore. This is
because of current leakage from the hull to the water and returning to the source.
Preventive measure: The incidence of corrosion can be avoided by shifting the
dc generator to the ship.
1. It is versatile with wide operating conditions possible.
2. It is effective even in high resistance soil.
3. The status of protective coating can be monitored.
1. It requires an external power source.
2. Stray current due to ICCP may affect nearby metallic structures.
3. Overprotection may lead to hydrogen embrittlement and disbanding
of coating (Cathodic Corrosion).
4. Design of anode bed needs to be carefully considered.
Note: The anodic reaction that occur on the surface of impressed current anode is not
metal dissolution rather it could be some other reactions:
2 H 2 O → O 2 + 4 H + + 4e
2Cl − → Cl 2 + 2 e
¾ Anodic
od c p
protection
otect o iss based oon tthee p
principle
c p e oof pass
passivation
vat o oof metals.
eta s. Thee
active passive metals such as iron, Cr, Ni, Ti and their alloys attains passivity
on anodic polarization and in this state their rate of corrosion decreases may
fold, sometimes by the order of 105-10
106.
¾ In anodic protection, the potential of the system is maintained in the passive
region with the help of a potentiostat. The structure to be protected is
connected to one terminal of potentiostat, the second terminal to be protected is
connected to an auxiliary cathode, and a constant potential is maintained with
respect to a reference electrode connected to the third terminal. The anodic
protection arrangement for a steel tank containing sulphuric acid is shown
below.
¾ Principle
p of anodic p
protection based on mix-potential
p theoryy
An active passive metal corroding in an acid medium has been depicted (fig II). For the
attainment of passivity the anodic current must be increased to icritical, which corresponds
t EPP. The
to Th currentt automatically
t ti ll adjusts
dj t itself
it lf as theth potentiostat
t ti t t sets t the
th potential
t ti l to
t
this value. The magnitude of applied current is iappl(1).
However, the
H th potential
t ti l has
h tot be
b raised
i d further
f th to t Eprot to
t maintain
i t i the
th metalt l in
i the
th
passive region and the corresponding applied current is iappl(2). A large difference in
magnitude of these two applied current values may be noted. The current required to
maintain
i t i passivity
i it corresponds
d nearly
l to
t ipassive which
hi h again
i indicate
i di t the
th rate
t off corrosion
i
under protection.
At the
A h corrosion
i potential,
i l applied
li d anodic
di current density
d i is i zero. Since,
Si at Ecorr and
d the
h
corrosion rate is 100 μ a/cm2. If the potential is increased to E1 with a potentiostat, an
applied anodic current density of (1000-10) or 990 μ a/cm2 is required.
At E2 corresponding to the icrit, an applied anodic current of approximately 10,000 μ
a/cm2 is required to maintain this potential, while at E3, applied current decreases to
0 9 μ a/cm2. At the optimum potentioal,
approximately 0.9 potentioal EA, (The optimum potential for
anodic protection is the midway in the passive range).
The applied anodic current is approximately 1 μ a/cm2, which is equal to the
corrosion rate at this potential.
The system illustrated in fig II can be cathodically protected by applying cathodic
current. Applied cathodic current density is equal to the difference between the total
reduction rate (ic) and total oxidation rate (ia).
If potential is shifted to Ec, an applied cathodic current density of (10000-1) or
approximately 10000 μ a/cm2 cathodic applied current produces the same effect or
reduction in corrosion rate as that of 1 μ a/cm2 applied anodic current.
{i appl ( cathodic ) = ic − i a }
This example demonstrates that anodic protection is much more efficient than
cathodic protection in acid solution.
1. It can be used in extremely corrosive environment.
2
2. The corrosion
i rate can be continuously
i monitored
i because corrosion
i rate is
i
proportional to the current flow.
3. Low currents are required
q once p
passivation is achieved.
4. Precise control based on polarization data.
5
5. E ll t currentt distribution
Excellent di t ib ti can be
b achieved.
hi d
11. It will not work if passivity is poor (aq: steel in HCl).
HCl)
2. Corrosion is never stopped completely.
3. There is no protection beyond water line.
4
4. High current is needed to passivate the metal in the beginning
beginning.
5. High cost is also involved.
6. All components must function properly for anodic protection to be
successful.
successful
7. It will not work in non-electrolyte environment like for corrosion in
soil, air or oil.
8
8. It cannot work for different metal in contact.
contact
1. Digesters in pulp/paper industry (where NaOH and Na2S
are presentt att 1750C).
C)
2. Phosphoric acid plant
3 Ammonia fertilizer plants for carbon steel.
3. steel
